|
Tuned Pipes & How They Work
May 1, 1996
As you have discovered, changing the tuned pipe on your RC Car, Boat, Airplane can make a drastic difference in performance. Why is this?
In order to answer this question, a general discussion of the characteristics of the tuned exhaust is necessary. A historical discussion will allow an easy understanding of each component of the tuned exhaust.
In the 1950's an engineer named Walter Kaaden was consulted by motorcycle racers asking him to help them "Go Faster". He discovered that by putting different length "Straight"pipes on a 2 cycle engine it changed the characteristics and performance. By adding a Divergent Cone (Figure 1) there was an even greater change in performance characteristics. You should view the 2 cycle engine as a "Sound Generator". Each time the piston uncovers the exhaust port the pulse of exhaust gases rush out the port can create a positive pressure wave. This wave radiates out of the exhaust port. The sound will be the same frequency as the RPM the engine is running. If the engine is turning 25000 RPM the exhaust will be generated at 25000 RPMs or 415 cycles per second. The length of the tuned exhaust is determined by the RPM the engine will reach and NOT the size(displacement) of the engine.
|
The original straight pipe attached to the exhaust port used the "negative pressure wave" created by the opening and closing of the exhaust port to suck out the charge in the cylinder. Since the sound waves that start at the end of the pipe travel to the other end of the pipe "at the speed of sound", there was only a small RPM range where the negative waves return to the exhaust opening at a useful time. At too low an RPM range, the wave would return too soon, bouncing back out the port. At too high an RPM the , the piston would have closed the exhaust port, doing no good.
This early pipe was easy to tune. You started with a long pipe and cut it off until it worked at the RPM range you wanted.
The next progression was to add a Divergent Cone on the end of the straight pipe. This intensified the sound wave and and in fact lengthened the returning wave, thus broadening the power band. The wave was not as strong as the straight pipe, but returned the wave over a LONGER TIME FRAME (Figure 2), thus making the chance that it would find the exhaust port open greater. The exhaust gasses could be sucked out if the exhaust port was found open by this returning wave. The divergent cone shape or angle was found to be important, since the steeper the angel the more intense the negative wave returned, but also the shorter the duration. The gradual divergent cone returned a less intense wave over a longer time. It was found that this negative wave was strong enough to suck the exhaust out and at the same time pull fresh mixture up through the intake ports into the combustion chamber. The addition of the divergent cone to the straight pipe produced great tuning advantages, but it had severe limitations. The broader negative wave from a "Megaphone" can arrive to early and pull fresh mixture out of the cylinder. How could we put this extra mixture back into the engine?
|
At this point another cone was added onto the divergent cone to make a "Positive Wave" reflect back into the open exhaust. These positive waved would follow the negative waves back into the open exhaust port and if properly timed would "Stuff" the fresh mixture into the combustion chamber through the exhaust opening just as the exhaust port was closing. This in effect supercharged the engine with no moving parts - only that addition off the tuned exhaust.
In addition to the straight Head Pipe, the Divergent Cone, the Convergent Cone. there are two other components of the tuned pipe. The middle section connecting the Convergent and Divergent cones, or the Belly of the pipe is the section of the tuned pipe which is changed in length to adjust the RPM Band which you want the pipe to operate. There is also the Stinger Section of the pipe which serves as a "Bleed Valve" to allow exhaust to escape.
The Belly section of the pipe determines the relative timing of the Negative and Positive waves. The timing of the waves is determined by the length of the straight pipe. If the belly section is too short, positive waves have a shorter distance to travel, and return to the exhaust port sooner. This can be good if the engine is to be operated at High RPM's only, but very bad if you want to have a broad RPM range to operate in. The diameter of the belly section is not important except to allow fit to the model.
The Stinger Section of the pipe is a pressure bleed valve to allow the exhaust gasses to eventually leave the pipe. Back pressure in the pipe, caused by a smaller or larger or longer or shorter stinger helps the wave action of the pipe, and can increase the performance of the tune pipe. This is due to the higher pressure created by the smaller stinger making the charge a denser medium. Sound waves will travel better in a denser medium. However, the more you restrict the stinger, the higher the heat retained. This can be a very detrimental characteristic of a two stroke engine, but the gains from a higher density medium can yield considerable performance increases.
As the two stroke tuned pipe has evolved, it has been found that if you have a gently divergent "Head Pipe" it will keep the gas velocity high near the exhaust opening, then a second medium divergent cone and a third high taper divergent cone attached to the belly section.
The design and tuning of expansion chambers is a relatively easy process if you can define the criteria needed in this design. You must know the RPM range you want to operate in, the top rpm, the temperature and thus the speed at which the gasses travel (dependent upon fuel used and temperature inside the pipe)
Built into "The Engine Analysis Program" is a pipe design module to design a single stage tuned pipe. There is the capability to adjust the wave speed constant, to adjust for temperature due to atmosphere and nitro content.
|
