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----- Original Message -----
From: Al
Gietzen
To: Rotary motors in aircraft
Sent: Thursday, May 22, 2003 5:10
PM
Subject: [FlyRotary] Tuned Induction
Al, I don't know much
about induction systems and they appear to be as much "Magic" as
science.
Obviously, I don’t either. But I’d
like to be able to get it out of the realm of magic.
My own
personal story is that when I had a Weber throttle body with two 2" dia
throats with a relative short length induction (about 1/2 my current length), my
static rpm was 5000 rpm, my top speed was 182 MPH TAS and rate of climb was
around 900 fpm. When I created an intake using smaller (1 5/8 and 1 3/8"
dia tubes) of the length that theoretically are tuned to provide the "dynamic
charging effect" at 5900 rpm, my static went up to 5300 rpm, my ROC went up to
1200 fpm and my top speed to 196 MPH TAS. So while I can't prove
that my current induction system is better "tuned" to my operating regime and
rpm - I sure like the improved results much better.
Given that speed
goes by the cube root of HP; the speed change suggests that your power increased
by 25%; so maybe something suspect there, or there was something really wrong
with your original setup. What else changed besides your runner
length?
Al,
The changes were runner length (longer by
approx 20%) and diameter of tubes (smaller by 25%).
After reading
Paul Yaw's tech stuff about the importance of the velocity of the air/fuel
mixture in an intake. I calculated that on my original manifold (two 2"
dia throats) that my intake velocity (assuming I was producing 180 HP which I
defintely was NOT!) would have been approx 71 MPH. According to what I
read, you want much higer air velocity. The theory (If I understood it)
was that the interia of a faster moving slug of air/fule would stuff
more air/fuel mixture into the chamber than slower moving air/fuel
mixture. My change in tube diameter and length more than double my air
velocity (calculated not measured) through the tubes and into the
chamber
Below is an extract of Paul Yaws very understandable and
excellent series on airflow for the rotary:
"....Velocity alone
determines at what rpm peak torque and horsepower will occur for a given intake
and exhaust duration. (Duration is the amount of time that the ports are open.)
As most readers know, the intake port on a rotary engine closes well after
bottom dead center.(BDC) What this means, is that the rotor is starting to
compress the mixture while the port is still open. When
velocity is low, some of the mixture will be squeezed back into the intake
manifold. If velocity is high, the inertia of the incoming mixture will overcome
the rising pressure of the chamber, and continue to pack mixture into the engine
well after bdc. This effect is called inertial supercharging, and you can
feel this while you are driving. At low speed, the engine does not make much
torque, but as rpm increases, the engine really starts to put you back in your
seat. The power increase that you are feeling is the result
of the extra mixture being packed into the chamber as a result of higher intake
velocity.
Now that you know how important velocity is, you must
also understand that there is such a thing as excessive intake velocity. It has
been determined that once the velocity in the induction system reaches
approximately .6 mach, (60% of the speed of sound) the engine will "top out".
Beyond this speed the additional airflow into the motor does not create enough
extra power to overcome the pumping losses, and the result is a decrease in
horsepower. It should be obvious now that velocity is critical, and must be
carefully balanced with airflow. "
So while my air velocity falls quite short of Mach 0.6, I think the
smaller diameter/longer runners did help quite a bit.
FWIW
Ed
Anderson
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