The 2 stroke crosshead engine works on exactly the same principle and cycle as the 2 stroke trunk piston engine. The disadvantages of the two stroke trunk piston engine are that although it has a low overall height, lubricating oil splashed up from the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and a risk of fire. There is also the likelihood of liner and piston skirt wear, allowing air into the crankcase. This can supply the required oxygen for an explosion should a hot spot develop. The crankcase oil must have additives which can cope with contamination from products of combustion, and the acids formed during combustion due to the sulphur in the fuel.
The most powerful diesel engines in the world are two stroke crosshead engines. Some of these engines have cylinder bores approaching 1metre with a stroke of over 2.5 metres. The crankshaft can weigh over 300 tons, with the engine weighing in excess of 2000 tons
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| The Crosshead |
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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.
The crosshead pin sits in the crosshead bearing which is either a forked type (shown left), where the bearing housings are mounted on the top of the connecting rod either side of the piston rod, or of the continuous type (shown above and below), where the bearing housing is formed by the top of the connecting rod. On the forked type, the piston rod passes through a hole in the crosshead pin and is secured by a nut. On the continuous type, the piston rod has a foot which is bolted onto the top of the crosshead pin. Modern engines are fitted with the continuous type of bearing. The crosshead bearing is difficult to lubricate effectively. Because the top of the connecting rod swings about the pin and changes direction each time the piston reaches mid stroke, the relative speed between bearing and pin at mid stroke is zero, accelerates to a maximum as the piston approaches top or bottom dead centre and then decelerates back to zero again as the piston approaches mid stroke and the con rod changes direction. This means that hydrodynamic lubrication, where the pin is separated from the bearing by a wedge of oil only occurs over part of the swing; i.e when the relative speed between the two components is high enough. The load on the pin is always downwards, so it is the bottom half of the bearing which is subject to wear. Because of the high loads the bearing material is a tin-aluminium alloy bonded to a steel shell. The pin is highly polished to a mirror finish. To accommodate the high downward load and to aid effective lubrication the pin has a large diameter. This increases the relative speed between pin and bearing. The bottom halves of the bearing shells have oil gutters cut in them to assist the distribution of oil. Oil is supplied to the crosshead using a swinging arm or a telescopic pipe and is sometimes boosted in pressure to aid efficient lubrication.
The crosshead slippers are mounted on stepped journals machined either end of the crosshead pin, secured in place by end plates. The slippers float on the journals, to allow for any slight misalignment in the guides. The rubbing surfaces are white metal lined. Oil is supplied to the slipper rubbing faces from the crosshead oil supply. The slippers have gutters machined in them to assist the spread of the lube oil. The guide surfaces are either machined into the A frames or are separately cast and machined, then bolted to the A frames. The alignment of the guides is very important as is the clearance between the guides and slippers. If this alignment is out of true or the clearances excessive then excessive wear will occur between piston rod and stuffing box and piston and cylinder liner. |
