Theoretical analyses of the cycle efficiency being based on the air standard cycles give rather poor information about engine behavior under various operating conditions. Considerations done by the author have shown that a limited usefulness of the air standard cycles results from the assumption that composition of the working gas expressed by the isentropic index is constant during the cycle.In the paper a new method of approach to the theoretical analysis of engine cycle efficiency has been presented. Unlike the air standard cycles, the method allows the values of isentropic index to be different for the three main stages of the cycle i.e. for compression, heat addition and expansion. Moreover, heat addition can occur at constant volume, pressure and temperature, or as their combination. Theoretical cycle with a changeable isentropic index can be presented in pressure-volume or temperature-entropy diagrams. However, to present it in T-s plane differences in the entropy values have to be considered. For that purpose Kirchoff's equation describing heat of reaction is to be employed to take into account change of the isentropic index of the working gas.With such an approach it is simply to pass to a real indicator diagrams since isentropic indices can be turned into polytropic indices. An example of application of the method for a turbocharged D.I. Diesel engine has been presented.