REFERENCE: http://www.howstuffworks.com/question90.htm
What does octane mean? [PARA]Answer
[PARA]If you've read How Car Engines Work, you know that almost all cars use
four-stroke gasoline engines. One of the strokes is the compression stroke,
where the engine compresses a cylinder-full of air and gas into a much
smaller volume before igniting it with a spark plug. The amount of
compression is called the compression ratio of the engine. A typical engine
might have a compression ratio of 8-to-1. (See How Car Engines Work for
details.) [PARA]The octane rating of gasoline tells you how much the fuel
can be compressed before it spontaneously ignites. When gas ignites by
compression rather than because of the spark from the spark plug, it causes
knocking in the engine. Knocking can damage an engine, so it is not
something you want to have happening. Lower-octane gas (like "regular"
87-octane gasoline) can handle the least amount of compression before
igniting. [PARA]The compression ratio of your engine determines the octane
rating of the gas you must use in the car. One way to increase the
horsepower of an engine of a given displacement is to increase its
compression ratio. So a "high-performance engine" has a higher compression
ratio and requires higher-octane fuel. The advantage of a high compression
ratio is that it gives your engine a higher horsepower rating for a given
engine weight -- that is what makes the engine "high performance." The
disadvantage is that the gasoline for your engine costs more. [PARA]The name
"octane" comes from the following fact: When you take crude oil and "crack"
it in a refinery, you end up getting hydrocarbon chains of different
lengths. These different chain lengths can then be separated from each other
and blended to form different fuels. For example, you may have heard of
methane, propane and butane. All three of them are hydrocarbons. Methane has
just a single carbon atom. Propane has three carbon atoms chained together.
Butane has four carbon atoms chained together. Pentane has five, hexane has
six, heptane has seven and octane has eight carbons chained together.
[PARA]It turns out that heptane handles compression very poorly. Compress it
just a little and it ignites spontaneously. Octane handles compression very
well -- you can compress it a lot and nothing happens. Eighty-seven-octane
gasoline is gasoline that contains 87-percent octane and 13-percent heptane
(or some other combination of fuels that has the same performance of the
87/13 combination of octane/heptane). It spontaneously ignites at a given
compression level, and can only be used in engines that do not exceed that
compression ratio. [PARA]During WWI, it was discovered that you can add a
chemical called tetraethyl lead to gasoline and significantly improve its
octane rating. Cheaper grades of gasoline could be made usable by adding
this chemical. This led to the widespread use of "ethyl" or "leaded"
gasoline. Unfortunately, the side effects of adding lead to gasoline are:
[PARA]* Lead clogs a catalytic converter and renders it inoperable within
minutes. [PARA]* The Earth became covered in a thin layer of lead, and lead
is toxic to many living things (including humans). [PARA]When lead was
banned, gasoline got more expensive because refineries could not boost the
octane ratings of cheaper grades any more. Airplanes are still allowed to
use leaded gasoline, and octane ratings of 115 are commonly used in
super-high-performance piston airplane engines (jet engines burn kerosene,
by the way).
--------------------------------------
Steven St.Laurent
C4i System Engineer
C4i Engineering Branch, PSD, MCTSSA
MARCORSYSCOM, U.S. Marine Corps
Office (760) 725-2506 (DSN Prefix: 365)
"Never be content with somebody else definition
of you. Instead, define yourself by your own beliefs,
your own truths, your own understanding of who
you are. Never be content until you are happy with
the unique person GOD has created you to be."
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