2 stroke slow speed

2 stroke sleeve

2 stroke crank

Uniflo and loop scavenging

	2 stroke engines with an exhaust valve mounted in the cylinder head are known as uniflow scavenged engines. This is because the flow of scavenging air is in one (uni) direction.
MAN B&W MC series uniflow scavenged engine

Some 2 stroke engines do not have exhaust valves; As well as scavenge ports in the cylinder liner, they are fitted with exhaust ports located just above the scavenge ports. As the piston uncovers the exhaust ports on the power stroke, the exhaust gas starts to leave the cylinder. When the scavenge ports are uncovered, scavenge air loops around the cylinder and pushes the remaining exhaust gas out of the cylinder. This type of engine is known as a loop scavenged engine. Note that the piston skirt is much longer than that for a uniflow scavenged engine. This is because the skirt has to seal the scavenge and exhaust ports when the piston is at TDC. Although simpler in construction with less moving parts, these engines are not as efficient or as powerful as uniflow scavenged engines. The scavenging of the cylinder is not 100%, and thus less fuel can be burnt per stroke. All modern large 2 stroke crosshead engines now being built are of the uniflow scavenged type.

SULZER RLA Loop Scavenged Engine

2stroke cycle

It may surprise you to learn that the biggest diesel engines in use operate on the two stroke principle. If you have experience of the two stroke petrol engine you will know that it causes more pollution and is less efficient than a 4 stroke petrol engine. This is because oil is mixed with the petrol to lubricate the crankshaft bearings, and a lot of unburnt petrol/oil/air mixture is discharged to the atmosphere. To learn more about the 2 stroke petrol engine cycle click here.

The two stroke Diesel engine does not mix fuel or oil with the combustion air. The crankshaft bearings are lubricated from pressurised oil in the same way as a four stroke engine.

The two stroke cycle is so called because it takes two strokes of the piston to complete the processes needed to convert the energy in the fuel into work. Because the engine is reciprocating, this means that the piston must move up and down the cylinder, and therefore the crankshaft must revolve once.

1. The crankshaft is revolving clockwise and the piston is moving up the cylinder, compressing the charge of air. Because energy is being transferred into the air, its pressure and temperature increase. By the time the piston is approaching the top of the cylinder (known as Top Dead Center or TDC) the pressure is over 100 bar and the temperature over 500°C	2. Just before TDC fuel is injected into the cylinder by the fuel injector. The fuel is "atomised" into tiny droplets. Because they are very small these droplets heat up very quickly and start to burn as the piston passes over TDC. The expanding gas from the fuel burning in the oxygen forces the piston down the cylinder, turning the crankshaft. It is during this stroke that work energy is being put into the engine; during the upward stroke of the piston, the engine is having to do the work.
3. As the piston moves down the cylinder, the useful energy from the burning fuel is expended. At about 110° after TDC the exhaust valve opens and the hot exhaust gas (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) begin to leave the cylinder.	4. At about 140º after TDC the piston uncovers a set of ports known as scavenge ports. Pressurised air enters the cylinder via these ports and pushes the remaining exhaust gas from the cylinder in a process known as "scavenging". The piston now goes past Bottom Dead Centre and starts moving up the cylinder, closing off the scavenge ports. The exhaust valve then closes and compression begins.

The two stroke cycle can also be illustrated on a timing diagram.

1 -2 Compression 2 - 3 Fuel Injection 3 - 4 Power 4 - 5 Exhaust Blowdown 5 - 6 Scavenging 6 - 1 Post Scavenging

1. approx 110º BTDC 2. approx 10º BTDC 3. approx 12º ATDC 4. approx 110º ATDC 5. approx 140º ATDC 6. approx 140º BTDC

In the 2 stroke trunk piston engine, the side thrust caused by the angularity of the connecting rod is transmitted to the liner by the piston skirt or trunk. It is therefore known as a 2 Stroke Trunk Piston Engine. The skirt of the piston also acts to seal the scavenge air ports when the engine is at TDC. This prevents the scavenge air from pressurising the crankcase.

