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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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