Numerical Analysis of the Role of Initial Combustion on Reduction of NO and Soot from DI Diesel Engines

Multidimensional simulation has been carried out to be clear the role of initial combustion in D.I. diesel engines on reduction of NO and soot emissions by reduction of initial injection rate or pilot injection. The multidimensional engine simulation code, FREC-3D(CI), which was developed by IKEGAMI group in Kyoto Univ. at 1988, was modified and was used in this study. The combustion submodel in this code was updated including ignition submodel that was formulated by a one-step global chemical mechanism to simulate measured ignition delay and initial combustion, sufficiently. In-cylinder NO and soot formation models were introduced by present authors. NO and soot were predicted by Zeldovich mechanism and Morel's soot formation and oxidation formulations, respectively. In result, computations demonstrated good agreement between measured and predicted trends of in-cylinder pressure, and rate of heat release, and showed a trade-off relationship between NO and soot emissions at pilot injection with high pressure injection. NO and soot emissions is greatly influenced by a dwell period between pilot and main injections. This is due to that local distributions of temperature and oxygen fraction are changed by a dwell period. Computations also showed that high turbulence kinetic energy in combustion chamber is sustained at the late combustion stage after fuel injection in the case of a higher initial injection rate or a injection without pilot injection, so that the soot oxidation process is promoted at the late combustion stage. And the reason why soot density increases in the case of the pilot injection or the reduced initial injection rate, which is a experimental result in previous study by one of the authors, was clarified by computed results.