University of Minnesota
University of Minnesota: Department of Mechanical Engineering
MEPS Laboratory


Active Research Projects:


The flywheel-accumulator is a novel energy storage device that integrates rotating kinetic and pneumatic energy storage into a single device. This approach addresses two primary challenges of conventional hydraulic accumulators: limited energy density and system pressure varying with energy storage. The flywheel-accumulator is a cylindrical pressure vessel containing compressed gas and hydraulic fluid, which are separated by a moving piston. Energy is added to the device by either adding hydraulic fluid, which increases the pressure and the moment of inertia, or by applying a torque to the device. Due to the centripetal acceleration acting on the hydraulic fluid, a pressure gradient is formed, causing the hydraulic fluid pressure at the center port to decrease with increasing angular velocity. Thus by modulating the method of adding energy to the device, the system pressure can be controlled independent of the energy stored. The combination of constant pressure and increasing the energy density by an order of magnitude make this technology revolutionary for hydraulic energy storage. Research in the MEPS group to date has resulted in steady-state system models, concept demonstration through a low-energy bench top prototype, and early development of a mathematical simulation of the transient behavior. Current work involves studying the fluid dynamics related to fluid swirl and addressing machine design challenges of a high-energy system.

Compressed Air Energy Storage - Open Accumulator

The open accumulator integrates compressed air energy storage into a hydraulic system through the addition of an air compressor/motor. The prime challenge of this approach is managing the heat transfer, viscous forces, and gas leakage when compressing air from atmospheric pressure to hydraulic system pressures. These issues are being addressed through a liquid piston compression approach, which is being applied to offshore wind turbines by a multi-University research team. Specifically, the MEPS group is responsible for the machine design aspects of the project including a novel adjustable linkage variable displacement pump, valving, and water hydraulics.

Variable Timing Pump/Motor Valves

Valve timing is important to the efficiency, power, and noise of hydraulic pumps and motors. The valves of current pumps and motors are fixed and tuned for a specific operating condition. When the operating conditions or fluid properties change, the performance of the pump motor suffers. The goal of this research is to achieve high volumetric efficiency through dynamically varying the opening and closing of the pressure and tank valves to match changing operating conditions, including pressure, angular velocity, fluid compressibility, and displacement. This is being achieved through three objectives:

  1. A simulation that creates the efficiency map for different operation conditions.
  2. Experimentally validate the efficiency map using valve prototype.
  3. Study of active valve timing on the impact of system volumetric efficiency.

Characterizing Check Valve Dynamics

The check valve is a passive device that allows uni-directional flow. Check valves are used in a variety of hydraulic circuits and components, including applications requiring fast valve response such as pumps, digital hydraulics, and servo hydraulic circuits. The dynamic performance of the check valve is critical for determining system parameters such as pressure pulsations and flow ripple. To design better hydraulic systems, the MEPS lab is developing a mathematical model of the fluid-mechanical dynamics of a check valve.

While the behavior of a valve could be predicted through a multi-dimensional multi-physics computational simulation, this approach is computationally expensive and not appropriate for iterative design and optimization. The need for a lumped parameter dynamic model of a check valve is being addressed through a coupled analytical and experimental approach.

A novel experimental setup has being developed to measure the position the valve moving element using a laser displacement sensor. The experimental results are being used to validate the model and construct appropriate empirical corrections. The validated model will then be used for design and optimize check valves for specific high speed applications.

Switch-Mode Hydraulic Circuits

Switch mode power converters act as a DC-DC hydraulic transformer, analogous to buck and boost converters in power electronics.  By utilizing a fluid inertial element, these converters can efficiently boost the pressure above a supply level or buck the pressure below it.  In application, a converter would be applied to modulate power at each actuator and supplied by a common pressure rail.  These circuits have the potential to eliminate throttling losses associated with the present metering valve approach, which can exceed 50% losses.  While the models of buck/boost circuits developed in the lab have demonstrated potential for 85%+ efficiency across their range of power, innovative approaches to rapid hydraulic switching and fluid compressibility analysis are needed before they can be physically realized. In the MEPS lab, we are approaching this problem with a multi-pronged approach. We previously demonstrated an active soft switch to absorb flow during valve transition to reduce transitional throttling losses. Current work involves designing a variable duty ratio hydraulic valve that will cycle at 100Hz.  Work in the near future will center around deriving models for fluid compressibility that account for gasses going in and out of solution during the rapid transients which are characteristic of switch mode hydraulics. With these pieces solved, the converter models will be updated and optimized for a laboratory scale prototype converter.

Switch-Mode Continuously Variable Transmission

Analogous to switch-mode hydraulics, a continuously variable transmission (CVT) with flywheel energy storage is enabled by applying switch-moded control to the rotating mechanical domain. In the switch-mode CVT, the energy stored in a flywheel is transferred to an output shaft through a high-speed clutch and a torsion spring. The primary limiting component of this system is a clutch capable of high frequency pulsation with low energy loss. To address this challenge, the MEPS group has designed and demonstrated a custom variable duty cycle clutch for this application. In addition, a custom torsion spring with high deflection and low inertia has been designed and tested. Current work includes system integration, more in-depth system modeling, and design for high energy and power levels.

Mobile Hydraulic Power Supply / Stirling Engine

There exists great potential to improve the compactness and efficiency of mobile hydraulic power sources. Leveraging the liquid piston gas compression concept from the open accumulator, the MEPS group is studying a liquid piston Stirling engine pump. This novel hydraulic power supply eliminates much of the kinematic linkage in the engine and improves heat transfer in the working chambers with the use of liquid pistons. The research team is currently approaching this problem through computational fluid dynamics studies of liquid piston gas compression, creating a computational model of the Stirling cycle for system optimization, and experimental studies of a liquid ring air compressor.

Hydraulic Applications: Hybrid Vehicles, Rescue Spreader, Etc.

The MEPS group is also involved in a variety of projects involving research challenges in specific applications of hydraulics. For example, the research team is developing a mobile hydraulic power supply for a hand-held hydraulic rescue spreader (Jaws of Life). In addition, the research team has extensive experience in hydraulic hybrid vehicles of parallel, series, and power-split architectures.