A model is presented which incorporates the key mechanisms in the formation and reduction of unburned HC emissions from spark ignited engines. The model includes the effects of piston crevice volume, oil layer absorption / desorption, partial burns, and in-cylinder and exhaust port oxidation. The mechanism for the filling and emptying of the piston crevice takes into account the location of the flame front so that the flow of both burned gas and unburned gas is recognized. Oxidation of unburned fuel is calculated with a global, Arrhenius-type equation.A newly developed submodel is included which calculates the amount of unburned fuel to be added to the cylinder as a result of partial burns. At each crankangle, the submodel compares the rate of change of the burned gas volume to the rate of change of the cylinder volume. If this ratio reaches a critical value during combustion, the mass fraction unburned at that time is used to quantify the amount of unburned fuel due to partial burns.The model is calibrated to engine data for two different engines and reasonable agreement is obtained as a function of engine speed, load, air-fuel ratio, EGR rate and spark timing. Results of a parametric study are presented which show the effect of several combustion chamber design variables on HC emissions. These variables include bore-to-stroke ratio, displacement per cylinder, compression ratio and crevice volume per unit displacement. The model predictions indicate that bore-to-stroke ratio has little effect on HC emissions while decreasing displacement per cylinder leads to increased HC emissions and increasing both compression ratio and crevice volume per unit displacement lead to increased HC emissions.