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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
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
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
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.
||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
||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.
||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
||/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?
||How about running the whole exhaust gas flow through a turbine, which smooths the gas flow? Turbos do this well.
||The balancing and timing of exhaust gas pressures is an important factor to engine performance. Messing with it may backfire.
||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?
||Good point basepair. I propose a cheap tyre noise cancellation
||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!
||"Attach an extra crank driven piston to the exhaust to cancel out the noise" The extra crank - that would be, um, waugsqueke?
||[energy guy] \\I propose a cheap tyre
||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.
||<off-topic> 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.