The incoming air is pressurised by a turbo-blower which is driven by the outgoing exhaust gases. The crankshaft is supported within the engine bedplate by the main bearings. A-frames are mounted on the bedplate and house guides in which the crosshead travels up and down.
The entablature is mounted above the frames and is made up of the cylinders, cylinder heads and the scavenge trunking. Comparison of two-stroke and four-stroke cycles The main difference between the two cycles is the power developed.
The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1. For a particular engine power the two-stroke engine will be considerably lighter—an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke.
The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil. Each type of engine has its applications which on board ship have resulted in the slow speed i. At this low speed the engine requires no reduction gearbox between it and the propeller.
There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator. The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake. The engine indicator An engine indicator is shown in Figure 2. A magnifying linkage transfers the piston movement to a drum on which is mounted a piece of paper or card.
The drum oscillates moves backwards and forwards under the pull of the cord. The cord is moved by a reciprocating up and down mechanism which is proportional to the engine piston movement in the cylinder. The stylus draws out an indicator diagram which represents the gas pressure on the engine piston at different points of the stroke, and the area of the indicator diagram produced represents the power developed in the particular cylinder.
The cylinder power can be measured if the scaling factors, spring calibration and some basic engine details are known. The procedure is described in the Appendix.
The cylinder power values are compared, and for balanced loading should all be the same. Adjustments may then be made to the fuel supply in order to balance the cylinder loads.
Torsionmeter If the torque transmitted by a shaft is known, together with the angular velocity, then the power can be measured, i. A number of different types of torsionmeter are described in Chapter Diesel engines 17 The gas exchange process A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process.
Scavenging is the removal of exhaust gases by blowing in fresh air. Charging is the filling of the engine cylinder with a supply or charge of fresh air ready for compression. With supercharging a large mass of air is supplied to the cylinder by blowing it in under pressure. Older engines were 'naturally aspirated'—taking fresh air only at atmospheric pressure. Modern engines make use of exhaust gas driven turbo- chargers to supply pressurised fresh air for scavenging and supercharg- ing.
Both four-stroke and two-stroke cycle engines may be pressure charged. On two-stroke diesels an electrically driven auxiliary blower is usually provided because the exhaust gas driven turboblower cannot provide enough air at low engine speeds, and the pressurised air is usually cooled to increase the charge air density.
An exhaust gas driven turbochargmg arrangement for a slow-speed two-stroke cycle diesel is shown in Figure 2. A turboblower or turbocharger is an air compressor driven by exhaust gas Figure 2. The single shaft has an exhaust gas turbine on one end and the air compressor on the other. Suitable casing design and shaft seals ensure that the two gases do not mix. Air is drawn from the machinery space through a filter and then compressed before passing to the scavenge space.
The exhaust gas may enter the turbine directly from the engine or from a constant-pressure chamber. Each of the shaft bearings has its own independent lubrication system, and the exhaust gas end of the casing is usually water-cooled.
Scavenging Efficient scavenging is essential to ensure a sufficient supply of fresh air for combustion. In the four-stroke cycle engine there is an adequate overlap between the air inlet valve opening and the exhaust valve closing.
With two-stroke cycle engines this overlap is limited and some slight mixing of exhaust gases and incoming air does occur.
A number of different scavenging methods are in use in slow-speed two-stroke engines. In each the fresh air enters as the inlet port is opened by the downward movement of the piston and continues until the port is closed by the upward moving piston. The flow path of the scavenge air is decided by the engine port shape and design and the exhaust arrangements.
Three basic systems are in use: the cross flow, the loop and the uniflow. All modern slow-speed diesel engines now use the uniflow scavenging system with a cylinder-head exhaust valve. The exhaust gases then travel down and out of the exhaust ports. Figure 2. In loop scavenging the incoming air passes over the piston crown then rises towards the cylinder head.
The exhaust gases are forced before the air passing down and out of exhaust ports located just above the inlet ports. The process is shown in Figure 2. With uniflow scavenging the incoming air enters at the lower end of the cylinder and leaves at the top. The outlet at the top of the cylinder may be ports or a large valve.
Each of the systems has various advantages and disadvantages. Cross scavenging requires the fitting of a piston skirt to prevent air or exhaust gas escape when the piston is at the top of the stroke. Uniflow is the most efficient scavenging system but requires either an opposed piston arrangement or an exhaust valve in the cylinder head. All three systems have the ports angled to swirl the incoming air and direct it in the appropriate path. Scavenge fires Cylinder oil can collect in the scavenge space of an engine.
