Investigation On The Pressure Fluctuation Characteristics Of The Pressure Amplification Common Rail System For Marine Low-Speed Engines

Common rail system has been widely used in the field of marine power because of its advantages,such as high injection pressure,fast response,and flexible and controllable injection rate.The pressure fluctuation characteristic is an important index to measure the working ability of the common rail system,and it is the key to determining whether the common rail system could successfully match the main engine.The structure of the pressure amplification common rail system for marine low-speed engines is relatively complex.The fuel pressure building process and the fuel injection process are controlled by the pressure amplification control valve and the injection control valve,respectively.Under the synergy of the double valves,the pressure fluctuation variation of the pressure amplification common rail system for marine low-speed engines is complex.It is of great significance to reveal the influential mechanism of system pressure characteristics for promoting the independent research and development process of the fuel system for marine low-speed engines.For this purpose,the pressure fluctuation characteristics of the pressure amplification common rail system for marine low-speed engines are studied by combining the experimental investigation and numerical simulation in the thesis.The main research work of includes the following aspects.Based on the injection performance test bench of the pressure amplification common rail system for marine low-speed engines,the effects of the control parameters on the pressure fluctuation charateristics and the moving characteristics of the pressure amplification piston of the pressure amplification common rail system for marine low-speed engines are explored under single-valve and double-valve modes,respectively.The system pressure characteristics under different modes are compared and analyzed.According to the variation of the system pressure fluctuation,the pressure characteristics indicators,such as the pressure peak before fuel injection,the pressure difference during stable fuel injection,and the pressure surge after fuel injection,are proposed.The corresponding relationship between the displacement of the pressure amplification piston and the system pressure fluctuation characteristics is obtained.Aiming at the pressure peak before fuel injection,the pressure difference during stable fuel injection,and the pressure surge after fuel injection,the mechanism of pressure fluctuation is investigated based on the pressure wave propagation theory with a closing boundary,the fluid network theory,and the water-hammer pressure fluctuation theory,respectively.The causes of system pressure fluctuation are revealed by comparing the theoretical analysis results with the experimental data.The calculated results of the pressure peak before fuel injection are in good agreement with the experimental results,and the average relative error is only 3.27%.As for the pressure difference during stable fuel injection,the maximum average relative error between the model calculation results and the experimental data under different working modes and working conditions is only 3.51%.The relative error between the calculated pressure surge after fuel injection and the test data is5.78%.According to the theoretical analysis results,it is concluded that the high fuel pressure rise rate is the main reason for the formation of the pressure peak before fuel injection.The pressure difference during stable fuel injection could be reduced by reducing the flow resistance and the fuel flow velocity.The pressure surge after fuel injection is caused by the water-hammer effect induced by the closing of the needle.In order to analyze the interactive influence of different parameters on the fuel injection pressure peak of the pressure amplification common rail system for marine low-speed engines,the predictive model of the fuel injection pressure peak considering the interaction of three factors is developed based on the polynomial response surface model and D-optimal design method.The determination coefficient and the adjusted determination coefficient of the developed fuel injection pressure peak predictive model are 0.997 and 0.984,respectively.On this basis,the significance analysis is carried out on the fuel injection pressure peak predictive model.The single,quadratic,and cubic factors that have significant effect on the fuel injection pressure peak are screened with a P-value less than 0.05 as the standard.The effects of the critical factors on the fuel injection pressure peak are analyzed.The essence of the fuel injection pressure peak of the pressure amplification common rail system for marine low-speed engines is revealed.Aiming at the problem of excessive fuel injection pressure fluctuation of the pressure amplification common rail system for marine low-speed engines,a multi-objective optimization study is carried out based on predictive models.A multi-objective optimization model is established to reduce the fuel injection pressure peak and increase the stable fuel injection pressure,and with the constraint condition that the cycle fuel injection quantity fluctuation is less than ±10%.The feasible solution set and the Pareto frontier of the optimization objectives are obtained by the multi-objective optimization algorithm within the design space,and an optimal solution selection method based on the minimum optimization rate factor is proposed.When the system operates in single-valve mode,the fuel injection pressure peak of the system after optimization no longer exists,the stable fuel injection pressure is increased by 12.02%,and the maximum fluctuation rate of cycle fuel injection quantity is only 3.84%.When the system operates in double-valve mode,the fuel injection pressure peak of the system after optimization is reduced by 35.55% at most,the steady fuel injection pressure is improved by more than 23%,and the maximum fluctuation rate of cycle fuel injection quantity is 9.87%.The fuel injection pressure peak and the stable fuel injection pressure could be optimized simultaneously under the premise of maintaining the relative stability of the cycle fuel injection quantity.