Effect Of The Fuel Stratification On SFI Combustion In A Novel Two-Stroke Gasoline Engine

The poppet-valved two-stroke gasoline engine as a novel type of two-stroke gasoline engine for passenger car,could play the advantage of controlled auto-ignition(CAI)combustion at low load,and give full advantages of downsizing at high load,which using its flexible valve mechanism.Stratified flame ignition(SFI)combustion could bridge the gap between CAI and lean burn combustion.Thus,the novel two-stroke gasoline engine provides a great potential technical solution to reduce fuel consumption and improve the thermal efficiency.Three-dimensional numerical simulation method was used in this study to simulate the cold flow,spray and combustion process in a self-designed poppet-valved two-stroke gasoline engine,which using vertical inlet and spray guided combustion system.The effect of direct fuel injection strategy and piston bowl structure on in-cylinder mixture formation process were studied at partial load,and further explained its mechanism of action to SFI combustion process.Firstly,the numerical simulation platform of the novel two-stroke gasoline engine was established.From the understanding of the in-cylinder air motion law,the effect of single injection timing,and the piston bowl structure under twice direct-injection condition on in-cylinder mixture formation process were studied.The results show that under the effects of in-cylinder two streams tumble vortex which in the opposite direction,it is difficult to form the suitable mixture for ignition near the spark plug at the end of the compression when the single injection timing is at the beginning of the compression stroke.And it is easy to form the suitable mixture for ignition near the spark plug when bowl pistons with smaller proportion of second direct injection under the twice direct-injection condition.On the basis of preliminary understanding of fuel and gas mixing process in cylinder,effect of piston bowl structure,the second direct injection timing and twice the proportion of injection on the SFI combustion process were further studied.The results show that it is easier to appear on the inlet side for auto-ignition,which affected by the reverse tumble in cylinder.The small size of bowl diameter could enhance fuel convergence capability after second injection,effectively reduce the temperature of the central region,result in that the auto-ignition is easier to appear on the peripheral zone which away from the flame.As the second direct injection timing delays to 320°CA step by step,the cooperation between bowl and fuel is better,and fuel is easier to convergence in the center of the combustion chamber.At the same time,the temperature in the peripheral zone is increase,auto-ignition time ahead step by step.Fuel stratified gradient between central zone and peripheral zone also is increase,which effectively reduces the PRRmax of SFI combustion,but slightly worse IMEP.With the increase of the second direct injection ratio,the central zone of the mixture in cylinder is richer,and peripheral zone of the mixture is leaner,PRRmax is reduce,and cylinder peak pressure peak becomes lower,IMEP is progressive deterioration.