| Hydraulic free piston engine is a new type of power unit that combines an internal combustion engine and a hydraulic pump.Compared with the traditional internal combustion engine,the crank connecting rod mechanism is cancelled,and it has the advantages of simple structure,adjustable compression ratio,low friction loss and high thermal efficiency.Among them,the opposed hydraulic free-piston engine has the advantages of low vibration and low heat dissipation loss.However,the inconsistent movement of the pistons on both sides will lead to deterioration of combustion,and it is necessary to control the movement.In this paper,a hydraulic synchronization mechanism based on tandem hydraulic cylinders is proposed,by means of simulation,its synchronization performance is verified and its energy loss is analyzed.A new synchronization control strategy based on PID feedback control is proposed for solving the problem of hydraulic synchronization mechanism,and the feasibility of the strategy is verified by simulation.According to the working principle of the hydraulic free piston engine prototype,a one-dimensional simulation model of the engine is built using MATLAB/Simulink,and the reliability of the model is verified by comparing the simulation and experimental results.The opposed hydraulic free piston engine is simplified to a simple physical model,and the simulation study is carried out.It is found that the influence of the phase difference on both sides is negligible compared with the displacement amplitude difference on both sides.Therefore,the amplitude difference should be mainly considered in the synchronous control.A hydraulic synchronization mechanism based on tandem hydraulic cylinders is proposed.The principle is that when the pistons on both sides move out of synchronization,the volume change speed of the hydraulic cylinders on both sides will be different,which will cause the hydraulic oil pressure to change,and the oil pressure will be used to promote synchronization.A one-dimensional simulation model is built to analyze the synchronizing performance of the hydraulic synchronization mechanism.The results show that under the action of the hydraulic synchronization mechanism,the engine can run stably,but there is a small difference between the displacement curves on both sides,and the difference in displacement increases with the difference in friction between the two sides.The energy loss of the hydraulic synchronization mechanism is calculated,and the results show that the proportion of viscous friction loss increases with the increase of the diameter of the coupler and the length of the contact surface of the coupler,and decreases with the increase of the gap of the coupler.The percentage of pressure loss decreases as the pipe diameter increases.The loss caused by the synchronization mechanism increases with the increase of output power,while the proportion of energy loss decreases with the increase of output power.Aiming at solving the problem of large energy loss of hydraulic synchronization mechanism,a new synchronous control strategy based on PID feedback control is proposed.The principle is to take the displacement difference between the top(bottom)dead center of the pistons on both sides or the difference between the actual displacement of the top(bottom)dead center and the set target value as a closed-loop control signal.By adjusting the air pressure or the constant volume valve connectted with the low pressure oil circuit to make the two sides operate synchronously,and the simulation research is carried out.The result is that by adjusting the air pressure or the volume of the constant volume valve,the control goal of balancing the top dead center or bottom dead center on both sides can be achieved,and the engine can run stably for a long time;under the condition of disturbance,there will be a large deviation in the displacement on both sides at the beginning,after several cycles of adjustment,the stability can be achieved again,so the control strategy has good robustness. |
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