Study On Operation Mechanism And Combustion Process Of Hydraulic Free Piston Engine

The operation mechanism of the hydraulic free piston engine(HFPE)is explored,and according to its characteristics,the strategy to improve its operational stability is proposed.According to the operating characteristics of hydraulic free piston engine,new combustion modes suitable for free piston engine is proposed,and the proposed combustion modes are studied through simulation and experiment.The prototype of a hydraulic free piston engine was built,and the ignition and continuous operation were successfully achieved.Zero-dimensional and threedimensional simulation models of the hydraulic free piston engine were established.The simulation method and experimental method were combined to explore the operation of the free piston engine mechanism and combustion characteristics.A synchronous control mechanism for the movement of pistons on both sides of an opposed hydraulic free-piston engine is proposed.The synchronization mechanism is analyzed by simulation,and the synchronization effect is verified.The results show that the synchronization mechanism can correct the piston position.When the friction force difference of two piston groups is 1,2 and 3 times,the displacement difference of bottom dead center can be reduced by 93.15%,87.69% and 74.32% respectively.The device can significantly improve the synchronization of piston movements on both sides of the engine and ensure the stable and continuous operation of the engine.According to the operation mechanism of hydraulic free piston engine,a hydraulic fixed volume piston mechanism is proposed to meet the requirements of the consistency of the top dead center during engine operation.The results show that the introduction of the mechanism can effectively improve the stability of top dead center.When the stroke of constant volume piston is 90%,the change of compression ratio can be reduced from 27.78% to 2.17%.According to the work principle of HFPE,an energy recovery system suitable for free-piston engine is proposed.The application of this system can effectively reduce the fluctuation of the bottom dead center position in the engine cycle operation due to the fluctuation of combustion conditions.The real results show that when the area of the energy recovery piston is 3 times of the sectional area of the plunger,the fluctuation of the bottom dead center position is reduced by 80.11%when the equivalence ratio increases from 0.3 to 1.The self-adaptive characteristics of piston motion during the operation of a freepiston engine are studied.The effect of this mechanism on engine stability,combustion,knocking,and energy conversion efficiency is explored.The characteristics of the free piston engine in terms of combustion constant volume,heat loss,and knocking are compared and analyzed.For hydraulic free-piston engines,it is necessary to cooperate with the fast-burning combustion mode to promote the improvement of thermal efficiency to a greater extent;the contact time of the hightemperature working fluid in the cylinder with the wall of the combustion chamber is shorter than that of the crank connecting rod engine,so its heat loss is less.Hydraulic free piston engine has a better anti-knock performance to a certain degree.According to the combustion characteristics of free piston engines,an ignition system combining a glow plug and a spark plug is proposed to trigger HCCI combustion.And the combustion mode was studied by the experiments.The experimental results show that the compression ratio range suitable for the DACI combustion mode is more extensive than that of the glow plug-assisted HCCI combustion mode,and the stability and flexibility of the combustion mode for ignition phase control is higher than that of the glow plug-assisted HCCI combustion mode.When the glow plug temperature is low,the heat release process is mainly controlled by the ignition timing of the spark plug;when the glow plug temperature is high,the heat release process is mainly controlled by the glow plug.The equivalence ratio suitable for the DACI combustion mode is between 0.6 and 0.8.When the equivalent is relatively large,the spark plug is used to control ignition,and when the equivalent is relatively small,the glow plug temperature is used to control ignition.A fast combustion mode with multi-point ignition is proposed,and the combustion mode is studied by simulation.The results show that when the number of spark plugs increases,the peak heat release rate increases.And the shape of the peak heat release rate also changes: the heat release rate changes from a slow increase to a rapid rise.At the same ignition phase,CA50 gradually advances and then delays as the spark plug distance increases,and when 4-point ignition is used and the spark plug distance is RD50,or 7-point ignition is used and the spark plug distance is RD70,the combustion duration is the shortest.At the same CA50,the anti-knock performance of the free piston engine is highest at 7-point ignition,followed by 4-point ignition,and singlepoint ignition mode.The indicated thermal efficiency of the free piston engine with multi-point ignition combustion mode is higher than that of the single-point ignition combustion mode.And in the 7-point ignition mode,when the edge spark plug is placed in the RD70,the highest indicated thermal efficiency can be obtained.The operating mechanism and combustion characteristics of the free piston engine are explored in depth.A scheme to optimize the operation stability of the free piston engine is proposed,and suitable combustion strategies are proposed according to the characteristics of the piston profile.It provides new ideas for improving the stability of the free piston engine.It provides a new direction for improving the combustion of the free piston engine and improving its thermal power conversion efficiency.