Crosshead & trunk engines


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Crosshead & trunk engines

  1. 1. DIFFERENCES BETWEEN CROSSHEAD AND TRUNK ENGINESComparisons Of Cross Head and Trunk Piston EnginesThere are two basic ways of connecting a piston to a crankshaft; • Crosshead construction (used by all slow speed two stroke engine manufactures} • Trunk piston construction (used in smaller four stroke engines)This handout is principally about the 2-Stroke design of diesel engines and some of theareas of differences that are to be expected when viewing this type of engine.Crosshead Engine ConstructionThe majority of 2-Stroke engines encountered at sea are of the "crosshead" type. In thistype of engine the combustion space (formed by the cylinder liner, piston and cylinderhead), and the scavenge space are separated from the crankcase by the diaphragm plate.The piston rod is bolted to the piston and passes through a stuffing box mounted in thediaphragm plate. The stuffing box provides a seal between the two spaces, stopping oilfrom being carried up to the scavenge space, and scavenge air leaking into thecrankcase.The foot of the piston rod is bolted to the crosshead pin. The top end of the connectingrod swings about the crosshead pin, as the downward load from the expanding gasapplies a turning force to the crankshaft.To ensure that the crosshead reciprocates in alignment with the piston in the cylinder,guide shoes are attached either side of the crosshead pin. These shoes are lined withwhite metal, a bearing material and they reciprocate against the crosshead guides, whichare bolted to the frame of the engine. The crosshead guides are located in-between eachcylinder.Using the crosshead design of engine allows engines to be built with very long strokes -which means the engine can burn a greater quantity of fuel per stroke and developsmore power. The fuel used can be of a lower grade than that used in a trunk pistonengine, with higher sulphur content, whilst high alkalinity cylinder oils with a differentspecification to that of the crankcase oil are used to lubricate the cylinder liner andpiston rings and combat the effects of acid attack. 1
  2. 2. Fig. 1 Crosshead DesignThe advantages of the crosshead design are: 1. Guide faces take side thrust; this is easily lubricated, wears little and takes side forces off the piston and liner running surfaces. 2. Uniform clearance around piston allows for better lubricating oil distribution reducing wear 3. Simplified piston construction designed for maximum strength and cooling. Extended load bearing skirts found on trunk pistons unnecessary 4. Due to gland lubricating oil may be optimised for crankcase and cylinder. High alkalinity oils used in cylinder allow poorer quality fuels to be burnt. 2
  3. 3. Trunk Engine ConstructionThe piston is directly attached to the connecting rod by a small end rotating bearing. Sidethrust is absorbed by extended skirts on piston.The main advantage is reduced engine height(See handouts – Principal stationary and moving parts for more details)The EntablatureThe entablature is the name given to the cylinder block which incorporates the scavengeair space and the cooling water spaces. It forms the housing to take the cylinder linerand is made of cast iron.The castings are either for individual cylinders which after machining on the matingsurfaces are bolted together to form the cylinder beam, or they may be cast in multi -cylinder units, which are then bolted together. The underside of the cylinder beam ismachined and then it is aligned on the A frames and fastened in position using fittedbolts.It is important to remember that the fitted bolts used to bolt the entablature, A framesand Bedplate together are for alignment and location purposes only. They are notdesigned to resist the firing forces which will tend to separate the three components.This is the job of the tie bolts (discussed later in this handout). 3
  4. 4. Fig. 2 EntablatureCrossheadThe purpose of the crosshead is to translate reciprocating motion of the piston into thesemi rotary motion of the connecting rod and so bearings are required. It is also necessaryto provide guides in order to ensure that the side thrust due to the connecting rod is nottransmitted to the piston. This also ensures the piston remains central in the cylinder thuslimiting wear in the liner.Two faces are required as the thrust acts in opposite directions during power andcompression stroke. Guide shoes positioned at the extreme ends of the crosshead pinprovided a large area and minimize risk of twisting.The crosshead pin connects the piston rod to the connecting rod. On either side of thecrosshead pin are mounted the crosshead slippers. The slippers run up and down in thecrosshead guides as the piston and rod are reciprocating and prevent the top of theconnecting rod from moving sideways. 4
