The burned zone excess air ratio λ_d in the diffusion combustion process was analyzed using the two-zone model, and it was compared with the excess air ratio λ_f estimated using a steady diffusion flame model of the fuel spray. It is found that λ_d is dependent on the fuel spray penetration and ignition delay. If the premixed combustion fraction is less than 50%, the NO formation is minimally influenced by the excess air ratio during premixed combustion and is dependent on the excess air ratio λ_d which determines the maximum temperature during diffusion combustion. It is clarified by analysis of the two-zone model that the large reduction in NO_x due to timing retard is mainly caused by decreases in both combustion temperature and combustion pressure, and the small reduction in NO_x occurring when the nozzle-hole diameter is decreased, is due to a small decrease in combustion pressure resulting from a decrease in the heat release rate during premixed combustion.
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Ishida et al. (1996) studied this question.