Two-stroke crosshead engine

A two-stroke crosshead engine completes a power cycle every revolution and separates the running gear with a crosshead, so the piston rod stays purely axial and the diaphragm isolates the dirty scavenge/combustion space from the clean crankcase. This lets the cylinder burn high-sulfur residual fuel while the crankcase keeps a separate clean lube system. Slow-speed (about 70 to 120 rpm) and large-bore, these engines couple directly to the propeller at the highest thermal efficiency of any marine prime mover.