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LESSON 13LESSON 13

MARINE MARINE MEDIUM MEDIUM SPEED(300—1000r/min)SPEED(300—1000r/min)

DIESEL ENGINE ⅡDIESEL ENGINE Ⅱ

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PistonsPistons

The piston is a The piston is a pot shaped(pot shaped( 筒状筒状 )) component. The crown is fairly thicomponent. The crown is fairly thick with the cylindrical parts ck with the cylindrical parts taperitaperingng to a thinner to a thinner section. section.

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The The combustion chambercombustion chamber is is enclosed between the cylinder enclosed between the cylinder head and the piston and the most head and the piston and the most

of of itit is contained in the top of the is contained in the top of the piston. piston.

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For this purpose the top surface of For this purpose the top surface of the piston crown may be the piston crown may be bowlbowl sha shaped or ped or toroidaltoroidal((环形的环形的 )) shapedshaped. .

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The The rimrim of the piston may contain of the piston may contain cut-out portions(cut-out portions( 避让坑避让坑 )) to to accaccommodateommodate the valves when they op the valves when they open. en.

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Bowl/Toroidal shape

Cut-off portion

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The cooling of the piston is The cooling of the piston is carried carried outout by circulating lubricating oil by circulating lubricating oil across the underside of the crownacross the underside of the crown and and inside the ring beltinside the ring belt or or through through specially shaped passagesspecially shaped passages. .

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The simplest The simplest meansmeans is by is by splsplash(ash( 飞溅飞溅 )) or or sspray(pray( 喷射喷射 )) on on the underside of the underside of the piston crowthe piston crown. n.

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A more complex A more complex design involves a design involves a chamber chamber specially specially constructed for constructed for the oil to circulate the oil to circulate throughthrough..

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In some designs In some designs this chamberthis chamber takes the form of a takes the form of a coil coil cast into cast into the material of the pistonthe material of the piston and and conductingconducting the heat away from the the heat away from the piston ring regionpiston ring region. .

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In another design In another design the chamberthe chamber is is open and constructed so that the open and constructed so that the oil splashes about violently. oil splashes about violently.

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This design is termed the This design is termed the ‘cocktail‘cocktailshaker(shaker( 震荡式震荡式 )’)’; the motion of t; the motion of the oil providing extremely good hehe oil providing extremely good heat transfer. at transfer.

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The pistons of high output(power) The pistons of high output(power) engines are often made in two engines are often made in two pieces bolted together to form pieces bolted together to form precisely shaped cooling precisely shaped cooling chambers. chambers.

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Pistons may be made in Pistons may be made in aluminiualuminium alloym alloy in order to keep the weigh in order to keep the weight down for balancing purposes at ht down for balancing purposes at high speed. In other designs, where igh speed. In other designs, where the necessity for light weight is not the necessity for light weight is not so important, they are made so important, they are made whollwhollyy of cast iron. of cast iron.

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The high output two-piece design The high output two-piece design is usually used is usually used in conjunction within conjunction with a heat resistant steel crown a heat resistant steel crown bolted bolted to a skirt portionto a skirt portion which may be of which may be of light alloy or of cast ironlight alloy or of cast iron. .

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The gudgeon pinThe gudgeon pin is carried in is carried in bossesbosses in the piston skirt. in the piston skirt. Generally the gudgeon pin is Generally the gudgeon pin is fully fully floatingfloating in both connecting rod in both connecting rod small end and the piston bosses. small end and the piston bosses.

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It is located It is located endwiseendwise, either by , either by padpads(s( 端盖端盖 )) fixed to the pistonfixed to the piston or by or by means of means of circlips(circlips( 卡簧卡簧 )) entered ientered into small end grooves at each end onto small end grooves at each end of the piston boss boref the piston boss bore. .

