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©FBI 2002
To begin with another update to the "Tillie" story, she finally, and
with some difficulty, made it into our garden, much to the bemusement of our neighbours, who have been asking which
direction we intend to fire our "cannon". She has indeed fired, and run for some length of time. On New
Year's Day, as part of the annual Worldwide Engine Crankup, she ran alongside a selection of smaller English and
American engines, despite below-freezing temperatures, resulting in a very happy owner! stationary-engine@atis.net
And so on to the discussion which attracted my attention this month on the ATIS Stationary Engine Mailing List:
* I was thinking about building my own muffler for my Galloway. My plan
is to use a NPT flange. Has anyone ever built one using this idea, and if so what goes inside the muffler i.e.
baffles?? I haven't figured out the "Top Dome" part yet, but possibly someone out there can help. They're
a little pricey and it would be fun to build my own.
This was certainly one which captured the imagination, and I could have just about filled an issue of GEM with
the replies this generated. Some replies deviated from the subject a little, discussing car mufflers, gun silencers
and some in-depth scientific information covering the expansion of the gases and resulting noise volume. So I had
to be ruthless and try to only use the emails which answered the original question.
* Try a flat steel plate with washers in between. The more washers, the louder it'll be.
Or, a piece of " pipe, with holes in it (small 1/4" holes, and a bunch of them), put a cap on one end,
and makes some sort of welded in screen in the bottom, then fill the pipe with steel wool...
* More washers, louder music?! I don't know how one would "lathe" a dome top ... rough it in and then
go with a file possibly.
I'm going to stay away from the steel wool as the exhaust will probably suck some depending on valve/seat condition
and timing.
* I have built mufflers for a lot of different engines and things. I use 8" thin wall pipe, have the inlet
in the center of the end and reach to about 2" from the end. use a cutting blade on a skill saw cut notches
about every 3" on opposite sides. On the outlet place where it is needed t-b-ls-rs cut the same. You can fill
with stainless steel the same as steel wool or put washers in the slots, largest at outlet end to smallest at inlet
end - they make a real mellow tone. The longer the better. This is just from no money experience.
It is worth what you pay for it but they don't have a life time guarantee like NAPA!!
* 8" thin wall ...you ever had one crack/break at the head or anywhere? I'm speaking of vibration cracking.
Or possibly you support the muffler to reduce vibration.
* For a "domed top", why not take a piece of flat plate, heat it up cherry red, and hammer it to a domed
shape with a flat rim? Bolt it to your flange with a flat washer or two as a spacer? You might also be able to
find some assorted cast iron bits (floor trap, etc.) at a plumbing supply house that would have a domed shape.
You'd probably need to weld some bits to it, but it would be a start. Have fun! Get creative. And you don't need
anything inside. The spacing between the two halves controls the amount of bark.
* I haven't figured out the "Top Dome" part yet, but possibly someone out there can help. They're a little
pricey and it would be fun to build my own.
* Someone mentioned using the bottom of a fire extinguisher for the dome, they're usually available at extinguisher
service centers as rejects.
* That was me! I attempted to fashion an authentic looking replacement for the muffler on my 10HP International
M type. As for using old fire extinguishers I'd make these few points. The old soda acid type water extinguishers
are generally a canister of thin brass and even the more modern water stored pressure types are fairly thin steel.
I used an old CO2 extinguisher for my dome and found it to be of surprisingly heavy gauge metal. I'd recommend
using the CO2 extinguishers because they are designed to hold a fair amount of pressure while most of the other
types of extinguishers are not. The heavier section will resist corrosion better and is less likely to sound 'tinny'.
* Rigged up a setup to pressure test those just today - a whole truck bed full of them. Very heavy tubing: they
would be run up to 600 PSI for half an hour to check for leaks.
* A good point. A muffler can fill up with fuel-air mixture and ignite with a heck of a bang. Mufflers can pop,
and big ones have exploded destructively in the past. 600 PSI is well above any pressure that you could generate
with an uncompressed charge (which I
wouldn't expect to give more than 45-60 PSI.)
* I found that length affects sound, too. Long = mellow, not really that much quieter, if I recall. Haven't made
a muffler since the sixties. I once made one for a Vespa with thin wall EMT, an orange juice can (they were metal
back then) and coarse steel wool. Worked great for a while, but didn't last on the road.
* A member of our club happened to find a discarded gas cylinder, the kind with the hemispheric bottom, and cut
both ends off to use for building a very convincing muffler.
