Research On Combustion Control Strategy Of Gasoline Compression Ignition With An Internal EGR Strategy At Low Load Conditions

Gasoline compression ignition(GCI)has become a research focuses in the field of internal combustion engine with the improved thermal efficiency and has the potential to reduce NO_X and soot emissions significantly.But GCI combustion mode has more incomplete combustion products and poor combustion stability at low load conditions,it is urgent to expand the stable combustion limit to lower load.Therefore,the i-EGR achieved by the exhaust rebreathing valve strategy is uesd.In addition,the i-EGR is coupled with intake heating,intake boost and multiple injection strategy to achieve the purpose of improving combustion stability,economy and emissions at low load conditions.First of all,the influence of i-EGR ratio on GCI combustion and emission characteristics at low load conditions were studied.The effects of i-EGR ratio on the stable combustion boundary were investigated at the lower fuel injection mass.The combustion and emission characteristics of GCI were investigated by increasing the intake temperature and pressure at different i-EGR ratios.In addition,based on the multiple injection strategy,the effects of pre-injection ratio at different i-EGR ratios on the mixture concentration distribution,the GCI combustion and emission characteristics were analyzed.Finally,the optimal combination of i-EGR ratio,pre-injection timing,injection timing and pre-injection ratio was determined by using the CCD response surface methodology(RSM)with 4 factors and 5 levels.Results show that with i-EGR ratio increases,the indicated thermal efficiency and HC emissions first increase and then decrease at 20mg/cycle of fuel injection mass.The lower NO_X emission can be achieved with i-EGR ratio increases,but it is not conducive to the control of CO and HC emissions.As the fuel injection mass decreases,the higher i-EGR ratio is needed to achieve stable combustion.When the fuel injection mass decreases to12mg/cycle,stable combustion is still difficult to achieve even if the i-EGR ratio more than75%.With the intake temperature increases,the maximum and minimum values of i-EGR ratio used to achieve stable combustion decreases gradually.Therefore,the maximum indicated thermal efficiency is achieved with the lower i-EGR ratio,and the maximum indicated thermal efficiency increases.However,the indicated thermal efficiency decreases at the higher intake temperature when more than 30%i-EGR ratio used.Besides,the i-EGR ratio used to achieve stable combustion increases gradually with intake pressure increases,the maximum indicated thermal efficiency and combustion efficiency are increased significantly,which effectively expands the load limit.However,the positive effect of increasing intake pressure on the emission performance is weakened with i-EGR ratio more than 35%,and the soot emission even increases slightly.The stable GCI combustion is easier to be achieved with the single-injection strategy at the 20%i-EGR ratio.As the i-EGR ratio and pre-injection ratio increases,the dispersion of mixture concentration decreases,the higher indicated thermal efficiency can be obtained at higher i-EGR ratio and pre-injection ratio.With pre-injection ratio increases,the GCI emission characteristics are improved effectively.On the contrary,emissions significantly increase when low i-EGR ratios and high pre-injection ratios are used.Therefore,the optimal combination is 43.93%i-EGR ratio,-60°CA ATDC pre-injection time,-20°CA ATDC injection time and 69%pre-injection ratio.