The paper presents the results of an investigation of gas engines used in the Polish system of natural gas transmission. The investigation concerned both four-stroke and two-stroke engines. The engines were fed with two kinds of gas fuel – low-calorific natural gas containing 54.5 % of methane, and with high-methane (up to 95 %) natural gas. Combustion in both types of engines with different methods of mixture supply into the cylinder was analysed for different parameters. The paper also presents numerical computations of basic physical values characterizing combustion of gas fuels in engines. The computations were made with Cantera numerical code based on the mechanism of elementary reactions occurring while burning methane GRI 3.0 for various molar fractions of methane in the gas fuel.