DIFFERENCES BETWEEN CROSSHEAD AND TRUNK DIFFERENCES BETWEEN CROSSHEAD AND TRUNK ENGINES Comparisons Of Cross Head and Trunk Piston Engines There 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 the areas of differences that are to be expected when viewing this type of engine. Crosshead Engine Construction The majority of 2-Stroke engines encountered at sea are of the "crosshead" type. In this type of engine the combustion space (formed by the cylinder liner, piston and cylinder head), 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 the diaphragm plate. The stuffing box provides a seal between the two spaces, stopping oil from being carried up to the scavenge space, and scavenge air leaking into the crankcase. The foot of the piston rod is bolted to the crosshead pin. The top end of the connecting rod swings about the crosshead pin, as the downward load from the expanding gas applies 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 with white metal, a bearing material and they reciprocate against the crosshead guides, which are bolted to the frame of the engine. The crosshead guides are located in-between each cylinder. 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 develops more power. The fuel used can be of a lower grade than that used in a trunk piston engine, with higher sulphur content, whilst high alkalinity cylinder oils with a different specification to that of the crankcase oil are used to lubricate the cylinder liner and piston rings and combat the effects of acid attack. Fig. 1 Crosshead Design The 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. Trunk Engine Construction The piston is directly attached to the connecting rod by a small end rotating bearing. Side thrust is absorbed by extended skirts on piston. The main advantage is reduced engine height (See handouts â Principal stationary and moving parts for more details) The Entablature The entablature is the name given to the cylinder block which incorporates the scavenge air space and the cooling water spaces. It forms the housing to take the cylinder liner and is made of cast iron. The castings are either for individual cylinders which after machining on the mating surfaces 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 is machined and then it is aligned on the A frames and fastened in position using fitted bolts. It is important to remember that the fitted bolts used to bolt the entablature, A frames and Bedplate together are for alignment and location purposes only. They are not designed 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). Fig. 2 Entablature Crosshead The purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the connecting rod and so bearings are required. It is also necessary to provide guides in order to ensure that the side thrust due to the connecting rod is not transmitted to the piston. This also ensures the piston remains central in the cylinder thus limiting wear in the liner. Two faces are required as the thrust acts in opposite directions during power and compression stroke. Guide shoes positioned at the extreme ends of the crosshead pin provided a large area and minimize risk of twisting. The crosshead pin connects the piston rod to the connecting rod. On either side of the crosshead pin are mounted the crosshead slippers. The slippers run up and down in the crosshead guides as the piston and rod are reciprocating and prevent the top of the connecting rod from moving sideways. Fig. 3 Crosshead Design Types of damage associated with the crosshead bearing There are two possible types of damage which may be sustained; 1 Wiping This is where part of the white metal contact faces are wiped out so that machining marks and oil grooves disappear, the material is displaced into the lubrication grooves where it forms '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. Possibly due, if occurring after trouble free operation, to particles in the lubricating oil. Roughness may also occur due to corrosion by weak acids forming in the lubricating oil. Water content above 1% can attack the white metal and cause formation of SnO which has the appearance of dark smudges on the surface. This must be removed whenever possible as the tin oxide can become harder than the metal of the journal causing obvious destruction of surface finish. 2. Cracking These may appear as individual cracks, hair line cracks, or densely cracked or crackled areas. The latter may be so dense so as to give the appearance of segregated grains. This can lead to scratching on the journal. The reasons for cracking may be insufficient bonding of white metal to the steel. Densely nested networks of cracks is due to fatigue fractures Stuffing Box Because the crankcases is separated from the cylinder and scavenge space by the diaphragm plate on a 2- Stroke crosshead engine, provision must be made for the piston rod to pass through the plate without oil from the crankcase being carried upwards, or used 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 leak into 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 a series of rings which are each made up of three or four segments. On the outside of each set of segments is a garter spring which provides the tension to hold the ring segments against the piston rod. There is a clearance between each segment to allow for wear. The rings are either bronze or can comprise of replaceable cast iron lamella fitted into a steel backing ring. The stuffing box is mounted on a ring which is bolted onto the underside of the scavenge air box. The stuffing box is taken out together with the piston rod during overhaul of the piston, but also can be disassembled for inspection in the crankcase with the piston remaining in position. The stuffing box housing is in two parts, assembled by a flanged joint. In the housing five ring grooves have been machined out of which the two uppermost ones accommodate sealing rings that