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[en] Highlights: • A hydraulic electronic unit injector in HFPE is developed and the test bench is established. • Effect of drive pressure on injection delay of hydraulic electronic unit injector are investigated. • Cycle fuel injection quantity is tested online and off-line engine operation. • The BDC control results in HFPE based on feed-forward compensation are acceptable. • The energy flow in HFPE is analyzed and the hydraulic output energy is optimized. - Abstract: The fuel injection system in two stroke engine is very important, therefore the hydraulic electronic unit injector system is developed and the injection characteristics of hydraulic electronic unit injector are investigated. Firstly the HFPE and the hydraulic electronic unit injector working principle are analyzed, and then PID control strategy is built by engine demand. In order to validate the feasibility of hydraulic electronic unit injector, the prototype test bench is established. The specific measurement principle is presented. Further the injection characteristics, such as the effect of injection pressure on injection delay and the effect of engine frequency on injection delay, are analyzed. In order to optimize the engine stability performance, the BDC control based on fuel injection control is investigated. The load control based on fuel injection is also discussed and the BDC feedforward control with the load variation is investigated. Experiment results of stead engine operation shows that the hydraulic electronic unit injector system based on PID control can be satisfied with the engine operation demand. In addition, cycle fuel injection quantity is tested online and off-line engine operation. It is obvious that the fuel injection quantity is affected by the hydraulic pressure. The fuel injection quantity variation can be improved with decreasing the fluctuation of drive pressure or adopting more suitable oil common rail instead of connected with exhaust valve hydraulic drive oil-way. The energy flow in HFPE is analyzed and the hydraulic output energy can be optimized by selecting suitable hydraulic valves parameters. The fuel injection quantity should be designed by the energy balance and the stable operation requirement in spite of the higher thermal efficiency.
[en] The cold start characteristic of hydraulic free piston diesel engine may affect its stable operation. Therefore the specific cold start characteristics, such as BDC or TDC positions, pressure in-cylinder, heat release rate, should be investigated in detail. These parameters fluctuate in some regularity in the cod start process. With the development of the free piston engine prototype and the establishment of test bench, the results are obtained. For the dynamic results, the fluctuation range of TDC and BDC positions is 8 mm and decreases with time. The thermodynamic results show that the combustion process is not stable and the pressure in-cylinder fluctuates largely in the cold start process. In addition, the combustion is rapid and knock happens inevitably. In order to investigate the reasons, a CFD model is established for temperature analysis in-cylinder and heat transfer conditions. It is found that higher start wall temperature will lead to more uniform temperature distribution. The delay period may decreases and heat release will move forward. This reason is analyzed by thermodynamic derivation based on the first law of thermodynamics. Finally, the improvement suggestions of cold start strategy are proposed. - Highlights: • The cold start behaviors of HFPE are investigated in detail. • CFD method is used for simulating temperature distribution in start process. • Thermodynamic derivation uncovers the compression temperature distribution. • The improvement suggestions of cold start strategy are proposed.