Numerical Study On The Combustion And Emission Characteristics Of Gasoline Compression Ignition (GCI) Based On Large Eddy Simulation

Gasoline Compression Ignition(GCI)has been regarded as one of the novel combustion modes to achieve clean and high efficiency combustion in a broad range of load.However,because of the high premixed combustion and consequently high heat release rate,especially at high load,some problems will arise,such as the high pressure rise rate,pressure fluctuation and high soot emission.Previous researches have shown that multiple injections can effectively reduce the pressure rise rate.However,even with multi-injection,high pressure rise rate was still observed in many GCI high load studies.In the meanwhile,the GCI simulations were mainly conducted using RANS turbulent model,thus both the detailed and insight understanding on the mixing and combustion mechanisms at high loads during load extension still need further investigation.Therefore,in this paper,LES turbulent model is used to explore the GCI combustion mechanism,on which the influence of different injection strategies and fuel characteristic on GCI combustion and emissions is investigated at high load.Firstly,the comparison between RANS and LES turbulent models for GCI mixing and combustion simulations were conducted on the single cylinder engine.The results show that the auto-ignition of GCI start from multiple locations and the combustion mainly located in the end of spray plume region,and the combustion of GCI is mainly controlled by the mixture stratification.LES turbulent model shows privilege advantages compared to the RANS model in term of mixing,combustion and emission predictions.With this LES-based GCI combustion simulation CFD model,it was found that increasing the injection pressure by 200 bar leads to high heat release rate and pressure rise rate.With the increasing of engine load,trade-off between thermal efficiency and pressure rise rate was observed with single injection strategy.In contrast,the double-injection strategy shows potential to resolve the trade-off between high pressure rise rate and low thermal efficiency for GCI at high load.With double injection strategy,both the pilot injection ratio and timing significantly affect the pressure rise rate.With 20% pilot injection at-83 °CA ATDC,the maximum pressure rise rate can be reduced to be lower than 10 bar/°CA and a44% thermal efficiency can be obtained.In addition,high octane number oxygenated butanol fuel was blended with gasoline in order to further suppress the soot emission at high load.It was found that the combustion phasing was delayed when butanol is added,mainly due to its lower reactivity compared to gasoline.With the addition of n-butanol and double-injection strategy,extremely low soot emission and 45% thermal efficiency can be obtained under high load condition(IMEP 19 bar),which were greatly improved compared to the gasoline combustion with single injection strategy.