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Exposing the Myths of High
Octane Fuel and the Definition of Detonation
It is often stated that " The higher the octane,
the slower the burn" AND there is PLENTY of discussion on the internet that
would support this statement... BUT... It is my opinion that it is really not
that "cut and dry"
First, a little tech info..
Octane is the fuel's ability to resist
detonation. The higher the octane, the higher its resistance to detonation..
This is understood.. It is not this article's intent to dive into how these
octane numbers are derived. Suffice to state that different companies/countries
derive these ratings quite differently. We can leave it at that for the time
being.
Detonation occurs AFTER IGNITION... This
is very important point in the discussion. MANY confuse/equate detonation to
pre-ignition. Pre-ignition is just that.. PRE IGNITION and occurs BEFORE
IGNITION.. So, detonation and pre-ignition are very different. Now,
pre-ignition will generally lead to full blown detonation.. but it is its own
entity and should be treated as such.
During the compression process, the fuel/air mix
is being "squeezed" into a small area. Shortly after this squeezing process
begins, ignition occurs. Ignition will start a flame kernal / flame front that
will propagate and expand from the ignition source (generally a spark plug in a
gasoline engine) outward towards the oncoming piston. Meanwhile, the
fuel/air charge is still being compressed and forced into the combustion chamber
area. This compression creates a pressure and this pressure creates heat. The
flame kernal is also creating heat and aiding in the rapid rise is pressure.
Side note: The speed at which
this flame kernal propagates is ,greatly, influenced by the head geometry, the
amount of pressure surrounding it, the residual "mix" present and its chemical
makeup.
OK.. back to the topic , at hand... So, we have a
rapid expanding flame kernal, a fast approaching piston, and a fuel/air mix all
occupying the same space and this space is getting smaller, VERY QUICKLY! This,
my friends, is compression in an internal combusted engine. When the temp
gets to a point that this fuel/air mix FULLY ignites, then we have combustion.
The key to proper combustion is to have it occur
at the best time to extract the most energy from the fuel/air mix , at the best
rod angle to impose the max amount of leverage on the rod, and to perform work
on the the crank train via force.
IF the combustion process before or after the
above criteria are met, then you still have combustion but it is not optimum
combustion. For this tech article, we will only concern ourselves with
combustion that occurs too soon with respect to the rod angle. This is what we
will term uncontrollable combustion or DETONATION.
Detonation from Wikipedia:
Detonation is a process of
combustion
in which a
supersonic
shock wave
is propagated through a fluid due to an energy release in a reaction zone. It is
the more powerful of the two general classes of combustion, the other one being
deflagration.
In a detonation, the shock compresses the material thus increasing the
temperature to the point of ignition. The ignited material burns behind the
shock and releases energy that supports the shock propagation. This
self-sustained
detonation wave
is different from a
deflagration,
which propagates at a subsonic rate (i.e., slower than the
sound speed
in the material itself). Because detonations generate high pressures, they are
usually much more destructive than deflagrations.
Detonations can be produced by
explosives,
reactive gaseous mixtures, certain dusts and aerosols.
In more simplistic terms (via RK Tek): Detonation
is when the charge is prematurely combusted due to very high heat/pressure. This
is more of an explosion than a combustion and the forces produced from this are
VERY large and are super-sonic in nature.
So, why is detonation so bad? After all, you have
extreme pressure occurring at very high speeds. This should produce some serious
force on the rod and spin that crank with much more authority. Yes, it produces
forces that we would love to have acting on our crank, unfortunately, these
great forces occur at the wrong time in the stroke. They occur at a rod
angle that causes great STRESS on the crank, piston, bearings, and rod. So, this
force becomes destructive vs. productive and since the force is so powerful, it
WILL cause engine damage if not addressed.
So, how do we address and prevent the onset of
detonation?
1) Proper combustion chamber design (i.e.
geometry and volume) See RK
Tek for this variable.
2) Proper ignition timing (adjust timing until
optimum)
3) Proper Fuel amount and Octane (resistance to detonation) fuel.
OK, FINALLY, we get to fuel octane...
The octane rating of the fuel is directly related
to its resistance to detonation. The higher the octane the higher pressures/heat
the fuel can withstand before ignition. The rate at which this fuel burns has
VERY little to do with the octane rating. The chemical soup of the fuel will
determine the fuel's burn rate under IDEAL conditions. Since we KNOW that
under combustion (active radicals, residual un-burnt mixes , squish action etc.)
is NOT IDEAL, then this burn rate can not be properly established nor
calculated. We do not need to know this burn rate because we will adjust our
ignition and combustion chamber design to accommodate ANY burn rate. So,
it is really irrelevant for us.