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circlip

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BearingsBearings

The main bearing shells are made of sThe main bearing shells are made of steel with a lining of bearing metal, whiteel with a lining of bearing metal, which may be ch may be white metalwhite metal, , copper-leadcopper-lead or or aluminium-tin(aluminium-tin( 锡锡 ) alloy) alloy; they often h; they often have ave aa thinthin flashflash of of leadlead or or indiumindium(( 铟元素铟元素 ) ) to provide a layer giving pto provide a layer giving protection against corrosion.rotection against corrosion.

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They(bearing shells)They(bearing shells) are held in po are held in position and shaped by the bore of thsition and shaped by the bore of the bedplate or frame. The external e bedplate or frame. The external circumferencecircumference of a pair of main be of a pair of main bearing shells is aring shells is slightly larger thanslightly larger than t that of the bore of the housing hat of the bore of the housing whicwhich receives themh receives them. .

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The difference is termed The difference is termed ‘nip(‘nip( 压压隙隙 )’)’. When the assembly is bolte. When the assembly is bolted up, the result is the d up, the result is the equivalent equivalent of of

interference fit(interference fit( 过盈配合过盈配合 )) b between the shells and the housing. etween the shells and the housing.

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The design of the connecting rod The design of the connecting rod large end bearing is similar to that large end bearing is similar to that of the main bearing. of the main bearing.

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The bearing shell generally has The bearing shell generally has tatags(gs( 标记标记 )) for location and oil gro for location and oil groovesoves round the center of its axial leround the center of its axial length ngth whichwhich correspond withcorrespond with groove grooves in the bearing housings in the bearing housing through through wwhich hich oil is transmitted up the rod toil is transmitted up the rod to lubricate the small end and to coo lubricate the small end and to cool the piston. ol the piston.

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The two halves are held together bThe two halves are held together by y bridges(bridges( 轴承盖轴承盖 )). The shells dep. The shells depend on the end on the accuracy(accuracy( 精确性精确性 )) of th of the bore to hold them in a e bore to hold them in a trulytruly circucircularlar shape. shape.

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AccordinglyAccordingly it is essential that the it is essential that the large end bolts are large end bolts are tightenedtightened

correctly to the designed pull correctly to the designed pull asas under- or over-tightening will under- or over-tightening will result in the housing bore result in the housing bore departing from its true circular departing from its true circular shape. shape.

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The correct tightening is obtained The correct tightening is obtained by the use of by the use of stretch gauges(stretch gauges( 拉伸拉伸仪仪 )) to measure the extension to measure the extension given given to the boltsto the bolts, or by using a , or by using a torque wtorque wrench(rench( 扭力搬手扭力搬手 )), or by means of , or by means of special hydraulic tightening equipspecial hydraulic tightening equipment. ment.

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Inlet and Exhaust ValvesInlet and Exhaust Valves

The exhaust valves open The exhaust valves open againstagainst th the pressure within the cylinder at the pressure within the cylinder at the end of the working e end of the working ststroke. roke.

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This pressure is considerably highThis pressure is considerably higher than that against which the inlet er than that against which the inlet valves have to open. Furthermore, valves have to open. Furthermore, the pressure of the exhaust gases the pressure of the exhaust gases aassistsssists, , once the valve is open,once the valve is open, in in expexpelling(elling( 排出排出 ) ) the gases through the the gases through the open valve. open valve.

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Because of these Because of these considerationsconsiderations it it is is notnot ununusual to find that the usual to find that the exhaust valves are designed to be exhaust valves are designed to be of smaller diameter than the inlet of smaller diameter than the inlet valves. valves.

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Being smaller also assists with Being smaller also assists with keeping them coolkeeping them cool which is which is important as exhaust valves often important as exhaust valves often give rise togive rise to thermal thermal problems. problems.

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The inlet and exhaust valve The inlet and exhaust valve seatsseats may become damaged during opermay become damaged during operation and ation and from time to timefrom time to time they h they have to be ave to be reconditioned(reconditioned( 修理修理 )) by by grindinggrinding in the valves. in the valves.

