Tuned pipe

A tuned pipe is a part of a two-stroke engine's exhaust system.

It should be distinguished from a muffler as a tuned exhaust pipe does more than muffling the sound. Its purpose is to retain the air/fuel mixture in the combustion chamber by using the pressure wave produced by the combustion process itself and bouncing it back to the exhaust port at the appropriate time, thus precluding the fresh charge, which comes through the transfer port/s, to follow the exhaust gases. The principle was invented by Limbach, a German engineer, in 1938. The main reason for it was not to create extra power, which is a secondary result, but to save fuel.

Function

Four-stroke engines have four separate intake, compression, combustion, and exhaust stages. On the other hand, two stroke engines only have two; 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 crankcase 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 crankshaft 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 is pushed out by the upward movement of the piston, and only some of the fresh mixture would be detonated. Two stroke engines perform poorly if not fitted with a tuned pipe.

How it works

A tuned pipe attached to a two-stroke engine, showing how the exhaust gases (in grey) are used to push the escaping fuel/air mixture (in green) back into the cylinder ready for the next ignition.

The tuned pipe is attached to the end of the exhaust pipe. When the exhaust port is opened right after the combustion of the fuel/air mixture, exhaust gas rushes out at great speed; this creates a pressure wave and the banging sound. Immediately before the exhaust port is blocked by the piston that is moving up, fresh mixture also escapes along with the exhaust gas. The first half of the tuned pipe is gradually flared, for easy extraction of gas. The end of the tuned pipe has a dish (or better a cone, see "Tuning the pipe") facing in the direction of the exhaust port. This acts as a wall to reflect the exhaust pressure wave back. A small hole at the centre of the end wall or in the middle of the tuned pipe lets the exhaust gas out. The returning wave pushes the mixture that just escaped the cylinder back in through the exhaust port. This back flow mimics the effect of supercharging or turbocharging to some degree, as a tuned pipe increases the fuel/air density in the cylinder.

Limitations

The timing of the return wave depends solely on the length of the exhaust to the point where the wave is reflected. However, the length of time the exhaust port is open alters with the engine's RPM. That is the reason for using the last converging cone (see below: Tuning the pipe).

Tuning the pipe

Because a tuned pipe cannot be effective over the full spectrum of the engine's RPM, it has to be "tuned" for a certain RPM range, just as a single-string/pipe musical instrument. To avoid a very narrow power band, a converging cone (the "baffle" cone) is used: it acts as a multiple series of plates of different diameter reverberating the waves to the exhaust port during a time lap that, if properly calculated, will act over a wider range, creating extra torque and power usually through the top half of the engine's total RPM range.