motor that converts thermal energy to mechanical work

a heat engine in which ignition occurs outside the chamber (cylinder or turbine) in which heat is converted to mechanical energy

a heat engine in which combustion occurs inside the engine rather than in a separate furnace; heat expands a gas that either moves a piston or turns a gas turbine

A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas (the working fluid) at different temperatures, such that there is a net conversion of heat energy to mechanical work.