Mathematical modelling of a hydraulic free-piston engine considering hydraulic valve dynamics

This paper describes the design of a single piston compression ignition hydraulic free-piston engine. An engine simulation model considering hydraulic valve dynamics is built. Extensive simulated results are presented and the major features of the engine are discussed. Experimental results from a full-cycle test of a prototype are also included and analysed integrated with simulation as well. The good agreement between experiments and simulations confirms the model can predict the engine performance. The engine takes more time in the suck phase for one cycle, which is helpful in sucking the low-pressure oil into the pump chamber. The dynamics of valves on the hydraulic chambers affect the chamber pressures. The pump chamber valve response lag compared with the piston displacement affects not only the chamber volumetric efficiency but also the engine fuel economy. The unchangeable piston motion trajectory makes the engine possible to get a high volumetric efficiency with fixed check valves. The rebound changes the compression stroke length and should be considered in the engine control. Asymmetric strokes appear when the engine is running under the piston self-excited vibration frequency.