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.
"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.
While clearly every engine model is different, I always thought that petrol engines reached maximum efficiency at higher revs (e.g. 4000 rpm) and diesel engines somewhat lower (perhaps 2500 rpm) so I must admit this article has confused me a little. Of course, that may be down to my ignorance. My own driving involves very little time between 700 and 1500 rpm. One is usually accelerating or decelerating through this range.
I should be happy if some expert wants to \'put me right\' on this matter.
Or what Honda has been doing for years on their 6 cylinder deactivation Oddessy van. GM acts like they were the first to do this, but copied Honda.
Based on my own experience with two GM produced SUV vehicles I would expect the Subaru to be less likely to need repairs during its useful life than the Equinox as well.
Unless you can put the cancelling signal at the same physical location as the source, you\'re always going to get spatial interference patterns.
Maybe the frequency is so low (wavelength so long) that this doesn\'t really matter. It might just be twice as loud for people you pass on the street.
Why not just fix the engine so it doesn\'t make so much noise in the first place?
Vibration is probably already taken care of. Back 20 years or so (I guess) when Oldsmobile built the Aurora and marketed it as a step forward in luxury cars, or some such fanfare, I remember that one of the things they learned from studying Mercedes was that their chassis had a frequency of 25000kHz, which is beyond our human range. Which means - when it hit a bump, or even just rolling down the highway, the car wasn\'t pummeling you with a constant vibration background noise which, even though you\'d get used to it, would still tire you out and irritate you. They built the Auroras to that new spec, and as I remember it was a first for the Big Three at the time. But I imagine that by now the noise and vibration guys have the low frequencies taken care of...I think our hearing runs out around 15-16000kHz, so they probably get close to that even on the less expensive cars. OK, I *hope* they have by now. I\'m an optomist.