MARINE ENGINES & PROPULSION
Ranger Hope © 2015
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and study only on the understanding that users exercise due care and do not
neglect any precaution which may be required by the ordinary practice of seamen
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accepted resulting from use.
Chapter 1: Principles of internal combustion engines
1.4 Timing-
valves and injection
1.5 Air supply
- turbo charging, after coolers
1.6 Engine
protection – devices
Chapter 2: Fuel storage and handling
2.1 Fuel storage
and survey compliance
Chapter 3: Fuel supply, injection and
control
3.3 Fuel injectors, combustion chamber
3.4 Diesel fuel
injection pumps
Chapter 4: Lubrication systems
4.1 Lubrication purpose and components
4.3 Contamination, analysis and servicing
5.3 Faults, maintenance and servicing
Chapter 6: Gearing and tailshafts
6.1 Gears and
clutch mechanisms
6.2 Reverse and
reduction gear boxes
6.3 The shaft
driven transmission system
Chapter 7: Propellers and
propulsion systems
7.2 Methods of
propulsion reversal
7.3 Other
propulsion systems -
Azimuth thrusters, Voith Schneider, Jets.
Chapter 8: Operating
an outboard motor
8.1 Outboard systems and performance
8.3 Engine protection and devices
8.4 Start up,
operation and shut down
8.5 Preventative maintenance schedule
Chapter 9: Operating an inboard
diesel engine
9.2 Engine protection and devices
9.3 Start up,
operation and shut down
9.5 Safety
aspects when working on engines
9.6 Preventative maintenance schedule
10.4 Loss of
lubricating oil pressure
10.6 Excessive vibration, fluctuation of engine
revolutions
Introduction
This book describes the main engines, auxiliary motors and propulsion systems found on small commercial vessels of less than 80 metres (less than 500 tons).
Marine engines are described by their crankshafts’ revolutions per minute (rpm) – typically as low speed engines (less than 400 rpm), medium speed engines (400 -1000 rpm) or high speed engines (over 1000 rpm).
As this small commercial fleet is principally fitted with high speed diesel inboard engines, or in the smaller craft increasingly with high speed petrol outboard engines, these are described most fully.
Definitions
The following technical terms are used in this text.
Bottom dead centre – the position where the piston has reached its furthest distance from the cylinder head, and will subsequently reverse its direction of stroke (bdc).
Calorific value - fuel contains heat energy which is released in the combustion process called the fuel’s calorific value, measured in joules per kilogram of fuel.
Cam lift - the distance that the peak of the cam’s lobe extends from the round which is the same as the valve opening plus the tappet clearance (valve lash).
The cam lift profile causes the valve to open rapidly then stay open momentarily before rapidly re-closing. Dwell is the angle of the cam over which the valve remains temporarily in the fully open position.
Compression ratio - is the comparison the volume of the cylinder as the piston sweeps from BDC position up to TDC position, sometimes called the ratio of the unswept volume to the swept volume of the cylinder. A compression ratio of 12:1 would mean that the air (in diesel engines) or air/fuel (in petrol engines) has been compressed to one-twelfth its original volume. Diesel engines need a high compression ratio heat the compressed air sufficiently to ignite the fuel.
Compression ratio =
maximum cylinder volume ÷ minimum
cylinder volume
Flash point - in order to catch on fire, a material must be heated
sufficiently to cause it to partially vaporize. The lowest temperature at which the vapour
arising from the fuel gives off a flammable mixture that will ignite when flame
is momentarily applied under defined test conditions is called its flash
point. Technically petrol and diesel are cocktails of hydrocarbons of
different flash points. Commonly from - 40°C to 0°C for petrol and 60°C for
diesel is regarded as the temperature that they will burn. (Diesel’s lower
volatility means it is safer to use than petrol). If a fire is cooled below its
flash point then the flame will not be sustained.
Force - an influence which tends to change the motion or direction of a body (such as pushing or pulling) measured in newtons (N). A force can either:
Start moving a body from rest or bring a moving body to rest.
Increase or decrease the speed of a body.
Change the direction of motion of a moving body.
Heat transfer - Heat moves (transfers) in
three ways:
Convection Heat moving within a liquid or
gas.
Conduction Heat moving through a solid.
Radiation Heat energy traveling out as
heat rays (direct heat).
The hottest air or water experiencing convection move
upwards, the coolest downwards, thus forming a gyre rotation in a sealed
system. Conducted
heat is movement away from its origin along metal components. Radiated heat (from
exhaust manifolds) may char or set it alight material that is close, consequently
removing surrounding flammable materials (boundary clearance) is essential to
limit spread of fire.
Power - is the amount of work or energy expended in a given time or the capacity to do work measured as Watts (W). A watt is work at the rate of one joule per second.
Power = Force x
Distance = newton/metres per second or joules per second.
Time in seconds
Force is in newtons (N), distance in metres (m) and time in seconds (s). Therefore the formula first answer is in units of Nm/s or j/s. However as1 Nm = 1 joule and 1 joule per second = 1 watt so the final answer converts to watts. Engine power is measured in kilowatts (kW) -1000 W = 1 kW.
Scavenging - describes the elimination of the burned exhaust gases from a cylinder, pushed out by the incoming stream of air induction. Valve overlap assists in the process.
Thermal efficiency - is a comparison of the work done at the flywheel to the amount of energy contained in the fuel, and is expressed as a percentage.
Top dead centre - the position when the piston has reached its least distance from the cylinder head, and will subsequently reverse its direction of stroke (tdc).
Torque - describes a force tending to cause a rotational movement about a point, also called a turning or twisting effort. Torque is the force exerted, but not moved, over a distance measured in newton/metres (Nm). For instance, the force the connecting rod exerts on the crankshaft.
Torque = Force x
Distance in newton/metres (Nm).
Turbulence - a rapidly swirling motion of the air as it enters the combustion chamber. In most engines it is deliberately induced as the violent movement helps ensure even mixing of the fuel and air. It enhances flame propagation speeding up the combustion process once the fuel has ignited.
Work - the amount of energy used to
overcome resistance in moving an applied force through a distance as Work = Force x Distance in joules
It could be described in newton/metres (Nm) as force is measured in newtons (N) and the distance is measured in metres (m).To prevent confusion between work and torque, the unit given to the formula for work is the joule (j), by conversion as:
One newton/metre =
one joule.
Volumetric efficiency - is the ratio between the swept volume of a cylinder (between tdc & bdc) and the actual volume of air drawn in during the induction stroke. The efficiency varies depending on the design, operating conditions and engine speed. A turbo charged engine will have a higher volumetric efficiency (in excess of 100%) than that of a normally aspirated engine.
Valve overlap - is the period which both the inlet valve and exhaust valve are open simultaneously. For instance, if the inlet and exhaust valve are both open at 10° before TDC to 3°after TDC the valve overlap is over a 13° angle.
The purpose of valve overlap is to ensure that are exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place. It also has a cooling effect.
Valve rotators - rotate a valve each time it opens to ensure even wear and prevent exhaust valves from localised burn out.
Chapter 1: Principles of
internal combustion engines
1.1 Operating principles
Combustion is the process in which fuel heated beyond its flash point ignites and gives off energy and the waste products as exhaust gasses (carbon dioxide, carbon monoxide and water). The three elements necessary for combustion (or fire) are:
Oxygen + Heat + Fuel =
Explosion or Fire
Internal combustion engines suck in air to access the oxygen and use hydrocarbon fuels including petrol, diesel and liquid propane gas.
The
internal combustion process
Internal combustion engines use successive explosions of atomised air and fuel mix to force a piston down a gas tight internal cylinder. The piston is connected through a big end bearing to a cranked shaft that is weighted by a flywheel to assist its rotary momentum. Thus with three primary moving parts (piston, connecting rod and crankshaft) the piston’s reciprocating motion drives the crankshaft’s rotary motion.
A sturdy metal casting of an engine block holds the internal parts and the many ancillary components required for smooth and continuous operation. These include:
A lubrication sump (oil reserve); a cylinder head with gas outlets and inlets; and in the drawing shown below, an over head camshaft driven timing gear within a rocker box. For it to work timing is everything. The fuel (diesel, petrol or gas) and air (oxygen) charge must be inserted, the exhaust removed and a compression seal reinstated at the precise instant for the explosive ignition. The heat and noise from multiple explosions as well as wear on metal surfaces must be minimised.
To ensure a gas tight compression seal, the cylinder liners in the engine block are bored to very tight tolerances and the barrel of the piston is grooved to accept close fitting sprung metal rings - that makes for a tight sliding fit against the cylinder walls.
In order to insert the fuel/air mix and exhaust the products of the combustion, sealable inlet and outlet passageways must be provided. Typically these passageways take the form of either ports (holes in the cylinder wall that are covered and uncovered by the passing of the piston) or by valves (steel stoppers that plug and unplug holes in the cylinder head at the correct timing intervals).
Basic features
The simplest engines often use ports and more the complex use valves, though hybrids like that shown below use both. The piston is connected to the big end bearing of the crankshaft by means of a connecting rod. The piston is forced down the cylinder by the expansion of combustion gases to turn the crankshaft. With the piston as low as it will go in the cylinder, it is said to be at bottom dead centre (bdc). The momentum of the flywheel weighted crankshaft rotating on its main bearings forces the piston back to the top of the cylinder, ready for the next explosion. When it is as high as it will go in the cylinder, it is at top dead centre (tdc).
The distance the piston moves from tdc to bdc is called the stroke of the engine.
The turning crankshaft drives the camshaft by means of timing gears (or by sprockets and a chain). In the drawing shown above, the side camshaft is driven by crankshaft gears. Oval cams on the camshaft open the inlet and exhaust valves at the correct time by means of push-rods actuating rockers. An inlet port (or alternately a valve) allows air or fuel/air mixture to be drawn into a cylinder ready for combustion, while an exhaust valve lets exhaust gas out of a cylinder.
The arrangement of ports or camshafts and valves differs depending on engine design. The drawing above shows a two stroke engine with its piston nearing tdc, its exhaust valve closed and inlet port covered, so compressing the fuel/air charge immediately prior to ignition. At the end of the power stroke the cam will open the exhaust valve to flush the cylinder ready for re-charge and re-ignition.
The work cycle
A sequence of four operations called the work cycle must occur for engines to work:
Induction – a charge of air (diesels) or air/fuel mix (petrol) is directed into the cylinder to provide the oxygen required for combustion.
Compression - the charge is compressed to optimise the result of combustion.
Power - the air/fuel mix (petrol) is ignited by a spark, or with diesels the fuel is injected to ignite in the superheated compressed air. The expansion of the gasses from this explosive ignition exerts a force upon the piston that turns the crankshaft.
Exhaust- The products of this explosion (the exhaust gasses) is ejected so the cycle can be repeated.
The work cycle of
induction (suck), compression (squeeze), power (bang) and exhaust (push) are
achieved by differing configurations or types of engines including four stroke or two stroke engines. The term stroke means the directional travel of
the piston along the cylinder from tdc - bdc or from bdc - tdc.
The four-stroke engine
takes two complete revolutions of the
crankshaft to complete the work cycle. The two-stroke does it in one complete revolution of the
crankshaft. This means that a two-stroke engine has twice as many power operations
as a four-stroke engine at the same speed, so it may produce more power and
acceleration but at the expense of fuel efficiency and reliability.
Petrol or diesel
Due to availability and convenience of handling, petrol (or
gasoline) and diesel (or distillate) are the most common small
vessel marine propulsion fuels. Larger vessels often use a cheaper and less
refined tarry fuel oil requiring heat to liquefy sufficiently for the
combustion process. Petrol and diesel are cocktails of hydrocarbons wth differing flash points, commonly from - 40°C to 0°C for
petrol and 60°C for diesel, this being the temperature that they will vapourize to ignite. Diesel’s higher flash point makes
it safer than petrol as any escaping fuel vapours can
pool in a boat’s bilges with consequent risk of explosion.
The diesel engine uses the piston’s sweep of the cylinder from bdc position up to tdc position to compress the inducted air, raising its pressure and consequent heat to such an extent (1000ºC) that when fuel is injected it immediately ignites. Diesel engines need a high compression ratio compressed the air sufficient to reach the temperature for ignition. A 16:1 to 20:1 compression ratio, common to small high speed diesel engines means that the air has been compressed to one-sixteenth or one twentieth of its original volume. The wall of the cylinder needs to be heavy gauge to contain the pressure.
Considerable power is needed to overcome that amount of compression to get a diesel engine started. Larger diesel engines have decompression levers that open the cylinder to the atmosphere so relieving the force needed for the starter mechanism to get the engine turning over. When the flywheel is spinning rapidly they are closed for the first compression and power operation to start.
The petrol engine, using the more flammable fuel, can ignite the fuel/air mix by an electrically generated ignition source at its spark plug, more fully described in Chapters 1.4 and 8.2. A compression ratio of 10:1, common to small high speed petrol engines means that the air has been compressed to one-tenth of its original volume, requiring less energy to turn over in starting than the diesel.
While advances in design are ever improving, the advantages and disadvantages of the typically ponderous but reliable diesel against the responsive but explosive petrol
is demonstrated in the proliferation of diesel engines for inboard installation (required for commercial vessels) and petrol engines for outboards. In short, the strengths and weaknesses of petrol versus diesel engines can be summarised as:
Petrol driven engines use fuel with high fire risk, require electrical sparking that is not ideal in a damp environment but can be low weight for power capacity with high acceleration performance.
Diesel driven engines operate using a fuel with lower fire risk, do not rely on electrical connections but can be heavy for power capacity with slow naturally aspired acceleration balanced by reliability and longevity. Though of a heavier build than petrol engines, in order to sustain the forces resulting from higher compression, they are known for producing working power (grunt) at lower speeds.
1.2 The four stroke engine
Four stroke engines can be diesel or petrol. The four strokes of a wet sump diesel engine are shown below with exhaust outlet marked (out), air inlet (in), piston (L), con rod (M) and crankshaft (N). This crankshaft revolves in a clockwise fashion.
Induction stroke-1
As the piston
nears tdc the inlet valve is opened by the camshaft’s
cam driven by the rotating crankshaft. As the crankshaft rotates further it
drags the piston down the cylinder sucking fresh air into the cylinder through
the inlet port and its inlet valve.
As the piston nears
bdc the inlet valve is closed, trapping the charge of
fresh air.
Compression-stroke 2
At near bdc both valves shut. As the crankshaft rotates it pushes the piston up the cylinder and so squeezes the trapped air (to about 5% of its normal volume). The compression of the air heats it up to about 500°C sufficient to ignite any diesel fuel.
Power stroke-3
Just before tdc the camshaft driven injector pump sprays a measure of fuel into the cylinder at high pressure through an injector. The atomised fuel sprayed mixes with the hot compressed air and explodes. As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel. The momentum energy of the heavy rotating flywheel keeps the engine rotating smoothly until the next power stroke.
Exhaust Stroke-4
Just before the piston reaches bdc, the exhaust valve opens and the pressure of the burnt gases is released into the exhaust outlet. As the piston passes bdc and rises up the cylinder it pushes the remaining burnt gases out of the cylinder through the exhaust valve. As the piston reaches tdc, the exhaust valve closes, and the engine is ready to repeat the work cycle.
The four stroke engine takes two complete revolutions of the crankshaft to complete the four strokes of this work cycle. A high speed engine’s crankshaft turns at more than 1200 revolutions per minute (rpm), or 20 revolutions per second, or 10 work cycles (40 strokes). Accurate valve timing is critical to efficient functioning. Typically wet sump type lubrication is used in four stroke diesel engines.
Four stroke cycle petrol engines
are similar in operation to four-stroke diesel
engines. Differences include that the fuel is pre-mixed with air in a carburettor. A fuel/air mixture rather
than pure air is drawn into the cylinder during induction though some petrol
engines use an injection system. In the power stroke, the petrol/air mix is ignited
by means of an electrical ignition system and spark plug, rather than the heat
of compression. The timing of the spark ignition for four-stroke petrol engines
is normally synchronised by the camshaft.
1.3 Two stroke engine
Simple valveless petrol two stroke engines
The work cycle of the simplest petrol engine pump or generator is shown below with exhaust outlet port marked (out), air/fuel inlet port (in), con rod (M), crankshaft (N), carburettor (K) and spark plug (B). This crankshaft revolves in a clockwise fashion. Two stroke engines take one crankshaft revolution to complete the four work cycle operations. The simplest two strokes achieve this by using ports in the cylinder wall that are sealed or exposed during the piston’s sweep along the cylinder. Typically a proportion of oil (ratio 16 to 30:1) is added to the fuel to lubricate moving parts, this system being described as dry sump lubrication.
Power and exhaust operations-stroke 1
Just before tdc the piston is positioned above both inlet and outlet ports so sealing the cylinder with its pressurised charge of fuel/air mix. The crankshaft synchronises the timing that releases a high voltage impulse to the spark plug to explode the fuel/air mix. As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel.
Half way down its first stroke the piston uncovers the exhaust outlet port and directed by the piston’s contoured head, exhaust gasses are partially ejected.
Induction and compression operations-stroke
2
The inlet port shown is a transfer port as it is open to both sides of the cylinder.
In the carburettor air and fuel are premixed and atomised. To improve the induction of fuel/air mixture into the crankcase, one-way valves (check valves, reed valves, or rotary valves) may be used in the intake port to allow the fuel air charge to get into the crankcase quickly, but prevent it leaking back out.
As the piston descends, the inlet port experiences a suction that inducts the fuel/air charge around the crankcase, in the process depositing a film of oil on the moving surfaces. Once past bdc and rising on its second stroke the piston pushes the charge up through the transfer port into the cylinder. This flow of the fuel/air charge into the cylinder forces any remaining exhaust out of the cylinder, a process called scavenging. Lastly, as the piston covers the inlet port, the fuel/air charge is compressed ready for the next power operation (ignition).
Valve and port diesel two stroke engines
Power (exhaust and induction) operations-stroke
1
Just before tdc the piston is positioned above the inlet port and the
exhaust valve is closed, so sealing the cylinder and its pressurised and heated
air. The crankshaft driven timing synchronises the injector to release a spray
of atomised fuel that explodes
instantly on mixing with the super heated air.
As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel.
With the initial force of the expansion expended, three quarters of the way down its first stroke the exhaust valve opens and exhaust gasses begin to be ejected.
An instant later the piston exposes the inlet port and fresh air is sucked in, often under pressure from a blower (supercharger or scavenger blower). This rush of fresh air further pushes the exhaust towards the outlet valve (scavenging).
Compression (exhaust and induction)
operations-stroke 2
After the piston passes bdc to begin the second stroke the fresh air compression completes the displacement of any remaining exhaust.
A quarter the way up the second stroke the piston covers the inlet port and next the exhaust valve closes. Now the air is fully compressed in the sealed cylinder ready for the next explosion.
Scavenging
Removal all burnt gases from the cylinder and replenishment with fresh charge (scavenging) is necessary to maintain the power and efficiency of the engine. The rapid operation of two strokes causes a flow of fresh air buffering against exhaust gasses to eject them. Improving the flow by careful design and a blower improves the scavenging efficiency. The cross flow type scavenging shown above uses a piston head profile directed towards the exhaust. Looped scavenging has inlets and outlets on the same side of the cylinder. Uniflow scavenging is arranged so the gasses all move centrally in the same direction towards the cylinder head outlet.
Engine designs
with transfer inlet ports positioned evenly around the cylinder improve
scavenging by allowing air to enter uniformly and push the exhaust towards the
exhaust valve.
Air, blowers and after-coolers
Small engines
are naturally aspirated purely by the suction created during the downward
stroke of the piston. However more complex engines use additional blowers to force
air into the cylinders and so increase their power and efficiency. Blowers may
be the supercharger or turbocharger type.
Superchargers
are driven directly by the engine and geared spin at up to twice the engine
speed. A stream of air from a supercharger can also partially cool the hot
running two-stroke.
Turbochargers
are driven by a turbine which is powered by the engine exhaust and may spin at
up to 100,000 r.p.m.
Balance and lubrication is critical. At high speed turbo vanes reach
high temperatures from the exhaust’s blast of flame. If the engine is instantly
shut down from high speed operation the rapidly spinning turbo will run-on for
a substantial time after the engine driven lubricating system has stopped. The heat
from the turbo vanes will conduct into the bearings with consequent damage. To
prevent these problems, hot engines should be allowed to idle for about 5 to 15
minutes before they are switched off, to allow the engine and turbo to cool
down.
Turbochargers also
heat the inducted air by compression. An after-cooler may be fitted to cool and
increase the density the air (providing more oxygen per unit of volume) before
it enters the engine, so increasing efficiency and cool running.
1.4 Timing – valves, injection and ignition
Diesel engines
Timing methods vary widely in different engine designs, but all must be set exactly as even a small timing error can stop the engine or seriously damage it. Injection, and the opening and closing of the valves, is accurately timed in relation to the position of the piston, by the gear or chain drive from the crankshaft to the camshaft. The meshed teeth of the gears ensure that injection and valve operations occur at the correct point of each work cycle. At speed engines work cycles occur so rapidly that valve and injection has to be initiated prematurely to counter the miniscule delay in actuating mechanism.
Four stroke engines have twice as many teeth on the camshaft gear (or sprocket) as there are on the crankshaft gear. This means the camshaft runs at half the speed of the crankshaft. The camshaft turns (injection and valves operate) only once for every two revolutions of the crankshaft.
With two stroke engines, injection occurs and the valves will open and close on each turn of the crankshaft. In two-stroke engines, the camshaft must run at the same speed as the crankshaft.
The specific opening and closing of the inlet and exhaust valves and the period of injection of the fuel can be taken from the engine manufacturers timing diagram. Examples are shown below:
Four stroke cycle diesel engine
The diagram below represents a Caterpillar turbo charged after cooled engine. The induction stroke commences when the inlet valve opens 10° before tdc when air is drawn into the cylinder as the piston moves down. The inlet valve closes 1° before bdc. The air is now trapped in the cylinder and as the piston rises on the compression stroke, the air is compressed. As the air is compressed, it rises in temperature. When the piston reaches 19° before tdc, the injection of fuel commences and continues until 73° after tdc.
The heat in the compressed air ignites the fuel and combustion takes place. The gases expand forcing the piston down on the power stroke.
The exhaust valves opens at 26° before bdc and the exhaust gases are discharged as the piston rises on the exhaust stroke. Most of the exhaust gases have been discharged as the piston nears tdc. However, at 10° before tdc, the inlet valve opens and air enters the cylinder and helps discharge any remaining exhaust gases until the exhaust valve closes at 3° after tdc. The whole cycle is then repeated.
Both the exhaust valve and inlet valve are open from 10° before tdc to 3° after tdc, an overlap of 13°. This is referred to as “valve overlap” and ensures that all the exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place.
Therefore there is one power stroke for every cycle or two revolutions of the crankshaft.
Two stroke cycle diesel engine
The two strokes of power followed by compression are required to complete one cycle. The events of injection, combustion, expansion and compression of the gases takes place as the four stroke engine, but the exhaust of the burnt gases and the induction of air take place at the bottom of its stroke, this being a chief difference between the two stroke cycle and the four stroke cycle.
There are variations in two stroke cycle engines. The
type described here is the most common to be found in marine engines. It has
inlet ports and exhaust valves. The
inlet holes or ports are in the lower section of the cylinder liner wall.
The piston uncovers the inlet ports as
it moves down the cylinder. The piston covers the inlet ports as it moves up
the cylinder. This action has the same effect as a valve opening and closing. An
engine driven scavenge blower is fitted and the incoming air is blown into the
cylinder through the inlet ports when they are uncovered by the piston.
The above timing
diagram represents a Detroit Diesel turbo charged inter cooled engine. Induction
commences at 49° before bdc when the piston has
uncovered the inlet ports. Air is forced into the cylinder by the scavenge
blower as the piston moves down to bdc and back up
again until it covers the inlet ports at 49° after bdc.
As the piston
rises, the exhaust valve closes at 62° after bdc. The
air is now trapped in the cylinder and as the piston rises the air is
compressed and rises in temperature.
Fuel is injected
before tdc and continues after tdc.
Detroit Diesel do not specify the period of injection as this will vary
depending upon the engine speed, the load and the size of the injectors. The
camshaft contains the exhaust valve cams as well as the unit injector cams.
Therefore, if the exhaust valve timing is correct, the unit injector timing
will be correct providing the injector follower is adjusted to a definite
height in relation to the unit injector. A special gauge is supplied to set
this height.
The heat in the
compressed air ignites the fuel and combustion takes place. The gases expand
forcing the piston down on the power stroke.
The exhaust
valve opens at 83° before bdc allowing the burned
gases to escape into the exhaust manifold. However, at 49° before bdc, the inlet ports are uncovered by the piston and air
enters the cylinder and helps discharge any remaining exhaust gases until the
exhaust valve closes at 62° after bdc. The whole
cycle is then repeated.
There is one
power stroke for every revolution of the crankshaft.
Most marine engines have several cylinders for more power and smoothness. (The cylinders are identified by numbering them in order, from the front of the crankshaft (inboard end) to the back (propeller shaft end).
Above, the cylinders are numbered 1- 4. The
timing assembly consist of rockers 5, valves 6, push rod 7, cams 8, camshaft 9
and timing gear 10. The crankshaft is 11 and the flywheel 12.
In engines with several cylinders, the
cylinders are designed to fire one after the other, to increase the smooth
delivery of power. They do not fire in consecutive order
(1,2,3,4,5,6,7,8), as this would have
the effect of twisting one end of the crankshaft while the other end tries to
catch up. Instead, cylinders are
designed to fire, first at one end of the engine, then at the other. In this way, the power thrust is more evenly
balanced on each end of the crankshaft.
Typical firing orders for internal
combustion engines are:
Four
cylinder engines 1,3,4,2 (sometimes 1,2,4,3)
Six
cylinder engines 1,5,3,6,2,4 (or 1,3,5,6,4,2)
Vee
eight cylinder engines 1,5,4,8,6,3,7,2
is most common
The cylinders are usually configured in a
straight line along the centre of the block, being called straight. Alternately they can be configured in the vee or v arrangement as shown below fitting greater power potential in a
more compact space.
Some auto engines arrange the cylinders in a
horizontal plane with pairs of pistons punching out to opposite sides of the
block, hence its name the boxer
engine, which provides balanced performance.
Valve timing
Valve timing is
the critical relationship between the position of the crankshaft and the
opening and closing of the inlet valves and exhaust valves. The valve train is
geared or has a chain drive with sprockets on the camshaft and crankshaft. Any
slight variation from the correct timing setting will result in loss of power
and overheating. Any large variation and the engine will not start.
To accurately
check the valve timing, it will be necessary to remove the timing cover to gain
access to the timing gears. The gears or sprockets are fitted to the crankshaft
and camshaft by keys so they can only be fitted in one position. However, they
can be incorrectly lined up to each other.
The operators
manual will indicate what the timing marks look like and in the case of chains,
what the sprockets should line up with. Typical lining up marks for gears are
shown below:
When timing has
been found to be correct, the tappet clearances (also referred to as valve
lash) should be checked. Whenever the cylinder head is overhauled, the valves
are reconditioned or replaced, or the valve operating mechanism is replaced or
disturbed in any way, the tappet clearance must be adjusted - also when the
cylinder head has been re-tightened after the initial run in period.
As the valve and
valve operating gear heats up in service, the clearance between the rocker arm
and the valve stem decreases. If insufficient clearance is allowed, the valve
will be prevented from seating. The correct clearance will be specified by in
the operator’s manual. Some manufacturers state clearances for an engine is at
its normal operating temperature, others when the engine is cold, while some
give both.
Clearances will
vary as much as 0.128 mm (0.005”) between a cold and the normal operating
temperature of an engine. Usually, an exhaust valve will have a greater
clearance than an inlet valve because of their different operating
temperatures.
Too much
clearance will cause excessive wear, noisy operation and altered valve timing,
that is, late opening and early closing. If the clearance is insufficient and
the valve does not seat properly, it will result in loss of compression through
valve leakage, burning and eroding of the valve and seat and general overheating.
In the extreme,
it is possible that the piston could strike the valve resulting in a bent valve
stem, damaged piston or worse if the valve or piston should break.
If the valve operating mechanism is
disturbed in any way and the engine is cold, but only a hot tappet clearance is
given, the tappet clearance must be checked. If required, further adjust when
the engine is at its normal operating temperature.
The most common
form of adjustment for tappet clearance is by means of a screw and lock nut
located in one end of the rocker arm. The clearance is measured by means of a
feeler gauge between the valve stem and rocker arm when the valve is in
the fully closed
position. This is usually done when the piston, under the valve being adjusted,
is on top dead centre at the end of the compression stroke.
An easy way to identify the above is as
follows:
On a six
cylinder engine with a firing order of 1 5 3 6 2 4, turn the engine over in the
direction of rotation. When the inlet valve and exhaust valves are rocking on
number 6 cylinder (i.e. the piston finishing its exhaust stroke and starting
its induction stroke) adjust the inlet and exhaust valve clearances on number 1
cylinder which will just be completing its compression stroke and commencing
its power stroke.
On the
crankshaft, the bottom end journals on numbers 1 and 6 are 180° to each other,
2 and 5 are 180° to each other, and 3 and 4 are 180° to each other.
What you are
doing is adjusting number 1 tappets while number 6 is rocking, then adjust
number 5 because it is the next one in the firing order to be on top dead
centre while number 2 is rocking, adjust number 3 while number 4 is rocking,
adjust number 6 while number 1 is rocking, adjust number 2 while number 5 is
rocking, and adjust number 4 while number 3 is rocking.
On a Detroit
Diesel, the exhaust valve/s can be adjusted on the cylinder on which the unit
injector follower is fully depressed. This means that fuel injection is taking
place so it is at the end of the compression stroke and the beginning of the
power stroke.
Timing a fuel injection pump
Early injection - If the injection occurs too early on the
compression stroke, it will result in high peak pressures. This will subject the engine to unsafe
stresses caused by the tendency of the pressure to reverse the rotation of the
engine and evidence by excessive detonation which is known as diesel knock.
Late injection - Retarded injection or late burning gives
incomplete combustion causing too low a power output and overheating.
Timing
instructions
It will be
necessary to follow the manufacturer’s instructions in the owner’s manual to
time the fuel pump to the engine as different methods are employed.
Timing principle - fuel injection commences on the compression
stroke just before top dead centre. With a four stroke, the piston also comes
up to top dead centre on the exhaust stroke. Make sure it is on the compression
stroke. As with timing inlet and exhaust
valves, the fuel injection pump must be timed to inject fuel at the correct
angle on the compression stroke. This means that the gear driven shaft to the
pump must also be lined up in the gear wheel train, otherwise, difficulty might
be experienced in lining up the holes in the drive coupling.
Timing engine to pump -The flywheel is usually marked with a tdc and with an injection mark that is before the tdc mark when turning the engine over in the direction of
rotation. Turn the engine over in the direction of rotation until its number 1
cylinder is on the compression stroke and the injection mark is lined up. The
fuel
injection pump
must also be lined up on number 1 element or port at the commencement of
injection. The owner’s manual will identify the position of the lining up marks
as brands of pumps differ. When the lining up marks on the pump meet the drive
couplings can be bolted together.
Alternative method of timing - Some manufacturers make provision for
locking the fuel injector pump shaft at a position corresponding to tdc for number 1 cylinder. A further pin is then located in
a hole in the camshaft timing gear that is tdc for
number 1 cylinder. The drive couplings can then be bolted together and the pins
removed.
As the pin is
located in a hole in the camshaft, it can only be on the compression stroke on
a four stroke engine.
Checking the timing of a fuel pump -The timing may be checked as follows:
Remove the
delivery valve and spring from number 1 element in the fuel injection pump. Open
the throttle to the full position. (If the throttle is left at the stop
position, the slot in the plunger will be in line with the spill port and no
fuel will be delivered.)
Rotate the
engine in its operating direction until number 1 cylinder is on the compression
stroke. Keep rotating the engine slowly and when the mark on the flywheel, indicating
the start of injection is lined up with the timing indicator mark, fuel will
immediately start to rise from where the delivery valve was removed. (This will mean the top of the plunger has
just covered the inlet and spill ports and injection is starting). If fuel
starts to rise before or after the timing marks are in line, the fuel pump
timing is out and will have to be adjusted.
Detroit Diesel unit injector - On a Detroit Diesel, the cam that actuates
the unit injector is on the same shaft as the cams for the exhaust valves. If
the exhaust valves are correctly timed, that is they open and close at the
correct angles, then the unit injector timing must be correct. It is then only
a matter of adjusting the unit injector follower to get the correct height in
relation to the unit injector body. A special gauge is supplied for this
purpose.
Cummins PT injector - On the Cummins PT system, it is only a
matter of setting the clearance between the rocker arm and the injector.
Petrol engines
Similar timing principles and mechanisms are used in petrol engines, with the fundamental differences that a pre-mixed fuel/air charge is introduced to the cylinder during the induction stroke and that the ignition requires an electrically driven spark to initiate the power stroke.
Smaller engines may use the magneto to produce the high voltage required for an electrical spark to arc across the spark plug gap. More complex engines will use a coil, distributor and contact breakers (or points).
The
magneto
A magneto
is an electrical generator using wire coils and magnets to produce alternating
current. It produces pulses of high voltage to activate the spark plugs of
small petrol driven engines such as light weight outboard motors and lawn mowers.
The shuttle magneto variation spins a wire
coil on its flywheel between the poles of a magnet whereas the inductor
magneto spins the magnet around a static wire coil.
The ignition
timing relies on a cam on the drive
shaft openning the
points (contact breakers) momentary interrupting the current and collapsing
a coil’s electromagnetic field. This induces a voltage across the coil, which
in turn supplies the energy for the spark plug firing. The size of the points’
gap opening can be adjusted to the manufacture’s recommendation. Its position
in relation to the cam can be adjusted to fine tune the ignition timing.
To prevent high
voltage arcing at the point’s contacts, leading to rapid decay, a capacitor (an electrical storage and
smoothing device) is placed across the points to absorb the energy burst. (Capacitors were previously called condensers, are built from two films of electrical conductors
separated by a film of electrical insulator).
The system is rarely
used for vehicles or vessels that have electrical accessories, except for aero
piston engines where simplicity and reliability are of advantage. The magneto
is not used to charge batteries in marine systems but in old outboard motors
may be wired to power an emergency light.
The battery, coil and distributor
More
complex multi cylinder engines will use an electrical system energised by a
battery bank that is kept charged by an alternator. This system induces a voltage across the coil,
which in turn supplies the energy for spark plug firing.
A crank driven distributor shaft revolves in a
cylindrical distributor body. The
shaft drives a cam that opens the points momentary
interrupting the current and collapsing the coil’s electromagnetic field. To
prevent high voltage arcing at the point’s contacts, leading to rapid decay, a capacitor (an electrical storage and
smoothing device) is placed across the points to absorb the energy burst. (Capacitors, previously called condensers, are built from two films of
electrical conductors separated by a film of electrical insulator).
The perimeter
of the distributor’s cap has contact
points individually connecting to each spark plug by a high tension lead. (Don’t touch them during running or you will
receive a high energy electrical shock!) As the distributor shaft rotates, the rotor arm fitted to its head brushes
past each contact point in turn to distribute the electrical energy to each
spark plug in the correct order and time.
The size of the
points’ gap opening can be adjusted to the manufacture’s recommendation. In
order to fine tune the ignition timing, its position in relation to the cam can
be adjusted by rotating the distributor body around the distributor shaft.
Further information on outboard engine petrol ignition is provided in Chapter 8.
1.5 Air supply
Turbo charging
A turbo charger (sometimes called a turbo
blower) can be fitted to both two and four stroke engines to increase the
volumetric efficiency and thus their power output. It uses the force of the
expelled exhaust gasses passing through a turbine to drive a rotor blowing air
into the air inlet.
Advantages
- The advantage of a turbo charger is that
fuel consumption is lower than that of a normally aspirated engine of the same
power output. In addition, the turbo charger utilises the exhaust gases of the
engine so no additional power from the engine is required to drive it.
The turbo
charger inducts a larger mass of air into the cylinder to that of a same cubic
capacity normally aspirated engine. This allows for a proportional increase in
the amount of fuel that can be injected and burnt in the cylinder thereby
providing an increase in the power output of the engine.
Components
of a turbocharger
A cut away of a
turbocharger’s components are shown below:
Rotor assembly - It has a rotor
shaft which has exhaust gas turbine blades on one end and air compressor blades
on the other end.
Casings - The exhaust gas
turbine blades are housed in a casing which is attached to the exhaust manifold
and to the exhaust pipe. Some casings are fresh water cooled to minimise the
heat radiated out into the engine space. This allows for a cooler engine space,
cooler air entering the engine air intake and therefore more power again. A
nozzle ring is fitted inside the casing to direct the flow of exhaust gases to
the turbine blades.
The air compressor blades are also housed in a casing which has an air
cleaner on the intake side and is connected to the intake manifold on the
discharge side. Where an engine is after cooled, the discharge side is
connected to the after cooler which is then connected to the intake manifold. Both
the above casings are attached to a centre casing which contains the bearings,
seals and method of lubrication.
Bearings and lubrication - The shaft may rotate in white metal bearings which can be lubricated from the engine driven oil pump. This method of lubrication also allows the oil to remove some of the heat in the turbo charger. One bearing locates the shaft and takes the small residual thrust, the other bearing allows the shaft to move longitudinally to accommodate the differential thermal expansion of casings and shafting.
Alternatively, the smaller turbo chargers usually incorporate a ball
bearing for positioning at the compressor end and a roller bearing to
accommodate axial expansion at the turbine end of the rotor shaft. The bearings
may have their own reservoir which forms part of the turbo charger. These
reservoirs usually have round oil level sight glasses with two horizontal lines
marked to indicate the high and low levels. Seals are fitted to retain the oil.
