Unburned Hydrocarbon Emissions from DISI Engines Under Stratified Charge Operation

摘要

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 point is interesting in that liquid film formation 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, and manifold pressure. 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 model uses CFD to predict the piston wetting and the mass of fuel distributed in extremely lean regions. The model predictions correlate well with measured UHC emissions, demonstrating the potential to estimate the propensity for UHC emissions in a proposed design.