Hi Svein,
> I do not know how much fuel is returned to the tank in a 914
> installation during ground operation.
The 914 has no mechanical fuel pump, only two electrical pumps. These
pumps pump at least 60 liters per hour through the system. Always,
including during idle. So the vast majority of the fuel returns to the
tank after passing the pressure regulator which is mounted on top of the
air box.
> consumption of 1 liter/hour it takes 12 minutes for 0,2 liter to flow
> through; at 5 liter/hour it takes 2.4 minutes.
For a 914 we are talking about seconds here.
> A pressure drop in itself will not cause any change in engine
> performance, but reduced pressure may cause too low fuel flow to the
> carb bowls.
On a 914 the fuel pressure is regulated over the airbox pressure. So, if
at full power the turbo kicks in and the carbs are pressurised, the fuel
pressure has to go up with it. On this particular mountain field the
ambient pressure was about 24" (this was indicated by my manifold
pressure gauge before starting the engine). The 914 has 40 inch of
manyfold pressure on take off power, slightly more in the airbox. The
fuel pressure has to remain 6 psi above this. See where this is going?
What I think has happened is that the mogas vaporised in the fuel
filters, due to the pressure drop, caused by the flow resistance of the
filters. The filters are before the fuel pumps. So any vaopur forming in
the fuel filters will flow to the pumps, which then turn into a void.
At lining up, I switched on both fuel pumps. Now two pumps where sucking
on the fuel filters, probably releasing a lot of bubbles at the instant
I switched on the second fuel pump. About 5 to 10 seconds later, the
engine hesistated. I think this was the moment where the vapour bubbles
reached the pressure regulator, at the very same time the turbo reached
maximum pressure and the fuel pressure had to go up in a similar fashion.
The only way to prevent this from happening (except for using avgas or
avoiding high and hot take offs) is to use a fuel pump mounted inside
the fuel tank, to get the pressure up before the fuel even leaves the
tank and avoiding the pressure drop at the suction side and filters
altogether.
> What speaks against vapour lock as the cause is your observation that
> the engine afterwards ran fine at low/medium throttle but not at high,
> and that the fuel pressure then dropped.
My fuel pressure gauge indicates relative fuel pressure, i.e. fuel
pressure on top of the airbox pressure. The pressure regulator keeps the
pressure at 6 psi above the airbox pressure at all times. So, regardless
of airbox pressure, turbo pressure or throttle setting, all I should see
is 6 psi of fuel pressure (which is the same as the carbs are seeing
because they live in a pressurised world; their vent lines are connected
to the airbox). In reality though the absolute fuel pressure has to go
up with the turbo pressure to maintain this relative pressure. This
relative fuel pressure could no longer be maintained when the turbo
pressure went up, hence the indicated pressure drop on the gauge. The
absolute pressure probably remained the same because it was during idle
already on the max that the pumps could deliver in this particular
circumstance. The vent line of the carbs is connected to the airbox, so
if the airbox pressure goes up but the fuel pressure doesn't follow, the
fuel will stop flowing into the carbs.
Hmm, the explanation is a bit chaotic but I think you will understand it
anyway.
If I had become airborn, I could probably have saved the day by
throttling back to a throttle setting where the airbox pressure would
fall below the fuel pressure. At worst this would have been 24" because
this was the ambient pressure at that altitude. I think I can maintain
altitude with 24" and probably even climb a bit, but sure it won't have
been fun. Provided of course that I had enough common sense to throttle
back against instinct before the engine stalled due to lack of fuel.
> Your question regarding AVGAS vs. MOGAS: Wikipedia states that /"Avgas
> has a lower and more uniform vapor pressure than automotive gasoline so
> it remains in the liquid state despite the reduced atmospheric pressure
> at high altitude, thus preventing vapor lock."/
Yep. I have seen that too. Lesson learned.
Frans
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