Bus? What Bus? Fuel-Cell Buses Quiet Compared With Diesels, UF Research Shows
July 1, 2002
GAINESVILLE, Fla. — Reduced air pollution and increased energy independence may not be the only advantages of the experimental fuel cells the federal government hopes will replace internal combustion engines.
Unlike their roaring diesel counterparts, fuel cell-powered buses also are remarkably quiet, according to research findings by University of Florida engineers published this spring in the Journal of Sound and Vibration.
A team of UF mechanical engineering faculty and students compared the noise produced by conventional diesel buses to the noise generated by an experimental fuel-cell bus owned by the university. Although fuel cells have long been recognized as being quieter than conventional engines, the research was an unusual attempt to quantify the difference, the engineers say. The result: The fuel-cell bus was at least nine decibels quieter than the quietest diesel bus – akin to the difference between a conversation and a busy machine shop, as one researcher put it.
“Really, the only noise comes from the fans and pumps and blowers and things that are part of the fuel-cell system or part of the bus, like the air conditioning compressor,” said Vernon Roan, a professor of mechanical engineering and member of the team. “All of that noise combined is still pretty quiet compared to a diesel engine.”
Roan said there are close to 75,000 municipal transit diesel buses nationwide. Replacing them would result not only in less air pollution, it also would mean less noise pollution, something many pedestrians and bus riders alike would find welcome, he said.
Fuel cells typically combine hydrogen and oxygen in a nearly pollution-free chemical reaction that produces energy. With some types of fuel cells, the only emissions are water and heat. Although major technical hurdles remain, the potential for this cheap, non-polluting energy source spurred the Bush administration in January to launch a major research program, the Freedom CAR project, aimed at making fuel cells a practical reality.
Roan, director of the UF Fuel Cell Laboratory, served as vice chairman of a federal committee on fuel cell-powered vehicles during the Clinton administration and has testified before congressional committees on the Freedom Car initiative as recently as June.
He said buses likely will be the first fuel cell-equipped vehicles to become widely used, with the first ones starting to replace diesel buses in as few as five years.
Fuel-cell buses are closer to practical use than cars for a number of reasons, he said. For one thing, current fuel cells are significantly larger than internal combustion engines, so it’s easier to fit them into buses, he said. For another, diesel buses are considered a major contributor to pollution in metropolitan areas, so getting them off the road is a top priority for state and local policy makers. It’s also easier to justify today’s relatively high cost for fuel cells in city buses, which have relatively long life spans and generate income from passengers, he said.
Scientists and fuel-cell companies have experimented with different versions of fuel-cell buses for years, with the UF-owned bus one of three experimental models built by a fuel-cell company in 1993 as part of a federally funded project. Most research, however, has focused on improving fuel-cell technology. The UF research zeroed on the oft-overlooked noise angle.
The team measured noise levels at several locations inside and outside the bus and compared the results to similar measurements from three diesel buses. The fuel-cell bus was consistently and significantly quieter. For example, from a distance of 2.5 meters, or about 8 feet, it produced 73 decibels compared to 82, 84 and 87 decibels for the diesel buses.
A 50-decibel noise is just audible while 90 decibels is considered very loud. So the gap between 73 and 82 decibels is “huge,” Roan said.
The researchers measured the noise generated by the buses only at idle because they did not have the equipment to do so while the buses were moving, Roan said. While the fuel cell produced little or no more noise while the bus was in motion, however, the diesel buses became much louder, he and other researchers noted.
“This really means that we took the diesel engine at its absolute quietest operation, so the difference to the public would be even more significant,” said Paul Erickson, a former UF doctoral student in mechanical engineering who participated in the study and now is an assistant professor at the University of California–Davis.
While the benefits of quieter mass transportation are clear, Roan pointed out one potential drawback. “It may be that fuel cell buses are too quiet,” he said. “By that I mean that if pedestrians can’t hear the buses coming, they could be in danger.”