In 1824 the French engineer S. Carnot published his work «Theoretical thermodynamic circular process», which became subsequently a basis of the theory of thermal engines. In this work the theoretical circular process comprises two isothermal and two adiabatic processes.
The degree of any engine perfection is defined by its real cycle correspondence to the theoretical. The theoretical cycle of Carnot proves that irreversibility of the cycle processes can be reduced almost to zero, if in isothermal process between points 1-2 on the temperature diagram of the working body is lower than the temperature of the hot source by infinitesimal quantity, and in isothermal process between points 3-4 the temperature of the working body is higher than the temperature of the cold source by infinitesimal quantity dT (Fig. 1).