MARINE PROPULSION DIESEL ENGINE

VARIABLE IGNITION TIMING (VIT) If an engine operate for long periods at reduced power or speed, the residual heat in the main components of the combustion chamber will decrease, causing a lower air temperature after compression. This will lead to an increase in the ignition delay of injected fuel, which will cause knocking or ‘rough running’ in the engine, with consequent damage from shock loads and poor combustion. This problem can be reduced by the used of variable ignition timing to advance the start of injection, allowing for the longer delay but maintaining ignition timing and the same peak pressure. In large, two-stroke engines, variable ignition timing is automatically superimposed on the normal fuel pump setting from the engine governor. Additional linkage from the governor will advance each pump setting over the range of lower speeds. Fuel pumps are designed to carry out necessary adjustment while the engine is running. This control can also be manually regulated to advance normal governor setting if it is known that fuel with low ignition quality is to be used.

Variable injection timing (VIT) MES In the MAN-B& W variable injection timing system, it operates on the same principle of control by a helix on the plunger but has further adjustment which make it possible to regulate the injection timing during operation of the engine. The pump barrel is able to be raised or lowered by a second rack which will alter the timing of the start of injection. 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 (barrel) in relation to the cam. The lower linkage rotates the pump plunger and thus the helix in order to vary the pump output. ITME MAN-B& W Variable injection timing (VIT) pump

Valve-controlled pump- Variable injection timing (VIT) MES Sulzer valve-controlled fuel pump 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 ‘S’

Valve-controlled pump- Variable injection timing (VIT) MES Sulzer variable injection timing system Figure ‘S’ above illustrates a valve-timed fuel pump as used in large Sulzer RTA engines. The pump plunger is raised and lowered by a follower on the cam. Spring-loaded suction and spill valves control oil to and from the pump chamber and each of these can be opened by lifting it from its seat by a pushrod moved by a lever operated from the plunger drive. Pivot points of the levers are positioned so that one pushrod moves up as the plunger rises while the other pushrod moves down. From the bottom of its stroke the plunger is moved upwards, the spill valve is closed, but the suction valve is held off its seat by its pushrod. No high pressure oil delivery takes place until the suction valve is able to seat as its pushrod moves down. From this point oil pressure is raised and injections continue as the plunger continues it’s up stroke. The spill valve pushrod will force it off its seat, releasing the oil pressure and ending injection. A non – return valve prevents loss of oil from the discharge. The plunger chamber is recharge during the downward stroke. The pivots of the levers are eccentric and by rotating these timing can be altered. Under normal operation the suction valve timing and start of injection is fixed, while spill timing used to control the power.

High speed engine fuel injection pump Higher speed engine which use camshaft pumps (or block pumps): those in which all the jerk pump elements are grouped into one or more complete units, each equipped with a common camshaft

Fuel Injection Timing & Adjustment CC Pounder The desired timing takes into account all the delays and dynamic effects between the cam and the moment of ignition. The desired timing is that which ensures that combustion starts and continues while the cylinder pressure generated can press on the piston and crank with the greatest mechanical advantage, and is complete before the exhaust valve opens at 110–130 degrees ATDC. The engine builder always specifies this from his development work, but it usually means that the only settable criterion, the moment of spill port closure (SPC), is about 20–25 degrees BTDC. In earlier practice this was done by barring (or manually turning over) the engine with the delivery valve on the appropriate cylinder removed, and with the fuel circulating pump on (or more accurately with a separate small gravity head), and the plunger rotated to a working position. When fuel ceased to flow, the spill port had closed. The accuracy of this method is, however, disappointing. There are differences between static and dynamic timing which vary from pump to pump and many makers now rely on the accuracy of manufacture, and specify a jig setting based on the geometry of the pump and the SPC point to set the cam rise or follower rise at the required flywheel (crank) position.

Typical Fuel Pump Timing Adjustment Adjusting timing by top clearance. Configuration If the cam is fixed it is of cam, tappet and pump at commencement of usually possible, within injection (A and C); and maximum cam and stated limits, to adjust follower lift (B and D). In each case the pump is the height of the tappet set correctly for the required injection point. A in relation to the cam. shows the case for an over-retarded cam, C Raising it has the effect that for an over-advanced cam within permitted that the plunger cuts off limits of top clearances. In either case Y (the the supply spill port (and residual lift) + Z = the timing dimension of the starts injection) sooner pump, T for a given cam position, and vice versa .