Operation
of the turbo charger on a diesel engine
In a four stroke
engine, exhaust gases flow from each cylinder into the exhaust manifold and
then past the turbine blades of the turbo charger. With the engine running at
full speed, the turbo charger can obtain speeds up to 100,000 revolutions per
minute (rpm). The air compressor blades will revolve at the same speed. Air is
drawn through the air cleaner and forced under pressure into the intake
manifold. When the inlet valve opens on the induction stroke, with the piston
descending in its cylinder, air is forced into the cylinder. It is necessary to
reduce the turbo charger speed in stages or slowly for two reasons:
If the engine
speed is reduced from full engine speed to stop quickly and the bearings of the
turbo charger are lubricated by the main engine driven lubricating oil pump,
the engine, on stopping, will cease to supply the lubricating oil to the turbo
charger bearings. Because of its high speed, it will take some time for the
turbo charger to come to rest and the bearings could be damaged. The exhaust
gas side of the turbo charger operates at a very high temperature. It is
preferable to reduce the temperature gradually rather than quickly to prevent
unequal contraction of the turbo charger parts as it slows down.
Monitoring
the performance
Normally, as
part of the purchase of a new engine, the engine distributor or dealer will do
an installation and pre-run check. The following will be recorded:
The speed of the turbo charger at a
nominated engine speed.
Air flow in.
Air flow out.
Air pressure after the compressor blades.
Exhaust gas flow
The flow of air
going into the turbo charger is important. The air is taken from the engine
room so sufficient ventilation to the engine room is required to ensure there
is enough for the engine as well as cooling the engine room. The exhaust gas
flow is also important. It ensures the installation of the exhaust piping is
within limits and not restricting the performance of the engine. As the above
is recorded, checks can always be carried out and readings compared with the
initial ones.
After coolers (charge air coolers)
An after cooler
is also called an inter cooler or a charge air cooler. An after cooler is
fitted where an engine is turbo charged, however it is not necessary to fit
one. Therefore an engine can be turbo charged or can be turbo charged and after
cooled.
The reduction in
air temperature will increase the density of the inlet air resulting in more
air entering the cylinder. More fuel can then be injected and burnt, giving
increased power.
The after cooler
is fitted between the air compressor side of the turbo charger and the air
intake manifold on the engine. In the after cooler, air passes over the outside
of the tubes while the engine cooling water or sea water passes through the
tubes usually in the opposite direction (contra flow). Fin plates are attached
to the outside
of the tubes to
increase the surface area for the air, thereby giving a better transfer of
heat.
Maintenance
Sea water
flowing through the tubes will tend to leave deposits in less time that if
fresh water was used. The end covers can be removed and a wire brush pushed and
pulled through the tubes. If the scale is not removed by the brush, the tube nest
will have to be chemically cleaned.
On the air side,
usually no maintenance is required if the air cleaner is doing its job and the
filter is changed regularly. A leaking tube will cause the cooling water to
pass into the air side. Depending on the design, the air may enter at the
bottom and leave at the top to prevent water carrying over with the air. A
drain cock is fitted at the bottom.
As the air
passes through the after cooler, its temperature may be reduced until it is
below the saturation temperature. Heavy condensation of water vapour may then
follow, this water being carried into the engine. If this is a problem, a water
separator can be mounted between the after cooler and the air inlet manifold.
1.5 Engine protection - devices
In best practice vessels of the 25 metres class (80 tonne) will be fitted with an audible warning device to indicate a dangerous condition associated with:
engine lubricating oil pressure;
engine jacket cooling water outlet
temperature; and
engine gear box lubricating oil pressure.
It should be noted that these protection devices give off an audible warning only and do not shut down the engine. Automatically shutting down an engine without any warning could have dangerous consequences and result in collision, grounding, or the loss of the vessel.
The alarm system may have a switch that must be turned on manually to put the system into operation. The danger of this system is the operator may forget to activate the system allowing the engine to run in an unprotected mode. It is preferable that there be no alarm switch. If there is an alarm switch, it is good practice to switch it on before starting the engine. It will sound until the engine is started and the minimum oil pressure registers. Similarly, it should not be switched off until the engine is stopped and the alarm sounds. This procedure also checks that the alarm is operational.
The gear box low lubricating oil pressure alarm operates in the same fashion as the engine low oil pressure alarm.
Low
oil pressure alarm
The oil pressure alarm consists of a pressure switch fitted to the pressure side of the lubricating oil system, usually into an oil gallery. The oil pressure acts on a diaphragm and spring which open the contacts in a micro switch. When the spring
pressure is greater than the oil pressure, the contacts will close and sound the audible alarm.
If an alarm switch is fitted, switch it on. When the engine is started, the oil pressure switch opens when the oil pressure reaches approximately 69 kPa (10 psi) and the alarm will cease to sound.
Likewise, if the oil pressure drops below the setting of 69 kPa (10 psi), the oil pressure switch will close the circuit and sound the audible alarm.
The alarm will continue to sound until the engine is stopped or the alarm switch, if fitted, is switched off.
High
temperature fresh water alarm
The high temperature fresh water alarm consists of a thermo switch. It has a bi-metal probe that activates contacts in a micro switch. It is installed in the side of the thermostat housing. When the engine is started and running at normal operating temperature, the contacts in the switch will be open. Should the engine coolant exceed 96° C (+ or - 3°) the water temperature switch will close the electrical circuit and sound the audible alarm and/or indicator light. The gear box low lubricating oil pressure alarm operates in the same fashion as the engine low oil pressure alarm.
A Detroit Diesel engine has an additional sensor fitted for the protection of their engines. In addition, advising the engineer of a slight loss of coolant, non circulation of the coolant or failure of the sea water cooling action, the alarm will also sound if there is a large loss of coolant.
A big and sudden loss in coolant may reduce the coolant level to below the probe in the thermostat housing. As the water is now not circulating over this probe, it will not detect the rise in temperature of the coolant. An additional sensor is fitted into the exhaust manifold outlet to detect the rise in temperature due to overheating.
The water temperature switch consists of a temperature-sensing valve and a micro-switch. The valve contacts a copper plug (heat probe) which extends into the exhaust manifold outlet. Engine coolant is directed over the power element of the valve and if the water temperature exceed its setting, the valve will close the contacts in the micro-switch thus closing the circuit and sounding the audible alarm. If a loss of coolant occurs, the heat of the exhaust gases will be transmitted through the copper plug to the temperature-sensing valve thus closing the circuit and sounding the audible alarm.
Emergency stop device
Numerous engines are fitted with a manually operated emergency engine shut down device, mounted in the air inlet housing, to stop the engine in the event an abnormal condition arises. If the engine continues to run after the engine throttle is placed in the no fuel position, or if combustible liquids or gases are accidentally introduced into the combustion chamber causing over speeding of the engine, the shut down device will prevent damage to the engine by cutting off the air supply stopping the engine.
The shut down device consists of an air shut off valve (flap) mounted in the air inlet housing which is retained in the open position by a latch. A cable assembly is used to remotely trip the latch. The shut off valve must be manually reset on the latch for restarting the engine after the malfunction has been rectified.
Chapter 2: Fuel storage and handling
2.1 Fuel storage and survey compliance
The standards
Compliance
To ensure safe vessels seafaring nations developed Classification Societies to keep registers (lists of approved safe vessels). Those organisations still determine rules (specifications) and conduct surveys for construction, equipment and maintenance for each vessel class according to its trade and sea area of operations.
American Bureau of Shipping AB Det Norske Veritas NV
Lloyds Register of Shipping LR Germanischer
Lloyd GL
Bureau Veritas
Nippon Kaiji Kyoka
China
Classification Society
For new vessels, a classification society or
survey authority will approve the specifications for design plans and check the
quality of materials and workmanship of all stages of the construction process
at the Initial Survey. To ensure
maintenance to the survey standards, regular ongoing inspections are scheduled,
called Periodic Surveys. These approved
specifications, the World’s best practice,
are supported in Australian legislation, regulated by the Australian Maritime
Safety Authority (AMSA).
The Australia domestic commercial vessel
fleet (under 80 mtrs long and under 200 nm offshore)
is transitioning from State survey authorities operating the Uniform Shipping Laws specifications
(USL code) to a National survey authority (AMSA Domestic Vessel Division)
operating the newer National Standards
for Commercial Vessels (NSCV). While drawing from the USL code, the NSCV
updates and provides the flexibility required by developers and operators. It
retains a prescriptive approach to compliance in its deemed to satisfy standards (standards
that shall be met) but also provides
a flexibility with performance based equivalent
solutions” (that can be proven to be as effective as those deemed to
satisfy).
Vessel survey
compliance will depend not only on its operations (trade and plying limits) but
on its survey authority their grandfathering exemptions from past survey
regimes or allowances from current equivalent solutions. In short, to confirm
your vessel’s survey compliance you will need to contact its Survey Authority.
However, some
features that are common in all domestic commercial vessels fuel system
installations that meet best practice in structural integrity, fire control,
environmental management and stability.
NSCV Survey
Due to the
greater explosive risk posed by petrol, inboard installations must be diesel
fuelled though petrol is
allowed for vessels fitted with outboards. Some special purpose petrol inboards
may be approved for petrol inboards (high powered wake and ski boats) where
risk mitigation includes starter interlocked engine room purging blower and
fuel management procedure.
NSCV surveys frequency is determined by risk level. Risk factors include age, attributes, operational area and nature, incident history of vessel class and performance of the operator. Greater risk category vessels include:
Class 1A, 2A, and 3A vessels (unlimited sea
areas);
Class 1 > 35m in measured length;
Class 1B/1C that berth one or more pax or berth >12 persons or carry more >36 pax;
Class 1D/1E that berth one or more pax or berth >12 persons or carry >75 pax;
Class 2B vessels > 35m in measured length
(workboats to 200nm offshore)
Class 2 tankers, dangerous goods carriers or
tug boats.
Survey
cycles
The periodic survey inspections of a vessel are
arranged in survey cycles of 5 years (inspections at 1, 2, 3, 4 and 5 years). These
periodic surveys are not intended to confirm the vessel’s compliance with every
requirement but to identify and verify the continued existence and
functionality including trialling machinery and its fuel systems.
1 yearly inspections -
for all vessels would include all pipe arrangements.
5 yearly inspections - for vessels 35 metres and over of deep tanks and double bottom tanks used exclusively for fuel oil, to be examined externally and tested to a head sufficient to give the maximum pressure that can be experienced in service.
One deep tank and one double bottom tank used exclusively for fuel oil to be surveyed internally every 5 years starting when the vessel is 10 years old.
10 yearly inspections – vessel hull thickness and examine larger fuel tanks internally
The term to examine
means a process that commences with a visual inspection that identifies the
evidence of damage, deterioration and/or modification (may require dismantling
if deficiencies are found).
The term to test
means the physical gauging of properties with the objective of ascertaining
continued readiness to function, condition or conformance with standards. E.g.
hammer tests, ultrasonic thickness measurements, oil analysis, starting of
machinery, turning of handles
The term to trial
means a specific type of rest of a system or component to ascertain functional
performance and/or compliance with applicable standards. E.g. machinery trials,
emergency generator trials, steering trials, fire hydrant appliance trials,
anchoring trials, evacuation trials.
The term to verify means to
ensure that an item exists and is as per the plan, meets or has been declared
as meeting an applicable standard by a an Authority.
Survey should not be confused with maintenance. A responsible owner will have a maintenance program to open up, inspect and repair at lesser intervals than required by the survey authority. Surveyors have the discretion to survey fuel oil tanks of the vessel not due for survey if he considers such action is warranted.
Fuel
storage components
Fuel
tanks
Fuel
tanks for storage of the fuel or for daily service tanks may form part of the
hull structure, be free standing and substantially constructed of carbon steel,
stainless steel, copper or marine grade aluminium. Fuel
storage arrangements on board are largely dependent on the intended service of
the vessel and location of refuelling stations within the area of operation of
the vessel. Typical arrangements include:
Short
distance -Tanks
port and starboard within the machinery space. These may be free standing or
form part of the hull structure plus a smaller tank for direct supply to the
main engine and generator engines. Fuel from the larger tanks is transferred to
the smaller tank as required.
Medium
distance - A
bunker or deep tank divided into port and starboard tanks forward of the
machinery space the aft bulkhead of the tank being the machinery space forward
bulkhead, with a small tank as for short distance, or:
Two small tanks both capable of supplying
the main engine and generator engines, each with independent lines to the main
and generator engines. The tanks would be used alternately.
Long
distance - as
for medium, but with the addition of a double bottom fuel tank/s.
Multiple tanks can have a cross over valve
fitted to either the fuel supply or return lines enabling the engines to be run
from either tank or in the event of contamination, to isolate an offending
tank. Care must be taken if redirecting a fuel return line to one tank only as
this effective fuel transfer can be rapid and may affect the vessels stability
or even overflow the tank.
Some vessels may have two day tanks, thus
the fuel return from the engines injectors should be changed over when the
delivery is changed. Similarly, it is wise to close cross over fuel supply
lines when refuelling from a high pressure fuel pump. The thrust of fuel
entering the port tank filler pipe may depress the fuel in the tank and even
force fuel up to overflow the starboard tank. The reverse will occur when the
filling stops as fuel from the starboard tank can surge back to spill out from
the port filler pipe.
Double bottom and void tank tops are prone
to corrosion but must be more regularly inspected. A weep of water entering a
double bottom tank through damage to the outer hull will suddenly become a
flood if the tank’s resisting internal air pressure fails due to the tank top
watertight seal corroding away.
Tank
components
Baffles
–
Perforated baffles (or not continuous baffles) are fitted inside the tank to
allow limited liquid movement but minimise free surface area effects of liquids
sloshing around as the vessel moves. Normally spaced not more than 1 m apart,
those fitted longitudinally will reduce free surface caused by the vessel
rolling and transverse baffles will reduce that caused by the vessel pitching.
Breathers-
Fuel tanks, containing flammable liquids, are required to be vented to
atmosphere (not into the vessel). This breather pipe will terminate in a gooseneck
or swan neck (a cranked pipe), which limits rain and spray from entering. If
the vent pipe is greater than 18 mm in diameter, the outlet is fitted with a
wire gauze for a flame trap.
Filler
pipes-
Filler pipes are arranged so spillage will not enter the vessel. The inlet or
delivery end of the pipe is located outside the vessel and will have a valve
and fuel tight cap. The pipe between the deck and the top of the tank may be
flexible, but must be reinforced and secured with twin corrosion resistant clips.
Fuel
pipes - Generally
unarmoured plastic or rubber flexible hose is not compliant.
Inspection
port -
The top or bottom of tanks, where water and condensation accumulate, are prone
to corrosion and need regular inspection. The bottom of the sounding pipe can
corrode or even jam the sounding device. Consequently fuel tanks of more than
800 litres capacity require opening up and inspecting at periods of not more
than 12 years through a manhole or inspection port. A larger tank may also have
modified vent pipes or fitted purging (by inert gas) pipes to ensure tanks are
evacuated of flammable gasses before opening up. The precautions of entering a
confined space must be applied.
Pumps
- are
used for transferring fuel between tanks. There may be a separate pump to
supply fuel at pressure to the engines. Stop valves are to be provided on the
suction and delivery sides of power operated pumps.
If the closed discharge pressure exceeds the
maximum design pressure of the system a relief valve discharging back to the
suction side of the pump shall be fitted.
Pumps located below decks shall be provided
with a means to stop the pump from a safe place outside the space.
Save
all –
Tanks that are fitted above machinery must have drip trays (savealls)
fitted to prevent leaks onto moving parts. Fillers, engines and gearboxes are
similarly fitted to stop oil reaching the bilge. Save alls also need drainage
arrangements.
Shut
off valves -
All fuel supply lines must have a means to be provided outside a propulsion
machinery space, (in an accessible position not likely to be isolated by a fire
in the space), to shut off the fuel to the main and auxiliary engines by means
of a fire safe valve or cock. In practice this means that a valve or cock
required to be fitted to each tank outlet can be operated from a safe position
outside the space by means of an extended spindle, or some other method of
remote operation. Any fuel
transfer or a cargo oil pump which is
located below deck in a machinery space shall be provided with a means to stop
the pump from both inside and outside the space.
Sludge
box & drain -
Sediment contaminants of water, algae and debris will gather at the tank bottom
where they must be periodically removed through a self closing sludge valve. In
the event of the tank rupture or for periodic inspections, all fuel tanks which
are not double bottoms must be fitted with a method of draining them into
another storage tank (not the bilge).
Sounding
and sight gauges -
Float fuel gauges are unreliable due to a vessel’s changing trim, so checking
the contents of the tank can utilise poking a calibrated stick (sounding rod)
down the filler pipe until it hits the bottom of the tank and reading off the
height of fuel that coats the retrieved rod (sounding the tank). An alternative
is to read the dry end of the retrieved rod showing the airspace above the fuel
(an ullage).
If the tank’s pressed up capacity is known then its remaining fuel can be
calculated. Whether a filler pipe or a dedicated sounding pipe is fitted, at the
tank bottom a reinforcing striker plate is welded to prevent a hole being
eventually battered into the tank bottom.
An alternative measuring technique is a
transparent sight glass spanning top to bottom whose fuel level reflects that
of the main tank. This clear plastic/glass tube is more vulnerable to fire and
impact than the main steel tank, so survey regulations specify that a self
closing valve be fitted in case of rupture. Under no circumstances must these
valves be left open. A recent variation is a non ferrous sight gauge containing
a steel float whose height (and tank volume) can be determined by magnetic
sensors. Tanks may be fitted with an overflow pipe which leads to an overflow
tank or relief double bottom fuel tank. These overflows can be fitted with a
sight glass and audible alarm. When re-fuelling, a safety managed procedure
that utilises pollution and spill control devices must be operated to prevent
spillage or fire.
Petrol and diesel fuel systems are more
fully described in Chapter 3.
2.2 Fuel transfer pumps
Unless the vessel’s fuel tanks are
positioned above the level of the engine or a day tank is installed at
sufficient height, fuel cannot be gravity fed to the engine’s fuel injection
pump. To assist in drawing fuel from the tank/s a fuel transfer pump is fitted
between the tank/s and the fuel injection pump. Fuel transfer pumps are also called
transfer, lift or charge pumps.
Diaphragm
type transfer pump
The diaphragm type transfer pump is
mechanically driven by a special lobe on the camshaft. The lobe pushes against
the lever causing the diaphragm to be pulled down against a spring pressure,
creating a partial vacuum.
A first check valve opens and draws in fuel,
filling the chamber between the diaphragm and check valves. As the lever moves
off the lobe of the cam, the diaphragm spring pushes the diaphragm up, closing
the first check valve forcing fuel through a second check valve and into the
fuel pump. An external lever is provided to permit manual operation of the pump
for priming purposes.
The pump will deliver more fuel than is
required. The fuel not being used will build up pressure in the line between
the fuel pump and the fuel transfer pump causing the second check valve to
close. The downward movement of the diaphragm will allow more fuel to enter
through the first check valve into the chamber. The first check valve will
close and as the return spring cannot overcome the pressure in the line between
the fuel pump and the second check valve, the lever will be held off the cam
until more fuel is required.
This diaphragm pump could be attached to the
side of the fuel pump and actuated by a cam on the camshaft for the fuel pump.
Alternatively, it may be attached to the block and actuated by a cam on the
main camshaft.
Plunger type transfer pump
The plunger type fuel transfer pump is
mechanically driven by a dedicated lobe on the camshaft. This pushes against
the plunger in the fuel transfer pump to create the pumping action. Check
valves control the direction of fuel flow, and prevent fuel bleed back during
engine shut down.
As the high point on the cam lobe rotates
away from the fuel transfer pump, the spring forces the piston towards the
camshaft. The pressure of the fuel in the piston bore closes the first check
valve and opens a second check valve forcing fuel to the low pressure supply
line. As the piston moves, a third check valve opens and fuel is drawn into the
spring cavity.
As the high point of the cam lobe rotates
towards the fuel transfer pump, the plunger and piston are forced towards the
inlet. The pressure of the fuel on the spring side of the piston causes the
third check valve to close and first check valve to open, allowing the fuel in
the spring cavity to flow to the other side of the piston.
A second plunger allows manual priming and
bleeding of air from the system. When the plunger is depressed, the first check
valve prevents back flow forcing fuel through the second check valve. When the
plunger is released, the spring forces the plunger outward. This action creates
a suction that causes the second check valve to close and the fuel is drawn
through the open first and third check valves.
If the pump supplies more fuel than is
required, the fuel will build up the pressure in the line between the plunger
pump and the fuel pump. The pressure build up will hold the plunger stationary
against the plunger spring an away from the arm, effectively stopping pump
operation until more fuel is required.
Gear
type transfer pump
This pump consists of two meshed gears in a
closely fitted housing with inlet and outlet ports opposite one another. One
gear is driven by the power source and in turn drives the other. As the gear
teeth separate and travel past the inlet port, a partial vacuum is formed. Fuel
entering the inlet port is carried to the outlet port in
pumping chambers formed between the gear
teeth and the housing. As the gear teeth mesh at the outlet there is no place
for the fuel to go but out.
Vane
type transfer pump
In the vane type pump, a slotted rotor
driven by a drive shaft rotates between closely fitted side plates, and inside
of an elliptical or circle shaped ring. Polished, hardened vanes slide in and
out of the rotor slots and follow the ring contour by centrifugal force.
Between succeeding vanes, pumping chambers are formed which carry oil from
inlet to the outlet. A partial vacuum is created at the inlet as the space
between the vanes open. Fuel is squeezed out of the outlet as the pumping
chamber size decreases.
2.3 Fuel handling
Refuelling
management
Fuel
arrangements must take into account its highly flammable nature, particularly
in the critical operations of loading, unloading and refuelling. The
only sure way to prevent accident is to ensure that personnel are trained and
competent in the refuelling operation.
All fuel filling stations must be positioned
outside of the machinery spaces and so arranged that any overflow cannot come
into contact with any hot surface or spill overboard. The safety plan
for refuelling should include but not be limited to the following:
Training - all personnel are trained to understand the safety plan and operate the safety equipment in order to comply with safety management and port regulations.
Mooring – moor the vessel securely
allowing for tide and wash, secure fuel lines and pad where there are sharp
edges. Pipe bends
should be smooth, not leak and if necessary be earthed.
Isolating – no naked flames or smoking
and have fire-fighting appliances in readiness. Plug
deck scuppers, ensure tank vents are clear and have clean up spill kit gear
ready. Sound tanks to ensure available
capacity. Physically check all isolation valves (just visual is insufficient)
and ensure incoming fuel is clean.
Connecting –approved transfer hoses and
equipment correctly. The
piping system should be flexible to enable pumping fuel direct from any tank to
another. When filling has been completed a container should be available to
catch spillage when the fuel supply line is disconnected. Both the tanker and
the ship must have non drip camloc fittings on their piping connectors to minimise this
risk. Metal tanks and lines must have electrical earthing. Metal delivery
nozzles must be released of potential static induced sparking by contacting against
an earth before placing in a filler pipe.
Draining can only be carried out if there is
space in other fuel tanks to take the fuel from the tank being drained, or it
is to be pumped a shore reception facility. If the tank is a double bottom tank
or deep tank, it can only be drained to the height the end of the suction pipe
is above the tank bottom. For tanks whose bottom is clear of any structure (eg. free standing), the valve for draining water from the
bottom of the tank can be opened and the remaining fuel drained into a
receptacle.
Maintaining - a constant watch to
monitor flow and prevent spills, close filler caps after fuelling and clean any
spills on deck. Fuel
is often supplied to small vessels from a road tanker where the amount of fuel
required can be measured by the fuel meter in the tanker discharge line to
minimise the risk of spillage.
2.4 Fuel contamination
Fuel contamination can be caused by:
Dirt;
Water;
Microbial
infection;
Dirt
- Dirt
can arise from dirty fuel taken on when bunkering. It is difficult to prevent
as the receiver of the fuel has no way of knowing if the fuel is clean or dirty
when being loaded. Effective filtration of fuel prior to use in the engine/s is
necessary. In new vessels it may be that
the tanks were not thoroughly cleaned prior to commissioning. Ensure the tanks
are thoroughly cleaned.
Water/Moisture
- Water
can be in the fuel taken on when bunkering. Care should be taken when loading
fuel in a new or remote location where fuel purity is not known.
An analysis of the fuel can indicate if the
water content is excessive; a figure of around 0.1 to 0.2 % by volume is
acceptable.
Moisture in the air space of the fuel tank can
build up water due to condensation. Drain off of any water in the tank/s on a
daily basis, and try to keep the tanks pressed up.
Microbial
infection - This
is a common problem in diesel and similar grades of fuel. Tests are available
to identify the presence of microbes and the fuel supplier can be asked to
supply a fuel which does not exceed defined standards of contamination.
It can occur due to taking infected fuel on
board, and is aggravated by failing to regularly drain fuel tanks of any water
that has accumulated in them. The microbes can only propagate in the presence
of water and feed on the fuel at the interface between the water and oil. The
waste they generate is deposited as a black slimy sludge.
On board a vessel their presence is first
detected in the fuel filters which clog up more frequently. In some cases the
slime gets past the filters and causes the fuel pump plungers or the injector
nozzle valves to malfunction due to rusting or partial blockage.
Elimination of the bacteria is difficult. Fuel
can be treated by chemicals but as noted above regular drainage of water from
the fuel tanks is essential to keep it under control. In case of severe
contamination it may be necessary to clean out and disinfect all affected tanks
and fuel lines.
2.5 Entering fuel tanks
Metal tanks are considered to be confined spaces in that the atmosphere
may be depleted of sufficient breathable oxygen to sustain life (due to the
rusting process) and/or may contain poisonous gasses (due to build up of fuel
vapours). In order to enter any confined space tank for maintenance or
inspection a confined space procedure must be followed by trained personnel.
This will include a specific risk assessment and control plan. Considerations
will include:
Substitution
–find
another way rather than physically entering the tank (cameras).
Isolation
–
remove all services into the tank (fuel, gas, vapours, electrics)
Administrative
measures –
training, signage, tagging, sentry.
Engineering
–
provide safe access and egress, use intrinsically safe lighting.
Ppe’s –
provide personnel protective devices and breathing apparatus.
Rescue
plan–
have rescue tripod available for personnel recovery and removal.
Gas
free
can be defined as when the atmosphere in an enclosed space or tank is the same
as that of the outside ambient air. Enclosed spaces or tanks are required to be
gas free to allow personnel to enter without danger, for the purpose of survey
and inspection, maintenance or repair work.
When the tank is considered to be gas free
an authorised inspector should test the space to verify the gas free condition
and issue a certificate declaring it to be so.
The standard method of making a space gas free is to ventilate the space for a
period of time either by forced or extractive ventilation. However additional
requirements may be necessary depending on the usage of the space.
Some methods adopted to gas free spaces are
as follows:
Fuel
tanks carrying diesel fuel - The
tank should be completely emptied of fuel.
Smoking or naked lights in the space
containing the tank prohibited. Warning notices should be placed at all
entrances to the space. The door or closure on any manhole or inspection
opening should be removed. A portable ventilation fan (preferably one to which
flexible ducting can be connected) should be situated near the tank opening,
the loose end of the flexible ducting placed into the tank and ventilation
commenced.
Special fans with ducting are readily
available for this work. Fans can be driven by enclosed electric motor (intrinsically
safe) or preferably compressed air drive. Gases will escape through the vent
piping and through the manhole. Dilution of the gases and the flashpoint of
diesel fuel is such that there is little danger of fire or explosion.
The time required to ventilate depends on
the size of the tank and fan.
Ballast
deep, double bottom and peak tanks - Tanks
used for ballast may be full or empty. An
empty tank should never be entered without gas freeing, especially if it has
been empty for a long time (a week or more).
The air becomes stagnant and any rusting that has occurred reduces the
oxygen content and pockets of carbon dioxide may have formed at lower levels.
A gas free certificate must be obtained. Many
gases in confined spaces have an anaesthetic action at quite low concentrations
and humans exposed to the gases experience the usual sequential effects of an
anaesthetic (or narcotic effect) such as drowsiness, loss of control, lack of
judgement, delirium and eventually loss of consciousness. When personnel enter
a tank a sentry (watch person) must be
at the entrance to the tank to maintain constant communication to ensure all
inside remain safe. Ventilation of the tank should be maintained whilst
personnel are in the tank.
If personnel do suffer symptoms of
anaesthesia as described, the sentry should call for help and rescue should
only be attempted by persons with breathing apparatus.
2.6 Flash point
The flash point of a fuel is the lowest
temperature at which the vapour arising from the fuel gives off a flammable
mixture that will just ignite (flash) when a flame is momentarily applied under
defined test conditions. The flash point is determined
under laboratory conditions using various
apparatus eg. the Cleveland; the Abel-Pensky; the Pensky-Martens
apparatus. The former determines the open flash point of a fuel and the latter
two the closed flash point. The open flash point is slightly higher than the
closed flash point.
The flashpoint is an indication of the
volatility of a fuel and determines the degree of fire hazard associated with
the fuel. The IMDG Code mentioned earlier, classes flammable liquids as Class
3. The class is divided into sub-groups 1 (highest hazard) to 3 (lowest hazard)
according to their flashpoints:
Class 3.1 - Low flashpoint liquids having a
flashpoint below - 180C
Class 3.2 - Intermediate flashpoint liquids
of -180C to, but not including 230C
Class 3.3 - High flashpoint liquids having a
flashpoint of 230C to and including 610C.
Liquids having a flashpoint above 610C
are not considered to be dangerous by virtue of their fire hazard. However,
vapours from Class 3 liquids are hazardous. If inhaled, they have a varying
narcotic effect. Prolonged inhalation may lead to unconsciousness and possibly
death.
Flash point should not be confused with the
ignition temperature. Ignition temperature is the temperature to which an
explosive vapour-air mixture must be heated to cause actual explosion. There is
no relationship between the flashpoint and the ignition temperature.
Chapter 3: Fuel supply, injection and control
3.1 Petrol fuel systems
Petrol engines use similar tank and supply arrangements to the diesel but dependant on the engine complexity and age may use:
Carburetion, or;
Petrol fuel injection.
Carburettors
Instead of the injector pump and injectors of the diesel engine, the fuel is atomised and mixed with air in a carburettor. The premixed fuel/air is then supplied to the cylinders through an intake manifold. The carburettor regulates the volume of air/fuel supplied to the engine at different speeds and can increase the percentage of petrol in the mix (make it richer) for easier cold starting. This control for this is called a choke.
Carburettors use the venturi principle of locally pressurising air in a narrowing tube. A fuel line (needle valve) opens into this air stream that sucks and atomises the fuel to pre-mix with the air. Carburettors are used in most outboards and small petrol engines. In the last twenty years larger petrol engines with electrical wiring circuitry have been increasingly fitted with electronics fuel injection systems because of their greater efficiency in meeting emissions control regulations. Most carburettors have the following main components:
Float bowl, float and needle valve - keeps a constant petrol level in the float bowl. The fuel pump delivers fuel to the float bowl. As the level in the bowl rises, the float rises on top of the fuel cutting off the flow of fuel into the bowl and forcing the needle valve shut. If the level drops, the float opens the valve and let in more fuel. The float bowl also helps to trap sediment, to prevent it blocking the jets.
Jets - fine brass jets regulate the amount of fuel which is delivered into the air stream drawn through the throat of the carburettor on induction. Several jets may be provided for idle, low speed, high speed, acceleration, etc.
Throttle butterfly valve - positioned in the main throat below the fuel venturi increases or decreases the flow of fuel/air to the engine (throttle).
Choke butterfly valve – positioned above the venturi restricts the flow of air, without reducing the flow of fuel. This chokes the engine for cold starts.
Further information on outboard engine petrol fuel systems is provided in Chapter 8.
Petrol fuel injection
Fuel injection is the modern alternative to the carburetion process that achieves greater control of fuel usage over a variety of throttle settings from idle to full power. This greater efficiency in fuel use meets regulatory standards for emissions controls and reduces fuel consumption.
Rather than being pre-mixed outside of the engine by induced
suction in a venturi, a metered quantity of fuel is either injected into the inlet
port/s or directly injected into the cylinder by a mechanical or electronic control
system.
Mechanical fuel
injection system (MFI)
The air is inducted through an inlet manifold to the inlet ports. A high pressure electric pump pressurises the fuel and passes it through an accumulator (reservoir) to a camshaft driven fuel distributor. This times the injectors to squirt fuel into the inlet port/s air stream during the induction stroke. The quantity of fuel injected is controlled by a throttle linked air intake flap valve that opens with throttle advance creating more piston induced suction. As the piston reaches tdc, the spark plug ignites the mix to initiate the power stroke. Carburettor type choking by restricting air flow to create a richer mix will not work with this system, so an arrangement for additional cold start fuel injection will be incorporated.
Electronic fuel
injection system (EFI)
The air is inducted through an inlet manifold to the inlet ports. Fuel is injected into the inlet stream of air during the induction stroke and this mix is compressed in the compression stroke. As the piston reaches tdc, the spark plug ignites the mix to initiate the power stroke. The injection point may be positioned at the inlet manifold of all the cylinders (throttle body injection or single point injection), receiving its fuel from a pressurised common rail, or may be separately injected into each cylinders inlet port (multi point injection).
The components of an EFI single point injection system as shown below include:
Electronic control unit - monitors engine demand and controls component response.
Pump and fuel
filter - supplies clean high pressure fuel to the fuel rail and injector.
Fuel rail - a
common fuel pipe feeding the injectors.
Pressure regulator - controls pressure in the system.
Throttle body- controls
air flow with a valve.
Injector - sprays
fuel into the inducted air flow.
Air filter and flow
meter –provides clean measured air flow.
Surge chamber - dampens the flow of air.
Direct fuel
injection (DFI)
Similar to the injector pump and injectors of the diesel engine, the fuel and air are supplied to the cylinder separately. An injector and a spark plug are fitted to each cylinder head. The air is inducted through an inlet port principally during the induction stroke while the direct fuel injector squirts in a metered spray of petrol that mixes with the air. The mix is compressed in the compression stroke and as the piston reached tdc it is ignited to initiate the power stroke.
The advantage of the system is greater control of the injection moment and richness as may be most efficient for the engine over its operating throttle range. For instance it is most efficient for:
Light load conditions (ultra lean burn) - to inject at the compression stroke.
Moderate load conditions (stoichiometric) - to inject during the induction stroke.
High load conditions (full power) - inject during more during the induction stroke.
3.2 Diesel fuel systems
Direct supply
A simple diesel
fuel supply system is shown below.
The fuel
tank with filler cap and venting breather is measurable by a sight gauge. Fuel is directed to the
mechanical fuel pump through the supply
line, along which primary and secondary filters ensure clean fuel only
reaches the mechanical pump. Additional water
separators are often fitted at the primary filter position. A manual lift pump is provided to assist bleeding
(priming) the fuel line of trapped air after servicing the fuel filters. The
cam driven mechanical pump forces the fuel to the injectors when it is required for combustion.
Other fuel line components include:
Baffle
- They are fitted to prevent free surface
effect. This affects the stability of the vessel and in extreme cases can cause
vessels to capsize.
Drain
valve - is fitted to the lowest part of the tank.
Its purpose is to drain water or sediment from the tank. A plug or cap is
fitted so, if the valve vibrates open, the fuel is not lost or causes a fire
risk. Water can be in the tank as a result of:
Coming
with the fuel supply;
Condensation
due to the level in the tank being kept low for a lengthy period;
Rain
or a wave entering through the unsecured deck fittings;
Being
mistaken for a water tank.
Emergency
fuel shut off - This is fitted to allow the fuel to be shut
off outside the engine room in the case of an emergency. It can be fitted
anywhere in the metallic fuel line. It cannot be fitted after the flexible fuel
line. Where fuel tanks are fitted outside the engine room and the fuel shut
offs are easily accessible, emergency shuts offs are not required. An extended
spindle can be fitted to the fuel shut off valve so it can be operated from
outside the engine room. The fuel shut off and the emergency fuel shut off are
then the one valve.
Filling
pipe - is fitted to the top of the tank and it is
preferable that it be piped continuously to deck level. It does not have to be
piped to the deck, if in the event of an overflow; the fuel will not run onto a
hot surface and ignite. The end of the pipe is to be fitted with a sealed cap
or plug.
Filter/water
trap - They can be a combined unit or separate
units. The unit provides a secondary means of filtering the fuel from sediment
and impurities while the water trap removes any moisture or water. The fuel
pump and injectors have very small clearances and any impurities or water in
the fuel will cause them to seize. (The fuel acts as a lubricant). In addition,
moisture could cause corrosion to those finely machined components. Sometimes
additional filters are fitted to the system.
Fuel
contents gauge – Several methods are used to measure the
amount of fuel. If a sight gauge is fitted the valves must be self closing. To
take a reading, open the valves against a spring or lift a weighted handle and,
on letting go, it will automatically close. If the glass breaks or the plastic
tube perishes, it prevents all the fuel in the tank running into the bilges or
in the case of a fire, prevents all the fuel in the tank feeding the fire. If a
sounding rod is used, a striking pad must be fitted to the bottom of the tank
to prevent damage to the tank through repeated soundings.
Modern arrangements now use magnetic flaps
to mark the position of a steel ball floating in a sealed levelling pipe. This
method resolves leakage and fire risk inherent in the sight gauge.
Fuel
pick up - is fitted above the bottom of the tank.
This is to allow a safety margin so as to reduce the amount of any water or
sediment flowing to the fuel filter. A valve or cock must be fitted directly to
the tank.
Fuel
return - Excess fuel from the injectors is returned
to the tank. It is good practice to operate from one tank at a time and the
excess fuel returned to this tank. In this case, the fuel return valve of the
tank not being used must be closed. In small vessels it is not practical to
operate off one tank as the vessel would develop a list, therefore engines
receive their fuel from the port and starboard fuel tanks.
Fuel
lift pump - Unless there is a day tank where the fuel
is fed by gravity to the engine, it will be necessary to have a fuel lift pump
to get the fuel from the tanks to the fuel pump. A fuel lift pump can be a
gear, diaphragm or plunger type.
Fuel
injector - It is a spring loaded valve located in the
cylinder head and allows the fuel, under pressure from the fuel pump, to enter
the combustion space. It enters in
an atomised form to allow it to mix
completely with the hot compressed air so that ignition can take place with
efficient combustion. Excess fuel is returned to the tank.
Fuel
injection pump - It accurately meters the fuel and delivers
it under high pressure at a precise moment to the spray nozzle of the fuel
injector.
Inspection
opening - is fitted in a position or a number may
be fitted to provide access to the whole tank. It allows the tank to be cleaned
and inspected.
