Effects Of Injection Strategy On The Performance And Emission In A GDI Engine

With the increase of car ownership year by year,energy problems and environmental problems have been paid more and more attention.The GDI engine has become the future development direction of the passenger car engine because of its good power,economy,emission and the transient response.However,the fuel and air of the GDI engine are separately into the cylinder.Compared to the PFI engine,the mixing time of fuel and air become short and mixing conditions of fuel and air deteriorate.Therefore,In order to obtain better Combustible mixture which is beneficial to the ignition and the rapid propagation of the flame,to improve the engine performance and to reduce emission,adjusting the injection strategy is very important.First of all,in order to illustrate the significance of the two stage injection strategy,this paper uses the research method which compared the single injection strategy and the two stage injection strategy.The advantages of the two stage injection strategy are obtained by analyzing the pressure curve of cylinder,the equivalent ratio near the spark plug and the emissions of the engine.The results showed that: with the proper two-stage injection strategy,all fuel can be injected by two times,which is effectively beneficial to the organization of the mixture and the reduction of fuel impingement caused by single injection.Meanwhile,the first injection can produce enough homogeneous mixture and the second injection makes a richer mixture near the spark plug,which is dramatically conducive to the ignition and strengthen the flame propagation velocity.So the peak of cylinder pressure is higher and rises rapidly.At the same time,the emissions of CO and HC can be reduced effectively and the NOx emission is also at a low level.Secondly,the effects of the first injection timing,the second injection timing,the ratio of two injections and the injection pressure on the formation of mixture,the performance and the emissions were studied at a high load and low speed condition.Thirdly,the effects of different speeds and different loads on the formation of the mixture,the performance of the engine and the emissions were studied.And this paper puts forward the improvement measures for the injection strategy under different speeds and different loads.At the same time,the following conclusions are drawn from this study:1.The effects of the injection strategy on the engine performance and emissions under the condition of 1500r/min,75% load.(1)The best first injection timing is 90°CA~110°CA.Under this condition,there is less fuel impingement and less rich and lean region in the formation process of the mixture in the cylinder.There is a rich mixture and a moderate turbulent kinetic energy near the spark plug when the spark plug is on fire.This makes the combustion heat release more concentrated and the combustion duration shorten.At the same time,the level of the constant volume of the cylinder increase.The cylinder pressure rises rapidly in a short time and the peak of cylinder pressure become larger.The emissions of HC and CO are the lowest,but the emission of NOx is slightly high.(2)The best second injection timing is 90°CA~110°CA.Under this condition,the combustible mixture with a certain concentration gradient can be formed in the cylinder which is conducive to the ignition and the rapid spread of flame.At the same time,the combustion performance is optimized.The emissions of HC and CO reduce and the emission of NOx is partially inhibited.(3)The best ratio of the two injections is 3:1 and 2:1.Under this condition,the formation process of the combustible mixture in the cylinder is better,and there is less fuel film on the wall.Emissions of HC,CO and NOx are at a low level.(4)The best injection pressure is 10.5MPa.The fuel evaporation ratio is higher under this pressure.A rich mixture near the spark plug can be formed,and the equivalent ratio is 1.183.At the same time,the release of heat is rapid and the fuel utilization rate is high.2.The effects of different speeds(1500r/min,2000r/min,3000r/min and 4000r/min)and different loads(20%,50% and 75%)on the engine performance and emissions were investigated.(1)With the increase of the speed,the airflow motion in the cylinder is enhanced.The turbulent kinetic energy increases.The fuel evaporation rate accelerates and the mixture can be rapidly and uniformly mixed.However,time of each cycle becomes shorter by second.The fuel film on the wall increases and the final fuel volatility decreases.Emission of CO increases with the increasement of the fuel film,but the change of HC is not obvious.Emission of NOX decreases with the increasement of speed,because the reaction time is shortened and temperature of the cylinder decreases.(2)For high speed conditions,the two injection timing is separately ahead of 30 degrees Crank Angle in order to increase the mixing time of gasoline and air.With the advance of fuel injection timing the fuel film on the wall decreases and the final fuel volatility increases.The fuel utilization rate increases and the average temperature in the cylinder increases.The emissions of CO and HC decrease,and the emissions of NOx increases.(3)With the increasement of the load,the mass of the fuel injection will increase.Then it makes the fuel injection duration longer.So the fuel penetration distance increases.The fuel impingement in the cylinder increases,and the fuel evaporation percentage decreases.With the increase of the load,the emissions of CO and HC increase and the peak value of NO increased obviously.(4)For the high load conditions,the two injection timing is separately ahead of 30 degrees Crank Angle in this paper.It makes the fuel impingement reduce,the equivalent ratio near the spark plug decrease,and the concentration gradient of the whole cylinder reduce.So mixture tends to be homogeneous.The average temperature of the cylinder increases and the emission of NOx increases.The emission of CO and HC decreases due to the uniform mixture in the cylinder.