Current state of the art variable displacement pump/motors have high efficiencies when operating at high displacements. However, as the displacement of the pump/motor is reduced, the efficiency significantly decreases. Digital pump/motors aim to increase the efficiency and range of operation of the fluid power system by minimizing leakages, friction losses and compressibility losses. It is based on the concept of actively controlling high speed on/off valves connected to each piston cylinder displacement chamber. This work involves the development of a coupled dynamic model of a digital hydraulic pump/motor that is crucial for understanding the design tradeoffs and operating characteristics of the digital pump/motor. This simulation model can be used to characterize and predict the efficiency, define the dynamic response and flow requirements of on/off valves required to provide significant improvements in efficiency and dynamic response over traditional pump/motors, and perform design optimization studies. The model can analyze different operating strategies (flow limiting and flow diverting) and characterize the effects on pump/motor efficiency and flow ripple. The simulation analysis shows that the sequential flow limiting strategy yields the lowest power loss in both pumping and motoring and that small variances in the valve response would cause a significant loss of power.

No abstract

No abstract

Hydraulic hybrid drivetrains, which are fluid power technologies implemented in automobiles, present a popular alternative to conventional drivetrain architectures due to their high energy savings, flexibility in power transmission, and ease of operation. Hydraulic hybrid drivetrains offer multiple environmental benefits compared to other power transmission technologies. They provide heavy-duty vehicles, e.g., commercial transportation, construction equipment, wagon handling, drilling machines, and military trucks, with the potential to achieve better fuel economy and lower carbon emissions. Despite the preponderance of hydraulic hybrid transmissions, state-of-the-art hydraulic hybrid drivetrains have relatively low efficiencies, around 64% to 81%. This low efficiency is due to the utilization of conventional variable displacement pumps and motors that experience high power losses throughout the drive cycle and thus fail to maintain high operating efficiency at lower volumetric displacements. This work proposes and validates a new methodology to improve the overall efficiency of hydraulic hybrid drivetrains by replacing conventional pump/motor units with their digital counterparts. Compared to conventional pump/motors, the digital pump/motor can achieve higher overall efficiencies at a wide range of operating conditions. A proof-of-concept digital pump/motor prototype was built and tested. The experimental data were integrated into a multi-domain physics-based simulation model of a series hydraulic hybrid transmission. The proposed methodology permits enhancing the overall efficiency of a series hydraulic hybrid transmission and thus allows for energy savings. Simulating the system at moderate load-speed conditions allowed achieving a total efficiency of around 89%. Compared to the average efficiency of the series hydraulic hybrid drivetrains, our simulation results reveal that the utilization of the state-of-the-art digital pump enables improving the total efficiency of the series hydraulic hybrid drivetrain by up to 25%.

High speed valves have an important role in many existing fluid power systems and are an enabler for many proposed digital hydraulic systems. One method commonly used to improve the dynamic performance of on-off valves involves modifying the electrical input signal to the solenoids to reduce the inductive lag and eddy current decay. This research examined two commercially available direct actuated and pilot-stage actuated cartridge poppet valves and the role of peak-and-hold voltage and reverse current input profiles on opening and closing switching times. A test stand was built to characterize the performance of these valves. The valves were placed between two high frequency pressure transducers and the pressure differential across the valves was recorded, allowing the calculation of transition and delay time. The peak and reverse voltage duration was tested over a range of zero to ten milliseconds and an optimum response was found at a peak duration of six to eight milliseconds. Peak voltages ranged from 50 to 55 volts, followed by a holding voltage of 12 volts. Reverse current profiles were used to turn off the valves with a maximum peak current of three amps. The reverse current was used to increase the decay rate of eddy currents thus improving the turning off performance of the valves. Commercial valves that had a range of 33 to 55 millisecond turn-on response without input signal modification; these same valves had response times reduced to a range of seven to nine milliseconds after applying the peak and hold method. The turn-off time was reduced from 130 milliseconds to a range of 16 to 50 milliseconds after adding reverse current inputs. This improvement in valve performance can lead to siginificant energy savings due to reduction of transition losses and can widen the useful application of the valves.