Vent
- the purpose of the vent pipe is to allow
the:
Escape
of air and vapours when the tank is being filled so it is not pressurised;
Entry
of air as fuel is used so a partial vacuum does not occur;
Normal
expansion and contraction of the fuel due to temperature change.
It is fitted to the top of the fuel tank at
the highest point when the vessel is in normal trim. This is to prevent an air
lock developing. An air lock is when the tank is being filled, air or vapours
become trapped in the top of the tank, are compressed, and when the pressure
exceeds the filling pressure, fuel is forced out of the vent or filling pipes
and a spill occurs. The smaller vent pipes terminate in a gooseneck, the end of
which must be higher than the filling point.
The end of the vent pipe has an anti-flash
wire gauze fitted to it. If the fuel vapours from the vent pipe ignite, the
flames cannot penetrate the gauze and ignite the contents in the tank providing
the size of the holes in the gauze are not too large.
Before a combustible substance will take fire, its temperature must first be raised to its point of ignition, and, if after it has ignited the temperature is reduced in some way below this point, the flame will be extinguished. A moderate flame can be extinguished by passing a current of air over it, for instance, blowing out a candle.
The reason for this is that more air than is required for combustion is supplied to the burning gas, the surplus tending to cool the flame below its point of ignition. In a similar way, gauze, which is a good conductor of heat, prevents the passage of flame, since it loses its heat very rapidly, and the flame upon coming into contact with it, is cooled below the point of ignition; consequently, no flame appears on the other side of the gauze. A good example is placing a lighted match under the gauze. The flame will not penetrate the gauze.
3.3 Fuel injectors, combustion chamber
A metered quantity of diesel fuel is pumped
by the fuel pump to the fuel injector where it is sprayed into of
the cylinder to ignite in its superheated compressed air, the resulting
explosion initiating the power stroke. Both the design of the fuel injector and
the shape of the cylinder’s combustion
chamber are critical to ensure even air/fuel mixing and efficient combustion.
Fuel
injector assembly
A fuel
injector is a spring controlled valve on the cylinder head that opens under
pressure from the fuel pump to spray fuel into the cylinder. The pressure and
fine passageways of the injector create an atomised spray to ensure good
mixing. Those fine passageways required for the atomisation can be easily
blocked, hence the particular precautions of filters and water separation in
the diesel fuel supply arrangements.
Spray
nozzle assembly
The fuel injector consists of a nozzle body
and valve. The nozzle body incorporates the valve seat and has holes or
orifices in it to atomise the fuel. The valve must seal effectively on the
valve seat to allow for a clean cut off of fuel to the cylinder. A leaking
valve causes misfiring and irregular speed, particularly on light loads. The
valve and nozzle body are lapped to form a mated assembly. Therefore the valve
and nozzle body cannot be exchanged individually. A nozzle cap attaches the
nozzle to the body of the injector.
Types
of spray nozzles
There are different types of spray nozzles. The type of spray nozzle used depends upon the design of the combustion chamber and the angle of the injector. Four types of spray nozzles are the single hole, multi-hole, pintle and pintaux.
The single and multi-hole spray nozzles eject
fuel when the valve opens through one hole in the case of the single spray
nozzle or through any number of holes at any angle in the case of the
multi-hole spray nozzle.
The pintle and pintaux are also similar. A pintle
on the valve projects past the valve seat and slightly past the end of the
nozzle. There is a slight but exact clearance between the pintle
and the injection hole. The pintle size and shape can
be varied so as to meet any spray pattern requirement. The pintle
prevents the formation of
carbon deposits in the injection hole. Pintle nozzles are used in engines with adequate air
turbulence such as pre-combustion chambers or turbulence chambers.
When the fuel pressure opens the valve, the pintle causes a conical spray pattern. It also allows a
relatively small proportion of the fuel to be injected as the valve starts to
open, followed by the bulk of the fuel thereby slowing down the pressure rise
in the cylinder bringing about smoother combustion and engine running. The pintaux differs in that it has a hole at an angle where
fuel sprays out separately from the conical pattern for pilot injection.
Nozzle
holder or body
A nozzle holder forms the body of the
injector. It is fitted with a flange to secure it to the cylinder head. It has
drilled passages for the fuel to flow to the valve and for the leak off from
the valve stem.
Spring
and adjustment screw
The valve is held on its seat by a spring
operating on a steel spindle. The compression of the spring can be adjusted by
a screw and a locknut so that the valve opens at the recommended pressure.
Cap nut - A cap nut is screwed onto the top
of the nozzle holder to enclose the adjustment screw and to seal the unit.
Combustion
chambers
Combustion chamber design, which includes
the shape of the cylinder head, the shape of the top of the piston and the air
flow through the inlet ports, is one of the most important factors in efficient
operation of the diesel engine. Because of the very short space of time
available in a diesel engine in which the fuel and air can mix, various methods
have been devised in an attempt to give improved mixing and combustion.
Combustion chambers can be of several
designs but all are concerned in creating turbulence to the air during the
compression stroke. In the diesel engine, the fuel is in the form of fine
particles sprayed into the cylinder after the air has been
compressed. To secure complete combustion,
each particle of fuel must be surrounded by sufficient air. The mixing of the
air and fuel is greatly assisted by the combustion chamber air turbulence. Some
engines have helical inlet ports to provide additional swirl.
Combustion systems can be classified as direct injection or indirect injection types.
The two most common types of indirect
injection are turbulence chamber and pre-combustion chamber. The larger, slow
speed engines and medium speed engines do not have the same difficulty in
achieving good combustion as small high speed engines.
Direct injection
With direct injection, the fuel is injected
directly into the combustion chamber which can be formed with a cavity in the
piston crown.
This cavity is carefully shaped to promote
air swirl and the direction of the injector nozzle ensures that rapid mixing of
the fuel and air assists complete combustion.
Advantages -
direct injection gives higher thermal efficiency with lower fuel consumption.
This is because no heat is lost or power wasted in pumping air through a
restricted opening into the separate chamber or in discharging the gases from
the chamber. This gives easier starting and generally this type of engine does
not require a starting aid device, such as glow plugs.
Disadvantages - this
kind of injection is prone to diesel knock (a rattling sound).
Indirect injection
The indirect injection or separate chamber
system is where a separate small chamber is connected to the main chamber by a
narrow passage or orifice.
The pre-combustion chamber and the
turbulence chamber (also called a compression swirl chamber) work on the same
principle. The main physical difference is the location and size of the
connecting passage.
With pre-combustion chambers only about 30%
of the combustion air is forced into the chamber, fuel is injected and primary
burning takes place in the chamber. This prevents too sudden a rise in pressure
which can contribute to diesel knock. The burning mixture of fuel and air is
vigorously expelled through the connecting passage into the main combustion
chamber or cylinder where an excess of air permits combustion to be completed.
Advantages -
lower injection pressures can be used, resulting in less wear of injector
nozzles; simpler design of nozzle equipment, which are easier to maintain, and
smoother idling of the engine. Engine manufacturers may in some instances use
either design in their range, depending on operating requirements.
Disadvantages -
not as efficient as direct injection. It can also be prone to pre-combustion
burn-out.
Fuel injectors - operation and faults
The fuel injector pump delivers a set amount
of fuel under pressure to the fuel injector. The pressure causes the valve to
open against the spring and the fuel to spray into the cylinder. When the helix
on the plunger of the fuel injector pump uncovers the spill port, the pressure
of the fuel drops quickly and the spring in the fuel injector causes the valve
to shut. The delivery valve in the fuel pump also closes and fuel is maintained
in the injector pipe. The needle valve is a neat fit in the nozzle and fuel
flows through the small clearance to lubricate it. This fuel is called leak off
and is returned to the fuel tank.
CAUTION:
Injectors deliver extremely high
pressures. You should exercise due care when dealing with this equipment as
there is a clear risk of serious and permanent injury.
Any injector problem at sea can be rectified
by replacing the injector with a spare. However, knowing how to identify faults
and how they are rectified is important.
Incorrect
opening pressure
Too low an opening pressure will cause the
valve to chatter on its seat. Fuel will be injected into the cylinder earlier.
It is caused by insufficient compression on the spring.
Too high an opening pressure will cause the
valve to hammer on its seat. Fuel will be injected into the cylinder later. It
is caused by too much compression on the spring.
The spring adjusting screw has a lock nut
which may have slackened off causing insufficient compression on the spring.
The spring may break. Replace the spring.
The correct opening pressure can only be
obtained by placing the injector in a test rig and adjusting the tension on the
spring until the correct opening pressure is obtained. Whilst on the test rig,
the spray pattern of the fuel leaving the nozzle can also be checked.
Distorted
spray form
Spray nozzle orifices are partially clogged.
Spray nozzles should be cleaned by first soaking them in either kerosene or
clean fuel to soften the dirt. The spray holes or orifices can be cleaned with
a pointed piece of wood. Do not use a piece of wire.
Dripping
injector
The valve is not sealing on its seat. Grind
it in with the finest grade of grinding compound. Excessive grinding causes the
valve to seat too deeply in its seat causing a lagging of the fuel admission
which results in late combustion and therefore loss of power. In addition, the
valve stem may be bent and this will cause the valve not to seal on its seat
and the valve stem will be tight in the nozzle body. The valve and nozzle body
are lapped to form a mated assembly. Therefore the valve and nozzle body cannot
be exchanged individually. Replace with a new valve and nozzle body. The opening
pressure will then have to be adjusted.
Dirt
between the valve and its seating
Spray nozzles should be cleaned by first
soaking them in either kerosene or clean fuel to soften the dirt. Do not use
anything metallic or abrasive to clean them. Grind it in with the finest grade
of grinding compound. Excessive grinding causes the valve to seat too deeply in
its seat causing a lagging of the fuel admission which results in late
combustion and therefore loss of power.
Injector
valve sticking in the nozzle body
The valve stem may be bent and this will
cause the valve stem to be tight in the nozzle body and the valve not to seal
on its seat. The valve and nozzle body are lapped to form a mated assembly.
Therefore the valve and nozzle body cannot be exchanged individually. Replace
with a new valve and nozzle body. The opening pressure will then have to be
adjusted.
Alternately, there may be dirt between the
valve stem and the nozzle body. It may be possible to clean the dirt away and
reuse the assembly. If however, there has been grit passing through the fuel
injector, it is most likely that there is pick
up (metal from one part is
transferred to its mating part) on the valve stem and body thereby scoring
them. Further operation in this condition could cause the valve stem to seize
in the nozzle body. Any pick up on the valve stem and nozzle body will require
the assembly to be replaced.
Too
much fuel escaping at the leak off pipe
Caused by excessive clearance between the
valve stem and the nozzle body resulting from wear or pick up from dirty fuel
or corrosion by water contaminated fuel. A fine clearance is required to
maintain the fuel pressure and allow some fuel to pass by to lubricate. Replace
the valve and nozzle assembly.
3.4 Diesel fuel injection pumps
Jerk type fuel injection pump
A jerk type pumps can have a separate unit
for each cylinder or multi-elements where several pump elements are housed together.
They are comprise of a:
Barrel;
Delivery
valve;
Rack
and pinion;
Camshaft
and spring
Barrel and
delivery valve - Each barrel is locked into the housing in such a way that the upper
section, which contains two ports placed at 180 degrees and known as intake
port and spill port, is completely immersed in fuel supplied by the fuel lift
pump. The barrel is closed at its upper end by a spring loaded pressure valve
known as a delivery valve. An injector
pipe is connected between here and the injector.
Plunger - The plunger which
operates within the barrel is driven on its upward stroke by a roller tappet
operating on a camshaft. Contact is kept
between the plunger and the tappet by means of a spring which operates in a
similar fashion to an inlet or exhaust valve spring. The plunger has a slot and a helix cut into
it near the top.
Rack and Pinion - A rack is fitted
to the pump to engage with a pinion on the outside of a sleeve. The sleeve fits over the plunger and has
slots engaging with keys. This allows
the plunger to be rotated by the fuel rack as the plunger moves up and
down. The end of the fuel rack is
attached to the governor.
Operation
Fuel
metering principle- When
the top of the plunger is below the inlet and spill ports, low pressure fuel
flows through the inlet and spill ports into the barrel. It fills the space
above the top of the plunger to the closed delivery valve and also down the
slot of the plunger and into the space below the helix.
The cam pushes
the plunger up and injection commences when the top of the plunger covers the
inlet and spill ports. As the plunger moves up, the trapped fuel is delivered
under high pressure through the delivery valve to the injector until the
helical grove on the plunger uncovers the spill port. This allows the fuel
pressure above the plunger to fall to the suction pressure through the vertical
slot. The plunger will rise further to complete its stroke but no fuel will be
pumped. As the lobe of the cam goes past top dead centre, the spring will cause
the plunger to return to the bottom of its stroke.
To vary the
amount of fuel injected into the cylinder, the plunger is rotated by the fuel
rack and this causes the helical groove to uncover the spill port earlier or
later depending on whether less or more fuel is required.
The fuel rack is
attached to the governor. If the propeller comes out of the water the engine
starts to speed up, the governor reacts by moving the fuel rack, causing the
helical groove to uncover the spill port earlier or cuts off the fuel
altogether. As the propeller comes back into the water, the engine starts to
slow down and the opposite occurs. To cut off the fuel to stop the engine, the
plunger is rotated by the rack until the vertical slot is in line with the
spill port so no fuel is delivered as the plunger moves up.
Calibration
and timing of a multi-element fuel injection pump
In a multi
element pump, each element is calibrated and timed on a test rig. To calibrate
a pump, each element is connected up to a calibrated test tube. The pump is run
and then each test tube is checked to ensure that each element has delivered
the same amount of fuel. Each element is timed to ensure that injection
commences at the precise time in the stroke.
If the injection
occurs too early on the compression stroke, it will result in high peak
pressures and will subject the engine to unsafe stresses. This is caused by the
tendency of the pressure to reverse the rotation of the engine and evidence by
excessive detonation which is known as diesel knock.
Rotary or Distributor Pump
The distributor
pump incorporates a single pumping element and automatic metering system. This
makes it unnecessary to calibrate and balance a number of pumping elements
which is the case of multi-element pumps. The main components of the
distributor pump are:
Internal transfer pump;
Metering valve;
Governor;
Rotor and cam ring assembly (pumping
element);
Timing advance mechanism;
Maximum fuel delivery adjustment.
Operation
The fuel
transfer pump (or fuel lift pump) draws the fuel from the fuel tank through a
pre-filter and pumps it to a filter head into a combined filter/water separator
where any contaminants and water are removed.
The fuel then is
pumped to the distributor pump which pressurises, controls timing, distributes
and meters an amount of high pressure fuel to the injectors.
The distributor
pump uses an internal transfer pump to increase the fuel pressure in relation
to engine speed. Fuel then flows through a solenoid valve to the timing advance
and the annular groove surrounding the rotor.
A metering valve
determines the amount of fuel made available to the pumping section of the
rotor. Fuel flow is either increased or decreased depending on rotation of the
metering valve by the governor.
A mechanical
shut down lever can also be used to move the metering valve to the closed
position, stopping fuel flow to the injectors and the remainder of the engine.
The governor is
connected to the engine throttle and the metering valve, controlling the fuel
flow in relation to movement in the engine throttle. Fuel from the metering
valve flows through a metering port into the charging port in the rotor and as
the rotor revolves these two ports fall out of alignment, trapping fuel in the
rotor.
As the rotor
continues to revolve the fuel is placed under increasing pressure and
eventually the rotor’s charging port aligns with delivery ports and the fuel
escapes to the injector.
Cummins
pressure time fuel injection pump
The
PT (pressure/time) fuel system is used by Cummins. It uses injectors which
meter and inject the fuel. Metering is based on a pressure/time principle accomplished
by a fixed size opening in the injector and the pressure of the fuel supplied
to the injector. The fuel is drawn from the tank through a filter by the fuel
pump, delivered to the injectors with much of the fuel being returned to the
fuel tank.
The main
components of the fuel pump shown below are:
Gear pump;
Pulsation damper;
Magnetic filter;
Standard governor;
Throttle shaft for the standard governor;
Variable speed governor;
Shut down valve
A gear pump
delivers fuel under pressure through a pulsation damper, which dampens out fuel
surges created by the gear pump action. A magnetic filter is used to remove any
metal debris from the fuel. The fuel flows from the magnetic filter to a
standard governor which controls the engine idle speed and the fuel pressure
delivered by the fuel pump.
The standard
governor is located in series with the variable speed governor, which is
positioned in the fuel pump housing. The variable speed governor controls the
fuel flow to the injectors in relation to the engine speed.
The amount of
fuel which flows to the injectors is dependent upon the fuel pressure from the
fuel pump and the time the feed port is allowed to remain open. Hence the
pressure - time principle of this fuel injection pump system.
Detroit diesel mechanical unit fuel injection system
Detroit
diesel fuel system
Fuel is drawn
from the supply tank through the strainer and enters the fuel pump on the inlet
side. On leaving the pump under pressure, fuel is forced through the fuel
filter into the fuel manifold and from there through fuel pipes to the inlet
side of the unit injectors. Surplus fuel returns from the outlet side of the
unit injectors through outlet fuel pipes into the return manifold, from where it
flows back to the supply source.
The fuel pump is
of the gear type with an in built relief valve. A non-return valve can be
installed between the fuel strainer and the source of supply to prevent fuel
draining back when the engine is not running. A restricted elbow is located at
the end of the outlet manifold to maintain pressure in the fuel system between
the inlet and outlet fuel passages.
Detroit diesel fuel
pump - mechanical unit injector
The unit
injector is a single unit which combines all the necessary components to
provide complete and independent fuel injection to each cylinder.
The unit
injector performs four functions of:
Creating the high fuel pressure needed for
efficient fuel injection;
Metering and injects an accurate amount of
fuel;
Atomising the fuel to assist mixture with
air in the combustion chamber;
Timing the injection of fuel into the
combustion chamber
Unit injectors
have the advantage that there are no high pressure fuel lines and the
continuous flow of fuel serves to cool injector components while also
preventing vapour pockets from forming.
Detroit
diesel mechanical unit fuel injector - Operation
Fuel enters the
injector through a filter cap and element and flows into a supply chamber. Fuel
also flows into space below the injector plunger. The plunger is moved by a
special cam, push rod and rocker assembly. As the plunger moves up and down
inside a bushing, fuel is fed through two ports in the bushing into the supply
chamber.
Detroit
diesel two stroke engine
For engine speed
the plunger can be rotated within the bushing using quadrant gear which meshes
with a control rack. Fuel metering is achieved by rotating the plunger which
varies the relationship between two helices machined into the lower portion of
the plunger and fuel inlet ports in the bushing. The positioning of the helices
and ports control the plunger’s stroke and the amount of fuel injected into the
cylinder.
On the pumping
stroke a portion of the fuel is forced through the lower port into the supply chamber
until the lower plunger helix shuts off the port. Fuel trapped below the
plunger is then forced through a central hole in the plunger and so through the
upper port into the supply chamber until that port is closed by the upper
plunger helix. With both ports closed fuel pressure builds up during the
remainder of the plunger stroke until it is sufficient to lift the injector
valve from its seat, at which point injection commences. The spray tip
incorporates a check-valve whose function is to prevent fuel dribble into the
combustion chamber after the injection cut-off point should the injector valve
fail to return to its seat.
On the injector
plunger’s return stroke, the high pressure area inside the bushing is again
filled with fresh fuel through the two inlet ports. This maintains a constant
circulation of cool fuel which helps in reducing injector temperatures and
effectively removes all traces of air. Excess fuel is fed back to the fuel
return manifold and subsequently, the fuel tank, through the injector outlet
opening which contains a filter element similar to the one on the fuel inlet
side. When the control rack is pulled back to the cut-off position the upper
port is not closed by the helix until after the lower port is uncovered.
Consequently, all fuel is passed back to the supply chamber and no injection
takes place.
When the control
rack is in the full injection position, the upper port is shut off shortly
after the lower port has been closed by the position of the helix. This rack
position is set to give maximum effective plunger stroke and maximum fuel
delivery.
Intermediate
throttle positions are provided by the relative position of the helical
contours to the inlet ports so that both the effective stroke of the plunger
and the commencement timing of the injection are altered.
Electronic unit injector
Increasingly, engine manufacturers are using electronic engine management systems to monitor and adjust engine performance, provide warning and improve control efficiency and data collection. The amount of fuel injected and the timing are determined by information fed into the microprocessor (Electronic Control Module) from sensors located on the engine.
The design
simplifies the plunger and bushing. It also replaces the mechanical rack with
an electronic solenoid. It allows precise metering and injection timing. Examples
of electronic engine management systems are:
Caterpillar’s CAT
Engine Monitoring System & CAT Engine Vision System
Detroit Diesel’s
Electronic Controls
CAT engine monitoring systems
These systems gather data collected from an Electronic Control Module (ECM), monitoring engine and transmission parameters, providing engine diagnostics and gauge readings. The information not only aids in improving fuel economy and engine performance, but provides a record of engine faults to assist with better maintenance.
Detroit Diesel Electronic Controls
Detroit Diesel Electronic Controls (DDEC) is a computerised electronic engine governing the fuel injection system that replaces mechanical controls in Detroit Diesel engines. In addition, within its on board computer, DDEC offers engine protection and self diagnostics to identify malfunctions in its components as well as the ability to trouble shoot engine problems.
Major components of the DDEC are the Electronic Control Module (ECM) and the Electronic Unit Injector (EUI).
Electronic Control
Module
It is a microprocessor that continuously monitors and analyses the DDEC system with electronic sensors during engine operation. A programmable read only memory (PROM) that provides instructions for basic engine control functions. Electronically erasable, programmable read only memory (EEP-ROM) that stores engine calibration values. A back up microprocessor to operate the engine should the main microprocessor fail. With this redundancy built into the ECM, reliability is assured.
Electronic Unit
Injector
It is built on Detroit Diesel patented mechanical unit injector design that simplifies plunger and replaces mechanical rack with an electric solenoid and allows precise metering and injection timing.
The EUI is actually simpler in the area of the plunger and bushing than the mechanical unit injector. The amount of fuel injected and the timing are determined by information fed into the ECM from sensors located on the engine.
3.5 Governors
The power
requirements of an engine may vary continually due to fluctuating loads, so some
means must be provided to control the amount of fuel required to hold the
engine speed reasonably constant during such fluctuations. To accomplish this, a
governor is fitted to the engine. There are a number of different types including:
Constant speed governor - It is used to maintain the engine at the
same speed. For example an auxiliary engine driving a generator may have a
fixed speed of 1800 rpm. The electrical load will vary. If the load is
increased, more fuel is required otherwise the speed will drop. The drop in
speed will cause the governor to alter the fuel pump to supply more fuel so the
1800 rpm is maintained.
Variable speed governor - It is used to maintain a set idling speed, a
maximum speed and any desired speed between these limits regardless of any load
change. The desired speed is set by a speed control lever or wheel. This type
of governor is used on propulsion engines. A simple mechanical and a hydraulic
type are described below.
Mechanical and hydraulic governors
Mechanical governors’
sensitivity is limited due to their flyweights not only limiting the speed, but
also performing the physical work of moving the fuel control mechanism. Additionally,
speed droop is inherent in them, so
they are incapable of maintaining constant speed with varying load without
manual adjustment.
Speed droop - The steady decrease in engine speed caused
by a load increase from no load to full load demand, without change in the
adjustment of the governor.
Isochronism -
The ability of a governor to regulate the engine speed so that there is zero
speed droop. A hydraulic governor of
the proper design is both isochronous
and extremely sensitive as the governor flyweights are used to limit speed
only, the work of moving the fuel control mechanism being performed
hydraulically. A mechanical
governor cannot make any adjustment to the fuel supply until the engine speed
has changed (ie. they cannot anticipate, they can
only correct).
Mechanical
variable speed governor
The governor is
engine driven causing the flyweights to rotate. When the engine is operating at
normal speed, the centrifugal force acts on the rotating flyweights and is
balanced by the vertical speeder spring force. The control sleeve remains
stationary.
If the engine load decreases, the engine speed increases (ie.
the propeller coming out of the water).
The centrifugal force acting on the flyweights also increases causing
the flyweights to move outwards and the control sleeve upwards. This moves the
fuel rack so less fuel is delivered. The upward movement in the control sleeve
increases the compression in the speeder spring and hence the speeder spring
force. This increased spring force and the control sleeve remains stationary in
the new position.
If the engine
speed decreases (ie. the propeller going back into
the water), the opposite occurs. Thus the control sleeve moves up and down as
the engine speed fluctuates because of load variations. The normal operating
speed of the engine can be manually adjusted by increasing or decreasing the
speeder spring compression and hence the speeder spring force by the speed
control lever.
Hydraulic
variable speed governor
The governor is
engine driven causing the flyweights to rotate. When operating at normal speed,
the centrifugal force acts on the rotating flyweights and is balanced by the
vertical ballhead spring force and the piston valve
remains stationary.
If the engine load increases, the engine speed decreases, the centrifugal force
acting on the flyweights also decreases causing the flyweights to move inwards
and the ballhead spring to move the piston valve
downwards. The piston valve, on moving downwards, will admit oil under the
power piston. This pushes the power piston upwards, compressing the return
spring and moving the fuel control towards more fuel. The movement of the
compensating lever slightly decreases the force on the ballhead
spring and returns the piston valve to its neutral position.
The normal
operating speed of the engine can be manually adjusted on the speed adjustment
wheel. To increase the engine speed, the force on the ballhead
spring is increased causing the piston valve to admit more oil under the power
piston which in turn increases the supply of fuel.
The compensating
lever is fitted to stop the governor from hunting.
Hunting is when the speed is below or above the control speed and the governor
continues to adjust the fuel control. To avoid hunting, a governor mechanism
must anticipate the return to normal speed and must stop changing the fuel
control setting slightly before the setting required for sustaining the control
speed has been reached.
Pneumatic
governor
The pneumatic
governor operates on the principle that air passing through a pipe tends to
create a vacuum in a part of smaller diameter.
The engine
suction through a venturi (a tube
with a narrowing throat or constriction designed to increase the velocity of
the gas or fluid passing through it) provides the
necessary
suction and in turn operates a diaphragm control connected directly to the
control rack of the fuel injection pump.
The pneumatic governor consists of two main parts being the venturi air flow control unit (mounted between the air induction manifold of the engine and the air intake filter) and the diaphragm unit (mounted on the end of the fuel pump housing).
The venturi unit - has a butterfly valve fitted at its throat and is actuated by the throttle. The butterfly valve is limited by stops for both the idle and maximum speeds. A vacuum pipe is taken from the same centre line as the butterfly valve to the diaphragm unit.
The diaphragm unit - has a diaphragm, a light spring dampening out any oscillations and keeping the fuel pump control rack in the full open position. A manually operated lever is fitted to the control rack and is used to stop the engine by drawing the control rack into the stop or no fuel position.
Operation
When the engine
is at rest, the lever is released and the spring forces the control rack into
the full load position. By setting the excess fuel device rack allowed to move
automatically to the extent of its travel, placing the fuel pump plungers into
starting position (maximum fuel delivery). The engine is ready to start.
The excess fuel device is located at the opposite end of the fuel control rack and consists of a plunger, or latch, which when released, allows the rack to move as previously stated. As soon as the engine starts and the governor takes charge, this device resumes its normal load position automatically, preventing the control rack from again going to the starting position.
The manual
control is set to running position and the engine started.
When the engine
starts and is idling, the butterfly valve is almost closed. A high vacuum is
immediately created in the corresponding pipe and airtight compartment in the
diaphragm unit.
The air in the
adjoining compartment is at atmospheric pressure, therefore the pressure on
this side of the diaphragm is higher than that existing in vacuum compartment.
This causes the diaphragm to move the control rack towards the stop position
until the engine is running at the predetermined idling speed.
When the
throttle is operated to increase the engine speed the butterfly valve is opened
wider, this decreases the velocity of the air passing the mouth of the
connecting tube.
The result is an
increase of pressure in the vacuum compartment of the diaphragm unit with the
spring forcing the diaphragm and the control rack towards the maximum speed
position, so increasing the amount of fuel delivered to the engine.
Fluctuations can
exist in the induction manifold. To dampen these fluctuations, an additional
adjustable spring controlling the diaphragm is fitted. This auxiliary spring
comes into operation at predetermined speeds and can be adjusted by a screw to
suit the engine requirements.
This governor is
extremely sensitive and efficient but has the bad fault that should any leakage
occur that will tend to destroy the vacuum, the governing effect will be lost
and the engine may race.
Electrical
/ hydraulic governors
The Electric
Fuel Control (EFC) governor is an electrical sensing system that can be
adjusted for isochronous engine speed droop. This governor will provide rapid
fuel rate changes to improve the transient response to the load change. It
consists of:
Magnetic pick up - This is an electromagnetic device that is
mounted in the flywheel housing. As the flywheel gear teeth pass the pick-up,
an alternating current (AC) voltage is induced, one cycle for each gear tooth.
This electrical signal is directly proportional to the engine speed and is fed
to the governor control.
Governor control - The governor control is an all electric
solid state module which compares the pulses (electric signal) from the
magnetic pick-up with a speed control reference point. A current output is
supplied to the actuator which rotates the actuator shaft to control the fuel
flow to the engine.
Actuator - The actuator is an electromagnetic rotary solenoid valve, the turning
action of the shaft regulates the fuel pressure and therefore determines the
engine speed and power. (In other governors, there are variations in that they
still have an electromagnetic solenoid valve but it is not a rotary type, and
it still controls the fuel pressure).
Inspecting, testing
and setting governors
Drive - Governors are driven by some part of the engine that rotates. It may be
off the camshaft, or be mounted on the scavenge blower and driven by the upper
blower rotor or be attached to the end of the fuel pump, or enclosed in the
fuel pump housing, or by some other method.
Lubrication - Mechanical governors are lubricated by oil
splash. Oil entering the governor is directed by the revolving flyweights to
the various moving parts requiring lubricating
Faults - Governor faults are usually indicated by speed variations of the engine.
However, speed fluctuations are not necessarily caused by the governor so if improper
speed variations occur, the unit should be checked for excessive load,
misfiring or bind in the governor operating linkage. Dirty oil is a common cause of hydraulic governor troubles.
Remote control of
governors - Some hydraulic
governors are equipped with a reversible synchronising motor which is mounted
on the governor cover. This motor makes a close adjustment of the engine speed
possible by remote control and is especially valuable for synchronising two
generators from a central control panel or bridge control.
Testing and
setting a mechanical variable speed governor
All governors
are properly adjusted before leaving the factory. However, if the governor has
been recondition or replaced, minor adjustment might be required.
As the procedure
for adjustment vary between makes and models of governors, the Owner’s manual must
be followed.
Caution: To prevent maladjustment, it is the practice of some manufacturers to
seal the governor mechanism after it has been adjusted on the test bed, and, if
the seal is broken, to decline responsibility for failure in performance.
Interference with the tension of the governor springs may cause the speed of
the engine to rise beyond the safety limit.
Interference with the maximum fuel stop may result in the injection of
too much fuel, thus causing excessive exhaust smoke and overheating.
Adjustments
The usual
adjustments are for the maximum no-load speed and the idling speed, although
there maybe a number of steps to affect a setting.
Adjustments
should only be made after the engine has reached normal operating temperature.
An accurate tachometer should be used for the engine speed.
Maximum no-load
speed
A stop is used
to limit the compression of the governor spring which determines the maximum
speed of the engine.
This adjustment
will only affect the maximum speed and have no effect on intermediate speed
control positions. Set the throttle at full speed and when it is running at
this speed, turn the adjusting screw so that the maximum speed, as recommended
by the manufacturer, is obtained. Tighten the lock nut on the adjustment screw.
Idling speed
A stop is used
to limit the travel of the fuel pump rack so that the slot in the plunger does
not line up with the spill port and stop the engine.
With the
throttle in the idle position, loosen the lock nut and turn the adjusting screw
until the engine is running at the manufacturer’s recommended idling speed.
Tighten the lock nut.
3.6 Troubleshooting
Exhaust
emissions
Exhaust is
directly emitted from the cylinder and therefore will indicate the operation
and condition of the engine and its combustion process.
Black smoke
For efficient
combustion, the ratio of fuel to air must be maintained otherwise incomplete
combustion will take place resulting in black smoke. This could be the result
of:
blocked or partially blocked air cleaner;
turbo charger not attaining sufficient speed;
poor compression;
incorrect fuel pump timing, Faulty fuel pump;
incorrect valve timing;
faulty injectors - dirty nozzle, opening
pressure, leak off; valve not seating;
engine overloaded
Blue smoke
Indicates that
lubricating oil is being burnt, caused by:
worn, broken or sticking piston rings and/or
worn cylinder liner bores;
worn valve guides;
valve stem seals leaking;
turbo charger seals leaking;
oil bath type air cleaner overfull
White smoke
white exhaust
vapour indicates water or moisture.
water in the
fuel;
moisture in the
air;
cold cylinder
liner bores and combustion space when first starting engine;
leaking cylinder
head gasket between cylinder and cooling water passage.
Changing fuel filters and bleeding a diesel fuel system
Changing Fuel Filters
Fuel
filters are a critical part of the fuel system that remove the dirt, water and
other foreign matter from the fuel supply, which could otherwise cause damage
and expense.
As the
quality of fuel can never be guaranteed, it is necessary to change the fuel
filters as part of the vessel’s maintenance program to the schedule in the
manufactures service manual, usually being from 200-400 running hours
intervals. This will ensure that the fuel supply remains uncontaminated. Follow this
common procedure to change a single in-line filter and bleed air from the
system:
Close - the nearest valves either side of the filter.
Open - vent and drain valves (if provided) and drain filter into a container.
Remove - filter cover or body as appropriate.
Clean - filter and re-assemble filter or fit new filter (if the renewable type).
Shut - drain.
Open - valves either side of filter.
Close - vent
when fuel appears.
It will
be necessary to bleed the system after changing the fuel filter. To assist the
process fill the filter and casing with fuel oil prior to bolting or screwing
the filter cover down. Given the many different types of filters (both
single and duplex), such a method is not always possible. Also, it will probably be necessary
to prime other equipment downstream of the filter (the fuel pump and possibly
injectors).
Bleeding a diesel fuel system
Diesel fuel systems rely on fuel’s incompressibility. An
injector pump forces fuel through the injector under great pressure. If any air
gets into the system, it will be compressed so preventing proper
injection. Air must be completely
removed from the fuel system from tank through to injector, a process called bleeding or priming. Bleeding is
necessary if you run out of fuel, change the fuel filter or air has been
introduced, possibly from a leaking connection. There are specific methods to
bleed depending on the engine type as described in its manufacture’s manual
along with the bleed access points or bleed
screws.
A Generic
procedure will ensure that the tank
has clean fuel available, that the shut off valve is open, and that personnel
protective equipment, rags and clean up materials are at hand.
Low pressure supply line – the fed water separator bowl is drained from
its underside cock. Next its top bleed screw is opened and re-closed once the gravity
driven solid stream of fuel without air bubbles flows freely. The same action
is taken with the next downstream component of the primary filter. The manual
lift pump is required to lift against gravity to push the fuel up to the
secondary filters. Here its top bleed screw is opened and re-closed once a
solid stream of fuel without air bubbles flows freely.
Continue pumping
to get fuel up to the bottom of the mechanical pump and similarly its bleed
screw is opened and re-closed once a solid stream of fuel without air bubbles
flows freely.
High pressure pump and distribution lines – low pressure lift pump cannot push fuel
through the high pressure mechanical pump. It will be necessary to turn over
the pump with the starter motor. The Detroit two strokes that use unit
injectors (called self priming) fed
by a common rail should start at this
point. Other jerk type fuel pumps may need to be bled at the pumps top bleed
screw/s or distributor lines by opening and re-closing once a solid stream of
fuel without air bubbles flows freely. If the engine still this does not start
or runs unevenly then the injectors must be cracked
(loosened and re-tightened once a solid stream of fuel without air bubbles
flows freely).
Caution - An
injector pump may apply thousands of kPa pressure to
force the fuel through an injector into the cylinder. In opening high pressure
side PPE’s must be used and great care taken against the flesh penetrating
capability and fire risk of squirting fuel oil.
Bearing in mind
the caution above, if the engine runs unevenly the faulty injector can be
identified by loosening one by one. If a loosened injector makes no difference
to even running, but all the others make running worse, then you have
identified the unserviceable injector that makes no difference.
Chapter 4: Lubrication systems
4.1 Lubrication purpose and components
The use of oil in an engine, called lubrication, assists in its efficient running by:
Lubrication
- providing a slippery film between moving parts that reduces wear.
Cooling
- dissipating and removing heat resulting
from friction.
Cleaning
- removing contaminants, debris
and residues of combustion.
It assists the piston rings to seal on the cylinder
liner walls, especially on the compression and power strokes where higher
pressures are involved. It acts like a detergent by removing metal dust and
carbon and keeps them in suspension so the filter can remove them. It prevents
the metal parts from rusting due to the presence of corrosive gas and/or
moisture.
Lubrication systems
Three systems are commonly used being the dry sump lubrication for small two
stroke engines, the wet sump for two
and four stroke engines and increasingly for larger outboards, oil injection.
Dry sump/oil
premix
The dry sump method uses pre-mixed oil in the
petrol fuel supply in ratios of from 40:1 to 20:1 as per the manufacturer’s
recommendation. The fuel/oil mix is inducted through the crankcase so
depositing a film of oil on the moving surfaces on its travel to the cylinder. Dry
sump engines have the advantage of lightness and simplicity. The cooling fins on
cylinder heads castings used by air cooled motors are not as ideal for slow
marine applications as for fast motorbikes, so water cooling is often required
leading to greater complexity in design. Two strokes premix engines are also prone
to oiling of the spark plug gap if start up is not immediate. Accurate mixing the
oil and fuel may not be convenient in a seaway.
Oil injection
The engine runs from a fuel tank but has a separate
tank for the oil. Under pressure from a crankshaft driven oil pump, the oil is
injected into the inlet port/s by way of the crankcase, so lubricating the
engine. In simpler engines the pump is operated by a cable co-joined to the
throttle that may be adjusted for volume with a fine tuning screw. While low
oil level alarms are fitted to sophisticated motors, their simper cousins’ oil
tanks must be visually monitored constantly to avoid the serious damage that
would result from running dry. In more complex engines electronic management
systems are independent and adjust the oil volume to the engines optimum
requirements throughout its throttle range, so providing smoke free operation
with ideal lubrication.
