Influence Of Injection Strategy On Combustion And Emission Characteristics Of Gasoline Compression Ignition At Low Load Conditions

Gasoline compression ignition(GCI)has the advantage of achieving low emission while maintaining high thermal efficiency,which is considered to be one of the promising combustion concepts to meet strict emission and fuel consumption regulations in the future,and has become an active research area in the field of combustion.However,due to the high octane number of gasoline,the low pressure and temperature in the cylinder at low load conditions,combustion instability and incomplete combustion products are easy to occur.Thus,how to obtain high efficiency clean combustion at low load conditions has become a great challenge for current GCI combustion mode.Therefore,numerical simulation technology is used to research the formation process of mixture of GCI combustion mode and the influence mechanism of combustion and emission characteristics with different fuel injection strategies.Firstly,the effects of multiple injection strategy on combustion and emission characteristics of GCI combustion mode at low load conditions were studied.Research shows that with pre-injection ratio increases,the combustion efficiency and indicated thermal efficiency at pre-injection timings of-120/-80/-40°CA aTDC first increases and later decreases,and the maximum one is about 60%.When the pre-injection timings were-80/-120°CA aTDC,the improvement of NO_X emission weakened with pre-injection ratio increased.The NO_X emission is the highest when the pre-injection timing is-40°CA aTDC,but three pre-injection timings can effectively reduce soot,HC and CO emissions.With pre-injection ratio increased,CA10has no obvious change,and the combustion duration shortened significantly by reducing the half-way duration of combustion(CA50-CA90).There is a high relevance between the indicated mean effective pressure(IMEP)and the half-way duration of combustion with pre-injection ratio increases.The combustion process can be effectively controlled by the main-injection timing.IMEP is affected by the combustion duration and CA50,it first increases and then decreases with the main-injection timing advanced.With injection pressure increases,the GCI combustion and emission characteristics can be improved,but the improvement effect weakened gradually.The effects of compound injection strategy on combustion and emission characteristics of GCI combustion mode at low load conditions were studied.The results show that the combustion efficiency and indicated thermal efficiency shown an ascending trend with PFI ratio increases.When the PFI ratio is less than 60%,the increase is more obvious,but the NO_Xemission is higher than the one in single injection strategy.When the PFI ratio is 60%,with direct injection(DI)timing advanced,CA10 first advanced and then delayed,the combustion duration is gradually shortened,and the combustion efficiency and indicated thermal efficiency reached the peak at the injection timing of-30°CA aTDC.With DI pressure increases,combustion efficiency and indicated thermal efficiency first increase and then decrease,NO_Xand soot emissions decrease significantly.Compared with multiple injection strategy and compound injection strategy,it is found that the multiple injection strategy can obtain better combustion and emission characteristics at lower injection ratio,and the compound injection strategy can obtain better combustion and emission characteristics at higher injection ratio.Compared with multiple injection strategy,NO_X and soot emissions can be reduced obviously by increasing injection pressure when the compound injection strategy is used,and the dependence on high injection pressure can be reduced by the compound injection strategy.