Numerical Study On The Effect Of Gasoline Direct Injection On RCCI Combustion

RCCI combustion can achieve ultra-low NO_x and soot emissions over a wide range of engine loads,with acceptable pressure rise rate and ringing intensity,and very high indicated thermal efficiency through a proper stratification of concentration and reactivity.However,one of the major issues of RCCI operation is the high load extension limited by the excessive pressure rise rate and high NO_x and soot emissions.Enhancing the in-cylinder mixture concentration stratification can be an effective way to reduce the pressure rise rate,which can be effectively controlled by the premixed ratio and fuel injection strategy.Therefore,adopting another gasoline direct injection in addition to the port-injection premixed gasoline to further increase the in-cylinder mixture concentration stratification can be used to control the combustion rate,the pressure rise rate and emissions.which also provides potential approach for the subsequent heavy load extension.Numerical simulations were conducted with 3-dimentional computational fluid dynamic(CFD)software CONVERGE to investigate the effect of fuel premixed ratio,gasoline injection timing,diesel injection timing and diesel fuel mass on gasoline/diesel RCCI combustion.Results show that the in-cylinder concentration stratification can be enhanced with gasoline port injection combined with direct injection strategy.An IMEP of 13.5 bar and pressure rise rate of 5.2 bar/°CA can be obtained with premixed and direct injection gasoline strategy to create stratified low reactivity fuel distribution,combined with diesel direct injection to trigger the ignition and combustion.With comparable IMEP,the pressure rise rate and NO_x can be reduced by 41%and 46%,respectively,with gasoline compound injection strategy,with slight penalty in soot compare to the baseline condition(fully premixed gasoline with direct injection diesel).Therefore,this can be an effective measure to control the pressure rise rate and to extend the high load limit of gasoline and diesel dual-fuel RCCI operation.Based on the optimization of fuel injection strategy,the influence of direct injection gasoline on the high load expansion of RCCI combustion mode was studied by optimizing the parameters,including total fuel mass,gasoline direct injection mass,gasoline premixing mass and gasoline and diesel start of injection timing.The results show that stronger stratification can be obtained using more direct injection gasoline with gasoline compound injection strategy(port+direct injection),prolong the combustion duration,reduce the in-cylinder peak pressure and pressure rise rate.The simulation results show that within the constraints of soot and NO_x emissions and fuel consumption,the IMEP can be extended to 16 bar;the soot exceeds the limit value and the fuel consumption also increases slightly if IMEP is higher than 17bar with this strategy.At this high load condition(IMEP 17 bar),gasoline premixed ratio could slightly improve the emission,accompany with higher pressure rise rate and in-cylinder pressure.