The characteristics of a hydraulic directional control method for the displacement controlled system are studied. The directional control is realized by the pilot-operated check valve. The reversing operation of the pilot-operated check valve system is investigated with simulation and experiment. The relationship between the valve spool motion and the circuit pressure has been analyzed during the directional control. The pressure reversing points during the directional control can be used to determine the operation state of the pilot-operated check valve system. The results indicate that the valve spool motion can be confirmed by the pressure reversing points. The response becomes faster with a smaller diameter control piston. A smaller hydraulic chamber volume is also an effective method to improve the dynamic response. Higher circuit pressures decrease the dynamic response. The results can be applied for the optimized design of the hydraulic directional control method with the pilot-operated check valve.

A hydraulic hybrid vehicle equipped with the hydraulic transformer is proposed. The hybrid system features in rotary swash plate type hydraulic transformer and pressure cross-feedback control. The full numerical model has been built. Extensive simulated and tested results are given. The contributions of the hydraulic transformer dynamics to the hydraulic hybrid vehicle performance are investigated. The proposed hydraulic hybrid vehicle can achieve the ideal vehicle dynamic performance by adjusting the hydraulic transformer controlled angle. The switching between the driving and the regenerative breaking of the hydraulic hybrid vehicle is realized by the pressure cross-feedback control. The propulsion mode of the hydraulic hybrid vehicle can be changed automatically. The larger hydraulic transformer controlled angle realises a higher driving pressure while the smaller one achieves a higher driving flow. Due to the small inertia of the hydraulic transformer, the hybrid system is able to satisfy the speed requirement of the driver.