It is possible to propose another method to obtain approximately isothermal processes of compression and expansion. This method is used in some multistage gas turbines where the gas is cooled during the compression stages and is heated during the expansion stages. The method can also be used in piston engines including relatively low-power engine but in piston engines it demands the use of valves and cannot be realized in Stirling engine, but can be realized in Ericsson engine. An example of Ericsson engine is described in [11]. The engine power is 10.8 kWt. It is based on the open cycle; that is, the air from atmosphere enters to the two stage compressor with intermediate cooling. Compressed air enters to the recuperator at the temperature of 146°C and the pressure of 600 kPa. In the recuperator it is heated up to 379°C and after that enters to the heater, where its temperature increases to 800°C. With this temperature the air goes to the expander where its temperature drops down to 405°C due to almost adiabatic expansion. After expander the air flows through the recuperator where it is cooled down to 172°C. At this temperature the air goes to the atmosphere.