Fuel Injection- The fuel injector The essence of a diesel engine is the introduction of finely atomized fuel into the air compressed in the cylinder during the piston’s inward stroke. It is, of course, the heat generated by this compression, which is normally nearly adiabatic, that is crucial in achieving ignition. Although the pressure in the cylinder at this point is likely to be anything up to 200 bar, the fuel pressure at the atomizer will be of the order of 1300–1800 bar. There is a body of evidence to suggest that high injection pressure at full load confers advantages in terms of fuel economy, and also in the ability to digest inferior fuel. Most modern medium speed engines attain 1200–1800 bar in the injection high-pressure pipe. Some recent engine designs achieve as much as 2300 bar when pumping heavy fuel. Working backwards from the desired result to the means to achieve it, the injector has to snap open when the timed high pressure wave from the pump travelling along the high pressure pipe has reached the injector needle valve. Needle lift is limited by the gap between its upper shoulder and the main body of the holder. Needle lift is opposed by a spring, set to keep the needle seated until the ‘blow-off pressure’ or ‘release pressure’ of the injector is reached by the fuel as the pressure wave arrives from the pump.

Fuel injector MES This figure shows a section through a hydraulically operated fuel injector as fitted to a large two-stroke diesel engine. The general design is similar for most engines and consist of spring loaded non-return needle valve operated hydraulically by a fuel pressure wave from the fuel pump to discharge fuel at high pressure through an atomiser nozzle. A typical fuel injector will consist of a valve body or nozzle holder to which the nozzle or atomiser is secured by a retaining nut. The valve body contains the spring and its compression nut, with an intermediate spindle if required. Surfaces between the body and atomiser are ground and lapped to form an oil pressure-tight seal. A dowel ensures alignment of the oil passages. Figure ‘IJ’

The fuel injector- operation ITME MES As in Figure ‘IJ’ above; The high-pressure fuel enters and travels down a passage in the body and then into Fuel injector needle valve and nozzle 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 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 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.

Typical Fuel Nozzles, Pintle (left) and hole (right).

Fuel Injector

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Section 3- Engine Systems and Components I LESSON PLAN 2 Lubricating Oil Systems ..\\..\\..\\..\\..\\July 2008\\Intro to ME\\Presentation Materials\\For Pedagogy presentation\\Presentation2. ppt

ITME Lubricating oils The modern lubricant must be capable of performing numerous duties. This is achieved through blending and additives. It must prevent metal-to-metal contact and reduce friction and wear at moving parts. The oil must be stable and not break down or form carbon when exposed to high temperatures, such as where oil cooling is used. Any contaminants, such as acidic products of combustion, must be neutralised by alkaline additives; any carbon build up on surfaces must be washed away by detergent additives and held in suspension by a dispersant additive. The oil must also be able to absorb water and then release it during purification, but meanwhile still protect the metal parts from corrosion.

Lubricating oils – continued ITME Lubricating oils are a product of the crude oil refining process. The various properties required of the oil are obtained as a result of blending and the introduction of additives. The physical and chemical properties of an oil are changed by additives which may act as oxidation inhibitors, wear reducers, dispersants, detergents, etc. The important lubricant properties will now be examined. Viscosity has already been mentioned with respect to fuel oils, but it is also an important property of lubricating oils. Viscosity index is also used, which is the rate of change of viscosity with temperature. The Total Base Number (TBN) is an indication of the quantity of alkali, i.e. base, which is available in a lubricating oil to neutralise acids. The acidity of an oil must be monitored to avoid machinery damage and neutralisation number is used as the unit of measurement.

Lubricating oils – continued ITME The oxidation resistance of a lubricant can also be measured by neutralisation number. When excessively oxidised an oil must be discarded. The carbon-forming tendency of a lubricating oil must be known, particularly for oils exposed to heat. A carbon residue test is usually performed to obtain a percentage value. The demulsibility of an oil refers to its ability to mix with water and then release the water in a centrifuge. This property is also related to the tendency to form sludge. Corrosion inhibition relates to the oil's ability to protect a surface when water is present in the oil. This is important where oils can be contaminated by fresh or salt water leaks.

