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1 June 2002

Automakers, engineers tune out engine noise

Columbus, Ohio—Drivers will soon enjoy a much quieter ride as Ohio State University engineers help automakers cut whistle noise in the car engine's air intake and exhaust systems.

Ahmet Selamet, professor of mechanical engineering at Ohio State, designed a pipe adapter that fits into the engine's intake ductwork and helps engineers study whistles. In experiments, they reduced the sound of a whistle by as much as 30 decibels, so the noise was no longer audible inside the passenger compartment of a car.

Whistle noise has long been an issue for the auto industry, Selamet explained. A car's intake and exhaust system contains a large number of branched pipes. Air streaming through the pipes, combined with the acoustic resonance in these branched structures, leads to whistles.

"Pipes can generate sound just like a flute or other wind instruments," Selamet said. "Changing the length, diameter, or location of a branched pipe changes the frequency and amplitude of the sound. Unfortunately, the sound created by an engine is not nearly as pleasant as the one created by a flute."

The engineers designed a T-shaped aluminum pipe adapter that fits into the intake system of an engine and allows for fast and easy investigation of airflow resonance. Acoustic sensors and software help the engineers understand where and how the whistles form. They then use that information to redesign the pipe configuration and eliminate the noise.

Initially focused on the auto industry, the same technology may also quiet other air circulation systems, improve the accuracy of air flow measurements in general, and prevent vibration-related failures in many engineering applications, including gas pipelines.

Selamet calls his invention a "generic" pipe adapter because it diagnoses whistle noise in any commercial vehicle.

Straight sections of the adapter connect in line with the engine intake duct under investigation. The branch sticks out sideways, and the user lengthens or shortens the device easily by hand. The user can adjust the diameter of the branch as well. This allows engineers to readily examine how different pipe designs affect whistle noise.

Sometimes a small metal ramp is all that's needed to deflect the air inside a pipe and cancel out a whistle, Selamet said. He used the new adapter to investigate several ramp shapes and find out which ones work best in different situations.

With three shapes of a ramp—a flat steep slope, a flat shallow slope, and a V-shaped slope—the engineers performed experiments with their pipe adapter on an air intake system of a full-sized Ford engine.

The auto industry has used ramps for this purpose in the past, Selamet said, but this is the first time the company has used Ohio State's adapter to quickly find the ideal ramp shape for a particular application.

In Selamet's laboratory setup, air flowing past the engine's throttle plate mixed with another stream from a nearby duct. The air streams swirled into a vortex that created sound waves. The resulting whistle noise rose to 135 decibels, just above the threshold of pain for human ears.

Of the three ramps, the one with the flat, steep slope was most effective for silencing the whistle. It reduced noise levels to 105 decibels, comparable to the normal levels for an engine intake system in a passenger car.

The adapter can save automakers time and money. Whistles often crop up late in the design stage of cars, as engineers rearrange pipes for a number of reasons, Selamet said.

"One pipe gets longer, another gets shorter, location and diameters change. Sometimes the arrangement becomes just right to create a noise that is particularly annoying. Suddenly, the car isn't salable," Selamet said.

With so many pipes as the potential source of whistles, engineers can have a difficult time figuring out which pipe is to blame. They often resort to expensive trial and error, Selamet said. That's why his team is developing reliable strategies to understand the physics of whistles and diagnose and suppress them quickly.

"Our goal is to close the gap between laboratory experiments and the actual vehicle by laying down the basic principles that apply universally to any model of car," he said.

Natural gas lines suffer from whistles, too. But in their case, rather than an annoying sound, the associated vibrations may cause greater concern. Vibrations in gas pipelines can cause damage over time and eventually lead to failures of valves and other components.