"Kevin R. Walsh" wrote:
I take issue with the statement "Slide throttles are difficult to make
because they
tend to stick when there is a large pressure differential across
them."
Lets assume you are running at WOT, and suddenly want to snap the slide
shut. The plate, if instantaneously closed, could see a pressure
differential of 14.7 psi. More if you are turbocharging your engine.
Using your example of a 3.125" circular throttle body, and the 14.7 psi
differential, the slide plate is 7.6699 in^2, and the total force on
the plate is 112.7 lbf. That's certainly a lot. I'm assuming that
we're talking about a flat slide throttle here, not a circular slide,
and thus this load must be taken across two support surfaces, one on
either side of the plate, parallel to the slide direction. This is,
nominally 56.37 lbf per bearing. This is within the capabilities of
most dry teflon based bearings of relatively small diameter (1/4") and
certainly bronze bushings or ball bearings as well.
For the design, three things are required:
1) a positive throttle return spring which will snap the throttle shut
even against the full manifold pressure.
2) a set of bearings which can support the load and a design of those
bearings such that they do not overconstrain the motion of the throttle
plate. This centers mostly around the design of the bearing geometries
and the selection of the bearing material.
3) A positive seal between the slide plate and the throttle body. This
is actually less required because at idle some leak around the plate
will just mean closing the idle stop a little, not a big deal.
I've worked on racing motorcycles, and they commonly use flat slide
throttles. They have fairly forceful throttle return springs, and
pull-pull cable actuation for positive motion. I've never witnessed
one being stuck open, and they fairly routinely go from WOT to full
closed. This is not to say taht improperly designed, built, or
maintained slides cannot get stuck. They certainly can.
I am confused about the advantages to using the flat side versus a much
more simple butterfly. It seems you could simply use a slightly larger
butterfly valve instead of a slide and have the same pressure drop at
WOT (which is, incidently, the only place this argument makes a
difference). The butterfly is a restricion, yes, but the guides for
the flat slide would interrup the flow near the walls just the same, in
my opinion.
Kevin
Thanks for the calculations.
Your numbers sound correct to me. Perhaps teflon will work fine.
I am still going with the little ball bearing wheels and
a car.
True, a slightly larger throttle body should compensate for
the pressure drop of the butterfly.
A slide throttle however is perfectly streamlined at WOT if it is made
long enough for a hole in the slide plate to align with
the tube holes. I decided not to that for space reasons.
I will radius the second hole.
Besides all that a slide throttle is trick looking, very
rugged and simple in my case with a simple push pull steel rod going
straight back to the instrument panel and a big old knob. No
moving parts or springs in the throttle linkage to fail
as there are none.
Paul Lamar
My .02:
How much power do you think can be gained by these exotic throttles? I'd bet its a fraction of one HP. Given all the other development issues one faces with any new engine application, is this a battle worth fighting? Why not just use a "big enough" throttle body and be done with it? BTW, for a typical rotary powered airframe, how much hp does it take to increase the top speed by one kt? (I know the answer for a boat, I think it is almost the same for a plane).
Jon
But of course Jon. Rotary engines are so easy to install and we have all the
major problems so well solved we are down to discussing trivia :-)>
As I recall top speed is proportional to HP cubed while it is inversely
directly proportional to drag.
Paul Lamar
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