Charged with the task of getting the fuel economy of the 2011 Chevy Equinox down to 32 mpg on the highway and beat out the 28 mpg-rated Honda CR-V, Toyota RAV4 and Ford Escape, GM engineers employed some rather unconventional fuel efficiency technology - active noise cancellation (ANC). By using the same technique used in noise cancelling headphones, the team was able to let the Equinox's engine run at a more fuel efficient rpm without the associated low frequency noise and achieve what GM claims is segment-leading fuel economy.
Noise and vibration engineer at GM, Jim Valance, points out that most car engines typically idle at under 1,000 rpm and operate at over 1,500 rpm when in gear. In between that rpm range the engine is more fuel-efficient but produces a lot of vibration and low frequency noise.
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"There's a boom, or very low frequency rumble that comes from the engine when it runs in that rpm range," said Vallance. "We knew if we could deaden those booms, we could run the engine at the lower rpm, which would provide a significant boost to fuel economy. So we started kicking around ideas and came up with noise cancellation like you see in some high-end stereo headphones."
The system Vallance and his team up with consists of an ANC module that kicks in when the engine is running in the 1,000 to 1,500 rpm range. Two microphones in the vehicle interior measure the amplitude and the phase of the engine noise sound waves and relay this information to the ANC module. To ensure it targets only the unwanted engine noise and not other noise inside the cabin, such as the stereo or passenger conversations, the module also takes into account the engine speed to calculate the target frequency that needs to be cancelled.
"We take a proactive approach and target only the unwanted sound in the vehicle, which in this case is linked with the engine firing frequency," says Vallance.
The ANC module then uses this information to generate a sound wave that has the same amplitude as the engine noise but with inverted phase. This inverted sound wave is directed through the speakers in the vehicle's front doors and the subwoofer in the rear so it combines with the original engine sound wave and the two cancel each other out.
GM provides the ANC system as standard on the 4-cylinder Equinox but it isn't the first time we've seen ANC technology employed in a car. Toyota used the same technique to reduce the interior engine noise of its Toyota Crown Hybrid when it was discovered that its engine, which also ran at a lower rotational frequency, was noisier than expected.