Study On The Motion Control Strategy And Stable Operation Of The Linear Free-piston Generator

Linear Free-piston Generator(LFPG)has been highly attractive to the specialists and scholars in the auto industry these years.As a new potential choice to the power units of the electric vehicle and other electric devices,the LFPG has merits of high fuel flexibility,alterable compression ratio,simple construction,high efficiency of energy conversion and so on,which are all brought in by its particular structure and motion.However,it is still difficult to maintain a stable state of the piston motion,and there occurs misfire or knock during the in-cylinder combustion at times.The investigation on the stable operation of the LFPG does matter in its further development and application.With the load-changing method,a control strategy on the stable operation of the LFPG was proposed in this paper,and the verification of its feasibility was taken on through theoretical analysis and simulation calculation.Based on the kinetic equation of the FPLG,a cyclic simulation model was constructed utilizing the SIMULINK and then employed to investigate the running performance of the LFPG under an stable state and how the fuel quantity and load coefficient affect its performance.After that,the condition of the SSO(Standard Stable Operation)was concluded on the basis of energy balance theory.While the altering fuel quantity(the altering Qin)as the interference factor and the stable compression ratio as the control objective,the means that regulating the load coefficient of the piston element by changing the resistance of the load circuit was brought forward to attain an SSO state.The exploration on the performance of the LFPG applying the above control strategy was conducted,which verified the feasibility of the strategy.Finally,the optimum analysis of the parameter in the controller was performed through a simulation method.Investigations indicate that when the LFPG operates under a SSO state,there exist a Qin of 271)and a ce of 260,and whether in the unstable state of a continuous operation(Qin ≥ 253)but Qin ≠ 271J)or a halting(Qjn<253J),the LFPG would achieve the SSO state finally by means of the load-following control strategy.In the unstable state of continuous operation,with greater disturbance variable,the achievement of the SSOtakes a longer duration and has a greater demand of the load resistance.While in the unstable state of halting,the stronger disturbance in the LFPG,the quicker breakdown happens in the system without a control module,and a longer period spends the LFPG spends to achieve the SSO state with a control module.In general,the less Qin from the fuel combustion,the less energy is left to drive the piston element motion and the smaller resistance of the load circuit is needed.But with a greater Qin,the LFPG would gain a shorter cycle,a higher frequency of energy output and a faster velocity of the piston element under the SSO condition.Additionally,the parameters of the controller makes great differences on the LFPG performance.The simulation calculation illustrates that as the proportionality coefficient increase,the measured compression ration curve comes out with larger range and number of the oscillations;but there occurs a more severe oscillation in the load resistance,which is of a greater amplitude in the initial stage and a shorter time to its peak value.Beyond these,the piston moves with a faster average velocity during the whole adjusting stage,shortening the time to pass by a certain distance and enlarging the oscillation frequency of the displacement curve of the piston element.When the kl increase,there is a smallest oscillation number of the load resistance at one "kI",but the oscillation amplitude of the curves present an uptrend.And because of the decrease in the "kI",the oscillation amplitudes of the load resistance and piston velocity change more fiercely,while the piston element gains a longer motion cycle,leaving a hysteresis phenomenon in the motion curves under a SSO state.