Ohio State University engineers are helping car manufacturers deliver a quieter ride by reducing whistle noise in the engine’s air intake and exhaust systems.
Ahmet Selamet, professor of mechanical engineering at Ohio State, and his students designed a pipe adapter that fits into engine’s intake ductwork and helps engineers study whistles. In experiments, they were said to be able to reduce the sound of a whistle by as much as 30 decibels, so that 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 resonances 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.’
Along with Ohio State research engineer Yale Jones and former graduate students Darius Kurniawan and Brian Knotts, Selamet collaborated with Jim Novak, senior technical specialist in powertrain operations of Ford Motor Co., which sponsored this research.
The engineers designed a T-shaped aluminium pipe adapter that fits into the intake system of an engine and reportedly allows for fast and easy investigation of airflow resonances. Acoustic sensors and software helped the engineers understand where and how the whistles form. They then used that information to redesign the pipe configuration and eliminate the noise.
Selamet calls his invention a ‘generic’ pipe adapter, because it can be used to diagnose whistle noise in any commercial vehicle.
Straight sections of the generic adapter connect in-line with the engine intake duct under investigation. The branch sticks out sideways, and can be lengthened or shortened by hand. The diameter of the branch can be adjusted as well, allowing engineers to 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 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-size Ford engine.
The auto industry has used ramps for this purpose in the past, Selamet explained, but this is the first time Ohio State’s generic adapter was used to quickly find the ideal ramp shape for a particular application.
In Selamet’s laboratory set-up, 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 dB, comparable to the normal levels for an engine intake system in a passenger car.
The adapter can save car manufacturers 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.
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.