Herein lies the disadvantage of this type of engine: although it has a low overall height, lubricating oil splashed up from the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and a risk of fire. There is also the likelihood of liner and piston skirt wear, allowing air into the crankcase. This can supply the required oxygen for an explosion should a hot spot develop. The crankcase oil must have additives which can cope with contamination from products of combustion, and the acids formed during combustion due to the sulphur in the fuel.

This design of two stroke is generally only used for the smaller lower powered 2 stroke engines - up to about 5000kW for a V16 engine with a 280mm bore and 320mm stroke.

Detroit diesels manufacture 2 stroke trunk piston engines as do Wichmann and General Motors. Sulzer used to produce a model which is sometimes found at sea as did Brons. A cross sectional drawing of their type GV engine is shown below.

2stroke crosshead engines

The 2 stroke crosshead engine works on exactly the same principle and cycle as the 2 stroke trunk piston engine.

The disadvantages of the two stroke trunk piston engine are that although it has a low overall height, lubricating oil splashed up from the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and a risk of fire. There is also the likelihood of liner and piston skirt wear, allowing air into the crankcase. This can supply the required oxygen for an explosion should a hot spot develop. The crankcase oil must have additives which can cope with contamination from products of combustion, and the acids formed during combustion due to the sulphur in the fuel.

The majority of 2 stroke engines encountered at sea are of the "crosshead" type. In this type of engine the combustion space (formed by the cylinder liner, piston and cylinder head), and the scavenge space are separated from the crankcase by the diaphragm plate. The piston rod is bolted to the piston and passes through a stuffing box mounted in the diaphragm plate. The stuffing box provides a seal between the two spaces, stopping oil from being carried up to the scavenge space, and scavenge air leaking into the crankcase. The foot of the piston rod is bolted to the crosshead pin. The top end of the connecting rod swings about the cosshead pin, as the downward load from the expanding gas applies a turning force to the crankshaft. To ensure that the crosshead reciprocates in alignment with the piston in the cylinder, guide shoes are attached either side of the crosshead pin. These shoes are lined with white metal, a bearing material and they reciprocate against the crosshead guides, which are bolted to the frame of the engine. The crosshead guides are located inbetween each cylinder. Using the crosshead design of engine allows engines to be built with very long strokes - which means the engine can burn a greater quantity of fuel/stroke and develop more power. The fuel used can be of a lower grade than that used in a trunk piston engine, with a higher sulphur content, whilst high alkalinity cylinder oils with a different specification to that of the crankcase oil are used to lubricate the cylinder liner and piston rings and combat the effects of acid attack.

The most powerful diesel engines in the world are two stroke crosshead engines. Some of these engines have cylinder bores approaching 1metre with a stroke of over 2.5 metres. The crankshaft can weigh over 300 tons, with the engine weighing in excess of 2000 tons

Turbo Charger

A two stroke crosshead engine must be supplied with air above atmospheric pressure for it to work. Although the first turbochargers were developed for aero engines in the first world war, it was not until the 1950s that large two stroke engines were turbocharged.

Before then the pressurised air needed to "scavenge" the cylinders of the exhaust gases and supply the charge of air for the next combustion cycle was provided by mechanically driven compressors (Roots Blowers), or by using the space under the piston as a reciprocating compressor (Under Piston Scavenging). This of course meant that the engine was supplying the work to compress the air, which meant that the useful work obtained from the engine was decreased by this amount.

Engine powers have increased phenomenally in the past 20 years. In 1980 an engine delivering 15000kW was a powerful engine. Today's largest engines are capable of delivering over 4 times this amount. This is due not only to improved materials and manufacturing techniques, but also to the improvements and developments in the design of the turbochargers fitted to these engines.

The amount of useful energy that an engine can produce is dependant on two factors; The amount of fuel that can be burnt per cycle and the efficiency of the engine.

Fuel consists mainly of Carbon and Hydrogen. By burning the fuel in oxygen the energy in the fuel is released and converted into work and heat. The more fuel that can be burnt per cycle, the more energy released.