* When it comes to a muffler I am not the least bit interested in keeping it looking original. I want to try to
get rid of the loud "BANG" that almost makes your ear drums bleed after a few hours. I am going to try
to get some different size reject fire extinguisher tanks. Next is to try to size one to the engine. With the tank
standing on the cart or skids, pipe the exhaust into the side of the tank. Have a hole cut in the top at least
the size of the pipe on the engine. This I will experiment with. Then weld a short piece of metal on the end of
a piece of pipe that will fit the hole in the top of the tank and drop it in the tank. The welded rod on the pipe
will hold it off the bottom of the tank. I will then run the engine and keep shortening the rod until the pipe
is close enough to the bottom of the tank to give a nice mellow "Woof" rather than a "Crack"
or "Bang". Then weld the pipe in the top of the tank. In theory the tank should absorb the loud noise
and the restriction at the bottom of the exhaust pipe will produce the mellow sound. This is how I think the Ruston
& Hornsby engine mufflers at our grain elevators were built. They made the sweetest stack music I have ever
heard! A side benefit might be that older gentlemen, wearing hearing aids, with great stories to tell might be
able to stay around and talk. Feel free to offer suggestions or criticism to the idea. It is still in the planning
stage.
* I'd looked at the "floor flanges" with the idea of building a muffler, several years back. A variation
that I came up with (but haven't built) is to bolt 2 floor flanges together with washers between, the circumferential
slit being the outlet. Opposite the inlet pipe is another pipe capped at the opposite end. It serves as a resonator
and is a well known type of "silencer" in acoustics.
If the pipe is of the proper length to reflect the main frequency of the sound back 180º out of phase at the
outlet slit, this "branch muffler" can be made almost 100% silent. To avoid the cumbersome length of
the pipe a Helmholtz resonator could be used instead, with a small pipe running into a large hollow chamber. I
looked at grease traps for this, also pipe expanders for larger diameter capped resonating pipes, thought about
drilling and welding a smaller pipe into a cap for a much bigger pipe. Or something like a small gas tank or fire
extinguisher casing. Lots of ways it could be done. This offers some additional expansion volume and would reduce
the back pressure offered by the floor flange simply having a baffle plate bolted up against it with a small exhaust
slit. It also might allow use of a wider exit slit without producing as much noise, and dropping the pressure would
tend to reduce the annoying hiss that these types of mufflers tend to make from gas escaping at the exhaust's sound
speed (supersonic relative to the cool ambient air) when the pressure gradient across the opening is "critical,"
or around 1 atmosphere or higher. Supersonic gas jets produce shock waves and turbulence that, on this scale, ends
up mostly producing lots of really high frequencies, much of the acoustic energy being ultrasonic. The reproduction
"cone muffler" I have been using on my Indian-built Lister clone works this way, by letting the gas escape
through a 1mm high slot around the circumference of a 4 1/4" circle (giving a total area similar to the cross-section
of a 1" pipe) and then turning the resulting sheet of supersonic gas upward with a pressed flange to basically
roll it into a cylinder with shear turbulence on both the inside and outside rapidly slowing the gas. It works
well enough, but I've grown weary of the hissing tone. Fairbanks-Morse mufflers and IHC LA/LB mufflers are similar,
as are many others.
* The "branch muffler" principle with a Helmholtz resonator is widely used in automotive mufflers, usually
as a hollow chamber at the rear of the muffler shell with no outlet, the inlet being a short piece of pipe lined
up with (but not in contact with) the muffler's main inlet pipe in the first reversing chamber right ahead of the
resonator.
Choice of the resonant frequency is critical; if wrongly selected it can really make the exhaust boom, but if done
right the sound waves cancel out at the point where they come out. Really a classical branch muffler would tee
out straight to the side of the pipe line, sound waves going out into it and coming back into the line delayed
from the initial impulse so it interferes destructively, but you can easily see that we're discussing the same
principle. That's another thought; just use a pipe tee (or "Y") with two outlet pipes of suitably chosen
lengths, one capped at the end and one open.
There are basically only 3 ways to "silence" exhaust noise: Reflection, restriction and absorption (as
with the fiberglass and steel wool already suggested.) How you use and combine these is limited only by your imagination.
* This discussion has raised another idea. A well know concept (but one that I haven't personally tried) for a
firearm silencer involves a length of pipe or tube with a stack of flat washers and a coil spring. The pipe is
capped on each end. One cap end is threaded to match the barrel, and one cap end is just open to allow the projectile
to pass. The caps keep the spring and washer stack in the tube. The expanding gas blast forces the washers apart
compressing the spring and thereby absorbing energy resulting in a quiet shot (ignore mechanical & bullet noises).
Seems that a similar arrangement could work as a "silencer" for an engine exhaust? It would definitely
require some experimentation with spring sizes / strengths, washer stack length, etc. and may work better on a
hit and miss than on a throttle governed engine.
