What is an octane rating and what does it mean?

You pull up to the pump and you are presented with a few choices, and we’re not talking about different fuel types here. We’re talking, of course, about octane ratings.

Those are large figures at the fueling station, and you know if you press the higher one you’re going to spend more money. Why is that and what does is all mean? Jason Fenske from is here to teach us about octane ratings, and their diesel cousin, the cetane rating.

An octane rating is a scale to show you the knock resistance of a given quantity of fuel. Knock resistance relates to the amount of compression the the fuel can take before it ignites. The greater the compression allowed then the greater the knock resistance, and a higher octane rating is earned. Initially, the octane rating scale was created on a 0-100 metric. Today, with various racing fuels, you can find octane ratings over 100.

Cetane is a rating applied to diesel fuel. Here we’re not looking at knock resistance but rather the ignition delay of the fuel. It’s similar to knock resistance but it’s measured in a different way. You can find a cetane number by figuring out the amount of time it takes for diesel fuel to ignite once it enters the combustion chamber. A long delay results in a lower cetane rating, while a shorter delay provides faster combustion and a higher cetane rating.

These numbers are useful to car and truck owners because they allow you to choose the proper fuel for your engine. The average modern gas-drinking car has a computer and air-fuel system that allow it to optimize engine performance regardless of which octane fuel is dumped into the tank. This changes, of course, if you’re dealing with a more complex engine such as the kind aided by forced induction or higher compression ratio engines.

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For diesel owners, higher cetane ratings result in greater performance through the rev range, more torque production, and a more complete burn of the fuel. That in turn results in less hydrocarbon emissions.

Fesnke, as usual, does a great job of breaking down all of the science behind these fuel ratings. It’s good information to know right now too, since there are currently lobbying efforts underway to raise our octane floor.

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