However, to burn more fuel, the amount of air supplied must also be increased. For example, a 10 cylinder engine with a bore of 850mm and a stroke of 2.35m must burn 1kg of fuel per revolution to deliver 38500kW when running at 105 RPM. (assuming 50% efficiency). This means that each cylinder burns 0.1 kg fuel per stroke. To ensure that the fuel is burnt completely it is supplied with 200% more air than theoretically required. Because it takes about 14kg of air to supply the theoretical oxygen to burn 1kg of fuel, 4.2kg of air must be supplied into each cylinder to burn the 0.1kg of fuel.

A lot of this air is used up scavenging (clearing out) exhaust gas from the cylinder. The air also helps cool down the liner and exhaust valve. As the piston moves up the cylinder on the compression stroke and the exhaust valve closes, the cylinder must contain more than the theoretical mass of air to to supply the oxygen to burn the fuel completely (about 100% or 2.8kg)

2.8kg of air at atmospheric pressure and 25ºC occupies a volume of 2.4m³. The volume of the cylinder of the engine in our example is about 0.74m³ after the exhaust valve closes and compression begins. Because the temperature of the air delivered into the engine is raised to about 50ºC, it can be calculated that to supply the oxygen required for combustion, the air must be supplied at 3.5 × atmospheric pressure or 2.5 bar gauge pressure.

NOTE: These figures are approximate and for illustration only. Manufacturers quote the specific fuel oil consumption of their engines in g/kWh. These figures are obtained from testbed readings under near perfect conditions. Quoted figures range between 165 and 175g /kWh. The actual specific fuel consumption obtained is going to depend on the efficiency of the engine and the calorific value of the fuel used.

About 35% of the total heat energy in the fuel is wasted to the exhaust gases. The Turbocharger uses some of this energy (about 7% of the total energy or 20% of the waste heat) to drive a single wheel turbine. The turbine is fixed to the same shaft as a rotary compressor wheel. Air is drawn in, compressed and, because compression raises the temperature of the air, it is cooled down to reduce its volume. It is then delivered to the engine cylinders via the air manifold or scavenge air receiver. The speed of the turbocharger is variable depending on the engine load. At full power the turbocharger may be rotating at speeds of 10000RPM.

MATERIALS Gas Casing: Cast Iron (may be water cooled) Nozzle ring and blades: Chromium nickel alloy or a nimonic alloy. Compressor casing: Aluminium alloy Compressor Wheel: Aluminium alloy, titanium or stainless steel

STARTING THE ENGINE

Because the engine needs to be supplied with air when starting up and running at low speeds, an auxiliary blower powered by an electric motor is provided. This automatically cuts out when the charge air supplied by the turbocharger is sufficient to supply the engine on its own.

Tie Bolts

To understand the importance of the role played by the tie bolts, it is necessary to appreciate what is happening inside the cylinder of the engine.

When the piston is just after top dead centre the pressure inside the cylinder can rise as high as 140 bar (14000kN/m²). This acts downwards through the piston rod and con-rod, pushing the crankshaft down into the bearing pockets. At the same time, the pressure acts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into the entablature prevent this happening and so this upward acting force tries to lift the entablature from the frames and the frames from the bedplate, putting the fitted location bolts into tension.

As the piston moves down the cylinder the pressure in the cylinder falls, and then rises again as the piston changes direction and moves upwards on the compression stroke. This means that the fitted bolts are under are cyclic stress. Because they are not designed to withstand such stresses they would soon fail with disastrous consequences.

To hold the bedplate , frames and entablature firmly together in compression, and to transmit the firing forces back to the bedplate, long tie bolts are fitted through these three components and then tightened hydraulically. To prevent excessive bending moments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to hold the crankshaft main bearing cap in position instead of conventional studs and nuts.

Operating the engine with loose tiebolts will cause the fitted bolts holding the bedplate, frame and entablature in alignment to stretch and break. The machined mating surfaces will rub together and wear away ( this is known as fretting). Once this has happened the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause the transverse girders to bend which could lead to cracking, and main bearing misalignment.

Once fretting between the mating surfaces has occurred, then tightening of the tie bolts will pull the engine out of alignment. The crosshead guides, the cylinder liner, and the stuffing box will no longer be in line and excessive wear will occur. Because the tie bolts will no longer be pulled down squarely they will be subject to forces which may lead to them breaking. If fretting has occurred, then the only solution is to remove the entablature or/and frame and machine the fretted mating surfaces (a very costly exercise).

