Piston oscillation characteristics of single piston hydraulic free-piston engines

Piston oscillation characteristics of a single piston hydraulic free-piston engine prototype are studied. Dynamical model of the piston assembly is built up and the oscillation of motion deduced. The amplitude-frequency characteristic is given and the stability of the piston motion is analyzed by the phase plane method. The piston motion is a kind of single-degree-of-freedom, variational damping and stiffness autooscillation oscilation. The piston vibration frequency has a monotonous increasing relationship with the amplitude and is limited by the compression ratio. The stable limit cycle area is determined by the position of air intake and the throw of piston. The realizing of self-excited vibration is confirmed by BDC's position lied in the stable limit cycle area. The assigned frequency can be regulated from zero to the self-excited vibration frequency.