Analysis Of Flow Characteristics Of High-pressure Common Rail Fuel Injection System

Under the strategic goal of "carbon peaking and carbon neutrality",higher requirements are put forward for the economy and emission performance of diesel engines.As the third generation diesel injection system,high-pressure common rail technology has incomparable advantages compared with previous fuel injection system.Its injection pressure is completely independent from the engine speed,with high injection pressure,fast jet speed and good fuel atomisation effect.The injection pattern can be flexible controlled according to the need to achieve clean and efficient diesel engine combustion.As the bridge connecting the pump and the injector,the pressure fluctuations in the common rail has a significant impact on the injection consistency.While the injector is the core executive component,its structural parameters directly influence the flow state and cavitation inside the spray hole,which affects the fuel atomisation quality.Therefore,the study of the structural parameters of the common rail system based on the influence of pressure fluctuations and flow characteristics is of engineering value,and is of great significance for the optimisation and design of the common rail system.Based on a diesel high-pressure common rail system with a maximum injection pressure of 180 MPa,this thesis studies the pressure fluctuations in the rail and the flow characteristics inside the spray holes as well as the cavitation phenomenon.Firstly,according to the working principle and basic structure of the common rail system,a one-dimensional simulation model of the common rail fuel system was established,and the accuracy and reliability of the model are verified by using the fuel injection rate test data.The influence of common rail system structure parameters on rail pressure fluctuations and fuel injection law was analyzed.Under the normal operation of the common rail system,the larger the engine speed,the greater the rail pressure fluctuations;the larger the plunger diameter of the pump and the smaller the plunger chamber volume,the stronger the ability to attenuate the pressure fluctuations in the rail;the pump with an odd number of plunger has a greater pressure fluctuations than the pump with an even number of plunger;the larger the inner diameter of the common rail pipe,the smaller the pressure fluctuations.On the single injector model,the influence of different structural parameters of the injector on the injection patterns was investigated:the larger the diameter of the spray hole,the smaller the volume of the control chamber,the larger the diameter of the inlet hole,the smaller the diameter of the drain hole and the larger the diameter of the control piston,the more the "slow first and then rapid" injection pattern can be achieved.Secondly,based on the Schnerr-Sauer cavitation model and the VOF multiphase fluid domain volume model,a three-dimensional simulation model of the nozzle was established.The accuracy of the model was verified by comparing the three-dimensional simulation results with the above one-dimensional simulation results.The quasi-steady-state method was used to study the flow characteristics and cavitation phenomena of the nozzle.The influence of different flow parameters and nozzle geometry parameters on the flow state and cavitation characteristics inside the nozzle was analysed,and the following conclusions were obtained:The larger the diameter of the spray hole,the greater the flow velocity,the more intense the cavitation phenomenon;the larger the ratio of spray hole length to diameter,the smaller the flow velocity,the weaker the cavitation phenomenon;the larger the radius of the inlet fillet,the smaller the pressure loss,the greater the flow velocity.It has a significant effect on reducing cavitation;the larger the inclination angle of the spray hole,the smaller the fuel flow velocity,and the more likely the cavitation will occur.