Tie bolts can break in service. To reduce the risk of this happening they must be checked for tightness; not overtightened; and the engine not overloaded. If a breakage does occur, this is not disastrous, as the engine can be operated with care for a limited period (the load on the engine may have to be reduced). The position of the fracture will dictate how the broken pieces are removed. However in the worst possible scenario where the bolt is broken at mid length, then the solution is to lift out the top half, remove the bottom nut, and then feed a loop of braided wire cable (about 7mm diameter) down the tie bolt tube, down the side of the broken tie bolt and once it emerges at the bottom a supporting piece can be fitted to the wire enabling the broken tie bolt to be withdrawn.

When checking the tightness of tie bolts, refer to manufacturers instructions for tightening pressures for the jacks and the order in which to carry out the check. The normal order is to start at the centre and work outwards checking the bolts in pairs. If the engine is fitted with bearing jacking bolts, then these must be slackened before tightening the tie bolts.

stuffing box

This photo of a split stuffing box showing the rings and springs was taken by Mr Dylan Wheel at the Ship Repair Exhibition at Olympia in Nov 2000. Thanks, Dylan

Because the crankcase is separated from the cylinder and scavenge space by the diaphragm plate on a two stroke crosshead engine, provision must be made for the piston rod to pass through the plate without oil from the crankcase being carried upwards, or used cylinder oil contaminated from products of combustion being carried downwards. It is also highly undesirable to allow the pressurized air in the scavenge space to leak into the crankcase.

The Piston rod passes through a stuffing box which is bolted into the diaphragm plate. The stuffing box casing which can be split vertically, as shown in the photo, contains a series of rings which are each made up of three or four segments. On the outside of each set of segments is a garter spring which provides the tension to hold the ring segments against the piston rod. There is a clearance between each segment to allow for wear. The rings are either bronze or can comprise of replaceable cast iron lamella fitted into a steel backing ring.

As the Piston rod passes up through the stuffing box, the oil from the crankcase is scraped off by the lower sets of rings and is returned via drillings to the crankcase. Any oil that passes this primary set is scraped off by another set of rings, and is led away through a drain to a tell tale open ended pipe into a tun dish outside the engine from where it drains to a recycling tank. As the piston passes down through the stuffing box, the top set of scraper rings will scrape off the contaminated oil into the bottom of the scavenge space, where it is drained away via the scavenge drains. However if these rings are faulty, then the oil may drain into the recycling tank.

By observing the open ended tell tale referred to above, a guide to the condition of the rings can be ascertained. If a large quantity of oil is draining out, then the lower set of rings are faulty. If air is blowing out, then the upper rings are worn.

Oil in the recycling tank can be purified back to the crankcase. However this is not necessarily a good idea. It may be contaminated by used cylinder oil which if mixed with crankcase oil causes an increase in viscosity of the crankcase oil. calcium deposits in the bearings lead to damage and the oil may carbonise and deposit on the underside of the piston crown when used as a piston coolant. Often this contaminated oil is just landed ashore or burnt.

Regular maintenance of the stuffing box will keep it in good condition. checking garter spring tension, ring butt and axial clearances, and replacing worn rings are all part of the overhaul procedure.

Excessive wear will take place if the crosshead guides are out of alignment or if the guide clearances are excessive. Worn stuffing boxes and excessive leakage can exacerbate the incidence of scavenge fires and increase the risk of a crankcase explosion.

scavenge fires

For a fire to begin there must be present a combustible material, oxygen or air to support combustion, and a source of heat at a temperature high enough to start combustion. In the case of scavenge fires the combustible material is oil. The oil can be cylinder oil which has drained down from the cylinder spaces, or crankcase oil carried upwards on the piston rod because of a faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinders and other similar causes. The oxygen necessary for combustion comes from the scavenge air which is in plentiful supply for the operation of the engines. The source of heat for ignition comes from piston blowby, slow ignition and afterburning, or excessive exhaust back pressure, which causes a blowback through the scavenge ports.

