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I agree with everything Jim is saying about alcohol, auto fuel
& vapor pressure.
But I want to stress that there is absolutely no reason you should have to
worry about these things if your fuel system follows a few simple
guidelines. The first of which is KEEP IT SIMPLE! A car's fuel
system is pretty simple and there are millions of them operating with no vapor
lock problems, including people who drive at high altitudes.
None of the people driving cars ever even thinks about
whether there is alcohol in the fuel or not. I don't either, even in my
airplane.
The issue of composite compatibility with alcohol is a separate
question. After experiencing problems with a couple of aircraft fuel tank
sloshing compounds, I distrust all of them. The only fuel sealing agent I
trust are the polysulfide compounds. ProSeal is one brand of polysulfide
sealant but there are others. It is impervious to anything you might find
in any fuel, including the stuff formed when alcohol is mixed with water.
Tracy
----- Original Message -----
Sent: Wednesday, February 16, 2005 1:33
AM
Subject: [FlyRotary] Re: Bad day at the
airport
Paul wrote:
Hi, Jim....yes, I use the same gasoline as I
use in my car. (Regular unleaded). I think Tracy and Ed Anderson also
use it, as they mentioned that their spark plug life is greatly deteriorated
when they are forced to use 100LL. Guys, please step in here and
correct me if I am wrong. Right
so far. Unleaded is MUCH better for your engine than Avgas.
Regarding
alcohol, I am not overly concerned with it. I coated my fuel tanks with 3
coats of the fuel tank sealant that comes with the kit. (Smells like dead
fish). It is supposed to be imperviant to auto fuel and
alcohol. Alcohol is not bad for
composites. Water is not bad for composites. Alcohol and water
mixed is VERY corrosive to composites. If you have alcohol, you WILL end
up with some alcohol / water mixed. The stuff you used on your tanks
might be OK if it doesn't leak AT ALL, but how do you find that out?
Your "bad day" keeps sounding like vapor lock, and some Autogas is much more
volatile (read prone to vapor lock) than Avgas. You need to check a
bunch of suppliers around and determine the vapor pressure of the gas they're
delivering to their clients. You want the lowest number you can
get. Avgas has a VP of 6.0 or so. Some unleaded is down around
7. Some more could be over 10. That's where you get into
cavitation / vapor lock / etc. problems.
Alcohol might very well
not be a factor where you live, but in any event it's very easy to
detect.
Time will tell. Paul
Conner
-----
Original Message -----
Sent:
Tuesday, February 15, 2005 12:28 AM
Subject:
[FlyRotary] Re: Bad day at the airport
<... I was using auto fuel
(regular). I have a fuel return line and it goes
...>
DAMN. I hadn't heard about that. Paul, that's a
really important factor. Maybe I lost it in all the chatter going
on. Auto fuel can be pretty close to avgas in volatility
(Vapor Pressure) but for the most part it's anyone's guess. Avgas is
regulated at max~ 6 psi Vapor Pressure. Mogas used to get as
high as 16-20 psi VP. That high a pressure can petty much guarantee
vapor lock under high under hood / cowl conditions. As time went on
and more and more cars got MPFI and environmental regulations progressed,
Mogas vapor pressures came down some. IIRC Calif has a 7-8 psi max
vapor pressure, and places with strict environmental regulations (like
cities who aren't making their air quality numbers) are more likely to
require lower VPs. Trouble is, VP can vary widely across various
sources of Mogas.
I would guess that anywhere that requires gas
pump nozzles that capture vapor as you fill your tank will have lower VPs,
but sadly, they are also most likely to have alcohol added to mogas.
Alcohol is a major NO NO for aircraft. Mogas fuel systems should be
very carefully designed to prevent vapor lock or you are apt to get into
some VERY SERIOUS SHIT!!. Like there have been maybe 80,000 Mogas
STCs issued to Lycs and Conts. NONE to ANY certified engine
that's injected.
Which is not to say that you can't make a system
that runs reliably with higher VP Mogas, but you need to be very VERY
attentive to the issue, both in design and operation. You also have
to figure out a way to find out what local VPs are. Alcohol is easy
to detect and distributors know how much they're adding to the fuel.
Good info on VP is more difficult to come by and it might vary some from
one distributor to another.
I've been researching Mogas and
recently gave a little presentation to my EAA chapter and a couple of
flying clubs. It's wonderful stuff for your engine (lead is really
terrible) but you need to be very careful to not allow ANY alcohol into
your airplane and be very careful about checking and monitoring
VP.
Unleaded Avgas is coming, but VP will be an issue with it ...
Jim S.
Mark R Steitle wrote:
Paul,
It looks like
the group has focused in on the fuel system, or more specifically the
sump tank as the likely culprit. I suspected you might have been
running mogas. Its my understanding that auto fuel has a much
lower vapor pressure than avgas and is therefore more likely to vapor
lock.
Just a SWAG,
but I think the engine and related systems got hot enough to heat the
fuel in the fuel rails so that once it passed the pressure regulator
(pressure drop) it started to boil, similar to when you open a hot
soda. With the return fuel going to an unvented tank, the boiling
fuel expanded enough to cause pressure in the header tank so as to
prevent fresh fuel from flowing into the tank. Eventually, the
header tank ran dry and the engine quit. You sure handled the
situation like a pro. Hope I never have to face that
scenario.
Mark S.
Hi, Mark...my humblest
appologies for taking so long to respond. I am whittling down the
200 plus messages in my inbox. I was using auto fuel (regular). I
have a fuel return line and it goes to the top of my aluminum sump tank
that is located on the cool side of the firewall (inside the cabin,
behind the rear seat). The return line is -8 in size. Hope this
helps. Paul
Conner
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