Hydrocarbon emissions from spark ignition engines are considered to arise primarily from four mechanisms: (1) flame quenching; (2) combustion chamber crevices; (3) oil absorption; (4) incomplete combustion. A detailed spark ignition engine model has been modified to investigate some of these mechanisms. The base model considered flame quenching as the mechanism for generation of unburned hydrocarbons and exhibited the known weaknesses of this type of model, namely, an increase in hydrocarbon emissions as speed increases, and a decrease in these emissions with advancing spark timing. The modified model considers the crevice mechanism and the oil absorption mechanism as the primary means of producing unburned hydrocarbons and is shown to correctly predict the response of these emissions to changes in speed and spark timing. However, whereas the base model correctly predicted the expected increase in hydrocarbon emissions in the very lean region, the modified model did not, thus exhibiting a deficiency common to models that ignore the flame quenching and incomplete combustion mechanisms.