Unburned fuel and carbon may also be blown into the scavenge space as a result of defective piston rings, faulty timing, a defective injector, etc. A build-up of this flammable mixture presents a danger as a blow past of hot gases from the cylinder may ignite the mixture, and cause a scavenge fire. A loss of engine power will result, with high exhaust temperatures at the affected cylinders. The affected turbo-chargers may surge and sparks will be seen at the scavenge drains.
Once a fire is detected the engine should be slowed down, fuel shut off from the affected cylinders and cylinder lubrication increased. All the scavenge drains should be closed. A small fire will quickly burn out, but where the fire persists the engine must be stopped. A fire extinguishing medium should then be injected through the fittings provided in the scavenge trunking.
On no account should the trunking be opened up. To avoid scavenge fires occurring the engine timing and equipment maintenance should be correctly carried out.
The scavenge trunking should be regularly inspected and cleaned if necessary. Where carbon or oil build up is found in the scavenge, its source should be detected and the fault remedied. Scavenge drains should be regularly blown and any oil discharges investigated at the first opportunity. Fuel oil system The fuel oil system for a diesel engine can be considered in two parts—the fuel supply and the fuel injection systems.
Fuel supply deals with the provision of fuel oil suitable for use by the injection system. Fuel oil supply for a two-stroke diesel A slow-speed two-stroke diesel is usually arranged to operate con- tinuously on heavy fuel and have available a diesel oil supply for manoeuvring conditions.
In the system shown in Figure 2. From the daily service tank the oil flows through a three-way valve to a mixing tank. A flow meter is fitted into the system to indicate fuel consumption. Booster pumps are used to pump the oil through heaters and a viscosity regulator to the engine-driven fuel pumps. The fuel pumps will discharge high-pressure fuel to their respective injectors. The viscosity regulator controls the fuel oil temperature in order to provide the correct viscosity for combustion.
A pressure regulating valve ensures a constant-pressure supply to the engine-driven pumps, and a pre-warming bypass is used to heat up the fuel before starting the engine. A diesel oil daily service tank may be installed and is connected to the system via a three-way valve.
The engine can be started up and manoeuvred on diesel oil or even a blend of diesel and heavy fuel oil. The mixing tank is used to collect recirculated oil and also acts as a buffer or reserve tank as it will supply fuel when the daily service tank is empty.
The system includes various safety devices such as low-level alarms and remotely operated tank outlet valves which can be closed in the event of a fire. Fuel injection The function of the fuel injection system is to provide the right amount of fuel at the right moment and in a suitable condition for the combustion process.
There must therefore be some form of measured fuel supply, a means of timing the delivery and the atomisation of the fuel.
The injection of the fuel is achieved by the location of cams on a camshaft. This camshaft rotates at engine speed for a two-stroke engine and at half engine speed for a four-stroke. There are two basic systems in use, each of which employs a combination of mechanical and hydraulic operations. The most common system is the jerk pump; the other is the common rail. Jerk pump system In the jerk pump system of fuel injection a separate injector pump exists for each cylinder.
The injector pump is usually operated once every cycle by a cam on the camshaft. The barrel and plunger of the injector pump are dimensioned to suit the engine fuel requirements. Ports in the barrel and slots in the plunger or adjustable spill valves serve to regulate the fuel delivery a more detailed explanation follows.
Each injector pump supplies the injector or injectors for one cylinder. There are two particular types of fuel pump in use, the valve- controlled discharge type and the helix or helical edge pump. Valve-controlled pumps are used on slow-speed two-stroke engines and the helix type for all medium- and high-speed four-stroke engines. Helix-type injector pump The injector pump is operated by a cam which drives the plunger up and down.
The timing of the injection can be altered by raising or lowering the pump plunger in relation to the cam. The pump has a constant stroke and the amount of fuel delivered is regulated by rotating the pump plunger which has a specially arranged helical groove cut into it. The fuel is supplied to the pump through ports or openings at B Figure 2.
As the plunger moves down, fuel enters the cylinder. As the plunger moves up, the ports at B are closed and the fuel is pressurised and delivered to the injector nozzle at very high pressure. When the edge of the helix at C uncovers the spill port D pressure is lost and fuel delivery to the injector stops.
A non-return valve on the delivery side of the pump closes to stop fuel oil returning from the injector. Fuel will again be drawn in on the plunger downstroke and the process will be repeated.
