Matching Research On Fuel Injection Strategy Of Gasoline Engine Under EGR Stratification Mode

With the rapid development of automobile industry,under the pressure of petroleum resource crisis and environmental pollution,the development of energy saving and emission reduction technology is facing great challenge.Traditional lightweight vehicles are powered by gasoline engines,so it is very important to improve fuel economy and reduce exhaust emissions by means of technology.Exhaust gas recirculation technology is an important way to improve fuel economy and reduce NOx emission of gasoline engine,however,the combustion quality will be affected when a high EGR rate is used during small and medium load,which will lead to lower fuel economy.To solve this problem,scholars have put forward EGR stratification technology.Exhaust gas and fresh air enter the cylinder seperated and stay stratified by means of special intake-port structure and reasonable control strategy,which will keep fresh air near and exhaust gas far away from the spark plug.The early flame propagation will not or less be affected by high concentration of exhaust gas under this circumstance,and the engine can work at higher EGR rate.In this paper,a 1.4T turbocharged direct injection engine was studied.At first,two EGR stratification plans,homogenization EGR of unilateral intake-port and EGR circuit of unilateral intake-port,were proposed.The effects of different EGR rate and pressure on the EGR stratification were studied in the homogenization EGR of unilateral intake-port mode.Different EGR circuit locations were studied in the EGR circuit of unilateral intake-port mode,and two seed plans,changing the direction of the EGR ciecuit and adding an air baffle,were proposed.Then CFD software FIRE was used to simulate EGR stratification effect of different plans,and the calculation results were analyzed,compared and optimized.The results show: when the homogenization EGR of unilateral intake-port was used,the EGR rate was too high near the spark plug.EGR stratification can't be optimized by reducing EGR rate of exhaust side.By reducing EGR pressure the EGR rate near the spark plug can be reduced,but it also lead to serious mixing.In the EGR circuit of unilateral intake-port mode,the EGR stratification effect was better when the EGR circuit location was left side of intake-port.Changing the direction of the EGR circuit and adding an air baffle can reduce the infuence of intake valve edge effect,and adding an air baffle had better optimization effect.When an air baffle was added,a reasonable control of EGR timing can increase the EGR rate and EGR concentration gradient by avoiding exhaust gas flow backwards.43% EGR concentration difference can be obtained when EGR circuit diameter was 22 mm and EGR timing was 0°CA-165°CA,and the EGR rate near the spark plug was about 14% at the same time.At higher speed,it can still form a very obvious stratification,but the stratification effect was poor.The specific performance was that the total EGR rate became low,the EGR concentration difference became smaller,and the concentration of EGR near the spark plug increased.Based on a better stratification plan,the combustion calculation was carried out in three different modes,such as PFI,GDI and a combined injection mode.At the same time,two different fuel stratification methods were proposed aiming at two different fuel injection ways,PFI and GDI,and detailed analyses of the combustion process of fuel stratification in EGR stratification mode were carried out.The results show: under the PFI mode,the cylinder pressure and heat release rate were higher than that of homogeneous EGR model when EGR stratification was adopted.The reasonable fuel stratification can greatly accelerate the engine combustion speed under the EGR stratification mode,which greatly increased the cylinder pressure and heat release rate,and the peak value of cylinder pressure improved 19.52% and 24.70% than that of the former two combustion modes.Under the GDI mode,compared with homogeneous EGR,the optimal fuel injection duration was shorter and the optimal injection timing was more advanced in the EGR stratification mode.The engine combustion can be optimized when EGR stratification coupling reasonable fuel stratification was adopted,which can make the peak value of cylinder pressure increase by 19.52% and 24.70% than that of the former two combustion modes.Under the combined injection mode of PFI and GDI,there was little difference between the best injection timing of different injection ratio,and the smaller the proportion of direct injection was,the smaller the optimal fuel injection duration became.The engine combustion can be optimized when the fuel stratification was used at the PFI part.The combustion speed was slow when the injection ratio was 1:4,and when the injection ratio was 1:1 and 4:1,the flame propagation velocity was faster.After fuel stratification was adopted,the peak value of cylinder pressure increased by 5.54%,2.60% and 7.29% when the injection rate was 1:4,1:1 and 4:1,respectively.