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# Cheap Exhaust Noise Cancellation

Attach an extra crank driven piston to the exhaust to cancel out the noise
 (+1, -2) [vote for, against]

The pressure in an exhaust pipe (before the muffler) increases and decreases repeatedly with each stroke of a piston.

The idea is to drive an extra piston from the crank in such a way that it generates the opposite pressure function exactly in time with that in the exhaust. Connecting the piston to the exhaust should cancel out the repetitive pressure variation (the loud noise) and leave just the average flow of exhaust gases.

The extra piston would be "sucking" when each cylinder's exhaust valve is open, decreasing back pressure and producing more power.

For the pressure to cancel correctly the piston would probably have to have some kind of variable cam or rocker to produce the correct amplitude pressure cancelation. I'm sure the engine computer could be programmed to provide this information.

 — stevewest, Mar 21 2005

Low-noise tarmac http://www.tarmac.c...cle.asp?id_news=120
Probably more useful than reducing exhaust noise. [Basepair, Mar 27 2005]

Yes, but it seems easier to employ the system that Cadillac had on their cars for a few years, and Bose has on their headphones right now: reverse wave generation. I don't know if that is the proper term for it, but the process consists of a speaker creating the exact sinewave opposite of any ambient noise. Two waves that are in 180 degrees out of sync cancel each other out, and the result is silence.
 — natewill, Mar 21 2005

 What you call "reverse wave generation" is precisely what I am suggesting, except using a piston instead of a loudspeaker and driven mechanically instead of electromagnetically.

Loudspeakers are notoriously inefficient, then you have the losses in the amplifier and alternator. What I am suggesting avoids all this complexity and inefficiency by using a direct mechanical linkage.
 — stevewest, Mar 22 2005

Wouldn't this require another four, eight or however many cylinders, plus computer-controlled variable cam drives? To my mind, that's gonna cost more than a couple of loudspeakers.
 — david_scothern, Mar 22 2005

You only need one extra cylinder, running at 4x or 8x the rate of the cylinders in the engine. It would connect to the combined exhaust of the complete engine.
 — stevewest, Mar 23 2005

 /only need/ That sounds like a lot to me. It could be built, though, if the mechanical drive and synchronization were worked out. Number of cylinders divided by two . . .

 The problem I see is that the exhaust noise does not come from the "pushing out" of the exhaust gases by each of the pistons. When the exhaust valves open, the gases come crackling out from their own immense pressure, leaving the piston comparatively little to push out. So it's not a "piston-shaped" noise, so to speak. The /variable cam or rocker/ could be made as a variable crankshaft linkage, which would be rather different.

How about generating immense amounts of compressed air, and feeding it into the exhaust pipe at a rate exactly opposite to the exhaust flow?
 — baconbrain, Mar 23 2005

How about running the whole exhaust gas flow through a turbine, which smooths the gas flow? Turbos do this well.
 — david_scothern, Mar 24 2005

The balancing and timing of exhaust gas pressures is an important factor to engine performance. Messing with it may backfire.
 — RayfordSteele, Mar 24 2005

How much of the noise made by a car comes from the exhaust? I know this seems like a silly question, but my house is next to a fast road and almost all the noise is tyre and (possibly) wind noise; only motorbikes seem to give audible engine noise. I guess it's more of an issue with slow traffic in towns?
 — Basepair, Mar 25 2005

Good point basepair. I propose a cheap tyre noise cancellation
 — energy guy, Mar 25 2005

Wow! I love these ideas! a tire noise cancelation system would be awesome for housing developments near an interstate. Why do we still use piston engines in our cars??????? We should build some hybrid turbine electric cars. These could use a small multi fule turbine to gennerate electricity for a battery stock which would power electric engines. When the batteries are at a low point the little turbine could kick on gennerating electricity untill the batteries are almost all the way charged, then shut down again... as mentioned turbos in the exhaust could decrease the sond, but using electric motors with a turbine electric hybird would decrease the noise even more... The technology is not that much of a leap from turbos really!
 — fity, Mar 27 2005

"Attach an extra crank driven piston to the exhaust to cancel out the noise" The extra crank - that would be, um, waugsqueke?
 — DrCurry, Mar 27 2005

 [energy guy] \\I propose a cheap tyre noise cancellation\\

I believe there is already a tarmac (? don't know if that translates - asphalt?) which greatly reduces tyre noise. It's pretty widely used on new developments and resurfacings here. It has a porous surface which reduces the noise coming from the compression and then expansion of pockets of air trapped by the tyre tread, and also absorbs more of any remaining noise. See link.
 — Basepair, Mar 27 2005

 Fity... why not gas turbines? As part of a hybrid system, their terrible part-load performance can be avoided, it is true, and their small size is an advantage. However, their efficiency is worse than a diesel, they require regular maintenance by skilled personnel and the noise that they make when running is an unpleasant high-pitched shriek. Turbo whine is nothing compared to the scream of 100hp at 100,000rpm.

Although they may seem like yesterday's technology, diesel hybrids are a lot more appropriate for the high efficiency that the regulators want, and the low maintenance that the driver wants. Gas turbines are best when high power density is needed, or when there is room for the regenerating equipment that makes them efficient.
 — david_scothern, Mar 28 2005

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