All jet engines and gas turbines are heat engines that convert thermal energy into useful work. The useful work may be in the form of mechanical power, as from a shaft which may be used to drive a propeller, a vehicle, a pump, an electric generator, or any other mechanical device. In Jet engine applications the work is in producing compressed air and combustion products which are then accelerated to provide reaction propulsion.
The thermal efficiency for a shaft application is calculated using the ratio of output mechanical energy divided by input thermal energy. For propulsive applications the definition of efficiency involves the velocity of the jet through the air since a stationary engine anchored to the ground produces no propulsive work. In such a case a propulsive efficiency is defined which relates the propulsive work delivered to the total mechanical energy produced by the engine.