Hello all,
I'm in the middle of designing my fuel system and wanted to see
what type of problems I might run into with the following
configuration. I've got an RV-4, plan on using two fuel pumps,
a purolator fuel filter/moisture separator, and was thinking of
using an electric 6 port switching valve. The idea was to keep
high pressure fuel lines out of the cockpit. I have since re-
thought the necessity of having the electric 6 port switching valve.
Eliminating it would eliminate one more thing that could fail along
with it's weight. So here's what it would look like:

One fuel pump per tank, either in the tank or close by, each
 feeding fuel to the fuel filter. This is assuming that the fuel
pump has an integral one way valve.(the filter has two inlets)
The fuel would flow from the filter to the two fuel rails,
 and the return would go to the manual three port valve mounted in
the cockpit. I'd have to select which tank to return fuel to manually.
A three position on-off-on switch would be used to select either pump,
which would in effect select tanks.

I'm somewhat leary of designing my own fuel plumbing, but I think
 that this design might have merit.
Please fire away if you see  any likely problem areas.

Brian Trubee
RV-4
Redmond,WA

Hi Brian,

This won't work. You tank up full for a long flight, start the engine and
idle out to the run way, by the time you get there you are pumping fuel out
of one tank breather like a garden hose. You are pumping fuel out of two
tanks,and returning nearly all of it (at idle most is looped back) to one
tank.

Paul had excellent points on electric valves over the years. Your mechanical
fuel valve gets a condition check every time you use it. If it gets stiffer,
you will notice it long before it seizes. The electrical valve will work
perfectly until the day the motor fails to move it. No warning.

I know as a fireman, you have seen things we haven't (thankfully), and
therefore have strong motivation not to put the HP fuel lines through the
cabin, but the easiest way to avoid this is to use the standard Vans fuel
system right up to the engine compartment, then mount the two pumps in
there, with return fuel (which is at tank pressure, not fuel rail pressure)
flowing back to the andair dual valve, and on to the selected tank.

The only modification to Vans standard is replacing the single pump with the
dual, and the extra return plumbing. There was a thread a while ago where a
guy got a dual valve made more cheapy.  You could also use two normal valves
and gang them on a common shaft or with a linkage.

Another excellent, proven option is Tracy's main tank/aux tank system, where
the fuel system draws and returns from one tank, and the other tank just has
an LP transfer pump feeding the main tank.

Ed uses the standard RV system firewall aft (correct me if I'm wrong Ed)
with fuel raill return fuel looping back to a header tank before the HP
pumps (same idea as the Subaru mentioned). Although this system has the
concern of vapour lock due to looping fuel heating up at low power settings,
Ed must have over 350 hours on his, and has operated into Florida heat.

Personally I'd rather have the HP pumps and filters in the chambers under
the floor on the RV-4 than forward of the firewall, as fuel spraying there
has lees ignition sources.

Regards,
Mark Wrathall

 
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