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<... Since the spring rate is an absolute value that is unaffected by ambient
pressure and the area of the valve face is fixed, I can see no mechanical reason
why this shouldn't work ...>
The pressure trying to OPEN the valve is MAP applied against the inside face of
the valve. The pressure trying to CLOSE the valve is spring tension PLUS ambient
pressure applied on the outside face of the valve.
You will have to somehow compensate for the ambient pressure outside the valve ...
Jim S.
Marvin Kaye wrote:
> "Russell Duffy" <13brv3@bellsouth.net> wrote:
>
> >>>I'd love to know how you're doing this, since it's exactly what I need.
> I've never seen any sort of valve that's referenced to MAP. The only thing
> I can imagine is an electric valve that's electronically controlled. <<<
>
> Since I'm nowhere near completion, this is one of those details that I've yet
> to work out... all I know is this is the right way to do this job and how I
> will attempt to setup my system. It seems to me that if the popoff valve is
> spring controlled, then it should be a simple thing to size and rate the
> spring to an absolute pressure. Let's go through this little bit of theory
> and you let me know where the holes are...
>
> Your recent post explained the relationship between MAP in inches of mercury
> and PSI of boost, that being 1psi = about 2" MAP. That says to me that at 36"
> MAP the deck pressure available is 18psi. Imagine that the popoff valve face
> has 2 square inches of surface area. Opposing that 18psi would require 9
> pounds of spring pressure against the 2sqin popoff valve flapper or poppet or
> whatever mechanical device is used to control the MAP inside the intake
> manifold. Once the MAP reaches 36" (18psi) the pressure on both sides of the
> valve flapper are the same... if the MAP goes higher it overpowers the spring
> and forces the flapper door open, venting the excess pressure beyond 18PSI.
> If you remember that the "A" in MAP stands for absolute, we are simply
> defining the amount of pressure available inside the manifold. Since the
> spring rate is an absolute value that is unaffected by ambient pressure and
> the area of the valve face is fixed, I can see no mechanical reason why this
> shouldn't work. (This is essentially how the alternate intake air doors work
> on most of the Lancair IV fleet... if the air filter clogs (ices up) and
> causes a bit of suction inside the air box, the little spring is overpowered,
> the alt air door opens and allows the intake to breathe using the air
> available inside the cowling.) It seems a simple problem in mechanics and
> not one of plumbing, at least to me. What am I missing?
>
> <Marv>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
--
Jim Sower
Crossville, TN; Chapter 5
Long-EZ N83RT, Velocity N4095T
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