Demonstration of Efficient Energy Recovery Systems Using Digital Displacement® Hydraulics

Heavy off-road vehicles using conventional hydraulic systems waste significant energy through the throttling of fluid to control the motion of their actuators. This paper demonstrates how Digital Displacement® Pump Motors (DDPMs) can be used to enable efficient hydraulic energy recovery systems for these vehicles by controlling the motion of actuators directly without the need of throttling. Experiments were carried out on a test rig consisting of a 10 tonne boom supported by a hydraulic ram designed to mimic the setup of a heavy off-road vehicle. In order to demonstrate the DDPM’s potential for energy recovery systems the round-trip efficiency was measured by lifting and lowering the boom. The round-trip efficiency was taken to be the ratio of the mechanical energy output from the DDPM, when motoring to lower the boom, to the mechanical energy input to the DDPM, when pumping to raise the boom, over a known ram extension. The results showed measured round-trip efficiencies of between 63% and 87% over a range of pressures, shaft speeds and displacement fractions. Measured data obtained during the test was used to simulate the test using different system architectures and components to determine the energy efficiency. Both load sense and displacement controlled systems were simulated using both swashplate and Digital Displacement pumps. Comparison showed that the Digital Displacement systems used between 1.1 and 10.8 times less energy than the equivalent swashplate based systems. This work forms the basis for further development of energy recovery system architectures using DDPMs. Future challenges include development of the actuator control valves and transformers required to implement such systems.