Numerical investigation of soot emission sources in a direct-injection spark-ignition engine based on comprehensive breakup model validation

Abstract

Direct injection system is widely adopted in spark-ignition engines to achieve higher thermal efficiency, but it accompanies a penalty in particulate emission, especially when engine is not fully warmed-up. Split injection strategy is known to be an effective measure to reduce engine-out particulate emissions. To better understand the role of split injections, this study aims to analyze the effect of split injection strategy on the sources of soot formation using computational fluid dynamics simulation. To accurately predict changes in particulate mass and number associated with split injection strategy, it is vital that spray models be carefully validated against the experimental data since spray dynamics govern the formation of soot emission sources, such as local fuel-rich mixtures and wall-deposited fuel-films. To this end, a set of spray experiments for free sprays is performed to measure liquid penetration length and droplet size distribution, and hence a comprehensive validation is conducted for spray breakup models. Then, engine simulations are carried out to predict the change in soot sources according to split injection, and the trend of simulation results is compared against the measured engine-out particulate mass and number. Simulation results indicate that breakup model validation using both penetration length and droplet size data is critical for predicting fuel spray dynamics and formation of sources of soot emission. It is also revealed that the piston wetting decreases as the number of injections increases because less amount of fuel is injected when piston is closer to the injector. Lastly, the late evaporation of heavy gasoline components from fuel-film appears to be a significant contributor to soot precursors formation.