Experimental Study of the Influence of Valve Timing on Hydraulic Motor Efficiency

The timing of the valves of a hydraulic motor plays an important role in determining the throttling energy. To reduce this dominating energy loss, the timing of the valves must allow the fluid in the chamber to be precompressed and decompressed such that there is minimal pressure differential across the transitioning valve. The optimal valve timing to achieve precompression and decompression is a function of the motor displacement, angular velocity, pressure, and air content of the fluid, thus to achieve high efficiency at all conditions, active valve timing is required. The valves in most hydraulic motor architectures are mechanically timed to the piston displacement, rendering it impossible to change the valve timing as a function of operating conditions. This paper presents one novel valve architecture that allows for such processes: a rotary valve that is controlled independently of the piston displacement, enabling active timing control. To validate the concept and test the motor valve at fixed timing and fixed displacement conditions, a prototype valve was installed on a single cylinder 3.5 cc/rev slider-crank piston motor. The nominal timing of the valve was optimized for operation for a pressure of 7 MPa, 2% entrained air by volume, and an angular velocity between 10 and 30 Hz. A model, including the pressure dynamics, leakage, compressibility, check valve dynamics, and geometry dependent parameters is developed, simulated, and compared to the experiment. The experimental system includes instrumentation for measuring the inlet and outlet flow rates, piston position, and pressure in the inlet, outlet, and cylinder. A comparison between the model and experimental data shows good agreement and demonstrate the large impact of valve timing on efficiency.