The gas as the working fluid does not undergo any phase change. The engines working on the gas cycles may be either cyclic or non-cyclic. The working fluid is assumed to obey all laws of a perfect gas.
In most of the gas power cycles, the working fluid consists mainly of air. Therefore, it is convenient to analyse the gas power cycles by devising an idealized cycle known as Air Standard Cycle, in which the working fluid is pure air.
The following assumptions are made in the air-standard cycles: