Investigation On The Piston Cycle Motion Properties In A Free-piston Hydraulic Engine

Compared with the piston motion in the conventional two-stroke crank IC engine,the piston motion in the HFPE has no analytical expression, no fixedtrajectory and no fixed limit position. It leads to the decrease of the cycle-by-cycle stability.The power density and the energy conversion efficiency are also reduced. The robustness ofthe system control is insufficient.The HFPE piston motion characteristics have an importanteffect on the system performance, understand the piston motion properties is one way toimprove the system performance.The hydraulic components properties of the HFPE which affect the piston motion isfirstly studied. For the issues of complex of the conventional HFPE hydraulic controlcircuit,a new hydraulic control circuit is proposed.Through the way of simulation andtest,the accumulator hydraulic storage characteristics are studied. Noting that in the lowcharge-discharge frequency condition, heat loss and hydraulic losses both exist, where thethermal efficiency is92%, hydraulic efficiency is98%; in the high charge-dischargefrequency condition, the accumulator loss mainly reflected in the hydraulic losses ofhydraulic efficiency about34%，and the thermal efficiency is almost100%.In clear of the hydraulic properties of the HFPE, the piston motion feature parametersare studied.Based on the dynamic properties of the accumulator, parameters match betweensystem parameters and accumulator parameters are studied，which shows that adjusting theaccumulator compression ratio of1to3can achieve the system compression ratio of5to30,which shows that parameter optimizing beween HFPE and accumulator is one of theeffective methods to improve system performance. The analytic expression of pistonmotion frequency is given, which notes that accumulator parameters and system structuralparameters both are influencing factors of piston motion frequency. In the expansion stroke,considering the combustion factor, characters of piston motion frequency are given.Pointing out that when the combustion energy release constantly, the combustion speed hasa great influence on piston motion frequency, which means the frequency characteristicscan be adjusted by way of improving combustion conditions.The hydraulic and mechanical coupling simulation model of the hydraulic free-pistonengine is established, stable operating characteristics of the system are studied. The resultsshow that, compared to conventional hydraulic circulation structure, the system efficiency gains a0.63%increase.Hydraulic pressure loss caused by the hydraulic system delayresponse dropped96.9%. The results provide theory support for research on the pistonmotion control propertise.