Four-Stroke Engine

The four-stroke diesel operates on an Otto cycle modified for compression ignition, completing induction, compression, combustion, and exhaust over two complete crankshaft revolutions. Its more complex valvetrain — with separate intake and exhaust valves in the cylinder head — allows precise control of gas exchange timing and makes the engine well suited to medium-speed operation between 500 and 1,000 rpm. On ships, four-stroke engines serve principally as diesel generator prime movers, but medium-speed models are also used as main propulsion engines coupled to the shaft through a reduction gearbox or driving electric motors in a diesel-electric arrangement. Because the crankcase and combustion space share the same engine oil in a four-stroke trunk-piston engine, lubricating oil alkalinity must be maintained to neutralise combustion acids, particularly when burning high-sulphur fuels. SOLAS II-1 and classification society rules require regular cylinder liner wear measurements and combustion analysis to verify that the engine remains within design parameters. Fuel injection equipment overhaul intervals and valve seat grinding schedules are published in the maker's maintenance manual and are inspected by class surveyors during periodical surveys.