The plunger may be rotated in the cylinder by a rack and pinion arrangement on a sleeve which is keyed to the plunger. This will move the edge C up or down to reduce or increase the amount of fuel pumped into the cylinder. The rack is connected to the throttle control or governor of the engine. This type of pump, with minor variations, is used on many four-stroke diesel engines.
Two linkages are actuated by the regulating shaft of the governor. The upper control linkage changes the injection timing by raising or lowering the plunger in relation to the cam. The lower linkage rotates the pump plunger and thus the helix in order to vary the pump output Figure 2.
In the Sulzer variable injection timing system the governor output is connected to a suction valve and a spill valve. The closing of the pump suction valve determines the beginning of injection. No helix is therefore present on the pump plunger. Common rail system The common rail system has one high-pressure multiple plunger fuel pump Figure 2. The fuel is discharged into a manifold or rail which is maintained at high pressure. From this common rail fuel is supplied to all the injectors in the various cylinders.
Between the rail and the injector or injectors for a particular cylinder is a timing valve which determines the timing and extent of fuel delivery. Spill valves are connected to the manifold or rail to release excess pressure and accumulator bottles which dampen out pump pressure pulses.
The injectors in a common rail system are often referred to as fuel valves. When the timing valve is lifted by the cam and lever the high-pressure fuel flows to the injector. The timing valve operating lever is fixed to a sliding rod which is positioned according to the manoeuvring lever setting to provide the correct fuel quantity to the cylinder. The high-pressure fuel enters and travels down a passage in the body and then into a passage in the nozzle, ending finally in a chamber surrounding the needle valve.
The needle valve is held closed on a mitred seat by an intermediate spindle and a spring in the injector body. The nozzle and injector body are manufactured as a matching pair and are accurately ground to give a good oil seal.
The two are joined by a nozzle nut. The needle valve will open when the fuel pressure acting on the needle valve tapered face exerts a sufficient force to overcome the spring compression. The fuel then flows into a lower chamber and is forced out through a series of tiny holes.
The small holes are sized and arranged to atomise, or break into tiny drops, all of the fuel oil, which will then readily burn. Once the injector pump or timing valve cuts off the high pressure fuel supply the needle valve will shut quickly under the spring compression force. All slow-speed two-stroke engines and many medium-speed four- stroke engines are now operated almost continuously on heavy fuel.
A fuel circulating system is therefore necessary and this is usually arranged within the fuel injector. During injection the high-pressure fuel will open the circulation valve for injection to take place. When the engine is stopped the fuel booster pump supplies fuel which the circulation valve directs around the injector body. Older engine designs may have fuel injectors which are circulated with cooling water.
Diesel engines 29 Lubrication The lubrication system of an engine provides a supply of lubricating oil to the various moving parts in the engine. Its main function is to enable the formation of a film of oil between the moving parts, which reduces friction and wear. The lubricating oil is also used as a cleaner and in some engines as a coolant. Lubricating oil system Lubricating oil for an engine is stored in the bottom of the crankcase, known as the sump, or in a drain tank located beneath the engine Figure 2.
The oil is drawn from this tank through a strainer, one of a pair of pumps, into one of a pair of fine filters. It is then passed through a cooler before entering the engine and being distributed to the various branch pipes. The branch pipe for a particular cylinder may feed the main bearing, for instance. Some of this oil will pass along a drilled passage in the crankshaft to the bottom end bearing and then up a drilled passage in the connecting rod to the gudgeon pin or crosshead bearing.
An alarm at the end of the distribution pipe ensures that adequate pressure is maintained by the pump. The fine filters will be arranged so that one can be cleaned while the other is operating. After use in the engine the lubricating oil drains back to the sump or drain tank for re-use.
A level gauge gives a local read-out of the drain tank contents. A centrifuge is arranged for cleaning the lubricating oil in the system and clean oil can be provided from a storage tank. The oil cooler is circulated by sea water, which is at a lower pressure than the oil.
As a result any leak in the cooler will mean a loss of oil and not contamination of the oil by sea water. Where the engine has oil-cooled pistons they will be supplied from the lubricating oil system, possibly at a higher pressure produced by booster pumps, e. Sulzer RTA engine. An appropriate type of lubricating oil must be used for oil-lubricated pistons in order to avoid carbon deposits on the hotter parts of the system.
