I have a question for people with electronic fuel injection
systems. My
question involves the tank setup and return lines. On a Cozy with a
20b.
I have a sump tank and I planned on having check valves between
the wing
tanks and the back seat sump tank. ( I am using Infinity retracts
) I'm
not
using a tank selector valve. Only a fuel system on/off valve. I
want it
simple yet right. I have 1/2" stainless tubing between the tanks and
1/2"
outlet from the sump. Now I'm concerned about the return line
from the
Fuel
Injection. Will the pressure of the heated return lines keep the
check
valves from allowing more fuel into the sump? I plan on a vented
system in
all tanks but then worry that any pressure could push fuel out of the
sump
vent line. I planned on having possitive air pressure on the vent
lines
during flight by facing the outlets into the wind or "jetstream"
I've got notes from other past questions on this group that
suggest the
check valves but I'm not sure they are using EFI and I wonder
where they
are
returning the fuel.
Rick Irwin
I'll give my take on your questions Rick, have a few questions of my
own. Where in the system is your fuel on/off located? Are your pumps
external to the sump? This has created problems for others, I suggest
pumps inside the sump. I do not have a check valve between selected main
and sump on my system. How large is your sump? The fuel returning to the
sump (if that's where you choose to send it) is not under high pressure.
I do see merit in your concern that ram pressure on the main tank vents
could force fuel out the sump tank vent. Perhaps the sump tank should be
vented back to a main tank or main tank vent?
I do have check valves external to the sump on the outputs of the two HP
fuel pumps before the outputs join to one enroute to the engine fuel
rail.
Perry
Perry thanks much for your response. My pumps will be directly
outside of
the sump tank. Lowest point, following an on./off valve and a final
filter
. I didn't want the pumps in the tank because I didn't want to have to
worry about a large seal surface. The sump tank is where the landing
gear
typically go in a Cozy. I'm using infinity retracts. My sump tanks is
7.5 gallons. I just increased it up from 4.9 to 7.5+. I planned on
returning the fuel to the right tank and I want 1/2" line connecting
the two
main tanks to the sump. All tanks are vented. I am thinking of not
using
check valves as of late. The On/Off valve would be on the firewall
preceeding the fuel pumps. I haven't closed the sump tank yet. I'm
anxious
to figure this out so I can get my spar back on and wings mounted.
Rick Irwin
Rick what are the dimensions of the tank? It is a problematic if it is
wide.
Keep it skinny and tall. The fuel pick up on a wide tank can be uncovered
under certain flight conditions. Large tanks are not necessarily the
right
answer. In fact under certain circumstances that can work against
you.
Keep the bottom of the sump tank as low as possible. Put it behind the
firewall
and extend it up above the spar. Tall and skinny.
This one was in a fatal accident and we are not sure what happened
but it was a fuel feed problem for sure. Long story. I investigated it
for the FAA and NTSB.
These return flow, high pressure fuel systems, are tricky. They are not
just plumbing problems. They are 3D problems as the exact positions
and sizes of tanks enter into the configuration. All flight attitudes
must be considered.
Paul Lamar
In the 3rd picture it appears the sump is mounted very high, just below
the spar. And the fuel lines appear to be going "uphill" to get to those
ports. This doesn't allow much head for the fuel to drain to sump by
gravity. You can see the NACA inlet in the photo. My sump is probably at
least 6" lower and actually protrudes a bit into the NACA.
Perry
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