Patent Issued for High-Inertance Liquid-Piston Engine-Compressor Developed at CCEFP

Achievement date: 

U.S. Patent # 8,297,237 was issued October 30, 2012 for the high-inertance liquid-piston engine-compressor–a project funded by the Center for Compact and Efficient Fluid Power (CCEFP), an NSF-funded Engineering Research Center (ERC) headquartered at the University of Minnesota.


This system represents a significant advancement in the field of compact pneumatic power. It has an energy density that is up to four times higher than a system actuated by a battery-powered motor; this means that an untethered device, such as a rescue robot, can be lighter and operate for longer periods of time. In a larger sense, CCEFP's work in compact pneumatics has the potential for enabling small and mobile applications, hence opening new markets for the fluid-power industry. The work also illustrates the benefits of inter-university collaboration, which ERCs foster.


Replacing batteries with the engine-compressor and replacing motors with high-power-density pneumatic actuators enable a robot that is more powerful, runs longer, and is ultimately more useful. These advantages are achieved by using a novel “liquid piston” trapped between elastic diaphragms. The device exploits the fluid-hammer effect of liquid in a long pipe–the same effect that bangs water pipes inside walls when one suddenly turns the faucet on or off.

Because of its unique configuration and operation, the engine-compressor can turn on instantly to produce power when needed or stay idle indefinitely without wasting fuel. The unique figure-eight arrangement of the liquid piston (see accompanying figure) self-balances the device to reduce vibrations and eliminate the need of opposing cylinders. The device features on-board electronics and seamless tubing to lower hydraulic friction and improve efficiency.