How a 2 stroke exhaust works
08-09-2007, 12:20 PM
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How a 2 stroke exhaust works
No I did not write this but is a very good explanation on how the pipes we run work. 
In reality, expansion chambers are built to harness sound waves (created in the combustion process) to first suck the cylinder clean of spent gasses--and in the process, drawing fresh air/gas mixture (known as 'charge') into the chamber itself--and then stuff all the charge back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere. This phenomenon was first discovered in the 1950s by Walter Kaaden, who was working at the East German company MZ. Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning.
An engine's exhaust port can be thought of as a sound generator. Each time the piston uncovers the exhaust port (which is cut into the side of the cylinder in two-strokes), the pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be be the same frequency as the engine is turning, that is, an engine turning at 8000 rpms generates an exhaust sound at 8000 rpms or 133 cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement.
Of course those waves don't radiate in all directions since there's a pipe attached to the port. Early two strokes had straight pipes, a simple length of tube attached to the exhaust port. This created a single "negative" wave that helped suck spent exhaust gases out of the cylinder. And since sound waves that start at the end of the pipe travel to the other end at the speed of sound, there was only a small rpm range where the negative wave's return would reach the exhaust port at a useful time: At too low of an rpm, the wave would return too soon, bouncing back out the port. And at too high of an rpm, the piston would have traveled up the cylinder far enough to close the exhaust port, again doing no good.
Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted.
So after analyzing this cut-off straight-pipe exhaust system, tuners realized two things: First, that pressure waves could be created to help pull spent gasses out of the cylinder, and second, that the speed of these waves is more or less constant, though it's affected slightly by the temperature of the air. Higher temperatures mean that the air molecules have more energy and move faster, so sound waves move faster when the air is warmer.
Putting a divergent cone on the end of a straight pipe lengthens the returning wave, broadening the power band and creating a rudimentary expansion chamber.
So, to sum up, when the negative wave reaches the exhaust port at the correct time, it will pull some of the exhaust gases out the cylinder, helping the engine to scavenge its spent exhaust gas. And putting a divergent cone at the end of the straight (parallel) "head" pipe broadens the returning wave. The returning negative wave isn't as strong, but it is longer, so it is more likely to find the exhaust port open and be able to pull out the exhaust gases. As with plain, straight pipes, the total length of the pipe with a divergent cone welded on determines the timing of the return pulses and therefore the engine speed at which they are effective. The divergent cone's critical dimensions are where it starts (the distance from the exhaust port to the start of the divergent cone is called the "head" pipe), while the length of the megaphone and the rate at which it diverges from the straight pipe determine the intensity and length of the returning wave--A short pipe which diverges at a sharp angle from the head pipe gives a stronger, more straight-pipe-like pulse.
Convergent cone. This is at the end of the belly section and reflects the pressure wave back toward the engine. The cone's taper angle influences how long the pressure wave takes to return along the tuned pipe. Again, a more pronounced taper causes a more intense pressure wave of short duration; a gentle taper results in a longer, less intense wave. A taper angle in the range of 15 to 20 degrees, or roughly double the angle of the divergent cone, is thought to be ideal.
Stinger. This is the last part of the tuned pipe. The stinger's diameter and length are important to performance but are largely irrelevant in RC car applications. The diameter and length of the stinger are governed by most sanctioning bodies, therefore, most pipes are designed to meet these regulations, and there is less variety of stinger sizes. The ideal stinger diameter is about 60 percent of that of the header pipe, and its length should be 10 to 12 times its own diameter.
Almost every system produced by us is a combination of divergent & straight pipe designs which in turn produces good power throughout the powerband. My company was started because of the horrible experiences from other homemade pipes which detroyed my engines. Every experience insired me to produce pipes that work and dont cause a hidden lean conditions thus avoiding a shortened engine life.
In reality, expansion chambers are built to harness sound waves (created in the combustion process) to first suck the cylinder clean of spent gasses--and in the process, drawing fresh air/gas mixture (known as 'charge') into the chamber itself--and then stuff all the charge back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere. This phenomenon was first discovered in the 1950s by Walter Kaaden, who was working at the East German company MZ. Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning.
An engine's exhaust port can be thought of as a sound generator. Each time the piston uncovers the exhaust port (which is cut into the side of the cylinder in two-strokes), the pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be be the same frequency as the engine is turning, that is, an engine turning at 8000 rpms generates an exhaust sound at 8000 rpms or 133 cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement.
Of course those waves don't radiate in all directions since there's a pipe attached to the port. Early two strokes had straight pipes, a simple length of tube attached to the exhaust port. This created a single "negative" wave that helped suck spent exhaust gases out of the cylinder. And since sound waves that start at the end of the pipe travel to the other end at the speed of sound, there was only a small rpm range where the negative wave's return would reach the exhaust port at a useful time: At too low of an rpm, the wave would return too soon, bouncing back out the port. And at too high of an rpm, the piston would have traveled up the cylinder far enough to close the exhaust port, again doing no good.
Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted.
So after analyzing this cut-off straight-pipe exhaust system, tuners realized two things: First, that pressure waves could be created to help pull spent gasses out of the cylinder, and second, that the speed of these waves is more or less constant, though it's affected slightly by the temperature of the air. Higher temperatures mean that the air molecules have more energy and move faster, so sound waves move faster when the air is warmer.
Putting a divergent cone on the end of a straight pipe lengthens the returning wave, broadening the power band and creating a rudimentary expansion chamber.
So, to sum up, when the negative wave reaches the exhaust port at the correct time, it will pull some of the exhaust gases out the cylinder, helping the engine to scavenge its spent exhaust gas. And putting a divergent cone at the end of the straight (parallel) "head" pipe broadens the returning wave. The returning negative wave isn't as strong, but it is longer, so it is more likely to find the exhaust port open and be able to pull out the exhaust gases. As with plain, straight pipes, the total length of the pipe with a divergent cone welded on determines the timing of the return pulses and therefore the engine speed at which they are effective. The divergent cone's critical dimensions are where it starts (the distance from the exhaust port to the start of the divergent cone is called the "head" pipe), while the length of the megaphone and the rate at which it diverges from the straight pipe determine the intensity and length of the returning wave--A short pipe which diverges at a sharp angle from the head pipe gives a stronger, more straight-pipe-like pulse.
Convergent cone. This is at the end of the belly section and reflects the pressure wave back toward the engine. The cone's taper angle influences how long the pressure wave takes to return along the tuned pipe. Again, a more pronounced taper causes a more intense pressure wave of short duration; a gentle taper results in a longer, less intense wave. A taper angle in the range of 15 to 20 degrees, or roughly double the angle of the divergent cone, is thought to be ideal.
Stinger. This is the last part of the tuned pipe. The stinger's diameter and length are important to performance but are largely irrelevant in RC car applications. The diameter and length of the stinger are governed by most sanctioning bodies, therefore, most pipes are designed to meet these regulations, and there is less variety of stinger sizes. The ideal stinger diameter is about 60 percent of that of the header pipe, and its length should be 10 to 12 times its own diameter.
Almost every system produced by us is a combination of divergent & straight pipe designs which in turn produces good power throughout the powerband. My company was started because of the horrible experiences from other homemade pipes which detroyed my engines. Every experience insired me to produce pipes that work and dont cause a hidden lean conditions thus avoiding a shortened engine life.
08-10-2007, 10:08 AM
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RE: How a 2 stroke exhaust works
Hope all are enjoying this article and to keep it up on top for a couple days I thought I would make post. 
