Triple Heaven
[size=5]Two-Stroke Pipes/Expansion Chambers[/size=5]

Okay, pipes are the misunderstood thing on a two stroke sled. I mean how can an aftermarket pipe be added to a sled and a horsepower gain insues? Well is all lies in the power of harmonics. That's right sound, sound carries engergy in the form of a wave. These waves are pressure differences in materials like liquids and gasses.

The first few pictures here are a precursor to the acutaly steps that affect the pipes ability to work. I will add an explanation later.






The pressure wave emminates from the port as the gasses of the burned fuel rush out of the port they create the wave. This wave moves down the ports of the motor and into the pipe. At this point the wave is positve. The exhaust gasses can expell themselves due to the pressure difference between the cylinder and the headpipe.




So now the positive wave travels down the header of the pipe to a divergent cone. Using divergent tubes or a megaphone the increase of the area of the pipe causes a negative pressure wave to be reflected in the opposite direction of the positve wave. So there is a negative wave travelling back toward the port and a positive wave continuing to travel away from the port.


What's the negative wave that's heading back toward the port for you ask? Well, when the exhaust port became exposed early on, the high pressure in the cylinder was able to force the exhaust gasses out, but now the piston is moving down to expose the intake port. However since the pressure in the cylinder has equalized with the pipe pressure, so now the fuel at the intake will need aid to flow into the cylinder and pipe. So the negative pressure wave is designed to pull the mixture into the cylinder and pipe this is called scavenging. The pressure wave will arrive hopefully as soon as the exhaust port opens, allowing the flow to start immediately. The mixture is drawn in and actually makes it into the headpipe.


The flaws of the two-stroke are now revealled as the piston begins it's upward stroke and begins forcing some of the mixture out the exhaust port. Now, back to the positive pressure wave still travelling through the pipe. It is now in the later stages of the Dwell Section. The Dwell Section is an area where the pipe does not expand or contract, it is just a tube with no variations. This Dwell Section is like a delay, the wave reflects nothing while in the Dwell Section. But, once the wave arrives at the Convergent section, the pressure wave reflects back a positive pressure wave back toward the port. So we have fuel in the headpipe with the piston forcing it out of the exhaust port as the piston begin the compression stroke and a positive pressure wave travelling in both directions. One is travelling toward the port, the other is travelling away from the port.


The stage is set for an effect that creates mass amounts of power in the motor. It's like a forces induction really. The positive pressure wave travelling toward the port now reaches the headpipe and begins to force the mixture back into the cylinder this effect is known as port blocking. As the piston is just about to close the port the pressure wave should have pushed the rest of the mixture back into the cylinder virtually force feeding the motor, similar to the turbocharger or the supercharger. The port closes, the piston reaches top dead center and then rolls over. The spark plug fires and the piston is forced down which opens the exhaust port and the whole thing starts over again.


So, it's not that the manufacturers don't have time to design a good pipe, it is a matter of tuneability for the average sledhead. The more general the pipe the easier it is to tune. With the aftermarket pipes, the measurements are much more accurate and the more precise the measurements the more power one can get from the pipe. Plus the stock pipe is designed to operate at lower RPM, which doesn't allow for maximum power, but maximum tuneability and reliability. But for the most part pipes are an engine tuning device. A pipe alows you to run at different RPM ranges. These ranges can vary based on the length of the pipe.



Now to discuss some of the other basics.
Law of Reflected Pressure Waves
If this wave encounters any change in cross section or temperature it will reflect a portion of its strength in the opposite direction to its travel. For example a high pressure wave encountering an increase in area will reflect back a low pressure wave in the opposite direction. A high pressure wave encountering a decrease in area will reflect back a high pressure wave in the opposite direction.
Length of Pipe
The length of the pipe is important to determining the tuned RPM of the pipe. The longer the pipe the longer the waves have to travel before they are reflected. So there needs to be more duration between the power strokes for longer pipes, thus lower RPM's. For short pipes, the power strokes can be closer together making them useful for high RPM engines.
Intensity of the Pressure Waves
The intensity or strength of the pressure waves are determined by the angles of the convergent and divergent cones. A high angle of convergence will cause a high intensity pressure wave. However the wave can never be any more intense than the wave that began it. If a positive pressure wave were traveling in a pipe with no angle and suddenly hit a 90 degree angle the reflected wave would be one of equal intensity.

