On 09/19/2013 07:07 AM, rparigor@suffolk.lib.ny.us wrote:
> The 914 returns a lot of fuel to the tank compared to the 912S. By
I have a 914 AND I have a one-way-valve in the fuel return line. No
problems, although with the engine at idle the fuel pressure tends to
climb to the upper edge of the green zone of my fuel differential
pressure gauge. But maybe that would even be the case without the one
way valve. At anything else than idle the fuel pressure is in about the
middle of the green zone.
> I think I remember reading it may
> not be a great idea to let fuel (especially auto fuel) cascade as I
> think it may somehow accelerate spoiling the fuel? Did you ever hear of
> this?
I have the same fear. Remember the returning fuel has an increased
temperature (might even be hot) due to its travel through the heated
engine bay, pumps, pressure regulator, etc! That's the reason why we
have a fuel return line after all; by keeping more flow in the system
than the engine requires we prevent vapor lock. Fuel consists of many
components, and they all vaporize at a different rate. It doesn't feel
right to have hot fuel cascading downwards from the top of the tank,
splattering around, having its components vaporize partly, then
condensing against the cold tank walls, and dripping into the fuel pool
again. Also the surrounding oxigen together with the high temperature of
the return fuel vapor might oxidize some of the fuel components at an
increased rate. We all know that fresh fuel is different than "old" fuel
and elevated temperatures and lots of contact with oxigen might play a
role in this "aging" process. My feeling is that it is healthier to
arrest the returning fuel immediately on arrival in a pool of cold fuel
without letting it get into contact with air and oxigen first.
For what it is worth, I try to use Mogas as much as possible, even when
flying in the French Alps (around FL100), have done this dozens of times
(Ilona's parents live there so we tend to visit them often with our
little aircraft) and I never had any problems with the fuel. Maybe it
just helps to keep the fuel temps down and not inviting too much
interaction with oxigen molecules.
> I suppose could put a dip tube to the bottom of the tank to
> prevent this?
So then what would be the point to route the fuel to the top of the tank
first while there is a shorter route to the same spot?
> On the 914 you don't want much in the way of back pressure
> as it can cause problems with proper functioning of the fuel pressure
> regulator.
One more reason to feed the fuel back at the underside of the tank.
Shorter hose, and less elevation of the fuel required to deliver it
where it ends up anyway.
Of course, a one way valve gives some resistance, but apparently it is
not too much. A valve can fail, but the most dangerous fail is when it
remains shut. If this happens, you will discover it during engine start
up. With the low fuel demand during that stage there is much return
flow, and if the valve stays shut it will quickly flood the engine
(apart from your fuel pressure needle pegging against the stop) Once
open, it seems utterly unlikely that the valve will spontaneously close
against the fuel pressure during flight. Once open, it will stay open as
long as fuel pressure is pushing against it and holding it there.
It gives some piece of mind that there is not an unclosable fuel hose
going to the engine bay. If the hose breaks, the fuel flow will stop by
itself.
Frans
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