Cylinder lubrication Large slow-speed diesei engines are provided with a separate lubrication system for the cylinder liners. Oil is injected between the liner and the piston by mechanical lubricators which supply their individual cylinder, A special type of oil is used which is not recovered. As well as lubricating, it assists in forming a gas seal and contains additives which clean the cylinder liner. Cooling Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine.
The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail.
Cooling enables the engine metals to retain their mechanical properties. The usual coolant used is fresh water: sea water is not used directly as a coolant because of its corrosive action. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problems.
As a result of its lower specific heat however about twice the quantity of oil compared to water would be required. Fresh water cooling system A water cooling system for a slow-speed diesei engine is shown in Figure 2. It is divided into two separate systems: one for cooling the cylinder jackets, cylinder heads and turbo-blowers; the other for piston cooling.
Diesel engines 31 Se» water in Figure 2. It is then pumped around the cylinder jackets, cylinder heads and turbo-blowers. A header tank allows for expansion and water make-up in the system. Vents are led from the engine to the header tank for the release of air from the cooling water. A heater in the circuit facilitates warming of the engine prior to starting by circulating hot water.
The piston cooling system employs similar components, except that a drain tank is used instead of a header tank and the vents are then led to high points in the machinery space. A separate piston cooling system is used to limit any contamination from piston cooling glands to the piston cooling system only.
Sea water cooling system The various cooling liquids which circulate the engine are themselves cooled by sea water. The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by sea water.
Some modern ships use what is known as a 'central cooling system' with only one large sea-water-circulated cooler. With less equipment in contact with sea water the corrosion problems are much reduced in this system. A sea water cooling system is shown in Figure 2. From the sea suction one of a pair of sea-water circulating pumps provides sea water which circulates the lubricating oil cooler, the jacket water cooler and the piston water cooler before discharging overboard.
Another branch of the sea water main provides sea water to directly cool the charge air for a direct-drive two-stroke diesel. One arrangement of a central cooling system is shown in Figure 2. The sea water circuit is made up of high and low suctions, usually on either side of the machinery space, suction strainers and several sea water pumps.
The sea water is circulated through the central coolers and then discharged overboard. A low-temperature and high-temperature circuit exist in the fresh water system. The fresh water in the high-temperature circuit circulates the main engine and may, if required, be used as a heating medium for an evaporator. The low-temperature circuit circulates the main engine air coolers, the lubricating oil coolers and all other heat exchangers.
A regulating valve controls the mixing of water between the high-temperature and low-temperature circuits. A temperature sensor is also used in a similar control circuit to operate the regulating valve which controls the bypassing of the central coolers. It is also possible, with appropriate control equipment, to vary the quantity of sea water circulated by the pumps to almost precisely meet the cooler requirements.
Starting air system Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use.
Up to 12 starts are possible with the stored quantity of compressed air. The starting air system usually has interlocks to prevent starting if everything is not in order. A starting air system is shown in Figure 2. Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system.
The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever.
When the air start lever is operated, a supply of pilot air enables the remote valve to open. This device is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder.
An interlock is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged. The remote operating valve prevents the return of air which has been further compressed by the engine into the system. Lubricating oil from the compressor will under normal operation pass along the air lines and deposit on them.
In the event of a cylinder air starting valve leaking, hot gases would pass into the air pipes and ignite the lubricating oil. If starting air is supplied to the engine this would further feed the fire and could lead to an explosion in the pipelines. In order to prevent such an occurrence, cylinder starting valves should be properly maintained and the pipelines regularly drained. Also oil discharged from compressors should be kept to a minimum, by careful maintenance.
In an attempt to reduce the effects of an explosion, flame traps, relief valves and bursting caps or discs are fitted to the pipelines. In addition an isolating non-return valve the automatic valve is fitted to the system. The loss of cooling water from an air compressor could lead to an overheated air discharge and possibly an explosion in the pipelines leading to the air reservoir.
A high-temperature alarm or a fusible plug which will melt is used to guard against this possibility. Control and safety devices Governors The principal control device on any engine is the governor.
It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
Governors for diesel engines are usually made up of two systems: a speed sensing arrangement and a hydraulic unit which operates on the fuel pumps to change the engine power output. Mechanical governor A flyweight assembly is used to detect engine speed. Two flyweights are fitted to a plate or ballhead which rotates about a vertical axis driven by a gear wheel Figure 2.
The equilibrium position or set speed may be changed by the speed selector which alters the spring compression. As the engine speed increases the weights move outwards and raise the spindle; a speed decrease will lower the spindle.
