What is a marine two-stroke engine crosshead?
A crosshead is a mechanical joint/connection used in marine two-stroke diesel engines to convert the rotating motion of the crankshaft into the linear motion required to move the piston and piston rod up and down within the cylinder liner.
Marine Slow-Speed Two-Stroke Diesel Engine Internals
Why is a marine two-stroke engine crosshead needed?
Crosshead type engines have a small bore-to-stroke ratio, which means that their stroke is long relative to the diameter of the piston crown. Due to the length of the stroke, the crankshaft also needs to have a large rotational diameter, but it is not economical to connect the piston directly to the crankshaft because:
- The engine crankcase would need to be very wide to accommodate the large rotational path of the crankshaft.
- The side forces acting upon the piston would be significant and aligning the piston within the cylinder would be difficult. If the piston is not correctly aligned with the cylinder, the piston rings will not press against the cylinder liner and seal correctly. Misalignment will also cause excessive wear on the cylinder liner and piston rings.
- The stresses placed upon the crankshaft would be excessive compared to if the force is first applied linearly to a piston rod, then to the crankshaft via a crosshead. The largest stresses are caused by the controlled explosions occurring within the combustion space.
Trunk engines are another type of marine two stroke engine, but they have a much larger bore to stroke ratio and therefore do not require a crosshead.
Crosshead and Trunk Type Engines