For an example, use this link and play with the different settings.
http://www.physics.smu.edu/~olness/www/05fall1320/applet/pipe-waves.html
Duration of Pressure Waves
The duration of the waves or how long they last is all determined by the rate of the convergent and divergent cones. If the cones are tapered gently they will pressure waves will last for long periods. If the taper is steep the duration will be short.
Duration vs. Intensity
In the last two sections we talked about intensity and duration. Of course the two are related because if we have a steep convergent cone, the intensity will be high but the duration will be short. We can be looking for two things then. One equilibreum. Or we can try and force the mixture back into the cyliner with a long wave or just an intense short burst.
How Temperature Affects the Waves
Temperature is important because it affects the speed of the waves. If the pipe is cold, the waves will travel slower. So the hotter the faster the waves travel. This is why pipes work better when they are warmed to operating temperature. So if the pipe is cold, the waves will arrive late to the ports, and if the pipe is too hot, the waves will arrive prematurely and won't be able to force all of the mixture back into the cylinder. Also this is why heat wrap works well along with the ceramic coating.
Back Pressure vs. Pressure Waves
Many mistake back pressure as the waves that force feed the motor. It's true that it could be considered back pressure, but these are two seperate phenomenon so I will define them here to show the differances. Generally the stinger and the silencer provide the residual back pressure seen in motors. They do this by restricting the flow of the gases. What this does is allow the hot exhaust to sit in the pipe an dissipate the heat it holds. Now as we know the more heat we have the faster the waves will travel. Pressure waves have to do with the differences in pressure in the pipe. As we know high pressure flows to lower pressure. Also these waves are created by the pressure emitting from the port just after it opens on a powerstroke.
Frequency of the Waves
Of course the frequency of the waves is all based off of engine RPM because the positive pressure wave doesn't begin until we have our first power stroke. So say we are running 6000 RPM. To get to the frequency in Hz. we need to divide by 60. So 6000/60=100 Hz. Or 100 pressure waves per second.
Dwell Area
The Dwell section of the pipe is important. It determines how much mixture the pipe will draw or scavenge and how much mixture the pipe will force back into the cylinder before the port closes or port block. Generally the fatter the dwell section the harder the pipe is going to suck. It will draw more of the mixture in. However, the fatter pipe does not support a strong final positive pressure wave to force the mixture back into the cylinder. So this means a smaller pipe will have a stronger positive wave. Really we want an equilibrium here, if we draw too much mixture and can't push it back into the cylinder it doesn't do us any good. Conversely if we draw too little we still aren't making the peak power.
Pipe Design Spreadsheets
If you would like to try your hand at pipe building, I've created a Excel spreadsheet to guide you. This is not an end all solution that will give you the exact results you put in, pipe building is still a trial and error business, so you may have to tweak the variables this program delivers to build a pipe with a big power gain! Also, I have a building aid that also calculates the angles and what not for the cones. This app was graciously sent to me by a guy who builds pipes for 300 cc bikes.

Here are the links.
www.freedomirishfootball.com/pipecalc.xls
www.freedomirishfootball.com/conecalc.xls

Here's a screen shot.



Bibliography

http://www.physics.smu.edu/~olness/www/05fall1320/applet/pipe-waves.html

http://www.motorcycle.com/mo/mcnuts/em-pipes.html

http://en.wikipedia.org/wiki/Expansion_chamber

http://www.liebold.com/rumipages/espansioni/twostroke.html

http://www.sae.org/servlets/productDetail?PROD_TYP=BOOK&PROD_CD=R-161

http://www.two-stroke-addicts.freeserve.co.uk/mick/hydropipe.htm

http://seniordesign.engr.uidaho.edu/2003_2004/SPR/snapshot%20-%20exp.%20chamber.htm
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