On-off motion of a hydraulic free-piston engine

The current paper presents the on-off characteristics of a compression ignition single-piston hydraulic free-piston engine. A pilot control circuit and a control method for on-off control of the engine are introduced. Detailed piston motion under on-off control of the engine is investigated. The results indicate that the piston on-off motion of 1 cycle can be divided into a pilot phase and a wave phase. The piston motion in the pilot phase is determined by the flow capacity of the frequency control valve. The piston acceleration during the accelerated compression process peaks in the pilot phase. The wave phase consists of a rebound stage, a decaying stage and a creep stage. The rebound stage determines the initial conditions of the other two stages and affects the piston frequency control. The control method proposed is immune from the bottom-dead-centre cycle variation. A high piston frequency results in a better engine fuel economy.