What is Octane?

In recent years, car manufacturers have been requiring or recommending premium gasoline (a high-octane grade of fuel) for use in more of their vehicle models. The difference in prices between premium and lower octane grades has also increased. As a result, more people are curious about what octane is and what those octane rating numbers on gas pumps mean.

What Octane Rating?

Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is actually the average of two different octane rating methods—motor octane rating (MOR) and research octane rating (RON)—that differ primarily in the specifics of the operating conditions. The higher an octane number, the more stable the fuel.

What type of fuel is in your car?

​Retail gasoline stations in the United States sell three main grades of gasoline based on the octane level:    

~Regular (the lowest octane fuel–generally 87)    

 ~Midgrade (the middle range octane fuel–generally 89–90)    

 ~Premium (the highest octane fuel–generally 91–94)

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Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all refer to the octane rating. The higher the octane rating, the higher the price.

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Stable v Unstable

Of the 18 isomers of normal octane (C8H18), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition (sponaneious combustion in your engine). This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C7H16) molecule is the 0 octane reference fuel.

​How about fuels for race cars?

​Race fuels, though, are a slightly different matter. Where street fuels are blended to meet a lower price point, race fuels feature better ingredients that lead to longer shelf life. If properly stored, race fuels can sit around for two years or more without degrading. Race fuels have octane ratings above 94 and are more stable.

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Next time you fill up, think about the chemistry involved!

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Source for info provided if you want to know more:  https://www.eia.gov/energyexplained/gasoline/octane-in-depth.php

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​A great video that explains engine knocking:

​https://www.youtube.com/watch?v=4ZysyokEU60