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SV: Europa-List: Vapour lock? Why?

Subject: SV: Europa-List: Vapour lock? Why?
From: Sidsel & Svein Johnsen <sidsel.svein@oslo.online.no>
Date: Tue, 31 Jul 2012 20:57:49
Frans,


In the tank bottom there are no bubbles at the temperature in question
(otherwise it would be boiling).  Fuel pressure here is atmospheric pressure
plus static pressure (negligible) from the fuel column above.  At the fuel
pump inlet, suction means lower pressure than in the tank, and here you
could get bubbles if the fuel temperature is not too far from the phase
shift point liquid-to-vapour, especially as the filter is upstream of the
pump (between the tank and the pump) since the filter restricts the flow a
little.  


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.  This phase shift follows
a certain temperature/pressure curve - there is no hysteresis (no difference
going from liquid to vapour, compared to from vapour to liquid).  With the
pressure you state in one of your postings, meaning considerably higher
pressure after the pump than at the bottom of the tank, I cannot see how a
gas bubble created due to suction can "survive" the passage through the
pump, unless heated up on its way through the pump/hose.


My posting earlier today described the movement of a vapour bubble that
exists downstream of (after) the pump.  If the heat downstream is such that
gas bubbles will form despite the relatively high pump pressure, then gas
bubbles will of course reach the carbs unless vented back through the return
line. 


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.


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.  Also T and X metal joints are insulated.
Aft of the fire wall I have used the heat sleeve several places as wear
protection, but it is so lightweight that one may use it all over.  You can
buy these at auto supply stores.  


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 LAA's restriction on Mogas use.  Replacing engine
oil and filter more frequently is an inexpensive consequence of using Avgas.
And never use Mogas purchased in the winter during the warmer period of the
year.


Regards,

Svein

LN-SKJ



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