A study of hydrocarbon emissions from a homogeneous charge spark ignition engine

The purpose of this research was to investigate the chemical and physical processes important for hydrocarbon (HC) emissions from homogeneous-charge spark ignition engines. Experiments were performed with nitric oxide (NO) addition to the intake manifold to determine whether NO has an effect on post-flame HC oxidation chemistry within an engine. No chemical NO/HC interaction was found, indicating that the majority of the HC consumption takes place at temperatures higher than 1500 K. Hydrocarbon emissions from a range of paraffinic and olefinic fuels were correlated by the inverse diffusion coefficient of the fuel, indicating that mixing of the HC with the high temperature bulk gas is important for post-flame HC consumption. However, HC emissions from methane, iso-butene and toluene were not correlated by the fuel diffusivity, which demonstrated that chemistry can also play a role in HC emissions.; Detailed chemical kinetic modeling indicated that at the temperatures of relevance for post-flame HC consumption in engines (temperature higher than 1500 K), the majority of the fuel decomposes through unimolecular decomposition, and the radical pool which consumes the smaller HC intermediates is produced from the decomposition of the parent fuel. Experiments were performed with paraffinic/aromatic fuel mixtures to further investigate the influence of chemistry on HC emissions. Hydrocarbon emissions varied non-linearly with the amount of paraffin in the fuel mixture. These results demonstrated that kinetic interactions exist between fuel components in a fuel mixture, and that the decomposition of the parent fuel is critical to generating the radical pool which consumes the smaller intermediate HC species.; The experimental results were used to develop a global HC consumption rate correlation that reproduced the measured HC emissions from the engine to within 10{dollar}sim{dollar}15%. The correlation uses only three parameters to correlate all of the data from both n-butane and iso-octane fuels. The overall activation energy obtained for post-flame HC oxidation in the engine was 10 kcal/mole, and was the same for all fuels tested. This low activation energy is similar to that obtained from detailed chemical modeling and is believed to be related to the thermal decomposition rate of the fuel at high temperature.