Sent: Wednesday, May 12, 2004 1:43
AM
Subject: [FlyRotary] Re: Runner
Length
Al, focusing on the losses at
these velocities and thinking that re-accelerating the column of air in the
intake tract is a bad thing misses the point entirely. That's like
saying a supercharger will drop horsepower because it takes power from the
engine to turn it.
Well; I don’t
think it misses the point entirely.
I was just pointing out that accelerating the air uses energy,
which is a negative in addition to other losses (reducing manifold
pressure). I didn’t mean to imply that you couldn’t get it back in a
greater charge – it’s just not free.
And, of
course we must keep in mind that inertial effect is separate from the
harmonic effect.
Here
is a link to a pretty good explanation of how to select the correct intake
runner
length:
http://www.hotrod.com/projectbuild/113_9907_efi/
They
say that the runner length should equal the third harmonic wavelength.
Al
I think one needs to use caution in attempting to apply "equations" and
"calculators" designed for tuning reciprocating engines to the rotary engine.
First, you need to know which port event they are using. For instance,
many (most?) used the pulse that rebounds from the closing port event and they
chose the runner length that will return the pulse at the proper time to the
originating port.
Some of them
return the pulse as the intake port is opening, some return it as the
intake port is closing. Clearly, these factor are factored into their
equations, but if you are uncertain about which they use you could be choosing
an non-optimum runner length. For example in the DIE of the rotary, it does
not return the pulse to the originating port but sends it over to
the other rotor's intake port. Also, the dominate "tuning" pulse used
in the DIE is not the "inertia rebound" (although it does play a role)
of the closing port, but
instead is the more powerful pulse generated by the residue exhaust gas
trapped during the exhaust phase which bursts out into the intake when the intake
port next opens.
All these factors can make a difference in what runner length is "right"
for your installation. So just need to be aware that apples and apples
are sometimes apples and oranges.
FWIW
Ed Anderson