The disadvantage of oil pumps driven by the crankcase is that they do not fully function until the engine is operating, thus allowing momentarily dry spots. While this is of minimal consequence for the small dry sump two stroke that supply oil/fuel for
the starting power stroke, in larger diesel engines electrically driven oil pumps may be fitted to pre-lube the system for some minutes prior to start up. Larger outboards use direct oil injection that is more fully described in Chapter 8.
Wet sump
The drawing below shows a basic wet sump lubrication system. Oil is stored in the wet sump from where it is sucked through a sediment strainer, through an oil cooler arrangement to the oil pump.
The oil pump passes it on to the oil filter to separate contaminants and debris. An oil pressure gauge is fitted to indicate the viscosity of the oil and the effectiveness of the system, along with a low pressure warning light and/or alarm. In the case that the filter or other lines become blocked and an over pressure bypass relief valve will open to return flow back to the bilge rather than damaging the pump or breaching the oil lines connections.
While some oiling occurs in the crankcase by the eccentric lobes of the crankshaft splashing the surface of the wet sump (splash lubrication), the body of pumped oil is
directed by internal drillings (galleries) to the crankshaft bearing journals of the bottom engine. Similarly the engine head gear including camshaft, rockers and valves is serviced through galleries. In the drawing above the oil is also directed the mechanical fuel pump. The oil supplied to the rockers can return down through drillings to the sump.
The following components are described in more detail:
Oil pumps
Relief
and regulating valves
Full
flow and bypass filters, magnetic filters
Heat
exchangers and purifiers
Oil pumps
Lubricating oil pumps are positive displacement of the gear, rotor or vane type. Being engine driven in most engines, they have the disadvantage that it takes several seconds for the oil to be pumped through the engine on start up. Wear takes place, especially on cold engines due to metal to metal contact until oil is received. Larger engines have electrically driven lubricating oil pumps so oil can be circulated for prior to the engine being started. The oil can also be heated to bring up the temperature of the engine and minimise differential expansion of the different metals.
Gear
type pump
Gear pumps consist of two meshed gears within a closely fitted housing that has inlet and outlet ports opposite one another. One gear is driven by the engine and in turning, drives the other. As the gear teeth separate and travel past the inlet, a partial vacuum is formed. Oil entering the inlet is carried to the outlet in the pumping chambers formed between the teeth and the housing. They withstand heat and will pump relatively viscous liquids at medium to high pressure.
Rotor
type pump
In a rotor type pump, there is an inner rotor driven by the engine. The inner rotor has a number of cam like lobes which mesh with mating parts in a rotor ring. The inner rotor causes the rotor ring to revolve within its housing with its inlet and outlet port positioned at 90° to each other. Oil entering the inlet is carried to the outlet by pumping chambers formed between the cam lobes.
Vane
type pump
A vane pump’s slotted driveshaft rotates between closely fitted side plates, and inside of an elliptical shaped ring. Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force. Between succeeding vanes, pumping chambers are formed which carry oil from the inlet to the outlet. A partial vacuum is created at the inlet as the space between the vanes open. Oil is squeezed out at the outlet as the pumping chamber size decreases.
The fixed rotor vane pump shown above is a modification suitable for hydraulic pumps for steering or winches. It operates by the solid vanes housed in a slotted rotor being flung by centrifugal force into the eccentric (nylon) housing on rotation. The drive direction (by belt, chain, air or hydraulic) determines the flow direction. It is best suited for clean fluids only. Lube vane pumps are unidirectional and of all metal construction to cope with metal debris in the oil, however the principles of operation are similar for a lube dedicated pump.
Control features
Gauges
and alarms
Standard commercial vessel practice requires control station positioned low pressure lights/buzzers connected to a sender unit on the engine block. When the starter panel is energised (the key is turned on) the sender will earth and activate the alarms. Once the engine fires and the alternator is charging, the electrical flow is reversed so that the circuit does not earth (the light will go out). High pressure alarms and oil filter pressure alarms may be fitted in more complex control systems.
Relief
valve
As the oil pumps are of the positive displacement type, a relief valve must be fitted to protect the pumps and the lubricating oil system from excess pressure. The relief valve is usually incorporated in the pump body but can be fitted externally to the pump. Upon opening, the relief valve will cause oil to discharge back to the suction side of the pump or back to the sump. At start up a cold engine has high oil pressure, (causing the relief valve to open) dropping as the oil thins out as the engine reaches normal operating temperature (causing the relief valve to close).
Regulating
valve
A pressure regulating valve is fitted to the system to maintain a pre-determined system oil pressure. The spring pressure can be adjusted to set the valve at the pre-determined pressure. The valve would normally be opened when the engine is at its normal operating temperature with the excess pressure being discharged back to the sump. Any drop in oil pressure caused by wear in the engine or in the pump, is automatically compensated by the pressure regulating valve until the spring causes the valve to shut completely.
Filters
An engine may have one or more oil filters of the following types:
Full
flow element type filter
The full flow filter requires all the oil to pass through it before entering the oiling channels. As all oil flowing to the engine must pass through this filter, a by-pass valve is incorporated in the design to prevent oil starvation in the event of the filter element becoming blocked. When the outlet pressure of the filter is below that of the inlet pressure by a predetermined amount, the by-pass valve will open allowing oil to pass. A typical opening pressures are 1.24 -1.45 bar (124-145 kPa or 18 to 21 psi). Whilst the oil now flowing will not be filtered, this if preferable to insufficient oil. To prevent this undesirable situation, filter element must be changed at the periods recommended by the engine manufacturer or less if required.
Bypass
element type filter
The bypass filter continually filters a small portion of the lubricating oil that is bled off and returned to the sump. The main portion of the oil goes to the engine. (Other types return the oil back into the flow to the cooler). Eventually all of the oil passes through the filter.
Centrifugal
type filter
This filter is a unit which does not employ an element. It can be used by itself or in conjunction with replacement element filters. These are centrifugal type filters and may be driven by the oil pressure or direct from the engine. Any solids in the oil are flung by a revolving drum to the sides of the rotor, where they will remain until the unit is dismantled for cleaning. Washing in a suitable cleaner is all that is required to put the unit back into service.
Magnetic
filter
A magnetic oil filter is used to remove small particles of metal usually from the result of wear. A new or overhauled engine will shed minute particles of metal until it beds in. A magnetic filter is therefore beneficial in this instance. Regular inspections of the filter may also draw early attention to a problem. The magnetic filter cannot be used on its own as it will not filter out the non metallic foreign particles.
The filter is a full flow type without a by-pass valve. It is so designed that metal particles fill horizontal gaps between the iron rings from the top and working downwards but still leave vertical spaces for the oil to flow. The element consists of a permanent magnet enclosed by a non-magnetic cylinder. A number of iron rings fit over and are attached to the cylinder. There are small gaps between the iron rings to attract the metallic particles. The element is situated in a non-magnetic casing.
The oil flow into the top of the filter and out at the bottom. Metallic particles are attracted and fill the horizontal gaps between the iron rings.
To clean the filter, the cover is taken off and the element is removed. One half of the iron rings are removed at a time to avoid demagnetisation and cleaned.
Installation
variations
Some engines are fitted with a full flow filter that removes the larger foreign particles without restricting the normal flow of oil plus a bypass filter to remove the minute particles of foreign particles that may be present.
Some engines may be fitted with three filters. The first removes the larger foreign particles, the third removes the minute foreign particles while the second removes the intermediate foreign particles.
When changing the elements on both of the above types of filter, the engine manufacturer’s recommendation as to the element number must be followed. This is to ensure that the filter has the correct degree of filtration. In addition, some elements have a built in relief valve while others don’t.
Some engines are set up with dual filters but only one is used at a time. Change over to the clean, from the dirty filter, can be accomplished whilst the engine is running.
Purifier
Where
a large quantity of lubricating oil is used in an engine, it is costly to carry
out oil changes. A purifier is therefore piped into the system to remove
impurities and water so it is possible to use the oil practically indefinitely.
Oils used in a purifying system are usually non-detergent oils as the purifier
removes the detergent as well as the impurities.
Where
a large quantity of lubricating oil is used in an engine, it is not cost
effective to carry out oil changes. A purifier is therefore piped into the
system to remove impurities and water so it is possible to use the oil
practically indefinitely.
Oils
used in a purifying system are usually non-detergent oils as the purifier
removes the detergent as well as the impurities.
It
is preferable to install the purifier in a continuous by-pass system. The oil
is drawn from the lowest part in the system, heated to approximately 80° C,
passed through the purifier and returned to the system. Heating the oil helps
to separate the water from the oil.
In
the purifier, provision is made to water wash the oil. Hot water together with
the oil is fed into the purifier. When it passes through the water seal of the
bowl, the two are separated and in doing so, a washing action occurs. Water
washing rids the oil of acids which are then discharged with the water.
The
purifier works on centrifugal action. It has a high speed revolving bowl filled
with cone shaped metal discs with holes in them. The oil is fed into the bowl
at the top, and flows down the centre of the discs. Due to centrifugal force,
the heavier solids are thrown out to the side of the bowl. The water and
lighter solids move between the discs to their outer edge and are discharged.
The clean oil, being lighter than the water, passes through the discs holes to
the clean oil discharge. The heavier solids build up on the side of the bowl
and regular cleaning is required. Most purifiers these days are of the self-cleaning
type.
Heat exchangers
Heat
exchangers or coolers for a lubricating oil system are commonly of types:
Shell and tube type or;
Plate type.
Shell and tube type:
Shell -The shell
has an oil inlet and outlet and contains the tube nest. Baffles may be fitted to increase the oil’s contact
period with the entire tubes’ surface. On some coolers, the oil on the inlet
side is prevented from direct contact with incoming oil on the tubes.
Tube nest - The tube nest consists of a tube plate at either end with tubes
fitted in between. Allowance for expansion is required due to the different
rates of expansion of the metals used in the construction and prevent any undue
stress on the tubes. One tube plate is secured at one end of the shell and expansion
allowance is by using twin O rings at
the other end for of the tube nest. The
gap between them will indicate a leak from either the oil or the cooling medium.
End covers
- An
end cover is fitted to each end of the shell. Where there is a flange on each
end cover, the sea water will flow in at one end and out the other end. Where
there is two flanges on one end cover, the sea water will enter and leave via
this cover. A division plate will be fitted in this cover and seals against the
tube nest across its diameter to separate the inlet and outlet flows. The other
end cover will have no flanges.
Electrolytic action - To protect the tube nest from electrolytic
action, a zinc anode is fitted in the sea water inlet.
Flow of liquids - In the typical shell and tube type cooler
shown below, oil enters and leaves by the flanges on the shell and circulates
around the outside of the tubes. Sea water is circulated through the tubes and
enters and leaves by the end cover/s.
The
flow of sea water is opposite to the flow of the oil. The best efficiency is
obtained by liquids moving in opposite directions to each other (by contra-flow).
Cleaning - The sea water will leave behind more
deposits than the oil as scale in the tubes so it will be necessary to
periodically clean them. The end covers can be removed and a wire brush pushed
and drawn through each tube, this being the reason why the sea water flows
through the tubes.
Plate type cooler
Construction - The plates are corrugated metal pressings of
horizontal or chevron pattern to make them stiffer but of thinner material. This
also increases turbulent flow so contributing to efficient heat transfer.
Turbulence, as opposed to smooth flow, causes more of the liquid contact with
the plates. It also breaks up the boundary layer of liquid which adheres to the
metal and acts as a heat barrier in smooth flow. However, turbulence can cause
plate damage due to erosion.
Joining the plates - Each plate is separated from the adjacent
plate with a joint material (nitrile
rubber), which is bonded to the plate with adhesive. The joint material is
positioned to stop the oil and sea water from leaking out of the cooler and to direct
the flow of both liquids along their correct paths. This allows the oil to be
pumped along the top of the plates and flow down every second pair of plates,
returning out of the bottom outlet. The sea water flows in at the bottom of the
plates and passes up the adjacent pair of plates, returning out of the top
outlet. The best efficiency is obtained by liquids moving in opposite
directions to each other (by contra-flow).
Cooler assembly - The inlets and outlets are attached to the
fixed end plate. The movable end sits in the horizontal carrying bars and the
plates are also located and supported by these. The flow ports at the corner of
the plates are arranged so that the cooling liquid and the liquid being cooled
pass between alternate pairs of plates.
Cleaning - If cleaning is needed to remove deposits,
use should be made of special soft brushes. Chemically cleaning may be
recommended where hard deposits have accumulated. Before cleaning, coolers are
isolated from the system by valves and blanks or by removing pipes and blanking
the cooler flanges. Flushing is necessary after the cleaning agent has been
drained from the cooler.
Advantages - Plate coolers are smaller and lighter than a
tube cooler giving the same performance. No extra space is needed for
dismantling (a tube cooler needs clearance at one end to remove the tube nest).
Plates can be added in pairs, to increase capacity and similarly damaged plates
are easily removed, if necessary without replacement. Cleaning is simple as is
maintenance. Turbulent flow helps to reduce deposits which would interfere with
heat transfer.
Disadvantages - In comparison with tube coolers in which
leaking tubes are easily located and plugged, leaks in plates are sometimes
difficult to find because they cannot be pressurised and inspected with the
same ease. Deteriorating joints may be difficult to remove and new joints difficult
to re-bond. Tube coolers are preferred for lubricating oil because of their
pressure differential. There are a large number of expensive joints and plates on
plate coolers.
4.2 Oils and additives
Oils
Types and grades of oil
Oils are slippery viscous liquids that do not mix easily with water but mix easily with other oils. They consist of long chain organic molecules principally of carbon, hydrogen and oxygen. They are derived from organic (plants or animals) or from mineral (fossilised plants or animals), the latter source being termed petrochemical. Oils used in engines have principally been derived from the distilled and purified petrochemical sources of crude oil or oil bearing shale deposits. Such natural distillation results in a mix of molecule lengths.
Increasingly organic and mineral oils are being mixed with polymers to create designer oils to specifications including homogenous molecular chain lengths that improve consistency in viscosity and slipperiness. These oils are called synthetic or semi-synthetic (the later when containing less than 30% synthetic oil).
American Petroleum Institute (API) tests and classifies oils by their quality into Groups I, II, III, IV, and V with another 3 sub categories. The Society of Automotive Engineers (SAE) grades motor oil viscosity as 5, 10, 15, 20, 25, 30, 40, 50 or 60. The numbers 5, 10, 15 and 25 are attributed with the letter W (winter) meaning they have a cold start viscosity appropriate to a lower temperature. Grade 20 indicates a hot viscosity grade and 20W indicates a cold viscosity grade.
Oils viscosity SAE may be described as single grade or multigrade (blends). This is to allow for the engine’s range of operational temperatures and the consequent viscosity of the oil. For instance, at start up on a very cold morning thin viscosity sufficient to freely oil all moving parts may require SAE 15W whereas on the hot afternoon SAE 30 will be the limit of thinness required, hence a blend of SAE 15W – 30 would meet the operational range.
The
engine manufacturer specifies the oil that is suitable for a particular engine
taking into account the operating conditions and the contamination problems
that could arise. This is stated by its API group and the SAE viscosity. It is
therefore essential that the correct type and grade of oil be used to prolong
the engine’s life.
General considerations
Premix
two stroke petrol engines require an oil designed to mix thoroughly but still deposit
onto the engine’s moving parts before the mix is combusted.
Generally
petrol engines require a lower viscosity than diesel engines as diesel engines,
relying on oiling to seal the cylinder for high compression, require heavy-duty
lubricating oils.
Oils
used for hydraulic situations must primarily be incompressible.
The
basic requirements of all oils include:
Lubricating quality;
High heat resistance;
Control of contaminants;
Lubricating quality - The reduction of friction and wear by
maintaining an oil film between moving parts is the primary requisite of a
lubricant. Film thickness and its ability to prevent metal-to-metal contact of
moving parts is related to oil viscosity. For instance, the optimums for GM Detroit
Diesel 471/671 are SAE 40 or SAE 30.
High heat resistance - Temperature is the most important factor in
determining the rate at which deterioration or oxidation of the lubricating oil
will occur. The oil should have adequate thermal stability at elevated
temperatures, thereby precluding formation of harmful carbonaceous and/or ash
deposits.
Control of contaminants - The piston and compression rings must ride
on a film of oil to minimise wear and prevent cylinder seizure. At normal rates
of consumption, oil reaches a temperature zone at the upper part of the piston
where rapid oxidation and carbonisation can occur. In addition, as oil
circulates through the engine, it is continuously contaminated by soot, acids,
and water originating from combustion.
Until
they are exhausted, detergent and dispersant additives aid in keeping sludge
and varnish from depositing on engine parts. But such additives in excessive
quantities can result in detrimental ash deposits. If abnormal amounts of
insoluble deposits form, particularly on the piston in the compression ring
area, early engine failure may occur.
Oil
that is carried up the cylinder liner wall is normally consumed during engine
operation. The oil and additives leave carbonaceous and/or ash deposits when
subjected to the elevated temperatures of the combustion chamber. The amount of
deposits is influenced by the oil composition, additive content, engine
temperature, and oil consumption rate.
Diesel
engines require special lubricating oil, because of the diesel fuel and the
higher pressures and therefore temperatures in the cylinder, compared to a
petrol engine. Additives are therefore used to assist the oil in performing its
duties and also in overcoming contamination problems and include:
Anti-oxidants or oxidation inhibitors - Prevent varnish and sludge accumulations on
engine parts. They also prevent corrosion of alloy bearings.They
decrease the amount of oxygen taken up by the oil thereby reducing formation of
acidic bodies. Additive generally oxidises in preference to the oil.
Anti-corrosives, corrosion preventative or
catalyst poisons - Prevent failure of alloy bearings and other
metal surfaces by corrosive attack. Such
additives inhibit oxidation preventing acidic body formation and providing
protective films on bearings and metal surfaces. Chemical film formation on
metal surfaces decreases catalytic oxidation of the oil.
Detergents - Keep engine surfaces clean and prevent deposits
of all types of sludge
by
chemical reactions. Oil soluble oxidation products are prevented from becoming
insoluble and deposited on various engine parts.
Dispersants - Keep potential sludge forming insolubles in suspension and prevent their depositing on
engine parts. Agglomeration of fuel soot and insoluble oil decomposition
products is prevented by breakdown into finely divided state. In colloidal
form, contaminating particles remain suspended in oil.
Extreme pressure agents - Prevent unnecessary wear of moving parts as
well as scuffing or scoring. By chemical reactions, film is formed on metal
surfaces which prevents welding or seizure when the lubricating oil film is
ruptured.
Rust preventative - Prevents rust in new and overhauled engines
during storage or shipment by preferential
wetting of metal surfaces through added adhesiveness.
Pour point depressants - Lower pour points by coating the oil’s wax crystals to prevent growth and oil absorption at reduced temperatures.
Viscosity index improvers - Lower the rate of change of viscosity with
temperature.
Improvers
are less affected by temperature change of oil. They raise viscosity at 93° C
(200° F) more in proportion than at 37° C (100° F).
Foam inhibitors - Prevent formation of stable foam and enable
foam to break up quickly.
4.3 Contamination, analysis and servicing
Contamination
Dust and metallic particles from wear
Dust
can enter through faulty or dirty air cleaner or leaks in the air intake system
and enter the cylinder. The dust deposits itself in the oil on the cylinder
wall and causes wear to the liner and piston rings. It can make its way to the
sump if there is wear in the liner or rings to deposit and/or form sludge.
Metallic
particles result from normal metal to metal wear, especially when the engine is
starting from cold and lacks initial lubrication. On an overhauled engine, the
high metal contact spots wear off creating debris. There will also be rust and scale from
storage tanks or pipes. Some engines have a magnet in the sump or a magnetic
filter to attract any metal particles and stop them from entering the system. Particles
may deposit and form a sludge that accelerates oxidation.
Fuel
A
leaking injector pipe situated under the rocker cover will allow fuel to drain
with the oil that is returning by gravity back into the sump. A leaking
diaphragm on the fuel lift pump or on some engines the seal on the fuel pump
will allow fuel into the sump.
Fuel
contamination will thin out the oil and it will run easily off the dip stick.
There will be a rise of the level in the sump. The dip stick will also have a
fuel smell. Fuel dilution of lubricating oil will cause a reduction in
viscosity and flashpoint. Lowering the viscosity impairs the oils lubricating
properties. Lowering the flashpoint increases the risk of a crankcase
explosion.
A
fuel dilution exceeding 2.5 % by volume indicates an immediate need for an oil
change and corrective maintenance action.
Incomplete combustion
Lack
of compression in a cylinder will result in insufficient air with lower
temperature causing combustion to be incomplete. An engine with worn piston
rings or cylinder liner will allow the products of incomplete combustion into
the sump, called blow by. A misfiring
engine will also create this type of contamination from incomplete combustion. Blow
by gases in the oil will cause oxidation which could lead to corrosion and
subsequent wear. A harmful varnish could adhere to engine parts. Carbon and
soot will contaminate the oil. The oil will go darker in colour and a sludge
could form.
(Oxidisation is when the oil is subjected to
a high temperature and intimate contact with air. The products of oxidisation
are acidic.)
Unburnt
fuel will wash the lubricating oil off the cylinder liner bore causing liner
and piston ring wear. It will dilute the oil in the sump.
Fresh or salt water leakage from cooling
systems
Water
created by a cold engine contaminates the oil. A normal by-product of
combustion is water and when the cylinder liner wall temperature is too low,
the water will condense in the cylinder and pass the piston rings or is scraped
off the cylinder liner walls, by the oil scraper ring, into the sump.
Fresh
water can also enter the sump from leaking water jackets, cracked cylinder
heads or liners, faulty cylinder liner seals (O rings), loose cylinder head or
a blown head gasket from a cylinder to a cooling water passage and the water
could leak into the sump when the engine is stopped. Fresh water contamination
of the lubricating oil will result in an increased level on the dipstick and a
drop in the fresh water level in the header tank.
Cooling
water could enter the sump from a leak in the tube nest of the oil cooler but
not while the engine is running. While running, the oil pressure is greater
than the cooling water pressure so any leak will cause the oil to flow into the
cooling water. However, when the engine is stopped, all the oil drains into the
sump. If it is sea water cooling and the sea water line is above the cooler, it
forms a head (pressure) on the sea water. It will flow through the leaking tube
into the sump.
If
it is fresh water cooling, the fresh water in the header tank forms a head
(pressure) on the fresh water. It will flow through the leaking tube into the
sump. Indications of a leak in the tube of the cooler cooled with sea water can
be seen in oil being discharged overboard with the sea water cooling whilst the
engine is running.
Indications
of a leak in the tube of the cooler cooled with fresh water can be seen in oil
floating on the top of the fresh water in the header tank.
Water
mixing with oil will result in emulsified oil which is grey/white or sometimes
described as milky in colour. Depending upon the degree of emulsification, it
can clog small openings and oil passages and prevents proper circulation and
heat transfer. It will form as sludge and corrosion will occur. The fresh water
could contain glycol which, when mixed with the oil, is damaging to the engine.
Lubricating
oil
analysis
Regular
laboratory analysis of the oil should be carried out, especially when a
purifier is fitted and periodic oil changes do not take place. The oil sample
tendered for analysis should be taken from the circulation system, preferably
while the engine is running, so that it is a true representation of the oil. If
this is not possible, it should be taken as soon as the engine is stopped. An
analysis will give valuable information as to the engine’s condition, any
change to the oil and the nature of contaminants.
A
laboratory will advise on the level of contaminants, what may be causing them
and what steps can be taken to remove them. The problem causing the
contamination must be rectified immediately. Contaminants may include:
Water;
Fuel dilution;
Foreign mineral matter;
Carbonaceous material; and;
Oxidation products
The
Detroit Diesel Operators Manual provides used lube oil analysis warning values
Including:
The
presence of ethylene glycol in the oil is damaging to the engine. Its presence
and need for an oil change and for corrective maintenance action may be
confirmed by glycol detector kits which are commercially available.
Fuel
dilution of the oil may result from loose fuel connections or from prolonged
engine idling. A fuel dilution exceeding 2.5% by volume indicates an immediate
need for an oil change and corrective maintenance action. Fuel dilution may be
confirmed by ASTM D-322 test procedure performed by oil suppliers or
independent laboratories.
Additionally,
if any of the following occur, the oil should be changed.
The
viscosity at 100° F of a used oil sample is 40 percent greater than the
viscosity of the unused oil measured at the same temperature (ASTM D-445 and
D-2161).
Iron
content is greater than 150 parts per million.
Pentane
insoluble’s (total contamination) exceed 1% by weight (ASTM D-893).
The
total base number (TBN) is less than
1.0 (ASTM D-664).
Note - The sulphur content of the diesel fuel used
will influence the alkalinity of the lube oil. With high sulphur fuels, the oil
drain interval will have to be shortened to avoid excessive acidity in the lube
oil.
Faults,
maintenance and servicing
Oil change
Time
between changes
An
engine manufacturer typically recommend the oil and oil filter change intervals
in each operator’s manual at between 200 and 500 hours, or less if the engine
lays idle for long periods. Kubota recommend for the 38 kW an initial run in
period of 50 hours and continuing at 200 hours intervals. Detroit Diesel
recommend 150 hours for their Series 53, 71 and 92 naturally aspirated and
turbo charged engines and 500 hours for their Series 149 naturally aspirated engines
and 300 hours for the Series 149 turbo charged engines. Detroit state that the
oil change intervals may be extended if supported by used oil analysis.
Factors
affecting oil change periods
The
amount of wear, oil contamination and efficiency of filtration are factors determining
oil change scheduling. Frequent long voyages at high speed with the resultant
high engine operating temperatures, may oxidise the oil and may result in the formation
of sludge and varnish. Short runs and in cold weather do not permit thorough
warming up of the engine, and water may accumulate in the crankcase from
condensation of moisture produced by the burning of the fuel and cold engine
parts. The use of effective filters, maintaining the engine in good condition
and preventing overheating of the oil, will improve the efficiency of the oil.
Additives
are used to increase the performance of the oil and some of them break down, in
preference to the oil, as they perform their duty. Some can only do so much
work before they must be removed from the engine along with the undesirable
side effects of combustion. Oil change should occur before the additives wear
out. This is not possible unless an analysis is carried out.
The
best policy is to follow the engine manufacturer’s recommendations on oil and
filter changes. These recommendations apply to normal engine operation. If the
operational conditions of the engine are not normal, change the oil
and filters more regularly. An oil change may be required at sea to enable the
vessel to reach port so sufficient spare oil should be carried on the vessel.
Changing the oil
Containment
precautions must be in place before the actual oil change including tagging off
any remote starting controls. Every effort to avoid and to deal with unexpected
spills into the bilge must be in place, and a plan to temporarily store then
dispose of waste oil ashore must have been negotiated with a shoreside disposal facility.
The
oil will flow better if the engine warms it up for a few minutes before draining.
Many vessels will have dedicated oil change manual pump that most conveniently
draws off the waste oil. Even so, it may be necessary to remove the lowest sump
plug to drain off any remnant sludge not ejected by the manual pump.
Once
the sump is dry the oil filter/s can be renewed. Disposable cartridge type
filters are removed using a suitably sized oil filter strap clamp tool and a
container to catch splash from any remnant oil. The used cartridge is disposed
of. The new cartridge is filled with clean oil, its rubber sealing washer
lightly smeared with oil and the cartridge is tightened back into the threaded housing.
Do not over tighten.
The process is the same for replaceable element filters except that the waste element is disposed of, the filter bowl cleaned and a new filter element installed. A formal record must be kept in the oil record book detailing quantity, disposal date and engine hours. Engineers will often write on the filter the date and engine hours as a future scheduling reminder, as shown below.
Faults in a lubricating oil system
Contamination – As has been described above.
Low
oil pressure - Oil
pressures will differ between types and makes of engines. As a guide, on a
Detroit Diesel, normal oil pressure at 2100 rpm is 276-414 kPa
(2.7 – 4.1 bar or 40- 60 psi) with a minimum oil pressure of 207 kPa (30 psi). At 1200 rpm, it is 207-414 kPa (30 to 60 psi) with a minimum oil pressure of 124 kPa (1.2 bar or 18 psi). A
lower minimum oil pressure is acceptable at a lower speed compared to the
minimum at a higher speed. This is due to the lower loadings on the bottom end
bearings whilst on the power stroke.
The
reduction in the normal operating pressure of lubricating oil can be a gradual
process or happen instantaneously. The total or significant loss of oil pressure
will cause the parts under most load to fail first, often the bottom end
bearings due to the load placed on them during the power stroke. In an emergency
situation where engine power is still required, reducing the engine speed will
reduce the load on the bearings. If there is still some oil pressure a
reduction in load maybe sufficient to save them. Should the oil pressure drop
instantaneously, the engine must be stopped immediately.
Should
there be no oil pressure within 10 to 15 seconds after starting the engine,
immediately stop the engine. Most modern vessels are fitted with low pressure
alarms but no equipment is 100% reliable, especially in a marine environment. The
engineer must rely on his/her senses to monitor the engine performance.
Insufficient level of oil in the sump - May cause a fluctuation of the oil pressure
as the vessel rolls, the pump loses suction and air enters it.
Lubricating oil pump strainer clogged - The additives in oil keep contaminants in
suspension for the filter to remove and minimise this problem.
Faulty lubricating oil pump - If the drive to the pump has sheared, there
would be no oil pressure at all requiring immediate engine shut down to avoid severe
damage. Should the pump gears/rotors be worn or have too much clearance between
them and the backing plate a gradual drop in oil pressure would be experienced.
Faulty pressure relief valve - It may stick open or its spring may have
broken. A cold engine on start up will have a high oil pressure causing the
relief valve to open. As the engine reaches its normal operating temperature the
oil pressure drops as the oil thins out resulting in the relief valve closing.
Should the relief valve stick in the open position or the spring break, the oil
pressure will drop below normal.
Filter partially clogged - With the filter being partially clogged,
the flow of oil will gradually be restricted. Lower oil pressure will occur and
be indicated on the pressure gauge until the filter by pass valve opens.
Oil temperature too high - A high oil temperature will thin the oil out
causing it to flow more easily with a resulting drop in oil pressure. This could
be caused by a worn engine which would have fresh water overheating as well.
Alternately, it could be caused by a dirty oil cooler on the sea water side.
Faulty oil pressure gauge - A faulty oil pressure gauge could indicate
a low oil pressure where in fact the actual pressure is correct. If the oil
pressure gauge is suspected, try another one.
Fractured lubricating oil pipes – This may result in a sudden pressure drop.
Excessive clearance in a bearing or bearings
- In
a main/bottom end bearing the clearance is very small and places a restriction
on the flow of oil increasing oil pressure. If the bearing clearance is
excessive, the oil is less restricted and its pressure will drop. Usually, the
bottom end bearing is the problem.
Water in the oil - Water mixing with oil results in emulsified
oil described as grey/white or milky in colour. Emulsified oil loses its
lubricating propertiesand the oil pressure will drop
below normal.
Fuel in the oil - Fuel contamination will thin the oil and it
will run easily off the dip stick. It will smell of fuel and there will be a
rise in the level in the sump. The oil will lose its lubricating properties and
the oil pressure will drop.
Chapter 5: Engine cooling
5.1 Marine
cooling systems
The purpose of a
cooling system is to maintain a constant temperature throughout the engine that
minimises hot spot expansion leading to seizing of the moving parts. Small
lightweight auxiliary engines use ambient air cooling with carefully designed cooling fins around the hottest
component, the cylinder head. More complex auto engines use a heat exchanger system of a wind cooled
fresh water radiator circulating fresh water coolant around the water
jackets within the engine block.
Water cooling
systems are universally used in marine main engines where speed driven cooling
is insufficiently constant for reliability. These systems include:
Direct water cooling (fresh or salt);
Heat exchanger cooling (tube or keel cooling).
Direct
water cooling
All vessels have
an unlimited supply of cooling water over the side. Simpler engines working in
fresh water have no difficulty in pumping water up into the internal water
jacket that surrounds the engine’s cylinder and dumping the used water back
overboard. The water pick up pipe will exclude large waterborne debris with a
clam shell external grate. A fine strainer and sacrificial zinc anode may be included.
Crude systems such as these rely on flow designed around an average water
temperature, and may cool the engine more than the optimum running temperature.
Outboards
typically use direct (salt) sea water cooling with its added problem of salt
induced corrosion and salt deposit build up. The cooling water pick up is
positioned in the outboard leg so designed that water is both directed towards
the gear box driven impellor pump and strained of debris. Anti corrosive
constructional metals, strategically placed sacrificial anodes and flow design
are critical in the piping and water-jacket layout. Moreover, a strict regime
of flushing out the motor with fresh water after every trip is essential.
Outboard motors have a leak off pipe from the cooling water system called a tell tale placed in a visible position
to indicate that water is circulating freely, a requirement being that the leg
and water pick up are constantly immersed. The outboard is more fully described
in Chapter 8.
Heat
exchanger cooling
The heat
exchanger cooling system maintains a constant temperature throughout the engine
by removing heat from the hottest part of the engine in the vicinity of the
combustion space and transfers it to the cooler parts. Used salt cooling water
is ejected directly overboard or via the exhaust. Marine systems are designed
to operate at temperatures of typically 85°- 90° C and may be pressurised to
raise coolant boil temperature and provide more efficient circulation. An
engine can operate intermittently up to 96° with a header tank cap pressure of
103 kPa (1.03 bar or 15 psi). The cooling water high temperature alarms are
set at around 96° C.
In the schematic
system shown below, engine driven pump sucks sea water from the sea chest through the cooler (header tank and heat exchanger) and another circulates fresh water through the
engine water jackets and the cooler. The close contact of the engine’s hot
fresh water is cooled (heat exchanged)
by the piped cold salt water on its way to be ejected (and cool) the exhaust.
The salt water or raw water components
include:
Rose or grid - The sea water intake is through a corrosion resistant (bronze or stainless steel 316 grade) rose or grid on the vessel’s hull in order to prevent large pieces of foreign matter entering or blocking the flow.
Sea cock and valve - is attached directly to the inside of the hull so that the sea water can be shut off during maintenance or repair whilst the vessel is afloat.
Multi sea cocks
are used on catamaran hulls allowing a cross over facility in case one strainer
(screen) gets blocked.
On some vessels
there is a high and low sea water intake valve. Only one is used at a time. The
high one is used for smooth water operations where there is shallow water so
that sand and mud are not sucked up into the system. The low one is used at sea
so when the vessel rolls, the intake does not come out of the water. It can
also be used on smooth water operations where there is deep water.
Strainer or screen- is fitted into the pipe work before the pump, to avoid small foreign matter being drawn into the system. The strainer may be fitted with a sight bowl. The strainer must be easily accessible for frequent cleaning and inspection. A sacrificial zinc anode plug is often fitted to the top of the sea chest.
Sea water pump - is an engine or belt driven positive displacement pump, usually of the flexible impellor type (often called after the common brand, Jabsco)
Fresh water cooler - The cooler is on the discharge side of the sea water pump and is usually protected against the corrosive effect of the water by a sacrificial zinc anode plug. To further protect against salt water corrosion the constructional materials used are non corrosive and consistent in their electrolytic reactivity.
Exhaust box or swan neck – In a wet exhaust system the raw water is ejected into the exhaust flow to cool and quieten the exhaust. It is injected into the exhaust pipe after a gravity lock (a bend or bucket) in such a way that water will not drain back through the exhaust manifold and into the cylinders. This is to prevent the potential for serious damage of turning over uncompressible water filled cylinders (hydraulicing) and/or introducing a corrosion source. This remains a possibility if an engine is repeated turned over without starting, as the build up of salt water may over flow the water lock. More complex engines may have a salt water leak off pipe with control valve whereby exhaust back pressure generated by the salt water injection into the exhaust can be optimally adjusted to manufacturer’s specification.
Some vessels use an insulated dry exhaust and eject the used raw water directly overboard.
The fresh water or coolant circulation
components include:
Fresh water or coolant circulation – The circulating liquid may be fresh water of a mix of water and additives. Coolant glycol based additive (often called antifreeze) assists to reduce corrosion, minimise freezing, raise boiling point and promote efficient coolant flow. Such corrosion inhibitors have a maximum service life and should be replaced as recommended by the manufacturer. In most high speed engines the fresh water system is pressurised both to raise the boiling point and to promote efficient pumping. Care must be taken when opening the cap of a warm header tank that the release of pressure does cause a gushing release of boiling water and steam.
The top of the cylinder liner and the inlet and exhaust valve area are where combustion takes place. Fresh water with additives called coolant is circulated under pressure through the block and cylinder head to cool this hottest part of the engine.
There are spaces around the cylinder liner and the block that are called jackets. The fresh water circulates through them. The fresh water then flows through the holes in
the top of the block and corresponding holes in the cylinder head gasket and cylinder head. In the cylinder head the water circulates around the inlet and exhaust gas passages to the thermostat.
The fresh water in some engines may be fed through the exhaust manifold and turbo charger. Cooling them minimises the radiant heat given off thereby keeping the engine space cooler and providing better engine power. Fresh water cooling of these parts does not leave the deposits that sea water cooling does.
Fresh water or coolant pump - is engine driven and as it is constantly immersed (needing no priming) is usually of the low maintenance centrifugal type.
Header tank or expansion tank - This allows for expansion of the fresh water as it heats. Modern engines typically also have a vent and additional overflow reservoir. The tank provides an adequate method of venting air and combustion gases from the fresh water system during engine operation. Since combustion gases may enter the fresh water system, it will be necessary to remove these gases before they cause deterioration of the engines cooling water system’s performance. Gases in the system reduce coolant flows and may result in engine failure if adequate venting capacity is not provided.
In many engines the header tank is situated above the fresh water cooler to form one unit.
Venting - Initial venting of the system is critical in order to ensure that the system is completely filled with coolant. Fill the engine slowly to allow the coolant to fill from the bottom up. Quickly filling the expansion tank can fill the vent pipes with coolant and result in slow or incomplete fill of the engine. If vent cocks are fitted, they are to be used to help any air to escape. After initial filling of the system, run the engine, keep checking and if necessary top up the water level until the coolant operating temperature is reached. Any trapped air should make its way up to the header tank in the form of bubbles.
