I was thinking about doing some evaporative test on avgas verses
different brands of auto fuel. I notice a distinct difference in the intake
manifold cleanliness  between the runners with the fuel injectors upstream

and the

fuel injectors in the center housing. With the injectors upstream
the intake runners feeding the outer ports are far more dirty and lacquered up.
What is that stuff in auto fuel that causes that mess?

Paul Lamar

Paul,

Modern auto gasoline is the product of fractional distillation, which
permits a lot more gasoline from a barrel of oil.  Basically they break
up a lot of the longer chain hydrocarbons into shorter chains and can
use these as part of gasoline.  A side effect is that the shattered shorter
hydrocarbons have broken ends that are active sites for polymerization.
The pieces want to join back together more like they were for millions
of years.  This is much of the ick found in intake systems and esp
in fuel tanks if auto gas sits for a long time.  There are stabilizers that
seem to work pretty well, but basically auto gas is a relatively unstable
mix of many different man-made hydrocarbon chains.

The original gasoline was a natural product that condensed out when
natural gas was being tapped.  They separated it and drained it on the
ground, of course finding out how dangerous this was.  This natural
gasoline is extremely stable (millions of years), but is a tiny amount from
a gas/oil well.

I understand that avgas is more stable because it is more of a uniform
non-broken hydrocarbon chain.  Of course, this is always changing.

I have used autogas for 15 years in my C150s, Long Ez and C182.
It runs fine, but I avoid leaving it in the tanks for extended periods.
If you fly rarely, fill up with 100LL and have fewer gunk deposits.
If you fly a lot, and your compr ratio is low enough, car gas works fine,
esp with gravity feed fuel systems.  Pumped systems are much more
prone to vapor locking.  Be  careful with pumped systems and auto gas,
my LE is semi-gravity pumped system (carb is about 4-6" below the
bottom of the tanks).  A true low-wing pumped system should have
the fuel pushed from the tanks rather than pulled.  Pushing (pump
in tank) has essentially solved auto vapor locking.

Bill
Long EZ builder & pilot
mechanical engineer

 
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