If anyone tries this method please report back. I'm really curious about how well or poorly it works and will probably
try it myself one day.
* I remember an exhaust system on a 64 Olds - that looked like two 14' lengths of exhaust tubing that had eaten
about five large rats, each. No mufflers or resonators, just fat places in the pipes.
* I read in one of my old books here in my library that a muffler should be 12 times the displacement of one cylinder.
I don't think that anything needs to go inside the chamber. As I understand things, the goal is to reduce the exhaust
gas velocity when it enters OUR space. I suspect the 12X chamber does this so that when the gas exits the chamber
the pressure is reduced as well as the velocity.
I also think that gas path diversions inside the chamber may well improve the efficiency, but that is just a guess.
I would love to try this on my 25hp Superior, but a 14,000 cubic inch chamber gets to be a little unwieldly to
play with.
* Many old books recommended 3-5 times the cylinder displacement as a suitable volume for a muffler of the expansion
chamber type. That'd drop the pressure at the outlet below the critical level, so no supersonic gas comes out of
the stack. With low speed engines (having slowly opened exhaust valves) it'd be quiet enough for most purposes.
Most aren't all that loud unmuffled. You get a lot sharper crack from that initial port shock wave as the exhaust
opens faster with higher speeds, aggressive cam profiles giving earlier and more rapidly opening valves, multiple
valves or piston opened exhaust ports. Think of a Jimmy Diesel's sound compared to a Cummins of similar power.
The gas flow's similar, but there's a lot more acoustic energy in that pipe with the Jimmy!
* Last weekend I made a muffler for a B&S 3HP engine in my Bantam tractor. The engine has a 3/4" NPT exhaust
port opening. The muffler I built looks like one that would be on a big tractor. 3/4 pipe at the bottom end, extending
into the main chamber about 4" and 96 3/32" holes drilled in this section. A cap was welded to the inner
end of this pipe so the hot exhaust gas would have to go through the small holes. The main body of the muffler
is made of a 12" piece of 2" dia. thin wall tubing. A washer type ring was made and welded to the main
body end and to the 3/4" inlet pipe. On the outlet end, I made another washer ring to fit the inside of the
2" and a hole in it to fit a piece of 3/4" EMT thin wall conduit. This was left open at both ends and
welded to the ring and body of the
unit.
Got time tonight to fire it up for the first time to see what she would sound like. I can't believe the difference
this made over just a piece of pipe. A nice mellow sound out of a Briggs and Stratton. Hard to believe but true.
I didn't use any type of packing inside.
* I know of a 1907 Otto gasoline model that is the quietest I've ever heard. Its muffler is a big cast iron pot
about the size of a propane bottle for a barbeque grill, with the inlet in the side and threads for an outlet pipe
in the top center that's much smaller than the inlet. I'd say it's a 3/4" or 1" water pipe, but I don't
have it at hand to measure it. He uses a piece about 4' long as the stack and unscrews it before loading up for
the trip home. If you want a quiet muffler, that's a good plan to follow. Use a big inlet and lots of expansion
volume to keep the backpressure down, but go small on the outlet. There's lots of time for the gas unloaded into
the expansion chamber at the beginning of the exhaust stroke to escape to the atmosphere with our low speed engines.
It doesn't have to blast out explosively to allow the engine to breathe reasonably well. Most guys building their
own mufflers use a big outlet to allow for gas expansion, but that allows the sound to escape from the outlet more
readily than to reflect back toward the engine. Think of the little pipes used by high speed engines like Briggs
& Stratton of similar horsepower. The average gas flow rate is about the same with the big slow engine, and
one of those little outlets will work fine if a big expansion chamber "soaks up" the large volume pulse
at exhaust valve opening.
* My suggestion is, use the biggest expansion chamber you can conveniently rig up, at least 5x cylinder displacement
volume, use an inlet pipe of the same size as your engine's standard exhaust, and an outlet pipe of the size used
by typical modern small engines of equivalent horsepower, e.g., 1/2" for 2-3HP engines, 3/4" for up to
8HP or so, " for 10HP and bigger. Averaged over time the larger but fewer exhaust pulses of the big engine
will make about the same flow rate as the small, fast and frequent ones from the little engine.
In either case the gas should flow out basically in the time of an exhaust stroke. When you look at ratios of displacement
and speed you can see that it works out pretty similarly for the flow time to be in proportion to the degrees of
crankshaft rotation for both situations if the engines are similar in power.
Even if you have no intention of fabricating a muffler yourself, some of this information may help to adjust the
sound your engine produces.
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©FBI 2002