Numerical Study On Combustion Process And Soot Emission Of Gasoline Compression Ignition (GCI)

Gasoline compression ignition(GCI)has been regarded as one of the potential concepts to achieve clean and high efficiency,and is currently a hot topic in internal combustion engine research community.However,GCI also has some issues that need to be resolved,including the combustion stability at low load,high soot emission and excessively high pressure rise rate at high load.In this paper,numerical study has been conducted explore the combustion process and soot formation mechanism within GCI combustion.A CFD simulation model for GCI combustion and emission predictions was built with Converge software package.Firstly,the prediction performance of three multi-step phenomenological soot models in spray combustion was compared on a constant volume vessel,and the combustion and soot models were further validated with experimental data from engine testbed.The simulation results from optical engine showed that improved combustion and higher IMEP could be achieved with stronger mixture stratification through injection strategy control.Stronger mixture stratification with late injection was beneficial to promote the auto-ignition and to suppress the peak pressure rise rate,accompany with high soot emissions;Although weaker mixture stratification induced from early injection can be helpful to improve combustion and to reduce soot emission,however,the excessively high combustion reaction rate results in high peak pressure rise rate,which would penalize the durability of the engine.It was observed that the combustion process gradually transit from HCCI like combustion with early injection to partially premixed diffusion combustion with late injection.Numerical simulations were further conducted on a single cylinder heavy-duty GCI engine at high load to explore the effects of gasoline surrogate component,soot model and injection strategy on the combustion characteristics and soot emissions at high load.It was found that the three component gasoline surrogate TRF coupled with Gokul's multi-step phenomenological soot model yielded the best results among tested surrogates and soot models.Compared to the single injection strategy,applying double injection strategy could extend the engine operating range and reduce the soot emission at high load.Pilot injection could advance the combustion phasing and consequently higher IMEP and thermal efficiency can be obtained.Double injection coupled with EGR could prolong ignition delay to improve the mixing process and to suppress the combustion reaction rate,while still maintaining reasonable combustion phasing,thus both low soot emission and low peak pressure rise rate can be achieved,and the soot emission can be further reduced with enlarged pilot-main injection interval.The overall results showed that high thermal efficiency,low soot and low peak pressure rise rate can be simultaneously obtained when double injection combined with 30% EGR was applied.