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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.

A fast response electromechanical sampling valve has been used to collect samples from the exhaust of a spark ignition engine. The variations in concentration of unburned fuel and other hydrocarbon species through the engine cycle have been analyzed by gas chromatography. The results have pointed out that most of the unburned fuel comes from sources located in the lower part of the combustion chamber, such as piston-ring crevices and oil film absorption/desorption. The appearance of non-fuel HC species in the exhaust have been related to the post-flame oxidation process of the unburned fuel. A high-frequency flame ionization detector (fast FID) was employed to measure the cyclic variation of the total exhaust hydrocarbons.