Mr Helmholz - simple model

Stuart Baly S.Baly at bom.gov.au
Wed May 19 02:13:10 GMT 1999


>If this opens a whole new box of problems, just ignore.  But, I've noticed
>what a huge change expansion chambers have had of VE in 2 strokes, and 
after
>seeing the Mercedes Benz GT (Sedan, whatever they call em) with the v-6, 
and
>3 into 1 exhausts, and running 2 expansion chambers (one per side), I just
>wonder, how they came up with that...
>   Is it the shape of the cones or volume that makes a "E.C." work, or
>both?.
>Grumpy

For a blue-smoke, it's the shape more than the volume. Things happen in 
this order:
1. the piston uncovers the exhaust port, sending a pressure pulse along the 
exhaust at the speed of sound. The exhaust gas begins travelling along the 
pipe somewhat slower than this.
2. the pressure pulse reaches the expanding part of the exhaust (i.e. 
increasing diameter). The change in diameter produces a reflected wave of 
lower pressure - this low pressure wave travels back towards the engine.
3. The piston, still travelling downwards, uncovers the transfer port. F  
uel-air mixture starts to displace exhaust gas from the cylinder.
4. The original pressure pulse reaches the reducing part of the exhaust 
(i.e. reducing diameter). This produces a reflected wave of higher 
pressure, travelling back towards the engine.
5. The reflected wave from pt. 2 reaches the engine - this draws the 
exhaust mixture from the cylinder, and in extreme cases, even draws some 
fuel-air into the exhaust.
6. The reflected wave from pt4. reaches the engine - this pushes the 
fuel-air back into the cylinder just in time for the rising piston to close 
the exhaust port off.

	I'm sure that the real world operation of a two-stroke exhaust system has 
many many more factors affecting it, and that this is a greatly sanitised 
version of reality, but it's a start.

Stuart Baly.

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