Engineers improve testing of auto parts for leaks

January 25, 2007
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ANN ARBOR—Testing auto parts for dangerous leaks could take a leap forward, thanks to improvements in a photoacoustic screening technique developed by two University of Michigan engineers.

David Dowling, assistant professor of mechanical engineering, and Serdar Yonak, a graduate student in that department, have found that by using two ultra-sensitive microphones and a C02 laser, they can detect gas leaks slow enough to fill a thimble in a work week.

“We believe that in the next year, a prototype system based on these results might actually be built and used in a factory,” said Dowling, whose project was funded by the Ford Motor Company.

In their experiment, Dowling and Yonak used a 12w CO2 laser, intense enough to ignite wood in less than a second, and passed it over an aluminum enclosure filled with sulfur hexaflourine gas. Because the laser photons—individual packets of energy that act as both particles and waves—are tuned to the exact frequency required to resonate sulfur hexaflourine, the beam heats up the gas, increasing the pressure locally near the leak and creating sound waves.

Using different sized “leaks”—essentially valves studding the surface of the enclosure—the researchers could let specific amounts of gas escape, then analyze their sound waves with a computer. “In effect, we detect a gas cloud, not a leak,” Dowling said. “But the high concentration part of the gas cloud can only reside at the leak location.”

So far, the acoustic method works equally well for small and large leaks because of the high-quality of current microphones. Indeed, one of the technological problems the researchers had to overcome was how to make the device ignore the inherent noise of the so-called raster scanning process, when the laser passes over the metal target.

In the future, said Dowling, automakers could use the technology to rapidly screen any part that contains gas, including components of the fuel system, exhaust components and valve seals. Current methods, which involve readings of atomic spectra or submerging the parts in a tank of water, are time consuming and lead to bottlenecks on the assembly line.

Dowling and Yonak’s method could be applied without stalling the manufacturing effort, he said. Yonak will be presenting the work at the 134th meeting of the Acoustical Society of America, which is being held this week in San Diego, Calif.

U-M News and Information ServicesUniversity of Michigan