Tuned pipe
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A tuned pipe is a part of a two-stroke engine's exhaust system. It should be distinguished from a muffler as a tuned pipe does more than muffling the sound. A tuned pipe's purpose is to obtain higher compression of the air/fuel mixture in the combustion chamber by temporarily creating a back flow in the exhaust pipe by bouncing pressure wave.
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[edit] Function
Four stroke engines have separate intake, compression, combustion, and exhaust stages. On the other hand, two stroke engines only have intake/exhaust and compression/combustion. After combustion, the piston goes down, and as it does so, exposes the exhaust port on the cylinder wall. Expanded exhaust gas rushes out through the exhaust port. The piston travels lower, and exposes the intake port, usually located further down and on the other side of the chamber. Because the exhaust is already flowing in one direction, and because the piston pushes down into the crank shaft housing where the fresh mixture is, fresh mixture flows in through the intake port. The fresh mixture flows in through intake port and some immediately flows out through the open exhaust port. The crank shaft continues to rotate due to inertia and pushes the piston up. As the piston rises, the exposed intake port is closed, blocking the flow of fresh mixture. However, the exhaust port is still open, and so gas is still flowing out of the chamber. If the exhaust port is open ended, the fresh mixture gets pushed out by the upward movement of the piston, and only some of fresh mixture would be detonated. Two stroke engines perform poorly if not fitted with a tuned pipe.
[edit] How it works
The tuned pipe is attached at the end of the exhaust pipe. When exhaust port is opened right after combustion of gas mixture, exhaust gas rushes out at great speed. This creates the banging sound. Right before the exhaust port is blocked by the piston that is moving up, fresh mixture also flows out along with the exhaust gas. The first half of tuned pipe is gradually flared, for easy extraction of gas. At the end of tuned pipe has a dish facing in the direction of the chamber. This last part acts as a wall to bounce off the exhaust pressure wave. A small hole at the center of the end wall or in the middle of tuned pipe lets the exhaust gas out. A tuned pipe is simply an empty chamber that allows pressure wave to bounce back. Returning pressure wave packs the fresh mixture that just flowed out of the chamber back into the chamber through the exhaust port. This back flow mimics the effect of supercharge or turbocharge to some degree, as tuned pipe puts more gas and air back into the chamber by bouncing pressure wave.
[edit] Limitations
The timing of the return pressure wave depends solely on the length of the exhaust to the point where the pressure wave is reflected. However, the length of time the exhaust port is open alters with RPM. Since the timing of the pressure wave is unaltered its return will no longer coincide with the exhaust port's closure. This means that tuned pipes only reach maximum effectiveness over a fairly narrow RPM range.
[edit] Tuning the Pipe
Because a tuned pipe cannot be effective over the full spectrum of the RPM, it has to be "tuned" for a certain RPM range, just as a musical instrument is tuned. Usually, it is tuned for the lower RPM because that's where most engines have weakest horse power and torque. By adjusting the exhaust pipe's total length, one could "tune" the pipe. For model engines this can be achieved by cutting the coupler that connects exhaust manifold and tuned pipe little by little and test driving the vehicle until improvements are seen at desirable RPM band. For full sized motorcycles, various manufacturers already have calculated the length of the exhaust pipe to suit the need of the particular model. However, one could obtain different length pipe to suit one's need in aftermarket. Generally longer pipe moves the effective band of tuned pipe to lower RPM range, and shorter length moves the effectiveness band to higher RPM.
