Diesel Combustion Analysis by Two-Zone Model (2nd Report, Examination of Excess Air Ratio in Burned Zone)

The excess air ratio of the burned zone, A_d, which was estimated by two-zone model analysis, was compared with the excess air ratio of the modeled steady-state fuel spray and diffusion flame. There was little correlation between A_d and the averaged excess air ratio in the cylinder Am, and A_d was mainly dominated by the characteristics of fuel spray and ignition delay. The excess air ratio during the premixed combustion period minimally influenced upon the NO formation fraction if the premixed combustion fraction was less than 50%. The major factors of NO_x reduction were investigated using the two-zone model. As a result, it was made clear that a large NO_x reduction due to timing retard was mainly caused by both a decrease of combustion temperature and a large decrease in combustion pressure on the expansion stroke, and a small NO_x reduction with the decreased nozzle hole diameter was caused by a decrease of combustion pressure based on the decrease of the heat release rate in the premixed combustion.