What is octane?  ­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.)    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, methane, propane and butane are all hydrocarbons.
Methane has 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.