Does the higher octane fuel burn at a slower rate
than lower octane fuel? IMO.. MAYBE, SOMETIMES, YES, and NO.. See above..
the rate of burn is dependant on many variables but to state that just because
the octane is higher, then the burn is slower, MAY NOT always be a correct
assumption.
Don't believe me?? Why should you? there are
countless articles contradicting what I just said.. Well.. see an interview from
the Sunoco Engineer below (I will highlight the best parts LOL)
Popular Hot Rodding Magazine, January 1998
By Scott Parkhurst
Octane is a measurement of a fuel's resistance to
ignition. Ideally, the air/fuel mixture will ignite at the proper time and burn
smoothly through the power stroke. The idea is that one powerful combustion of
the air/fuel mixture is better than several randomly-ignited small flame fronts.
When you can precisely control the point at which the fuel will ignite, maximum
performance of the engine can be achieved, and power-robbing knock and ping will
be eliminated. Knock and ping are a result of abnormal ignition, or multiple
flame fronts colliding within the combustion chamber during the compression
stroke.
All reputable fuel manufacturers determine the octane
rating of their gasoline in the research lab using a special, dedicated single
cylinder engine. Comparing the gasoline to a series of standard reference fuels
in the test engine results in either a research octane number (RON) or a motor
octane number (MON) depending on a set of operating conditions. The RON is
determined with the test engine operating at 600 rpm, at standard barometric
pressure, and the intake air temperature set at 125 degrees Fahrenheit. RON is
primarily used to address part-throttle knock and ping problems. The MON
addresses wide open throttle operation and is determined with the test engine
spinning at 900 rpm, also at standard barometric pressure, and the intake air
temperature pumped up to 300 degrees.
The best predictor of a fuel's performance in a
street/strip machine is the Anti-Knock index (AKI). This is simply the average
of the RON and MON numbers, or (RON + MON) / 2. Most all octane ratings posted
at the pumps are determined by this AKI formula, and are the minimum values you
could expect to see. The minimum octane requirement
of your engine is determined by several variables besides the compression ratio.
The engine and cylinder head configuration, air/fuel mixture, timing, coolant
temperature, atmospheric pressure, relative humidity, and ambient air
temperature will also affect the octane required to make your mill produce
maximum power.
The burn rate of a fuel is a measurement of the time
required for complete combustion of the air/fuel mixture. The notion that octane
ratings affect the burn rate of fuel is about 180-degrees from reality; burn
rate is a function of several variables, and the two are completely independent,
although there is generally a correlation between octane ratings and burn rates.
To give you a good example of this, we contacted Jim
Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal,
which is our fastest burning fuel, and coincidentally one of Sunoco's highest
octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for
those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel
has to burn pretty quickly to achieve complete combustion".
Octane boosters offer little help in the quest for
higher octane. Most popular street-legal octane boosters claim increases in
octane ratings up to five points, and those boosters intended for off-road use
only claim up to seven points. That's a lot of octane to hope for simply by
pouring an additive in a tank. Sunoco told us that before they launched their
GT-100 Unleaded retail pilot program, they wanted to be sure that a 100 (R+M) /
2 octane street legal fuel would be of value, and that enthusiasts would not be
able to get the same (or better) results using an octane booster. Nine of the
most popular retail octane boosters were put through a series of tests to
determine where the consumer could get the most bang for the buck. The test
results were verified by an independent testing facility, using several brands
of regular unleaded and premium gasolines, just to make sure everything was
legit.
According to Mark Borosky, Vehicle Test Engineer for
Sunoco,"Of the nine octane boosters tested, none showed a significant increase,
and one actually lowered the octane number of the test gasolines."Testing
repeatedly showed a maximum increase in octane of 3.5 points by only two of the
six street legal octane boosters when the recommended treatment rate was blended
with lower base 87 octane gasoline. The best the remaining four products could
muster was less than a one point increase. "While clearly no one would actually
use an octane booster in a low base octane fuel, we wanted to give the
manufacturers the benefit of the doubt relative to their claims of five-to-seven
point increases," explained Borosky.
When tests were performed using 93 and 94 octane fuel,
even the two best products from the previous tests produced a disappointing 1.5
to 2 point maximum increase. The remaining four street-legal octane boosters
showed less than a .5 point increase. Those products designated for offroad use
only didn't fare any better than the street-legal products. Subsequent tests
where the dosage of octane booster was doubled, tripled, and even quadrupled
produced only minimal improvements in octane, regardless of the base octane
number of the test gas. In fact, quadrupling the treatment rate of the most
powerful additive produced only a 3.5 point increase in octane when added to 93
premium, resulting in a cost of $3.25 a gallon.