INDICATIONS

Indications of a scavenge fire are loss in power and irregular running of the engine, high exhaust temperatures of corresponding units, high local temperature in scavenge trunk, surging of turbocharger, and sparks and smoke emitted from scavenge drains. External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles. If the scavenge trunk is oily the fire may spread back-from the space around or adjacent to the cylinders where the fire started and will show itself as very hot spots or areas of the scavenge trunk surfaces. In ships where the engine room is designed as UMS, temperature sensors are fitted at critical points within the scavenge spaces. activation would cause automatic slow down of the engine.

ACTION

If a scavenge fire starts two immediate objectives arise- they are to contain the fire within the scavenge space of the engine and to prevent or minimize damage to the engine. The engine must be put to dead slow ahead and the fuel must be taken off the cylinders affected by the fire. The lubrication to these cylinders must be increased to prevent seizure and all scavenge drains must be shut to prevent the discharge of sparks and burning oil from the drains into the engine room. A minor fire may shortly burn out without damage, and conditions will gradually return to normal. The affected units should be run on reduced power until inspection of the scavenge trunking and overhaul of the cylinder and piston can be carried out at the earliest safe opportunity. Once navigational circumstances allow it, the engine should be stopped and the whole of the scavenge trunk examined and any oil residues found round other cylinders removed. The actual cause of the initiation of the fire should be investigated

CO2 Scavenge Fire Extinguishing Installation

If the scavenge fire is of a more major nature, if there is a risk of the fire extending or if the scavenge trunk is adjacent to the crankcase with risk of a hot spot developing it sometimes becomes necessary to stop the engine. Normal cooling is maintained, and the turning gear engaged and operated. Fire extinguishing medium should be applied through fittings in the scavenge trunk: these may inject carbon dioxide, dry powder or smothering steam. The fire is then extinguished before it can spread to surfaces of the scavenge trunk where it may cause the paint to start burning if special non inflammable paint has not been used. Boundary cooling of the scavenge trunk may be necessary. Keep clear of scavenge relief valves, and do not open up for inspection until the engine has cooled down.

After extinguishing the fire and cooling down, the scavenge trunking and scavenge ports should be cleaned and the trunking together with cylinder liner and water seals, piston, piston rings, piston skirt, piston rod and gland must be inspected. Heat causes distortion and therefore checks for binding of piston rod in stuffing box and piston in liner must be carried out. Tightness of tie bolts should be checked before restarting the engine. Inspect reed valves if fitted, and scavenge relief valve springs. Fire extinguishers should be recharged at the first opportunity and faults diagnosed as having caused the fire must be rectified.

PREVENTION

To prevent scavenge fires good maintenance and correct adjustment must be carried out. Scavenge trunking must be periodically inspected and cleaned and any buildup of contamination noted and remedied. The drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludges which do not drain down so easily and which are a common cause of choked drain pipes. Scavenge drains should be blown regularly and any passage of oil from them noted. The piston rings must be properly maintained and lubricated adequately so that ring blow-by is prevented. At the same time one must guard against excess cylinder oil usage. With timed cylinder oil injection the timing should be periodically checked. Scavenge ports must be kept cleared

The piston-rod packing rings and scraper rings should also be regularly adjusted so that oil is prevented from entering the scavenge space because of butted ring segments. This may and does occur irrespective of the positive pressure difference between the scavenge trunk and the crankcase space.

Fuel injection equipment must be kept in good condition, timed correctly, and the mean indicated pressure in each cylinder must also be carefully balanced so that individual cylinders are not overloaded.

If cylinder liner wear is up to maximum limits the possibility of scavenge fires will not be materially reduced until the liners are renewed.

piston

The Piston comprises of two pieces; the crown and the skirt. The crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures. Steel, alloyed with chromium and molybdenum is used, and some pistons have a special alloy welded onto the hottest part of the crown to try and reduce the erosion caused by the burning fuel. The crown also carries the 4 or 5 piston ring grooves which may be chrome plated. The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides). A forged steel piston rod is bolted to the underside of the piston. The other end of the piston rod is attached to the crosshead pin. Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.

Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return.

