Study on reduction of exhaust emission in diesel combustion

摘要

Mechanisms of nitric oxide formation and emission for a conventional high-speed direct-injection diesel engine have been studied by a stochastic model that takes chemical kinetics into account. In calculations cetene was used as a fuel and itsinitiation processes were expressed by a one-step global mechanism. Detailed reduced chemical mechanisms were employed for the processes of heat release and NO formation. The calculated NO concentration coincides with a value measured in an exhaust gaswithin 10%. It is shown that most NO is formed locally at temperatures higher than 2000 K under a range of excess air ratios, 1<λ<3.The effects of EGR and water addition on reducing NO and soot emission were also discussed. Results show that EGR up to 10% shifts the local excess air ratio from those points where NO is formed intensively to fuel rich regions without almost anytemperature drop. Thus, EGR reduces NO emission without sacrificing thermal efficiency. EGR, however, reduces the local oxygen concentration and this causes the increase in soot emission. On the other hand, water addition shifts the local mixing ratiofrom the NO forming region to lower temperature region without decreasing local oxygen concentration in a fuel rich region. For this reason, water addition for reducing NO emission basically results in a drop in fuel economy but it does not increase sootemission. Furthermore, the effects of CO{sub}2 addition on NO and soot emission were discussed and it is shown that CO{sub}2 addition shifts the local mixture to rich atλ>1 and to lean atλ<1. It suggests that CO{sub}2 addition is able to reduce NO andsoot emission without the drop in thermal efficiency.