On 07/31/2012 06:24 AM, Bill Sisley wrote:
> Have you checked the finger filter in the entry side of the fuel pump?
Good point. I didn't.
Since home I have digged deeper into this subject. I didn't mention that
I topped off the tank with 40 liters of mogas. I just assumed that apart
---From the ethanol and lower octane rating and absence of lead that the
fuel would have a similar boiling point. I now learned that this
assumption is totally wrong. Given the altitude and temperature it is
fully understandable that I experienced vapor lock, mogas just has to
vaporize in these conditions.
So, this is a lesson learned. I should have learned more about it
instead of blindly pouring in mogas because it is best for the engine
and so far always worked very well.
Which now poses a dilemma. I try to avoid using avgas as much as
possible because of the lead. But now I should try to avoid using mogas
when the temperatures are high and the altitude is high. Given my
personal preferences regarding weather and flying altitude this applies
to most of the longer trips.
It looks like the vapour lock develops at the inlets of the electrical
fuel pumps. The fuel has to be sucked through the fuel filters and this
drop in pressure causes formation of bubbles. I have actually observed
this happening when I was building the airplane and put the fuel system
in use on a hot day: I spent a lot of time searching for a supposed air
leak because of all the bubbles I saw passing the fuel filters, which in
the end turned out to be "just" fuel vapor bubbles.
Reading more about this subject I have learned that the best way to
prevent this from happening is to put a fuel pump *inside* the fuel tank
to get the pressure up before the fuel even leaves the tank, and not to
suck fuel through the filter but to push it through the filter. This way
one prevents the pressure drop which causes the fuel to vaporize.
It seems that this is a common practice in cars with fuel injection, to
have the fuel pump literally inside the tank or at least directly at the
tank outlet.
Of course such a fuel pump could fail (especially because it lacks a
fine filter before the inlet) so it is an additional risk. Then again it
is possible to use it only on one of the fuel tank outlets. Switching to
the other outlet then bypasses the failed pump.
I now just wonder how many of us are actually using mogas in hot
weather? Am I the only one struggling with this dilemma?
Frans
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