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Pumping fuel backwards through an electric fuel pump
I've installed a 12-gallon fuel cell aux tank in the tailcone of my
4-place conventional low-wing plane -- intended for go-fast trimming when flying solo more than for its extra fuel capacity. As you know, a typical 4-place is terribly nose-heavy with only front-seat passengers and no baggage. The tank would be emptied if flying with a full load of passengers and baggage. Now I'm considering how to plumb it. My engine is gasoline, not fuel-injected. The simplest approach would be to vent the tank it to the outside, and run a single 3/8 hard tube between my left main and the aux tank. The aux tank is positioned towards the ceiling of the turtledeck so there's a fair amount of gravity-feed available for it in normal flying attitude to drain back into that same tank. I'd have a single electric pump to pump from the main uphill to the aux tank, with a shut-off valve in the line to keep it there. Opening the valve would let it gravity-feed back into the main tank ... slowly. That is, if it's OK to allow it to gravity feed "backwards" through the fuel pump. (I need to consider the possibility of the main tank gravity-feeding back into the aux tank in a prolonged steep climb. I'll need to remember to shut off the valve whenever a fuel transfer isn't wanted.) I'm wondering about installing a second electric pump in series at the aux tank end, pointing back towards the main tank, to considerably speed up the draining. Only one pump would be run at a time of course -- either to fill or to drain the aux tank. But when either pump was on, it would be pumping fuel "backwards" through the other one. I understand that the simple Facet pumps have no check-valve so this should be possible. But is it harmful to such pumps? I've got a more elegant approach (a little complicated to draw here) that would use a single pump and a double-stacked selector valve as used on a fuel-injected engine. The selector valve makes it so that in the "fill" position, the pump would pump fuel from the main to the aux, and in the "drain" position, that same pump would pump fuel from the aux to the main. This is the slick set-up, but is considerably more complex/expensive/heavy to implement. The fourth approach would use separate fill and return lines, with separate pumps, check valves, and shut-off valves. This would avoid venting overboard. But it's even more complex, and I hope to not have to go there. Surely others have solved this simple plumbing challenge before me. I welcome your ideas. :-) Greg |
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