Stirling engine
A Stirling engine is a heat engine that is operated by the cyclic compression and expansion of air or other gas (the "working fluid") at different temperatures, resulting in a net conversion of heat energy to mechanical work. More specifically, the Stirling engine is a closed-cycle regenerative heat engine with a permanent gaseous working fluid. "Closed-cycle", in this context, means a thermodynamic system in which the working fluid is permanently contained within the system, and "regenerative" describes the use of a specific type of internal heat exchanger and thermal store, known as the "regenerator". Strictly speaking, the inclusion of the regenerator is what differentiates a Stirling engine from other closed-cycle hot air engines.