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Thread: Tuned Pipe

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    Registered User mstull's Avatar
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    Tuned Pipe

    I have an idea for a tuned exhaust system. I'm hoping one of you is an exhaust tuning expert who might know if it could work:

    Small, single cylinder engines can get huge power gains (over a narrow RPM range) with an expansion chamber type, tuned exhaust system. But our in-line twin cylinder engines commonly use a relatively untuned exhaust. If each cylinder had its own separate expansion chamber, we could reap similar gains. But our larger displacement and lower RPM engines would need two rather huge expansion chambers, where drag, exhaust cracking, and weight, would probably make them not worth their power gains.

    Tuned exhausts are available for our twin cylinder engines. They are not too common on aircraft because they have similar problems. But I'm wondering if there might be a better way. My understanding is that the goal of a tuned exhaust is to pack some extra charge (fuel air mixture) into the combustion chamber from the exhaust pipe.

    At the end of the power stroke, the exhaust port opens, letting a burst of high pressure exhaust go down the pipe. A fraction of a second later, the transfer ports open, allowing fresh charge into the cylinder. Some of that charge follows the exhaust pulse out the exhaust. A pressure wave, in a tuned exhaust, reflects back towards the engine, just as the exhaust port is starting to close, and packs that charge back into the cylinder.

    My idea is to not use a 2 into 1 manifold, and not have separate tuned pipes either. What if there was a U shaped pipe that connected the two cylinders' exhausts. This U shaped pipe's length could be such that the exhaust pulse from one cylinder would go down the pipe, around the U, and reach the other cylinder right at the critical time.

    So exhaust would be resonating back and forth in that pipe. We'd be using an actual pulse, rather than the reflection of a pulse, to do the work. In the middle of the U, there would need to be a Tee that takes the pressure off into a silencer/muffler.

    To keep this pipe from being too long, it could expand to a larger diameter. It would be conical, like most of our pipes are now, so it ends up fat where it goes around the U bend. So it could be tuned with its length and/or diameter. To picture it, there would be 90 degree bends pointing down at the exhaust ports, feeding twin conical pipes going down just past the bottom of the engine, connecting in a big, fat U, that has a Tee that feeds into a muffler.

    If this system works, the engine would need to be designed for it. If we simply added it to an existing engine, the extra heat from the increased power could create a problem. And detonation could be a problem if we don't adjust the compression ratio of the engine.

    But if we provide for those things, the advantages are:
    1. Improved fuel economy from recovering most of that lost charge in the exhaust.
    2. Lighter weight, because we can use smaller displacement to get the same power.
    3. Less vibration, because the smaller pistons and connecting rods have less reciprocating mass.

    Remember that a smaller displacement engine needs smaller, lighter crankshaft flywheels and a smaller, lighter crankcase. So the whole engine can be significantly smaller and lighter, to produce the same power. And a smaller displacement engine can operate safely at higher RPMs, allowing a smaller exhaust system.

    My MZ201 is a good example of a relatively untuned engine. It uses brute displacement (626 cc) to get decent power from high torque, but has a fairly light weight exhaust system. A CorsAir M25Y is just the opposite. It uses a tiny 173 cc displacement with a bulky, heavy, tuned, exhaust system to get its power at high RPMs. Both engines work fine, but the CorsAir engine gets better fuel economy and vibrates less.

    It would be interesting to find out how long or fat the U pipe would need to be, and if it could be designed to have a reasonable range of RPMs where it would be efficient. A good engine to test this on would be a Rotax 377. It's already designed to take the heat and power of the 447.
    Mark E. Stull
    mstull@wtxs.net

  2. #2
    Registered User jumpinjan's Avatar
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    Re: Tuned Pipe

    A twin cylinder 2-stroke will fire alternately, I think. When the exhaust exits one cylinder the other cylinder is going on it compression stroke. This wave travels at the speed of sound, so the wave arrives in the second cylinder out of time on its compression stroke. So, how does this work again?
    Jan
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  3. #3
    Moderator Dana's Avatar
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    Re: Tuned Pipe

    Hmmm, intriquing idea, but I think Jan's right... the ports are only open for a relatively short time at the bottom of the stroke, so the pipe would have to be very long to delay the pulse from one cylinder long enough to arrive at the right time for the other... to the point that it would offer no advantage over separate tuned pipes.

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  4. #4
    Registered User mstull's Avatar
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    Re: Tuned Pipe

    Yes, in-line, twin cylinder, two stroke engines have alternately firing cylinders... that each fire once per revolution. So at 6,000 RPM, there's 12,000 firings per minute, which is 200 firings per second. So each cylinder fires 0.005 seconds after the other.

    The exhaust pressure pulse travels through the exhaust pipe much slower than the speed of sound. Yes, the SOUND of the exhaust port opening would reach the other cylinder at the speed of sound, through my U pipe. But the pulse of wind itself travels much slower.

    Let's say for example you blew air into a pipe at 100 mph. The air would only move through the pipe at 100 mph. But if you sing into the pipe, the sound would travel through it at the speed of sound... even though the air from your breath would only travel as fast as you're blowing wind. The same thing happens in the exhaust pipe.

    Now if that pipe was conical, increasing to 10 times its cross-sectional area, then the air in that larger part of the pipe would move 1/10 as fast: 10 mph... while the speed of sound would continue un-slowed. That's the reason existing tuned pipes are conical and have larger diameter expansion chambers. They're trying to slow the pulse's speed so the tuned pipe doesn't have to be so long.

