Simulation of Effects of Valve Pockets and Internal Residual Gas Distribution on HSDI Diesel Combustion and Emissions

Experiments show that intake flow details have a significant influence on High-Speed Direct-Injection (HSDI) diesel engine soot emissions. Four different intake modes were simulated using the combination of the CFD codes, STAR-CD and KIVA-3V, to investigate spray-intake flow-emission interaction characteristics. The simulation results were compared to steady-state flow bench data and engine experimental data. It was found that it is difficult to accurately predict the timing of the small pilot and main combustion events, simultaneously, with current simplified ignition models. NOx emissions were predicted well, however, an insensitivity of the soot emissions to the details of the intake process was found, mainly due to the deficiencies in predicting the ignition delay. The results show that a strong swirling flow causes the formed soot to remain within the bowl, leading to high soot emissions. The presence of valve pockets in the piston helps spread the soot into the squish region because the presence of valve pockets reduces the swirl. It was shown that the start-of-first ignition timing was changed, and the NOx and soot emissions were changed noticeably by the details of the residual gas distribution. Improvements in ignition and combustion predictions were also pursued using detailed chemistry in the combined Kiva/Chemkin code. It was found that ignition delay predictions are very sensitive to the chemistry mechanism. NOx predictions obtained by the extended Zel'dovich model agreed with the detailed mechanism. For late injections NO2 emissions become significant and comparable to the NO emissions, leading to underestimated NOx predictions by the extended Zel'dovich model.