Influence factor of diesel soot formation in diesel engine combustion predicted by multistep soot model with highlight in soot surface activity

Abstract

As more strict regulations on soot emission with increasing emphasis on the emitted soot particle size have been imposed, diesel engine partially premixed combustion had been proven to achieve ultra-low NOx and soot emissions with high thermal efficiency simultaneously, by synergy control of mixing and chemical parameters. It calls for further study in the effect of combustion boundaries on soot formation. In this study, soot formation characteristic was investigated by CFD code coupled with reduced n-heptane model and improved multistep soot model. History of acetylene, which is taken as the main species of PAH formation and soot surface growth, plays more important roles on soot prediction. The revised parameter of fraction of active sites α CH was introduced as the indicator of soot surface activity in diesel soot formation. The effects of combustion boundaries on soot surface activity and soot surface growth were explored in this study. When the mixture was quite homogeneous, lowered combustion temperature was the main factor for reduced soot formation due to the lowered specific surface growth rate R CH , in spite of α CH increasing slightly due to the slowed decay rate of surface activity. As the inhomogeneity of the mixture was increased, more unburned hydrocarbons were produced, promoting the formation of acetylene and soot surface activity. It was the dominated reason for higher soot surface growth rate, resulted worsened engine-out soot. In addition, residual of CO in later combustion phase impeded the re-oxidation of soot.