Thermostat - A thermostat is a heat sensitive valve that enables the engine to reach its normal operating temperature quicker to minimise warming up wear. When the engine is cold, the closed thermostat closes the coolant off from the heat exchanger. When the engine reaches its normal operating temperature the thermostat opens and directs the water to the heat exchanger.
Heat exchanger or fresh
water cooler – The heat exchanging process takes place below the header
tank. When the thermostat is open, fresh water will flow through the bath of
the cooler (shell) and be cooled by
the nest of tubes through which the raw water flows, hence the name of a tube nest and shell heat exchanger.
Shell
and tube nest heat exchanger
Shell -The shell has a fresh water
inlet and outlet and contains the tube
nest. Baffles may be fitted to increase the fresh water contact period with
the tubes’ surfaces.
Tube nest - The tube nest consists of a tube plate at either end with tubes
fitted in between. Allowance for expansion is required due to the different
rates of expansion of the metals used in the construction and prevent any undue
stress on the tubes. One tube plate is secured at one end of the shell and
expansion allowance is by using twin O
rings at the other end for of the tube nest. Any gap between them will indicate
a leak from either the raw water or coolant.
End covers - An end cover is fitted to each end of the
shell. Where there is a flange on each end cover, the sea water will flow in at
one end and out the other end. Where there is two flanges on one end cover, the
sea water will enter and leave via this cover. A division plate will be fitted
in this cover and seals against the tube nest across its diameter to separate
the inlet and outlet flows. The other end cover will have no flanges.
Cleaning - The sea water will leave behind more deposits than the fresh water as
scale in the tubes so it will be necessary to periodically clean them. The end
covers can be removed and a wire brush pushed and drawn through each tube, this
being the reason why the sea water flows through the tubes. Dismantling and
cleaning heat exchangers is a scheduled service task.
Keel
cooling
In this system coolant
is circulated through pipes or tanks mounted externally on the hull below the
waterline. Seawater on the outside of the keel cooling pipes cools the fresh
water on the inside. These coolers may be a channel or half a round pipe welded
to the hull or may be a copper tube grid. A sea water pump can eliminated
though attention to convectional flow in design is paramount.
Keel cooling has
the great advantage that there is no external water inlet that may become
clogged and restrict engine cooling, particularly if the vessel is operating in
silty shallow areas. The disadvantages include that marine growth on the pipe
will impair heat transfer requiring slipping to clean the keel cooling pipes.
Unless shielded the external the pipes are subject to impact damage from
flotsam or grounding. A large fresh water tank and fresh water reserves are
required.
Due to the disadvantages of a fully keel cooled system hybrids are commonly found, particularly when auto engines are modified for marine applications.
The hybrid shown above uses a closed system salt water circulation cooling the fresh water by way of a standard heat exchanger. The advantage of this design is that replenishment of the reservoir can be by fresh or if necessary while at sea with readily available salt water. Also damage or leaks in the external pipes will not immediately affect the systems cooling efficiency.
Additional cooling features
Other coolers - Sea water may be piped through an oil cooler before it goes into the fresh water cooler, as described in Chapter 4. A turbo charged engine fitted with an after cooler may also be cooled as described in Chapter 1. If the engine employs a wet exhaust system, a proportion or all of the sea water will then be pumped directly into the exhaust pipe. In a dry exhaust system, all the sea water would be pumped overboard as it leaves the fresh water cooler or after cooler.
Other pumps - On large engines, the fresh and sea water
pumps are driven by electric motors rather than the engine. No thermostat is
fitted, the fresh water circulates through the engine as well as the cooler.
The advantage is that the fresh water can be pre-circulated through the engine
and its temperature gradually raised until at normal operating temperature so when
the engine fires and the sudden rise in temperature is not so great. To control
the temperature of the fresh water, a control valve is fitted for the sea water
to by-pass the cooler until it is required.
5.2 Pumps
Two types of
pumps are employed in cooling water systems:
Flexible impeller type; (raw water)
Centrifugal type pump (fresh water).
Flexible impeller type pump
This rotary positive displacement pump is so widely used for cooling and bilge systems that the type is often called by its trade name, a Jabsco pump. The bronze casing in which the neoprene or rubber impeller revolves is not uniformly circular, having a constriction (or cam) over a third of its diameter between the inlet and outlet. As the impeller blades pass the cam a partial vacuum is created because the space between the impeller blades expands around the inlet (drawing water in) and contracts around the outlet (pushing water out).
The partial vacuum
created is so good that it is a self priming action which makes this type of
pump popular as a bilge pump.
The engine driven shaft is sealed by packing or a mechanical seal. All suction side connections must also be air-tight as leaks will stop or slow flow through the pump.
A cover plate over a gasket gives easy access to the casing and impeller. The impeller is a drive fit onto a splined shaft or one with a keyway. Although it is a self-priming pump the flexible impellor relies on the pumped fluid for lubrication so it will be damaged if the pump runs dry.
The impeller is clearly a delicate component that will easily be stripped of its vanes if a nail or metal fragment is sucked up from the inlet. Other failures of flexible impellers result from chemical attack (from polluted bilge water), water flow cavitations (from narrow piping or over speed pumps) or more traumatically solid materials that evade the inlet gratings and screens and are drawn into the pump. A sudden increase in wet exhaust engine noise is a sign that salt water cooling has dramatically failed, and the impeller is a prime suspect. Pumps that are not used for extended periods can develop misshapen and brittle impellers that need to be replaced and can adhere to the pump cover.
Spare impellers sets should
always be onboard so timely replacement can be carried out by removing pump
cover and gasket beneath and sliding the impeller off the splined drive shaft
to inspect for damage. They can be reluctant to let go and may have to be
carefully prised off with levers. Check for broken blades, impeller end
clearance, worn casing wear plate and leaking seals. The end plate and impeller
must be a good fit to pump and self prime. Old end plates may have become
grooved so will have to be honed flat again. Repairs may include attention to
the gasket or replacing a worn bearing.
To separate the bearings from the shaft use a wood block to support the
unit while tapping out the shaft.
A new impeller can be just as reluctant to squeeze back onto the shaft and into the casing. A smear of soap and the assistance of a rubber mallet may be required.
Before starting the pump ensure that drive belts (if fitted) do not slip. It may be necessary to initially prime the system especially if the pump is fitted high in the vessel. Smaller portable electric pumps are unlikely to pump up to more than one to two metres so outlets may have to be initially positioned by trial and error.
Common faults
Loss of pressure
is usually due to too much clearance between the impeller and the back cover
allowing water to escape from the pumping chambers.
If the pump is
allowed to stand idle for a considerable period, the rubber impeller blades
will stick to the housing and tear when the pump is started.
The water
flowing through the pump helps to lubricate and keep them cool. Running a pump
dry will result in a damaged impeller. In a cooling water system, this would
only happen if there is air in the system.
Metal debris
will strip an impeller in seconds.
Centrifugal type pump
A centrifugal
type pump must be full with the medium it is pumping, whether gas or liquid. It
is not a positive displacement pump and will not self prime. As a water pump the
water enters at the centre of the impeller and is thrown radially out of the
impeller by centrifugal force to the periphery from where it is discharged. The
centrifugal pump can therefore be recognised by the suction or inlet entering
the centre of the pump housing and the outlet or discharge on its circumference.
The centrifugal
pump consists of a shaft attached to an impeller. The impellor will have
internal curved vanes if it is hollow or curved vanes on one or both sides if
it is not hollow.
Centrifugal type
pumps are generally used for large quantities of water at low pressures. The
pressure can be increased by having a number of impellers in series, although
this is not required in a cooling water system. It is preferable that a
centrifugal pump has a positive suction (the level of liquid is above the inlet
of the pump) or is used when the lift is very small. They are not self priming
pumps. They are suitable for closed fresh water cooling systems.
Faults
To be effective
the impeller must have the minimum of clearance between the suction and
discharge openings.
Erosion is a
problem in centrifugal pumps caused by the flow of the fluid, especially if
used for sea water in shallow sandy conditions.
Cavitation can
also be a problem.
5.3 Faults, maintenance and servicing
Faults
Corrosion and electrolysis in cooling water
systems
Galvanic corrosion (often incorrectly referred to as
electrolysis) is the corrosion of the more active (or less noble) member of a
pair of metals in physical contact in a corrosive environment. The more active
metal is the anode, whereas the less active metal (more noble) is the cathode
and does not corrode. Although the current flowing is often very small, it is a
continuous process and the attack is made worse if the anode is small and the
cathode large. Where the more corrodible metal is very much larger in area (e.g.
stainless steel bolts in an aluminium hull) galvanic corrosion may not happen.
Where corrosion on
dissimilar metals occur, the particular metal that will corrode can be determined
from the Galvanic Series, a table in
which the common construction metals are listed with the active (or anodic)
metals at the top and the noble (or cathodic) metals
at the bottom.
Active Metals (anodic)
Magnesium and
magnesium alloys
Zinc and galvanised
steel
Aluminium
Cadmium
Aluminium alloys
Iron, cast iron,
mild steel
Stainless steels
(activated)
Lead, tin, tin-lead
solders
Naval brass, high
tensile brass
Manganese bronze
Nickel and High
nickel alloys (activated)
Copper, Admiralty
brass
Phosphor bronze,
silicon bronze, gun metal
Cupronickel
Monel, Inconel, high nickel alloys (passivated)
Stainless steels (passivated)
Silver
Titanium
Platinum, gold
graphite
Noble Metals (Cathodic)
If two metals
are coupled, the metal nearer the top of the series will be the anode and
suffer increased corrosion, whereas the metal nearer the bottom will be the
inert cathode. Generally, the further apart the two metal are in the Galvanic
Series, the worse will be the galvanic corrosion of the more active metal.
Where two metals are listed close together in the series, there is little
likelihood that damaging galvanic corrosion will occur.
As can be seen
from the above, when two dissimilar metals are placed in sea water, galvanic
action loosely referred to as electrolysis will take place. In an engine
cooling system, a number of different metals are used which are close together
in the Galvanic Series. However to prevent the slight galvanic action
(electrolysis) that will take place, a sacrificial anode is placed in the fresh
water, if an inhibitor is not used, and sea water inlets of the cooler (heat
exchanger). Regular inspections should
take place of these anodes. They should cleaned with a wire brush so they are
more effective. If they are excessively eaten away, they are to be replaced. If
in doubt about the condition of the anode, strike it sharply against a hard
surface. A weakened anode will break.
Sea
water system faults causing rise in temperature
A gradual rise
is where the temperature rises over a period of time. It could be caused by a
gradual build up of scale on the cooling water surfaces or a strainer gradually
becoming clogged.
A sudden rise in
temperature could be caused by the thermostat stuck in the closed position, a
pump impeller revolving on its shaft or the engine overloaded.
Sea water temperature too high - Not normally a problem in southern Australian waters, however must be considered when a vessel is operating in warmer northern waters. The engine speed should be reduced to bring the temperature back to its normal operating temperature.
Sea water intake rose or grid - Could become clogged over a period of time so there would be a gradual
increase in the fresh water cooling temperature.
Reduce the engine speed until the normal operating temperature is obtained.
Alternately, a plastic bag may get sucked onto the grid and a sudden rise in temperature would occur. Gradually slow down the engine to reduce the heat slowly and stop the engine. With no suction holding the plastic bag on the grid and with the vessel moving through the water, the plastic bag will come away from the intake grid. Start the engine and let it idle until temperatures stabilise.
Clogged sea water strainer - Could become clogged over a period of time so there would be a gradual increase in the fresh water cooling temperature. Reduce the engine speed gradually and stop the engine. Clean out the strainer. Start the engine and let it idle until temperature stabilises. Alternately the vessel may have voyaged through matter which quickly clogged the strainer. Effect the same remedy as before.
Faulty impeller in
sea water pump -The rubber
flexible impeller in the sea water pump could be damaged. Damage usually occurs
when the pump is run dry. Indications would be the pump discharge pipe would be
warm and not at the same
temperature as
the sea water. In addition there would be no or a reduced sea water discharge
overboard.
Reduce the
engine speed gradually and stop the engine. Replace the impeller. Should you be
at sea and have no replacement impeller, it may be possible to reach port at
reduced speed if the impeller is only partially damaged and still can pump some
water. Alternately, a sea water hose from the fire pump or the wash deck hose
could be connected up to the system at the discharge side of the sea water pump
to get the vessel back to port.
With a
centrifugal type pump, the pin holding the impeller onto the shaft may have
sheared. The indications would be as above. It should be possible to make up
and fit a new pin.
Faulty seal in sea water pump - Sea water will leak out and will cause no problems provided it does not
spray over anything, especially electrical equipment. The seal can be
replaced when the vessel gets back to port.
Air in sea water cooling system - On a lot of vessels, air is trapped in the sea water cooling system when the vessel re-enters the water after slipping.
With the engine stopped, the air can be bled off by slackening off the backing plate on a jabsco pump or loosening a join in the seawater cooling pipe on the suction side of the pump that is below the water line. If it is a jabsco pump and has run dry until the engine overheated, the rubber impeller will be severely damaged.
Insufficient speed of sea water pump - On some vessels the sea water pump is belt driven from the engine. The
adjustment of the belt may cause it to slip.
Reduce the engine speed gradually and stop the engine. Adjust the belt tension. Start the engine and let it idle until temperature stabilises. It may be that the pump does not attain sufficient speed as the driver or driven pulleys may be the wrong size. Reduce the engine speed gradually until normal operating temperature is attained. When back in port, change the pulleys.
Dirty or fouled tubes in the cooler - The sea water discharged overboard would be restricted. It is unusual
for the cooler to be completely blocked. Reduce engine speed until
normal operating temperature is attained. Stop engine and clean the cooler or
return to port under reduced speed.
Electrolysis in the cooler - It would appear that the zinc anode/s have wasted and require replacing.
Leaking sea water hoses or pipes - Sea water will leak out and will cause no problems provided it does not spray over anything, especially electrical equipment.
The hose can be replaced, or a piece of rubber fastened with a hose clip, can be fitted to the leaking pipe.
Keel cooling pipes not effective due to marine growth - This causes a gradual increase in the fresh water temperature. Reduce the engine speed gradually until the normal operating temperature is obtained. The vessel will have to be slipped to clean the keel cooling pipes.
Keel cooling pipes leaking - Corrosion or electrolysis on the keel cooling pipes could cause a leak to develop. The fresh water would flow into the sea water as the fresh water pressure is greater than the head of sea water, even when the engine is stopped, that is until the fresh water level drops to the same level as the water line of the vessel. If the leak is not too bad, the engine can be run provided the fresh water tank is kept topped up. The vessel will have to be slipped to repair the leak.
Fresh
water system faults causing rise in temperature
Fresh water cooling level is too low - A leak has developed in the fresh water system causing a loss of water in the header tank. It could be a leak in the piping, seal in the pump or a blown cylinder head gasket. Reduce the engine speed gradually and if the fresh water system is the unpressurised type, very slowly top up the header tank to its correct level. If the fresh water system is of the pressurised type, reduce the engine speed gradually and stop the engine. Let the engine cool down further before placing a rag over the header tank cap. Turn the cap anti-clockwise until it reaches the position where the pressure is released. When the pressure is released, remove the cap. Start the engine and very slowly top up the header tank to its correct level.
If there is very little water in the header tank, it is advisable to let the engine cool right down before adding fresh water. When the engine is hot and the fresh water level in the header tank is low, cold water should be introduced very slowly whilst the engine is running. The cold water will then be heated sufficiently before it circulates around the combustion space. Cold water suddenly coming into contact with the hot cylinder liner and cylinder head may crack them.
If possible, the leak in the piping should be stopped or the pump seal replaced.
In an emergency or in distress the engine could be run with a blown head gasket between the cylinder and a cooling water passage to get the vessel back to safety providing the leak is not too severe and the engine is not stopped. If the engine is stopped, water could make its way into the cylinder and hydraulic the engine resulting in severe damage.
Thermostat not opening fully - The thermostat is in the closed position when the engine is cold and first started. In the closed position, the water is circulated through the engine only. As the engine reaches its operating temperature, the thermostat opens and now allows the water circulating through the engine to pass through the fresh water cooler or the keel cooling pipes. Should the thermostat stay in its closed position or not open fully, the engine will overheat. Feeling the pipe from the thermostat housing to the fresh water cooler will indicate whether or not water is flowing through it. Reduce the engine speed gradually and stop the engine. When the engine has cooled down replace the thermostat. Start the engine. Should you be at sea and have no replacement thermostat, the engine can be run without one to get the vessel back to port.
Faulty impeller in fresh water cooling pump
- The rubber flexible impeller in the fresh water pump could be damaged.
Reduce the engine speed gradually and stop the engine. Replace the impellor. Should you be at sea and have no replacement
impellor, it may be possible to reach port at reduced speed if the impellor is
only partially damaged and can still pump some water.
Alternately, the
impeller from the sea water pump could be used if it is the same size and a sea
water hose from the fire pump or the wash deck hose could be connected up to
the system at the discharge side of the sea water pump to get the vessel back
to port.
Faulty seal in fresh water pump - Fresh water will leak out and will cause no problems provided it does
not spray over anything, especially electrical equipment, until the level in
the header tank is low and overheating will start to occur. See Fresh water cooling level is too low above.
Leaking fresh water hoses or pipes - Fresh water will leak out and will cause no problems provided it does
not spray over anything, especially electrical equipment, until the level in
the header tank is low and overheating will start to occur. The hose can
be replaced, or a piece of rubber fastened with a hose clip, can be fitted to
the leaking pipe.
Build up of scale on cylinder water jackets
- Fresh water contains impurities which will come out of solution at high
temperatures and adhere to hot surfaces. The hottest part of the engine is in
the combustion space at the top of the cylinder. Scale will deposit on the cylinder
liner walls in this area, on the passages to the cylinder head and around the
exhaust valve. The scale will stop the transfer of heat from the combustion
process to the fresh water cooling and in the case of passages, will restrict
the flow. This will be a gradual process.
Reduce the engine speed until normal operating temperature is attained. Back in port, the cooling water system will have to be chemically cleaned.
Air in fresh water cooling system – This is not normally a problem when the engine is running. Air can get
into the system when repairs are carried out and the cooling system is
refilled. On starting the engine, bubbles will be sighted in the header tank as
the air makes its way out. As the water replaces the air, the water level in
the header tank will drop. As it drops, it can be topped up slowly.
The cooling system is like any other part of machinery. It requires routine maintenance and preventative maintenance to minimise costly engine overhauling due to overheating. The cooling water system should always be maintained at its correct level in the header tank. Inhibitors are used in the cooling water to provide corrosion protection, pH control, water softening, prevent freezing and increasing the boiling point.
The manufacturer’s instructions should be followed as to what type of inhibitor should be used. It is recommended that one brand of inhibitor be used as mixing different brands may have serious consequences if they are not compatible. Care should be taken to insure the inhibitor is only used at the correct strength, is pre-mixed prior to filling the engine and is used when topping up the system. The inhibitor has a maximum service life and should be replaced as recommended.
Hoses on the cooling water system tend to deteriorate due to age and heat. In marinised auto engines they may not be of a suitable durable marine quality and may be fastened with ferrous (rust prone) hose clamps. A hose found to be soft, is bulging or its outside surface is covered with minute cracking should be replaced immediately. It is recommended that all hoses be replaced regularly in order to avoid possible engine overhaul, water damaging electrics and inconvenience.
Repair leaks as
they occur as they will only get worse.
Keep a regular
check on the cooling water temperature. A rise in temperature could be
attributed to a dirty cooler. The cooler should therefore be cleaned regularly,
especially if the vessel is to proceed to an area of higher sea water
temperatures.
Any anodes are
to be inspected regularly, cleaned, checked for weakness and replaced if
circumstances dictate.
Flexible
impeller replacement is a service scheduled item.
Setting a high temperature alarm
The cooling
water temperature alarm consists of a thermo switch. It has a bi-metal probe
that activates contacts in a micro switch. It is usually situated in the
thermostat housing. The manufacturer will state at what temperature the engine
operates at and what temperature the alarm will sound.
The probe can be
inserted into a container of water with a thermometer making sure they do not
touch the container.
Heat and agitate
the water to ensure uniform water and probe temperature. Place a meter over the
contacts of the micro switch. The contacts should close when the thermometer
reaches the temperature within the required tolerances. If the water is left to
cool, the contacts will open.
Should the
contacts not close within the required tolerances, the micro switch will have
to be adjusted. However, most of them are sealed units and cannot be adjusted.
A replacement would therefore have to be fitted.
Manufacturers
supply a shut down device that would energise a solenoid to shut off the fuel
when the cooling water temperature is at the alarm setting.
Chapter 6: Gearing and tailshafts
Ships with low speed engines typically use direct drive from the crankshaft to the tailshaft with its propeller. Smaller domestic commercial vessels typically operate with high speed engines where direct drive shafts would revolve too fast to drag water across their propellers without efficiency losses due to cavitation (bubbling in the water flow). Consequently a gearbox is used to provide reduced speed of the shaft, reverse propulsion and through a clutch mechanism, disengaged propulsion.
6.1 Gears and clutch mechanisms
Principles
of marine gear boxes
Gearing is used in drive trains (sets of intermeshed gear wheels) to alter direction, position or speed/mechanical advantage of propulsion and auxiliary equipment such as winches, pumps and steering. The gear and shaft driven directly by the motor can be called the input and the final gear and shaft it drives can be called the output. Drive train A shown below illustrates reduction and reversal of the output, drive train B illustrates reduction only with further displacement (to the right) of the output, drive train C illustrates reduction and splitting into two outputs.
The efficient crankshaft speed of a small high speed motor is greater than the efficient speed of a propeller that revolves in relatively dense salt water. The solution to this is to use of a drive train with an input shaft and drive gear wheel enmeshed with a set of follower gear wheels connected to an output shaft (a gear box). The output shaft follower (to the propeller) has more gear teeth than the input shaft driver (from the crankshaft), and therefore it rotates more slowly. The number of teeth in each gear wheel has a direct relationship to not only the speed of the follower but also the transfer of torque. The reduction ratios shown in the drawings above are calculated by the:
number of gear teeth (cogs) in the large
wheel = 16 = 2 or a Reduction Ratio of 2:1
number of gear teeth in the small wheel 8
Therefore if the engine speed is 1000 rpm and the propeller speed is 500 rpm then:
rpm of the engine = 1000 = 2 or a Reduction Ratio of 2:1
the rpm of the propeller 500
While the speed has been halved, the mechanical advantage has been doubled (not allowing for efficiency losses due to friction and heat). Marine gear boxes are designed to reduce the speed of the propeller shaft but some applications require a greater speed from the output shaft. This is simply achieved by using a smaller follower to give higher output speed with less mechanical advantage. It must also be noted that by adding a following gear wheel it will turn in the opposite direction to its driving wheel (in reverse).
Gear
selection and engagement
Gear box arrangements to select reverse gear include mechanically (using a gear lever directly or through a cable or linkage), electro-mechanically (using electrical solenoids which change the gears directly or hydraulically through clutches) or by hydraulic operation alone. The gearbox control and engine throttle (speed) may use separate levers for thrust and speed, or be combined in a single control lever as drawn below.
Marine transmission systems are heavy and generate considerable momentum so changing gear is smoothed by clutches. To avoid clashing of gears and overload on clutches the sequence of changing requires a delay for the propeller and shaft to slow down. Modern systems operate sequentially, or have override safety systems, however good practice remains to follow the manufacturer’s instructions always.
Clutches
In most operations it is convenient to be able to positively engage and disengage an input shaft from an output shaft. The mechanism that smooths this operation is called a clutch.
Basic
clutch mechanisms
Dog clutch - The simplest
arrangement as found on windlasses and winches is the dog clutch. Shown below, a collar is splined to the driving motors
shaft that can be pushed by the clutch lever to engage hard against a lug in
the winch drum’s side. This arrangement only allows engagement when winch and
motor are stopped or both moving slowly at the same speed (a neat trick if at
all possible for the arrangement shown below).
Dog clutches are not used in propulsion engines.
Cone clutch - The cone clutch does allow for engagement while the motor is running and is suitable for slow running operations such as capstans, windlasses and winches.
The warping drum (rope carrier) or gypsy (chain carrier) is machined with a tapered central hole. The drum is fitted over a matching tapered axle and can revolve smoothly when in the disengaged mode. The tapered axle can also be turned by a directly fitted motor. When the drum is tightened down onto the axle it grips (engages) and will turn with the motor driven axle. The system can snatch as it engages and although usually of heavy duty build, as with all friction type clutches there will always be momentary slip before engagement resulting in heat and wear.
More sophisticated systems are held in engagement with a heavy coil spring that can be manually compressed to disengage the drum from the axle. For a better grip the driving axle may be machined to mate inside a wide tapered drum and the mating cone’s surface be coated with a friction lining material. Cone clutches are rarely used in propulsion engines.
Centrifugal clutch - A driving shaft and output coupling drum can revolve independently at their common bearing. The driving shaft has hinged weighted friction pads that at rest are restrained clear of the drum’s surface by springs. When the driving shaft is revolved the friction pads are thrown outwards by centrifugal force sufficient to overcome the springs and bind onto the drums surface (engaging the coupling). Light duty pumps, power tools and the smallest outboards often use a centrifugal clutch for its light weight and economy.
Double de-clutching - Before the modern synchromesh gearbox the earliest motor cars used a crash gearbox. To change gear, a lever is first applied to rip the currently engaged gearwheel away from the output gear train and into neutral (disengaging the engine from the road wheels). While temporarily in neutral the accelerator is revved to boost the engine speed to match the speed of the lower gear wheel (determined by the road wheel speed). This is called double de-clutching. When the driver judges that the speeds are matched the gear lever is applied to enmesh the lower gear by crashing it into the engine gear wheel, with hopefully not too much crunching of the gear teeth. Such foot and hand control is not available in marine gearboxes so some means of smoothing the change of gears is required.
Synchronising
clutch mechanisms
Single plate clutch - a clutch plate disc is held firmly squeezed by heavy duty springs between friction pads on the flywheel and on the pressure plate disc.
A sliding muff coupling can be withdrawn using a lever mechanism to release the spring tension and enable disengagement. Not shown in the drawing for simplicity are the photos of the flexible links between the clutch plate and the output shaft that allow for shock loads and natural oscillation in engine output.
Multiple plate clutch - The single plate clutch mechanism operates externally of the gear box and is typical for auto applications. For weight and space saving, marine applications usually have clutches that operate within the gear box. The large single clutch plate is replaced by several smaller ones enabling equivalent surface area contact in a more compact unit.
Hydraulic oil is pumped in through the input shaft which forces the internal piston disc to overcome the hold off resistance of the return spring and move towards the output assembly. This in turn squeezes together the plates that are alternately splined (embedded) inside the rim of the input shaft assembly and on the outer rim of the output shaft assembly. Adjusting the hydraulic pressure enables accommodation for heavy loading operations as may be required for low gear or reverse gear.
Clutch slip - Clutch slip describes when there is incomplete engagement between the driving and driven side of the clutch. The power developed by the engine is therefore not fully transmitted to the propeller. Wear of friction plates or linings is unavoidable due to momentary slippage when clutch is being engaged. Each engagement results in a slight loss of the friction material. In the course of time, this accumulated wear allows slippage. With regular maintenance, adjustments can be made to the clutch to reduce this problem but eventually wear will be such that the friction material will need to be replaced.
Fluid (hydraulic) coupling – while a hydraulic linkage may be the activating mechanism to enable positive engagement in the previous clutch types, the fluid coupling transfers thrust by smooth fluid action. Consider a household electric fan pointed towards another. The second fan will slowly rotate in the breeze. This induced motion in the driven turbine (second fan) enables a cushioned engagement/disengagement between a driving motor shaft and a driven shaft albeit with some slip (poor thrust transfer) at low speed.
A fluid coupling transmits rotation from one shaft to another by accelerating hydraulic fluid inside its housing containing closely fitted rotors; the input driving shaft pump (impeller) and the output driven shaft turbine (runner). The flywheel, impeller and housing (shell) are fixed and the cavity filled with hydraulic fluid. The impeller spins the fluid from the centre where the velocity is low, to the periphery where the velocity becomes high. A net force of multiplied torque on the turbine causes it to rotate with the direction of the impeller.
Fluid couplings
have a stall speed where the impeller is turning but not fast enough to coax
the turbine into rotation (i.e. when
the car driver stopped at traffic lights selects the gear while his brakes are
still on). In this condition for excessive periods or if
the runner becomes jammed, the engine's power could transfer its energy as
overheated fluid, possibly leading to damage. A fluid coupling only achieves
about 94% transmission efficiency due to fluid friction and turbulence.
Torque converter -
Unlike the two rotor fluid coupling, the torque converter has at least three
rotors- the impeller (motor driven), the turbine, (load
driving)
and positioned between them the stator (that modifies oil flow returning from
the turbine to the impeller).
The fluid flow returning from a fluid coupling’s turbine can oppose the
direction of impeller rotation during slip, causing lost efficiency. The torque
converter uses its stator to redirect the returning fluid to assist the
rotation of the impeller so improving efficiency and output torque.
Though simple stators can be fixed often
they are mounted on a one way clutch
allowing forward motion only. Since the returning fluid is initially travelling
in a direction opposite to impeller rotation, the stator will likewise attempt
to counter-rotate as it forces the fluid to change direction, an effect
prevented by the one-way stator clutch. The typical torque converter in
automatic transmissions has three stages of operation:
Stall-
the motor turns the impeller but the turbine cannot rotate. (When the car
driver stopped at traffic lights selects the gear while the brakes are still
on).
Acceleration-the motor accelerates with the impeller spinning at a greater rate
than the turbine.
Coupling-The turbine has reached
approximately 90% of the speed of the impeller. Torque multiplication has
ceased and the torque converter is behaving like a fluid coupling.
Lock-up clutches such as the Voith TurboSyn coupling overcome the problem of slip and improve fuel
efficiency by
coating the periphery of a multi-section impeller with friction plates.
Centrifugal force throws the impeller onto the turbine at higher revolutions,
creating a positive clutch lock at speed.
6.2 Reverse and reduction gear boxes
The gear system below has an input shaft and forward shaft (often combined), a reverse shaft and an output shaft turning within an oil bath called the sump (not shown). The forward and reversing shafts have pinions (driving gear wheels) engaged by clutches. When disengaged as below the reverse pinion gear wheel rotates freely on its shaft.
Ahead mode operation - The forward clutch pushes the forward pinion to engage on the forward/input shaft. The forward pinion then drives the driven shaft as shown above. The reverse clutch is not active so the reverse pinion rotates freely on the reverse shaft.
Astern mode operation - The reverse clutch pushes the reverse pinion to engage on the reverse shaft. The reverse pinion then drives the driven shaft as shown above. The forward clutch is not active and the forward pinion rotates freely on the forward/input shaft. An input shaft driven oil pump provides oil flow for lubrication and operation of the clutches. Oil pressure is regulated by a pressure relief valve before distribution to the gears, bearings, clutch and control via passages in the gearbox. A control box is mounted on the top of the gearbox and connected by linkage (electronic, hydraulic, pneumatic) to the throttle and forward/reverse control at the wheelhouse and gearbox (for manual emergency operation). An oil cooler and oil filters may be mounted on the gearbox.
Epicyclic or planetary gearing
Epicyclic or planetary
gearing systems provide the advantages weigh saving, balance and compactness
for a gear box suitable for marine applications. Variations of three basic
geared components of a sun gear, planet gear and annulus are utilized.
One or more
outer gears (planet gears)
revolve about a central gear wheel
(sun gear). In more complex systems groups of planet gears can be
mounted on a movable arm (carrier)
which as a body can rotate around the sun gear. An outer ring (annulus) with inward-facing teeth
meshes with the planet gear. In operation one of the three components (star,
planet or annulus) is held stationary, another is used as a power input and the last component is used
as the output. The ratio of
input to output is dependent upon the number of teeth in each gear, and upon
which component is held stationary.
In the
reversing gearbox shown below the output shaft can revolve in ahead mode
(anticlockwise) with the annulus free to rotate within the shaft. If however the annulus is locked on the
output shaft by the outer tightening
collar (brake band) the annulus and shaft are rotated in reverse (clockwise) by
the planet gears.
Epicyclical gear arrangements can be
simple or compound. Compound arrangements enable large reduction ratios, weight
saving and flexibility. They may be meshed-planet
(at least two or more planets in mesh within each planet train), stepped-planet (a shaft connects two planets
within each planet train), or multi-stage
structures (two or more planet sets).
The main thrust bearing is located on the output shaft near the aft end of the gearbox. Its function is to carry the reaction to the thrust of the propeller and transmit it through the gearbox casing/frame to the framework of the vessel. This ensures the gears and engine is not affected and remains in the correct longitudinal alignment.
Gear
box maintenance
Assuming that normal maintenance has been carried out on the gearbox, for daily pre-departure checks it should be sufficient to:
Check the oil level in the gearbox sump.
Run the engine with the gearbox in neutral
position and check for leaks.
Operate the gearbox in the head and astern
mode to ensure satisfactory operation after ensuring that the lines holding the
vessel are secure during testing.
Gearboxes are fairly reliable. Faults tend to be due to lack of normal maintenance such as:
Adjustment of clutches and operating
mechanism
Maintenance of filters
Cleaning of oil coolers
6.3 The shaft
driven transmission system
General description
The traditional transmission system of a
vessel is a system of gears, bearings, and couplings that drive a shaft
(solid metal rod) onto which a
propeller is fitted, called a shaft driven transmission.
Direct
driven propellers - The
thrust block/bearing is a short length of shafting with a single collar with
tilting white metal faced thrust pads either side of the collar. The forward
side pads resist the thrust of the propeller when operating in the ahead
direction and the aft pads when going astern. The shaft is supported on both
sides of the collar by bearings and all are carried in a housing bolted to the
vessel frames. The pads are fitted in carriers that transmit the loads through
the housing to the frames. In some vessels self aligning roller thrust bearings
are used. The rollers are often angled to combine both thrust and bearing
loads. The load is transmitted through the roller carriers through housing to
the engine frames.
Reduction
gear driven propellers - The
reduction gear is interposed between the engine and the propeller to reduce the
speed of the propeller relative to the engine speed. This is to allow the
engine to run at its optimum or design speed. Propellers running at lower
speeds are generally more efficient. The
thrust bearing is in many cases built into the aft end of the reduction gear.
The purpose of the thrust bearing is to
prevent the thrust of the propeller from being transmitted to the engine. A
thrust load on the crankshaft would result in longitudinal misalignment of main
bearings with the crankshaft and, crankpins with pistons and connecting rods
resulting in overheating of the parts and premature wear.
The intermediate shaft is a length (or
lengths) of shaft between the thrust block and tail shaft. They are required
when the engine is located well forward of the stern. The shaft is supported by
a bearing (plummer block).
The propeller
shaft (screwshaft
or tubeshaft)
is the final link between the engine and propeller. It
must pass through the hull of the vessel and the arrangements must be such that
water cannot enter the vessel. This is done by enclosing the shaft in a tube
that passes through the aft bulkhead of the engine room through the peak tank
and the stern plating or frame, the outer diameter of the tube being made
watertight at the aft engine room bulkhead and where it passes through the
stern.
The shaft is supported in the stern tube by
bearings at each end of the tube. The forward end is flanged and connects to
the intermediate shaft or thrust block. At the forward end of the tube where it
passes through the aft engine room bulkhead, a packing gland or mechanical seal
is fitted to prevent water entering via the annulus between the tube and shaft.
The purpose of the propeller shaft is to
carry the propeller. To achieve this the shaft protrudes through the stern
sufficient to fasten the propeller to it. The most common fastening arrangement
is to cut a taper with keyway and key in the shaft end with a threaded extension; a matching
taper and keyway is cut in the bore of the propeller. The propeller is pushed
on to the shaft taper and the nut is made tight; the nut is then prevented from
turning by fitting a suitable locking device.
Couplings and intermediate shaft
Muff
Coupling
A muff coupling is a sleeve joining two in-line shafts. Unlike flange type couplings which butt solidly together, the muff coupling allows for a gap between the joining shafts so that expansion of one shaft (due to heating) can be accommodated. It is therefore used where one of the shafts must expand longitudinally due to heating (such as steam turbine shafts). The coupling shown below is heated and shrunk on. To remove, oil is pumped under pressure into grooves assisting release of one half of the coupling.
Alternative arrangements include keyed half couplings that are a press fit on shafts or half couplings that are forged integral (are part of) with the shaft
Flange
Coupling
A flange is fitted to the end of a shaft and bolted to a similar flange on the end of the adjoining shaft. The flange may be an integral part of the shaft or separate from it. If separate it has to be keyed and secured to the shaft. The mating flanges are butted and bolted together using fitted bolts with accurate alignment to avoid wear on the bolts.
Faulty couplings are rare, the problem usually being misalignment or loose bolts, the later often created by excessive vibration from misalignment. Care should be taken in alignment adjusted on the slipway with older wooden boats as the hull may alter shape when fully supported when back in the water.
Flexible
couplings
Some flange type couplings (known as flexible couplings) have bolts that are secured in one half of the coupling and loose in the other. The loose half bolt has a rubber or synthetic sleeve which allows for flexibility in alignment. This type of coupling is often used as a less than ideal last resort where satisfactory alignment has not been successfully achieved in a marine shafting.
The stern tube contains the propeller shaft, and shaft bearings and seals. The stern tube may be water lubricated, or oil-filled.
Water lubricated - The water lubricated stern tubes (as below) allows the stern tube to fill with sea water, and the shaft runs in water lubricated bushes. Some systems can also use pumped engine cooling (raw) water from the jabsco supply to inject water around the stern gland housing. A gland sealed by compressible rings of rope like gland packing material the inboard end stops too much sea-water leaking into the vessel, but some dripping is acceptable as this helps to lubricate the gland.
Some systems provide a grease nipple on the inner stuffing box. Though the grease will initially stop leaking it should be used with discretion as grit and sand can create a paste that actually wears the shaft and ultimately increases leaking. The gland can be periodically checked and tightened slightly to reduce dripping if necessary. Feel the bearing after tightening for excessive heat due to friction, and loosen if required.
The stern gland can be re-packed as the gland packing material wears away. Dig out the old gland packing carefully to avoid scoring the bearing. New gland packing coils must be cut to the correct length from the continuous lengths supplied by your marine dealer.
