Research On Injection Stability And Pressure Fluctuation Of High-pressure Common Rail System

High pressure common rail system is the most potential fuel supply system today.The system injection stability directly affects the working performance stability and reliability of the matched diesel engine.It is one of the important evaluation indexes of the engine fuel supply system.In this paper,the definition of injection stability is: under the condition of the same rail pressure and the same excitation pulse width,The degree of difference in injection characteristics of the same injector between injection cycles.The main components of the high-pressure common rail system include high-pressure oil pump,common rail pipe and fuel injector.The process of pumping oil and fuel injection is accompanied by the flow of fuel and the opening and closing of needle valve,which will cause the change of system pressure and the change of fuel injection volume between engine fuel injection cycles.This phenomenon will further lead to uneven engine output and affect the emission and working characteristics of the whole machine.Therefore,it is of great significance to study the injection stability of high-pressure common rail system.Firstly,this paper studies the important factors affecting the injection stability of high-pressure common rail system,and it is clear that the pressure fluctuation effect in the system is the root cause of the problem of injection stability;Then,the simulation models of pressure fluctuation effect in different dimensions are established,the formation mechanism and propagation law of pressure fluctuation effect are explored through the research method of three-dimensional simulation,the influence law of pressure fluctuation characteristics is explored through the joint simulation method of Amesim and Simulink,and the potential to optimize the injection stability of the system from the perspective of hydraulic factors and control factors is clarified;Then,a new pressure regulating valve and a rail pressure control strategy around the problem of injection stability are proposed to improve the injection stability of the system from the perspective of hydraulic factors and control factors respectively.The research work will provide technical support for the optimal design and rail pressure control of high-pressure common rail system,and is of great significance to promote the self-study of diesel engine electronically controlled fuel supply system.The main work of this paper is as follows:In order to explore the main factors affecting the injection stability of high-pressure common rail system,the cyclic injection quantity is analyzed based on the calculation formula of cone needle valve injection rate.The results show that the pressure state between different injection cycles is very important to the injection stability;The change law of fuel injection characteristics of high-pressure common rail system under various working conditions is studied by experimental research method,and the fuel injection law and cycle fuel injection prediction model are summarized;The joint test method of kurtosis and skewness is used to prove that each stability characterization parameter conforms to the normal distribution.Then,the stability characteristics of the system under various working conditions are quantitatively analyzed by using range,relative range,kurtosis and variance,and the stability performance of high-pressure common rail system under different working conditions is obtained.Both theoretical analysis and experimental results show that the pressure fluctuation effect is the main factor affecting the injection stability of the system.Based on the three-dimensional simulation research method,the water hammer effect in the high-pressure common rail system caused by the opening and closing process of needle valve in the process of pump oil and fuel injection in the high-pressure common rail system is explored,and the pump oil and fuel injection process leading to the pressure fluctuation of the high-pressure common rail system are analyzed,The results show that the pressure fluctuation effect in the high-pressure common rail system can be attributed to the mutual transformation of fuel kinetic energy and pressure potential energy.The inconsistency between fuel flow velocity and pressure wave propagation velocity leads to the repeated transformation of kinetic energy and pressure potential energy,that is,the pressure fluctuation effect is formed.Based on the high-precision hydraulic model and control strategy,the effects of various parameters of high-pressure common rail system on the pressure fluctuation characteristics of the system are studied,the effects of common rail pipe structural parameters,system working parameters and control strategy on the pressure fluctuation characteristics of the system are analyzed,and a pressure fluctuation characteristic analysis method based on multiple regression model and sensitivity analysis is proposed,The effects of hydraulic factors and control factors on pressure fluctuation characteristics are compared horizontally.The results show that under a certain common rail pipe volume,increasing the pipe diameter is an effective method to reduce pressure fluctuation,and the length and diameter of common rail pipe have little effect on the fluctuation period;Compared with changing the structural parameters of common rail pipe to reduce the system pressure fluctuation,the influence of control strategy on the system pressure fluctuation is more significant.Based on the observation of a large number of pressure fluctuation curves,three standard modes of pressure fluctuation are proposed,and dimensionless parameters are proposed,which can quantitatively characterize the system state and clarify the influence weight of control strategy on pressure fluctuation due to hydraulic structure in the system.In order to improve the injection stability of the system from the hydraulic structure,the working principle of the used injector and the propagation mode of pressure fluctuation are analyzed,and then a new pressure fluctuation suppression device is designed combined with the pipeline vibration elimination measures under high oscillation frequency,Finally,combined with one-dimensional hydraulic simulation,the pressure fluctuation suppression effect and injection stability improvement effect of the new pressure fluctuation suppression device are evaluated.The results show that the new control valve can improve the injection stability of the original system by 10%-85%under different working conditions.Generally speaking,the larger the circulating injection quantity,the more obvious the control valve can improve the injection stability.Based on the mass flow conservation equation,the rail pressure control strategy is established,and the rail pressure control model is deduced and modified;Then,the rail pressure control strategy is programmed,and the main ideas and implementation forms of the control strategy are introduced;Finally,based on the joint simulation of the highpressure common rail system model and the rail pressure control strategy,the off-line simulation of the rail pressure control strategy is completed,and the feasibility of the rail pressure control strategy is verified in combination with the starting conditions,conventional conditions and transient conditions of the diesel engine.The results show that the rail pressure control strategy based on the flow conservation equation can meet the requirements of rail pressure control under various working conditions,and make the system have good injection stability.Although the control strategy based on flow conservation has good injection stability,the strategy needs all links in rail pressure control to be accurate,especially the MAP diagram needs to be accurate enough.However,this is almost impossible in the long-term use of high-pressure common rail system.Therefore,a control method based on reinforcement learning is proposed,and a rail pressure control revenue mechanism is established The rail pressure control based on reinforcement learning is realized by action selection strategy and value function updating method.Finally,the control strategy based on reinforcement learning is verified.The verification results show that the rail pressure control method based on reinforcement learning can improve the injection stability of common rail system under different working conditions;In the process of continuous use,the algorithm will find the control parameters to improve the injection stability of the system more and more quickly;Under different disturbances,the control parameters can be quickly adjusted based on this strategy to keep the system in a state of high injection stability.