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# PiezoElectric Engine Muffling

Convert noise into electricity without reducing engine power
 (+3, -2) [vote for, against]

Recently an article appeared on the 'Net (see link) indicating that it might be possible to power a cell phone by the sound-energy of talking. At last the "hot air" of politicians can be useful for something!

Anyway, sound-energy is sound-energy, and many types of vehicals have a noise-reduction device typically called a "muffler". It is interesting that most propeller-airplane engines don't; apparently a muffler reduces the total engine power available. If my understanding is correct, this is because, during the exhaust-stroke of a piston engine, some of the engine's energy is used to force the exhausted gas through small holes in the muffler, thereby breaking up sound-waves. Safe airplane flight requires engines that aren't wasting energy.

Obviously, then, if there was an alternate way to reduce the noise of a piston engine, the average automobile's gas mileage could go up a bit.

So, how about an exhaust pipe, longer and larger-diameter and curvier than usual, lined with piezoelectric transducers? Each exhaust-stroke of the engine will have little difficulty pushing gases into the pipe (because of its diameter), and each pressure-wave of sound will affect the transducers in a way that would absorb sound-energy and create some electricity. By the time the sound-waves reach the end of the pipe, they could be diminished enough that no other muffling is needed.

Of course the electricity produced could be another way to keep the vehical's battery charged, meaning that the engine could have a smaller "alternator"/ generator, and another small increase in gas mileage would be the result of that (less engine power being used to directly produce electricity).

------------------------

As an extension of the original notion above, it is possible to "run a piezo crystal backward", feeding electricity into it and obtaining vibrations thereby.

Therefore it might be possible, toward the tail-end of the exhaust pipe, to apply some computer-controlled pulses to the last crystals in the pipe, to create "anti-sound waves" to do actual noise-cancellation as the exhaust gases enter the air. It would be really neat if the energy to power that one-chip-computer and create those anti-sound-waves could come from the transducers in the first part of the exhaust pipe.

 — Vernon, Dec 04 2008

Talk-powered Cell Phones http://www.networkw...ommunity/node/35879
As mentioned in the main text. [Vernon, Dec 04 2008]

Sound Power Level http://en.wikipedia...i/Sound_power_level
[afinehowdoyoudo, Dec 07 2008]

A good reference for this sort of work http://www.scribd.c...gineering-acoustics
start on page 20. [WcW, Dec 07 2008]

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you don't really explain how this would work.
 — WcW, Dec 04 2008

 // most propeller-airplane engines don't //

 Piston-engine aircraft have relatively short exhaust "stubs". Gas turbine (turboprop) aircraft are similar but are mechanically different powerplants.

The length of the exhaust stub is carefully engineered to provide efficient scouring and scavenging of the cylinder in the peak power band. Cars with damaged exhaust systems experience signifiant LOSS of performance as the induction system is no longer "balanced" with the exhaust. This is particularly noticeable with 2-stroke piston engines.
 — 8th of 7, Dec 04 2008

 Any mechanism which removes energy from the exhaust gas may potentially reduce engine effeciency. Exhaust gas momentum is used to scavenge the combustion chamber (suck out remaining exhaust gas and begin sucking in the next gas charge).

 While most piston engined aircraft don't use this effect, that has more to do with the volume and weight of the exhaust system required.

 A clear example is the catalytic converter. These devices require the exhaust gas to pass through a fine (and restrictive) matrix. Removing the catalytic converter from a 1995 VW passat diesel increased MPG from average 58 to average 64.

While energy could be created by piezo crystals, it might be much more effective to use them in place of the bulky and heavy sound insulation found in the doors, bonnet, bulkhead, underfloor etc. In these locations the sound energy really is 'surplus'.
 — Twizz, Dec 05 2008

 [8th of 7], this Idea is more about piston engines in general, than about airplane engines. For those, an exhaust system both decreases power and increases weight --and planes need maximum power-to-weight ratio. But for other engines (boats, lawnmowers, autos, etc.) this could be useful (ok, maybe not so much for lawnmowers :).

