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Forum index » Science and Technology » Physics » Acoustics
length of exhaust pipe should be 1/4 wavelength?
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BobG
science forum beginner


Joined: 12 Sep 2005
Posts: 7

PostPosted: Sat Apr 29, 2006 2:01 am    Post subject: length of exhaust pipe should be 1/4 wavelength? Reply with quote

I want to 'tune' the exhaust of a generator to peak the effciency at
3600 rpm.Thats 30 bangs per sec, and if the bang travels 1120 ft/sec
down the pipe, a 1/2 wavelength pipe would be about 15 ft long! If the
cylinder had 6 in^3 volume, and the piston comes up in 8ms, and the
diam of the pipe was about 3/8", the pressure wavefront would travel
down the pipe about mach 1.This pulse of exhaust would be half way down
the pipe when the exhaust valve closed. I heard that the idea was for
the 'momentum' of this pulse to create a lo pressure at the exhaust
valve to help scavenge the cylinder when it opens again in 24ms. Does
that mean the pipe has to be a full wavelength long? 30 feet?!? I
thought you could get a couple concentric tubes and try to tune it like
a trombone slide... look for a dip in the fuel consumption at the
resonant point. Would it crack like an airplane prop if the pulse
exited at mach 1? Thats good news and bad news. The generator uses less
fuel, but it gets louder.
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Greg Locock
science forum Guru Wannabe


Joined: 28 May 2005
Posts: 111

PostPosted: Sat Apr 29, 2006 6:41 am    Post subject: Re: length of exhaust pipe should be 1/4 wavelength? Reply with quote

"BobG" <bobgardner@aol.com> wrote in
news:1146276080.089448.137150@e56g2000cwe.googlegroups.com:

Quote:
I want to 'tune' the exhaust of a generator to peak the effciency at
3600 rpm.Thats 30 bangs per sec, and if the bang travels 1120 ft/sec
down the pipe, a 1/2 wavelength pipe would be about 15 ft long! If the
cylinder had 6 in^3 volume, and the piston comes up in 8ms, and the
diam of the pipe was about 3/8", the pressure wavefront would travel
down the pipe about mach 1.This pulse of exhaust would be half way
down the pipe when the exhaust valve closed. I heard that the idea was
for the 'momentum' of this pulse to create a lo pressure at the
exhaust valve to help scavenge the cylinder when it opens again in
24ms. Does that mean the pipe has to be a full wavelength long? 30
feet?!? I thought you could get a couple concentric tubes and try to
tune it like a trombone slide... look for a dip in the fuel
consumption at the resonant point. Would it crack like an airplane
prop if the pulse exited at mach 1? Thats good news and bad news. The
generator uses less fuel, but it gets louder.


The pressure pulse will travel at M1 anyway.

The way to think about it that probably helps is that the pressure pulse
reflects and inverts at the outlet to atmosphere, so you want 2L/c to
equal the interval between the relevant exhaust valve events. 4L/c would
do as well, but will be less effective..

If the volumetric flow rate achieves M1 (and this is quite common) it
will make a noise like a raspberry. There are several solutions, given
the option the simplest way is to increase the size of the pipe.

Your trombone idea is excellent, we often use that for air intake
tuning.

Cheers

Greg Locock
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Angelo Campanella
science forum Guru Wannabe


Joined: 08 May 2005
Posts: 226

PostPosted: Wed May 03, 2006 1:55 pm    Post subject: Re: length of exhaust pipe should be 1/4 wavelength? Reply with quote

BobG wrote:
Quote:
I want to 'tune' the exhaust of a generator to peak the effciency at
3600 rpm.Thats 30 bangs per sec, and if the bang travels 1120 ft/sec

First of all, 1120 ft/sec belongs to room temperature, which exhaust gas
is not. Figure 49*SQRT(459+F) ft/sec where F is the Fahrenheit degree
temperature of the exhaust gas... maybe 1000F or so (check it out).

Quote:
thought you could get a couple concentric tubes and try to tune it like
a trombone slide...

Trombone slide (Use Bob Burns' "Bazooka" as a design guide)

Quote:
look for a dip in the fuel consumption at the
resonant point.

That would take a long time and a lot of fuel. Better to disable the
speed regulator, and look for a maximum of RPM.

Quote:
Would it crack like an airplane prop if the pulse
exited at mach 1?

Often yes. Long tubes will do that.

The 'blatting' or staccato cracking sound heard when cars with no
muffler (just a single straight pipe from the headers to the tail pipe
end) make that very sound on acceleration (a boutique car aesthetic in
some warped minds). These are traditional one dimensional shock waves
that form as soon as the impulsive sound pressure level approaches one
atmosphere. The longer the tube, the lower the pressure needed. (A
garden hose about 10' long , fed by a trombone mouthpiece, blown like a
trombone easily makes the same shock wave sounds.

