MARINE PROPULSION DIESEL ENGINE

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, 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 running maneuvered on/with diesel oil or even a blend of diesel and heavy fuel oil. The mixing tank is used to collect re-circulated oil and also acts as a buffer or reserve tank as it will supply fuel when the daily service tank is empty. safety devices are fitted in the system such as:- 1. low-level alarms of daily service tank. 2. remotely operated tank’s outlet valves which can be closed in case of fire.

Fuel injection. The function of the fuel injection system; ❑ To provide the right amount of fuel at the right moment and in a suitable condition for the combustion process. ❑ There must have correct fuel supply by proper timing of the delivery and the atomization of the fuel. ❑ The injection of the fuel is achieved by the position 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 type of fuel injection system used in diesel engine are;-Combination of mechanical and hydraulic operations. 1. the jerk pump (the most common) 2. the common rail.



From the services tanks the treated oils is pumped through a pressurised fuel system to the engine. With the oil temperatures necessary for high viscosity fuel, and the possibility that a trace of water may still be present, it is necessary to maintain the engine pump suctions and circulating connections under pressure to inhibit boiling, gasification and cavitations. The oil first passes to the primary or supply pump which raises its pressure to about 4 bars; this pressure is maintained in the circulating returns. The circulating or booster pumps draws oil from the primary discharged, raising its pressure to 10/12 bar and delivering it through the heater, viscosity regulator and fine filter to the main engine fuel pumps.

The heater reduces the oil’s viscosity for efficient combustion. The temperature required will depend upon the oil quality, but to avoid fouling it should not exceed 150°C. In large two-stroke engines the fuel injectors will circulate fuel during the periods they are not actually injecting it to the cylinder. This ensures the system remains fully primed and at uniform temperature. The circulated oils is returned to a buffer or venting tank from which it passes either back to the low pressure part of the system before the circulating pump suction, or into the service tank.

Jerk pump system (FO injector pump). Separate injector pump exists fitted 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. The needle valve in the injector will lift at a pre-set pressure which ensures that the fuel will atomize once it enters the cylinder. There are two types of fuel pump in use: 1. The valve controlled discharge type for slow-speed two-stroke engines. 2. The helix or helical edge pump for high-speed four-stroke engines.

Helix-type injector pump This type of pump is used on many four-stroke diesel engines. Function of pump This 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 2.12). As the plunger moves down, fuel enters the cylinder. As the plunger moves up, the ports at B are closed and the fuel is pressurized 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 down stroke and the process will be repeated. The plunger is 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.









In its simplest form, the barrel has a supply port at one side and a spill port at the other. • The plunger is actuated by the fuel cam through a roller follower. Before, and as the plunger starts to rise, Figure (a), the chamber is free to fill with fuel and to spill back into the gallery in the pump housing outside until the rising top edge of the plunger closes the supply and spill ports, Figure (b). • Thereafter the fuel is pressurized and displaced through the delivery valve towards the injectors. The initial travel ensures that the plunger is rising fast when displacement commences, so that the pressure rises sharply to the desired injection pressure. When the plunger has risen far enough, a relieved area on it uncovers the spill port, the pressure collapses, and injection ceases, Figure (c).

• The relief on the plunger has a helical top (control) edge so that rotation of the plunger by means of the control rod varies the lift of the plunger during which the spill port is closed, and therefore the fuel quantity injected and the load carried by the engine, as shown in Figures (c, d, e). • At minimum setting the helical edge joins the top of the plunger and if the plunger is rotated so that this point, or the groove beyond it, coincides with the spill port, the latter is never closed. In that case no fuel is pumped and that cylinder cuts out, Figure (f)







Valve-controlled pump In the variable injection timing (VIT) pump used in MAN B&W engines the governor output shaft is the controlling parameter. 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.(Figure2.13). 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. Operation of the spill valve will control the end of injection by releasing fuel pressure. No helix is therefore present on the pump plunger.

Figure 2.13 Variable injection timing (VIT) pump

Common rail system. The common rail system has one high-pressure multiple plunger fuel pump (Figure 2.14). 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.

Figure 2.14 Common rail fuel injection system

Timing valve The timing valve in the common rail system is operated by a cam and lever (Figure 2.15). 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 maneuvering lever setting to provide the correct fuel quantity to the cylinder.

