What is octane?

Octane ratings of 87, 89, 93

You pull into a gas station and are presented with three options: regular, mid-grade, or premium —all gasoline of course, as true fuel choice isn’t currently a reality in America. But have you ever wondered what those three numbers — 87, 89, and 93 or some similar variation — on the pump itself mean?

Sure, maybe you’ve heard that those numbers represent the rating of this thing called octane. But unless you know what octane is, and what it does, that answer is about as useful as telling someone unfamiliar with the stock market that the Dow is up three points.

The short answer is that octane is the measure of how much compression a fuel can withstand before igniting. Or, in layman’s terms, the higher the octane rating, the less likely the fuel is going to pre-ignite (read: explode unexpectedly) at higher pressures and damage your engine. That’s why performance cars with higher compression engines require higher octane (premium) fuel. In essence, higher octane fuels are compatible with higher compression engines that can increase efficiency and performance while potentially reducing emissions by combusting the fuel more completely.

That’s the short answer. The long answer is a bit more complicated, and requires some understanding of how, exactly, our cars turn the liquid fuels in our tanks into the energy that propels them down our roads. So put your learning caps on folks, class is officially in session.

Four-stroke engine in motion

In the image to left, we have an animation of what powers the vast majority of internal combustion engines (ICEs) on the road, known as the four-stroke cycle. During Step 1, the right tube (known as the intake valve) opens up while the piston simultaneously moves down. This brings a mixture of air and fuel (represented by blue) into the cylinder. Then in Step 2, the piston moves back up, compressing the air and fuel, increasing the temperature and pressure in the cylinder. Step 3 begins after the air/fuel mixture is ignited by the car’s spark plug. It’s this step that creates the energy that actually powers the car, with the force generated by the exploding air/fuel mixture pushing the piston back down, turning the car’s crankshaft, and ultimately propelling the car into motion. This is followed by Step 4, where the piston moves back up and pushes what’s left of the combusted air/fuel (known as exhaust and represented by brown) out of the left tube, or exhaust port. Then the cycle starts all over again.

So what does this have to do with octane? Everything, actually. As mentioned above, the higher the octane rating, the more the fuel can withstand greater pressures without pre-igniting. This allows for a variety of options that can increase power and efficiency while adding the ability to decrease emissions as well. These options include increasing the compression ratio, changing the timing of spark ignition, injecting less fuel into the cylinder, and more. Additionally, engines designed to work with higher octane also burn the fuel more completely — meaning less harmful emissions in exhaust. One MIT study even estimated that if higher octane fuels and engines designed for them became more widespread, the U.S. could cut our annual CO2 emissions by 35 million tons.

With that knowledge in hand, we can now come back to those numbers — 87, 89, and 93. 87 is the standard octane rating that most car engines are designed for. Or put another way, an octane rating of 87 or higher can withstand the compression ratios used in most cars. Anything lower and you risk engine damaging pre-ignition or knocking — processes that occur when the compression ratio generates more pressure and heat than the fuel can handle, causing it to combust early and push down against the piston while it is still moving upward in Step 2. And since 87 is the standard octane rating for the fuel most engines use today, the 89 and 93 octane rated fuels are only different because they can handle more heat and pressure before igniting on their own. This, in turn, means they can run in performance engines designed to use higher compression ratios. It also means that while higher octane fuels won’t hurt engines designed to run on 87 octane fuel, they won’t provide a benefit either.

And there you have it. Octane is simply a measure of how much heat and pressure a fuel can withstand before exploding, and — in tandem with a properly designed engine — a higher octane fuel can increase performance and efficiency while decreasing emissions.

Class dismissed.

 

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9 replies
  1. Ferne
    Ferne says:

    Wonderful site you have here but I was wanting to know if you knew of any community forums
    that cover the same topics talked about here? I’d really love to be a part of
    online community where I can get comments from other experienced individuals
    that share the same interest. If you have any suggestions, please let me know.
    Kudos!

    Reply
  2. Daniel
    Daniel says:

    Great article ! Really appreciate thanks for your value information, I’ve never known about this I usually fill the gas with only low grade because I think it’s not necessary by the way premium gas is cost too higher after read this article just wake me up ,
    So is it ok if I’ll change to fill gas from lower grade to premium grade ?
    Really appreciate for your response ! Thanks !!!!

    Reply
    • Nathan Taft
      Nathan Taft says:

      Hi Daniel, Thanks for the response! What type of vehicle do you have? It only makes sense to switch to premium if you have a vehicle that can take advantage of the higher octane.

    • Nathan Taft
      Nathan Taft says:

      No worries! I would consult your owner’s manual to be sure, but I believe that your Lexus RX300 could benefit from higher octane fuel. Hope that helps—have a good one.

  3. Joe Ambrosino
    Joe Ambrosino says:

    Fairly accurate explanation except running high octane fuel in a car not designed for it won’t burn the fuel completely and will create a carbon build up prematurely clogging your catalytic converters. A simple test is know is simply rub your finger inside your tail pipe and you should not have a heavy soot build up

    Reply
    • Rich Lorimer
      Rich Lorimer says:

      Higher octane fuel explodes explodes and burns slower. The spark plug is fired slightly before the piston reaches the top (advanced) so that the peak of the explosion occurs right after the piston starts back down. Higher octane requires the ignition timing to be more advanced or the explosion will occur too late for full power.. In the old days, it was easy to readjust ignition timing and carburetors to use higher octane gas in any car or truck engine and get more power, cleaner burning, and 15-20% better fuel economy. I did it on a number of vehicles. Modern cars use fuel injection and settings can only be changed in the computer program. Some vehicles have octane sensors as well as knock detectors so that the computer will make changes as needed. .

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