The cycle shown above is an idealized version of the cycle that real sterling engines run. When heat flows from the hot reservoir to the cold reservoir then the cycle is run in the clockwise direction as shown on the diagram and the result is work done by the output shaft. When work is done on the output shaft the cycle is run in the counterclockwise direction and heat flows from the cold reservoir to the hot reservoir. The processes on the top and bottom of the cycle are isothermal and the processes on the right and left are isovolumetric. When the cycle is run in a clockwise direction Heat is added during the left isovolumetric and the top isothermal processes (shown in
red) and heat is rejected during the left isovlolumetric process and the bottom isothermal process (shown in
blue)(for a full derivation see the
theory section).
The volume of a real sterling engine is changing constantly so the isovolumetric processes in the diagram above are not isovolumetric in a real engine. The isothermal processes though can be quite close to isothermal with real engines. Real engines have reduced efficiencies due to friction and conduction around the gas.