The hydraulic unit is connected to this vertical spindle and acts as a power source to move the engine fuel controls. A piston valve connected to the vertical spindle supplies or drains oil from the power piston which moves the fuel controls depending upon the flyweight movement. This reduces fuel supply to the engine and slows it down. It is, in effect, a proportional controller see Chapter The actual arrangement of mechanical engine governors will vary considerably but most will operate as described above.
Electric governor The electric governor uses a combination of electrical and mechanical components in its operation. The speed sensing device is a small magnetic pick-up coil.
The rectified, or d. This unit will then move the fuel controls in the appropriate direction to control the engine speed. A spring holds the valve closed and its lifting pressure is set by an appropriate thickness of packing piece Figure 2.
Only a small amount of lift is permitted and the escaping gases are directed to a safe outlet. The valve and spindle are separate to enable the valve to correctly seat itself after opening. The operation of this device indicates a fault in the engine which should be discovered and corrected. The valve itself should then be examined at the earliest opportunity.
Crankcase oil mist detector The presence of an oil mist in the crankcase is the result of oil vaporisation caused by a hot spot. Explosive conditions can result if a build up of oil mist is allowed. The oil mist detector uses photoelectric cells to measure small increases in oil mist density.
A motor driven fan continuously draws samples of crankcase oil mist through a measuring tube. An increased meter reading and alarm will result if any crankcase sample contains excessive mist when compared to either clean air or the other crankcase compartments. The rotary valve which draws the sample then stops to indicate the suspect crankcase.
The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air. The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for crosshead type engines and the level model for trunk piston engines. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
Various designs and arrangements of these valves exist where, on large slow-speed diesels, two door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used. One design of explosion relief valve is shown in Figure 2. A deflector is fitted on the outside of the engine to safeguard personnel from the outflowing gases, and inside the engine, over the valve opening, an oil wetted gauze acts as a flame trap to stop any flames leaving the crankcase.
After operation the valve will close automatically under the action of the spring. Turning gear The turning gear or turning engine is a reversible electric motor which drives a worm gear which can be connected with the toothed flywheel to turn a large diesel. A slow-speed drive is thus provided to enable positioning of the engine parts for overhaul purposes. The turning gear is also used to turn the engine one or two revolutions prior to starting.
This is a safety check to ensure that the engine is free to turn and that no water has collected in the cylinders. The indicator cocks must always be open when the turning gear is operated. Medium- and slow-speed diesels Medium-speed diesels, e. It provides high powers, can burn low-grade fuels and has a high thermal efficiency. The cylinders and crankcase are isolated, which reduces contamination and permits the use of specialised lubricating oils in each area. The use of the two-stroke cycle usually means there are no inlet and exhaust valves.
This reduces maintenance and simplifies engine construction. Medium-speed four-stroke engines provide a better power-to-weight ratio and power-to-size ratio and there is also a lower initial cost for equivalent power. The higher speed, however, requires the use of a gearbox and flexible couplings for main propulsion use. Cylinder sizes are smaller, requiring more units and therefore more maintenance, but the increased speed partly offsets this. Cylinder liners are of simple construction since there are no ports, but cylinder heads are more complicated and valve operating gear is required.
Scavenging is a positive operation without use of scavenge trunking, thus there can be no scavenge Fires. Better quality fuel is necessary because of the higher engine speed, and lubricating oil consumption is higher than for a slow-speed diesel. Engine height is reduced with trunk piston design and there are fewer moving parts per cylinder. There are, however, in total more parts for maintenance, although they are smaller and more manageable.
The Vee engine configuration is used with some medium-speed engine designs to further reduce the size and weight for a particular power. Couplings, clutches and gearboxes Where the shaft speed of a medium-speed diesel is not suitable for its application, e.
Between the engine and gearbox it is usual to fit some form of flexible coupling to dampen out vibrations. There is also often a need for a clutch to disconnect the engine from the gearbox. Couplings Elastic or flexible couplings allow slight misalignment and damp out or remove torque variations from the engine. The coupling may in addition function as a clutch or disconnecting device.
Couplings may be mechanical, electrical, hydraulic or pneumatic in operation. It is usual to combine the function of clutch with a coupling and this is not readily possible with the mechanical coupling.
Diesel engines 4i Clutches A clutch is a device to connect or separate a driving unit from the unit it drives. With two engines connected to a gearbox a clutch enables one or both engines to be run, and facilitates reversing of the engine.
