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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