Practical application of thermal-cycle mathematical model of internal combustion piston engines, which takes into account only the thermal energy of the working fluid that is used directly for the production of mechanical energy is considered. It is proposed to make the construction of a mathematical model and calculation of the thermal cycle on it for a number of agreed ratios of the heats of that supplied at constant volume and constant pressure as a function of the degree of expansion, followed by a preliminary analysis of the results.