Octane Explained

by Ask Bob: The Auto Answer Man

Dear Bob,
I have a project I have to do for school on Octane. I need to know how the octane rating of gasoline is determined and which octane ratings are available today? I've searched the internet and can't get any straight answers, so if you could help, that would be great. Thanks!

Your answer will vary greatly depending on where you live (region and country). For a definitive discussion on octane ratings, look up the procedures in the SAE (Society of Automotive Engineers) publications (major public libraries may carry it).

The octane rating is basically an anti-knock rating. Engine "knock" is also sometimes referred to as "ping". It is basically pre-detonation of the fuel before the piston is in the correct position (either near top of it's stroke or on its way down on the power stroke). Ping occurs while the piston is on the upstroke. Essentially, it is the elevated combustion cylinder temperature mixed with the now elevated cylinder pressure (compression) that will cause fuel it ignite without the spark.

In the United States, pump octane is an average of 2 ratings, research octane (RON) and motor octane (MON). If you look at any gas pump in the United States, you will see a yellow sticker that says "octane by R + M / 2" That is the basic formula for an average. These 2 numbers mean different things. You could make an analogy to that of a blood pressure reading (systolic and diastolic). Research Octane number is always higher than Motor Octane number.

In Europe, they only report the RON. You may hear people discussing that in Europe, the octane is higher. Well, that is not exactly true. You see, in Europe, you might find 96 octane at a local gas station. (wow 96, highest we have here is 94). Well, that 96 is equivalent to 92 here in the States.

The octane value of a fuel can be varied by several different processes. It can be refined to that value (usually more costly) or enhanced with different additives. Different people will argue which method is better. I hope that gives you a good basis for study!
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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