Working medium is a kind of dielectric substance (such as gas, steam, etc.) Among them, heat energy is converted into mechanical energy in a heat engine, and work can only be obtained through its state change (such as expansion) in a heat engine.

In order to get more work in the expansion of the working medium, the working medium should have good expansibility. In the continuous operation of the heat engine, in order to facilitate the inflow and discharge of the working medium, it is also required that the working medium has good fluidity. Therefore, among the solid, liquid and gaseous substances, gaseous substances are ideal working media. At present, steam is the main working medium in thermal power plants.