Mr Helmholz - simple model

[Bruce Plecan]

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



> The Helmholz resonating system was originally developed for wind
acoustical
> instruments as a model.  We now routinely use it to create and size
resonant
> intake systems.  But some may still have trouble visioning it.
>
> The pure model is a ball  "resonating volume"  with a pipe "tuning pipe"
> attached.   Changing the temperature changes the local speed of sound
within
> the resonator - and thus the frequency of resonance changes.  Changing the
> volume within the resonating volume changes the frequency - larger lower.
> Changing the area/length of the tuning pipe changes the frequency.
>
> So everyone should see a ball with a pipe.   We now add some pipes - sort
of
> like porcupine spikes sticking out of the resonating volume - but part of
the
> resonating volume.  N -1 spikes are of the length and volume of the header
> pipes running to the collector.  The Nth is the length and volume
including
> the volume of the cylinder with the exhaust open during overlap  N is the
> number of cylinders firing into the common volume.
>
> The headers/runners can be individually ram tuned to a frequency somewhat
> higher than the highest helmholz frequency and thus extent the top end as
this
> resonance swamps the helmholz effect.
>
> When a pulse of sound ( exhaust gas acoustic pressure front ) hits an open
> volume it generates a negative pressure wave that travels back up the
header
> to the exhaust valve.  We want the negative peak of this wave to arrive at
the
> time both valves are open "overlap" to gain maximum effect.  That is the
> theory behind ram tuning exhaust.  With the ability to swamp that effect
with
> the Helmholz resonance, think ringing a bell - with a variable sized
bell - we
> can retune the resonance downward as much as practical.
>
> Since the only thing that matters ( assuming a reasonably well designed
> otherwise exhaust system ) is the arrival of this negative pulse, we need
only
> measure the instantaneous port pressure in this crankshaft angle region
and
> then tune the system to a maximum negative pressure.
>
> Adding blind volume to the resonating volume simply lowers the resonant
> frequency -  a good thing since the additional volume does not have to
flow
> gas.   Adding area/shortening the length of the tuned pipe raises the
resonant
> frequency.  Perhaps multiple switched exhaust pipes?
>