Stable operation of the diesel engine at all speeds is provided by the adjustment of the fuel equipment, the structural parameters of the combustion chamber, the physical and chemical properties of the fuel, etc. To increase the range of operating speeds of the engine and increase its injectivity, studies are being conducted aimed at optimizing the processes of fuel combustion, improving the adjustments of the fuel equipment, shape and size of the combustion chamber. This paper presents an analysis of the performance of the diesel fuel combustion process in a 2F 10.5/12.0 diesel cylinder at rated load at various speed modes of operation with a fixed mechanism for controlling the cycle fuel supply. When conducting a laboratory study, measurements were made of hourly fuel and air consumption in all modes of operation. The diesel was installed on a loading stand with a weight mechanism, with which the torque measurement was carried out. The results of indexing the working process of the diesel engine, the characteristics of heat. Calculation of heat release rate and gas temperature indicator was carried out by the CNIDI method.