  5. 5. Fig. 3 Crosshead DesignTypes of damage associated with the crosshead bearingThere are two possible types of damage which may be sustained;1 WipingThis is where part of the white metal contact faces are wiped out so that machining marksand oil grooves disappear, the material is displaced into the lubrication grooves where itforms stubble or may fill them completely. Providing adequate lubrication is present,this may be caused by two high a degree of roughness of the crosshead journal. Possiblydue, if occurring after trouble free operation, to particles in the lubricating oil. Roughnessmay also occur due to corrosion by weak acids forming in the lubricating oil. Watercontent above 1% can attack the white metal and cause formation of SnO which has theappearance of dark smudges on the surface. This must be removed whenever possible asthe tin oxide can become harder than the metal of the journal causing obvious destructionof surface finish. 5
  6. 6. 2. CrackingThese may appear as individual cracks, hair line cracks, or densely cracked or crackledareas. The latter may be so dense so as to give the appearance of segregated grains. Thiscan lead to scratching on the journal. The reasons for cracking may be insufficientbonding of white metal to the steel. Densely nested networks of cracks is due to fatiguefracturesStuffing BoxBecause the crankcases is separated from the cylinder and scavenge space by thediaphragm plate on a 2- Stroke crosshead engine, provision must be made for the pistonrod to pass through the plate without oil from the crankcase being carried upwards, orused cylinder oil contaminated from products of combustion being carried downwards.It is also highly undesirable to allow the pressurized air in the scavenge space to leakinto the crankcase.The Piston rod passes through a stuffing box which is bolted into the diaphragm plate.The stuffing box casing which can be split vertically, as shown in figure 4, contains aseries of rings which are each made up of three or four segments. On the outside of eachset of segments is a garter spring which provides the tension to hold the ring segmentsagainst the piston rod. There is a clearance between each segment to allow for wear. Therings are either bronze or can comprise of replaceable cast iron lamella fitted into a steelbacking ring.The stuffing box is mounted on a ring which is bolted onto the underside of the scavengeair box. The stuffing box is taken out together with the piston rod during overhaul of thepiston, but also can be disassembled for inspection in the crankcase with the pistonremaining in position.The stuffing box housing is in two parts, assembled by a flanged joint. In the housing fivering grooves have been machined out of which the two uppermost ones accommodatesealing rings that prevent scavenge air from blowing down along the piston rod. In thelowermost grooves scraper rings are fitted which scrape the lubricating oil of the pistonrod. The oil is led through bores in the housing and back to the crankcase. 6
  7. 7. Fig. 4 Stuffing Box stuffing box in engineFig. 5 Stuffing Box in Engine 7
  8. 8. Fig 6 Stuffing Box ArrangementBetween the two uppermost ring grooves, for the sealing rings, and the three lowermostgrooves, for the scraper rings, a cofferdam has been machined out which, through a borein the housing and a connecting pipe, communicates with a control cock on the outside ofthe engine. It can be checked by opening this control cock that the scraper and sealingrings are functioning correctly. 8
  9. 9. Tie BoltsTo understand the importance of the role played by the tie bolts or tie rods, it isnecessary to appreciate what is happening inside the cylinder of the engine.When the piston is just after top dead centre the pressure inside the cylinder can rise ashigh as 140 bar (14000kN/m2). This acts downwards through the piston rod and con-rod,pushing the crankshaft down into the bearing pockets. At the same time, the pressureacts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into theentablature prevent this happening and so this upward acting force tries to lift theentablature from the frames and the frames from the bedplate, putting the fitted locationbolts into tension.As the piston moves down the cylinder the pressure in the cylinder falls, and then risesagain as the piston changes direction and moves upwards on the compression stroke. Thismeans that the fitted bolts are under cyclic stress. Because they are not designed towithstand such stresses they would soon fail with disastrous consequences.To hold the bedplate, frames and entablature firmly together in compression, and totransmit the firing forces back