Two Stroke Crosshead Diesel Engine

The two stroke crosshead type diesel engine works on the same principle as

two stroke trunk type diesel engine. The majority of two stroke diesel

engines at sea are of the “Crosshead” type. In this type of engine, the

combustion space (formed by the liner, piston and cylinder head) is separated

from the crankcase by a diaphragm plate.

The piston rod is bolted to the piston and passes through a stuffing box mounted in the diaphragm plate. The stuffing acts as a seal between the two spaces; stopping oil from being carried to the scavenge space and preventing scavenge air from 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 on to the crankshaft.

To ensure that the crosshead reciprocates in alignment with the piston in the liner, guide shoes are attached on either side of the crosshead. These shoes are lined with whitemetal, a bearing material and a reciprocate against crosshead guides, which are bolted to the frame of the engine.

Using the crosshead design, allows the engine to be built with very long strokes. This enables engines to burn greater quantity of fuel/stroke and develop more power and also enables engines to run at slow speed so that it can be directly coupled to screw propellers, which drive ships without the use of reduction gears (higher the speed of these screw propellers, the lower their efficiency – today the rpm of these engines about 100 and lower). Non use of reduction gears results in savings in cost, space and weight.

Also the fuel used in the engine can be of an inferior grade having higher sulphur content than used in a trunk type engine. Higher alkalinity and viscosity cylinder oil having different specification from crankcase oil are used to lubricate the cylinder liner and piston rings and combat the affect of acid attacks and high temperature of the combustion space.

The most powerful diesel engines are two stroke crosshead type engines. Some of these engines have bores reaching upto one metre and strokes over 2.5 metres. The crankshaft may weigh upto 300 tons and engine may weigh over 2000 tons.