[Twizz], the momentum of a fluid flow, and pulses in the fluid flow, are two different things. The pulses are the noise in this case; diminishing them won't significantly affect overall flow/momentum of the exhaust gases. OR, it will certainly affect it less than an ordinary muffler. I don't object to using piezo sound-conversion in other places, though, as you mentioned.
 — Vernon, Dec 05 2008

Well, given the unmuffled engine operates at 130 Db, and your energy transforming methods are 100% efficient, then tell me, please, where will you use the extra 10 Watts?
 — MikeD, Dec 06 2008

Those 10 watts could be used to boost the stereo, where they will certainly be necessary.
 — Srimech, Dec 06 2008

[MikeD], are you sure the figure is only 10 watts? Why does it take so many more watts than that to create (with amplifier and speaker) 130db? Modern electronic sound-production systems aren't THAT inefficient! Anyway, if the watts produced is that small, then I'd say about the only place to use it would be at the end of the exhaust pipe, making those final noise-cancellation waves. Because I don't expect 100% conversion of noise to electricity. It might be worth noting, though, that at the engine the exhaust pipe is pretty hot, and maybe some thermo-electric energy conversion could be possible there. So, probably enough watts available to ensure the engine noise is sufficiently muffled.
 — Vernon, Dec 06 2008

How are you going to isolate the high frequency energy from the low frequency pulsing without inhibiting flow? This is akin to saying "build a muffler that uses hydraulic rams to dampen sound" without including any mechanism or concept. Use fairy feathers to damp sound, use mist nozzles to damp sound, use tiny turbines to damp sound... how?
 — WcW, Dec 06 2008

[Vernon], one answer, right off the top of my head, whereas your question is concerned, is; Stereo systems not only have to move the air, but the cone that moves the air. The reciprocation of the air is probably the least of the forces required when factoring in the reciprocation of the mass of the actual speaker. It has been said by one of my physics professors that if one could harness the energy of sound with 100% efficiency, it would take a stadium full of people screaming at the top of their lungs to light a 60 Watt incandescent bulb. That might be hyperbole, but even so...
 — MikeD, Dec 06 2008

 [WcW], piezo electric crystals can come in various sizes and shapes, but mostly they all have at least one flat surface that can be pressed-against by some external force. If you read the linked article, you might get the impression that the sound-absorbing crystals are fairly thin, so that differences in air pressure by passing sound waves can apply enough force for the crystals to generate a voltage. So, I described lining the exhaust pipe with such crystals. Perhaps I should have specified the inside of the pipe as preferable. I don't think the crystals care much about the frequency at which sound waves apply changing pressures to them, but it should be clear that as the exhaust passes down the length of the pipe, sound waves traveling in the gases will have plenty of opportunity to apply changing pressures to many energy-absorbing piezoelectric crystals. After all, if the sound waves can apply changing pressures to your eardrums after escaping an ordinary unmuffled exhaust pipe, then they can apply changing pressures inside the pipe.

[MikeD], OK.
 — Vernon, Dec 07 2008

so you think that a simple thin coating of ablative material will be adequate to muffle engine noise even if it includes no restriction to flow? A quick review of the literature is needed here! Making a muffler using only flexible surfaces to ablate the high frequencies just does not work unless you significantly restrict the flow, in which case it works poorly.
 — WcW, Dec 07 2008

If you look at my link you will find a quite astute model for analyzing the effectiveness of acoustic dampening that treats acoustic energy using the same rules that we use in electric circuits. There it is illustrated how hard it is to develop a damper that will ablate sound using a passive electromagnetic circuit (as on page 30., your idea in a nutshell). Simply put, if it is compliant enough to capture the wave motion then the frictional loss to the wave is to minimal to ablate it, but if we increase the ablation we no longer follow the wave motion and thus capture no energy. The higher the back pressure (muffler restriction) the more wave amplitude is available to our damper but this comes at the cost of engine performance.
 — WcW, Dec 07 2008

[WcW], why are you confusing piezoelectric material with 'ablative material'? They are not the same thing! Also, why the focus on high frequencies when most of the noise of IC engines is lower-frequency? Finally, I did specify a curvier-than-normal exhaust pipe. It seems to me that at each curve there is something like this:
___/
If the gas flow is going from left to right, then when it hits the curve, there is a sort of angled "face" that the sound waves impact more directly, which offers an opportunity to better-extract energy, by piezo crystals on that surface.
 — Vernon, Dec 07 2008

First of all the high frequencies are the primary target of ablative mufflers as they are the most irritating noises. Second of all I am simply pointing out that if you plan to ablate a noise with anything we must examine the ablative properties of that material. The text i provide uses the model of ablating sound using an electromagnetic medium. Your piezo surfaces are simply electromagnetic resonators with a very low compliance.
 — WcW, Dec 07 2008

 [WcW], ok, you are using "ablate" in a way that I was not previously familiar with. Or perhaps you are confusing it with "abate"?

