Fundamental Investigation of NOx Formation in Diesel Combustion Under Supercharged and EGR Conditions

Aim of this study is to clarify the NOx formation mechanism in diesel combustion under high-supercharged condition. Effects of ambient conditions and fuel injection parameters on diesel combustion were investigated using a constant volume chamber. NOx formation process was investigated using a total gas-sampling device. The results indicate that by using the above experimental setup it is possible to realize entirely diffusion combustion like what seen in the highly supercharged condition. Increasing ambient pressure up to 8MPa with high injection pressure shortens the ignition delay and offers a heat release rate proportional to the fuel injection rate with a short combustion duration. Increasing ambient pressure gives a higher NOx formation rate and final NOx concentration. This is due to enhancement in the fuel-air mixing which promotes the heat release. Increasing injection pressure increases the NOx formation rate during the heat release, however reduces the final NOx mass per released heat. Lowering oxygen concentration in the range of this study shows slight effects on the combustion trend, however, it greatly reduces the NOx formation rate and final NOx concentration.

To better understand the above phenomena, a spray combustion model was developed based on stochastic mixing model including NO formation scheme. Generally the calculation results well reproduce the experimental results. The analysis of calculated result reveals the role of fuel-air mixing in the NOx formation under entirely diffusion combustion with a low overall equivalence ratio. In the early stage of combustion, rich mixtures are diluted with air and fall into the equivalence ratio range of high NO production rate. Then, the mixtures are further diluted and shift to the state of freezed NO formation. According to this mechanism, the experimental results on effects of ambient pressure and injection pressure are reasonably described.