Research On Sealing Performance Of Needle Valve Assembled In Electronically Controlled Fuel Injector

In order to increase the fuel injection pressure of the diesel engine,the leakage of the injector should be reduced.Needle valve assembly is one of the precise couplings in the electronically controlled common rail injector,which plays an important role in the sealing of the high-pressure oil circuit of the injector.In this paper,the working principle of the needle valve assembly was analysed and the mathematical model of the needle valve assembly was established using the principles of fluid mechanics,heat transfer and tribology.Based on theoretical analysis and data fitting for the pressure fluctuation phenomenon of the high-pressure pipeline,a single injection-based pressure fluctuation model was established.By using numerical simulation,the influence of structural parameters and working parameters on the sealing performance of the dynamic lower needle valve coupling was studyed.The study found that under the condition of single changing factor,the sealing property of the needle valve assembly is reduced by increasing the fit clearance of the assembly,reducing the length of the sealing face or increasing the pressure of fuel.The eccentric or tilting of the needle valve assembly and the pressure fluctuation of the pipe also reduce the sealing property,and the pulse width of the injection has little effect.Based on ANSYS Workbench,the one-way fluid-structure interaction and fluid-solid-heat coupling numerical simulation for static leakage of needle valve parts were carried out.The results show that the effect of thermal deformation on the gap oil film is less than that caused by pressure.The numerical trends of the gap oil film obtained by the two numerical calculation methods are almost the same.Under 120 MPa fuel pressure,compared with fluid-solid coupling,the mean oil film thickness of the outlet cross section of concentric,eccentric and inclined needles increases by 18.28%,17.09%,and 21.61%,respectively,when thermal deformation is considered.The calculation method can reflect the deformation of the needle valve more accurately.Based on the thermo fluid solid coupling,the flow field,temperature field and deformation of needle valve assembly when concentric,eccentric and inclined under different pressure were analysed.It is found that the higher the pressure and the gap oil film outlet temperature,the greater the elastic deformation and thermal deformation of the needle coupling.Compared with the concentric condition,the eccentric and inclined condition of needle make the gap oil film's average velocity and temperature rise higher,which lead to the increase of leakage rate of clearance fuel.Improving the linear expansion coefficient and strengthening the heat transfer of the needle valve assembly can reduce the thickness of the gap oil film and improve the sealing performance of the needle valve assembly.The analysis of the average oil film change of needle valve assembly during eccentric and inclined condition was made by the thermo fluid solid coupling method.The results show that the eccentricity of the needle valve reduces the average thickness of the gap oil film,and the incline of the needle valve raises the average oil film thickness,and the trend is more obvious with the pressure rising.When the needle valve is eccentric and inclined,the uniformity of the oil film along the circumferential gap is better with the pressure rise.