Four-stroke engine

A four-stroke engine takes two crankshaft revolutions per power cycle, with distinct intake, compression, power, and exhaust strokes controlled by poppet valves. Marine four-strokes run medium-speed (about 400 to 1000 rpm) and trunk-piston, driving generators directly and propellers through a reduction gearbox. They give higher power density and lower mass per kW than two-strokes, at slightly lower peak efficiency, and dominate auxiliary, ferry, and diesel-electric installations.