The results of mathematical modeling of the operating process of a spark ignition engine when running on gasoline and two carbon-free fuels: hydrogen and ammonia are presented. To carry out the computational study, a software package developed at MADI was used. In this model, the factor m in the Wiebe formula that corresponds to each fuel type is determined. This value selection is based on the condition of coincidence of experimental and calculated data on the maximum cycle pressure and the highest rate of pressure increase. In the second stage of computational studies, the impact on the performance of a spark ignition engine from switching to hydrogen and ammonia was assessed. The results show that, with the transition to hydrogen, the maximum value of the heat release rate increases by 17.5% compared to gasoline, while with ammonia increases by 1.6%. Engine operation on ammonia has the highest hourly fuel consumption, which is 2.6 times higher than that of gasoline and 6.2 times higher than that of hydrogen.