 Anyway, remember that this Idea was sparked by that other notion of converting some sound-energy of speech into enough electricity to power a future cell phone. Your objections would appear to make that other notion impossible; you wrote: "if it is compliant enough to capture the wave motion then the frictional loss to the wave is to minimal to ablate it". However, it CERTAINLY is impossible to use energy of one type to create energy of another type, without reducing the total quantity of energy of the first type (unless you know of a problem with the Energy Conservation Law?).

I therefore conclude that your objection is flawed (probably because "friction" is not the critical factor here).
 — Vernon, Dec 08 2008

Piezo's are remarkably energy efficient. There is a school of thought that says if overunity is to be achieved it will come from this field. It is why Piezo's were outlawed on the 'bakery for a while..they get very close to 100% efficiency. What you do with that energy afterwards I can't comment on. Their frequency range seems a bit restricted for this application. The losses will come from trying to squeeze a low amplitude, high frequency energy into a high amplitude low frequency band. Now *that* I haven't seen without added energy, except in the electomagnetic realm.
 — 4whom, Dec 08 2008

Oddly enough, my micromanipulation stage has piezo motors and they make a terrible whine. If I could use an internal combustion engine to muffle them, I'd be quite merry.
 — MaxwellBuchanan, Dec 08 2008

[4whom], there's plenty of room inside an exhaust pipe for different-sized piezo crystals, for dealing with different sound-frequency ranges.
 — Vernon, Dec 08 2008

I guess I wasn't clear. I don't care what form the energy takes once you remove it from the air: efficiency beyond that point is trivial. What i care about is the assertion, that i judge to be false, that the energy will be absorbed at all. A piezo crystal may well efficiently convert all of the force applied to it into electricity; the fiberglass matting is very near 100% efficient converting that energy into heat. However we must craftily trap that energy and spread it over a huge area for simple ablation to even capture a fraction of the energy (really mufflers are only weak filters). Sound doesn't behave like a bullet, wasting itself against the first surface it hits it likes to slip around, you know, fluidly and stuff. It isn't "banging" against the sides of your exhaust pipes just waiting to waste itself on the first mildly soft smooth surface it comes across. To wit: If sound did behave the way you describe no noise would survive the long curvaceous soft fleshy journey into our bleeding ears.
 — WcW, Dec 09 2008

 [WcW], then you are essentially saying either that the notion of powering a cell phone via voice energy and pizoelectric crystals won't work at all, or that this variant Idea just can't absorb ENOUGH sound energy to make a significant difference. Fine; this is still the HalfBakery, remember?

 Also, your argument still contains an error; even without absorbing any sound energy at all, a muffler is going to get hot because the exhaust gas is hot. And per [MikeD], the EXTRA energy of the sound is only 10 watts or so. It doesn't take the kind of heat sink you describe, to handle 10 watts. I suspect part of the large size you talk about has to do with reducing the corrosive effects of hot exhaust gas, by cooling it. I note that piezo crystals are quartz, an already-fully-oxidized substance and therefore nearly corrosion-proof (I'm aware that the requisite electrical conductors are not likely to be so lucky, unless they are made out of gold or platinum, heh).

So really, the only way to find out for sure how impractical this Idea is, is for somebody to build one. Maybe the conductors can be made from a conductive compound that can withstand the heat (there are several, such as magnetite or one of the copper oxides, or even fools gold), coated with a ceramic glaze to withstand the corrosion.
 — Vernon, Dec 09 2008

"So really, the only way to find out for sure how impractical this Idea is, is for somebody to build one." [Marked For Tagline]
 — WcW, Dec 09 2008

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