Research On Pressure Fluctuation Characteristics Of High Pressure Pipe In Ultra-High Pressure Common Rail Fuel System

As the most critical component of diesel engine,electronic control fuel injection system plays an important role in improving diesel combustion optimization and emission control.Electronic control high-pressure common rail fuel injection system with its flexible and controllable injection timing,stable and independent injection pressure,has become today’s the most popular fuel injection system.With the continuous upgrading of emission regulations in various countries,higher requirements are put forward for the fuel injection system.Increasing the fuel injection pressure to achieve ultra-high pressure fuel injection,has a positive effect on improving engine combustion,reducing pollutant emissions and improving fuel economy.When the fuel injection pressure reaches 2000 bar and above,the pressure fluctuation in the high-pressure pipe is more intense,and higher requirements are asked for the injection control,high-pressure sealing and so on.In this thesis,the performance of ultra-high pressure common rail fuel injection system with injection pressure up to 2500 bar was studied by building up an ultra-high pressure system test platform.The main contents of this thesis are respectively the pressure fluctuation characteristics in high-pressure pipe and multi-injection characteristics of ultra-high pressure common rail fuel injection system.Firstly,this thesis analyzed the pressure fluctuation in the high-pressure pipe of ultrahigh pressure system with the working pressure in the range of 1800 bar ～ 2500 bar.Different fluctuating characteristic parameters were chosen to characterize each stage of pressure fluctuation,and their changes have also been studied.The change law of the pressure drop amplitude,the fluctuation amplitude,period and its decay time were obtained at the time of injection.Results show that,with the increase of injection pressure,when the injection was finished,the water hammer pressure waves generated by the seating of the needle valve were more intense.Therefore,the amplitude of the pressure drop and fluctuation increased,but the period of fluctuation decay decreased,those would facilitate multiple injections with shorter dwell-time.At the same time,the influence mechanism of fuel injection pulse width on fuel pressure fluctuation in high-pressure pipe was studied.Secondly,this thesis studied the multi-injection(pilot-main injection and main-after injection)characteristics at the condition of ultra-high injection pressure,the change law of pressure fluctuation and the mass of fuel injected in second injection has been studied with the changed dwell-time between two consecutive injections.Results showed that,as the dwell-time changed,the opening pressure and injection mass of the second injection,the pressure drop amplitude and the minimum pressure during injection,all these characteristic parameters showed a periodic fluctuation.The influence mechanism of the second injection condition by the water hammer pressure waves generated by the seating of the needle valve at the end of the previous injection has also been analyzed.