Stirling engines

A Stirling engine is a heat engine in which closed working gas such as air or helium is alternately heated and cooled from the outside in two different locations in order to produce mechanical energy. The Stirling engine operates on the principle of a closed cycle and is an example of energy conversion of a poorly usable form of energy (thermal energy) to a more usable mechanical energy. The Stirling engine can utilize any external heat (or cold) source.