Study On The Effect Of GDI Injector Nozzle Structure Parameters On The Injection Charactersitics

With the addition of the new limitation on the number of exhaust particles in the emission regulations,there is no doubt that higher requirements have been put forward for the combustion and emission characteristics of gasoline engines.The inhomogeneous mixing of fuel and air in engine cylinder and the difference of fuel injection between cylinders and cycles limit the further improvement of emission characteristics.In order to improve the emission performance and fuel consumption,in-cylinder direct injection(Gasoline Direct Injection,GDI)fuel system has become one of the important development directions.Compared with the Port Fuel Injection system,the GDI fuel system has the advantages of accurate control of circulating fuel injection and good responsiveness of each cylinder.However,at present,high-performance GDI injectors mainly rely on foreign imports or foreign enterprises for processing and production.In order to make them localized,it is necessary to conduct in-depth research and Analysis on GDI injectors.During the development and research of GDI injector,it was found that the structural parameters of the nozzle have a great influence on the injection characteristics.Because most areas of injector are filled with mainly single-phase continuous flow,while the flow of nozzle area is complex gas-liquid two-phase flow.The gas-liquid ratio at the outlet of the nozzle and the jet velocity directly affect the initial crushing state of fuel,and then affect the mixing of fuel with air in the cylinder.As a result,it is particularly important to obtain the machining consistency of the structural parameters of the injector nozzle.In the previous research work,the macroscopic spray characteristics of imported and domestic GDI injector samples were compared with the constant volume bomb visualization platform.By comparing the variation of spray diffusion angle and spray penetration,it is found that the spray characteristics of each spray hole were quite different.However,due to the experimental installation deviation,measurement error,the interaction of fuel bundles in each hole and image processing error,it was difficult to realize the quantitative analysis of the influence of spray hole processing difference on spray characteristic.As a result,the technical route of using multiphase flow simulation analysis method to study the nozzle structure was put forward.In order to verify the effectiveness of the simulation analysis model of multiphase flow,it needs to be validated by the experimental results of gas-liquid flow.However,due to the processing of small transparent holes is very difficult,experimental study on cavitation in spray nozzle are always carried out based on similarity Criterion to establish a simplified and magnified transparent nozzle structure.Simplified Experimental Study is used to study the generation and development of cavitation phenomenon in nozzle when the fuel flow through narrow gaps.In this paper,based on the previous experimental results of simplifying the flow process and flow characteristics in the nozzle,the same grid model as the experiment is established,and the numerical analysis is carried out under the same experimental conditions.The cavitation phenomena under different pressure differences in the experimental and simulation results are compared to verify that the cavitation model can be used to describe the gas-liquid two-phase flow in micro-holes.The validity of the turbulence model is verified by comparing the flow characteristics under the same conditions.Based on the verified simulation model for describing the motion of multiphase flow,the influence of GDI injector nozzle structure parameters on fuel injection characteristics is simulated in this paper.Firstly,the fluid domain model is abstracted from the real injector head structure,the influence of injector hole structure parameters on fuel injection characteristics was quantitatively analyzed by using the method of single factor sensitivity analysis.The structural parameters of the nozzle mainly include nozzle inlet chamfer r,inner hole diameter d,inner hole length L1,inner hole taper K,nozzle angle a,outer hole diameter D,outer hole length L2 and outer hole shoulder chamfer R.The gas-liquid two-phase flow in the nozzle with different nozzle structures is simulated,and the following conclusions are obtained:In the study range,with the increase of nozzle diameter,the increase of circulating fuel injection can be described by linear fitting equation.This shows that the sensitivity of injection characteristics to nozzle diameter remains unchanged within the study range at the same injection pressure.And the formulas for calculating the amount of circulating fuel injection under different injection pressure and injector hole diameter are obtained.It finds that the gradual increase of the inlet chamfer can effectively inhibit the generation and development of cavitation in the injector nozzle.The local resistance coefficient reduces as the increase of nozzle inlet chamfer.Moreover,the sensitivity of the injection fuel mass to the inlet chamfer increases first and then decreases slowly with the increase of the inlet chamfer,when r=0.02mm,the sensitivity is the strongest.For the inner hole taper of the spray nozzle,the increase of the inner taper can restrain the development of the cavitation phenomenon,which leads to the increase of the effective flow cross-section area at the exit of the spray nozzle,while the increase of the outer taper leads to the expansion of the cavitation area,but directly reduces the local resistance of the spray hole and leads to the increase of flow coefficient.The