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A computer model has been developed to predict exhaust hydrocarbon emission levels from spark ignition engines. The model incorporates the two presently accepted main sources of unburned hydrocarbons: the top land piston-ring crevice and the oil film absorption/desorption process. The main innovation shown by the model is the consideration of the combustion period in the calculation of the unburned hydrocarbons concentration. Both the oil film and the crevice are divided into small volume elements. The flame position during combustion is monitored, so that it can be determined for each volume element if absorption or desorption in the oil film is taking place, and if the flow to the crevice is of burned or unburned mixture. The model shows that failure to consider the combustion event leads to an underestimation of about 18% of the HC desorbed from the oil film, and to an overestimation of around 14% of the HC released from the crevice.