The much discussed EFI return problem now rears it's ugly
head. After sorting through all the opinions I'm ready to go with a
transfer
pump system as shown at http://kgarden.com/cozy/fuel.jpg

Paul Comte wrote:

***let me say first that this is not a Cozy discussion.  It is a discussion
of a fuel system in an experimental AC which Mr.. Slade is happily building.
I am delighted to add my two bits to the thread...

I surely must have missed a detail in here somewhere because I don't think
you have a problem.  If I've understood you correctly, you want to switch
the sump's "refill" source between the left and right tanks as you have on
previous AC.  I also understood you need to provide for return of the excess
fuel from the EFI distribution.

Isn't that fuel still under considerable pressure after the distribution
point?
Doesn't the sump get refilled via gravity?
And the fuel you need to return was recently drawn from the sump?
So...  What resistance (to return flow) will the EFI return line accept?
(they really don't spray return fuel into the top of Auto fuel tanks do
they?)
(Doesn't the EFI system pump push bypass fuel through at least ten feet of
line back to the fuel tank in an auto?  There has to be return line
resistance there...)
So what is the sump "pressurized" to by 26 Gallons of fuel in the (assumed
to be full) Strake?

My point is that (not claiming to be engineer...legal disclaimer etc...) I
can't see why you can't simply return the excess fuel by pumping it into the
sump.

You are drawing fuel from a tank containing the same head pressure that you
ideally will return the fuel to.  Say it is 10psi  (I have no idea what it
really would be) and you mechanically raise the pressure to 90psi, run past
the EFI system and then back to the sump.  The return fuel has potentially 9
0psi which could easily overcome the sumps head pressure.  The sump will
have refilled itself from which ever tank you have selected, but remember
that even while you are trying to return fuel to the sump, you are still
drawing fuel off that same sump. After the system is filled, I think the
drawing off and returning of fuel is simultaneous.

I think your net difference is null with the exception of the sump head
pressure variable dictated by the amount of fuel in the selected tank...
I can see no time when you will be returning MORE fuel to the sump than you
will be drawing out of it.  It is the same fuel, same lines and except for a
bit of heat expansion, the volume should not change...

If you didn't get my point, try this image.  A toy train running on a small
loop of track.  You can add cars until the engine almost touches the
caboose.  Even if they touch, the train can still run around and around,
never getting longer just because they went through the tunnel... ;-)

I would however, give lots more thought to a sump tank vent line.  If you
installed some type of "one way" check valve in the lines between the strake
tanks and the sump you will maybe want to vent the sump back to one tank or
the other.  It would be prudent to anticipate a situation where excess fuel
would not be able to exit the sump via the left or right feed lines...  In
that case you would possibly add a few psi to the sump, maybe not a good
thing.

Paul Comte
Milwaukee, WI
making flat stuff

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