Quote:
Thats good news and bad news. The generator uses less
fuel, but it gets louder.

A little of both; surely more noise; maybe less fuel consumption.

Angelo Campanella
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BobG
science forum beginner


Joined: 12 Sep 2005
Posts: 7

PostPosted: Wed May 03, 2006 2:50 pm    Post subject: Re: length of exhaust pipe should be 1/4 wavelength? Reply with quote

Very interesting Angelo... I agree that the speed is temperature
dependent... 1000 to 1120 ft/sec variation could be dialed in with the
tuning....
Is the pressure wave + and - 14.7 psi? Ahead of the pulse its 29.4 and
behind it its zero? What SPL is that in dB? 140? Where does air
compressibility come into play? Trying to tie together some audio and
aerodynamic terms and concepts.... get the big picture.....
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Ken Plotkin
science forum beginner


Joined: 10 Jun 2005
Posts: 41

PostPosted: Thu May 04, 2006 12:01 am    Post subject: Re: length of exhaust pipe should be 1/4 wavelength? Reply with quote

On 3 May 2006 07:50:47 -0700, "BobG" <bobgardner@aol.com> wrote:

Quote:
Very interesting Angelo... I agree that the speed is temperature
dependent... 1000 to 1120 ft/sec variation could be dialed in with the
tuning....
Is the pressure wave + and - 14.7 psi? Ahead of the pulse its 29.4 and
behind it its zero? What SPL is that in dB? 140? Where does air
compressibility come into play? Trying to tie together some audio and
aerodynamic terms and concepts.... get the big picture.....

Compressibility definitely enters into it. The pressure pulse is
nonlinear. It steepens as it goes down the tube. The reflection is
an expansion, which spreads out as it propagates back toward the
engine.

The idea is that the expansion will reach the exhaust valve at the
right time to help empty the cylinder. It may also propagate through
the cylinder (during the overlap period) and help draw the fresh
charge in.

Really more of a gasdynamic problem than an acoustics problem. It's
also a thermodynamics problem. With the elevated temperatures, and
the cooling in the expansion, some of the exhaust energy is recovered.

Ken Plotkin
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Angelo Campanella
science forum Guru Wannabe


Joined: 08 May 2005
Posts: 226

PostPosted: Sat May 06, 2006 7:12 pm    Post subject: Re: length of exhaust pipe should be 1/4 wavelength? Reply with quote

BobG wrote:
Quote:
I agree that the speed is temperature
dependent... 1000 to 1120 ft/sec variation could be dialed in with the
tuning....

70 degrees F = about 1100 ft/sec.
Exhaust gasses will vary up to 1000F or more,, increasing the speed to
1800 ft/sec and the wavelength accordingly

Quote:
Is the pressure wave + and - 14.7 psi?

It depends on the minor explosion that is let into the exhaust pipe.

Quote:
Ahead of the pulse its 29.4 and
behind it its zero?

The way that negative pressures (really values less than 14.7; it can't
go below zero absolute).

Quote:
What SPL is that in dB?

One atmosphere is 196 dB simplistically. If one had a peak-to-peak
pressure of 0-14.7 psi, it would be 1/3 that (10 dB less) or 186 dB ("RMS".

If one presumed that it would be two atmospheres 0-to-peak (0-29.4 psi),
it would be 6 dB more or 192 dB )("RMS").

Quote:
140? Where does air
compressibility come into play? Trying to tie together some audio and
aerodynamic terms and concepts.... get the big picture.....

Especially on the negative side, if the volume is doubled, the pressure
does not become zero, it becomes only halved, pursuing a "hyperbola"
shaped curve, this is the nonlinear nature, with the harmonics therefor.

That is the beginning of nonlinearity. More and new phenomena arise as
this pressure wave travels down a long pipe. Since the compressions (and
rarefactions) occur adiabatically (no time for the pipe metal to cool or
heat the exhaust gas), the local sound velocity inside the wave is
faster or slower. This makes the pressure peaks travel faster than the
pressure valleys.

The traveling positive pressure region has no alternative but to become
a genuine shock wave, while the trailing negative region merely
stretches out thinner (lower density). (Unlike ocean waves, the air
cannot form a spillover and white cap... the air must pile up against a
shock front where the density is greater behind the shock front, and
less before the shock front within the lesser pressure wake of the shock
one wavelength ahead of it.

These shock fronts, when emitted from the tailpipe of a car without a
muffler, sound like a blatting trumpet or trombone.

Angelo Campanella

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