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This is required much more often This is required much more often in the case of the exhaust valves in the case of the exhaust valves becausebecause of operation at higher of operation at higher temperaturetemperature andand becausebecause the gases the gases flowing throughflowing through may contain may contain particles of carbonaceous matter particles of carbonaceous matter which occasionally get which occasionally get trappedtrapped under the valve seatunder the valve seat causing causing pittingpitting. .

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To assist in To assist in frequentfrequent reconditionin reconditioning of exhaust valve seats g of exhaust valve seats itit is not un is not uncommon on engines common on engines designed to rudesigned to run on heavy fueln on heavy fuel for the exhaust valfor the exhaust valves to be enclosed in separate ves to be enclosed in separate cagecages(s(外壳外壳 )) whichwhich……

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whichwhich can be removed from the can be removed from the cylinder head without the need for cylinder head without the need for dismantling the whole component. dismantling the whole component.

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On large medium speed engines it On large medium speed engines it is sometimes the case that both is sometimes the case that both exhaust and air valves are exhaust and air valves are enclosed in cages enclosed in cages which are which are separate from the head. separate from the head.

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The The lifelife of of an exhaust valvean exhaust valve between twice of reconditioningbetween twice of reconditioning can can be extended if be extended if the thermal loadsthe thermal loads toto whichwhich itit is subjected is subjected can be can be evenly distributed around the evenly distributed around the valve. valve.

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This is accomplished by rotating This is accomplished by rotating the valves slowly as the engine is the valves slowly as the engine is working. working.

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Valve rotators(Valve rotators( 转阀装置转阀装置 )) which cwhich carry out this movementarry out this movement have a type have a type of of ratcheting mechanism(ratcheting mechanism( 棘轮机棘轮机构构 )) which which indexesindexes the valve the valve roundround a small amount every time it is opera small amount every time it is operated by the ated by the rockerrocker geargear..

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Valve Operating Gear(Valve Operating Gear( 配气机构配气机构 ))

The motion of The motion of the cam follower levthe cam follower leverer or or tappet(tappet( 梃杆梃杆 )) is transferred t is transferred to the valve by o the valve by push rodspush rods and and rockerocker leversr levers. .

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Rocker lever

Push rod

cam

tappet

Adjusting screw

valve

Valve seat

Valve operating gear

roller

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An adjusting screwAn adjusting screw is provided at is provided at one end of the rocker lever by one end of the rocker lever by means of means of whichwhich the clearance the clearance betweenbetween the rocker lever and the the rocker lever and the valve cam can be adjusted. valve cam can be adjusted.

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Some clearance is essential for Some clearance is essential for satisfactory operation of the valve. satisfactory operation of the valve. The valve must be held on its seat The valve must be held on its seat by the gas pressure in the cylinder by the gas pressure in the cylinder during the periods when it is to during the periods when it is to remain closed. remain closed.

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If there were no clearance, the valIf there were no clearance, the valve could be ve could be propped(propped( 支撑支撑 , , 维持维持 )) open slightly allowing gas to leak topen slightly allowing gas to leak through the seat and cause damage. hrough the seat and cause damage.

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On the other hand, the clearance On the other hand, the clearance must not be excessive must not be excessive because the because the valve would then be valve would then be hammeredhammered by by the rocker gear when being lifted the rocker gear when being lifted off the seat and again off the seat and again hammerhammer on on to its seat when the rocker gear left to its seat when the rocker gear left it during the re-seating movementit during the re-seating movement causing damage to the valve.causing damage to the valve.

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The valves and components The valves and components forming the valve operating gearforming the valve operating gear are almost certain to expand with are almost certain to expand with the result the result that when the engine is that when the engine is warmwarm thethe clearanceclearance between the between the valve rocker and the valves valve rocker and the valves is is different from different from thatthat when the when the engine is coldengine is cold. .