sensitivity of injection characteristics to small taper is strong,and the sensitivity of injection characteristics to small taper decreases with the increase of taper.Relatively speaking,the sensitivity of injection characteristic to internal taper is greater than that of external taper.The sensitivity of injection characteristics to the nozzle angle increases with the increase of the nozzle angle.When the angle increases to a=70°,the sensitivity increases obviously.Therefore,the control of processing accuracy under the condition of large nozzle angle should be strengthened.On this basis,the influence of eccentricity deviation of nozzle axis on injection characteristics is studied and analyzed.The results show that the negative nozzle angle can easily lead to cavitation and enhance the sensitivity of circulating injection fuel volume;positive orifice angle can reduce the cavitation area and the sensitivity of cyclic fuel injection.For the inner-hole length L1,outer hole length L2 and outer hole diameter D,it finds that the sensitivity of cyclic fuel injection rate to them is weak.The existence of shoulder chamfer R of the outer hole can reduce fuel accumulation and carbon deposition through local eddy current at the dead corner of the outer hole.Based on the single factor analysis results of the nozzle structure and combined with the current processing accuracy of the nozzle structure parameters,The order of magnitude of the influence of each structural parameter on the circulating fuel injection quantity is compared,and the key structural parameters are determined as the nozzle inlet chamfer r.the inner hole diameter d,the taper K of the outer hole K and the nozzle angle a.Based on the results of single factor sensitivity analysis,the influence of coupling of key structural parameters on injection characteristics is studied by using two-factor coupling analysis method.The results show that increasing the nozzle entrance chamfer will increase the sensitivity of the injection characteristics to the entrance chamfer r.On the contrary,increasing the nozzle diameter will also increase the sensitivity of the injection characteristics to the entrance chamfer r;Increasing the entrance chamfer r reduces the sensitivity of injection characteristics to the nozzle inside cone,but enhances the sensitivity of injection characteristics to the nozzle outside cone.In addition,the chamfering grinding at the entrance of the external cone nozzle is easy to change or destroy the minimum flow cross section area of the nozzle,so the grinding time should be strictly controlled;The increase of nozzle diameter d enhances the sensitivity of injection characteristics to nozzle taper,and the effect on inner taper is more obvious;Under the chamfer of the nozzle inlet with r>20 ?m,the cycle fuel mass is less sensitive to the nozzle angle,but with the decrease of the nozzle entrance chamfer,the sensitivity of the circulating injection volume to the entrance angle of the nozzle increases gradually;Increasing the nozzle angle reduces the sensitivity of injection characteristics to nozzle diameter,but the sensitivity of injection characteristics to nozzle diameter remains basically the same under small nozzle angle.Increasing the nozzle diameter enhances the sensitivity of cyclic injection quantity to large nozzle angle(a>70°);The change of nozzle angle has little effect on the sensitivity of injection characteristics to inner taper.For external cone nozzles,the sensitivity of injection characteristics to external cone taper decreases gradually only when the injector angle is larger(??70°).Based on the above results of two-factor coupling analysis,the influence of the mutual variation of injector nozzle structure parameters on fuel injection characteristics is clarified,which provides theoretical guidance for the optimization design of injector hole structure.In this paper,a single hole in the axis direction is machined on the injector body.The flow characteristics of a single hole are measured and a simulation model similar to that of a single hole structure is established.The simulation is carried out under the same boundary conditions,and the flow rate between the experiment and the simulation is compared.At the same time,according to the optimal design results,several GDI injectors are manufactured and measured on the developed oil pump nozzle performance evaluation test bench.The experimental results indirectly verify the reliability of the numerical analysis results in this paper.The influence of injector nozzle structure parameters on fuel injection characteristics based on Single-factor and multi-factor coupling analysis.The sensitivity of injection characteristics to entrance chamfer of nozzle is studied in this paper.It is clear that the sensitivity of injection characteristics is the strongest when the chamfer radius of nozzle entrance is near 20 um.This conclusion can be used to guide the choice of nozzle grinding time.Combined with the distribution area of cavitation area in the spray hole,the sensitivity of circulating fuel injection to the eccentric distance between the injector nozzle axis and the ball center of the valve seat is analyzed.It is pointed out that the positive eccentricity of the injector nozzle axis in the direction of the needle valve stem is beneficial to improve the consistency of fuel injection characteristics.The optimum region of nozzle structure parameters in the design stage of GDl injector is determined,and the fitting formulas for calculating the difference of circulating injection quantity caused by the deviation of nozzle diameter under different injection pressure are summarized,which can guide the selection of nozzle structure processing accuracy.