DIESEL COMBUSTION ANALYSIS BASED ON 2-ZONE MODEL - (EXAMINATION OF EXCESS AIR RATIO IN BURNED ZONE)

The burned zone excess air ratio lambda(d) in the diffusion combustion process was analyzed using the two-zone model, and it was compared wi th the excess air ratio lambda(f) estimated using a steady diffusion f lame model of the fuel spray. It is found that lambda(d) is dependent on the fuel spray penetration and ignition delay. If the premixed comb ustion fraction is less than 50%, the NO formation is minimally influe nced by the excess air ratio during premixed combustion and is depende nt on the excess air ratio lambda(d) which determines the maximum temp erature during diffusion combustion. It is clarified by analysis of th e two-zone model that the large reduction in NOx due to timing retard is mainly caused by decreases in both combustion temperature and combu stion pressure, and the small reduction in NOx occurring when the nozz le-hole diameter is decreased, is due to a small decrease in combustio n pressure resulting from a decrease in the heat release rate during p remixed combustion.