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In setting the clearanceIn setting the clearance allowanceallowance(( 余量余量 )) must be made for this fact must be made for this fact and careful note (must be) taken tand careful note (must be) taken that the clearance is set hat the clearance is set in accordanin accordance withce with the temperature of the engi the temperature of the engine. ne.

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Some makers give cold clearance, others give clearance appropriate to the engine at running temperature.

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READING MATERIAL

FUEL INJECTION EQUIPMENT

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There are two types of fuel injection system in use.

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One is the common rail system, the other is known as the jerk pump system, in which the fuel is metered and raised in pressure by a separate fuel pump for each cylinder and which is timed to force it(fuel) through the injector into the cylinder at the appropriate moment.

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The great majority of medium speed diesel engines use the latter system.

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Fuel Pump

The fuel pump plunger塞 ) is actuated by a cam and roller tappet follower.

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When the follower is on the base circle of the cam, the pump plunger is at the bottom of its stroke and the inlet port in the barrel is uncovered allowing fuel in the gallery( 腔 ) which surrounds the barrel to flow into the portion of the barrel above the plunger.

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Delivery valve

barrelplunger Helix groove

projection

rack pinion

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As the cam rotates the plunger rises and seals off the inlet port. At this point of the stroke the pumping action starts.

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Further upward movement of the plunger causes the fuel to be raised in pressure and expelled through the delivery valve to the injector.

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A helical groove extends from the top of the plunger part way down its cylindrical surface.

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Angular( 角的 ) positioning of the plunger about its axis so locates this groove in relation to the inlet port to give a greater or lesser stroke between the sealing of the port by the top of the plunger and its communication with the helical groove.

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When the port is opened to the groove the high pressure in the fuel above the plunger is released to the low pressure in the gallery and pumping ceases although the plunger continues to move upwards.

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The amount of fuel delivered will vary in accordance with the effective length of stroke determined by the angular position of the plunger.

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This angular position is controlled by a rack and pinion, the latter forming a sleeve having axial slots to receive a projection on the plunger. The rack position therefore, determines the quantity of fuel supplied.

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Most medium speed engines have individual fuel pumps, one to each cylinder, operated by cams mounted on the same camshaft that carries the inlet and exhaust cams.

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The timing of the injection is controlled by the instant that the pump plunger closes the inlet port.

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This instant can be adjusted with reference to the camshaft and crankshaft position by raising or lowering the plunger by the screw in a tappet. Raising the level of the screw will advance the point of injection.

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Fuel Injector

After leaving the fuel pump delivery valve, the fuel is conveyed( 传送 ) by high-pressure steel piping to the injector.

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The pressure generated by the fuel pump acts upon the annular part of the needle valve exposed below the guided portion.

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When the upward force on the needle, due to this pressure, exceeds the force exerted by the valve compressed spring and the needle lifts off its seat.

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The fuel pressure is then able to act over the whole cross sectional area of the needle which immediately snaps(突然移动 ) up to the fully open position.

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The fuel is squirted(喷出 ) at high velocity through small holes in the injector nozzle causing it to divide up into a fine spray which penetrates(穿入 ) throughout(遍及 ) the combustion chamber.

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When the pressure in the fuel pump is released by the spill action the needle valve is immediately re-seated by the spring.

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The opening pressure at which the spring is set is important; it is usually about 200 atmospheres (bar) for medium speed diesel engines.

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The delivery valve of the fuel pimp plays an important part in ensuring a clean end to the injection of fuel.

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As well as acting as a non-return valve to prevent total back flow of fuel from the high-pressure pipe connecting the pump to the injector, it is designed to withdraw a small amount of fuel from this pipe as it closes.

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This causes the pressure at the injector to fall more steeply(急剧地 ) than would otherwise be the case, thus providing a sharp end to the injection and helping the injector needle to reseat without causing dribble(滴下 ) at the nozzle holes.

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This action is brought about by a piston portion formed on the valve. The clearance between this piston and the bore in the body is an important dimension which should be maintained when reconditioning delivery valves.