Yes I guess you could get air trapped between the pump
and the injectors but it should escape through the injectors
when they are pulsed. On the other side of the pump it
is a different story.
High pressure pumps are notoriously hard to prime. That is why
they must be as close to the tank as possible.
The ultimate is having the pump in the tank like they
do it in the automotive world.
Paul Lamar ...No rotor no motor.
Ok, so apparently add another thing I need to plumb into the fuel
delivery system -- a pressure regulator. It's a real problem that I am
so clueless on EFI fuel delivery system design -- I keep discovering
things I need. This area is a missing piece on the website, Paul -- not
a word to be found. What little I have found elsewhere on the web is of
little use as well. Ah well... I suppose I will muddle through it all
eventually.
Here's my design so far -- those who have a clue, please chime in & tell
we where I am going astray...
Mark
Very important. Please ask. Not everything is on the web site
yet. Mark Wrathal has a good do's and don'ts on his web site.
http://members.aon.at/wrathall/rotary/
Looks like the high pressure pumps need to go next to
the shut off valve before the gascolater. The one way valves
are built into the pumps. What brand of pump are you
using?
Paul Lamar ...No rotor no motor.
Paul,
Can I ask an apparently dumb question? Why aren't people motivated to
imitate the experience of the auto world and put the high pressure pump
in or very near the tank?
Perhaps people are reluctant to run electricity into the tank for fear
of explosion. Is that a realistic problem when I never hear of exploding
gas tanks in cars? What's wrong with just outside the tank? I'm looking
at my Mazda OEM in-tank pump (which enters through the top of the tank,
with a gasket to seal it from the inside of the passenger compartment),
where the design has the wires external to the tank under the back
seat/hatch.
Is condensate or ill-fitting gas caps more of a problem for planes than
cars and that's the reason that we're trying to fit that gascolator in
there? (I have drained water on occasion from the 172s I've flown, so
it's not just theoretical concern.) But is that a fear left over from
carburetor days when a fuel-bowl full of water was an unsolvable problem
in the air? If high pressure pumps pulverize the water/fuel mix and push
it through the injectors, is that fear of water still a problem on the
same scale? (I'm thinking of the irony of working so hard to keep to
keep water out at the same time some high-compression racers install
water injection systems to avoid detonation.)
In a related question, if one had in-tank electric fuel pumps on each
tank (each with one-way check valves) feeding a common fuel supply line,
couldn't an electric switch right/left/both replace the expensive
mechanical fuel valve (assuming that the same switches triggered a
solenoid-controlled valve to switch the return lines to the appropriate
tanks)?
Electric fuel pumps could still be next to the tank with a small sump
with a drain in the lowest part of the tank to get rid of water. Yet I
rarely see fuel schematics that look like that. People want to make them
look like imitations of a carb-based aircraft design rather than an
fuel-injection-based auto design. How come? What am I missing?
I'll shut up now and re-take my seat at the back of the room.
Barry Gardner
Wheaton, IL
I think you are right about water in fuel injection systems.
The engine might briefly stumble but only briefly if you get a
dollop of water now and then. Easy to test on the ground.
IMHO the fundamental problem is maintenance of the pump. You
need a big enough hole in the wing root rib to get it in
and out of there. Most people don't want to cut a hole large
enough in such a critical structural area. I personally would
hesitate to do it with out the blessing of the designer or an
FEA on the structure. I am sure it could be done with enough
reinforcement around the hole. Also automotive pumps are designed
to be installed from the top and not the side of the tank.
That presents a streamlining problem. Perhaps something can
be modified to work. Also my guess is the pumps should be
replaced every five years whether they need it or not.
I would feel more comfortable with two pumps in each tank.
One being a back up. The critical scenario being out of
gas in one tank and having the single pump in the other
tank fail on a long cross country over inhospitable territory.
Paul Lamar ...No rotor no motor.
Paul,
I agree with the dual internal fuel pumps, and you don't reset
breakers when those babys pop.
R/ Tim
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