Modeling soot emissions from wall films in a direct-injection spark-ignition engine

Due to the new challenge of meeting number-based regulations for particulate matter in spark-ignition engines, and the important contributions of wall films to soot emissions, a computational fluid dynamics model was formulated and improved to enable the prediction of soot emissions (mass and number density) from spark-ignition engines, with consideration of multi-component fuel surrogates to represent real gasoline and with full three-dimensional cylinder engine grids. A chemistry mechanism including n-heptane/iso-octane/toluene/polycyclic aromatic hydrocarbon (89 species and 506 reactions) was formulated, and the model was also validated based on available experimental data from laminar premixed flame burners and the SANDIA engine combustion network spray combustion data. Fuel pyrolysis was also found to be a significant process for soot formation in direct-injection spark-ignition engines. The vaporization of the wall films plays a significant role in soot formation, and a grid-independent wall film model was achieved for predicting soot emissions near wall films. The improved models were then applied to study of soot emissions from late-injection strategies in a four-valve single-cylinder gasoline direct-injection spark-ignition engine, and the trends seen are consistent with experimental data.