All gas stations have these Octane Ratings listed on
their pumps; they generally offer three forms of gasoline, regular, super,
and premium. To really understand what they do, you must understand
how gasoline engines work. This project contains a description of this;
to see it.
The spark plugs in gasoline engines are timed
to give off a spark at a specific moment, so that the pressure in the
engine cylinder, caused by the energy from the burning gasoline, is at its
greatest at a point after the cylinder is moving down in the engine. To
do this, it must spark before the cylinder has reached maximum compression.
Compressing matter creates heat, and sometimes this heat is enough to
cause the octane to spontaneously combust before the cylinder has reached
maximum compression. This creates an enormous amount of pressure forcing
the cylinder backwards, essentially trying to turn the engine backwards. This
pressure causes the cylinder to jam back up against the drive shaft, producing
a thunking noise. When this situation continues for several piston
cycles, a knocking sound can be heard. This is called 'Engine Knock'.
Because of the immense strain it causes on the engine, it should be
avoided. The octane number was created so that you can match your engine
with an octane ratio that is known to work well in your engine.
Octane ratings are determined by running two separate
tests in a special one-cylinder engine. Iso-Octane is mixed with n-Heptane,
with increasing amounts of n-Heptane, until the engine starts to knock.
The percent of octane in the fuel mix is then noted. The two
tests are run with different engine conditions such as faster engine speeds
(specifically, 600 vs. 900 revolutions per minute), and the two octane numbers
are averaged to produce the octane rating for that fuel. The engines
are varied to simulate different types of engines, so that engine manufacturers
know what octane to recommend. Trace amounts of many chemicals do exist,
but they generally do not affect the octane number greatly. Lead actually
does affect the octane rating; this is why leaded airplane and racing fuels
can have ratings as high as 110% or higher.