Consult your manufacturer’s instructions for the correct size (usually at least three coils of gland packing sized appropriately to the shaft diameter). When cutting the coils make a diagonal cut (scarf) to increase the overlap at the ends. A minimal wipe of grease on the shaft may be acceptable to ease fitting, but don’t overdo it. Tighten the gland and monitor for heat and dripping for a few days until it settles in. Re-adjust as required.
The propeller fits tightly onto a tapered shaft and is positioned with a keyway. This keyway needs to support the propeller over 1.6 x diameter of its hub length along the shaft (i.e. a shaft diameter 50 mm x 1.6 = 80 mm keyway).
Routine management - Check the stern gland for leakage. Though it is common practice to allow a slight leakage from the gland to keep it cool, many modern stern gland packings (due to their low coefficient of friction) will operate satisfactorily without leakage. Listen to the noise generated by the rotation of the propeller. Increase in noise generally means wear in the after stern tube bearing.
Oil-filled
stern tubes - give better lubrication and reduce corrosion. Sea-water
is kept out of the stern tube by a complex outboard seal with springs and o rings that retains the oil bath. An
inboard mechanical
or standard packing gland
seals oil inside the stern tube to lubricate the white
metal bearings. The stern tube oil header tank should be regularly
checked and topped up. A
pump to circulate the oil around the system may be provided and valves for
venting and draining are also provided.
Routine management - Check header tank to ensure there is no loss of oil. Check oil for water dilution. Most oils are designed to emulsify if water enters the stern tube. If the forward gland is the packing type there should be no leakage. Adjust to stop leakage if necessary. Mechanical seals at the forward end are usually trouble free but should be regularly checked. A save all should be provided to catch any oil leakage thus preventing it from entering the bilges.
Mechanical shaft seals or PPS –The shaft is contained in an oil bath within a rubber boot and sealed with o rings running in grooves. These are often called drip-less shaft seals with the obvious advantage of maintaining a dry bilge. They are very common in smaller craft with small diameter shafts.
Outboard engines have a sealed, oil filled foot containing gearbox, bearings, shafts and seals. The oil level in the foot should be regularly checked and periodically changed as shown in the engine maintenance schedule. The differences between oil lubricated and water lubricated stern tubes include:
|
Water
lubricated stern tubes - |
|
|
Advantages |
Disadvantages |
|
A tried and tested sturdy system. Simple to repair wearing parts or replace, but usually requires dry dock for repair. No complex mechanical seals. |
High wear rates at the aft bearing and shaft liner or shaft as it carries the weight of the propeller. Carbon steel shafts have to be protected from corrosion by a sleeve or liner. On smaller ships the shaft is often made from corrosion resistant steel or monel and a liner is not required. |
|
Oil
lubricated stern tubes - |
|
|
Advantages |
Disadvantages |
|
The shaft runs in oil. Friction is reduced and more power is available at the propeller due to white metal bearings. Less wear and longer life from reduced friction. For carbon steel shafts, liners are not required for protection of the shaft from corrosion as the shaft is not exposed to sea water. |
Initial cost is greater as additional piping. An oil header tank is required. After end of the stern tube requires a mechanical seal which, to ensure reliability, is of a fairly complex design. The forward end can be fitted with either a mechanical seal or conventional packing gland. |
6.4 Maintenance
Running
checks should include:
Gear box - oil level; inspect for leaks around casing; ensure pins and bolts in mechanical linkages are secure.
Shaft bearings - check oil level in bearing sump; leakage through end seals; temperature of bearing.
Stern gland - a leaking stern gland is a normal routine and is adjusted as a matter of course. See stern tubes in section above.
Propeller shaft - check noise level to determine whether it is increasing.
Regular maintenance schedules will include:
Gearboxes
- Adjust
clutches and operating mechanism, clean filters and clean oil coolers.
Drive
train – Engine
and shaft bearing moutings, loose shaft coupling bolts (perhaps due to mis-alignment)
may need to be tightened. Mis-alignment of
intermediate bearings will show up by overheating of the bearing.
Survey servicing and repair schedules will
include:
Weardown survey- Allowance is determined by the manufacturer, but rules of thumb range from 3% of diameter as due for replacement to 6% being condemned (3% of a 50mm diameter shaft is 1.5mm permissible movement within the bearing).
Water lubricated tailshaft wear - The aft bearing is accessible via the small gap between the aft end of the stern tube and the front end of the propeller boss. If a rope guard is fitted over this gap, it must be removed. The shaft rests on the bottom half of the bearing. The gap between shaft and stern tube bearing can be measured by using long feeler gauges inserted at the top of the shaft. Many bearings have longitudinal grooves to allow water to circulate. Ensure measurement is taken at the bearing surface not the groove. If the gap above is too small to allow access of the feeler gauges another method of measuring the weardown is to clamp a dial indicator gauge to the hull with the pointer resting on the top of the shaft between the stern tube and propeller. Note the dial reading, then jack up the prop until resistance to jacking increases. Note the new reading. The difference is the weardown (the sum of the wear in the bearing and shaft wear).
Oil lubricated tailshafts wear - These shafts can have a mechanical seals at each end. The standard method of checking wear is by depth gauge. A collared plug in the gland housing or the stern tube just forward of the gland is removed. The depth from the face of the plug boss to the top of the shaft is measured. This should be compared to the original measurement when the shaft was installed. The difference is the weardown.
Checking
intermediate shaft alignment
Misalignment will cause noise and vibration. Gearbox and shaft bearings can overheat and collapse more quickly. The flange couplings of the intermediate shaft of a vessel in service can be checked for parallel, concentricity and alignment using:
Dial indicators and/or
Feeler gauges and straight edges.
Utilising a dial indicator -
1. Inspect the intermediate shaft. If supported on two bearings go to step 2. If supported on a single bearing it is necessary to provide a temporary support to ensure the shaft does not tip. This could be a vee block set up to ensure the shaft maintains the same alignment.
2. Remove the coupling bolts at either end. If muff couplings are fitted, dismantle them and slide the muff/s away from the shaft ends.
3. Using an indicator gauge, clamp the gauge on the engine/gearbox side half coupling. Set the pointer on the top of the intermediate shaft coupling flange and record the reading on the indicator dial. Rotate the engine gearbox shaft and take readings of the dial at 90, 180 and 270 degrees. For perfect alignment, the indicator reading should remain the same at all angles. The procedure for a muff coupling is similar except that the indicator gauge is clamped on the shaft and the pointer set on the intermediate shaft.
4. As a double check transfer the dial indicator to the intermediate shaft. Set the pointer on the engine gearbox shaft and repeat the action in Step 3. The readings should be the same as before.
Using feeler gauges and straight edge -
1. Inspect the shaft. If supported on two bearings go to step 2. If supported on a single bearing, provide a temporary support to ensure the shaft does not cant or tip. (This could be a vee block carefully set up to ensure the shaft maintains the same alignment.)
2. Remove the coupling bolts at either end. If muff couplings are fitted, dismantle them and slide the muff/s away from the shaft ends.
3. Lay the straight edge across the outer diameters of the half couplings at 0, 90,180 and 270 degrees. Any mis-alignment will show as a gap between straight edge and one half coupling. This gap can be measured by feeler gauge.
4. Again as a double check, rotate one of the shafts 90 degrees and repeat step 3. Allowable mis-alignment will depend on the diameter of the shaft. The vessels operating manual or other documents will often provide the information.
Stern gland and outer bearings
Tailshaft (outer) bearings - Water
lubricated bearings wear, the after one more than the forward one. Problems are
usually due to misalignment, but grit in the after bearing can accelerate the
wear. As they are not accessible nothing can be done in water. In extreme cases
this may result in premature slipping to resolve the problem. Wear of bearings
usually shows up as an increasing noise level around the stern.
Measuring stern tube bearing and tail shaft wear
Two bearings usually support the shaft. One is located at the forward end where the shaft penetrates the aft engine room bulkhead and the other is at the after end of the stern tube. The forward bearing is not accessible unless the shaft is removed. As it only carries part of the weight of the shaft, wear is not usually a concern. The aft bearing has to take part weight of the shaft and the whole weight of the propeller so considerable wear takes place in this bearing. Weardown is a measure of the sag (deflection) of the shaft due to bearing and shaft wear.
Shafts should be withdrawn for inspection every 5 years. Increased vibration at the vessel’s stern with persistent stern gland leaks (despite adjustments) are symptoms of excessive weardown. If exceeded, the consequences include fracture of the shaft with propeller loss.
Setting
up for propeller shaft alignment
The shaft coupling must accurately line up with the gearbox coupling. The engine & gearbox unit position can be adjusted by loosening its mounting bolts and adding or removing shims (metal spacers) to lift/lower the gearbox & coupling for correct tilt & elevation (alignment). Alternately flexible engine mountings have an adjustment facility - these mountings have a service life before softening demands replacement.
If the shaft is drawn (removed) while the vessel is on the slipway, alignment can be positively adjusted by stringing a taut piano wire through the dead centre outer bearing, the inner bearing and carried on to the gearbox flange. If the wire does not meet the dead centre of the gearbox flange then either the engine & gearbox unit must be adjusted with shims, or the outer bearing bracket moved (the latter being more problematic.)
Chapter 7: Propellers and propulsion systems
7.1 Propellers
Propeller
theory
|
Pitch (geometric pitch) |
The action of a propeller is similar to a carpenter’s auger rotating to drill a hole through a piece of timber. The drill digs a hole equal to the angle of the shavings clearance spiral (the pitch) of the auger with each complete rotation. Unlike the auger, a propeller is not a continuous spiral but sections from it (blades) are placed around a central hub. As it rotates in a fluid medium the hole the propeller digs in the fluid (the actual advance of the propeller) is less than the geometric pitch. The difference is termed the slip. |
|
Pitch
angle |
Ø is the pitch divided by the circumference of the prop at a given diameter. (i.e. Tan Ø = pitch/p x diameter of the propeller). |
|
Diameter |
Means the maximum diameter. With an even number of blades, it is the distance from the tip of one blade to the centre of the blade. Directly opposite for odd numbers, it is twice the distance from the centre of the propeller to the tip of any blade. |
|
Cavitation |
The results from pressure variations on the back (forward face) of a blade forming vapour filled cavities (bubbles). This leads to a loss of thrust/torque and increase in revolutions until the bubbles collapse. The collapsing bubbles produces intense local pressure on the blade that results in erosion and surface pitting. |
|
Excitation |
Where the natural frequency of the propeller blades is the same as frequencies derived from other sources. Examples include: The variation in torque of the engine; Pressure variation as a blade passes the
stern; The flow of the water into the propeller; The natural hull frequency. The synchronised result can produce resonance in the blades and unacceptable vibration of the propeller or shaft. Sometimes the blades generate disturbing noises called a singing propeller. |
Propeller
calculations
Description - Viewed from the stern of a vessel the shaft may be configured to turn in clockwise direction (right handed) or in an anticlockwise direction (left handed). Due to the differential water density and aeration experienced by a propeller’s uppermost submerged blades and the lower submerged blades, a right handed prop will create a paddle wheel effect. This pushes the vessels head to port when ahead propulsion is first engaged and pushes the vessels stern to port when astern propulsion is first engaged. The inertia of a vessel accelerating from a stall (being stopped in the water) accentuates this effect.
With twin propped vessels the starboard prop is usually configured to be right handed and the port prop to be left handed. Some props may be set up as twins on the same shaft, counter rotating to negate this paddle wheel or transverse thrust effect. The prop may have two, three or multiple blades. Consequently a propeller it is described by its number of blades, the direction it turns, its diameter and its pitch.
A four
bladed, right handed, 75 cm diameter x 150 cm pitch propeller.
Gear and propeller ratio –The gearbox reduces the drive speed from the engine to the shaft and propeller to reduce the likelihood of excitation and slip. For an engine that turns at 2400 rpm with a propeller designed to turn at 800 rpm, the following can be found:
Gearbox ratio =
Engine speed = 2400 = 3:1 Ratio
Propeller speed 800
This relationship is also described by the formula:
Gearbox ratio = Engine speed =
Engine ratio
Propeller speed Propeller ratio or
Engine speed =
Propeller speed x Engine ratio
Propeller ratio or
Propeller speed =
Engine speed x Propeller ratio
Engine ratio
Using a 75 cm diameter x 150 cm pitch propeller at theoretic speed (discounting slip) for every revolution it could move the boat 1.5 mtrs forward through the water. The vessel’s speed would be:
Propeller pitch x
revolutions per minute = distance travelled per minute or
Theoretical speed in knots = Propeller pitch x
propeller revolutions x 60 minutes
1852 mtrs per nautical mile
Theoretical speed in knots = 1.5 x 800
rpm x
60 = 38.87 nautical
miles/hour
1852
With a 50 cm diameter x 75 cm pitch propeller the theoretical speed for every revolution could move the boat 0.75 mtrs forward. Assuming 350 rpm prop speed:
Theoretical speed in knots = 0.75 x 350
rpm x
60 = 8.5 nautical miles/hour
1852
The process of the vessel through the water (the apparent speed) is often measured by a log (an impeller device recording water flow past the hull). The log suffers inaccuracies due to calibration error, water flow anomalies and the envelope of water that is dragged along with the vessel’s hull (the wake). The log will read higher or lower than the theoretical speed. The difference between these two distances is called the apparent slip. For example our 8.5 knots theoretical speed read 7.7 miles on the log.
Theoretical speed - Apparent speed (Log speed) = Loss/gain in speed (apparent slip)
8.5 - 7.7 = 0.8 knots Positive apparent slip
(Where log speed is less than theoretical speed it is called positive apparent slip.)
The True speed of the vessel between two points can be found from traditional position fixing or electronically by GPS or radar. The difference between the True speed and the Theoretical speed is the True Slip. The relationship of the terms is shown below:
Fixed pitch
propellers
The propellers may have two, three or multiple blades of varying sizes and pitch. The aim of the ship designer is to match the engine, gearbox, shaft and propeller to enable the most efficient propulsion system for the intended trade and sea area.
All displacement vessels will have a predetermined maximum speed at the point when the movement the hull through the water creates a displacement wave matching the vessel’s underwater length. To go faster the vessel would have to climb over its own displacement wave requiring expediential and inefficient use of power. These vessels usually have shafting parallel or nearly parallel to their water line, allowing horizontal thrust from the propeller.
Planing vessels have sufficient power with flat bottoms that climb over and escape their wave of displacement to skim over the water surface. But the shaft often has to dip steeply down to reach the water, and the consequent clearance between outer bearing and the hull bottom restricts the size of propeller that can be fitted. The thrust from extreme variations of this set up can have a significant vertical (downward) component.
The selection of propellers is complex and can be aided by software calculators. General principles are that slip is reduced by a coarser pitch. High revving props and ones required for high torque operations such as towing will usually have a coarser pitch.
Controllable
pitch propellers
Controllable pitch propellers have blades that rotate on their axis around the hub.
This provides a method of adjusting the pitch of the blades from the normal full ahead position to zero pitch then to full astern. With these props the transverse thrust is always in the same direction, as the shaft always turns in the same direction independent of ahead of astern propulsion. Controllable pitch propellers including folding props also allow refining the pitch for versatility in multitasking operations such as towing, trawling, sailing or making good speed back home with the catch.
There are many types of adjusting mechanisms. Some rely on oil pressure provided by an oil distribution/control box mounted around a section of the after most intermediate shaft, or on the end of the aft end of the reduction gear. Oil pressure is generated by a shaft or electric motor driven pump. A control mechanism is mounted on the distribution box. A piston rod passing down the centre of the shaft actuates a servo motor in the propeller hub which, by means of sliders/cranks, turns the blades.
Fitting and removing propellers
Fitting a Propeller - A machined taper of usually 1 in 12 on the propeller end of the shaft and a matching taper in the bore of the propeller positively position the propeller on the shaft. A keyway over a distance of 80% and key is provided in the shaft with a matching keyway in the propeller. The small end of the shaft is extended and threaded to take a nut. The propeller is hardened up on the shaft taper by tightening the nut. A locking device is fitted to prevent the nut from slackening that can be a Setscrew or Allen screw that penetrates both the nut and the propeller boss. Often a castellated nut and split pin or cotter pin is used.
Think
safety, as propellers can be very
heavy and may have sharp edges. Always beware that as the propeller becomes
free, it can quickly slide along the shaft. You should take precautions to
ensure that the moving propeller does not fall off the shaft causing personal
injury or damage.
To remove a large propeller first fit lifting arrangements to support the prop. To remove a small propeller remove rope guard and fairing piece from propeller nut, then remove locking device and nut. The prop may be stuck fast on the shaft. That means that when it does come off it may do so suddenly, so refit the propeller nut and tighten for a loose fit sufficient to stop the prop jumping off the shaft when it finally becomes free.
Fit
the drawing gear (two or more long studs that screw into threaded holes) in the
back of the propeller boss and a strongback. Place the strongback
over the studs so that it bears on the end of the shaft and run nuts down on
studs. Tighten the nuts to increase load on studs. At some point, the load will
hopefully overcome the friction between propeller and shaft taper and the
propeller will dislodge. If the propeller is holding, a sharp hit with a large
hammer on the strongback will often produce results. In
extreme cases heat may have to be applied to break the seal of corrosion.
7.2 Methods of propulsion reversal
Reversal of the thrust of a propeller can be
achieved by various methods such as:
Engine reversal of rotation
Reversing rotation of the drive shafting by
means of gears and clutches.
Controllable pitch propellers which can
change the blade pitch.
Steerable thrusters that rotate the
propeller through 3600.
Voith Schneider (VS) props using rotating vertical blades.
Engine
Reversal of Rotation - This
is not common in smaller vessels. To provide engine reversal, cams for correct
timing, opening and closing of inlet and exhaust valves, fuel injection, and
air start have to be provided in addition to the cams for ahead operation.
For a four stroke engine, this is usually
done by providing ahead and astern cams side by side on the camshaft/s and the
camshaft is designed to slide longitudinally
In the normal position the ahead cams are in
line with the cam followers. When astern rotation is
required the camshaft is moved longitudinally and the astern cams are moved
into line with the cam followers.
In many engine designs the air start
distributor is separate from the camshaft but a method of cam changeover from
ahead to astern operation similar to moving the camshaft is adopted.
The operating gear for changing direction is
mostly mechanically operated with pneumatic assistance. Engine manufacturers
each have their own particular method of achieving this. Whichever method is
used, interlocks are required to ensure that fuel is injected only after the
engine is turning on air in the required direction.
Reverse
Reduction Gear - On
most smaller engines, it is more convenient (and cheaper) to use a reverse
reduction gear box rather than fit a reduction gear and reversing gear for the engine.
Various systems of gears such as, spur,
bevel or epicyclic are used in combination with
clutches (in the case of epicyclic gears-brake bands)
to effect speed reduction and reversal.
Manufacturers use various arrangements of
gears to achieve reversal.
Controllable
Pitch Propellers - These have separate blades that are designed
to rotate on their axis around the propeller hub. This allows the pitch of the
blades to be adjusted from the normal full ahead position to zero pitch, then
to full astern.
There are many types of adjusting mechanisms
but all rely on oil pressure provided by an oil distribution/control box
surrounding a section of the aftmost intermediate
shaft or, on the aft end of the reduction gear. Oil pressure is generated by a
pump driven by electric motor.
A piston rod passing down the centre of the
shaft and propeller shaft actuates a servo motor in the propeller hub which, by
means of sliders/cranks, turns each blade the same amount.
A control mechanism is mounted on the
distribution box and operated from the wheelhouse by hydraulic,
electrically/electronic, or mechanical linkage. It can also be operated in an
emergency at the distribution box.
7.3 Other propulsion systems
The
saildrive
A compromise between shaft and outboard drive is the sail drive or leg.
This system uses an outboard style leg with gearbox attached to a conventional motor, either petrol or diesel. The advantage obtained being a smaller engine space is required and greater immersion and clearance of the propeller is possible without the need of a deep rounded hull. Alignment is integral. The motor to leg connection can be bottom hung (as on fin sailing hulls) or horizontally attached at the transom. The latter requires a sealing rubber boot that is prone to damage from impact and the effects of barnacle growth.
Thrusters
Bow
and stern thrusters
Strategically positioned tunnels in the vessels side give provide a placement for
propellers to thrust a vessel for fine manoeuvring.
Typically positioned at bow and stern, the
bow thruster is most effective when constructed as a continuous tunnel from
port to starboard, allowing uninterrupted directional sucking and blowing of
linked propellers through the vessel. In
smaller vessels they can be electrically driven but more usually hydraulic
drives, or even dedicated diesel motors may be employed.
Steerable
Thrusters
Variations of steerable thrusters systems
are called azimuth thrusters, Z
thrusters, rotatable kort nozzles, rudder propellers, duckpellers or dynamic position systems.
The propeller is driven by a vertical shaft
through bevel gears and a swivelling arrangement allows the propeller to rotate
through 3600 thus providing thrust in any direction and dispensing
with the need for a rudder.
The lower bevel gears and propeller shaft
are enclosed in a pod and the vertical shaft in a trunk. In the upper part
(inside the vessel) the trunk is surrounded by an outer casing and swivelling
of the trunk (and propeller) is effected by a worm and wheel gearing, the wheel
being fixed to the top of the trunk. The pod, trunk and outer casing is filled
with oil. The source of power is fitted by bevel gears at the top the vertical
shaft and can be coupled directly to a diesel engine, electric or hydraulic
motor. The many off shore supply, dive support, dredging and drilling vessels that
require accurate geographical positioning use multiple azimuth thrusters linked
to GPS feedback and are now commonplace for such operations.
Voith schneider (VS) propulsion
The VS propeller is a vertical axis
propeller that consists of a number of blades. The blades are located
vertically on a horizontal rotating disc. The disc is driven by a motor with a
vertical shaft, or the drive can be changed to the horizontal by means of bevel
gears in which case a diesel engine provides the power.
Each blade can be turned around its longitudinal axis by a special mechanism that feathers the blade causing a thrust in any required direction. The mechanism is complex and fairly noisy. With conventional rudders the turning force drops away and the maximum angle of helm is generally limited to around 350. At greater angles, the rudder can stall and an increase in angle gives no increase in manoeuvrability. Thrusters and VS propellers provide exceptional manoeuvrability as the thrust remains virtually constant at any angle. They are used on low speed craft, tugs ferries and any operation where exceptional manoeuvrability is required.
Water jet propulsion
The water jet propulsion system was developed in New Zealand by Bill Hamilton to provide a propulsion system that would propel a vessel on fast flowing and shallow rivers.
They are now used throughout the world in a range of vessels from pleasure craft to large sea-going ferries where high propulsive efficiencies are required. Vessels in excess of eighty metres in length are now capable of speeds up to 40 knots using large water jets powered by either gas turbine or large diesel engines. There are several companies now producing water jets with the two most popular being Hamilton Jet and KaMeWa.
The principle of a water jet is to draw water from beneath the hull and through an impeller which then forces the water at great speed through the rear of the jet.
Newton’s third law states every action has an equal and opposite reaction. The jet propelled vessel has no propeller- it is reaction propelled. This principle is similar to a jet aircraft except that the medium used is water and not air. A good example is the thrust felt when holding a water hose of high pressure. This is the reactionary force harnessed to propel the jet aircraft and also a vessel propelled by a water jet.
The thrust from a jet depends on the weight and velocity of the medium expelled rearwards. In the case of a water jet the medium is water, a compact heavy liquid which is also used to eliminate noise and heat from the jet’s operation. The jetstream does not push against anything to generate thrust with the water merely thrown back clear of the vessel. Once free of the jet unit nozzle, the water has no further effect on the craft.
As there is nothing under the vessel in the form of propellers or rudders, water jets are widely used for marine rescue vessels. This is because the risk of injury to persons in the water is greatly reduced.
Water jets are designed with impellers matched to the performance of the engines used. This is so that the engine operates in its most economical range as specified by the manufacturer. By changing an impeller, it is possible to either limit engine rpm or allow the engine to operate above its maximum specified rpm. In most cases, it is advisable to allow the engine to only operate up to the manufacturer’s specification.
|
Part |
Function |
|
Intake Part Housing |
Directs the water from beneath the hull into the impeller. |
|
Mainshaft |
Connected to and rotated by the engine and onto which the impeller is fitted. |
|
Impeller |
Is keyed onto the mainshaft and forces the water towards the rear of the jet. |
|
Stator |
Straightens the water after it has passed through the spinning impeller. |
|
Nozzle |
Directs the straightened water out of the rear of the jet after it leaves the stator. |
|
Steering Nozzle |
Redirects the water causing the vessel to steer. |
|
Reverse Bucket |
When lowered over the nozzle, causes the water to deflect towards the bow and the vessel reverses. |
While a water jet consists of many parts the seven of particular importance are listed above and shown in the diagram below.
Steering and reverse
Together with providing thrust, the jetstream is also used to give the vessel direction. As the steering wheel is turned, the jet unit moves the jet stream to port or starboard.
The more power exerted through the jetstream, the greater the steering effect and the sharper the turn. If the engine is not operating, there will be no jetstream out of the water jet, and steering will not be available.
All water jets use a reverse bucket which is lowered into the jetstream, redirecting the stream towards the bow of the vessel causing it to reverse. On smaller jets this may be done manually with a cable. On larger jets this may be done hydraulically.
Never stop the engine or disengage the drive to the water jet when approaching a mooring or at any time when steering will be required. There is great stress on the bucket mechanism and in old jets the expectation of a rapid change from ahead to astern may result in disappointment.
Approximately half the amount of thrust is available in reverse than in forward. This is due to the jetstream not heading directly towards the bow, but at a slight downwards angle. Even with the reverse bucket lowered, full steering is available and the vessel can also be held completely still in the water. This is called the zero point and is available at any engine speed by simply changing the bucket position.
It is possible for the water jet to be directly connected to the engine flywheel by the jet input coupling.
A marine reversing gearbox can be used to either obtain a more suitable rotation speed entering the jet, or to backflush the jet should it become clogged with debris, such as seaweed. If the water jet will operate in water with a lot of floating weed or debris, then it is advisable to consider fitting a marine gearbox, even if the ratio is 1:1. However, debris is not usually a serious problem to water jets providing the vessel is travelling at any speed.
A marine gearbox will add weight to the vessel and, in most cases, will absorb approximately five percent of the engine power. This must be taken into account when selecting the correct size water jet for the vessel and its use.
Other methods for clearing debris
Should the intake screen become blocked:
Lower the engine speed to slow the vessel -the debris should fall away from the intake screen.
Select reverse for a few seconds if a reversing gearbox is fitted, - this will backflush the water jet and usually remove the debris.
Water jets may also be fitted with manually operated screen rakes. When operated, debris (such as weed) will be pushed away from the screen.
Disconnecting the marine jet from the engine
If the engine is to be operated with the vessel either out of the water or independent of the water jet for maintenance reasons, then the jet must be disconnected from the engine.
This will require either:
The removal of the drive shaft (for a
directly driven water jet) or
The selection of neutral if a reversing
gearbox is fitted.
If the neutral position is selected and there is still a slight turning of the water jet shaft, disconnect the coupling at the gearbox output.
Any turning of the shaft will cause thrust, even if only very small.
Perhaps the most important factor in the performance of any jet boat is the power to weight ratio. It is vital that the all up weight of the vessel is known. This weight must include, boat, weight, passengers, fuel, fresh water, provisions, safety gear plus any other item normally carried. You also need to know the vessel’s required speed.
From this information, it is then possible to select the correct size water jet and engine power required for the application.
If the vessel’s weight is incorrectly calculated, the power to weight ratio will be affected, resulting in the vessel performing below expectation. All too often, the water jet is blamed for the lack of performance.
Vessel size and shape will also need to be taken into consideration when determining the correct size water jet.
Starting and
immobilisation
Water jet units are not themselves a source of power. They need to be coupled with an engine which transfers power to the water jet, which in turn converts that power to propulsion.
The water jet will commence operating as soon as the engine is started and will cease operating immediately as the engine stops.
Apart from the standard checks you will be required to undertake on the engine, you should follow these procedures before using the water jet.
Clear any debris which may be attached to the intake housing by selecting reverse gear and backflushing the water jet or using the manually operated screen make (if fitted).
As the water jet will also provide the vessel with steering, ensure that the steering nozzle has unrestricted movement and is responsive to steering wheel movement.
Check operation of the reverse bucket. The water jet will stop when power from the engine is cut. If a gearbox is fitted, the propulsion from the water jet will cease if the gearbox is placed in neutral.
Chapter 8: Operating an outboard motor
8.1 Outboard systems and performance
General
description
Outboard motors rival inboard propulsion as the most commonly fitted for small craft. Their great advantages are that you don’t need to drill a prop shaft hole in an otherwise perfectly sound hull, and that an engine, gearbox and propeller unit occupies space outside of the vessel proper where they can easily be accessed or removed for maintenance. Being a directional thrust device they also replace a steering system and improve reverse steering performance. Their disadvantages are that they add higher weight at the extreme aft in the vessel placing extreme point loads on the transom attachment and their noise emission is at ear height. They are exposed to collision damage and lack the large deep slow revving props that give the thrust needed by tugs or trawlers. They do not conveniently provide for auxiliary drives for bilge/fire pumps, generators or refrigeration.
In mitigating these disadvantages stunning advances include remote vertical trimming, quiet four strokes, diesels and power take offs. Marine outboard engines range from small electric driven trolling units to 350 horsepower (hp) V8 units - 500 hp units have been produced. Note – Outboards are commonly rated in horse power. Hp x 0.74 = kilo Watts.
Outboard engines are best fitted on smaller commercial vessels such as water taxis, water authorities, water police or small charter vessels (fishing or scuba diving) that have planning or semi-planing hulls with flat buoyant after sections. On larger sporting vessels, commercial and government vessels twin and sometimes treble outboard engines of the same power rating are used on the one vessel for increased performance and security against one engine’s breakdown. They are also used as auxiliary engines on some vessels for their convenience rather than for performance, particularly sailing vessels.
Outboards are built for the rigours of marine use but require maintenance and care to remain in good working order. Shown below with a general description are the major components of small (8hp) and medium sized outboards (150hp), including:
Powerhead – This is the engine and power source of an outboard. Energy is transferred from the powerhead via a driveshaft, gearing and propeller shaft to the propeller.
Midsection/leg – Houses the driveshaft and exhaust channels/ports.
Engine mounting – Small outboard motors use clamp mounting brackets to attach the motor to the transom of the vessel. Large motors are bolted to modified transom well deck or transom extension pods. The motor is fitted to the bracket with a bushed rotatable bearing enabling directional propeller thrust and steering.
A trim and tilt adjustment with locking mechanism facilitates vertical angling so the motor can be lifted clear of the water or positioned to provide the desired vertical thrust. The tilt lock ensures that the motor does not lift when in reverse. Hydraulic rams may be fitted to adjust motor trimming during passage.
Hydrofoil – Also called an anti-ventilation plate, it moderates the water flow around the propeller limiting cavitation (ventilation). Cavitation is the aeration of water around an over fast revolving propeller caused by pressure differential around the blade tips, and further exacerbated by disturbed surface water being dragged down into the stream.
Gearbox – Houses the bevel gearing which transfers engine power to revolutions of the propeller shaft to give thrust.
Water intakes – Sea water is sucked in by a rubber impeller pump at this point and flows through the engine for cooling purposes. A tell-tale (water outlet) is positioned in easy view at the top of the midsection to indicate to the driver that the cooling system is operating.
Propeller – As the propeller blades revolve through the water the engine/vessel are thrust in the opposite direction to the water.
Remote motor control – On smaller engines throttle control and the emergency stop switch are located on a tiller arm attached to the engine. This arm is also moved left or right to alter the direction of the vessel. In order that the motor is disabled if the driver is thrown from the vessel he/she wears wrist lanyard that operates an emergency stop switch (kill switch).
Larger engines have remote controls located in the wheel house. These controls incorporate the ignition switch, choke, control handle (forward, reverse, neutral) and remote trim control. In larger configurations radio control kill switches are now fitted that allow the operator movement around the vessel.
Powerheads
The types of powerheads include:
Trolling electrical motors;
Carburetted petrol driven engines;
Direct fuel injected or electronically fuel
injected petrol engines;
Diesel driven engines.
Trolling electrical motors – These motors rely on externally recharged batteries and consequently have power and endurance limitations, being used primarily for rod and line recreational fishing. Lithium battery technology is improving performance particularly in power for weight compared to traditional lead acid batteries. Solar charging is a probable future development. For more on electrical motors see the section 8.2 on starter motors.
Carburettor petrol engines – Small and older two stroke powerheads are almost exclusively fitted with carburettors to supply the fuel/air mix. Fuel and air is mixed in the carburettors and channelled into the cylinder where combustion occurs. A description of how carburettors work is found in Chapter 3.1. Usually a water separator bowl is fitted allowing build up of condensation in the fuel line to be observed and drained off. A combined or separate fuel filter to entrap debris will also be fitted. An air filter ensures clean dry air and is easily removable allowing access for fine adjustment to the fast and idle speed carburettor jets.
These smaller engines typically have a vertically mounted block housing the cylinders and pistons, with a vertically mounted cylinder head that holds a spark plug for each cylinder. Shown below is a straight 2 configuration – straight 2 - 4 and v 6 are not uncommon.
Outboards on vessels without dedicated battery electrical systems will use a pull starter cord around the flywheel and a magneto to generate the spark. Some have a limited power take off lead for navigation lights.
Vessels with a
dedicated electrical system will often use coil, breaker points and distributor
ignition. A further description of petrol ignition systems is found in Chapter 8.2.
Most simple two stroke outboards will use premix fuel/oil lubrication with a dry sump. However, newer models may employ separate oil injection, having an oil reservoir and/or oil tank under the powerhead cover that must be kept topped up.
Direct fuel injected or electronically fuel injected petrol engines – Due to their greater efficiency at fuel burn and emission compliance, larger and newer four stroke powerhead engines are almost exclusively fitted with DFI or EFI that meter the fuel directly into the air inlet manifold. A water separator bowl is fitted allowing build up of condensation in the fuel line to be observed and drained. A fuel filter to entrap debris is also be fitted. An air filter ensures clean dry air and is easily removable for cleaning. A description of how DFI and EFI work is found in Chapter 3.1.
These larger engines typically have a vertically mounted block housing the cylinders and pistons, with a vertically mounted cylinder head that holds a spark plug for each cylinder. Straight and v cylinder configurations are common - shown below is a v 6 EFI configuration. DFI only outboards on vessels with a dedicated electrical system will use coil, breaker points and distributor ignition. A further description of petrol ignition systems is found in Chapter 8.2.
Four stroke outboards will use a wet sump lubrication system with a dip stick, filler point and oil filter as shown above.
Diesel driven engines – lightweight diesel engines are increasingly being fitted to powerheads of even relatively small horse power, thus removing reliance on electrical ignition. More about diesel engines is found in Chapters 3 and 9. The configuration of the outboard remains the same as for petrol driven types.
Leg,
gearbox and propeller
shaft
Outboards are constructed
with increments from standard short (15 inch) to ultra long leg lengths (30
inch) to match the distance required from the outboard’s mounting position to
the propeller’s ideal immersion depth – generally taken to be with the
anti-cavitation plate being just below the transom bottom. UUL lengths of 35
inches are rarely available.
Consequently the drive shaft from the powerhead must be a long extension
from the crankshaft to a 90º bevel geared connection with the propeller shaft in
the bottom of the leg. The gearbox is a sealed oil bath. The propeller fits
onto the propeller shaft and is bolted into place with a knurled nut and
retaining split pin. In order to prevent extreme propeller damage in the case of
grounding a skeg is fitted and an over torque
propeller to prop shaft release mechanism is included, simplistic versions
including a spring loaded clutch or a weak link propeller retaining shear pin.
A replaceable streamlined sacrificial anode is bolted onto the underside of the anti-cavitation plate. Here also an adjustable steering trim tab fin enables the worst of the helm bias created by transverse thrust to be minimised.
Above the gear box is the water intake where cooling sea water is sucked and, by way of a drive shaft driven impeller, is pump water around the water jackets of the engine block and head. It is partially ejected below the powerhead as a tell-tale to visually confirm that the cooling system is operating, and the remainder is ejected with the exhaust, just above the anti-cavitation plate.
Propeller
action - steering and trim
A standard outboard engine uses a right hand revolving propeller to thrust water to give forward propulsion (with the prop geared to turn clockwise) and reverse propulsion (with the prop geared to turn anti-clockwise). The entire motor is mounted on a bracket that is vertically hinged (the trim adjustment and lock) and horizontally hinged (the steering and tiller). When in forward propulsion, the prop is thrust towards and pushes against the vessel’s transom. In reverse propulsion the motor would suck itself away from the transom and out of the water if not fitted with a trim lock that keeps it positioned in the water.
The outboard is turned on its horizontal hinge by a tiller arm that can be turned by hand, by a rotating steering wheel drum operating a push pull cable or steering wheel hydraulic pump actuating hydraulic rams. This horizontal thrust and its directional capability gives outboards impressive steering while the prop is turning but virtually none when it is not. Skegs are fitted on the leg to enable some steering when in neutral and give protection for the propeller against grounding.
As a propeller turns its upper blades work in aerated less dense water than the deeper blades. The consequence is greater traction from the lower blades and a turning force on the vessel called transverse thrust or the paddle wheel effect.
Transverse thrust causes a right hand propped vessel in ahead to experience a force deflecting its bow to the left and its longitudinal trim heel to port.
The small fast props of outboards exhibit minimal transverse thrust. Steering deflection is largely unnoticed as it is counteracted by the steering trim tab and/or slight bias by the operator in helm angle. However, a resultant heel force to port with powerful right hand outboards at speed can be high, sufficient to noticeably create imbalance. Solutions to this problem include fitting twin outboards with left and right hand rotating props that negate each other’s transverse thrust effect. Single outboards configurations may be fitted with a contra rotating propeller (twin props on the same shaft rotating in different directions).
Shown below, the angle of vertical trim of an outboard has a resultant vertical thrust component. A propeller facing downward will force the bow of the vessel down, and a propeller facing up will force the bow up. In moderation, a motor and prop trimmed down will grip the deep water for improved towing and assist with pushing the bow of the vessel through a head sea. Overdone it will swamp the vessel at the bow. In moderation, a motor and prop trimmed up will keep the bow of the vessel buoyant and throw off head seas. Overdone it will cause the bow fly skyward on accelerating or cause the prop to lose its traction due to lifting clear of the water. This thrust is also used to adjust for longitudinal trim thrown out by uneven conditions of loading.
