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Charlie,
I also would encourage you to consider transferring fuel directly into the main tank. It is unlikely that the high pressure pump used as a transfer pump will be able to pump air against the ~40 psi head pressure in the rail when
an auxiliary tank runs dry. So you probably would not have the engine stumble then. But to be able to transfer fuel by selecting another tank, the air would need to be purged from the transfer pump to get it working again. This could be done by shutting
the normally running injection pump off until the rail pressure dropped enough that the air in the transfer pump could be purged through the injectors. This might require more than just a stumble. The engine would need to be turning (probably windmilling
at cruise speeds) so the fuel injectors would be active.
My fuel system has only the two wing tanks. It initially used a low pressure Facet pump to transfer fuel. I didn't like the low flow rate of that pump. So I replaced it with a third high pressure injection pump. That worked
well but the possibility of developing more than 40 psi in the wing tank in case of a vent malfunction bothered me. The tank would most likely fail at pressures much less than this. So I ended up with a duplex valve and returning fuel to the selected tank.
A homemade duplex valve was used for several years while I saved up the $ for the Andair valve. This was a solution for me since I just had the two tanks to deal with.
I still have the extra injection pump and will set up a transfer system like you propose on my test stand just out of curiosity since what makes sense to me may not be how the actual system behaves.
Steve Boese
If I was going to go with the transfer fuel plan I would just have a separate inlet port into the main tank. That would eliminate any chance of air getting into the system.
I once had a very complicated fuel system as I have 6 tanks (inboard main, outboard auxiliary, wingtip aux-auxiliary) and I figured going to a common header tank would simplify things. But it was prone to vapour lock. But I had a solution for that which created
another issue, which i had another solution for, etc. Things got very complex.
Even when it was perfect on the ground I had several engine outs when in flight during my test period due to vapor lock or air.
Managing 6 tanks is always going to be more complex, but I simplified it a lot by going to a return-less system (deadheaded) like you are proposing. It worked well and eliminated all issues.
During my current rebuild I modified this again, but mostly just to clean it up as when I first modified it I was at the airport where I didn't have the shop resources that I have now.
Most modern vehicles now use a returnless system for emissions requirements so this is nothing new, so I think your plan to deliver fuel from the main tank is simple and reliable. I would just change the transfer line. Even if it seems like more work, it
will be worth it because if you don't do it now while you are at home in your shop, then if later you find out you need to do it after you are at the airport you will be cussing. Unless you are one of those lucky SOBs that have a fully equipped private hangar!
But still it is easier to do it while building.
If you are determined to try it with the transfer plumbed into the delivery line, then at least install an inlet port with a cap in the main tank so that it will be easy to change later. This is probably the best course of action, as then you get to experiment
which is what we're doing here, but have an easy fallback to planB.
Hi Andrew,
I get you on the 'complicated system' issue. That's one reason I'm considering this idea. And if I had any way to set up a gravity fed sump for the aux tanks, I'd eliminate the aux tank selector valve. :-)
Yeah, pumping air is the big vulnerability. If I go down this path, I'll definitely include a GEM optical sensor ahead of the transfer pump(s). I'll also do some 'pumping air' testing. One in-tank injection pump will be running during any transfer operation,
so testing will determine whether air from the xfer pump will cause engine stoppage, or just a stumble.
Thank you for the input. Anybody else? I'm not married to this, but I am giving it serious consideration.
On 3/20/2017 8:51 PM, Andrew Martin wrote:
Charlie, I think that would work. probably better to re-route the lines though, so fuel from Aux tanks naturally wants to go through the regulator rather than to the fuel rail. eventually you will leave a transfer pump on and suck air.
In effect you've got 4 efi pumps, maybe just a little bit extreme, especially if someone turns them all on.
Andrew
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