Fuel 101

[mike.davis]

This is a message that was posted to the airsoob
list, and I thought that anyone considering an
auto conversion for their Rebel project might
find it interesting...

Mike

--------------------
OK, I promised more on fuels: This is a
compilation of information gathered from
several sources and my experience. I used to be a
teacher, and hopefully I've
presented this in a reasonably easy to follow
format.

As engines become more and more "high
performance" oriented, the quality of the
gasoline used in them is becoming more and more
important. Everyone wants to use low cost fuels
that allow maximum life from their engines. Most
Fliers know that you eventually have to "pay the
piper". Whether that payment is made buying
Avgas, or paying repair shops is a matter of
intelligent choosing. The following will help you
to make the most intelligent fuel choices for
your plane. To understand why we have different
grades of fuel, it's necessary to take a detailed
look at exactly what happens during combustion.
First, on a two stroke engine, as the piston
starts its upward motion from bottom dead center,
the fuel/air mixture fills the cylinder and
pushes out the previously fired exhaust gases. As
it continues upward, the piston finally closes
off the exhaust port to trap these fresh gases in
the top of the cylinder. From here on, any upward
motion increases the amount of pressure in the
cylinder. Along with this pressure increase the
fuel/air mixture's "instability" increases. The
fuel mixture becomes unstable, or volatile
or "ready to burn without actually burning".
Ideally, the ignition spark should ignite this
super unstable fuel mixture just milliseconds
before it ignites on it's own. By igniting the
charge at this ideal moment, the combustion takes
place in the most effective manner. This means
the flame front moves through the combustion
chamber instantly in one quick, smooth bang that
exerts maximum pressure on the top of the piston.
On a four stoke engine, the fuel enters on
the intake stroke as the piston descends in the
bore with the intake valve open, and valve timing
is set so the intake valve closes when the
maximum amount of fuel has entered the cylinder.
This usually occurs some number of degrees or
crankshaft rotation after the piston has started
back up the cylinder. From here on in, 2 stroke
or 4 stroke behave the same until after ignition.
All of this ideal mechanical genius depends
on an engine design that consistently produces
the ideal level of instability in the fuel charge
for that crucial moment of ignition.
Unfortunately the temperatures in the combustion
chamber vary greatly. These temperature
variations can adversely effect the instability
of the fuel charge. Sometimes the combustion
chambers get so hot that a hot spot in the
combustion chamber can ignite the unstable fuel
charge without benefit of a spark. This is known
as "pre-ignition". Sometimes the shock waves
caused by the first milliseconds of ignition can
ignite the very unstable "end gasses" at the
outer diameter of the combustion chamber causing
an uncontrolled flame propagation. This is
called "detonation" (or pinging). While pre-
ignition and detonation are technically different
problems, they are both similar in cause. The
cause is a gasoline that is ready to explode
before the optimum moment. The problem for fuel
chemists is to develop and produce a gasoline
that resists pre-ignition and detonation yet
burns instantly when ignited. The result of this
development is a fuel with a higher "Octane
Rating".
The "octane rating" refers to the fuel's
resistance to pre-igniting under very high
temperature and pressure. In the early 1900's
chemists at the Ethyl Corporation learned they
could accomplish this easily and inexpensively by
adding varying amounts of "tetra-ethyl lead" to
regular (or white) gasoline. This is why high
octane fuels were referred to as "Ethyl". The
lead "Hi-Test" not only acted as an octane
improver, but (in four stroke engines) it also
lubricated valve stems and valves seats. As we
now know, today, the lead produced unacceptably
toxic exhaust emissions. While engineers worked
at making engines more efficient, chemists have
been given the job of increasing the "octane
rating" of gasoline with less toxic substances to
reduce emissions. This has not been easy, but the
end result has been affordable 92 octane (and
premium priced 105 octane) unleaded fuels. Two
stroke engines do not benefit from lead content
of fuels - and actually last longer without lead
which causes cyl deposits which can, by
