The principle of operation of this engine is the work done by the expansion and contraction of a gas, typically helium, hydrogen, nitrogen, or simply air. It must follow a cooling cycle in a cold focus shrinks, and the heat in a light bulb hot, it expands. That is, it is necessary the presence of a temperature difference between the two foci and is, therefore, of a heat engine.
The work cycle consists of 2 isochores (heating and cooling at constant volume) and 2-isotherms (compression and expansion at constant temperature). The hot air engine Stirling uses a heat source fixed to heat air in your cylinder. It can be considered as an external combustion engine for gear motors as it is governed by an adiabatic process, since it does not require burning fuel in its interior and to operate does not transfer heat to the environment.