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High power density, good controllability, and low cost are the most appealing characteristics that make hydraulic systems the best choice for many applications. Current state-of-the-art hydraulic variable displacement pumps show high efficiency at high displacement while they have poor efficiencies at low displacement. This paper proposes a novel alternating flow (AF) variable displacement hydraulic pump to: 1) eliminate metering losses by acting as a high-bandwidth pump for displacement control; 2) achieve high efficiency across a wide range of operating conditions and displacements; and 3) allow multiple units to be easily common-shaft mounted for a compact multi-actuator displacement control system from a single prime mover. A dynamic model using first principles describes the cylinder pressure, flows between pairs of cylinders, and net inlet and outlet flows as a function of the pumps phase-shift angle. The model captures hydraulic check valve dynamics, the effective bulk modulus, leakage flows, and viscous friction. Piston kinematics and dynamics are discussed and energy loss models are presented. A first prototype of the AF pump was constructed from two inline triplex pumps that were modified so that three piston pairs were created. Experiment tests were conducted and used to validate the model. After model validation, simulation results from the model can offer an initial evaluation of this novel pump concept and potential applications and can be used to guide the design for future prototype of the AF hydraulic pump. [ABSTRACT FROM AUTHOR]

A digital displacement hydraulic motor uses actively controlled on/off valves instead of a valve plate for porting fluid. Displacement can then be varied by varying the timing of the on/off valves. To minimize throttling energy loss, the valve timing must also consider the precompression and decompression of the compressible fluid. In this paper, a rotary on/off valve, whose rotation controls the valve timing, is used. The desired motion of the rotary valve that encodes the ideal valve timing is a function of, and is periodic with respect to, the motor crank shaft angle. As the desired valve trajectory is not periodic in time unless the motor speed is constant, the conventional internal model based repetitive control is not applicable. A recently developed angle-domain repetitive control is used instead to control the rotary valve motion to achieve the desired valve timing. Experiments show that the proposed controller achieves good tracking performance for a broad range of varying motor speeds. The maximum error is reduced by six to ten times with the new control scheme over baseline fixed period repetitive or proportional-integral controllers. This in turn leads to efficiency gains for the digital hydraulic motor up to 45%. [ABSTRACT FROM AUTHOR]

Switch-mode hydraulic control is a compact and theoretically efficient alternative to throttling valve control or variable displacement pump control. However, a significant source of energy loss in switch-mode circuits is due to throttling during valve transitions. Hydraulic soft switching was previously proposed as a method of reducing the throttling energy loss, by absorbing, in a small variable volume chamber, the flow that would normally be throttled across the transitioning high-speed valve. An active locking mechanism was previously proposed that overcomes the main challenge with soft switching, which is a lock mechanism that releases quickly and with precise timing. This prior work demonstrated a reduction in energy losses by 66% compared to a control circuit. In this paper, a numerical model is developed for a switch-mode virtually variable displacement pump (VVDP) circuit, utilizing the proposed soft switch. The model is then used as a means of designing a proof of concept prototype to validate the model. The prototype design includes methods for controlling the soft switch spring preload, travel distance, piston displacement required to unlock the soft switch, valve command duty cycle, switching cycle period, and load pressure. Testing demonstrated that the soft switch circuit performed as expected in a baseline condition. The operating region for this prototype was found to be quite narrow. However, the model does a good job of predicting the displacement of the soft switch. [ABSTRACT FROM AUTHOR]

Switched inertance hydraulic systems (SIHS) use inductive, capacitive, and switching elements to boost or "buck" (reduce) a pressure from a source to a load in an ideally lossless manner. Real SIHS circuits suffer a variety of energy losses, with throttling of flow during transitions of the high-speed valve resulting in as much as 44% of overall losses. These throttling energy losses can be mitigated by applying the analog of zero-voltage-switching, a soft switching strategy, adopted from power electronics. In the soft switching circuit, the flow that would otherwise be throttled across the transitioning valve is stored in a capacitive element and bypassed through check valves in parallel with the switching valves. To evaluate the effectiveness of soft switching in a boost converter SIHS, a lumped parameter model was constructed. Simulation demonstrates that soft switching improves the efficiency of the modeled circuit by 42% at peak load power and extends the power delivery capabilities by 77%. [ABSTRACT FROM AUTHOR]

The efficiency and power density of gas compression and expansion are strongly dependent on heat transfer during the process. Since porous media inserts can significantly increase heat transfer surface area, their addition to a liquid piston compressor/expander has been hypothesized to reduce the time to complete the compression or expansion process and hence the power density for a given thermodynamic efficiency; or to increase the thermodynamic efficiency at a fixed power density. This paper presents an experimental investigation on heat transfer with porous inserts during compression for a pressure ratio of 10 and during expansion for a pressure ratio of 6. A baseline case without inserts and five cases with different porous inserts are tested in a compression experiment: 3 interrupted ABS inserts with plate spacing of 2.5, 5, and 10 mm and 2 aluminum foam inserts sized with 10 and 40 pores per inch. The 2.5 mm and 5 mm interrupted plate inserts were also tested in expansion experiments. Porous inserts are found, in compression, to increase power-density by 39-fold at 95% efficiency and to increase efficiency by 18% at 100 kW / m 3 power density; in expansion, power density is increased three fold at 89% efficiency, and efficiency is increased by 7% at 150 kW / m 3 . Surface area increase is found to be the predominant cause in the improvement in performance. Thus, a liquid piston compressor/expander together with a porous medium may be used in applications requiring high compression ratios, high efficiencies, and high power density such as in an open-accumulator compressed air energy storage (CAES) system or a compressor for compressed natural gas (CNG). [ABSTRACT FROM AUTHOR]