Exhaust theory Pt1

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Tue Jan 06, 2009 9:47 pm

Basic theory:

The Expansion Exhaust has two waves forms that are reflected, one is negative (vacuum), one is positive (pressure)
The negative wave is reflected to the exhaust port to arrive to assist drawing in the incoming fuel charge (down stroke)
The positive wave is reflected to the exhaust port to assist in compressing the fuel charge (upstroke)

Tuned exhaust physics – Helmholtz resonator:

The amount of power increase (increase in volumetric efficiency) can be 10-50% over a straight pipe. The effect only occurs at a resonant frequency (powerband)
The speed of a wave traveling in medium depends on the density (pressure and temperature) in the medium
A sonic wave (wave in air) will be reflected by the end of a pipe. If the pipe is closed, the reflection is the same as the wave (pressure), if the pipe is open, the reflection is opposite of the wave (vacuum).

Parts of the pipe:

Header – Important in determining the tuned length.
Diffuser – the negative wave is reflected here
Main Chamber – “expansion chamber”
Convergent cone – The positive wave is reflected here
Stinger – Allows some of the pressure to be bled off

Now lets look at the Math there again are a few different calc here which give different answers.

The first thing is the Tuned length of an expansion unit.

Basic formula is as used by a lot of people

Lt = (EDx42545) / RPM

ED = Exhaust duration in Degrees.

Now this is a good basic calc BUT it doesn’t take into account the true temperature of the exhaust gases
As it uses the constant of 42545 for the local speed of sound using a std temp I believe of 340deg C.

The Temperature is a major factor in the timings of the pressure waves in an exhaust system and as you are very aware the Temps in a Lambretta system get a lot hotter then this. Id say 450-600 deg C

So with this in mind here is the formula to be used which takes into account of the temperature.

Speed of Sound SS = Sqrt(401.8 x (Tex + 273)) m/s

Tex = temp of exhaust in Deg C the 273 changes this to Kelvin.

Once this has been calc the following can be used

Lt = (83.3 x SS x ED)/RPM

So for example:

ED = 190 deg
RPM = 10000
Tex = 550 degC

Basic Lt = 808mm

Temp comp Lt = 910mm

A big differance
Main function of the above is the factor of Temperature which is very often not taken into account

So there you have it for now Ill carry on with other parts later

Hope Ive explained myself properly

Smiffy (RinB)
If its made of Metal Fettle it !!

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