An alternative method of cooling uses a nozzle plate and nozzles. Note that the oil goes up the annular space formed between the oil tube and the bore in the piston rod, and returns down the centre. The oil is sprayed up matching bores onto the underside of the crown. This allows the crown to be made as thin as possible, to allow for maximum heat transfer while maintaining strength, and combined with the "cocktail" shaker effect caused by the reciprocating motion, gives efficient cooling.

When overhauling the piston it is important to check the thinning of the piston crown due to burning/erosion/corrosion. The piston should be dismantled to check the cooling space. If this is subject to a build up of carbon (in an oil cooled piston) or scale (in the case of a water cooled piston) then this may have led to thermal stressing of the piston, which in its turn can lead to cracking of the piston crown. If the cooling oil is allowed to leak into the combustion space then the consequences could be disastrous.

PISTON RINGS

The Piston Rings are made of alloying cast iron with chromium, molybdenum, vanadium, titanium, nickel and copper. They are harder than the cylinder liner in which they run to give them a maximum life.

Piston rings seal the gas space by expanding outwards due to the gas pressure acting behind them. They also spread the lubricating oil up and down the cylinder liner and transfer heat to the liner walls.

When overhauling the piston it is important to check the ring grooves for wear and the piston ring condition. The axial and butt clearances should be measured and recorded.

man b&w cam reversing

The fuel pump cam on the MAN B&W MC series engine is designed to raise the plunger on the injection stroke and then keep the plunger at the top of its stroke while the follower stays on the peak of the cam until just before the next delivery stroke when the follower returns to the base circle of the cam, and the fuel pump plunger moves down on its suction stroke.

The animation on the left shows the cam follower just beginning to move up the slope of the cam with the camshaft rotating in anticlockwise direction. (i.e. start of injection)

If the engine direction is reversed at this point, then air will enter the pneumatic cylinder as shown and will move the piston to the right. The cam follower will be moved across and would finish in the position shown which would be at the correct fuel pump timing for running astern.

It should be noted that the reversal of the follower only takes place while the engine is rotating. If the engine had been stopped from running ahead, and then started astern, the fuel pump followers would move across as the engine starts to rotate, and before the fuel is admitted by venting the fuel pump puncture valves.

A micro switch shown on the LHS detects whether the follower has moved across. If not, an indicator light is lit in the control room, However the engine will still start if a follower fails to move, perhaps due to corrosion in the servo cylinder. A high exhaust temperature deviation alarm would operate within a short time. Allowing the engine to start in this situation could be useful during manoeuvring in confined waters.

lost motion

On a two stroke engine, the fuel pumps must be retimed when the engine is required to turn in the opposite direction (i.e. astern).

If one cylinder of the engine is considered (left), the piston is just before TDC with the engine running ahead and the crankshaft rotating clockwise. The piston is moving up towards TDC. The picture on the right shows the fuel cam at this point; where the cam follower is rising up the lift of the cam as it rotates clockwise. This point can be considered as the start of injection.

The fuel pump cam follower is moving up the rise of the cam on the delivery stroke. The cam is correctly in time with the engine

If, at this point the engine is stopped, and is started in the reverse direction (astern), the crankshaft now moves in an anticlockwise direction. Then the piston in this particular unit is now moving down the cylinder and is just after TDC. At this point fuel injection should have just finished. However, by studying the picture of the cam (right) it can be seen that the camshaft has reversed direction (because it is directly driven from the crankshaft), and is also rotating anticlockwise. In the picture the follower is moving down the cam which means the fuel pump plunger is just finishing the suction stroke; i.e completely out of time with the engine.

Here the fuel pump cam is in the wrong position. When the piston is just after TDC, fuel delivery should have finished and the follower should be approaching the peak of the cam.

So that the Fuel Pump cam is timed correctly with the crankshaft when the engine is reversed, the fuel cams are rotated by a hydraulic servomotor which changes the position of the cams relative to the crankshaft. The angle through which the cams are turned is known as the Lost Motion angle. Although this can be made to happen when the engine is still rotating, it is probably easier to think of the engine stopped as shown left and the camshaft moving as shown on the below. Once the fuel cams have moved, the engine can then start running in the reverse direction (anticlockwise). Because the engine is started using compressed air admitted through the air start valves, the operating mechanism for these must also be retimed.

The angle that the cams move through is the lost motion angle.