Study On Dynamic Injection Characteristics Of High Pressure Common Rail System Of Marine Diesel Engine

As the "heart" of diesel fuel system,high-pressure common rail system(HPCRS)is the core equipment of ships.It has been the bottleneck to limit the development of marine diesel fuel system for a long time.Especially for the characteristics of long fuel injection duration,large cyclic fuel injection quantity,and complex system structure of marine diesel engines,the dynamic injection mechanism is unclear,and the fuel injection stability is difficult to control,which limits the further improvement of marine diesel power,economy,and emission performance.Meanwhile,the electronic fuel injector as a key core component,the cavitation flow characteristics within its internal control valve and nozzle also have a complex impact on the precise control of the injection process,dampening the consistency and stability of the system dynamic injection.There are the technical problems that needs to be solved urgently for the current HPCRS.This paper conducted research around the HPCRS of marine diesel engines independently developed in China,and proposed four HPCRS configurations: monorail integrated,dual rail parallel,dual rail series and trackless distributed.Meanwhile,the multi physical fields numerical models of the HPCRS were established.With the establishment of the HPCRS dynamic injection performance test bench,the fuel supply characteristics,fuel injection characteristics and pressure fluctuation characteristics were validated.And further research was conducted on the influencing factors and mechanisms of dynamic injection characteristics at the system and component levels,providing theoretical support and method reference for China’s self-developed marine HPCRS.The specific research work is as follows:Firstly,the research was conducted on the impact of system configuration on dynamic injection characteristics,and the trends and influencing mechanisms of system fuel injection consistency and pressure fluctuation characteristics under different configurations were obtained.It was found that among the four configurations,the dual rail series system had best consistency in fuel injection quantity for multiple cycles of a single cylinder(fuel quantity deviation is 0.06%),and the stability of steady-state pressure was relatively high(fluctuation difference is 3%).And the stability performance of a system is related to its effective pressure storage capacity,mainly influenced by the internal hydraulic configuration of the system.The influence of high-pressure chamber pipeline distribution on fuel injection consistency and pressure fluctuation characteristics was studied.It is illustrated that the change in high-pressure volume near the fuel supply end mainly affects the transient regulation ability of the system,and the closer the high-pressure volume near the fuel injection end,the more significant the impact on fuel injection consistency.Further research was conducted on the influencing factors of cycle fuel injection quantity fluctuations under all operating conditions,and the major influencing factors and mechanisms of cycle fuel injection quantity fluctuations were obtained.It was found that the difference in fuel quantity is mainly caused by the difficulty in controlling the closing process of the needle valve,which is mainly affected by the sealing leakage characteristics and control accuracy at the control valve and the injection stability at the nozzle,both of which are related to the cavitation flow characteristics at the corresponding positions.Then,the research was conducted on the unsteady gas-liquid two-phase flow characteristics in the control valve under high pressure difference and the mechanism of cavitation formation in the variable cross-section area.A three-dimensional two-phase flow cavitation numerical model was established for the control valve.The accuracy of the model was verified through hydraulic characteristics and flow characteristics experiments.On this basis,the influence of pressure variation on the formation process of cavitation under steady and transient flow,as well as the influence of ball valve lift viraition on the formation area of cavitation within the variable flow interface were implemented respectively.The conversion law of the minimum throttling area in the transient control process was discovered,and the generation mechanism,spatial distribution structure and dynamic evolution law of control valve cavitation phenomenon were derived.To further suppress the formation of cavitation within the control valve,a study was conducted on the impact of structural changes in the control valve on the flow cavitation characteristics during the dynamic injection process.It was found that the gradually shrinking fuel return orifice had a significant inhibitory effect on its internal cavitation,diminishing the peak volume fraction of cavitation at the inlet from0.21 to 0;the rounded control valve effectively suppressed cavitation in the return fuel throttle and guide hole,and the peak volume fraction of cavitation at the inlet fell off from 0.21 to0.025 and from 0.68 to 0.5,respectively;an increase in the valve seat angle would lessen the level of cavitation at the flow surface between the ball valve and the valve seat to some extent.Finally,the research on the internal flow and dynamic injection characteristics of the HPCRS nozzle was carried out,and the injection test was complished with a real size visual nozzle.The pressure fluctuations at the nozzle inlet,nozzle internal flow,and injection characteristics under different injection strategies and pressures were analyzed.The research demonstrated that the spray cone angle at the main injection stage represented a boot shaped trend from slow to fast,which is mainly due to the form conversion between geometrically induced cavitation and string cavitation.During the pre-injection and post-injection processes,throttling instability caused by fluctuation in needle valve lift is the main reason for low fuel quantity fluctuations.Meanwhile,the causal relationship between multi-frequency pressure fluctuations and transient flow and cavitation characteristics within the nozzle under multiple injections was studied,as well as the impact law on the injection characteristics was revealed.It was illustrated that the formation and transformation of cavitation affect the frequency characteristics and propagation speed of pressure fluctuations by affecting the bulk modulus of fuel.Further research was conducted on the flow cavitation characteristics and formation mechanism of string cavitation within the nozzle under different nozzle structures.Based on the relevant research theory of vortex cavitation,it was demonstrated that in addition to low-pressure induced cavitation,vortices have the effect of converging gas nuclei and stabilizing them at the center of the vortex nucleus to form string cavitation.The single vortex basin in the sac is the key to forming vortex cores and generating stable string cavitation,and the transition between single vortex and double vortex flow fields affects the stability of string cavitation formation.