Hi everyone, I know I saw some information on internal fuel pumps as
opposed to external pumps. But can't find the emails. Paul I believe
it was you who said the internal pump would be better for the P Port
fuel injected engine. Let me get to the question I'm in the process of
building my first tank and think if I'm going to install an internal
tank I should make provisions for it now. How ever I'm not sure what
that entails. Any feed back here would be greatly appreciated. Thanks
T.W.
Internal pumps are nice as you never have to worry about prime or vapor
lock. Trouble is you need two in each tank as a back up. You also need
an access panel large enough to get them in and out.
I assume this is a low wing airplane.
Paul Lamar ...No rotor no motor.
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RV8A, Leading edge tanks, two in each tank will make it tight. Getting
them in and out through the planned access panel would mean dropping a
wing to replace the pump.
How should they be connected electrically, two separate switches for
the back ups,
or both run all the time when on that tank. And the plumbing, they
should feed a single line out of the tank? So that means check valves
between pumps and the suction lines will be separate? So many
questions, I'm pretty sure I can construct the tank with the exception
of the rear baffle and inside rib while I work out the details and that
way I can move on to the right wing.
Paul what size of pump for the pport, Tracy sells the 95 PSI pump for
$121. But I believe that is and external pump. I need to pump a
minimum of 33 gal per hour for full power I think. So if I use two
pumps per tank then I could use smaller pumps. What if I used a single
internal pump for the primary and an external pump for the secondary.
Could I pull fuel through the failed pump? I could easily mount the
secondary on the firewall. Would fuel push through the secondary with
out to much pumping loss. Yeah I think I'm over my head here. I'll
have to make some sketches to figure this out. Stand by for more
questions. Thanks T.W.
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Well at 250 HP level you will need about 30 gallons an hour.
Both right and left tank pumps can be on for takeoff with the four pump
system.
Two switches, center off up for primary and down for back up could be
used to select the primary or the backup pumps. One switch for the
right tank and the other for the left tank. The pumps should have one
way valves built in to the output side. Most do. as a backup you can
install two one way valves out side the tanks before the supply lines
are teed together. You tee the outputs together in the tank or outside
the tank and then tee the outputs again to the high pressure fuel rail.
Do not attempt to use dynamic pressure type vents to help the flow. In
other words vents facing forward into the air stream.
In some case over about 150 MPH dynamic pressure can force fuel through
the system and out the other tank's vents.
The return flow MUST be switched with a valve back to the tank you are
drawing fuel from. VERY IMPORTANT! If you don't you can empty a tank in
a matter of a just few minutes by pumping fuel overboard through the
vents in the tank not selected.
DO NOT use a header tank with the return fuel going to the header tank
with this system. The header tank will be pressurized to 50 psi and the
return flow system WILL NOT WORK. Beside that a header tank would have
to be very robust indeed to withstand 50 psi.
Another advantage of the four pump system is the plumbing is about as
simple as one can get with a high pressure fuel system.
Red is return flow and green is supply.
External high pressure pumps might work but they are VERY VERY sensitive
to restriction or lack of head on the intake side. Not good on a low
wing airplane.
CAUTION! High pressure fuel systems are complicated and subtle. All
scenarios must be investigated very carefully indeed.
Paul Lamar
Having no experience at fuel system design, I wonder if it would be
simpler/lighter/safer to use a 3 pump scheme. 2 high pressure submerged
pumps, and one low pressure transfer pump.. If one of the HP pumps fail,
you run the engine from the other one, and periodically transfer fuel from
the tank with the bad pump to the tank with the good one.. I haven't
thought about how to make the plumbing work.
Matt-
That is almost the way Tracy does it. He uses two external high pressure pumps
and one low pressuer transfer pump. No selector valve. Fuel is returned to
the tank it cam from.
Paul Lamar ...No rotor no motor.
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