An exhaustive study in plastic

  • 31-Mar-2008 06:05 EDT
woco-resonator.jpg

Woco’s patented all-plastic resonator (shown) won the 2006 Society of Plastics Engineers’ award in the powertrain category. The prototype plastic exhaust system also uses wide-band resonator technology.

A muffler operates in a high-temperature environment bathed in water and acids, which underscores why passenger vehicles are fitted with metal—not plastic—exhaust systems. By the end of the decade, though, vehicles could be adorned with a plastic exhaust system.

“We have found a way to isolate the plastic from the steel pipe, and the resulting exhaust system has an advantage in packaging size and weight as well as an acoustic improvement,” Udo Gaertner, Vice President of Woco Motor Acoustic Systems, said in an exclusive product-reveal interview with AEI.

Woco has tested its patents-pending plastic exhaust system on two different passenger vehicles. Both vehicles were powered by a four-cylinder, 2.8-L, turbocharged diesel engine. “We chose a diesel engine to start with because the exhaust temperature is approximately 600°C, which is cooler than gasoline engine exhaust,” said Gaertner.

In the first case study, the benchmarked multiple-weld exhaust system had a mass of 36 lb (16 kg) and a resonator volume of 25 L (0.9 ft3), while Woco’s hand-built prototype exhaust system was 27 lb (12 kg) with a resonator volume of 12 L (0.4 ft3). “With the outer muffler shell made of plastic, corrosion is not an issue,” said Gaertner.

Dynamometer testing was done at Woco’s Warren, MI, facility to compare the all-metal exhaust system to the plastic exhaust system. Outside-of-vehicle acoustic measurements showed a 2 dB(A) drop on the passenger side and a 1 dB(A) drop on the driver’s side when the test vehicle was fitted with the plastic exhaust system. With the engine running between 2500 and 4000 rpm, the plastic exhaust system recorded a more than 5 dB(A) drop in interior noise.

Noise reduction occurred, in part, because the plastic exhaust system incorporated a patented wide-band Woco resonator. In addition to the in-house tests, the Woco system underwent 1500 mi (2400 km) of on-road vehicle testing. “There was no melting or degrading of the plastic. There was no malfunction of any kind,” Gaertner said.

The next case study benchmarked a 22.5-lb (10.2-kg) all-metal exhaust system with an interference resonator in which exhaust gas traveled 8 ft (2.4 m) from entrance to exit. Woco’s second hand-built plastic exhaust system—tested on a softtop convertible—had a mass of 16.5 lb (7.5 kg), and its inline resonator meant exhaust gas moved only 1.7 ft (0.5 m) from entrance to exit.

“A resonator that is the same length as the exhaust pipe means less exhaust-gas restriction, so the turbocharger could be dialed down, which in turn would mean a fuel-economy improvement because the engine could be recalibrated to a specified horsepower,” said Gaertner.

Acoustic tests in the second study also accented sound performance. “With just 40% of the size of the benchmarked muffler, the Woco plastic exhaust system showed no degradation in the interior or exterior noise behavior,” Gaertner said. Heat tests conducted at Woco’s facility included a non-interrupted, 4-h dynamometer run time with different throttle positions and soak phases.

“The maximum temperature on the steel at the entrance of the muffler was 400°C, while on the plastic it was 110°C. The vehicle was also street-driven for 200 mi with a peak temperature on the plastic muffler of 58°C. So far, the Woco plastic exhaust system has accumulated more than 9000 mi in static and dynamic testing, and there haven’t been any breaks, leaks, or melting of the plastic,” Gaertner said.

In both case studies, the Woco solution bypassed the need for a companion part. “Since a muffler heat shield and the corresponding assembly are not needed, that’s a cost savings to the automaker. The OEM will also realize extra packaging space because the plastic exhaust system is at least 50% smaller than a conventional muffler,” said Tim Droege, Director of Sales and Marketing for Woco Motor Acoustic Systems.

Woco has chosen DuPont as the thermoplastic supplier, while another company will supply an endless single woven glass-fiber thread. “We went from concept to first prototype in eight weeks. What remains to be done is on-vehicle testing by an OEM,” Gaertner said.

A prototype plastic exhaust system for a gasoline-engine powered vehicle is in the works, as is a plastic exhaust system for a motorcycle. The first production-ready plastic exhaust system could reach the market on a 2009 model year vehicle.

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