The effects of engine operating parameters on the unburned hydrocarbon in the residual gas in a cylinder of a spark ignition engine were investigated. Since the residual gas was not homogeneous in the cylinder, a method was developed to estimate the mean hydrocarbon concentration in the residual gas by the gas sampling technique. It was shown that the mean hydrocarbon concentration in the residual gas was much higher than in the emissions. In addition the analysis of net and gross combustion efficiencies was made. The influence of compression ratio on the hydrocarbon in the residual gas and combustion efficiencies was also studied.