A mathematical model of an ideal thermotropic cycle is proposed, which extends the idea of real working processes in the internal combustion engine (ICE). The thermotropic cycle the same for diesel, gasoline and gas ICE, and also provides significantly better correspondense to the real cycles than classical cycles. Heat supply to the cycle is carried out in the form of a combined process "compression-expansion". The combined process consists of two incomplete thermotropic processes and with a high degree of approximation reproduces the laws of real combustion processes in the ICE, from compression to expansion. The mathematical model of the basic thermotropic process is based on the laws of molecular kinetic theory and thermodynamics of ideal gases. The main thermotropic process is based on three fundamental equations of thermodynamics: differential equations of the first law of thermodynamics and the law of heat exchange of the external source of the working medium, as well as the equation of state (Clapeyron). The heat of the process in the thermotropic process, in contrast to the polytropic, is an independent value and takes into account fuel consumption. The equation of the main thermotropic process has additivity, which allows us to consider its parameters as the sum of the adiabatic and thermal components of the process occurring in the gas mixture. An important feature of the thermotropic process is also the variable heat capacity of the process. The gas laws of classical thermodynamics are special cases of a new process. The advantage of the new cycle model is mathematical simplicity. The initial system of equations made it possible to perform precise integration and express the equations of processes and cycles in elementary functions. Precise integration ensured high accuracy of estimation of influencing factors contained in the model and absolute convergence of thermal and material balances of the cycle. A number of elements of the theory of thermotropic processes and cycles can be used to replace isochoric-isobaric-polytropic combustion models in undergraduate and specialist studies. This is facilitated by the absence of the need for complex computer programs that significantly complicate the development of educational material. In General, models with more complex laws of heat supply can be useful for undergraduates and graduate students to pre-evaluate the effectiveness of the proposed project activities to improve the internal combustion engine.