An Investigation On The Characteristics And Influence Factors Of NO₂ Formation In DMDF Engine

Nitrogen dioxide?NO₂?is a kind of air pollutant,but due to the strong oxidizing property,it plays an important role in after-treatment system.For example,NO₂ can oxidize the soot particle captured by the Diesel Particulate Filter?DPF?and realize its passive regeneration.Therefore,in recent years,the research concerning generation and emission characteristics of NO₂ have attracted wide attention of scholars around the world.According to previous study,compared with conventional diesel engines,the diesel/methanol dual fuel?DMDF?engine could significantly increase NO₂ emission.However,the detailed formation process and influence factors of NO₂ in Diesel/Methanol Dual Fuel?DMDF?engines have not been reported previously.In order to investigate the mechanism of increased NO₂ and effect factors of NO₂ emission in DMDF engine,following work was conducted:Firstly,in light of previous research,a hypothesis concerning the reason for increased NO₂ in DMDF engine was presented.With computational fluid dynamics?CFD?software CONVERGE,the hypothesis was demonstrated and the formation characteristics of NO₂ in DMDF engine were revealed.Secondly,with an in-line four-cylinder,inter-cooling,turbocharged,electronically controlled unit injection pump DMDF engine,the experiments about the effect of different operation conditions on the NO₂ emission were conducted,and the influence factors of NO₂ emission in DMDF engine were summarized.Main investigation results are shown as following:1.The methanol injected from intake manifold in DMDF engine directly leads to the increased NO₂ emission.The methanol injected from intake manifold can form premixed zone of methanol in cylinder.Because at same level of equivalence ratio,methanol can produce more hydroxyl peroxide?HO₂·?radicals than diesel,and the middle temperature atmosphere in methanol premixed zone is beneficial to the accumulation and conservation of HO₂·,the formation of NO₂ is prompted.2.The effect of temperature on NO₂ emission mainly lies in that on formation reaction of NO₂.The effect of NO₂ destruction in DMDF engine is negligible for the NO₂ emission.Plenty of research show that temperature can significantly influence NO₂ emission,so the the impact of temperature on the NO₂ emission is mainly due to that on NO₂ formation.3.The control of NO₂ in DMDF can be realized by regulating the forming conditions of NO₂.Different operation conditions can change NO₂ emission through regulating the influence factors of NO and HO₂·formation and contact time of two products,such as changing in-cylinder temperature,combustion duration,equivalence ratio of methanol,oxygen content and so on.4.The control of NO₂ emission is more complicated than that of NO₂/NO ratio.The effect of in-cylinder temperature on NO and HO₂·are completely opposite.The competition between the changes of the two products determines the absolute value of NO₂ emission,which makes the control of NO₂ emission more complicated.However,the variation tendency of NO₂/NO ratio is consistent with that of HO₂·.Therefore,the control of the NO₂/NO ratio can be achieved by regulating the formation and accumulation of HO₂·.