Frans=0Aif the fuel boils in the carb bowls where does the vapour go? On a
912 it goes into the inlet manifold anyway so will cause rich mixture.=0AAn
d displace some air. In the 914 it will be at manifold pressure so may forc
e its way into the needle jet and again cause rich mixture.=0AGraham=0A=0A
=0A________________________________=0A From: Frans Veldman <frans@privatepi
lots.nl>=0ATo: europa-list@matronics.com =0ASent: Tuesday, 31 July 2012, 21
:47=0ASubject: Re: SV: Europa-List: Vapour lock? Why?=0A =0A--> Europa-List
message posted by: Frans Veldman <frans@privatepilots.nl>=0A=0ASvein,=0A
=0A> If the fuel pressure after the pump is equal to or higher than the=0A>
pressure in the bottom of the tank, and there is no heating of the fuel=0A
> on its way so far, the bubbles will disappear.=C2- The transition from
=0A> liquid to vapour or from vapour to liquid is virtually instantaneous.
=0A=0AI thought (but I'm in no way an expert) that for a fluid to boil you
=0Aneed some contamination which acts as some sort of trigger. Just like=0A
bubbles in a glass of cola seem to form at certain spots. Once just a=0Abit
of vapour has accumulated, other molecules hook in and the bubble=0Akeeps
growing.=0ASo if the fuel temp is on the edge, once vapour bubbles exist it
is=0Aeasier for other molecules to take the same route.=0A=0AOn the other
hand, I see what you are saying. If there is no hysteresis=0Athen bubbles s
hould disappear under pressure. Then again, it would be=0Aimpossible to get
vapor lock unless you heat the fuel. (Which is what=0Ayou are saying).=0A
=0A> My posting earlier today described the movement of a vapour bubble tha
t=0A> exists downstream of (after) the pump.=0A=0AOk, I believe you. Then t
he problem would only be at the pump inlet and=0Anot beyond. Still, the pum
p would then turn into a void. With a bubble=0Ainside the pump the pump can
not maintain pressure, so the transition=0Afrom gas to fluid doesn't take
place.=0A=0A> Given the composition/characteristic of the fuel on board and
given the=0A> fuel pressure, the only variable we can do anything with is
the *fuel=0A> temperature*. =0A=0AThe engine was just started. The oil temp
was about 60 degrees. We where=0Aanxious to take off because of the heat i
n the cockpit. There was no=0Aheat anywhere in the fuel system except for h
eat created by the sun.=0AAgain, this is a 914 with a very high fuel return
flow. Any heat=0Asomewhere in the system will be carried away quickly.=0AT
he only thing remaining is the fuel pressure drop inside the suction=0Aarea
, i.e. the fuel filters.=0A=0A> In addition to fire sleeves on all engine c
ompartment fuel and oil hoses=0A> and good shielding from direct exhaust pi
pe heat, I also use heat=0A> protection sleeves (silvery outer layer over i
nsulation layer) placed=0A> outside the fire sleeve forward of the fire wal
l.=0A=0AI have this too. Keep in mind that the 914 has no mechanical fuel p
ump.=0AThe fuel enters the engine compartment to the airbox where the press
ure=0Aregulator is, and then returns back to the firewall. It is a very sho
rt=0Aroute, and fuel is flowing there with a rate of 1 liter per minute.=0A
Vapor lock between the regulator and carbs is of no consequence, as the=0An
eedle valves of the carb just let any gaseous fuel through until some=0Aflu
id matter raises the floats.=0A=0A> Given a choice of fuel, Avgas in hot we
ather/high altitude seems to be=0A> the best option to avoid vapour lock, a
s also confirmed by the very=0A> interesting posting today regarding LAA
=99s restriction on Mogas use. =0A> Replacing engine oil and filter more
frequently is an inexpensive=0A> consequence of using Avgas.=0A=0ADespite
changing the oil and filter, the engine contaminates=0Anevertheless. There
are some interesting documents with pictures on the=0Ainternet from the Bri
tish Rotax dealer. Avgas is just bad for the Rotax.=0AAnd it is more expens
ive. Enough reasons to investigate how we can=0Aoperate more reliably on mo
gas.=0A=0AThanks for sharing your insight.=0A=0AI guess it is time to wait
for an exceptional hot day, pour some mogas=0Ain the poor thing, switch on
the electric pumps, and then make some=0Apictures at various places in the
fuel system about what is travelling=0Athrough the fuel lines. With our ins
ights combined we would see a lot of=0Abubbles in the fuel pump inlet, and
=========================0A
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