Stirling engine,somewhat working gas as isolated works with repeat heating and cooling process .
Adventages:
Thermo is exterior and fuel-air mixture burning must control more correct.
It is suitable for being air engine.Quiet,more less harmful to environmental,efficiel,reliable,more less vibration,more less risk of explosion.
A sample application;
Four cylinder stirling engine had done with shaving foam
(source:http://www.wikipedia.org)