Exhaust System Reduces Auto Emissions, Boosts Engine Performance

March 01, 1999

COLUMBUS, Ohio -- A new automobile exhaust system reduces pollution and boosts engine power at the same time.

The single design takes the place of multiple parts in the standard auto exhaust assembly, including the manifold, muffler and catalytic converter. A study showed the new system increased overall engine performance by 5 percent while also cutting emissions by 15 percent.

Ahmet Selamet, associate professor of mechanical engineering at Ohio State University, developed the design with his former graduate student Kris Norman, now a technical specialist at Ford Motor Co. Together with James Novak, senior technical specialist at Ford, the researchers described their work in a recent issue of the Journal of Sound and Vibration. They labeled the new design a "perforated muffler manifold catalyst," or PMMC.

According to Selamet, consumers want new engines to perform better, consume less fuel, create less pollution, and run quietly -- all at the same time. Some of these features compete with each other during vehicle design.

"When engineers incorporate a number of components into commercial exhaust systems to meet emissions and noise regulations, it's not unusual for the changes to cut engine performance by about 15 percent. For a six-cylinder, 200-horsepower engine, that means a loss of about 30 horses, and that's a lot of power," said Selamet.

He added that the new design would help vehicles recoup part of that loss.

The typical exhaust system resembles a noisy tangle of pipes fraught with twists, turns, and internal passages. As air whistles through the pipes, pressure waves created by the engine's pistons reverberate inside, ringing as loud as 190 decibels -- well past the human ear's threshold of pain. Silencers muffle the sound for the outside world, but inside the pipes, the pressure works against the engine, which expends energy to push the exhaust through.

"Any time we change the cross-sectional area of a pipe, it will reflect waves back to the source, in this case the engine," said Selamet.

Gases flow more easily through the new exhaust system, because it contains shorter pipes with fewer internal divisions. In the laboratory, the PMMC lowered air pressure within the pipes by 30 percent, which boosted engine power by 3 to 5 percent.

Selamet said that the extra power would help an engine perform better and make it more fuel efficient. The overall performance of an engine using the PMMC would improve about 5 percent.

Tests also revealed that the PMMC cleans 15 percent more pollutant emissions from vehicle exhaust with no new filtering technology -- it simply makes the most of a standard catalytic converter.

"In my judgment, when it comes to catalytic converters, we aren't getting what we pay for," said Selamet.

He explained that engines release the bulk of their contaminants during the first few minutes of operation, before the typical converter starts working. That's because the chemical reactions that clean the exhaust can't begin until the converter heats to about 250°C, which for the standard exhaust system may take as long as two minutes. During those two minutes, contaminants sail through the filter untouched.

Selamet and his colleagues found a way to make the converter heat faster -- by installing it closer to the engine.

"We thought that, instead of just losing the heat generated by the engine, we could use it to our benefit," said Selamet. In tests, the catalytic converter within the PMMC heated 40 percent faster than its commercial counterpart.

The researchers made another change -- they widened the pipes leading to the converter, so that exhaust hits the entire surface area of the filter. Normally, narrow pipes leading into converters direct the exhaust over a fraction of the surface area.

The shorter heat-up time and wider filter area added up to a 15 percent reduction in overall emissions during a 22-minute simulated drive. Emissions of carbon monoxide, hydrocarbons, and nitrogen oxide were reduced by 16, 27, and 9 percent, respectively.

In addition, the PMMC weighs about 15 percent less than a typical exhaust system, which also improves gas mileage.

Selamet said these improvements would have a large impact on the environment and the economy, considering the millions of cars sold in the U.S. every year.

"If we improve the performance of certain areas ever so slightly, they directly cause an increase in fuel efficiency," said Selamet. "Any slight improvement in a single vehicle translates into enormous savings in the overall picture."

Ford Motor Co., sponsor of the research, has applied for a patent for the PMMC design, and incorporated variations of this concept into some of its prototype vehicles.

Ohio State University

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