to the bedplate, long tie bolts are fitted through these threecomponents and then tightened hydraulically. To prevent excessive bending moments inthe transverse girders, the tie bolts are positioned as close to the centre of thecrankshaft as possible. Because the tie bolts are so close to the crankshaft, some enginesemploy jack bolts to hold the crankshaft main bearing cap in position instead ofconventional studs and nuts.Operating the engine with loose tie bolts will cause the fitted bolts holding the bedplate,frame and entablature in alignment to stretch and break. The machined mating surfaceswill rub together, corrode and wear away (this is known as fretting). Once this hashappened the alignment of the engine running gear will be destroyed. Loose tie bolts willalso cause the transverse girders to bend which could lead to cracking, and main bearingmisalignmentOnce fretting between the mating surfaces has occurred, then tightening of the tie boltswill pull the engine out of alignment. The crosshead guides, the cylinder liner, and thestuffing box will no longer be in line and excessive wear will occur. Because the tiebolts will no longer be pulled down squarely they will be subject to forces which maylead to them breaking. If fretting has occurred, then the only solution is to remove theentablature or/and frame and machine the fretted mating surfaces (a very costlyexercise).Tie bolts can break in service. To reduce the risk of this happening they must bechecked for tightness; not over tightened; and the engine not overloaded. If a breakagedoes occur, this is not disastrous; as the engine can be operated with care for a limitedperiod (the load on the engine may have to be reduced). The position of the fracture will 9
  10. 10. dictate how the broken pieces are removed. However in the worst possible scenariowhere the bolt is broken at mid length, then one solution is to lift out the top half,remove the bottom nut, and then feed a loop of braided wire cable (about 7mmdiameter) down the tie bolt tube, down the side of the broken tie bolt and once itemerges at the bottom a supporting piece can be fitted to the wire enabling the brokentie bolt to be withdrawn. 10
  11. 11. Fig. 7 Tie bolt Arrangement 11
  12. 12. Chain DrivesRotation of camshafts in an engine may be by gears or by chain turned by the main crank.The disadvantage of using gears is difficulty in alignment, lubrication and disadvantageto wear from foreign materials as well as their increased cost. The disadvantage of chainsis the requirement for tensioning and their finite life. Although for large installations thiscan be very long.Wear on the chain pins, bushes as well as the chain sprockets can all lead to a slackeningoff of the chain. This can lead to slap and changing of cam timing. This alters the leadsof the fuel pumps and exhaust valves. The degree of angular displacement can bechecked using a manufacturer supplied poker gauge.Chain damage occurs if the chain is too tight or too slack and the result is fatiguecracking of the links. If the tension is too tight, then this adds to the working stress of thechain. Insufficient tension leads to slap with resultant damage to chain and rubbingstrips. Vertical misalignment of the sprockets means rubbing at the side plates resulting inreduction of thickness and possible failure.Recommended limit on stretch is about 1.5 to 2%, if maximum movement of thetension is reached before the chain has reached its maximum stretch then a pair of linksmay be removed. When maximum stretch is reached, or if the chain shows signs ofdamage then the chain should be replaced.The simplest method is to break the old chain and attach the new chain to it. The engineis then turned and as the old chain is paid off, the new chain can be paid in. Thismaintains approximately the correct timing; the tension of the chain can then be set.Final adjustment of the timing can be made following manufacturers instructions; thisgenerally means turning the engine until No1 is at top dead, then checking by us ofpointer gauges the position of the cam.The cam drive is adjustable and can be slackened off, by hydraulic means on largemodern engines, the section of cams can then be turned relative to the crankshaft angleand the timing restored.The chains are lubricated by the injection of a spray of oil between the chain wheels andthe chain rollers just before the rollers are about to engage the wheel. Thereby an oilcushion is formed to dampen the impact 12
  13. 13. Fig. 8 Typical Chain Drive ArrangementChain stretch and hence reduction in tension can be accounted for by movement of atensioning wheel. The tension usually being checked by movement to and fro at thecentre of the longest free length 13
  14. 14. Fig. 9 Chain Construction 14