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Excerpted from
Group K Technical Documents.
See also
Alcohol in Fuel. At the high power we extract from these tiny, lightweight motors,
we have to care for them lest they start letting us down. The
following will help you to make the most intelligent fuel choices for
your favorite propulsion.
Combustion Details
To really understand the importance of different quality gasoline's,
it's necessary to take a trip with the piston.
We join the piston just as it starts moving upward from bottom dead
center. The fuel/air mix pushes out previously fired exhaust gases. Still
moving upward, the piston finally closes off the exhaust port to trap
these fresh gases in the upper part of the cylinder bore. From this point
on, each millimeter of upward motion increases the amount of pressure in
the cylinder. Along with this pressure increase, comes a significant
increase in the fuel/air mixture's "instability."
Instability refers to how volatile or how "ready to burn without
actually burning" the mixture is. Under ideal circumstances, the spark of
the plug should ignite this super unstable gas charge just a few
milliseconds before it ignites on it's own. By igniting the charge at this
ideal moment, the combustion takes place with the maximum amount force in
the minimum amount of time. That is, the flame front moves through the
combustion chamber instantly in one big perfectly timed bang that drives
the piston downward with the maximum amount of force.
All of this perfect mechanical wonderfulness hinges on an engine
combination that consistently maintains the fuel charge at the ideal level
of instability for that crucial moment of ignition. Unfortunately, we do
not fly in a perfect world. Combustion chamber temperatures vary
greatly—variations that affect the instability of the fuel charge.
Pre-Ignition & Pinging
Combustion chamber temperatures can peak enough to cause a hot spot
that will prematurely ignite the unstable fuel charge without the spark of
the plug. This is known as "pre-ignition". It also sometimes happens that
the shock waves caused by the first milliseconds of ignition can detonate
the very unstable "end gasses" at the outer diameter of the combustion
chamber. This is called "detonation" (or pinging).
While pre-ignition and detonation are technically different problems,
they are both promoted by the same problem. That problem is a gasoline
that is ready to explode before the optimum moment. The problem for fuel
chemists is to come up with a gasoline that resists pre-ignition and
detonation, yet still burns instantly when ignited. Chemists have learned
that they can accomplish this by "raising the octane rating" of the fuel.
OCTANE
Gasoline's "octane rating" refers to the fuel's resistance to
pre-igniting under very high temperature conditions, not the amount of
lead it contains. Early 1900's chemists learned that they could accomplish
this task easily and inexpensively by blending in varying amounts of
"tetra-ethyl lead". This additive is why high octane fuels were referred
to as "Ethyl". The lead in these gasoline's not only acted as an octane
rating enhancer, but (in four cycle engines) it also acted as a lubricant
for valve stems and a cushion for valves seats. However, as we know today,
the lead resulted in unacceptably toxic exhaust emissions. While engineers
struggled to make engines more completely burn each charge, chemists have
been given the job of increasing the "octane rating" of gasoline's with
less toxic substances. This has been no easy task, however the end result
has been affordable 92 octane (and premium priced 105 octane) unleaded
fuels. Since two cycle engines do not gain any side effect benefits from
leaded fuel, the absence of lead is no problem.
PUMP GAS ENGINES...RACE GAS ENGINES What's the difference? - In years
past, high performance engines required race fuel. The engine builders of
that time prepared engines with unnecessarily radical port layouts. This
poor style of porting resulted in very high peak rpms, and very poor low
end power. Compression was then hiked up to restore some of that lost low
end. The result of these two wrongs was not a right...it was a piston
eating engine. In fact, in many cases, even race gas couldn't solve the
terrible temperature problems that these engines had. Race gas just made
the problem less noticeable.
During the middle 1980's, Group K technicians were the first to develop
personal water craft (pwc) engine kits that were "octane specific". We
quickly learned that low octane did not mean low performance. In time we
found the few variables that permitted 92 octane engines to produce lots
of usable power. We found that with a given octane fuel, reliable
operation depended on the correct combination of three "operating
temperature" variables. They are peak rpm, compression ratio, and ignition
advance. Most stock engine formats could safely tolerate significant
increases of any two, but not all three. In time we learned that the best
results in performance and reliability were available by a well chosen
balance of all three. With a balance of this kind, these pump gas engines
were able to operate at nearly full output all the time, without any
detonation or pre-ignition. However we also learned that any significant
increase in just "one" of these variables would immediately create
temperatures that required the need for cooler running race gas.
It bears noting that upgraded cooling systems on 92 octane engine
formats could offer only slight decreases in excessive operating
temperatures in this situation. The technically preferred solution is
always to reduce the excessive heat at the source...the combustion
chamber. At a certain point, no cooling system on earth can exchange away
the excessive engine heat as rapidly as it is being generated.
Maintaining a "temperature balance" with a group of matched
modifications permits an engine to produce much more overall horsepower
with excellent long term reliability. That's why all Group K modifications
are designed and sold as packages or kits. Just about any mix of
miscellaneous bolt on parts can result in a performance gain of some kind.
However, an accidental wrong mix of incompatible parts and mods can result
in a string of chronic temperature related engine failures. Group K 92
octane and 105 octane kits net the maximum performance and reliability
because they are designed to approach all the limits of temperature
without ever stepping over the "excessive heat" limit.
Aviation Fuel
Aviation gasoline (or "avgas") is blended specifically for use in small
aircraft. It's also commonly used by many high performance engine owners
because of it's high stated octane rating (usually 100-110) and the
relatively low price compared to racing fuel. Unfortunately this fuel is
not all it appears to be. Avgas octane is rated on a different scale than
gasoline's intended for ground level use. What is 100 octane "av", is not
necessarily 100 octane "ground level". Besides this, there is also a big
chemical difference. Normal ground level race fuels are made up 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 somewhat weak engines often have difficulty taking
off with any extra weight. To help reduce this weight problem, aviation
gasoline's are blended with no heavy molecule end. This makes a gallon of
avgas weigh substantially less than a gallon of ground level fuel. 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 pwc racing engines, there is defiantly a compromise in power. This,
despite the fact that many pwc owners experience the desirable cooler
operating temperatures that avgas offers. In addition, some blends of
avgas will quickly separate from some oils used in premix situations. For
the above reasons, we do not recommend the ongoing use of 100% avgas, and
we will not prepare any "avgas" engine kits.
Despite all this bad news, running avgas (accepting the slight power
loss) is usually a better choice than burning down 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.
Marine Gas
The unusually high price of marina gas does not reflect some higher
quality, rather its a function of the captive customers. The normally low
grade of marina fuels are no better than regular auto fuel. Unless you
have personal knowledge of the marina's fuel, the Group K folks recommend
against using it.
Octane Boosters
Octane booster additives cannot turn a gallon of average quality fuel
into a gallon of racing quality fuel. They are essentially flame
retardants. They raise the octane 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 high
performance engine without damaging it but with a noticeable power
loss. Group K recommends the use of octane boosters only in very extreme
or emergency situations. In those situations we recommend using as little
as possible (no more than 3 oz. per gallon).
For more technical articles related to 2-stroke engines, please visit
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