Power cycles are useful for performing work. Those that use a working fluid that is in the gas phase throughout the cycle are called gas power cycles. Other cycles involve liquid-gas and gas-liquid phase changes of the working fluid, and these are treated in the next chapter.
Engines operating on gas cycles can be classified as open or closed cycles, depending on whether the working fluid enters from and leaves to the environment (an open cycle) or the working fluid is wholly contained within the engine throughout the cycle (a closed engine cycle). Actual gas turbine and internal combustion engines are open engine cycles. The Stirling engine is an external combustion engine that cyclically heats and cools a working fluid that is wholly contained within the engine in a closed engine cycle. For a Stirling engine, a particular gas can be chosen as a working fluid contained within the closed system to best meet the engine requirements.