Multi-dimensional Numerical Simulation On PAHs Formation Mechanism During Diesel Engine Combustion

With the increasingly stringent exhaust emission standards of the diesel engine as well as the development of the diesel engine technology, it's very important to study soot formation mechanism and control smoke emissions. The formation and growth processes of polycyclic aromatic hydrocarbons (PAHs), which are suggested as soot-precursors, have been paid more attention to. This paper studied and improved the existed combustion mechanism of n-heptane, and then simulated the in-cylinder combustion process of a diesel engine and formation mechanism of PAHs with multi-dimensional model which has coupled the reduced chemical kinetic model and CFD model.The formation mechanism of PAHs was investigated, and it was formed via H-abstraction-C2H2-addition mechanism. The new reduced kinetic model with PAHs formation mechanism was constructed by adding PAHs formation mechanism to improved Patel model and combining with the reactions of PAH precursors C2H2 and C3H3, and it can well describe the course of PAHs formation.The key reactions, which have tremendous effect on the system temperature and ignition timing, have been extracted from the reduced kinetic model of n-heptane with PAHs formation mechanism by using temperature sensitivity analysis techniques. In order to improve the predictions of ignition timing and combustion process, the kinetic parameters of the key reactions were optimized. The simulations were carried out by using the optimized model, and the simulated results of the ignition timing, NTC region, in-cylinder temperature, pressure and the mole fraction of some important species agreed well with ones by using the detailed chemical kinetic model.Three-dimensional CFD model coupled with reduced chemistry with PAHs formation mechanism was established to simulate the in-cylinder combustion and PAHs formation of the diesel engine. The comparison of experimental results and simulated ones has validated the exactitude of the model. Simulation was performed to study the effect of initial conditions on the diesel engine performance. With regard to the combustion of the diesel engine, intake swirl had great influence on the velocity fields of in-cylinder gas flow. Taking a certain intake swirl could increase the evaporation rate of the fuel on the wall of the combustion chamber. However, if the swirl ratio was too large, the utilization rate of the in-cylinder air decreased because the spray and flame were limited in the piston bowl. In-cylinder PAHs decreased with the increase of swirl ratio. Under the supercharged diesel engine conditions, fuel combusted completely for more oxygen in the cylinder and the formation of PAHs decreased. Ignition delay was longer and alleviation phenomenon of PAHs formation in the cylinder was evident at lower intake temperature. Under the lower fuel injection pressure, the fuel injection duration was so longer that PAHs formation became faster at later period. As EGR rate was enhanced, the ignition delay was longer, the in-cylinder pressure and temperature reduced, and the period of PAHs formation alleviation was shortened. Compared with other initial conditions in question, EGR had highly significant effect on the engine-out emissions of PAHs.