Charlie,

I installed a system similar to your proposed diagram on my test stand today.  I used one gerotor fuel injection pump taking fuel from a separate source and tee'd it into the line between the fuel pressure regulator and the dead-ended fuel rail.  When this pump was turned on, the fuel pressure in the rail increased by about 8 psi (that setup uses 5/16 OD steel tubing so the pressure increase with -6 tubing would most likely be less).  When the inlet to that transfer pump was unported, the fuel pressure returned to normal at which time I turned the transfer pump off.  With the transfer pump turned off, there was a dribble of fuel coming out of the that pump's inlet.  Apparently, that pump does not completely stop the reverse flow of fuel through it when there is pressure in the fuel rail.  As a result, when the inlet to the transfer pump was placed back below the level in the auxiliary source and the pump was turned back on, it immediately began pumping fuel into the pressurized line.  Any air drawn into the pump had been purged back out the pump's inlet.

If your transfer pump has the same imperfect check valve action, your proposed system appears to work fine in spite of my air lock reservations.  Your selector valve with the "off" position would prevent long term reverse flow of fuel through the transfer pump when it is shut off.  

FWIW

Steve Boese  

At the point you're at, what would be the downside to simply putting a pump in each tank, and using a rotary switch to select?  Having the main tank with the return wired in with the ignition, so that it is always running to avoid the return overfilling the tank.

On Mar 20, 2017 5:26 PM, Charlie England <flyrotary@lancaironline.net> wrote:

1st, let me define my delivery architecture. I like Tracy's idea of feeding the engine from one tank, and transferring aux tanks to the main tank. I understand the downside of being unable to switch tanks, but many a/c (including turbines) supply from one tank, so that's my choice. It avoids the need for a duplex fuel selector, which I like. Effectively, I have 3 'aux' tanks, and I'm using the stock (3 inlet) Van's RV fuel selector to feed redundant transfer pumps in parallel. 

I know that most have a separate return port in their tank(s) for regulator return. With my need for aux transfer, my original plan was to 'T' the aux transfer line into the regulator return line, which I'm pretty sure has been done before.

I'm currently working on installing both injection pumps in the fuel tank, conceptually similar to standard auto practice for the last couple of decades: no risk of vapor lock with the added bonus of a very clean installation. The regulator will be in the wingroot, just outside the tank, with the manifold pressure line running to that location for pressure control. (Deadheading fuel to the fuel rail has been done on both cars and a/c successfully; I believe it's an option on the new SDS system being marketed to Lyc drivers now.)

The recent thread on fuel pressure changes while running both injection pumps got me thinking. If it's typical to see only a couple of PSI change when running both pumps, has anyone considered running the transfer line into the regulator? The reason I'm considering this is twofold. It provides a 'final option' for short term fuel delivery if both injection pumps are lost, and, because the regulator is in the wingroot, I would need to run only one fuel line to the supply tank.

I'll be using gerotor style transfer pumps (positive displacement) and the aux selector has an 'off' position, so backflow won't be an issue.

A quick & dirty sketch is attached, diagramming the idea.

There would never be more than 2 pumps running at any time, since transfers would only happen in cruise flight. Can anyone poke holes in this arrangement? 

Charlie

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