Variable Restrictiveness Exhaust - Meet Mr Helmholz

[Robert Harris]

To review the basics  A Helmholtz resonant intake consists of a largish
diameter intake pipe, a resonating volume (plenum) and individual cylinder
runners.  The individual cylinder runners are length/diameter tuned to the
highest power rpm desired and the helmholz resonator extends the rpm range
downward.  

Reverse it.  Individual cylinder runners, resonating volume ( collector ) and
a largish tunable pipe.  Size the exhaust cylinder runners for the high rpm
diameter and length.  Fire them into a collector and then out a pipe. 

Most  exhausts can be reduced to an inverted helmholz resonator.  We all know
about sizing the collector by running a high power run on a freshly painted
collector and cutting the collector off an inch past where the paint burned
off to establish the size.

With the awareness that the exhaust system is essentially a helmholz
resonator, we can progress past simple length diameter tuning and begin to
draw strange and exotic systems that baffle and befuddle and yet work
incredibly well.

>From Advanced Engine Technology
Heinz Heisler ISBN 0-340-56822-4
5.12.2  Helmholz Resonator Cylinder Charging


                         C        /    A
Helmholz  N =  ---       /    -------
                       2pi   V      L V

N = resonant frequency Hz
C = velocity of speed of sound m/s ( at the temperature of the gas )
A = cross sectional area of tuned pipe m2
L = length of tuned pipe m
V = Resonating volume m3

RPM to HZ   Rpm x number of cylinders divided by 120 ( 4cycle )

5.14.4 Velocity of sound in a gas.

Excellent discussion of calculating the velocity of sound in a gas.  In
essence, the velocity is proportional to the square root of the absolute
temperature.  For an initial play value, he recommends 518 m/s for a 400c
average system exhaust temperature.  Your milage may vary
.
Th individual runners are included in the resonating volume as well as the
collector "plenum".  

Again, looking at the exhaust as a Helmholz resonator, we immediately see that
by varying the volume we can tune the resonant frequency.  This can be done by
using a small collector and coupling blind volumes in and out.  It can also be
accomplished by butterfly valve switching in/out tuned stubs or using a vacuum
headlight actuator to vary the length of a slippy pipe stub coupled to the
collector. 

We can also change the effective rpm that this resonance affects by varying
the number of cylinders - say split banking such that we feed two or four or
eight cylinders into each chamber.

Finally, we can change the temperature of the exhaust and change the
resonance.

http://www.msdignition.com/2new.htm#Switch

This switch just might be the ticket for thinking - if you can twist your
sister and put on a strange hat.   Water injection for exhaust gas temperature
control - not anally extracted - but from a reputable manufacturer in
production - just for another application.

The control system only needs look at four items .  

Feed Forward or predictive:   RPM and EGT at the resonator

Feed Back or corrective :  Camshaft angle and pressure applitude/sign

Set point - sign and camshaft angle of peak pressure.

Should be very doable if someone cared to try to take advantage of maximum
scavenging thru a very wide region.   But then - who would want to do that?


Habaneras - not just for breakfast anymore