Simpler outboard configurations will have a vertical trim adjustment that once found by trial and error is locked off. More complex set ups are fitted with hydraulic trim adjustment rams allowing for in passage refinement of trim for loading and sea condition variation. Twin outboard vessels can tweek the vertical trim adjustment and consequent vertical thrust forces of port and starboard outboards, so balancing both longitudinal trim and the list/heel of the vessel as may be the result of loading or prop induced transverse thrust. This ability is particularly marked with catamaran hulls.
8.2 Starting methods
Battery operated trolling outboards need no starter, merely the activation of their driving DC electric motor. The common methods for starting internal combustion outboards are pull start or electric starter motor.
Pull
cords
The simplest of outboards start up is achieved using a pull cord that wraps around a pulley directly connected to an extension of the crankshaft. These machines do not have a dedicated electrical storage system, relying on a spark inducing mechanism called a magneto. When the cord is pulled the crankshaft spins and the magneto provides the spark for the engine work cycle to begin and continue.
A magneto is an electrical generator
using wire coils and magnets to produce alternating current. It produces pulses
of high voltage to activate the spark plugs of small petrol driven internal
combustion engines such as light weight outboard motors and lawn mowers. The shuttle
magneto variation spins a wire coil on its flywheel between the poles of a
magnet whereas the inductor magneto spins the magnet around a static
wire coil.
A cam on the drive shaft opens the points (contact breakers) momentary
interrupting the current and collapsing the coil’s electromagnetic field. This induces
a voltage across the coil, which in turn supplies the energy for the spark plug
firing. To prevent high voltage arcing at the point’s contacts, leading to
rapid decay, a capacitor (an
electrical storage and smoothing device) is placed across the points to absorb
the
energy burst. (Capacitors,
previously called condensers, are
built from two films of electrical conductors separated by a film of electrical
insulator).
The system is rarely used for vehicles or vessels
that have electrical accessories, except for aero piston engines where
simplicity and reliability are of advantage. The magneto is not used to charge
batteries in marine systems but in old outboard motors may be wired to power an
emergency light.
Pull cords are subject to rot and chaffing, a
spare should be carried. The whip like effect of the flailing cord with its
retaining knot at the end of the pull
can inflict injuries on users and observers alike.
The battery, coil and distributor
More complex
multi cylinder engines may use an electrical system energised by a battery bank
that is kept charged by an alternator. This system induces
a voltage across the coil, which in turn supplies the energy for spark plug
firing. While an emergency pull start cord may be
fitted more usually a starter motor is fitted.
A crank driven distributor shaft revolves in a
cylindrical distributor body. The
shaft drives a cam that opens the points momentary
interrupting the current and collapsing the coil’s electromagnetic field. To
prevent high voltage arcing at the point’s contacts, leading to rapid decay, a capacitor (an electrical storage and
smoothing device) is placed across the points to absorb the energy burst. (capacitors or condensers, are built from films of electrical conductors separated
by a film of insulation).
The perimeter of the distributor’s cap has contact points
individually connecting to each spark plug by a high tension lead. (Don’t touch them during running or you will
receive a high energy electrical shock!)
As the distributor shaft rotates, the rotor arm fitted to its head brushes past each contact point in
turn to distribute the electrical energy to each spark plug in the correct
order and time. The
size of the points’ gap opening can be adjusted to the manufacture’s
recommendation. In order to fine tune the ignition timing, its position in
relation to the cam can be adjusted by rotating the distributor body around the
distributor shaft. Further information on engine ignition including electronic ignition is provided in
Chapter 3.4.
Electrical starters
The battery operated starter motor is standard
on larger engines. Due to the high compression pressures mentioned previously,
starter motors draw a very high current for a short period of time. The
connections on the batteries and starter motor must be clean and tight. The
starting motor leads must be kept as short as possible to prevent voltage drop
and the wire size must be sufficient to take the high current.
The starter motor reverses the dynamo’s principle of using motion within a field to generate electricity by using electromagnetism to create motion. These motors have very heavy work to do albeit for a short time so the cabling must be thick, of a short run and have clean contacts with large surface area to avoid resistance, sparking and consequent overheating.
The solenoid forces the pinion outward to temporarily engage with the engines flywheel. On the motors starting and release of the starter switch the pinion springs back clear. Common faults with starter motors are poor connections. The outboard starter motor is a lightweight miniaturised device outside unsuitable for user servicing.
8.3 Engine protection and devices
Outboards are fitted with immobilisation devices and alarms. The in-gear immobilisation and kill switch features are detailed in Chapter 8.4.
Alarms
Best practice is that an outboard engine be
provided with a visual and/or audible warning device to indicate a dangerous
condition associated with:
Engine
jacket cooling water outlet temperature;
Two
stroke direct injection lubrication;
Engine
gear box lubricating oil.
These protection devices can give off a
flashing light and/or audible warning. Automatic devices that could shut down
without any warning, for instance when crossing a bar entrance, could result in
loss of the vessel so are not usually fitted. Some outboards will automatically
slow the engine speed (i.e. to 2000 rpm) to reduce ultimate engine damage but still
enable an emergency manoeuvre to be completed before the operator can shut down
manually. The sound and light sequence of alarms will be individual to the
outboard make and model, a full description being available in the manufacturer’s
user’s manual.
A
high temperature fresh water alarm - consists of a thermo
switch. It has a bi-metal probe that activates contacts a micro switch. It can
be installed in the side of the thermostat housing. When the engine is running
at normal operating temperature, the contacts in the switch will be open.
Should the engine coolant exceed 96° C (+ or - 3°) the water temperature switch
to earth closing the electrical circuit and switching on the audible alarm
and/or indicator light. The gear box low lubricating oil pressure alarm
operates in the same fashion as the engine low oil pressure alarm.
Two
stroke direct injection lubrication - consists of a low level
switch fitted in the lube oil reservoir tank and/or the lube oil tank.
A
low engine oil pressure alarm - consists of a pressure
switch fitted to the pressure side of the lubricating oil system, usually into
an oil gallery. The oil pressure acts on a diaphragm and spring which open the
contacts in a micro switch. When the spring pressure is greater than the oil
pressure, the contacts will close and sound the audible alarm. When the engine
is started, the oil pressure switch opens as the oil pressure reaches
approximately70 kPa (0.7 bar or 10 psi) and the alarm
will cease to sound. Likewise, if the oil pressure drops below the setting of 70
kPa (10 psi), the oil pressure switch will close the
circuit and sound the audible alarm.
8.4 Start up and shut
down
Pre-departures
Always consult the manufacturer’s operating manual before operating an outboard engine. Before starting it is important to check the following:
Fuel and air supply;
Lubricating oil;
Outboard attachment and
steering;
Fuel
If the outboard is fitted with a portable fuel tank, it will be necessary to loosen the vent at the top of the tank to allow sufficient air to enter the tank. It may also be necessary to prime a carburetted motor’s fuel lines using the inline rubber bulb primer pump. Do not over do squeezing this device, a few times will be sufficient until it firms up or you could risk flooding the system. An internal tank will need to be sounded to ensure there is sufficient fuel, or filed to the known capacity. Air filters should have been included in the servicing schedule
Oil
Two stroke premix fuel - Most small outboards are two strokes. These require a specific fuel and two stroke oil ratio to be mixed in the fuel tank prior to being used by the engine. This will also apply to some larger two stroke outboards. This should be done while ashore to ensure accurate measuring for levels. It is imperative that the fuel/oil ratio is correct at the time of fuelling as you will not be able to verify the ratio later. The oil content is the sole means of lubrication within the engine. If you are unsure of the fuel/oil mixture in a tank, replace it with a fresh amount. Do not simply top up the contents of a tank unless you are sure of the original mixture. It is also important not to leave two stroke fuel sitting for long periods (more than four months).
Oil injection two stroke - Increasingly two stroke engines are being fitted with oil injection. These systems automatically regulate the fuel/oil ratio required by the engine and receive oil from a reservoir and/or tank separate from the fuel tank. The level/s must be checked at regular intervals to ensure oil usage and the level topped up when refuelling with the manufacturer’s specified oil.
Oil pump four strokes - The four stroke petrol and diesel engines also require lubricating oil. The oil in this case does not become mixed with the fuel. It is circulated through the engine via a pump from a main reservoir and then returned to the reservoir. This also requires checking prior to starting the engine and topping up with the manufacturer’s specified oil type.
Attachment
and steering
Outboards are generally attached to the vessel by clamps or by bolts. These must be inspected for integrity before start up. The tiller arms with their hydraulic rams
must be activated by hard over the steering in both directions while listening for smooth travel and checking for no fluid leaks.
Clamp screw mounting and safety chain - The method uses hand tightened screw
clamps that trap the vessel’s transom (mounting plate) between the engine’s
mounting bracket and the hand screws. A safety/lanyard chain is usually fitted
to secure the engine’s bracket to the vessel. The hand screws must be checked
for tightness prior to starting the engine and the chain fastenings and chain
length checked to ensure the engine will not become submerged if it falls from
its mounting.
Bolted on - This method securely fastens the motor to a purpose built transom/well or to a pod that is bolted to and extending out from the transom, so providing the foundation for steering tiller arm, hydraulic rams or cabling attachment. It is a far more secure than clamps with its engine bolted through the transom plate, usually in no less than four places. This system must be checked for tightness of mounting bolts and a visual check made for any sign of the bolts pulling through the hull.
Bolts into pods provide a space
that can be flooded over time, so these voids must be periodically checked for
dryness.
Start up
Make sure the motor is tilted into the water. A reverse tilt lock is fitted to smaller outboards without hydraulic trim tilt. This mechanism is used to prevent the engine from tilting out of the water when being operated astern. The tilt control mechanism can be tested by selecting astern with the engine stopped and attempting to tilt the engine - if working correctly the engine will not tilt.
The gear selection lever must be placed in the neutral position prior to starting the engine. This is to avoid possible injury to any persons or property from the propeller or from the sudden propulsion of the vessel. Most outboards will have an in-gear start prevention mechanism but these should not be relied upon. If you are unsure if the motor is in neutral with a small outboard, remove the emergency stop lanyard and clip, tilt the engine up and with light finger pressure try to rotate the propeller.
The start choke and throttle settings must be selected to avoid damage to the engine due to over-revving of the engine on start up. Systems vary from differing brands from tiller twist grip control with starting marks with pull chokes (shown above), to integrated remote motor control boxes (shown below). The latter have a choke knob and a lift bar to temporarily raise idling speed. They will usually have a release button so the lever can be operated in the throttle mode but not linked to the gear shift to enable full throttle testing and adjustment.
This emergency engine stop lanyard and clip must be used on all outboard engines.
If the clip is not inserted into the stop switch the engine will not start and if removed the engine will immediately stop. Its purpose is to quick stop the engine if the operator falls overboard, so he/she is not run over by the crewless circling vessel, or worse, is left far from land with the vessel motoring off. The clip is secured to an engine stop switch prior to starting and the lanyard attached to the operator.
After the engine starts, check cooling water
tell-tales to ensure the cooling system
is working. If the engine has a thermostat, tell-tale flow may be delayed for
45 seconds. Take care, as a cooling system failure may destroy the engine. Reduce
engine to normal idle speed before engaging gears.
Shut
down and flushing the cooling system
To prevent salt
and/or silt build-up in the outboard cooling system, it should be flushed with
fresh water. A garden hose attachment must be available to feed water directly
into the engine water inlets. Make sure
the water feed is adequate by turning the hose on until water escapes around
the hose attachment. Alternatively, place the engine leg
in a large drum or tank. Fill the tank until the level of fresh water is well
above the inlets and the run the engine in neutral for 3 to 5 minutes. This
will pump water through the system and remove salt and silt deposits. While
flushing the engine, leave the motor in neutral as an exposed spinning
propeller is a hazard.
8.5 Preventative maintenance schedule
The electronic ignition systems
and fixed fuel jets often used with modern outboards need less maintenance than
earlier outboards. However, regular maintenance is essential for maximum life and reliability. Maintenance must be
carried out according to the manufacturer’s handbooks, but normally includes
regular inspection, routine servicing and troubleshooting of symptoms or
faults.
Regular
inspection
Whenever the engine is running, ensure that the telltale is flowing well. A reduced flow may show a blocked cooling system, or worn or damaged water pump that would result in serious damage. Flush system with fresh water after use in salt or muddy waters. Clean the engine with a rust preventative regularly. Additionally:
Check fuel lines for cracks or damage;
Check spark plugs and wiring for cracks in
the insulation or corroded terminals;
Check propeller and shaft seal for damage
and lock pin for tightness;
Check the anodic plate and replace if it is
badly eroded;
Check the anodic plate is not missing,
painted over, or coated in oil or grease;
Inspect for loose, damaged or missing parts.
Routine
servicing
Maintenance or repairs to an outboard motor
are difficult, and almost impossible to carry out on a pitching vessel at sea.
Ignition and fuel system repairs and adjustments are awkward, and are best done
in a test tank on dry land.
If you make sure your outboards are properly checked and maintained, the risk of having to do repairs at sea is considerably reduced. Engine servicing requirements will be specified in the manufacturer’s maintenance manual.
Ignition System
There is little maintenance either required or possible on electronic ignition systems. However, the spark plugs must be periodically cleaned or replaced to keep the engine starting and running well. The spark plug gaps must be set according to the manufacturer’s specifications before plugs are replaced in the engine. Any damaged plugs must be replaced. When replacing the spark plugs, screw them in by hand until finger-tight, then use a spark plug spanner to tighten them another ¼ turn to bed them against the gaskets. Do not over-tighten the spark plugs. For earlier magneto
ignition systems, the breaker points will need to be periodically checked and adjusted according to the manufacturer’s handbook.
Fuel System
Periodically
clean or replace fuel filters according to the manufacturer’s handbooks.
Some engines may
have fuel jets which are screw adjustable to make the fuel/air mixture richer
(more fuel) or leaner. Adjusting screws
for Low Speed mixture and High Speed mixture may be provided. An Idle
Speed adjustment is also often provided. If adjustment to the mixture is
needed, carefully follow the manufacturer’s handbook, as minor changes may
cause starting and running problems.
Lubrication and
greasing
Lubricate grease points and control mechanisms. Lubricate all pivots and linkages
according to the engine handbook.
Routinely check, drain and refill gearbox with the correct marine gear oil. To prevent air locks, the gearbox is filled from the bottom with the top vent plug removed. The gearbox is full when oil flows from the vent. When checking or re-filling the gearbox, the following conditions may mean trouble, and must be checked out:
Water or milky
brown oil in the foot;
Metal particles on
the magnetic fill plug;
There is evidence
that the gearbox oil level was very low.
Key areas to inspect for free play and free from corrosion are shown below. The areas must be maintained by keeping well greased:
Checking propeller assembly
Periodically the propeller should be removed and inspected along with the drive shaft. Fishing line and weed can become entangled around the thrust bushing and cause shaft wear. Any slack in the propeller shaft bearing will indicate the need for its replacement. Any small dints along the prop blades edges may be smoothed over with emery cloth. Larger blade damage will need to be assessed and repaired or the prop even replaced by a mechanical engineer.
If the blades show loss of paint or corrosion around the tips then cavitation is occurring. The cause will need to be established which could be due to the motor being fitted too high in the water, over revving, too greater pitch of propeller or obstructions in the water flow. Often a wider hydrofoil (anti-cavitation plate) will reduce the quantity of air sucked down around a cavitating prop. The cotter pin should be inspected and replaced if defective as with the shear pin if fitted when refitting the repaired propeller. At this opportunity the anode should be replaced and if the steering trim tab has been removed then it must be refitted in the same position as before to maintain the steering bias.
Troubleshooting
Checks if an engine
fails to start
There are many reasons for an engine failing to start. It’s a good rule of thumb to start with the simple checks in a logical order. If the engine does not start, the following checks should be conducted. Check that:
Emergency stop clip
(if installed) is correctly connected;
Fuel tank is not
empty, fuel tank breather open if manual breather is fitted;
Fuel bulb is primed,
engine is not flooded, filter is not clogged;
Throttle and choke
positions are in correct position, as manufacturer’s specifications;
Gear selection
lever is in the neutral position;
Start battery is in
good condition (if engine is electrically started).
Warm up periods
A warm up period is recommended after starting the engine, to help prevent cold seizure and premature wear of engine components. During the warm up period, the engine should be allowed to idle without load or excessive revving.
Cold seizure happens when heat from friction and combustion within the cylinder expands the piston at a much greater rate than the cylinder, resulting in the piston seizing in the cylinder bore. The warm up period allows time for the piston and the cylinder to expand at a similar rate. The cooling water flow is controlled by a thermostat which allows the cooling water to remain in the engine. This assists with warming the cylinder until operating temperatures are reached, and then maintains the water cooling temperature.
Engine overheating
All engines become hot. The water cooling system is designed to control rising temperatures. An overheated engine can cause extensive damage, so it’s important to watch for early signs. If steam is seen rising from the engine exhaust or steaming off the engine cooling water discharge, it probably indicates an overheating engine.
Many outboards are fitted with audible alarms, temperature gauges or engine revolution limiters which will indicate an overheat problem. The engine should be shut down if weather and water conditions allow, or at least reduce the engine revolutions and load to an absolute minimum.
The most common reasons for overheating are either obstruction to the cooling water intakes or water circulation pump failure. Obstructions generally occur when floating plastic bags or similar matter, wrap around the gear leg blocking the water intakes. These can be removed by shutting down the engine, disconnecting the emergency stop clip from the engine, tilting the engine and removing the obstruction. The engine may then be restarted and run at low speed and load until normal operating temperatures are obtained.
The water circulating pump failure can be recognised on start up of the engine so that an overheat situation can be avoided. This check is made by ensuring cooling water is being discharged from the cooling water pump indicator (tell-tale) or the engine exhaust ports. This type of pump failure is often the result of the engine being stored for periods without use, allowing the impeller to perish and/or stick to the pump housing.
A table of symptoms and faults for the petrol engine that are primarily used in outboard motors is complied below. A similar table for diesel engines is found at the end of Chapter 9.
|
Troubleshooting - Ignition |
||
|
Symptoms |
Possible Causes |
Correction |
|
Engine will not start |
Faulty spark plugs – remove plug, clean tip and set gap correctly, or just replace as plugs are the most common ignition problem. |
Replace spark plugs |
|
Engine will not start or runs badly |
Faulty ignition system – remove a plug. With the lead attached and plug body earthed to the engine, crank over and check for good spark at the plug gap. A thin weak spark shows a faulty system.This may be caused by burned and dirty magneto points. Modern electronic ignition system (EIS) engines often replace the points with electronics, and these are more reliable, but cannot normally be serviced. |
Clean/adjust magneto points or replace faulty components EIS will need specialist service engineer. |
|
Troubleshooting - Fuel |
||
|
Engine will not start |
Fuel supply problems – Check fuel level, fuel cock, fuel lines and joins. |
Refill or repair |
|
Engine will not start |
Stale fuel, too much oil in mix, or oil and petrol not properly mixed – Always check that two-stroke fuel is fresh and correctly mixed. |
Replace if you are not sure |
|
Engine will not start |
Engine flooded – if the crankcase fills with excessive fuel mix, the engine may not start. Remove and check a spark plug. If it is very wet, the engine may be flooded with too much fuel. Open the choke, turn the fuel off, and allow it to evaporate to dry the engine out. |
Dry out & then try again later. Choke is only needed when the engine is cold. |
|
Engine will not start, runs badly, or stops |
Fuel blockage due to jets blocked by water or dirt – remove and check spark plug, does it look moist or dry. A dry plug after several starting attempts may indicate blocked jets |
Clean & flush tank, fuel lines, carburettor bowl and jets. |
|
Engine will not start, runs badly, or stops |
Mixture adjustments incorrect – some carburettors have screw adjustable jets. Before fiddling around with a mixture adjustment, count how many turns it takes to screw the adjustment fully in. This will allow you to restore the original adjustment if necessary. |
Have the engine tuned up |
|
Troubleshooting - Mechanical Problems |
||
|
Poor starting |
Poor compression – can you detect strong compression during cranking |
Major repairs |
|
Troubleshooting - Cooling System |
||
|
Overheating |
Raw water intake blocked |
Check, clean |
|
Overheating |
Water pump impeller worn –check tell-tale |
Replace |
|
Overheating or not warming |
Thermostats regulate water flow. If these stick, they may cause these symptoms |
Replace |
Chapter 9: Operating a diesel inboard engine
9.1 Starting Methods
The four common
methods for starting marine diesel engines include:
Hand start, electric starter motor,
hydraulic start or air start.
Hand start
Hand starting is
usually fitted to smaller engines although some manufacturers do provide them
on larger engines. On the smaller engines, a starting handle engages in the end
of the crankshaft. The starting handle is turned until the engine has
sufficient cranking speed for it to fire.
The starting handle should be held with all
fingers and the thumb on one side of the handle. If the thumb is placed on the
opposite side to the fingers, and the engine back fires, the thumb could be
broken.
The compression
pressure of a diesel engine is higher than that of a petrol engine, therefore
more effort is required. To make the engine easier to turn, decompression
levers may be fitted to relieve the pressure in one or a number of cylinders, not
necessarily all cylinders. A decompression device is a mechanism that is
actuated by a lever, which holds the inlet or exhaust valve off its seat. This
means that when the piston ascends on its compression stroke, no compression or
resistance is there because the air is flowing straight out the inlet or
exhaust valve (whichever one is decompressed).
The
decompression lever/s are conveniently placed to the starting handle so one
person can operate both by:
The decompression lever/s are engaged;
The engine is hand cranked until it attains
sufficient cranking speed;
The decompression lever/s are released
allowing full compression pressure;
The engine fires.
On the larger
engines, the starting handle is operated by sprockets and a chain with a ratio
to provide easier turning. The decompression levers are operated by a second
person. This means of starting on the larger engines is an additional emergency
method. The main means is an electric starter motor.
Electrical
starters
The dynastart
A dynamo or
generator is an electrical generator, belt driven by the motor, using wire
coils and magnets to induce alternating electrical fields which are converted
to direct current at the commutator (a segmented rotary electrical connection/switch using carbon brushes
for continuous contact). It has a stationary outer casing
providing a magnetic field and a set of windings on an armature (supporting
axle assembly) rotating within the field. The magnetic field is created with
either permanent magnets or electromagnets (field coils). Dynamos use their
rotary switch on the armature, the commutator, to output DC current, and an
external voltage regulator to control output over the range of speeds of the
driving motor.
The dynamo operates at any
speed with low output at engine idling. The alternator (described below) cuts
in at a minimum speed but has greater output than the dynamo at idling.
Additionally, the complexity and maintenance requirements of the dynamo’s commutator and brush mechanism have resulted in it being
largely superseded by the alternator for automobile and marine DC battery
charging.
A dynastart is a combined dynamo/starter motor championed in Stuart
Turner’s marine petrol engines, renowned for their simplicity and reliability.
The single unit with fields and coils can reverse switch its polarity
connections to achieve both functions (through a voltage regulator and control
box) of starting the motor, or in an emergency of driving the prop shaft for a
few crucial minutes until the battery is exhausted.
The engine starter
motor
The battery operated
starter motor is now commonly used on large engines of high kW power. Due to
the high compression pressures mentioned previously, starter motors draw a very
high current for a short period of time. The connections on the batteries and
starter motor must be clean and tight. The starting motor leads must be kept as
short as possible to prevent voltage drop and the wire size must be sufficient
to take the high current.
The starter motor reverses the dynamo’s principle of using motion within a field to generate electricity by using electromagnetism to create motion. These motors have very heavy work to do albeit for a short time so the cabling must be thick, of a short run and have clean contacts with large surface area to avoid resistance, sparking and consequent overheating.
The solenoid forces the pinion outward to temporarily engage with the engines flywheel. On the motors starting and release of the starter switch the pinion springs back clear. Common faults with starter motors are poor connections. The pinion can jam in engagement with the flywheel in older worn starters. If the solenoid indicates that it is working by a click on turning the ignition starting key then a tap on the rear end of the starter motor with a rubber hammer may free it until the required service and repair can be undertaken.
Hydraulic
starting systems
A hydraulic starting system is normally used on a stationary engine that drives an emergency pump, or an engine on a vessel that needs to be started first to get all machinery running. It consists of connecting hoses and fittings to the:
Oil reservoir;
Hand pump;
Accumulator and pressure
gauge;
Hydraulic driven
starter motor.
Operation
Hydraulic oil from the oil reservoir is pumped by the hand pump into the accumulator.
The accumulator is divided by a synthetic diaphragm which is filled with compressed air of around 1,400 kPa (14 bar or 200 psi). The diaphragm is sealed to stop the air leaking out. The oil is pumped into the lower portion of the accumulator and increases the pressure of the air. A manually operated control valve on the bottom of the accumulator allows the oil to flow to the starter motor which rapidly accelerates the engine to a high cranking speed. The fluid returns directly to the reservoir from the starter motor.
|
Starting procedure - Steps |
|
|
1 |
Ensure oil in the reservoir is at the correct level. |
|
2 |
Use the hand pump to raise the pressure in the accumulator to provide adequate cranking speed to start the engine. |
|
3 |
Set throttle to the start position. |
|
4 |
Push the control valve lever on the accumulator to allow the pressurised oil to flow to the starter motor. |
|
5 |
Close the control valve quickly when the engine starts to conserve the accumulator pressure and to prevent excessive overrunning of the starter drive clutch assembly. |
Detroit Diesels
have a hydraulic starting system called a hydrostarter. It is basically the
same as above, but uses nitrogen in the accumulator instead of compressed air.
In addition, it has an engine driven pump so the system is automatically
recharged after each start. It still has a hand pump so it can also be manually
recharged. There are also different types of hydraulic starter motors.
Air start method
Of a number of
variations, a distributor air start type is shown below. Compressed air is
forced into the cylinder to push the piston down and rotate the engine fast
enough so that it fires. That air must be fed into the cylinder when that piston
is on its power stroke at 10 to 15 degrees past top dead centre. On the power
stroke, both inlet and exhaust valves are closed, so the compressed air cannot
escape when it is fed into the cylinder.
The air is fed
into the cylinder via an air start valve situated in the cylinder head. There
is normally one air start valve for each cylinder. On some V12 and V16 engines,
air start valves are only fitted to one bank of cylinders to save on costs.
The air start
valve is a spring loaded valve. It opens when compressed air is fed to it and
closes when the air is shut off to it.
The air is
supplied to each air start valve via an air distributor. The distributor is
mounted on the end of the camshaft. It has a steel disc with a hole is drilled
through it. As the disc rotates, it lines up with holes in the distributor
body, the number of which are equal to the number of cylinders. Pipes are
connected from these holes in the distributor body, in the firing order
sequence, to each air start valve. The air is stored in and supplied from an
air receiver. A multi stage compressor is required to supply the air to the
receiver. The compressor is usually connected to a diesel engine that is hand
started, or by a battery operated starter motor.
|
Starting
procedure - Steps |
|
|
1 |
Ensure the air receiver (bottle) is fully charged with compressed air. |
|
2 |
Drain off any water and oil in the air receiver. |
|
3 |
Check the indicator cocks are open and bar the engine over at least one complete revolution to ensure there is no fresh water in a cylinder. Close the indicator cocks. |
|
4 |
Open the stop valve on the air receiver. |
|
5 |
Place the fuel lever in the start position. |
|
6 |
Place the air starting lever in the start position. Air will now flow to the cylinder that has just started its power stroke. The pressure of the air will open the air start valve and air entering the cylinder will force the piston down. |
|
7 |
The rotation of the engine will cause the distributor to shut off air to that cylinder and supply air to the next cylinder in the firing order. |
|
8 |
When the engine fires, release the air starting lever. (It is usually spring loaded). |
|
9 |
Shut the stop valve on the air receiver. |
|
10 |
Recharge the air receiver. |
Precautions with air start engines
Where the
propulsion engine is fitted with a gear box, the air receiver stop valve must
be closed whilst the engine is running.
The air start
valves must be checked to ensure they are seating. This can be checked by
placing a hand on the air pipe at each air start valve. If the valve is
leaking, the hot gases of combustion will pass the valve seat and flow into the
air supply pipe. A bad leak will cause the air supply pipe to glow red.
Should the air
compressor be pumping oil, this oil will finish up in the air receiver and an
explosion could occur.
A non-return
valve is fitted in the air line between the air receiver stop valve and the
distributor. This is to prevent the hot gases going back into the air receiver
and possibly causing an explosion.
An air receiver
is a pressure vessel and should be treated with respect. It is essential to
drain any water out of it regularly to prevent internal corrosion. At the same
time, oil would be drained from it, sometimes in the emulsified form, to
prevent explosions. Should the oil be excessive, the air compressor should be
overhauled. Survey requires the air receiver to be inspected internally and the
relief valve set annually. The relief valve must be set and floated in the
presence of the surveyor. A safety valve should never be altered or interfered
with.
9.2 Engine protection and devices
Alarms
Best practice is
that a propulsion engine be provided with an audible and visual warning device
to indicate a dangerous condition associated with:
Engine lubricating oil pressure;
Engine jacket cooling water outlet
temperature;
Engine gear box lubricating oil pressure.
As noted, these
protection devices give off a warning only. Automatic devices that could shut down
a propulsion engine without any warning, for instance when crossing a bar
entrance, could result in loss of the vessel so are not recommended. The older alarm
systems may have a disabling switch that must be turned on manually to put the
system into operation. The danger of this system is the operator may forget to
activate the system and operate in an unprotected mode. Automatic alarms that
alert at start up before self-cancelling are preferable to notify the operator
that the system is working properly.
The gear box low
lubricating oil pressure alarm operates in the same fashion as the engine low
oil pressure alarm.
A low oil pressure alarm - consists of a pressure switch fitted to the
pressure side of the lubricating oil system, usually into an oil gallery. The
oil pressure acts on a diaphragm and spring which open the contacts in a micro
switch. When the spring pressure is greater than the oil pressure, the contacts
will close and sound the audible alarm. When the engine is started, the oil
pressure switch opens as the oil pressure reaches approximately 70 kPa (0.7 bar or 10 psi) and the alarm will cease to sound. Likewise,
if the oil pressure drops below the setting of 70 kPa
the oil pressure switch will close the circuit and sound the audible alarm.
The alarm will
continue to sound until the engine is stopped or if an alarm switch is fitted,
it is switched off.
A high temperature fresh water alarm - consists of a thermo switch. It has a
bi-metal probe that activates contacts in a micro switch. It is installed in
the side of the thermostat housing.
When the engine
is started and running at normal operating temperature, the contacts in the
switch will be open. Should the engine coolant exceed say 96° (+ or -3° C) the
water temperature switch will close the electrical circuit and sound the
audible alarm and/or indicator light.
The alarm will
continue to sound until the temperature drops below the above mentioned
setting.
A Detroit Diesel
engine has an additional sensor fitted for the protection of their engines. It
will also sound the alarm on a large loss of coolant. A big and sudden loss in
coolant may reduce the coolant level to below the probe in the thermostat
housing. As the water is not now circulating over this probe, it will not
detect the rise in temperature of the coolant. An additional sensor is fitted
into the exhaust manifold outlet to detect the rise in temperature due to
overheating.
The water
temperature switch consists of a temperature-sensing valve and a micro-switch.
The valve contacts a copper plug (heat probe) which extends into the exhaust
manifold outlet. Engine coolant is directed over the power element of the
valve. Should the water temperature exceed its setting, the valve will close
the contacts in the micro-switch. This closes the circuit and sounds the
audible alarm. If a loss of coolant occurs, the heat of the exhaust gases will
be transmitted through the copper plug to the temperature-sensing valve,
closing the circuit and sounding the audible alarm.
Emergency stop device
Some diesel
engines are fitted with a manually operated emergency engine shut down device.
This is mounted in the air inlet housing, to stop the engine if an abnormal
condition should arise. If the engine continues to run after the engine
throttle is placed in the no fuel position, or if combustible liquids or gases
are accidentally introduced into the combustion chamber causing over speeding
of the engine, the shut down device will prevent damage to the engine by
cutting off the air supply and thus stopping the engine.
The shut down
device consists of an air shut off valve (flap) mounted in the air inlet
housing which is retained in the open position by a latch. A cable assembly is
used to remotely trip the latch. The shut off valve must be manually reset on
the latch for restarting the engine after the malfunction has been rectified.
During heavy weather it is imperative that there be no engine failure as loss of propulsion power may cause the vessel to broach and possibly capsize. A radio broadcast or a visual observation will warn of approaching bad weather. Precautions would include:
Secure everything that may move in heavy
weather;
Derricks or cranes should be lowered and
secured;
Hatch covers, weather tight doors, port hole
covers if fitted, engine room skylights should be secured to prevent the entry
of water, possible flooding and/or putting the machinery out of action;
Any watertight door should be closed;
Ensure there is no water in the bilges or
compartment to cause a free surface effect.
It is preferable that the fuel tanks not being used be full or empty to avoid free surface effect. Reference should be made to the stability book for any special conditions of loading that apply.
Ensure that engine oil and water levels are up to their full marks. A lubricating oil level in the engine sump that is at the low mark on the dipstick may cause the lubricating oil pump to lose suction when the vessel rolls.
Drain the fuel tanks, fuel filters and fuel/water separators of any water or sediment. In heavy weather it will mix with the fuel as it surges in the tank and may clog the filters.
In consultation with the Master, speed should be reduced to prevent undue stresses on the hull and machinery and so the engine does not over speed when the propeller come out of the water. Keep a constant watch on the hull, machinery, cargo and passengers.
9.3 Start up, operation and shut down
Start up procedure
The checks and
procedures to be carried out before starting an engine depend on:
Whether the engine has just been repaired or
overhauled and
Whether you were the last person to run the
engine.
The certified
engineer has full responsibility for both pre-departure checks and running checks
whilst the vessel is under way. Described below are main propulsion checks (less
the complete auxiliary equipment checks of a full pre-departure). The composite
examples given below is for a large diesel engine with: lubricating oil pump,
fresh water cooling pump and the sea water pump driven by electric motors and with turning gear & motor* (to
revolve crank to position the pistons for pre start) air start engine** or
electric motor start*** with decompression levers. Firstly, carry out all the
necessary checks and procedures for starting the engine genset
and bringing it on line (supplying electrical power to the vessel).
|
Maintenance
checks |
Comments |
|
Check
the fuel filter is clean. Check there is sufficient fuel in the fuel tank for
the intended voyage plus a reserve. |
Open
the fuel tank drain valve and drain off any sediment or water. Open the fuel
tank outlet valve. If a water separator is fitted, drain off any accumulated
water. |
|
Prime
the fuel system and bleed off any air if a filter has to be changed. |
This
is not always necessary except after a repair has been carried out to the
fuel system or long periods of shut down. |
|
If
the fuel injection pump has its own sump, check the level in the sight glass
is at the upper line. |
|
|
Check
for clean sea water strainer & open the sea connection valve plus
overboard discharge valve. Start up the sea water pump. Check for leaks. |
The
sea water will by-pass the fresh water and lubricating oil coolers until the
temperatures rise above the normal operating temperatures. |
|
Check
the fresh water in the header tank is at the correct level. |
Open
valves - start the fresh water circulating pump and ensure there are no fresh
water leaks. Apply heat to the fresh water to gradually raise the
temperature. |
|
Check
the oil in the sump is at the correct level. If a turbo charger with its own
lubricating system is fitted check the sight glass level is at the upper
line. |
Open
valves - start the lubricating oil pump and ensure there are no leaks. Apply
heat to the lubricating oil and gradually raise the temperature. |
|
Check
the condition & tension of all vee belt drives. |
Check
the condition of all flexible hoses. |
|
Grease
or oil any linkages. |
|
|
Ensure engine maintenance work is complete, isolation tags removed & engine clear of rags, especially exhaust. |
No tools, materials or parts lying about, all guards in place. Check the whole engine is free from fuel and oil. |
|
Start
up checks |
|
|
Gear
box is in neutral. |
Check
the movement of the hand throttle. |
|
Old
large engines may have indicator cocks fitted to each cylinder to take
indicator cards (performance diagrams). These allow the power of the cylinder
to be assessed against a manufactures ideal power diagram for performance diagnosis.
