Study On Stratified Mixture Formation And Combustion Characteristics In A GDI Engine

With increasing concerns about strict legislations on emissions and global energy crisis,Gasoline direct injection engine has become an important trend of vehicle engines because of its good power,fuel economy and transient response characteristics.In order to further improve the performance of GDI engine and improve the quality of stratified mixture formation、reduce the wall oil film,and improve the engine’s indicated thermal efficiency,the optimization of fuel injection strategy was studied in this paper.A three-dimensional CFD numerical model of the engine was established employing CATIA and AVL-Fire software according to the GDI engine geometry parameters.The constant volume bomb spray test was carried out to complete the calibration of the spray sub-models,and the combustion model was verified by combining the engine bench test data.Subsequently,the optimization study of injection strategy was carried out under different operation conditions.The main contents and conclusions are as follows:1.Effect of injection strategy on the formation quality of stratified mixture and combustion characteristics under full load conditions of 2000r/min(1)Under the two-stage injection strategies,too early and too late of the second injection time resulting in unsatisfactory stratification of the mixture and poor combustion performance,When the second injection timing is 70°CA BTDC,the mixture stratification effect is the best,the rapid burning period is the shortest,the combustion rate is the fastest,and the highest indicated thermal efficiency can be gained;With the increase of the second injection ratio,the concentration of the mixture near the spark plug increases at the ignition timing,and the quality of the wall oil film decreases first and then rises,When the ratio of the two injections is 7:3,The quality of the stratified mixture is better,the wall oil film has the lowest quality,and the indicated thermal efficiency is the highest,which is 34.22%.(2)Adopting three-stage injection strategies,when the ratio of three injections is 5:3:2,and the second injection time is 150°CA BTDC,the quality of the stratified mixture is the best,the oil film is produced less,and the flame spread more quickly.The cylinder peak pressure is higher,and the combustion center of gravity is closer to the compression top dead center.Meanwhile,the indicated thermal efficiency is increased to 36.78%.(3)Compared with the single injection strategy,the split injection strategy promotes the evaporation of fuel and reduces the quality of the wall oil film,In particular,the three-stage injection strategy reduces the mass of oil film by 52.81%;compared to the homogeneous mixture formed by single injection strategy,the stratified mixture formed by split injection strategy is more conducive for flame propagation,employing three-stage injection strategy,the indicated thermal efficiency is increased by 2.58% compared with the single-injection strategy.The end-mixing gas is thin under split injection strategies,effectively suppressing the tendency of knocking phenomenon.2.Optimization of injection times under different working conditions(1)For low-speed-load conditions,split injection strategies significantly improve fuel evaporation characteristics and reduce oil film quality,achieving higher cylinder peak pressures and higher fuel utilization ratio.The indicated thermal efficiency of twostage injection strategy is slightly higher than three-stage injection and the control strategy three-stage injection is more complicated,so the second injection strategy is the best.(2)Under medium and low speed,medium load conditions as well as medium and high speed,high load conditions,the quality of mixture under the two-stage and threestage injection strategy is better than single injection strategy.Especially,the shortest combustion duration and the fastest combustion rate can be realized under the three injection strategies.Besides,the combustion center is closer to the compression top dead center,so that the indicated thermal efficiency is further improved.Therefore,the three injection fuel strategy is optimal under these conditions.