| The influence of different post-flame fuel oxidation rates on the hydrocarbon emission formation process in a spark ignition engine was investigated using both experimental measurements and model simulations. Because the engine was fueled by gaseous fuels, the investigation focused on the combustion chamber crevices, in particular the piston top-land crevices as a source of hydrocarbon emissions. An engine test system was set up, from which the regular exhaust emissions CO2, CO, O 2, NOx, total hydrocarbon, as well as the speciated hydrocarbons from C1 to C6, were measured under different engine operating conditions for a variety of fuels including pipeline natural gas, pure methane, ethane, propane, and iso-butane.; Three stages were found for the in-cylinder hydrocarbon development during the post-flame period, namely the blowdown process, the displacement process before the hydrocarbon vortex reaches the vicinity of the exhaust valve, and exit of the vortex through the exhaust valve.; The computational model can reasonably predict the regular exhaust emissions and speciated hydrocarbon emissions. The fuel's oxidation chemistry plays a complicated and critical role in the eventual hydrocarbon emission formation. Although the main component of the mixture of engine-out hydrocarbons is fuel itself, the fraction of the intermediate hydrocarbons increases as the fuel carbon number increases. Most of the intermediate hydrocarbons are generated and flow out of the cylinder during the blowdown and the displacement process. Most of the hydrocarbons in the vortex remain unoxidized. The cut-off temperatures, i.e. the temperature below which hydrocarbons that are intermediates in the fuel oxidation process begin to appear in the bulk gas, for different fuels are 1480 K for methane, 1300 K for ethane, 1400 K for propane, 1450 K for n-butane, and 1470 K for iso-butane.; A strong interaction between hydrocarbon emissions and NO x emissions was identified using the model. Direct interaction and indirect interaction mechanisms were defined. It was found that the reactions between radicals CH3, CH 2 and NO, and reactions between radicals O, H and NO are the dominant reasons for the interaction for low carbon number hydrocarbon fuels. (Abstract shortened by UMI.)... |
