Hilditch, J., Han, Z., and Chea, T., "Unburned Hydrocarbon Emissions from Stratified Charge Direct Injection Engines," SAE Technical Paper 2003-01-3099, 2003, doi:10.4271/2003-01-3099.
The sources of unburned hydrocarbon (UHC) emissions in direct injection stratified charge engines are presented. Whereas crevices in the combustion chamber are the primary sources of UHC emissions in homogeneous charge engines, lean quenching and liquid film layers dominate UHC emissions in stratified charge operation. Emissions data from a single cylinder engine, operating in stratified charge mode at a low speed / light load condition is summarized. This operating point is interesting in that liquid film formation, as evidenced by smoke emissions, is minimal, thus highlighting the lean quenching process. The effects of operating parameters on UHC emissions are demonstrated via sweeps of spark advance, injection timing, manifold pressure, and swirl level. The effects of EGR dilution are also discussed. Spark advance is shown to be the most significant factor in UHC emissions.A semi-empirical model for UHC emissions is presented based on the analysis of existing engine data. The UHC model uses computational fluid dynamics (CFD) to predict the mass of fuel distributed in extremely lean regions and the piston wetting - believed to be the primary sources of UHC emissions in stratified charge operation. When corrected for spark advance, the model predictions correlate well with measured UHC emissions, demonstrating the potential to estimate the UHC emissions propensity for proposed design changes.