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The oxidation chemistry of paraffins, aromatics, olefins and MTBE were examined. Detailed chemical kinetics calculations were carried out for oxidation of these compounds in the engine cycle. The oxidation rates are very sensitive to temperature. At temperatures of over 1400 K (depending on the fuel), all the hydrocarbons are essentially oxidized for typical residence time in the engine. Based on the kinetics calculations, a threshold temperature is defined for the conversion of the fuel species to CO, CO2, H2O and partially oxidized products. The difference in the survival fraction between aromatics and non-aromatics is attributed to the higher threshold temperature of the aromatics.

The engine-out (EO) total and speciated hydrocarbon emissions data from the Auto-Oil Program Phase II Heavy Hydrocarbon Study had been analyzed. The methodology was to first investigate the stabilized EO emissions (Bag 2) of a specific vehicle (Vehicle 04B, a 1989 Model Year Ford Taurus); then the vehicle-to-vehicle differences in Bag2 emissions were considered. Finally, the differences in the Bag2 and the starting/warm-up EO emissions (Bag1) were examined. The speciated emissions may be interpreted as a “feed-through” part due to the unreacted fuel species, and an “offset” part due to the decomposition products. The significant non-fuel emitted species were methane and the olefins. The HC emissions for vehicles with different total emissions were similar in species composition. For both the total and speciated emissions, there was no substantial difference between the Bag1 and Bag2 values for Vehicle 04B.

About 50-90 percent of the hydrocarbons that escape combustion during flame passage in spark-ignition engine operation are oxidized in the cylinder before leaving the system. The process involves the transport of unreacted fuel from cold walls towards the hotter burned gas regions and subsequent reaction. In order to understand controlling factors in the process, a transient one-dimensional reactive-diffusive model has been formulated for simulating the oxidation processes taking place in the reactive layer between hot burned gases and cold unreacted air/fuel mixture, with initial and boundary conditions provided by the emergence of hydrocarbons from the piston top land crevice. Energy and species conservation equations are solved for the entire process, using a detailed chemical kinetic mechanism for propane.