What is the difference between a two-stroke and a four-stroke marine engine?

The fundamental difference between a two-stroke and a four-stroke marine engine is how many piston strokes it takes to complete one full working cycle, and that single difference cascades into very different size, speed, mechanical design and role aboard ship. A two-stroke engine completes intake, compression, power and exhaust within just two strokes of the piston, one up and one down, so it fires and produces a power stroke on every single crankshaft revolution; it typically uses scavenge ports low in the cylinder liner to admit fresh air and a single exhaust valve, often hydraulically or electronically actuated, at the top to expel burnt gas, with no separate camshaft driven inlet valve needed. A four-stroke engine needs a full four strokes, intake, compression, power and exhaust, so it fires only on every second revolution, and uses a camshaft to mechanically operate both separate inlet and exhaust valves in a more conventional automotive-like arrangement.

In marine practice the two engine types have settled into quite distinct and largely non-overlapping roles:

• Large low speed two-stroke crosshead engines dominate main propulsion on big merchant ships, tankers, bulk carriers and container ships. They run slowly, connect directly to the propeller shaft without a reduction gearbox, burn heavy fuel oil economically thanks to their low speed and long stroke, and their crosshead design, which keeps side thrust off the piston and cylinder liner, lets them run reliably for years on lower grade fuel between major overhauls.
• Medium and high speed four-stroke trunk piston engines are more compact, mechanically simpler in some respects despite the extra valvetrain, and run considerably faster. This makes them the standard choice for auxiliary diesel generators supplying a ship's electrical power, and for main propulsion on smaller vessels, where they connect to the propeller through a reduction gearbox since their higher running speed does not suit direct drive.
• Two-strokes are generally regarded as more thermally efficient for main propulsion at large scale, given enough time to optimize a huge slow running engine, while four-strokes offer better power to weight ratio, faster response, and the flexibility of running multiple smaller units in parallel, which is exactly what a ship's electrical plant needs for redundancy.

Because most large merchant ships need both a slow, efficient, direct drive main engine and fast, flexible electrical generation, they typically carry both engine types working together rather than choosing one over the other, a two-stroke crosshead main engine for propulsion and several four-stroke generator sets for auxiliary power, each doing the job it suits best rather than either type being simply superior overall.