Last October, a U.S. patent was issued for a device developed by researchers affiliated with the Center for Compact and Efficient Fluid Power. The device, a free piston engine compressor, directly converts hydrocarbon fuel to compressed air and can be used to power new portable technologies that use pneumatic actuators.
“There really wasn’t much guidance out there for an engine and compressor combined into one thing,” said Eric Barth, a professor at Vanderbilt University who helped develop the device. “All previous examples of engines that would then be linked to serve that function were separate modules that could be bolted together.”
Replacing traditional battery-powered electric air compressors with this new power supply allows for further development in applications such as un-tethered robotics, portable hand tools and more. The compressor uses a homogenous charge compression ignition (HCCI) and a two-stroke free-piston configuration to reduce its size while enhancing its efficiency. Using this design means the engine does not require an idle and can provide power on demand without wasting fuel.
“It’s an interesting engine cycle because it’s not a two-stroke engine and it’s not a four-stroke engine, but it has two strokes to it,” said Barth. “It’s like the intake and compression stroke of a normal four-stroke engine is done by injecting high pressure air and fuel very quickly and combusting it.”
Despite the prototype being just 12 cm long, the compressor was able to pressurize a 530-ml air tank to 97 psi in 38 seconds. The target specifications for the device, as set by the CCEFP, include a length of 20 in. with a diameter of 3 in. and a weight of 3 lbs. The average power output is set at 80 to 100 psi of compressed air at 100 W. These design goals were set in order to closely replicate the needs of an un-tethered rescue robot.
The device reduces vibrations by combining a figure eight design and a novel “liquid piston” trapped between elastic diaphragms. This design exploits the fluid-hammer effect, a surge in pressure caused when a fluid in motion is suddenly forced to stop or change direction.
“The liquid piston aspect of the device allows you to do a balancing of the engine,” said Barth. “You essentially have a one-cylinder engine that you can balance because you can have different parts of the ‘piston,’ which is the liquid section, moving in one direction and in other sections moving in the opposite direction.”
Check out this related article on free piston engines: http://www.mobilehydraulictips.com/ccefp-update-hydraulics-free-piston-engines/