The principle of combustion stimulation by a gas jet is to inject a small amount of the incombustible gas in the SI engine combustion chamber. Depending on the moment of the gas injection with respect to the moment of the spark ignition, the jet (introduced in the right direction) stirs unburned charge (injection before ignition) or goes through the flame kernel (injection after ignition) and removes chemically active combustion products towards farther parts of the chamber. Previous investigations revealed that in both cases the jet can result in combustion intensification, in increase of the indicated mean effective pressure (IMEP) and in significant increase of the cycle peak pressure. Besides, the action of the gas jet can affect knocking combustion, cyclic variability and can decrease pollutant emissions.Previous investigations explained, by means of visualisation of the combustion process, the mechanism of most of the phenomena caused by the gas jet. However, explanation for some test results, for example the decrease of exhaust gas temperature, was still supposed. The heat release analysis of the gas jet stimulated combustion allows to elucidate many such problems. In addition, the heat release analysis confirmed that previous conclusions, for example those related to the increase of the indicated mean effective pressure, were quite correct. Further, the magnitude of the heat release rate in both cases (without injection and with injection) was estimated.