The control of an opposed hydraulic free piston engine

•The control method of an opposed hydraulic free piston engine was proposed.•Grey model prediction can provide better performance in piston position control.•The piston motion on one side influences that of the other side.•A satisfactory synchronization of the compression stroke can be achieved.•Better real-time control performance was obtained in the hardware-in-the-loop test. An opposed hydraulic free piston engine is under design at the Tianjin University in China because of the advantages of this engine compared to the conventional engine. The main challenges presented by such engines are the control of the piston motion and synchronization. This study investigates the fundamental control context and control method of the opposed hydraulic free piston engine using an AMESim simulation model. The responses of the engine to force disturbance and setpoint change are studied using proportional–integral–derivative (PID) controller, simplified energy equation feedforward and grey model-modified energy equation feedforward. Grey model-modified energy equation feedforward, along with PID, is found to provide satisfactory response performances in piston position control. Synchronization is also discussed, and an acceptable synchronization performance is found to be achievable. The results of hardware-in-the-loop testing show that the control strategy works and that the controller is stable.