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Octane Ratings vs. Quantity of Octane



    [Gasoline pump showing octane numbers]


    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; click here 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 [Sparking spark plug] 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.


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