The paper presents the thermodynamic analysis of the engine supplied with small and large diesel fuel doses while increasing natural gas quantity. The paper presents changes in the combustion process thermodynamic indexes and changes in the exhaust gas emissions for dynamically increased share of the gaseous fuel. The cylinder pressure history was subject to thermodynamic analysis, . based on which the mean indicated pressure, the heat release rate, the quantity of heat released as well as the pressure rate increase after self-ignition were determined. These parameters were also referred to the subsequent engine operation cycles by specifying the scope of the change per cycle. The relationship between the engine load and the start, the center and the end of combustion while increasing the gas amount supplied to the cylinder was indicated. The presented analysis of the results indicates significant impact of the changes in the supplied gas amount on the engine operating indexes and exhaust emissions. The conducted tests made it possible to confirm that significant cooling of the cylinder (determined by the constant temperature of exhaust gas) does not allow combustion of a dual-fuel mixture during dynamic change of the engine fueling. It was shown that there is a relation between the thermal history of the cylinder (steady state conditions), the engine fueling rate and the possibility of combustion of a diesel-methane mixture.