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A new model to describe diesel combustion process has been developed. In this model diesel combustion field is divided into two zones, premixing and combustion. Turbulent mixing process is described by the stochastic approach in each zone separately. Comparison of calculations with experimental results showed that this model can predict the entire course of heat release and nitrogen-oxide formation precisely, under wide-spread conditions. Two-dimensional flame temperature distributions in the combustion field by the two color method were compared with simulation results. Both the measured and the calculated flame temperature distributions showed good agreements with each other. In the diesel combustion process, the injected fuel mixes with air entrained inside the spray. The mixture is thus formed, and ignites at several points. Random expansion of flamelets accelerates both mixing and combustion. Following this, fairly moderate diffusion combustion proceeds.

The effects of fuel injection conditions (injection pressure, nozzle orifice diameter and fuel injection quantity) on NOx formation in direct-injection Premixed Charge Compression Ignition (DI-PCCI) combustion were investigated using a constant-volume vessel and a total gas-sampling device. The results show that promotion of fuel-air mixing reduces final NOx mass accompanying a delayed hot flame. In particular, under low oxygen mole fraction conditions, in addition to the hot flame delay, the promotion of fuel-air mixing results in a lower heat release rate. In this case, the final NOx mass is further reduced. For a fixed nozzle orifice diameter, the final NOx mass is reduced with increasing injection pressure. This effect is remarkable for smaller nozzle orifice diameters. Regardless of the oxygen mole fraction, under the low injection fuel quantity condition, enhancement of fuel-air mixing reduces the final NOx mass per released heat.