A differential compound engine is described which indicates it to be a significant advance over other traction prime movers. Its engine rating, as confirmed by both theoretical and experimental analysis, represents an increase of approximately 150% over the corresponding naturally aspirated engine. Furthermore, the compound mode of operation implies feedback of surplus power to the output shaft and therefore overall efficiencies in excess of engine efficiency, at least over part of the load range. Values of 40% in the neighborhood of the design point can be expected, experimental work having demonstrated engine brake thermal efficiencies in excess of 42-43%. Additional advantages include rising torque characteristics with decreasing output shaft speed, the incorporation of effective engine braking, and response characteristics superior to those of a turbocharged engine.