An alternative path to octane euphoria is to blend
gasolines of different octane levels yourself. It's easier than you may think,
safe, and the results are predictable. The formula for mixing gasolines of the
same type is pretty straightforward. When you mix a 50/50 blend of two unleaded
fuels, simply average the two octane ratings to determine what's in the tank If
you mix 94 and 100, you get 97. The same generally holds true for leaded
gasolines, assuming the lead content is
nearly equal.
Blending a leaded fuel with unleaded, however, pushes
the octane up a bit more than the math would suggest, due to the effect of the
lead. Just a gram or two of lead blended into the unleaded fuel will raise the
octane number significantly. Commercial leaded racing fuels contain anywhere
from a trace to six grams of lead per gallon. If you were to mix 50 percent 110
octane leaded fuel with 100 octane unleaded, you would actually end up with an
octane number around 106 to 107. Keep in mind that even the smallest amount of
lead or leaded gas line with unleaded, could spell the end of your catalytic
converter or oxygen sensor. The same holds true for using octane boosters
intended for off-road use only. A word to the wise, check for any lead content
in all the additives you might mix with your unleaded gasoline. And check with
your state emissions regulations for street use.
We asked Sunoco's Wurth about using aviation fuel in an
automobile engine. He was emphatic when he said, "Don't do it. Even though
Sunoco is a major producer of aviation fuel, this fuel is specifically blended
for aircraft engines. Aircraft operate under very different conditions than
automobiles, and the fuel requirements are quite different as well. Aircraft
engines generally.... run within a very narrow rpm range. There's no need for
transient throttle response in an airplane because after the pilot does the
initial engine run-up, the throttle is set in one position and the rpm doesn't
normally change until landing. Also, airplanes fly where the air is cold and
thin, and the atmospheric pressure is low. These are not even close to the
conditions your street machine will see on the ground. Also, since most
piston-driven aircraft cruise at 3,000 rpm or so, the burn rate of aviation gas
is much too slow for any high performance automotive applications."
What is it that makes race gas so different? What's it
made of? Sunoco tells us their GT PLUS 104 octane unleaded race gas is only
15-20 percent traditional gasoline, and about 85 percent additives! Actually
there are about 120 different chemicals in GT PLUS. One reason it isn't street
legal is the high oxygen content. The EPA requires that the oxygen content of a
street legal fuel cannot exceed 2.9 percent. GT PLUS is about 3.5 percent
oxygen. This fuel is light in weight at only 6.14 lbs-per-gallon. The high
oxygen content improves the octane, and when the induction system is properly
calibrated, this fuel will help make additional horsepower. The high oxygen
content has a supercharging effect, since 3.5 percent oxygen is the equivalent
to about 17 percent more air. Different fuels can actually alter horsepower
5-to-10 percent or more.
We wanted to to know more about the different types of
race gas Sunoco had, and didn't realize there were five different types of
racing fuel alone.
GT-100 Unleaded, is a clear fuel with a pump octane of
100, and will handle compression ratios of up to 12:1, and is street legal in
all 50 states.
GT PLUS, is also unleaded, and is rated at 104 octane.
It is suitable for compression ratios up to 14:1 and is colored light blue. It
will not harm oxygen sensors or knock sensors in computer controlled engines. It
is not street legal.
STANDARD, is a leaded fuel rated at 110 octane, is
colored purple, and is intended for drag racing, road racing, and race boats.
SUPREME, is also a leaded fuel and is dark blue. It was
developed to help resist vapor lock and meet the demands of sportsman, modifieds,
offshore powerboats, and endurance racing where engines regularly run in excess
of 7000 rpm.
MAXIMAL, we mentioned earlier, is colored red, has 116
octane, and is leaded. It is intended for exceptionally high performance
applications like Pro Stock where extremely high cylinder pressures are common.
Its extremely fast burn rate is satisfactory where rpm exceeds 10,000.
Now that you're an expert on gasolines, you probably
would like to know where to buy and store the stuff. If you are fortunate enough
to live in the mid-Atlantic states, you can take advantage of Sunoco's GT-100
Unleaded retail pilot program and get 100 octane race fuel at pumps located at
select Sunoco stations. The rest of us have to purchase from local speed shops,
at race tracks, or directly from Sunoco distributors.
When you plan on buying fuel in quantity, say a
55-gallon drum, you'll be happy to know that racing fuel has a shelf life of
about a year, if you store it properly. The container must conform to all safety
standards, and should be made from metal or polymer. Make sure the container is
opaque and solid in color. The white plastic jugs we see at the track should be
used for short-term storage only. They let in sunlight, which will affect the
fuel. The lead in leaded fuel and other chemicals in unleaded fuel are
photosensitive, and will dissipate if they am exposed to the sun. Keep any
container tightly sealed to prevent evaporation.
So, the next time somebody tells you
that higher octane fuel burns slower, just refer him to this article.
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