Study On PAH Formation Mechanism During Combustion Process Of Diesel Engines

Soot is a main pollutant produced by diesel engines. It's harmful to human health and environment. Polycyclic aromatic hydrocarbons (PAH), which are found to be mutagnenic and carcinogenic, have been suggested to be the precursors to soot. So, more and more attentions have been paid on their formation and growth. It's necessary to use chemical kinetic method to study on the PAH formation mechanism in internal combustion engines. Because existing combustion mechanism of n-heptane doesn't contain PAH, the major goal of this paper was to build a new reduced kinetic model with PAH formation included.All computations were performed using CHEMKIN software and SENKIN mode. First, detailed mechanism of n-heptane was employed and the influence of mixture concentration on precursors of PAH was analyzed. The result showed that C3H3 was much sensitive to mixture concentration and the peak mole fraction was small, so it didn't contribute to the formation of benzene significantly. It was also established through sensitivity analysis and reaction rate analysis that the key pathways to C2H2 and C2H4 production are reactions C2H3+O2<=>C2H2+HO2 and C2H5+O2<=>C2H4+HO2.Then reaction pathways relevant to benzene, naphthalene, phenanthrene and pyrene were discussed based on a detailed kinetic model of aromatics formation in laminar premixed acetylene and ethylene flames. They are as follows: the production of aromatics precursors, the formation of the first aromatic ring, the growth of PAH and finally the oxidation. There are two dominant channels of benzene and phenyl production: one is recombinations of C3H3, the other is n-C4H3+C2H2=A1-. From benzene, multi-ring aromatics can then be formed via H-abstraction-C2H2-addition mechanism. These reactions were added to the reduced n-heptane kinetic model and combined with it by the reactions C2H3+O2<=>C2H2+HO2 andC2H5+O2<=>C2H4+HO2. Thus, a new reduced kinetic model with PAH formation mechanism was constructed, and it involved 64 species and 78 reactions. Finally, the model was validated against detailed kinetic model on ignition timing, in-cylinder temperature and pressure, and important species'mole fraction. The trend of PAH agreed well with experimental results. And a high temperature and fuel-rich mixture lead more PAH production and the mixture concentration affected PAH greatly.Experimental research showed that PAH emissions from diesel engines had two sources: one from the combustion process of diesel engines, the other from unburned fuel components.