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Good post, Todd.
Certainly any vapor lock is a dangerous situation,
whether in a Lycoming or Auto conversion installation. As Todd
mentioned I do use a ventless, very small header tank mounting on my lower
firewall. The two EFI pumps are plumbed to the bottom of this header tank
made out of an 8" length of 3x1 1/2" rectangular tube with plates welded
across both ends and fittings - including separate feed for each pump,
return from pressure regulator, inlet from wing tanks and a sump drain. I
also have plates inside the header tank with numerous 1/8" holes drilled in
them. The theory is that as he hot bubbly return fuel plashes down on the
plashes upon returning from the injector fuel rails, the bubbles will have
a better chance to collapse and return to liquid form.
I found when doing runs on my test stand that the
two EFI pumps produced a tremendous pressure differential inside the sump.
On one occasion, I had a heavy duty marine plastic fuel tank sitting on the
ground 24" below the EFI fuel pumps, I notice the engine began to run roughly
and checking around discovered I had forgotten to open the vent on the plastic
tank. The pressure differential created was sufficient to collapse this
heavy duty tank. I was impressed!. After than I knew there would be
sufficient pressure to pull fuel from my wing tanks without a boost pump.
But, I installed a boost pump (6 psi) never the less. The boost pump is
located between my wing tanks and sump and produces approx 6 psi.
My first encounter with a possible vapor lock
problem was when doing high powered stalls (high engine power/low cooling
airflow) on a hot summer day during my test phase. The engine started to
surge and I noticed my fuel pressure fluctuating and dropping as low as 20 psi
in its swings. I turned on the boost pump and the pressure stabilized and
back to flying. After that - I did three things which may account for
why I now have 260+ hours of flying in the Southeast and have had not further
problems.
1. I shielded the header tank, pumps and
filters from exposure to radiant exhaust heat by enclosing them in a foiled
covered fiberglass box.
2. I routed a 1 1/2" dia blast tube from a
NACA duct to this fiberglass box for cooling air.
3. I used 100LL avgas which has a much less
tendency for vapor lock (I started using 100LL for a completely different reason
than vapor lock..another story).
The only thing I would do differently had I to do
it over, is to mount the header tank is a cooler place like in the wing
root area. Understand I am not advocating this system - the potential for
problems if you overlook some factor is there - make no doubt of
it.
I do agree with Todd that my use of a very small
header tank may reduce the vapor lock problem. True, due to the small
amount of fuel, it could get heated to the vapor point quicker - however, I
believe that since the volume is small I am replacing that quantity of fuel with
fresh wing tank fuel quickly and frequently which would tend to preclude too
much of a heat build up as the fuel is quickly gone. If you have a much
large tank then it would take longer for the fuel to be heated to that point -
but, then also longer for the problem to be overcome.
I believe someone, Jim S?, mentioned that if there
is a vapor lock problem it is almost certainly occurring before the EFI pumps,
and I agree. Many EFI automobiles today are using a "no-return" fuel
system. But in those systems the fuel system is pressurized to high
pressure all the way from the in-tank pump to the engine - so with 20-40 psi
fuel pressure in the lines it considerably reduces the probability of vapor
forming. Although if you heat any fuel sufficiently (like a line next to
an exhaust header pipe) you can get vapor bubbles.
Todd's experience would indicate that no or very
small header tank eliminates or considerably reduces the problem.
IF anyone should decide to use a set up similar to
mine (which again, I am not advocating), I would strongly
recommend:
1. Use a small header tank, less than a quart
- perhaps around a pint.
2. Place it in a cool
location
3. Ensure that cooling air is routed to the
fuel system components (pumps/filters)
4. Have a boost pump between tanks and header
tanks (turned on for all take-offs and landings)
In Paul's case, I am not certain it was vapor
lock since he mentioned he found the his header tank cool to the touch.
Certainly not a definitive temperature sensor - but a reasonable one.
Certainly not to be discounted as a possible source of the problem and it
possibility needs to be confirmed or eliminated.
FWIW
Ed A
----- Original Message -----
Sent: Saturday, February 12, 2005 6:18
AM
Subject: [FlyRotary] Re: Vapor lock in
sump/header tank.
Hi
Paul;
Caution this is a long story relating my previous vapour
lock issues and how they may relate to yours. Recent comments have differed
about the need to vent or not to vent a sump/header tank. As I understand it
you have a sump tank in which it is gravity fed from your mains, then pumped
up to your engine. However IIRC a previous post in which you said you had
copied Ed's system which is a header tank that has boost
snip
Can you see where all this was going? I surmise that the greater
capacity of the header tank (as compared to Ed's) allowed some heat absorption
as it took longer to become affected, but once heated it was far less
manageable. My pilot workload was too great and became centred around fuel
management. I had so much time, effort and $$$ invested in making that header
tank work that I didn't want to let go of the idea, but one day I just had
enough and tore it out of there. I've now converted to a returnless system
with no header tank and I couldn't be happier.
What I'm trying to show is that there is more going on
with heat in the fuel system than expected. I'm not familiar with your system,
but the way I understand it, you have your main tanks draining by gravity into
your sump tank. Your FI return line feeds into this tank bringing heat from
the engine with it. I surmise that it is bringing enough heat to begin a
vapour build-up in your sump tank overcoming the gravity feed. How long after
your emergency landing did you restart your engine? If it was immediately and
it ran fine, then this theory may not be valid, however if there was sometime
elapse (while you kissed the ground & changed your shorts :-), then it
could be that the tank had time enough to cool and/or relive the pressure
allowing more fuel to enter the tank.
One way that I could see overcoming this would
be.....
A
vent line from the top of your sump tank up to the top of both of your
main tanks, but not tied into the main tank vent system. This would allow any
fuel vapours to immediately flow out of the sump tank, eliminating any chance
of vapour build-up, while allowing the cool fuel to continue to be gravity fed
to the sump tank. These fuel vapours would then flow into the main tanks where
they should immediately condense, preventing the loss of any fuel through a
direct atmosphere vent system. This would (may) only work if you have
your main tanks located above the sump tank. Having your FI pumps located at
or below the sump pump would surely be a help as well. But I would not expect
the tank to work without a vent or with a vent to
atmosphere.
I hope some of this is relevant to you and
helps.
Todd
Bartrim (top posted all the way to the
bottom)
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