In order to understand better the operation of spark-ignition engines during the warm-up period, a computer model had been developed which simulates the thermal processes of the engine. This model is based on lumped thermal capacitance methods for the major engine components, as well as the exhaust system. Coolant and oil flows, and their respective heat transfer rates are modeled, as well as friction heat generation relations. Piston-liner heat transfer is calculated based on a thermal resistance method, which includes the effects of piston and ring material and design, oil film thickness, and piston-liner crevice. Piston/liner crevice changes are calculated based on thermal expansion rates and are used in conjunction with a crevice-region unburned hydrocarbon model to predict the contribution to emissions from this source.Comparison of model results to existing test data for a similar engine configuration has shown that the model predicts the correct component temperature trends during warm-up. Based on the results of this study, crevice region hydrocarbons are a substantial contributor to unburned hydrocarbons during cold start. Also, it was shown that piston expansion is the major controller of the rate of change of the crevice hydrocarbons during warm-up.