prevent scavenge air from blowing down along the piston rod. In the lowermost grooves scraper rings are fitted which scrape the lubricating oil of the piston rod. The oil is led through bores in the housing and back to the crankcase. Fig. 4 Stuffing Box stuffing box in engine Fig. 5 Stuffing Box in Engine Fig 6 Stuffing Box Arrangement Between the two uppermost ring grooves, for the sealing rings, and the three lowermost grooves, for the scraper rings, a cofferdam has been machined out which, through a bore in the housing and a connecting pipe, communicates with a control cock on the outside of the engine. It can be checked by opening this control cock that the scraper and sealing rings are functioning correctly. Tie Bolts To understand the importance of the role played by the tie bolts or tie rods, it is necessary 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 as high 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 pressure acts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into the entablature prevent this happening and so this upward acting force tries to lift the entablature from the frames and the frames from the bedplate, putting the fitted location bolts into tension. As the piston moves down the cylinder the pressure in the cylinder falls, and then rises again as the piston changes direction and moves upwards on the compression stroke. This means that the fitted bolts are under cyclic stress. Because they are not designed to withstand such stresses they would soon fail with disastrous consequences. To hold the bedplate, frames and entablature firmly together in compression, and to transmit the firing forces back to the bedplate, long tie bolts are fitted through these three components and then tightened hydraulically. To prevent excessive bending moments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to hold the crankshaft main bearing cap in position instead of conventional 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 surfaces will rub together, corrode and wear away (this is known as fretting). Once this has happened the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause the transverse girders to bend which could lead to cracking, and main bearing misalignment Once fretting between the mating surfaces has occurred, then tightening of the tie bolts will pull the engine out of alignment. The crosshead guides, the cylinder liner, and the stuffing box will no longer be in line and excessive wear will occur. Because the tie bolts will no longer be pulled down squarely they will be subject to forces which may lead to them breaking. If fretting has occurred, then the only solution is to remove the entablature or/and frame and machine the fretted mating surfaces (a very costly exercise). Tie bolts can break in service. To reduce the risk of this happening they must be checked for tightness; not over tightened; and the engine not overloaded. If a breakage does occur, this is not disastrous; as the engine can be operated with care for a limited period (the load on the engine may have to be reduced). The position of the fracture will dictate how the broken pieces are removed. However in the worst possible scenario where 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 7mm diameter) down the tie bolt tube, down the side of the broken tie bolt and once it emerges at the bottom a supporting piece can be fitted to the wire enabling the broken tie bolt to be withdrawn. Fig. 7 Tie bolt Arrangement Chain Drives Rotation 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 disadvantage to wear from foreign materials as well as their increased cost. The disadvantage of chains is the requirement for tensioning and their finite life. Although for large installations this can be very long. Wear on the chain pins, bushes as well as the chain sprockets can all lead to a slackening off of the chain. This can lead to 'slap' and changing of cam timing. This alters the leads of the fuel pumps and exhaust valves. The degree of angular displacement can be checked using a manufacturer supplied poker gauge. Chain damage occurs if the chain is too tight or too slack and the result is fatigue cracking of the links. If the tension is too tight, then this adds to the working stress of the chain. Insufficient tension leads to 'slap' with resultant damage to chain and rubbing strips. Vertical misalignment of the sprockets means rubbing at the side plates resulting in reduction of thickness and possible failure. Recommended limit on stretch is about 1.5 to 2%, if maximum movement of the tension is reached before the chain has reached its maximum stretch then a pair of links may be removed. When maximum stretch is reached, or if the chain shows signs of damage then the chain should be replaced. The simplest method is to break the old chain and attach the new chain to it. The engine is then turned and as the old chain is paid off, the new chain can be paid in. This maintains approximately the correct timing; the tension of the chain can then be set. Final adjustment of the timing can be made following manufacturers instructions; this generally means turning the engine until No1 is at top dead, then checking by us of pointer gauges the position of the cam. The cam drive is adjustable and can be slackened off, by hydraulic means on large modern engines, the section of cams can then be turned relative to the crankshaft angle and the timing restored. The chains are lubricated by the injection of a spray of oil between the chain wheels and the chain rollers just before the rollers are about to engage the wheel. Thereby an oil cushion is formed to dampen the impact Fig. 8 Typical Chain Drive Arrangement Chain stretch and hence reduction in tension can be accounted for by movement of a tensioning wheel. The tension usually being checked by movement to and fro at the centre of the longest free length Fig. 9 Chain Construction PAGE 12