Study On The Effects Of Injection Strategy On Combustion And Emission Characteristics Of A Direct Injection Gasoline Engine

Gasoline direct injection(GDI)engine has great potential to reduce fuel consumption and emissions,so it has become the mainstream of the development trend of gasoline engine.Optimizing injection strategies to organize the ideal gas mixture in cylinder effectively,so that the GDI engine can operate in different combustion modes to meet the needs of different conditions and then give full play to the advantages of energy saving and emission reduction,has become one of the research hotspots of GDI engine.In this paper,the effects of different fuel injection strategies on the combustion and emission characteristics of GDI engine were studied on a single-cylinder GDI optical engine,combined with the analyses of high-speed photographic images and bench test data.The following results showed that: In the medium speed and load conditions,when using a single injection strategy,injection time too early or too late would cause the fuel wall wetting phenomenon.As the injection timing delayed,the fuel and gas mixing time shortened,the nonuniformity of mixture increased,which was not conducive to improve the quality of combustion.The emissions of CO,THC and soot gradually increased,while NOx emissions decreased gradually on the whole.When the injection time was 290 ° CA BTDC,in the early stage of intake stroke,and the injection pressure was 10 MPa,the fuel and gas mixture was relatively uniform,combustion was more complete and emission characteristics were better.If the injection pressure was too low,it was not conducive to fuel atomization,however,if the fuel injection pressure was too high,it was more likely to cause the wall wetting,which would have a negative impact on combustion and emission characteristics.In the medium speed high load condition,the double injection strategy was adopted and the first injection timing was set at 290 °CA BTDC,the early stage of the intake stroke.As the second injection timing delayed,combustion phase delayed gradually,the peak value of heat release rate also decreased,THC emissions gradually increased.In addition,the appropriate delay of second injection time could reduce CO,NOx and soot emissions.When the injection timing of the double injection strategy was set to 290 ° CA BTDC and 270 ° CA BTDC to form the homogeneous stoichiometric mixture,a proper reduction of the fuel proportion of the second injection could increase combustion speed and reduce cycle variation of combustion and the emissions of CO,THC and soot.However,with the decrease of fuel proportion of second injection,the emissions of NOx gradually increased.When the injection ratio was 2:1,the combustion and emission characteristics were better.When the injection timing of the double injection strategy was set to 290 ° CA BTDC and 90 ° CA BTDC to form the stratified stoichiometric mixture,with the decrease of second injection fuel ratio,combustion temperature and heat release increased,while combustion cycle variation decreased,so combustion quality was improved.In addition,the emissions of CO,THC and soot decreased gradually,while NOx emissions increased.When the injection ratio was 3:1,the engine could form a more reasonable stratified stoichiometric mixture and had better characteristics of combustion and emission.