The hydraulic or fluid coupling uses oil to connect the driving section or impeller with the driven section or runner Figure 2. No wear will thus take place between these two, and the clutch operates smoothly. The runner and impeller have pockets that face each other which are filled with oil as they rotate. The engine driven impeller provides kinetic energy to the oil which transmits the drive to the runner. Thrust bearings must be provided on either side of the coupling because of the axial thrust developed by this coupling.
A plate-type clutch consists of pressure plates and clutch plates arranged in a clutch spider Figure 2. A forward and an aft clutch assembly are provided, and an externally mounted selector valve assembly is the control device which hydraulically engages the desired clutch. The forward clutch assembly is made up of the input shaft and the forward clutch spider. The input shaft includes the forward driven gear and, at its extreme end, a hub with the steel pressure plates of the Circulation vortex Recirculating Shrouding oil drain cover holes Figure 2.
Thus when the input shaft turns, the forward driven gear and the forward clutch pressure plates will rotate. The forward clutch plates are positioned between the pressure plates and are spline-connected to the forward clutch spider or housing. This forward clutch spider forms part of the forward pinion assembly which surrounds but does not touch the input shaft. The construction of the reverse clutch spider is similar. Both the forward and reverse pinions are in constant mesh with the output gear wheel which rotates the output shaft.
In the neutral position the engine is rotating the input shaft and both driven gear wheels, but not the output shaft. When the clutch selector valve is moved to the ahead position, a piston assembly moves the clutch plates and pressure plates into contact. A friction grip is created between the smooth pressure plate and the clutch plate linings and the forward pinion rotates. The forward pinion drives the output shaft and forward propulsion will occur.
The procedure when the selector valve is moved to the astern position is similar but now the reverse pinion drives the output shaft in the opposite direction. Reduction ratios range from about to on modern installations. Pinion and gearwheel arrangements will be similar to those for steam turbines as described in Chapter 3, except that they will be single helical or epicyclic. Reversing Where a gearbox is used with a diesel engine, reversing gears may be incorporated so that the engine itself is not reversed.
Where a controllable pitch propeller is in use there is no requirement to reverse the main engine. However, when it is necessary to run the engine in reverse it must be started in reverse and the fuel injection timing must be changed.
Where exhaust timing or poppet valves are used they also must be retimed. With jerk-type fuel pumps the fuel cams on the camshaft must be repositioned. This can be done by having a separate reversing cam and moving the camshaft axially to bring it into position. Alternatively a lost-motion clutch may be used in conjunction with the ahead pump-timing cam. The fuel pump cam and lost-motion clutch arrangement is shown in Figure 2. A period of dwell then occurs when the fuel pump plunger does not move.
A fully reversible cam will be symmetrical about this point, as shown. The angular period between the top dead centre points for ahead and astern running will be the 'lost motion' required for astern running. The lost-motion clutch or servo motor uses a rotating vane which is attached to the camshaft but can move in relation to the camshaft drive from the crankshaft.
The vane is shown held in the ahead operating position by oil pressure. When oil is supplied under pressure through the drain, the vane will rotate through the lost-motion angular distance to change the fuel timing for astern operation.
The starting air system is retimed, either by this camshaft movement or by a directional air supply being admitted to the starting air distributor, to reposition the cams. Exhaust timing or poppet valves will have their own lost-motion clutch or servo motor for astern timing.
The bedplate is single-walled and arranged with an integral thrust bearing housing at the aft end Figure 2. Cross members are steel fabrications although the centre section, incorporating the main bearing saddle tie-bolt housings, may be a steel forging. To resist crankshaft loading and transverse bending, the main bearing keeps are held down by jackbolts. The crankcase chamber is arranged by using individual A-frames for columns which are also the mountings for the double-slippered crosshead guides.
The A-frames are joined together by heavy steel plates and short angle girders to form a sturdy box frame. Individual cast-iron cylinder blocks are bolted together to form a rigid unit which is mounted onto the A-frames.
Tie bolts extend from the top of the cylinder block to the underside of the main bearing saddles. The crankshaft is semi-built, with the combined crankpin and crankweb elements forged from a single element. The journal pins are then shrunk into the crankwebs. Taylor Sad to say, at the moment we don't have got details about the actual performer D. Even so, we might enjoy if you have virtually any information about that, and so are prepared to supply the idea. Send out the item to us!
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