The Ultimate Low-Maintenance Engine

Any backyard mechanic will tell you that the internal combustion engine depends on a complicated array of rods, valves, springs, and other components. Now scientists have built an engine that's nearly as efficient--but contains no moving parts. The invention, described in this week's Nature and at a recent meeting,* turns heat into intense sound waves that can compress gases or power generators.

The idea came from a heat engine invented by Robert Stirling, a mechanically gifted reverend, in 1816. It consists of a cylinder in which heated air expands to drive a piston. Once the heated air has done its work, it is rammed into a second chamber by a "displacer piston," where it cools and is recycled back into the cylinder. Scott Backhaus and Gregory Swift, physicists at Los Alamos National Laboratory in New Mexico, thought they could eliminate the pistons by using sound waves.

The tabletop engine consists of a resonator, a tube in which sound resonates at a frequency of 100 Hz. In other words, at every point of the tube the pressure rises and falls 100 times a second as gas molecules rush back and forth over a few centimeters. The sound also travels into another tube, which is shaped as a closed loop and connected to one end of the resonator. At one point in the tube the gas is heated, and every time the pressure peaks, it is allowed to expand into an adjacent stack of cold wire mesh, which cools the gas. During the second half of the sound wave, the cooled gas molecules rush back to the heated area in the tube, become heated, and the cycle begins once more, amplifying the sound waves. The engine generates up to 710 watts in sound energy and wastes about 70% of the heat added to the system--an efficiency of 30%. "For the first try, I think we should be very proud of it," says Swift.

This efficiency represents a "major breakthrough," says acoustics researcher Steven Garrett of the Pennsylvania State University in State College. He believes that improved thermoacoustic Stirling engines could become more efficient than internal combustion engines, which max out at about 40% efficiency. The next step, says Swift, is to build larger engines that use the sound pressure to liquefy gases now "flared off," or wasted, during oil production. The acoustic energy could also be harnessed to push and pull a coil around a magnet and generate electricity, he says.

* The joint 137th Meeting of the Acoustical Society of America and the 2nd Convention of the European Acoustics Association integrating the 25th German Acoustics DAGA Conference, Berlin 14 to 19 March.