ITME Oil treatment Both fuel oils and lubricating oils require treatment before passing to the engine. This will involve storage and heating to allow separation of water present, coarse and fine filtering to remove solid particles and also centrifuging. The centrifugal separator is used to separate two liquids, for example oil and water, or a liquid and solids as in contaminated oil. Separation is speeded up by the use of a centrifuge and can be arranged as a continuous process. Where a centrifuge is arranged to separate two liquids, it is known as a 'purifier'. Where a centrifuge is arranged to separate impurities and small amounts of water from oil it is known as a 'clarifier'. The separation of impurities and water from fuel oil is essential for good combustion. The removal of contaminating impurities from lubricating oil will reduce engine wear and possible breakdowns. The centrifuging of all but the most pure clean oils is therefore an absolute necessity.

MES Fig. 5.11 Typical Lubricating oil system

Lubricating Oil Systems- a typical system MES Fig. 5.11 shows a lubricating oil system for a large main engine. Pressure pumps, strainers and fine filters are in duplicate, one set being used while the other acts as standby. Fine filters should be capable of being cleaned without interruption of the oil flow. Mesh size will depend upon the bearing materials and clearances: in most large engines it is 50 microns. Capacity of the system must be adequate for the type of installation. If the engine has oil-cooled pistons the capacity and throughput will be increased accordingly. 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.

ITME Figure 2.7 Typical Lubricating oil system

Lubricating oil system- a typical system ITME 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 2.17). 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. Pumps and fine filters are arranged in duplicate with one as standby. 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.

Lubricating oil system- continued ITME 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.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. Cylinder lubrication 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. It is fed through openings in the cylinder wall. In most large engines, it is timed to enter between ring segments when piston is at or near the end of its stroke. 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.

CC Pounder The function of the cylinder lubricant: • To assist in providing a gas seal between the piston rings and cylinder liner. • To eliminate or minimize metal-to-metal contact between piston rings, piston and liner. • To act as a carrier fluid for the functional alkaline additive systems, particularly that which neutralizes the corrosive acids generated during the combustion process. • To provide a medium by which combustion deposits can be transported away from the piston ring pack to keep rings free in grooves. • To minimize deposit build-up on all piston and liner surfaces.

Lubricating oil centrifuging ITME Lubricating oil in its passage through a diesel engine will become contaminated by wear particles, combustion products and water. The centrifuge, arranged as a purifier, is used to continuously remove these impurities. The large quantity of oil flowing through a system means that full flow lubrication would be too costly. A bypass system, drawing dirty oil from low down in the oil sump/ drain tank remote from the pump suction and returning clean oil close to the pump suction, is therefore used. Since this is a bypass system the aim should be to give the lowest degree of impurity for the complete system, which may mean running the centrifuge somewhat below the maximum throughput.

CC Pounder Lubricant Testing Lubricants are a valuable indicator of the overall functioning and wear of machinery. Under normal engine operating conditions the deterioration of a lubricant takes place slowly, but severe conditions and engine malfunctions promote faster degradation. Monitoring lubricants in service and analysing the results determine not only their condition but that of the engine and auxiliary machinery they serve. All the major marine lubricant suppliers offer used lube oil testing services to customers, with results and advice transmitted by email, fax or post to owner and ship from specialist laboratories. The range of analyses undertaken on engine lubricant samples submitted typically embraces checks on: • Water content: even in small quantities, water is always undesirable in a lubricant since it can act as a contaminant. A check on the water content will indicate defects in the engine, purifier and other auxiliary machinery. Where water is present, testing determines whether it is fresh or sea water.

Lubricant Testing- continued • Insoluble products: lubricants in an engine are contaminated by a number of insoluble products. Monitoring their presence provides a very good indication of the engine’s operating condition or the effectiveness of the purifier or filter; it also safeguards against breakdowns since a sudden increase in the insoluble products present indicates a malfunction. • Viscosity: a measure of the ability of a lubricant to flow, viscosity cannot be used in isolation to assess a lubricant’s condition but it yields useful information in conjunction with other determining factors, such as the level of oxidation or contamination by fuel, water or other elements. • Flash point: the temperature at which the application of a flame will ignite the vapours produced by an oil sample under standard conditions. Below a certain value there is a high risk of very serious incidents in service, including crankcase explosion. The flash point must therefore be monitored very carefully.

