Abstract: To control the piston motion of a single piston hydraulic free-piston engine, control methods are investigated. The piston moving mechanism is analyzed in theory. The test equipment for a prototype has been provided. The theoretical basis, control circuit and control method for the piston motion are analyzed. The piston frequency can be controlled by changing the delivery time of the compression power. A novel pilot control circuit is provided and a free-piston feedback modulation control method has been developed. The flow capability of hydraulic valves is fully utilized by the circuit. The piston extreme position is also limited by the circuit. The throttle and the leakage are main losses of the cyclic energy in the circuit. The sufficient condition for the steady operation of the single piston hydraulic free-piston engine is that the integrality of the expansion process should be achieved.

Key words: Dynamic analysis, Free-piston engines, Motion control, Self-excited oscillation, Helical joint, Li group theory, Parallel mechanism, Virtual center of motion, Articulated moving platform