    Now the wind exiting the exhaust port may be going at the speed of sound, or even much faster, for the instant when it first starts to open. But at that instant, the cross-sectional area of the partially open port is much smaller than even the header pipe. So it is slowed a lot in the header pipe. It's like blowing air into the end of a large pipe with a compressed air gun. In the first instant, the air in the end of the pipe would swirl like crazy and quickly build enough pressure so it starts to push down the pipe.

    Yes there is a sound wave that speeds down the pipe. But it's the much slower pressure pulse that is used for exhaust tuning.

    Now in real life, the speed the pressure pulse moves through the header pipe might work out to be a function of the average of the exhaust port opening area, and the average of the speed of the flow out the port, divided by the cross-sectional area of the header pipe... along with other factors.

    It may still work out that the length and/or diameter of my U pipe would be too large to be practical. That's what I'm asking (of an exhaust tuning expert, if there's one out there).
    Mark E. Stull
    mstull@wtxs.net

  5. #5
    Super Moderator Midniteoyl's Avatar
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    Re: Tuned Pipe

    Sounds like an 'H' pipe..
    Jim

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  6. #6
    Registered User jumpinjan's Avatar
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    Re: Tuned Pipe

    I think you have some confusion here. A pressure wave will travel very quickly, much, much quicker than a volume of exhaust gas. Consider it the speed of sound (which will vary with temp, density, and pressure, I think). The diverget cone is a "megaphone" to help extract gasses and the convergent cone is to send an "echo" wave to the exhaust port. Its as simple as that. All this has to be "Timed" correctly for it to work. Maybe you should look at this page:
    http://www.americanrcboats.com/howto...dpipeworks.htm
    Jan Servaites (Dayton OH - The Birthplace of Aviation)
    (Where we had the brains and not just the wind to make flight possible!)

  7. #7
    Registered User mstull's Avatar
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    Re: Tuned Pipe

    Thanks for the link, Jan.

    That explains a lot. It would be interesting to see how that fits with our typical 2 cylinder engines with a 2 into 1 manifold. Is there enough space between the two cylinders' exhaust pulses so the expansion chamber can deal with each cylinder separately? From that explanation, it sounds like the whole process happens in the brief time that the exhaust port is open, without any effect from the previous pulse... other than the residual pressure in the chamber. If that's the case, and the article is correct about the pressure waves traveling at the speed of sound, then my U shaped pipe would have to be too long to be practical. I'll stick with using stock exhaust systems.
    Mark E. Stull
    mstull@wtxs.net

  8. #8
    Registered User WileEZ's Avatar
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    Re: Tuned Pipe

    Well y'all got me thinking...

    Mind you, I just thought of this and did a quick drawing.

    U pipe to bounce the exhaust from one cylinder to the other and vis-versa.

    Additional pipe from the inside of the "U" for actual exhaust.

    If one notice, the outer part can slide inside the pipe from the cylinder exhausts, kinda like a trombone, to adjust optimum RPM.

    Bonus, the area inside the "U" can be enclosed and made into a muffler.

    Note my name!
    Attached Thumbnails Attached Thumbnails Tuned Pipe-u-exhaust_pipe.jpg  
    Last edited by WileEZ; July 1st, 2008 at 09:46 PM.
    WileEZ
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  9. #9
    Super Moderator Midniteoyl's Avatar
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    Re: Tuned Pipe

    You'd be feeding one exhaust into the other.. what would that do? You want the exhaust gas to escape in an expeditious manner, but the exhaust pulse to travel back up the tube, hit the exhaust valve, and 'just' be heading back down the pipe when it opens. This adds energy to the exhaust, aiding its evacuation from the cylinder.
    Jim

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  10. #10
    Moderator Dana's Avatar
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    Re: Tuned Pipe

    Quote Originally Posted by Midniteoyl View Post
    ...You want the exhaust gas to escape in an expeditious manner, but the exhaust pulse to travel back up the tube, hit the exhaust valve, and 'just' be heading back down the pipe when it opens. This adds energy to the exhaust, aiding its evacuation from the cylinder.
    There is no exhaust valve in a 2-stroke engine.

    A 2-stroke tuned exhaust uses the returning pulse to force some of the excess fuel/air mixture that has "spilled" out into the exhaust back into the cylinder, providing a slight supercharging effect.

    -Dana

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  11. #11
    Registered User WurlyBird's Avatar
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    Re: Tuned Pipe

    So, with rough calculations, sound moves about 5 feet in .005 seconds. I would love to see a Rotax 337 with an exhaust pipe loop sticking 2.5 feet out the side.

    I don't really think this would work out any better, or even as well as a tuned pipe. If the compressional wave hits the second exhaust port too soon it will impede the exhaust evacuating the cylinder and the incoming gases would probably mix with the remaining exhaust and give a less then ideal mixture, and low efficiency. Too late and it is like no exhaust system at all. Interesting idea though.

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  13. #13
    Registered User wally's Avatar
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    Re: Tuned Pipe

    what the go-kart racer guys used to use is a retangular megaphone. small at the engine and gets larger at the opening. Very, very loud but effective.
    Wally

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    Re: Tuned Pipe

    When was that, Wally? The last megaphones I remember at kart races was in the Sixties. And was the engine you saw a 2-stroke or 4-stroke?

  15. #15
    Registered User wally's Avatar
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    Re: Tuned Pipe

    Yup it was back in the '60s.

    I never did have one on a go-kart but a guy gave me a West Bend super-bee (2 stroke) with a belt reduction and prop for powering a hang glider. It also had the swoopy megaphone exhaust. I fired ut up once on a stand just to see if it would run. Dang, it was loud. This was maybe 10 years ago.

    I ran a Homelite KL-82 engine on my kart in the good old 1960's. No clutch, just push it to start and go. I once saw a guy that had 4 Mac 101's on his kart!!

    Wally

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