Definition of compound engine
1 : an engine (as a steam engine) in which the working fluid is expanded successively in two distinct phases so as to minimize losses (as from cylinder condensation) and so allow a high ratio of expansion to be used, the working fluid (as steam) after expanding in the high-pressure cylinder being exhausted into a low-pressure cylinder and then exhausted usually into a condenser
2 aeronautics : a propulsive system consisting essentially of a reciprocating engine, a steady-flow gas turbine, and a compressor so arranged that (1) the exhaust from the reciprocating engine drives the turbine, the exhaust from the turbine being directed rearward for jet propulsion, (2) the turbine drives the compressor, the excess turbine power being delivered to the engine shaft through gearing, and (3) the net shaft power of the system is converted to propulsive power by means of a propeller
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