How does a marine diesel engine work?

A marine diesel engine works on the compression ignition principle: air alone is drawn into the cylinder and compressed to a ratio high enough, generally in the range of roughly 12:1 to over 20:1 depending on engine type, that its temperature rises well above the fuel's self ignition point, and only then is fuel injected as a finely atomized spray near the top of the stroke, where it ignites spontaneously on contact with the hot compressed air rather than needing a spark as in a petrol engine. The rapidly expanding combustion gases drive the piston down the cylinder, and a connecting rod converts that linear motion into rotation of the crankshaft, which ultimately turns the propeller shaft directly or through a reduction gearbox, or drives a generator for electrical power.

Large ships rely on two broad engine families built around this same principle. The great low speed two-stroke crosshead engines, which power most large tankers, bulk carriers and container ships, complete a full power stroke every single revolution of the crankshaft: fresh scavenge air is admitted through ports low in the liner while the piston is near the bottom, and burnt gas is expelled through a hydraulically or electronically actuated exhaust valve at the top, with a crosshead bearing isolating the piston's side thrust from the connecting rod so the engine can run reliably on lower grade heavy fuel oil for years between major overhauls. These engines run slowly and their low speed and direct drive to the propeller are exactly what makes them efficient at ship propulsion.

Medium and high speed four-stroke trunk piston engines instead need four full strokes, intake, compression, power and exhaust, to complete one cycle, so they fire only every second crankshaft revolution, use camshaft operated inlet and exhaust valves rather than ports, and run considerably faster. Their smaller size and higher speed make them well suited to auxiliary generators supplying the ship's electrical power, and to main propulsion on smaller vessels, usually connected to the propeller through a reduction gearbox since their higher running speed does not suit a propeller directly.

Both engine families depend heavily on turbocharging: exhaust gas, still under significant pressure and temperature as it leaves the cylinder, drives a turbine that in turn drives a compressor forcing considerably more air into the cylinders than could be drawn in by the piston alone, which is what allows these engines to burn enough fuel per cycle to produce their rated power. Fuel injection timing and quantity, controlled either by a traditional camshaft driven jerk pump system or by modern electronically controlled common rail and hydraulic systems on newer engines, governs both the power developed and how cleanly the fuel burns, which increasingly also has to satisfy emissions requirements met through engine tuning, scrubbers or selective catalytic reduction depending on the ship and the regulations it trades under.