The Numerical Simulation Of Internal Flow And Atomization Characteristics With High Injection Pressure For GDI Nozzle And Parameter Optimization

In cylinder Direct Injection(GDI,Gasoline Direct Injection)engine gradually become one of the mainstream automotive engine with its excellent power performance and fuel economy,however its particulate emissions limits the GDI engine emissions.By increasing the injection pressure and changing the nozzle geometry parameters to improve the quality of fuel spray are the key methods to solve this problem.In this paper,numerical simulation was used to study the influence of nozzle shape parameters on the nozzle flow and atomization characteristics when the needle valve moves under high injection pressure,so as to provide a theoretical basis for improving the quality of GDI engine mixture.Firstly,a GDI flow calculation model was established to verify the accuracy of the model.The changes of flow and spray characteristics in cylindrical nozzle under different injection pressure and in the conical,the elliptic-shaped and hyperbolic-shaped nozzle with varying longitudinal cross sections under high injection pressure were studied.The results show that for cylindrical orifice,with the increase of injection pressure,the velocity and turbulent kinetic energy at the nozzle exit increase significantly,and the turbulent kinetic energy disturbance becomes more intense in the opening and closing stages of the needle valve,but the flow coefficient of the nozzle orifice decreases slightly.For conical orifice,as the taper K coefficient decreases from positive to negative,the flow coefficient of orifice decreases obviously,the turbulent kinetic energy at the orifice outlet increases sharply,and the cavitation state inside the orifice gradually develops from no cavitation to supercavitation.For the elliptic-shaped with varying longitudinal cross sections,with the increase of the size of the variable-cross-section,the flow coefficient of the jet orifice increases obviously,and the cavitation density in the orifice decreases sharply.The farther the variable section position is from the orifice entrance,the smaller the orifice flow coefficient,orifice exit velocity and turbulent kinetic energy are.However,the cavitation density inside the orifice increases sharply.For the hyperbolic-shaped nozzle with varying longitudinal cross sections,the cavitation in the orifice is mainly distributed near the outlet of the jet orifice.As the size of the variable section increases,the turbulent vorticity in the orifice and the turbulent kinetic energy decreaseat the nozzle outlet gradually,but the flow coefficient increases sharply,and the position of the variable section has no influence on the flow coefficient.Secondly,a GDI nozzleinternal flow coupled jet atomization model was established,and a GDI spray test bench was set up to verify the accuracy of the model.The effects of nozzle shape parameters on primary atomization and secondary atomization were studied with different injection pressure.The results show that the spray SMD(SMD,Sauter Mean Diameter)and the spray penetration decrease rapidly and the spray-breaking is more violent when the injection pressure increases for the cylindrical orifice.For conical orifice,as the nozzle taper K coefficient decreases from positive to negative,the primary atomization increases sharply,but the secondary atomization degree is weak.For the elliptic-shaped with varying longitudinal cross sections,as the K_m increases,the spray SMD increases in the stage of the primary crushing,but decreases in the secondary atomizing stage.Forthe hyperbolic-shaped nozzle with varying longitudinal cross sections,with the decrease of Km,the spray SMD decreases in the whole fuel atomization stage.The spray SMD decreases,as the distance between the variable section position and the nozzle inletincreases.Finally,in order to obtain the GDI injector with excellent performance,orthogonal test optimization method was used to optimize the shape parameters of the nozzle.Three nozzle shape parameters(nozzle taper K coefficient,variable section size K_m,variable section location X_m)were selected as optimization factors in the optimization design.The research results show that the effect of the varyiable section size K_m and the effect of the K coefficient are the largest and the smallest respectively with the spray SMD as the optimization target.After optimization,the spray SMD of the orifice was significantly reduced.