I agree. As one pulse travels through the collector it's inertia
cause a low pressure pulse in the other pipe which of course
increases the exhaust flow out of the other rotor 180 degrees
out of phase.
Paul Lamar
If the 13B, which is 180 degrees out of phase needs a 31" header for maximum
power at 6500 rpm, then what header length would a 3-rotor, which is 120
degrees out of phase need in order to maximize power at the same rpm? Seems
to me that something less than 31", like maybe 20-24" would provide more
scavenging. Or, am I missing the bigger picture here?
First a collector system won't work on a three rotor.
Tuned pipes will help of course.
The exhaust pulse travels down the pipe at the speed of sound.
As the RPM changes the distance between the pulse changes.
Higher RPM shorter distance. Optimum length occurs as one
pulse is leaving the collector and another is being emitted by
the engine.
The speed of sound is about 880 feet per second.
Each rotor fires once per rev. At 6000 RPM or 100 RPS
pulses are every 1/100 of a second apart or .01 second.
It takes the pulse about .003 seconds to travel 32 inches.
That is an exact multiple of .01 second. So the pressure
wave is bouncing back and fourth in the pipe three times
and the low pressure part of the wave arrives back at
the exhaust port just as the port opens again. This
also sync's in the collector enhancing the low pressure
part of the wave in the other pipe. Multiples of 32 inches
also work such as 16 inches at 6000 RPM.
Paul Lamar
Hi Paul,
I need some clarification on terminology & exhaust tuning techniques.
Perhaps you & Lynn can either either refute the following or expand on it.
The stuff I've read about tuning says that there are basically 2 kinds
of tuning, 'inertial' & 'sonic'. It sounds like what you are describing
is 'sonic', where the sound energy (at the speed of sound) tries to drag
the air along with it. Sonic is supposed to work with either
independent pipes or collectors but isn't as strong as inertial tuning.
Inertial tuning requires a collector & uses the actual flow of the
exhaust gas into the collector to effectively create a pulsed vacuum on
the other pipe, kind of like a venturi effect. The speed of the gas is
much slower than the speed of sound but is supposed to be much more
powerful than sonic tuning. The pipe length to the collector is
supposedly determined by the speed of the actual gas flow, or time it
takes the gas to get from the port to the collector. It should enter
the collector timed to create the vacuum event in the other pipe as it's
port opens. Obviously an rpm design range must be selected.
The holy grail would be to have a pipe length that would have inertial
tuning & be one of the sonic multiples so that sonic will augment the
inertial tuning.
Collector (inertial) systems obviously work on V-8's with 4 cylinders
feeding one collector. I would think that the same principle would apply
to a 3 rotor rotary if pipe length is adjusted to compensate for the
more closely spaced events relative to the 2 rotor. In old hotrod engine
tuning, the diameter & length of the collector had marked effect on the
effectiveness of the header.
I remember Tracy saying that the dynamic chamber (intake) tuning method
won't work on a 3 rotor. To my rather limited brain this makes sense if
the intake system sees its plumbing as a closed system & the pressurized
slugs of air are bouncing between the rotors in a 2 rotor but get
dispersed between the other 2 rotors (destroying the timing) in a 3 rotor.
So, Lynn & Paul, is my interpretation of what I've read about tuning
more or less correct? Does the 32" optimum length of exhaust pipes match
what would be needed for inertial tuning described above?
That is essentially correct Charlie. You can have a pipe closed at both
ends that resonates. There will be no net flow out of the pipe.
There can be flow out of the pipe and the pressure waves will
still bounce back and fourth. This is probably why the diameter
of the pipe affects the tuning almost as much as the length.
Larger pipes... slower flow. See the attached chart.
That is also why you can not hear noises as well when you are
up wind of them.
Also the larger the pipe the slower the flow the less flow velocity
lost due to wall friction.
It is the pressure waves that move at the speed of sound
within the moving gases. The actually flow speed of the gases
can be much slower. Both phenomena combine to increase or decrease
the power. Lower pressure at the exhaust port means less force is required
to expel the hot gases and consequently more net torque is available
at the e-shaft.
Correct. The dynamic chamber will not work on a three rotor.
The dynamic chamber is irrelevant anyway on an aircraft engine
that is mainly concerned with power at 6000 RPM. In effect it
doubles the length of the intake pipe while sacrificing
a bit of flow resistance and hence power at the top end. As far as I
can tell, right at the moment, the RX8 engine does not use the dynamic
chamber effect.
Paul Lamar
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