increasing compression ratios and holding heat,
actually cause pre-ignition and detonation.
In the past, it was assumed that any aircraft
engine had to be run on expensive leaded aviation
fuel. From the inception of leaded fuel, aircraft
engine manufacturers raised compression to
increase the amount of power they could expend
from the engines.
It has since been learned that low octane
need not necessarily mean low performance. It has
been found that with a given octane fuel,
reliable operation depended on the correct
combination of three "operating temperature"
variables; peak rpm, compression ratio, and
ignition advance. Most engine designs can
safely tolerate significant increases of any two,
but not all three. It has been determined that
the best results in power and dependability were
available with a well-chosen balance of all
three. With a balance of this kind, engines were
able to operate at nearly full output all the
time, without any detonation or pre-ignition.
However, any significant increase in just "one"
of these variables would immediately create
temperatures that required higher-octane gas.
Detonation or pre-ignition disturb
the "barrier layers" that insulate piston tops
and cyl heads from the direct heat of combustion,
causing rapid component overheating and failure.
Aviation gasoline (or "avgas") is blended
specifically for use in small aircraft. Av gas
octane is rated on a different scale than
gasoline intended for ground level use. What is
100 octane "avgas" is not necessarily 100 octane
"Mogas". There is a significant difference in the
composition of "Av-Gas" and "Mo-Gas". "Mo-Gas" is
composed of gas molecules that have a "light end"
and a "heavy end". The light end of the molecule
ignites easily and burns quickly with a low
temperature flame (as a piece of thin newspaper
would burn). The heavy end of the molecule is not
so easily ignited, but it burns with a much more
intense heat (as an oak log would). This heavy
end of the gasoline molecule is responsible for
the hotter, more powerful part of the combustion
process.
Small aircraft are constructed as very
weight conscious vehicles. That's because their
low horsepower engines often have difficulty
taking off with any extra weight. Av-Gas is
blended with little or no heavy molecule end.
This makes a gallon of avgas weigh substantially
less than a gallon of Mo-Gas (lower Specific
Gravity). They also have less of the very light
ends usually present in Mo-gas for easy cold
weather starting and fast warm-up. This helps
prevent vapour-lock at altitude. Since small
plane engines turn very low rpms and produce so
little power, the omission of the heavy end is
not a horsepower issue. However, for high output
auto and 2 stroke engines, there is definitely a
compromise in power. For air-cooled engines, this
can be an advantage, as with lower specific
power, the fuel also burns cooler. In addition,
some blends of avgas will quickly separate from
some oils used in premix situations so straight
"Av-Gas" is not recommended for use in pre-mix 2
strokes.
However, running avgas (accepting the slight
power loss) is usually a better choice than
damaging a high output engine on regular pump
gas. In this situation, the best choice is
usually a 50/50 mix of pump and avgas. That
provides "some" heavy molecule ends for the
engine.
There is a great convenience to buying
premixed marina fuel for machines that consume as
much gas as outboards, snowmobiles and 2 stroke
ultra-lites. You do not always get what you pay
for. The high price paid at a marina is more a
reflection of their awareness of a captive
customer than an indication of higher quality
fuel. In any case, the normally low grade of
marina fuels can work fine in the low rpm, low
power output motors in most boats. However, your
2 stroke ultra-lite may require much better
octane than most marinas offer. Unless you
have personal knowledge of the high quality of
the premixed fuel at your ride spot, I strongly
recommend not to use it!
Octane booster additives cannot turn a
gallon of average quality fuel into a gallon of
racing quality fuel. These additives are
essentially flame-retardants. That is, they raise
the octane rating of a fuel by making it
resistant to burning ... not by improving the
high temperature stability. Pump gas, with an
octane additive, can permit you to run a race gas
engine without damaging it. However it does so
with a noticeable reduction in power. Standard
premium unleaded fuels are achieved in



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