As standards for hydrocarbon emissions from vehicles become more stringent due to environmental concerns, considerable effort has been devoted to investigate the mechanisms of formation, transport, and oxidation of unburned hydrocarbons in spark ignition engines.
In order to understand controlling factors in the processes, a transient one-dimensional reactive-diffusive model has been formulated for simulating the oxidation process taking place in the reactive layer between hot burned gases and cold unreacted air-fuel mixture, and for estimating exhaust hydrocarbon emission levels from natural gas spark ignition engines.
The main innovation shown by the model is the consideration also of the expansion of crevice gas in the axial direction. The model takes into consideration the contribution of the top land piston-ring crevice phenomenon, this being the main unburned hydrocarbon source in natural gas engines.
A comparison of the simulation results with experimental emission measurements shows an underprediction of the calculated remaining hydrocarbons in cylinder, probably because of the one-step kinetic mechanism employed for the oxidation of methane.