Figure 2.15 Timing valve

Fuel Injector / Fuel Valve

The fuel injector A typical fuel injector is shown in Figure 2.16, It can be seen to be two basic parts; 1. nozzle 2. nozzle holder or body. ❑ 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 mitered seat by an intermediate spindle and a spring in the injector body. ❑ The spring pressure, and hence the injector opening pressure, can be set by a compression nut which acts on the spring. ❑ The nozzle and injector body are manufactured to give a good oil seal. ❑ The two are joined by a nozzle nut.

Figure 2.16 Fuel injector / Fuel Valve



❑ 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 will atomize, 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 2-stroke engines and many medium-speed 4-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.

END Q&A

Lubrication System Purpose: To supply of lubricating oil to the various moving parts in the engine. Lubricating oil in a diesel engine is used for the following purposes:  To prevent metal-to-metal contact between moving parts.  To aid in engine cooling.  To act as a cleaner  To form a seal between the piston rings and the cylinder wall.  To aid in keeping the inside of cylinder walls free of sludge and lacquer.

Figure 2.17 Lubricating oil system of Slow Speed 2-stroke Engine













Lubricating Oil System (Refer to fig 2.17)  Lubricating oil for an engine is stored in the bottom of the sump tank or in a drain tank located beneath the engine (Figure 2.17).  The oil is drawn from this tank through a strainer by LO pumps, and drawn into 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 is fitted at the end of the distribution pipe to ensure that adequate pressure is maintained by the pump.  Pumps and fine filters are fitted in duplicate with one as standby.

 The fine filters is arranged in such a way 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.  If there is any leaking in the cooler will mean a loss of oil and not contamination of the oil by sea water will not happen.  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.g. 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.

Typical LO lubrication System





Cylinder lubrication system.  Large slow-speed diesel 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. Cylinder lubricating oil helps to assists in forming; 1. A gas seal to prevent hot gas by pass. 2. Contains additives which clean the cylinder liner. The function of the cylinder lubricant; (i) To assist in providing a gas seal between the piston rings and cylinder liner. (ii) To eliminate or minimize metal-to-metal contact between piston rings, piston and liner. (iii) To act as a carrier fluid for the functional alkaline additive systems, particularly that which neutralizes the corrosive acids generated during the combustion process. (iv) To provide a medium by which combustion deposits can be transported away from the piston ring pack to keep rings free in grooves. (v) To minimize deposit build-up on all piston and liner surfaces.

A direct metal-to-metal moving contact has an action that is comparable to a filing action. This filing action is due to minute irregularities in the surfaces, and its harshness depends upon the finish and the force of the contacting surfaces as well as on the relative hardness of the materials used. Lubricating oil is used to fill these minute irregularities and to form a film seal between the sliding surfaces, thereby preventing high friction losses, rapid engine wear, and many operating difficulties. Lack of this oil film seal results in seized, or frozen pistons, wiped bearings, and stuck piston rings. The high-pressures of air and fuel in diesel engines can cause blow-by of exhaust gases between the piston rings and cylinder liner unless lubricating oil forms a seal between these parts. oil is used to assist in cooling by transferring or carrying away heat from localized hot spots in the engine. Heat is carried away from bearings, tops of the pistons, and other engine parts by the lubricating oil. It is the volume of lubricating oil being circulated that makes cooling of an engine possible



Lubricating oil pressure pumps draw oil from the engine drain tank through suction strainers, the tank suction being clear of the lowest point to avoid picking up any water or sludge which may have settled. The pumps discharge at pressure through the oil cooler, ensuring that sea water at its lower pressure cannot leak into the oil system in the event of fault in the cooler. The oil then passes through the fine filters to the engine. It will be distributed to all bearings, piston cooling, sprayers, exhaust valve actuators, control system etc. Various sections of the lubricating system may require different pressure and to accommodate this engine driven booster pumps may raise the supply pressure, while pressure reducing valves and restricted orifices may reduce pressure or flow to other parts. Used oil drains to the bottom of the crankcase and passes through strainers by gravity to the drain tank. A bypass centrifuge system is fitted to purify oil from the drain tank to remove water, sludge and insoluble.