Soot formation and oxidation mechanisms in a diesel engine separated swirl combustion system

Abstract

The separated swirl combustion system (SSCS) is a new combustion system developed to improve fuel efficiency and reduce soot emissions in diesel engines. While soot emissions from the SSCS can be tested in a real-world single-cylinder engine, soot formation and oxidation cannot be; therefore, the mechanisms behind those processes have not yet been researched. To understand the mechanisms of soot formation and oxidation in the SSCS, soot evolution must be investigated using a simulation model. To bridge this gap, this study analyzed the combustion and emission performance of the SSCS under different speeds in a real-world single-cylinder engine. Then, a new phenomenological model was developed and used to simulate soot formation and oxidation and reveal the mechanisms behind those processes. The SSCS with the new model was validated against a double swirl combustion system (DSCS). The experiment results show that at 2100 r/min, the SSCS significantly reduced fuel consumption (about 6.54 g/(kW h)) and soot emission (0.17 FSN) by 2.89% and 6.31%, respectively. The simulation results show that the DSCS generates more incipient soot particles and soot mass than the SSCS, and surface soot oxidation is faster in the SSCS. The mechanisms analysis shows that the equivalence ratio is smaller in the SSCS than in the DSCS, so fewer incipient soot particles are generated, and the temperature is higher, which speeds up soot oxidation. Furthermore, the combustion duration is much shorter in the SSCS than in the DSCS, which means that the time between the end of combustion and the exhaust valve opening is longer, granting more time for soot oxidation. Due to the decreased soot formation and faster soot oxidation, the soot mass is also lower in the SSCS than in the DSCS.