Lubricant Testing- continued • Base number (BN) or alkalinity reserve: during the combustion process the sulphur contained in the fuel can produce acidic products that can damage the engine; this acid must be neutralized to avoid extensive corrosive wear of the cylinder liner or piston rings. Neutralization is the role of specific additives in the lubricant, the amount and type of which are measured by the BN which represents the alkalinity reserve. Close monitoring of the BN in service is essential to ensure that the lubricant still has sufficient alkalinity reserve to perform correctly. • Wear metal elements: monitoring changes in metal elements (measured in ppm) provides vital information on how patterns of wear develop inside the machinery. Regular monitoring detects any sudden increase in a wear element, such as iron or copper, which would indicate that excessive wear was occurring.

Typical Lubricating and Cooling Oil System MEK

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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 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

GLMA

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. Fuel pump quantity control

GLMA

CC Pounder (Figure 8.7) Principle of operation of the fuel injection pump

Principle of operation of the fuel injection pump CC Pounder In its simplest form (Figure 8.7) the barrel has a supply port at one side and a spill port at the other. (In early practice these ports were combined.) The plunger is actuated by the fuel cam through a roller follower. Before, and as the plunger starts to rise (Figure 8.7(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 8.7(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 8.7(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 8.7(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 8.7(f)).

In its simplest form, the barrel has a supply port at one side and a spill port at the other. (In early practice these ports were combined.) The plunger is actuated by the fuel cam through a roller follower. Before, and as the plunger starts to rise (Fig. (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 (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 (Fig. (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)).

Helix-type injector pump - operation The fuel is supplied to the pump through Figure ‘P’ ports or openings at B, Figure ‘P’ below. 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.

Fuel injection systems have a significant influence on the combustion process and hence a key role to play in improving engine fuel consumption and reducing noxious exhaust emissions. The following characteristics of an injection system are desirable in achieving these goals: Σ Injection pressures during the whole process should be above 1000–1200 bar for a good spray formation and air–fuel mixture; a tendency in practice to 1600–1800 bar and higher is noted. Σ Total nozzle area should be as small as possible in relation to cylinder diameter for good combustion, particularly at part load. Σ Total injection duration should be 20 degrees of crank angle or less for achieving a minimum burning time in order to exploit retarded combustion for reduced NOx emissions without loss in efficiency. A high compression ratio is desirable. Σ High pressures at the beginning of injection promote reduced ignition delay, while increased mass flow can result in an overcompensation and increased pressure gradients. Consequently, rate shaping is necessary in some cases, particularly with high speed engines. 242 POUNDER’S MARINE DIESEL ENGINES AND GAS TURBINES Σ High aromaticity fuels cause increased ignition delay in some cases. Pre-injection with high injection pressures is necessary and can achieve non-sensitivity to fuel quality. Σ Electronically-controlled adjustment of injection timing should be applied for optimised NOx emissions at all loads, speeds and other parameters. Σ The load from the torque of the injection equipment on the camshaft and/or the gear train should be as low as possible in order to prevent unwanted additional stresses and noise. Σ For safety reasons, even a total breakdown of electrical and other

SECTION 3 Fuel System

Fuel oil system The fuel oil system for a diesel engine can be divided in two parts:- 1. the fuel supply and 2. the fuel injection systems. Fuel supply deals with the provision of fuel oil suitable for use by the injection system. (Contact engine maker). Fuel oil supply for a two-stroke diesel A slow-speed two-stroke diesel is usually arranged to operate continuously on heavy fuel and diesel oil supply for manoeuvring conditions. In the system shown in Figure 2.11, the oil is stored in fuel oil tanks in the double bottom tanks from which it is pumped to a settling tank and then heated to the required temperature.

After passing through centrifuges/purifier the cleaned and heated oil is pumped to a daily service tank. 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.



In a typical system, both oils are stored in double bottom tanks. From there the fuel oil is pumped to a settling tank and then heated. A series of centrifuges is used to clean the oil before it is pumped to a service tank. When needed the fuel oil is then pumped through a heater and viscosity meter which regulates the fuel oil temperature to ensure it arrives at the injection system at the correct viscosity for combustion. Similarly the diesel oil system is pumped from the double bottom tank to a centrifuge before a settling tank. The diesel oil is then pumped direct to the injection system via a three-way valve, which only allows one type of oil into the engine at a time. As diesel does not need to be as hot as fuel oil it is important to change from one fuel to another gradually to allow the engine temperature to stabilize



LO/FO Purifier