Can I use Piezoelectronics to dampen Engine Exhaust

You can certainly use noise canceling techniques to quiet an engine - the trick would be finding piezo-electronic devices that could deliver the power and frequency response that you need. They do use them in noise canceling headphones, though.

Idea by Bhankiii is good one. Try that if your have enough resources.

Noise cancellation devices require extra energy and silencers work without putting any extra energy. A good design of silencer or dynamic pressure wave absorber or smoothening device will be much better idea. Speakers are often used to cancel noise from motors but they are so-so effective. Look at the gun silencers and similar technology of the defense equipments on discovery channel and you will get more ideas. Those want to sell the defense stuffs create noise about their silencers so you can pick that noise to make your own silencers.

You can also find the noise source and redesign it by removing component that causes noise.

As an alternative: It's common to use differing path lengths for a divided exhaust stream. When rejoined the two are out of phase, & cancel each other.

If elecronic controls were to modify the length of one or both tracts, keeping the proper relationship through variing engine speeds...

What a great idea!

Along the same lines (sadly two-strokes have all but died out) changing the positioning of the cones in a two-stroke expansion chamber could give both good low end power and good top end power (and everything in between). Years ago, I custom built expansion chambers for two strokes, and could get very high peak powers (and correspondingly low power outside the power band). Your system would provide good power across the band -- as well as lower emissions. This, and accurate fuel injection, could revive two strokes... maybe.

A two stroke with an expansion chamber naturally does exhaust gas recirculation -- the standard method for reducing Nox. So if one positioned a lambda sensor at the exhaust port...

That would be what we called "slippy pipe" (USA, Karting, Enduro).

As an aside; when a 2-stroke cycle tuned exhaust was really doing it's job, the noise amplitude went markedly down.

The mention of headphones brings a question to mind, but first...

Seems to me that the idea would be a non-starter from a cost benefit standpoint. The weight and power penalty aspects have already been mentioned. The cost of reliable (hardened) feedback circuitry is also likely to be prohibitive--consider the price of the best hearing aids. Then there's the odd circumstance that the beneficiaries would mostly be strangers--everyone but the vehicle (and IC engine) purchaser (for whom lighter, cheaper sound elimination is already available)! Plus, there are the many car buyers for whom the sound is part of what sells the car/engine--they want to be seen and heard.

As to the personal approach--noise cancelling headphones--I have considered their effectiveness for preventing left-ear driver hearing loss (right ear for UK and such). But the thing I wonder about are laws in many states, including mine, which prohibit (collect money for) wearing of phones while driving. Does anyone here have experience with whether or not noise cancellation phones are exempt from such restrictions? Or is it just not worth it because of the potential nuisance of repeatedly being pulled over and having to explain...?

Many moons ago I saw an item on somethin like Beyond 2000 or one of those Future Tech shows.

One guy had developed a system with two (2) high powered speakers plumbed into the Exhuast just after the Muffler.

He had a pick-up mic in the muffler and used electronics to drive the Speaker to generate an inverse wave form to the exhaust.

The effect was less than 5db at 1meter.

The downsides at the time were:

- It was heavy, right at the rear of the vehicle

- It required a larger power source to drive big enough speakers

These limitations may not still be an issue, but audio is not one of my strong suites.

Sapper.

what about using the pressure fluctuations to move the piezo device and generate electricity! (I've seen worse ideas on this site)

That's interesting - I read an article just yesterday about using piezo's to generate electricity from falling rain.

I like it!

The idea is to reduce energy in the exhaust, so absorbing energy to generate electricity makes sense -- and that absorption alone should already start to reduce the sound level. Then, using that electricity to drive the cancelling device would further reduce the sound level. In a simple system, the pulse from the generator would simply be delayed appropriately to show up at the cancelling transducer at the right time, with nothing more than a capacitor needed for the temporary storage.

Gut feel says that the scale of things might not work well -- at least you don't see piezoelectric woofers, but you do see piezoelectric tweeters all the time.

Just what I was thinking...

I can see using pressure devices to generate the required electricity.

Also, the sound vibrations that are targeted for dampening, might be able to generate electricity.

Where would I look for sample candidates to narrow the selection of piezo devices and potential mounting and implementation so that I could do a test?

<capability of generating a canceling vibration that would significantly reduce the combustion noise of an engine?>

Maybe there.

Say your noise cancelling counter measure cancelled everything to a low Purring sound---to somebody standing outside

How does that improve the Engine's thermal efficiency?

May not be a viable effort

the deal with noise cancellation is that it is very individual. we could be sitting side by side and hearing entirely different noise. So a device that cancels the noise for both of us would result in unwanted noise for each of us. Air conditioning systems are sound-deadened in auditoriums and recording studios by having baffles that cancel out the pulsation caused by the vanes of centrifugal fans. So you could put a chamber in the muffler where a sound detector sits--something that is able to deal with extremely high temperatures and pressures--and matching the speed of sound to the speed of the circuitry, you could have a baffle down the line that vibrates in such a way that it maintains a constant pressure so from there downstream only a hushed flowing sound could be heard. But that would do nothing for the actual sounds in the engine compartment, only the exhaust. You'd need something attached to the frame and to the engine block that would do the same thing--probably to the body as well. But every part of the body, frame, and engine would have its own resonant quality to be dampened. And if you were standing closer to the dampener than to the body part or the engine, the dampener would be just as noisy as the engine or the resonance in the body part. You just can't have a big hunk of iron with four to eight pendulums inside operating at very different moments 3600 times a minute without creating some noise. So if you want to deaden the noise, the dampener would be better if it were attached to the person hearing the noise than to the thing making the noise.

The muffler itself is a dampening device that absorbs a good deal of the vibration. I've ridden in many cars where there was so little engine noise that all I could hear was the road surface on the tires (and Foghat blasting on the stereo, of course), so muffling works pretty well, if not perfectly. Bigger mufflers with gynormous plenum chambers would probably further reduce the tailpipe noise, but I'll bet engineers have already addressed this and the muffler as it is currently manufactured is pretty close to the point of diminishing returns. You don't want a muffler bigger than a car, for example. And if having a muffler ten times as big as the current size would only improve dampening by 10%, you can see why people haven't pursued that.

You could begin by using a wankel engine which has virtually no vibration to dampen, use a modern constant adjustment transmission that uses a belt and variable pulleys instead of gears, enlarge the muffler till it takes up the entire trunk area, maybe bubble the exhaust through a fluid like water, put velvet on the tires to reduce road noise, perhaps pave the freeway with velvet as well, and then apply huge numbers of noise cancelling baffles to every part of the car and you'd have a pretty quiet car. Put a video camera on each side of the car and project its output on the opposite side of the car and soon you'd have a nearly invisible car as well. Use radar/sonar in place lights and windows and you'd have yourself a stealth car.

It may be better to sound dampen the car and keep the sound out. There are many aftermarket products on the market to accomplish this. Hushmat.com Very good product and the owner is areally good guy.

If you really want to play with piezo use them or some other type of electrical vibration energy conversion device to enhance charging of the electrical system. An extra 20 amps would be a great practical level goal, although difficult to reach.