Svein,
> If the fuel pressure after the pump is equal to or higher than the
> pressure in the bottom of the tank, and there is no heating of the fuel
> on its way so far, the bubbles will disappear. The transition from
> liquid to vapour or from vapour to liquid is virtually instantaneous.
I thought (but I'm in no way an expert) that for a fluid to boil you
need some contamination which acts as some sort of trigger. Just like
bubbles in a glass of cola seem to form at certain spots. Once just a
bit of vapour has accumulated, other molecules hook in and the bubble
keeps growing.
So if the fuel temp is on the edge, once vapour bubbles exist it is
easier for other molecules to take the same route.
On the other hand, I see what you are saying. If there is no hysteresis
then bubbles should disappear under pressure. Then again, it would be
impossible to get vapor lock unless you heat the fuel. (Which is what
you are saying).
> My posting earlier today described the movement of a vapour bubble that
> exists downstream of (after) the pump.
Ok, I believe you. Then the problem would only be at the pump inlet and
not beyond. Still, the pump would then turn into a void. With a bubble
inside the pump the pump can not maintain pressure, so the transition
---From gas to fluid doesn't take place.
> Given the composition/characteristic of the fuel on board and given the
> fuel pressure, the only variable we can do anything with is the *fuel
> temperature*.
The engine was just started. The oil temp was about 60 degrees. We where
anxious to take off because of the heat in the cockpit. There was no
heat anywhere in the fuel system except for heat created by the sun.
Again, this is a 914 with a very high fuel return flow. Any heat
somewhere in the system will be carried away quickly.
The only thing remaining is the fuel pressure drop inside the suction
area, i.e. the fuel filters.
> In addition to fire sleeves on all engine compartment fuel and oil hoses
> and good shielding from direct exhaust pipe heat, I also use heat
> protection sleeves (silvery outer layer over insulation layer) placed
> outside the fire sleeve forward of the fire wall.
I have this too. Keep in mind that the 914 has no mechanical fuel pump.
The fuel enters the engine compartment to the airbox where the pressure
regulator is, and then returns back to the firewall. It is a very short
route, and fuel is flowing there with a rate of 1 liter per minute.
Vapor lock between the regulator and carbs is of no consequence, as the
needle valves of the carb just let any gaseous fuel through until some
fluid matter raises the floats.
> Given a choice of fuel, Avgas in hot weather/high altitude seems to be
> the best option to avoid vapour lock, as also confirmed by the very
> interesting posting today regarding LAAs restriction on Mogas use.
> Replacing engine oil and filter more frequently is an inexpensive
> consequence of using Avgas.
Despite changing the oil and filter, the engine contaminates
nevertheless. There are some interesting documents with pictures on the
internet from the British Rotax dealer. Avgas is just bad for the Rotax.
And it is more expensive. Enough reasons to investigate how we can
operate more reliably on mogas.
Thanks for sharing your insight.
I guess it is time to wait for an exceptional hot day, pour some mogas
in the poor thing, switch on the electric pumps, and then make some
pictures at various places in the fuel system about what is travelling
through the fuel lines. With our insights combined we would see a lot of
bubbles in the fuel pump inlet, and less (or none) after the fuel pump.
Frans
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