If
fitted engage the turning gear*, switch motor on & roll engine to start
position (just after tdc power stroke) to clear any
cylinder water/oil. Turn off. |
Opening
the indicator cocks on this occasion allows the compression pressure to go to
atmosphere. This allows the turning gear to rotate the engine easier. If the
volume of any fresh water in a cylinder exceeds the clearance volume of the
cylinder, it will discharge out the indicator cock while the engine is being
rotated on the turning gear. This problem will have to be rectified before
continuing further. Turning gear option* |
|
Activate
the alarm system. |
|
|
When
the fresh water cooling water and the lubricating oil have reached their
operating temperatures, shut off the heat source. |
|
|
Ensure
the air receiver/s are at their correct pressure.** |
Run
the air compressor to bring to correct pressure. Drain off any water and/or
oil from the air receiver. Air start fitted option** |
|
Open
up the air receiver stop valve to supply air to the starting air distributor.** |
Air
start fitted option** |
|
Roll
the engine over slowly on air at least one complete revolution to ensure
there is no fresh water in any cylinder.** |
Water
may discharge from the indicator cocks. If there is water in a cylinder, the
problem must be rectified immediately. |
|
Place
the fuel lever in the start position. |
|
|
Engage the air start lever**. When the engine fires release
the lever air start fitted option. Activate electrical starter motor.*** Open decompression levers,
the engine and close off the levers as the spinning engine reaches starting
speed. |
Air
start fitted option** Electrical starter motor option*** Listen
for any unusual noises, especially hard metallic knocks. |
|
Check
for fresh & sea water, lubricating oil, fuel oil and exhaust leaks. |
Check
the colour of the exhaust gas. |
Running checks
It is assumed it is a manned engine room so all machinery will be monitored regularly.
|
Running checks: |
|
Engine oil pressure. |
|
All lubricating oil levels. |
|
Engine fresh water temperature. |
|
Fresh water level in the header tank. |
|
For fresh water, sea water, lubricating oil, fuel oil and exhaust gas leaks. |
|
Pumps circulating fresh & sea water and lube oil. Electric motors driving them. |
|
All flexible hoses for leaks and possible deterioration. |
|
Exhaust temperatures |
|
For any overheating. |
|
Listen for any unusual noises, especially hard metallic knocks. |
|
Maintain the log book. |
Use your senses
when you carry out checks.
|
Smell |
Hot oil, exhaust gas leaks, insulation melting on electrical cables, paint on an overheated surface and overheating problems give off individual smells that can be recognised. You can smell the combustion gases of a blown head gasket if blown between the cylinder and the outside of the engine. |
|
Hearing |
You can hear an injector pipe rattling in a clip. If you don’t stop it rattling, it will either wear through or fracture probably resulting in oil being sprayed onto the exhaust manifold and igniting. You can hear a metallic knock from a bottom end bearing with too much clearance. You can hear the pulsations of a blown head gasket if its blown between the cylinder and the outside of the engine. |
|
Sight |
As well as hearing the above mentioned injector pipe rattling, you can also see it rattling. Sight allows you to see most things that are wrong such as locking devices missing, excessive movement due to slackness or wear, etc. You can also see exhaust gas leaks, especially on starting an engine, before it fires. Soot deposits at a flange will also indicate an exhaust gas leak. |
|
Feel |
Placing your hand on a part will indicate its temperature. You will quickly learn what the normal temperature feels like, therefore overheating can be detected. You can feel the pulsations of a blown head gasket if its blown between the cylinder and the outside of the engine. |
Shut down procedures
It is important
that engine temperatures be reduced gradually to prevent unequal contraction of
the different metals found in an engine. Otherwise cracking may occur in the
block or cylinder head. If an engine is fitted with a turbo charger, it is
necessary to reduce its speed and idle for some time before shutdown.
|
After
shut down checks |
Comments |
|
Deactivate the
alarm system. |
|
|
Shut the fuel
off at the tank. |
|
|
Open the
indicator cocks. |
|
|
Engage the
turning gear and start its motor. |
|
|
Stop the sea
water circulating pump. On a lot of vessels, this valve is left in
the open position from one survey to the next. This is bad and dangerous
engineering practice. The purpose of the valve is to stop the ingress of sea
water should a hose or pipe fail. If the valve is not worked (opened and
closed) regularly, marine growth will make it difficult to close and
impossible to seal. |
A number of
engine rooms have flooded, whilst no one was on board, because the sea valve
was left open and a pipe or hose burst. A notice
should always be placed in a prominent position indicating whether the valve
is opened or closed as a useful reminder in the pre-departure check. |
|
Shut the sea
water discharge valve. |
|
|
Keep on
circulating the fresh water and the lubricating oil and turning the engine
until it cools down to the ambient temperature. |
|
|
Switch off the
turning gear motor. |
|
|
Stop the fresh
water and lubricating oil pumps and close any valves in the systems. |
|
|
Close the
indicator cocks. |
|
|
If electrical
power to the vessel is not required, shut down the engine driving the
alternator. |
|
9.4 Engine room log book
An engine log
book assists in trouble shooting and the maintenance of all machinery providing
the relevant details are entered regularly. They can be used in an
investigation into an incident so are an official document, and to be completed
in accordance with best practice. Best practice requires that entries are
dated, legible, truthful and recorded by an authorised crew person. Errors
found later should not be rubbed out, but struck through and re-entered with
the correct information and initialled and dated by the correcting officer.
Entries should
be made in a regular manner with information such as relevant fuel consumption,
pressures and temperatures so a pattern can be easily detected in change of
engine behaviour. The log book may also record maintenance and servicing,
though this is often more conveniently displayed on a engine room white board
or digital record keeping system. Certain parts may need to be inspected at
more frequent intervals, whilst the condition of the parts may indicate that
longer running periods may be allowed. Therefore, records should also be kept
of all maintenance carried out, the running hours and date at which it was
carried out.
Records should
also be taken of any measurements. For example, if the piston rings are
renewed, but the liners measured are found to be within tolerances stated by
the manufacturer, the measurements taken shall be recorded so the rate of wear
can be ascertained. It can be calculated in running hours when the wear will
exceed the tolerances. The engine log will indicate the operating conditions of
the engine during any particular period.
Computerised sensors
can scan all important points of an engine and are increasingly being used on
modern engines. Service intervals can be set so maintenance is carried out when
needed. The computer can tell the operating conditions of the engine and where
the critical wear and tear is or has taken place.
9.5 Safety aspects when working on engines
Potential hazards can be identified in the engine room workplace including:
Fire and explosion with catastrophic
consequence;
Being caught in moving machinery;
Heat causing burns or scalding;
Oil causing slippery conditions for manual
handling;
Unsatisfactory completion of maintenance
work;
Breathing poor or contaminated atmosphere;
Loud noise leading to temporary/permanent hearing
impairment.
When you are
working on an engine, ensure that the engine cannot rotate. Such an occurrence may
result in the loss of fingers or worse. There are various ways in which an
engine can be rotated, depending on the size, method of starting and number of
propellers.
Advise personnel
that work is being carried out on an engine. Be aware of any personnel not
present at the time of the instruction, so they can be advised.
Place warning
signs in the relevant positions if the engine may not be operated.
Disconnect the
source of supply and tag out the method of rotating the engine. e.g. disconnect
batteries, tie up the air receiver stop valve.
A vessel may
have one engine driving a propeller and making way through the water whilst the
other engine is stopped. The vessel moving through the water can cause the
stationary propeller to revolve and turn the stationary engine. In this type of
situation a shaft brake would be fitted and should be applied and the turning
gear engaged.
Job scenario considerations
You are aware
that fuel dilution of the lubricating oil is taking place and suspect a leaking
injector pipe under the rocker cover. With the engine stopped, the rocker cover
is removed. There is no indication of which pipe is leaking. There is no
provision to bar the engine over, so it is necessary to start the engine to
locate the exact position of the leak.
Fire and explosion with catastrophic
consequence - In starting
the engine, it is possible for fuel to spray in any direction as the rocker
cover is not containing it. Should it spray onto a hot surface, it could
ignite. In addition, it could spray into a person’s face and due to its high
pressure, cause eye damage. Eye protection should be worn.
Being caught in moving machinery - One person should start the engine and stop
it as soon as it fires, while the other person locates the fuel leak. Remember,
the valve rocker gear will be operating so make sure no part of your clothing,
hair, part of your body, rag in the hand can get caught.
Oil causing slippery conditions for manual
handling - Oil causing
slippery conditions will be splashed over the engine. When working on an
engine, it is difficult not to have patches of oil lying about. Oil could cause
you to lose your grip or footing and sustain injury. Any spilt oil must be
cleaned up immediately.
Heat causing burns or scalding - Being burnt on a hot exhaust pipe is bad
enough, but being scalded by hot water is serious. The engine may have
overheated. Working on the cooling water system before the engine cools down is
dangerous. Equally dangerous is removing the cap on the cooling water header
tank before releasing the pressure in the tank.
Unsatisfactory completion of maintenance work
- When any work has been
carried out on an engine, a check should be made to ensure that the work has
been completed and that there are no tools or rags lying in, around or on top
of the engine before a test run is carried out. Completing the work means that
all connections, couplings, flanges and components have been tightened up, all
locking devices are in place, access doors and guards are back in place, plus
oil and water levels are satisfactory. Failure to take these precautions could
result in fire, or maybe damage to the engine and injury to personnel should a
component come adrift.
Breathing poor or contaminated atmosphere – The potentially hot and fume prone engine
room may not only be uncomfortable to work in but if contaminated or oxygen
depleted may alter a worker’s perception and judgement. Stay atmosphere aware.
Loud noise leading to temporary/permanent
hearing impairment – The noisy
engine room may not only restrict communication and perception but over long
exposure cause permanent hearing loss.
9.6 Preventative maintenance schedule
Wear occurs in all
engines. It should be rectified at regular intervals rather than allow it to
accumulate until the inevitable breakdown occurs. Preventative maintenance
schedules are laid down to ensure the continued trouble free performance of the
engine and to maintain it in tune.
Engine
manufacturers issue a maintenance schedule for each model of engine. The
recommendations for preventative maintenance should be followed as closely as
possible to obtain long life and best performance. However, the periods between
inspections are a guide only as much depends
on operating conditions.
The maintenance
periods are usually expressed in running hours. However, most manufacturers
recognise that some engines will do less running hours in a given period than
others. An engine that is not used regularly will have its own problems.
Therefore manufacturers also specify time periods to overcome these problems.
Below, a sample
maintenance schedule is shown. This schedule does not go into depth, such as
when the cylinder head should be removed. The maintenance schedule indicates
which part needs cleaning, changing and checking.
Reference should
be made to the Operators Manual for
detailed instructions. These instructions deal with procedures, cautionary
measures and useful hints. Such instructions help to carry out the work quicker
and safely. The Chart also gives specifications for the fuel, oil, grease and
coolant. It also refers to other information for more complete and detailed
instructions.
|
Lubrication and Maintenance Chart |
||
|
Item/Hours |
Checks
& Procedures |
Lubricant |
|
10
hour service |
|
|
|
Engine |
Inspect engine
for leaks/loose connections |
|
|
Engine crankcase |
Check the oil
level - add oil if needed |
EO |
|
Cooling system |
Maintain
Coolant level |
|
|
Air starter |
Lubricator oil
level - add oil if needed |
EO (SAE10W) |
|
Clutch shift
collar |
Lubricate |
MPGM |
|
Fuel and oil differential
pressure |
Fuel filter
and oil filter differential gauges |
|
|
Governor |
Maintain oil
level |
EO |
|
50
Hours Service |
|
|
|
Zinc rods |
Inspect -
replace zinc rods (sea water) |
|
|
125
Service Hours |
|
|
|
Shift lever bearings |
Lubricate |
MPGM |
|
Pilot bearing |
Lubricate |
MPGM |
|
Main shaft bearing |
Lubricate |
MPGM |
|
Clutch |
Check - adjust
if required |
|
|
250
hours service |
|
|
|
Cooling system |
Add cooling
system conditioner or replace coolant conditioner element. |
|
|
Batteries |
Maintain
electrolyte level. Clean top of batteries and cable connections |
|
|
Engine crankcase
|
Change oil and
oil filters. Take sample for analysis. See oil and oil filter change interval
|
EO |
|
Belts |
Inspect all
belts for wear and proper adjustment |
|
|
Hoses |
Inspect hoses
for wear and loose connections |
|
|
Fuel tank |
Drain water
and sediment |
|
|
First oil change
|
|
|
|
New/rebuilt engines |
Inspect -
adjust valve lash. Observe rotation of valves. Inspect - adjust injector
timing |
|
|
1000
hours service |
|
|
|
Fuel system |
Clean primary
fuel filter. Change final fuel filters. |
|
|
Governor |
Change oil |
EO |
|
Breather |
Clean
crankcase breather elements |
|
|
Governor |
Lubricate 2
fittings |
MPGM |
|
Shutoff controls |
Inspect engine
shutoff controls. The shutoff controls must be inspected periodically so that
they will function properly when required. Only authorised personnel should
perform the inspection. Contact your Caterpillar dealer. |
|
|
2000
hours service |
|
|
|
Valve lash |
Inspect -
adjust valves. Observe valve rotation. |
|
|
Jacket water
Thermostats |
Inspect -
Replace if necessary. |
|
|
Turbochargers |
Inspect
turbochargers. |
|
|
Engine mounts |
Inspect engine
mounts & torque test bolts. |
|
|
Damper |
Inspect damper
for damage. |
|
|
4000
hours service |
|
|
|
Cooling system |
Drain and
clean cooling system. Add coolant conditioner pre-charge. |
|
Chapter 10: Troubleshooting
Exhaust smoke;
Low operating power;
Loss of lube oil pressure;
Overheating of engines;
Excessive vibration;
Fluctuation of engine revolutions;
Crankcase explosions.
10.1
Engine will not start
An internal
combustion engine needs a starting system to get it operating. It requires
fuel, air and ignition for the combustion process. A full charge of air must
enter the cylinder for compression and the fuel must be injected in an atomised
form into the cylinder at a precise moment. There must be no restriction in the
flow of exhaust gases.
An engine not turning over quickly when the
starter motor is engaged:
|
Starter |
Comment |
|
Battery capacity
low |
Try starting on the other or both battery banks. Never continue to use a battery if the starter motor is sluggish. High discharge rates will buckle the battery plates. Check that electrolyte level is above the plates. Check the specific gravity of each cell of the battery. (A fully charged specific gravity reads 1.26, flat reads 1.15 with a variance tolerance between cell of less than 0.030). A lower reading on one cell usually indicates the battery needs replacing. Multimeter voltage checks must be carried out under load. |
|
Poor battery connections
|
Check that the battery connections are clean and tight. A dirty/loose connection can be identified by the heat it generates. |
|
Poor starter
motor connections |
The starter motor draws the most load on the battery. This is especially so on diesel engines because of their high compression ratios. The electrical connections must therefore be tight and clean. |
|
Faulty starter
motor |
The starter motor could be burnt out or the pinion is not engaging with the ring gear on the flywheel |
|
Incorrect grade
of lubricating oil |
If the oil is too thick, the engine will not attain sufficient speed on the starter motor. In turn, this will not generate the amount of heat required on the compression stroke to ignite the fuel. |
|
Engine has been
overhauled and is tight |
The parts of an overhauled engine are brought back to their correct clearances. In these clearances there will be high spots to wear away as the engine is run in. The engine will not attain sufficient speed on the starter motor to generate the amount of heat required to ignite the fuel. When the engine is run in, it will turn easily. |
There must be
sufficient air and no restriction in the exhaust gas system:
|
Gas flow |
Comment |
|
Air cleaner
restricted |
The air cleaner is choked restricting all or most of the air required by the engine. |
|
Exhaust gas
restriction |
Could be caused by a bucket left on the outlet of a vertical exhaust pipe to prevent rain water entering the engine; or by the automatic flap valve fitted for this purpose and is stuck in the closed position. Occasionally a baffle could come loose in a silencer and block the passage of exhaust gas. |
The air must be
compressed to a high enough temperature to ignite the fuel.
This is usually due to low or poor compression. Compression pressure can be checked by replacing each fuel injector in turn with a compression gauge. Poor compression throughout the engine could result from one or more of the following:
|
Compression |
Comment |
|
Incorrect valve
timing |
The inlet valve is not opening or closing at the correct moment in the cycle. This is because the engine has not been correctly timed after maintenance work has been carried out. (The timing of the exhaust valve would be out as well.) |
|
Worn cylinder
liner bores |
Normal wear takes place on the cylinder liner where the piston rings come into contact with it. The wear is more pronounced near the combustion space where the heat burns the lubricating oil. The wear is also oval due to the thrust of the piston on the cylinder wall. The piston rings will not seal against the cylinder liner walls and, on the compression stroke, air will pass the piston rings into the crankcase. |
|
Cold engine |
The air entering the engine and the piston, cylinder liner and cylinder head are so cold that they take away the heat of the compressed air before it can reach sufficient temperature to ignite the fuel. If an engine is fitted with heater plugs, they can be utilised. Other alternatives are to use an air heater or a starting fluid to assist ignition of the fuel. |
There must be fuel:
|
Fuel |
Comment |
|
Fuel tank empty |
Fuel
piping could develop a leak empting the contents of the fuel tank into the
bilges. |
|
Blocked fuel feed line |
The
suction valve on the fuel tank could have vibrated closed or someone could
have closed the emergency fuel shut off valve. |
|
Faulty fuel lift pump |
Fuel
is not being delivered from the fuel tank to the engine. If of the diaphragm
type, the diaphragm could be perished or damaged. The drive to the pump could
be damaged. |
|
Choked fuel filter |
The
fuel filter has choked up with foreign matter preventing the full flow of
fuel. The filter may not have been changed at its recommended period. A bad
batch of fuel may have been received. The filter would require changing at
more frequent intervals than recommended until the system is clean. |
|
Air in fuel system |
Air
is compressible. Fuel is not. Air in a fuel system will cause the engine to
malfunction or not start. Air usually enters the fuel system when repairs are
carried out or where there is a fuel leak. This air must be bled off until a
bubble free fuel is obtained. Some fuel systems have a manual priming handle
on the fuel lift pump or on the fuel injection pump. In addition, there are
bleed valves throughout the system, such as on filters or water separators. |
|
Faulty fuel injection pump |
The
fuel pump is not delivering fuel to the injectors. |
|
Incorrect fuel pump timing |
The
fuel is not being delivered to the fuel injector at the precise moment in the
cycle. The engine could have been overhauled and the timing of the fuel pump
was incorrectly carried out. |
10.2
Exhaust smoke
The colour of
the exhaust gases can indicate problems with the engine.
Some electronic
controlled fuel systems have the provision for the exhaust emissions to be
monitored. It allows for the fuel metering and timing to be altered to suit the
load and have the most efficient combustion possible:
|
Smoke |
Comment |
|
Black smoke |
Indicates
excessive fuel for the amount of air. It causes incomplete combustion and
black smoke is emitted. The problem
may be: Engine overload. Blocked air
cleaner. Faulty
injector/s. Faulty fuel
pump. Incorrect fuel
pump timing. Incorrect
valve timing. Lack of
compression. |
|
Blue smoke |
Indicates
lubricating oil is being burnt. The
problem may be Worn cylinder
liner/s. Worn or faulty
piston rings. Excessive
clearance in valve guides. Oil seals in
turbo charger not sealing. If fitted, oil
bath type air cleaner overfilled. |
|
White exhaust vapour |
Indicates
water or moisture. The problem may be: Water in the
fuel. Moisture in
the air. Cold cylinder
liner bores when starting the engine. Fresh water
leak into the combustion chamber. |
10.3
Low operating power
Low operating
power is caused by wear in the engine and combustion being incomplete.
Therefore, fuel and air problems are involved. All in the following table
result in lack of compression:
|
Power |
Comment |
|
Restriction in air cleaner |
Element
is clogged restricting the flow of air into the engine. Replace element or
clean air cleaner. If oil bath type, check cleanliness and level of oil. |
|
Cylinder head gasket
leaking |
Could
be leaking between two cylinders, between a cylinder and the outside of the
engine or between a cylinder and a cooling water passage. |
|
Fuel injector |
The
fuel injector body may not be sealing properly in the cylinder head allowing
the compressed air to escape. |
|
Air start valve |
The
air start valve may not be sealing in the cylinder head allowing the
compressed air to escape. |
|
Incorrect tappet adjustment |
The
tappet adjustment is such that there is no clearance between the inlet or
exhaust valve stem and the rocker arm. The inlet or exhaust valve is not
closing on the compression stroke. (Referred to as riding). |
|
Sticking valves |
The
cam, through the cam follower, push rod and rocker arm, causes the valve to
open. The spring causes the valve to shut when the cam follower moves off the
lobe or peak of the cam. A
sticking valve is caused by combustion being incomplete. Alternatively, the
engine may have overheated. Carbon finds its way between the valve stem and
guide until the spring cannot exert sufficient pressure to close the valve. A
broken valve spring will not close the valve. On the compression stroke, air
will pass the valve. It could be an inlet or exhaust valve. |
|
Pitted valves and seats |
The
exhaust valve and seat is more prone to being pitted. Carbon, from incomplete
combustion, is hammered between the valve and seat when the valve closes. On
the compression stroke, air will pass the valve. |
|
Valves not seating
correctly |
Can
be caused by the head of the valve being bent on its stem due to the head
being too thin from continual grinding. It
can also be caused by exhaust gases scouring the valve face and/or valve seat
in the head. On the compression stroke, air will pass the inlet or exhaust
valve. |
|
Incorrect valve timing |
The
inlet and exhaust valves are not opening and closing at the correct time in
the cycle. Caused when the timing is being set. Check the timing marks are in
line on the crankshaft and camshaft gearwheels or sprockets. |
|
Worn cylinder bores |
Normal
wear takes place on the cylinder bores. This is especially towards top dead
centre where the high temperatures of combustion tend to break down or burn
the lubricating oil. The cylinder bores will also wear oval due to the thrust
of the piston on the power stroke. Replace the cylinder liner. |
|
Broken, worn or sticking
piston rings |
The
piston rings expand and seal against the cylinder liner walls. Normal wear
takes place and will in time become excessive. Piston rings are also subject
to breakage in service or when installing. They will also stick in their
grooves due to the carbon from incomplete combustion or overheating. In all
cases air will pass the piston rings, on the combustion stroke, into the
crankcase. |
|
Piston ring gaps in line |
Installation
of the piston rings may have gaps which were not equally separated or the
ring gaps came into line during the running of the engine. Piston ring gaps
in line will cause the air from compression to enter the crankcase. |
|
Turbo charger |
Reduction
in the turbo charger speed can be attributed to worn or faulty bearings or
carbon build up on the exhaust gas turbine blades. This will cause a
reduction in air supply to the air intake manifold resulting in a loss of
engine power |
|
Restricted fuel supply |
Could
be caused by fuel tank outlet valve not being fully opened. Vibration coupled
with loose gland packing could cause it to start closing. The fuel tank vent
pipe anti-flash gauze could be clogged. This will cause the fuel pump to pull
a vacuum on the fuel tank. Open the outlet valve and tighten the gland
packing. Clean the anti-flash gauze. |
|
Faulty injector |
Caused
by blocked nozzle hole/s, valve not seating properly, broken spring, injector
not opening at correct pressure, valve stem sticking in nozzle, leaking
injector pipe, excessive fuel return. Replace the injector. |
|
Faulty fuel pump |
Caused
by incorrect timing, wear between plunger and cylinder, delivery
valve not seating properly, incorrect calibration, broken spring. Retime the
fuel pump and if that is not the problem, replace the fuel pump. |
|
Fuel lift pump |
Fuel
lift pump is not delivering sufficient fuel from the tank to the fuel pump.
If of the diaphragm type, the diaphragm could be perished. In gear or plunger
type pump it will most probably be wear. Overhaul the pump. |
|
Fuel filter |
Fuel
filter could be blocked. Clean filter. |
|
Water in the fuel |
Drain
the water from the fuel at the fuel tank and at the separator. |
|
Restriction in flow of
exhaust gases |
Can
be caused by excessive build up of carbon or a baffle plate in the silencer
coming adrift and partially blocking the flow. |
|
Restricted air supply |
Could
be caused by the fire flaps and hatch cover in the engine space being closed
resulting in insufficient air flow to the engine. |
10.4 Loss of lubricating oil pressure
The reduction in
the normal operating pressure of lubricating oil can be a gradual process or
happen instantaneously. The loss of oil pressure will cause those parts under
the most load to fail first. This would be the bottom end bearings, due to the
load placed on them on the power stroke. By reducing the engine speed, the load
on the bearings is reduced. If there is still some oil pressure there, the
reduction in load maybe sufficient to save them.
Should the oil
pressure drop instantaneously, the engine must be stopped immediately:
|
Lube |
Comment |
Recommended |
|
Insufficient level of oil
in the sump |
May
cause a fluctuation of the oil pressure as the vessel rolls, the pump could
lose suction and air enters it. |
Reduce
speed. Top up to the correct level. |
|
Lubricating oil pump
strainer clogged |
Uncommon
problem as the additives in the oil keep the foreign matter and sludge in
suspension for the filter to remove. Will usually be a gradual drop in
pressure. |
If
possible, clean the strainer |
|
Faulty lubricating oil pump |
If
the drive to the pump has sheared, there would be no oil pressure at all. The
engine must be stopped immediately otherwise severe damage could occur. Should
the gears or rotors or vanes of the pump be worn or too much clearance
between them and the backing plate, there will be a drop in oil pressure,
usually a gradual drop will occur. |
Repair
the pump. |
|
Faulty relief valve |
The
pressure relief valve may be stuck in the open position or its spring may
have broken. A cold engine on start up has high oil pressure which will cause
the relief valve to open. The engine’s oil pressure drops as the engine
reaches its normal operating temp and the oil thins out. This results in the
relief valve closing. Should the relief valve stick in the open position or
the spring breaks, the oil pressure will drop below normal. |
Free
up the sticking relief valve or replace the relief valve spring. |
|
Filter partially blocked |
With
the filter being partially blocked, the flow of oil will gradually be
restricted. Lower oil pressure will occur and be indicated on the pressure
gauge until the filter by-pass valve opens. Most
oil filters have a by-pass valve in them to prevent oil starvation in the
event of the filter element becoming blocked. Whilst the oil flowing through
the by-pass is not filtered, this is preferable to insufficient oil. When the
by-pass valve opens, the oil pressure would return so the engineer should be
aware of why the oil pressure has dropped slightly then increased. |
Replace
the filter element or clean the filter (centrifugal type). |
|
Oil temperature too high |
A
high oil temperature will thin the oil out causing it to run more easily with
a resulting drop in oil pressure. Could be caused by a worn engine which
would have fresh water overheating as well. Alternately, it could be caused
by a dirty oil cooler on the sea water side. |
Run
at a slower speed until the normal operating oil pressure is obtained for
voyage home or clean the tubes in the oil cooler. |
|
Faulty oil pressure gauge |
A
faulty oil pressure gauge could indicate a low oil pressure where in fact the
actual pressure is correct |
If
the oil pressure gauge is suspect replace |
|
Fractured lubricating oil
pipes |
Will
result in a gradual or sudden drop in pressure if the pipe splits. |
Carry
out repairs to rectify the leak. |
|
Excessive clearance in a
bearing or bearings |
The
small bearing clearance places a restriction on the flow of oil that causes
the oil pressure. If the bearing clearance is excessive, the oil is less
restricted and its pressure will drop below normal. Usually, a bottom end
bearing will be the problem. |
Providing
the journal is all right, replace the bearing. |
|
Water in the oil |
Water
mixing with oil will result in emulsified oil. It is grey/white or sometimes
described as milky in colour. Emulsified oil loses its lubricating
properties. When a certain amount of emulsification takes place, the oil
pressure will drop below normal. |
Stop
the water leak and change the lubricating oil. |
|
Fuel in the oil |
Fuel
contamination will thin out the oil and it will run easily off the dip stick.
There will be a rise in the level in the sump. The dip stick will also have a
fuel smell. Fuel contaminated oil loses its lubricating properties and the
oil pressure will drop below normal. |
Stop
the fuel leak and change the lubricating oil. |
10.5
Engine overheating
An engine
overheating can be identified by the fresh water cooling temperature gauge, the
exhaust temperature and by the operator’s sense of touch. Experience can
develop a sense of touch by brushing the palm of a hand lightly over the parts
of the cooling water system. It enables the operator to know exactly at what
temperature each part works best and enables him or her to tell when there is a
difference in operating conditions. In determining the cause of an engine overheating,
consideration should be given as to whether it is a gradual process or there is
a sudden rise in fresh water temperature.
A gradual rise
is where the temperature rises over a period of time caused by:
Wear
Gradual build up of scale on the cooling
water surfaces;
Sea water strainer gradually becoming
clogged.
Sudden rise in
temperature could be caused by:
Thermostat stuck in the closed position
Pump impeller revolving on its shaft or
Engine overload.
When the engine
is hot and the fresh water level in the header tank is low, cold water should
be introduced very slowly whilst the engine is running. The cold water will
then be heated sufficiently before it circulates around the combustion space.
Cold water suddenly coming into contact with the hot cylinder liner and
cylinder head may crack them. The following should be considered:
|
Heat |
Comment |
Recommended |
|
Scale build up of on
cylinder water jackets, etc. |
Fresh
water contains impurities and they come out of solution at high temperatures
and will adhere to hot surfaces. The
hottest part of the engine is in the combustion space at the top of the
cylinder. Scale will deposit on the cylinder liner walls in this area, on the
passages to the cylinder head and around the exhaust valve. The scale will
stop the transfer of heat from the combustion process to the fresh water
cooling and, in the case of passages, will restrict the flow. This will be a
gradual process. |
Reduce
the engine speed until normal operating temperature is attained. The water
passages will have to be chemically cleaned to remove the deposits. |
|
Engine parts may be too
tight causing friction |
A
new or overhauled engine normally runs hotter because it is tight. As the
engine is run in, the high spots disappear and the engine turns easily. This
reduces the operating temperature. |
Reduce
the engine speed so that it runs at its normal operating temperature. |
|
Blown cylinder head gasket |
Leaking
between the cylinder and a cooling water passage will be indicated by bubbles
in the header tank whilst the engine is running. The extent of the leak will
determine the amount of bubbles. When checking for bubbles, remember the
above for pressurised and unpressurised systems. Whilst the engine is
running, the pressures inside the cylinder exceed that of the leak and the
water. The heat will turn the water into steam and be discharged with the
exhaust gases. However, when the engine is stopped, there is no pressure in
the cylinder. The header tank is above the cylinder thereby putting pressure
(a head) on the water. The water would then flow through the leak in the
cylinder head gasket into the cylinder. |
Should
the piston be below top dead centre, sufficient water could flow into the
cylinder and hydraulic it. The
water level would drop in the header tank and the procedure would be the same
as that above. However, remember that if the engine is stopped for a period
of time, it may hydraulic the cylinder. |
|
Low compression |
Low
compression causes the engine to overheat. Some of the heat in the combustion
gases by-passes the piston rings and goes into the crankcase. The cooling
water is not taking away the heat caused by combustion and overheating of the
engine occurs. |
Reduce
speed gradually until the normal operating temperature is attained. Rectify
the cause of low compression. |
|
Engine overloaded |
An
engine that is overloaded will overheat. An engine can be overloaded by a
dirty hull, a rope around the propeller, a bent propeller blade or too large
a pitch propeller. |
Reduce
speed until the normal operating temperature is attained. Clean the hull
growth, remove the rope from the propeller, straighten the propeller blade,
alter the propeller pitch or replace the propeller with one of the correct
pitch. |
|
Fuel injection into
cylinders may be too late |
The
timing of the fuel injector pump is out causing the fuel to be injected into
the cylinders too late. This will cause the engine to overheat. |
Reduce
speed until the normal operating temperature is attained. Stop engine and
adjust fuel pump timing. |
10.6 Excessive vibration, fluctuation of
engine revolutions
Vibration is a
back and forth movement and may only appear at certain revolutions of the
engine. It is usually the result of some component being out of balance like
the rotor shaft of a turbo charger.
The crankshaft
of an engine can be fitted with a vibration damper or harmonic balancer. They
reduce the quick increases in crankshaft speed caused by the firing impulses of
the cylinders and dampens out the crankshaft vibration.
Flexible engine
mounts that have lost their resilience will also cause vibration.
When the engine
is not lined up with the shafting (intermediate or propeller), vibration will
occur.
A damaged or
bent propeller blade or a rope caught around the propeller will cause
vibration.
A misfiring engine
will cause vibration.
Fluctuations in
the engine speed can be caused by the governor.
A hydraulic
governor should be checked for sufficient supply of oil, that dirt has not
entered the oil spaces, the linkages are not binding or have excessive wear.
A mechanical
governor should be checked that the lubricant is sufficient, and that the
linkages are not binding or have excessive wear. Fluctuations in the engine
speed can also be caused excessive engine load and the governor not being able
to cope.
Intermittent
misfiring by a faulty fuel injector can also cause speed fluctuation.
10.7 Crankcase explosions
In an engine
crankcase, there is fuel in the form of a mist of lubricating oil and oxygen.
Only heat is required for an explosion to occur. If there is fuel
contamination, the risk is higher. Heat can be supplied from a hot bearing,
combustion gases passing the piston rings (blow
by) or by partial seizure of a piston with its cylinder liner. When the
three components come together, an explosion will occur.
An engine may be
fitted with a crankcase relief valve to relieve any violent increase of
pressure in the crankcase. They are so constructed as to open smartly and close
promptly and decisively. Closing promptly ensures a minimum of fresh air enters
the crankcase which might cause a further explosion.
A hot bearing is
caused by insufficient clearance or lack of lubrication. The journal may have
to be ground. A new slipper bearing would be required.
Blow by is
caused by worn liner and/or piston rings. Replace the worn parts.
Partial seizure
of a piston with its cylinder liner is caused by overheating which also causes
a reduction of lubrication in the cylinder. Depending upon the degree of pick
up, a severe case will require the cylinder liner and piston to be replaced.
Engine oil
diluted by fuel will result in the oil thinning out and losing its viscosity.
Stop the fuel leak and replace the oil. As stated previously, oil diluted by
fuel increases the chances of a crankcase explosion.
Glossary
|
Term |
Definition / information |
|
Alternator |
A generator which produces electrical power. |
|
Amp meter |
Measures the amount of amps in an electrical system or
device. |
|
Back flooding |
Occurs when fluid passes in reverse through a pump or
valve back into the fluid reservoir or compartment. |
|
Bleeding |
A process of removing air from fluid lines or pipes. |
|
Circuit breakers |
A safety devise used in
electrical systems to cut the supply of electricity when a fault is evident. |
|
Cock |
A valve |
|
Combustion |
This process occurs when trapped air is compressed and
heated in the cylinder igniting fuel which is injected into the cylinder. |
|
Compression |
The pressure generated in the engine cylinder as the
piston rises forcing trapped air to the top of the cylinder. |
|
Condenser |
This unit cools refrigerant gas giving up heat to the
surrounding fresh air and returning the refrigerant to liquid state. |
|
Contamination |
Occurs when foreign materials or impurities enter the
fuel, oil or water. |
|
Drive train |
All the mechanical equipment from the gearbox to the
propeller including for example the propeller shaft, bearing, and couplings. |
|
Electrolyte |
A solution of sulphuric acid found in batteries. |
|
Evaporator |
This is a cooling unit where heat from the
refrigeration plant evaporates liquid refrigerant, turning it into gas. |
|
Exhaust gases |
Gases given off by the combustion process. |
|
Fuel injection |
The process where fuel is injected under pressure into
the engine cylinder. |
|
Fuel/oil ratio |
The mix of fuel and oil which is used in most two
stroke outboard engines. |
|
Gearbox |
A system of gears which alter the ratio between the
revolution of the engine and the propeller shaft. By using a gearbox the
engine and the propeller shaft will revolve at different speeds. |
|
Hydrometer |
A devise which measures electrolytes in battery acid. |
|
Intake rose |
A grid attached to the vessel’s hull which covers the
inlet for sea water into the cooling systems. |
|
Keel |
The bottom of a vessel. |
|
Keel cooler |
A water cooling system where hot engine water passes
through pipes which run under the vessel’s keel and are cooled by the sea
water. |
|
LPG |
Liquid petroleum gas (LPG) is an alternative fuel to
diesel or petrol. |
|
Lubrication |
The use of an agent such as oil, to prevent contact
between surfaces; reduce friction, wear and heat; and remove foreign
materials (metal shavings). |
|
Oily water separator |
This is a filter unit which separates oil and water. |
|
Outboard engine |
A self contained engine, gearbox and propulsion unit
which is attached externally to the vessel’s stern. |
|
Propeller shaft |
The steel shaft which runs from the gearbox to the
propeller. Sometimes referred to as the tailshaft. |
|
Refrigerant |
A gas/liquid material used in refrigeration systems to
absorb heat from the refrigeration plant. |
|
Relief valve |
A connection used in piping or hoses which is forced
open by fluid pressure to avoid any dangerous build up of pressure. |
|
Term |
Definition / information |
|
Hydrometer |
A devise which measures electrolytes in battery acid. |
|
Intake rose |
A grid attached to the vessel’s hull which covers the
inlet for sea water into the cooling systems. |
|
Keel |
The bottom of a vessel. |
|
Keel cooler |
A water cooling system where hot engine water passes
through pipes which run under the vessel’s keel and are cooled by the sea
water. |
|
LPG |
Liquid petroleum gas (LPG) is an alternative fuel to
diesel or petrol. |
|
Lubrication |
The use of an agent such as oil, to prevent contact
between surfaces; reduce friction, wear and heat; and remove foreign
materials (metal shavings). |
|
Oily water separator |
This is a filter unit which separates oil and water. |
|
Outboard engine |
A self contained engine, gearbox and propulsion unit
which is attached externally to the vessel’s stern. |
|
Propeller shaft |
The steel shaft which runs from the gearbox to the
propeller. Sometimes referred to as the tailshaft. |
|
Refrigerant |
A gas/liquid material used in refrigeration systems to
absorb heat from the refrigeration plant. |
|
Relief valve |
A connection used in piping or hoses which is forced
open by fluid pressure to avoid any dangerous build up of pressure. |
|
|
|
|
Rudder stock |
The rod which is attached to the rudder and is turned
to rotate the rudder. |
|
Sacrificial anodes |
Zinc blocks placed on the vessel’s hull, in the engine
or in the cooling systems so that the anodes corrode first, reducing the
amount of corrosion to the main component. |
|
Scale |
Residue or foreign materials which are left behind by
water (particularly sea water) and build up within the engine or in pipes. |
Sea water strainer |
A filtering system which captures foreign materials
from entering the cooling systems with fresh sea water. |
|
Short circuit |
Occurs when a positive and negative electrical current
intersect, bypassing the proper electrical route. |
|
Stern gland |
A packing which surrounds the propeller shaft as it
passes through the vessel’s hull and prevents sea water from entering the
vessel through the hole in the hull. |
|
Survey |
Inspection conducted by marine or waterways regulating
authorities on all commercial vessels. In order to pass survey a vessel must
meet certain standards and carry specific equipment such as fire safety
systems and life saving equipment. |
|
Thermostat |
A valve that opens or closes in response to
temperature. |
|
Throttle |
Hand accelerator unit on an outboard engine. |
|
Term |
Definition / information |
|
Tiller arm |
An arm extending from the rudder stock which can be
used to manually turn the rudder. |
|
Tube nest cooler |
A water cooling system where fresh sea water passes
through internal tubing, cooling hot engine water that flows around the
tubing. |
|
Turbo charger |
Uses the flow of exhaust gases to spin the blades on a
small turbine which in turn spins rotary air compressor blades forcing a
large mass of fresh air into the engine cylinders. This produces greater
combustion and increases power from the engine. |
|
Voltmeter |
Measures the amount of volts in an electrical systems or device. |
Acknowledgements
This text contains extracts & edits of material
courtesy of A.N.T.A. publications, GM Detroit, Cummins, and AMSA. The
assistance and guidance of Atef Jaber is gratefully acknowledged.
Ranger Hope © 2015