The typical thermal efficiency of the most efficient large diesel engines is about 50%. Thermal efficiency measures how much of the fuel’s chemical energy becomes useful mechanical output rather than heat lost through exhaust, cooling, friction, and other processes.
Large, slow-speed two-stroke marine diesels are especially efficient because they run near a carefully optimized operating point, use very high compression, and are often directly coupled to a ship’s propeller. Their low rotational speed also reduces some friction and pumping losses. Modern examples can exceed 50%, with peak figures commonly reported in the 50–55% range.
The number is a peak engine figure, not a promise that every diesel engine—or an entire ship—converts half its fuel energy into useful work in every situation. Passenger-car diesels are generally less efficient, and efficiency falls when an engine operates far below its ideal load. Waste-heat recovery can improve the efficiency of a larger propulsion system, but that is distinct from the engine’s own thermal efficiency.