Model of stirling engine realized by a syringe and a test-tube.

In the family of heat engines, 'Stirling engine' defines a closed-cycle regenerative hot air engine (or other permanent gas) engine, though the term is often used incorrectly to refer generically to a much wider range of hot air engine types. In this context, "closed-cycle" means that the working fluid is permanently contained within the system, whereas "open-cycle" engines such as internal combustion engine and some steam engines, exchange working fluid with their surroundings as part of the