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The natural gas high pressure direct injection (HPDI) engines have attracted attention due to their high combustion stability and thermal efficiency comparable to diesel. However, the nitrogen oxides (NOx) emissions from HPDI engines remain high. In this study, the coupling effects of the exhaust gas recirculation (EGR) and pilot diesel quantity were investigated on a four stroke HPDI engine by simulation. Three pilot diesel quantities and four EGR rates were involved, and the combustion and emission characteristics of the engine were compared and discussed. The results show that with increasing EGR rate, the maximum combustion pressure (Pmax) decreases with different pilot diesel quantity, and the crank angle corresponding to the maximum combustion pressure (CPmax), the crank angle corresponding to the maximum heat release rate (CHRRmax) and the crank angle corresponding to the 50% cumulative heat release (CA50) are all pushed back. NOx emissions decrease gradually with the increase of EGR rate, but soot, unburned hydrocarbon (HC) and carbon monoxide (CO) emissions and methane escape all increase and the indicated thermal efficiency (ITE) decreases. The traditional “trade-off” relationship between NOx and soot emissions still exists on HPDI engine and becomes more obvious as the quantity of pilot diesel increases. NOx and soot emissions can be well balanced at moderate EGR rate. Increasing the quantity of pilot diesel can partially offset the hysteresis effect of EGR on the combustion process. And when the EGR rate is controlled within 30%, increasing the quantity of pilot diesel is beneficial to reduce HC and CO emissions and methane escape. By increasing the quantity of pilot diesel coupled with a moderate level of EGR rate, it is possible to significantly reduce NOx emissions while maintaining the ITE, methane escape and unburned diesel, HC and CO emissions at the same level as without EGR. But soot emissions will increase substantially.

Numerical Study on the Effects of Pilot Diesel Quantity Coupling EGR in a High Pressure Direct Injected Natural Gas Engine