Simulation Study On The Effects Of Compound Injection Strategy On The Soot Formation Process Of Gasoline Compression Ignition (GCI)

Gasoline compression ignition(GCI)could control combustion phasing effectively by combining the gasoline characteristics of low reactivity and high volatility with the strategy of direct injection near top dead center to get a higher premixed ratio,and hence achieve high thermal efficiency and low emissions during the combustion process.However,under heavy load conditions,it is difficult to completely separate the fuel injection and combustion process.There is a large proportion of diffusion combustion inevitably,resulting in higher soot emissions.The injection strategy is an effective strategy to control the mixing and stratification of fuel and air in the cylinder and improve the combustion control and soot emission of GCI under heavy load.Therefore,a three-dimensional computational fluid dynamics(CFD)simulation method coupled with chemical reaction dynamics and multi-step phenomenological soot model was used in this paper,and the effects of compound injection strategy on the soot formation process of GCI were studied,so as to provide theoretical guidance for high-efficiency and clean combustion of GCI under heavy load.First of all,the effects of the compound injection strategy of port fuel injection(PFI)combined with pilot-main injection on the combustion process and soot formation process of GCI were studied,and the injection parameters were optimized.The results show that the combustion duration can be shortened and the heat release can be more concentrated by increasing the PFI ratio,advancing the main injection timing and increasing the pilot-main injection interval.The change of the PFI ratio has a great impact on the soot formation,and advancing the main injection timing can improve the soot oxidation.In addition,it is necessary to select appropriate pilot-main injection interval for different main injection timings,thereby suppressing the nucleation process of soot and reducing the emission of soot and its number density to some extent.On this basis,the compound injection strategy and exhaust gas recirculation(EGR)were coupled to study their effects on GCI combustion and soot formation.The results show that increasing the EGR rate is helpful to reduce the maximum pressure rise rate and NO_x emission,but it will reduce the soot oxidation rate and increase the soot mass and its number density emissions.When the PFI ratio is 40%,the main injection timing is-8°CA ATDC,the pilot-main injection interval is 15°CA and the EGR is 20%,it can achieve low emission with the soot emission of 0.0209g/(k W·h)and the NO_x emission of 3.213 g/(k W·h),and high thermal efficiency(46.798%)with the maximum pressure rise rate lower than the limit value.The results of the study on PFI combined with main-post injection strategy show that the use of a smaller main-post injection interval and post injection mass can promote the soot oxidation in the late combustion stage,thus reducing the soot emission.When the EGR rate is 20%,the PFI ratio is 40%,the main injection time is-8°CA ATDC,the main-post injection interval is 10°CA,and the post injection mass is 5 mg,the higher thermal efficiency and lower soot and NO_x emissions can be obtained.Finally,the compound injection strategies of PFI combined with pilot-main injection were compared with that of PFI combined with main-post injection.The results show that under the condition of GCI heavy load,the compound injection strategy coupled with EGR can achieve low soot and NO_x emissions and ensure high thermal efficiency with the maximum pressure rise rate lower than the limit value.