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Posted for "Ed Anderson" <eanderson@carolina.rr.com>:
Hi Scott, Welcome.
I'll take a crack at answering your questions.
Hello All...
After lurking on the list for a while I see most folks
are using some combination of the lower intake
manifold with custom fabricated runners. It seems
everyone is using metal and not composites for the
intakes. Are there a particular reasons for this?
Like heat, vibration, etc.?
In a word, yes. Heat in particular is a concern as the rotary intake is in
close proximity to the exhaust. Some folks have experimented with epoxy
intakes for the aircraft rotary, but no one (to my knowledge) is flying with
one yet. High temperature (and therefore expensive) epoxy resins are
required and also post curing of the composite intake needed to give it a
higher operating temperature.
I believe there are still a couple of folks still working on a composite
manifold, but have heard nothing about progress recently.
I understand the dynamic supercharging idea, but what
is causing this pulse(s) to come back through the
intake? To utilize the dynamic supercharging effect
on the intakes is there a particular length the intake
runners should be? Is the amount of supercharging
significantly effected by the diameter of the intake
runners?
The Mazda enginers discovered that when the intake port of one combustion
chamber is abruptly closed by the rotor the air stream (that may have had a
velocity of close to 200-300 fps (or possible more) in the intake runner) is
suddenly stopped. This creates a shock wave/pulse of air which is reflected
back from the closed intake port through the intake runner. The length of
the intake manifold from the closing port, which causes the pulse, to the
intake port of the second rotory can be made so that for a certain rpm (or
narrow rpm range) the pulse arrives at the intake port of the second rotor
just as it is opening. This causes a bit more air/fuel mixture to be shoved
into the intake of the second rotor creating a small "supercharging" effect.
The Mazda engineers found the increase in power attributed to this dynamic
charging to be worthwhile taking advantage of and designed several manifolds
to do so including one with a valve which controlled the "Lenght" of intake
manifold the pulse "Saw" at different rpms/manifold pressures.
However, most people believe that getting a good flowing intake manifold
outweighs any benefit of dynamic charging with a "Bad" intake manifold.
Best I can tell is that it doesn't seem to hurt and may slightly improve the
HP.
The lenght of the intake tubes does depend on the engine RPM, Manifold
pressure and inlet temperature. The calculation are basically dependent on
the speed of sound in the medium through which it is propogating (the intake
manifold) which varies with pressure and temperature of the air/fuel mixture
in the intake. But, these are relative minor considerations with the
dominate one being engine rpm. To get the dynamic charging effect around
6000 rpm the runner distance from intake port to throttle body opening of
each rotor is on the order of 19-21 inches. The higher the rpm the shorter
runner distance required to capture the effects of dynamic charging.
There you have it as I understand it.
Ed Anderson
RV-6A N494BW 170 rotary hours
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