A Study On Modeling Of Aromatic Hydrocarbon Emissions Forming Process In TWC Of Gasoline Engine

To meet the emission standards, the unleaded gasoline and three-way catalyst (TWC) technology are used in gasoline engines. Then, there are more aromatic hydrocarbons in unleaded gasoline to improve the octane number. Although, TWC can reduce benzene emissions up to 98% at ideal conditions, the benzene, as a secondary pollutant, is intensely formed at high load and high speed conditions.But at present, it is not clear completely for the catalytic reactions of aromatic hydrocarbon. According to the principles of reaction kinetics for TWC and statistical physics, the reaction kinetics model is established by using the method of Monte Carlo, and is confirmed by comparing with the experiments. The results show:(1) The predicted emissions from this model such as CO, NOX, THC and abnormal pollutants such as benzene, toluene, ethyl-benzene are in well agree with the test emissions. So the TWC reaction kinetics model is reliable, and the reaction exhaust gas and process in TWC, the Monte Carlo steps and the size of modeling lattice used in this model are concise and feasible.(2) It also shows that the benzene is the secondary formation in TWC, when the pre-TWC exhaust gas temperatures are from 549 to 693℃andλis below 1. Also, there are two paths for the formation of benzene: one is the dealklation of alkybenzenes, the other is the partial oxidation of alkybenzenes. And at fuel-rich conditions, the first path is the dominant. Besides, because the second path can produce water and CO except benzene and the internal steps for the first path are fewer than the second, the benzene is more effectively produced by the first path.(3) At fuel-rich conditions, since there are a few oxygen free radicals, the conversion reaction for CO is believed to prefer via a water gas shift reaction to via oxidation reaction directly. Therefore, the conversion reaction for CO produces a large number of the hydrogen free radicals which promote the reactions speed for the formation of benzene.(4) At fuel-rich conditions, with the pre-TWC exhaust gas temperature rising, the conversion reactions for the normal pollutants are becoming more and more intense. Besides, the reduction reactions speed for toluene and ethyl-benzene is more rapidly than their oxidation reactions speed, which promotes the forming reactions speed of benzene. (5) When operating the catalyst in a critical pre-TWC exhaust gas temperature of 549-630℃, due to the reactions speed for ethyl-benzene converting to toluene being more rapidly than that for toluene converting to benzene, the secondary formation of toluene is observed.