Octane ratings explained

So now we know what engine knock is and how to prevent it. We are ready to look at how anti-knock properties are measured: octane ratings.

There is only one way to measure the anti-knock properties of a fuel: burn it in an engine and see how much it knocks. Obviously, fuel companies don't just tip samples into the gas tank of an old Camaro and drive around listening for the pinging sound. They use special engines which are precise scientific instruments costing in the region of a million dollars each! (The exact price isn't advertised, because if you have to ask, you can't afford it.) These engines allow scientists to precisely measure the amount of knocking caused by a fuel, but they need some kind of standard scale to compare fuels to. This is why the octane rating was developed.

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The octane rating isn't really anything to do with octane, but rather its cousin iso-octane that we met on the previous page. (So strictly speaking, an octane number is actually a 2,2,4 trimethylpentane number, but octane is less of a mouthful.) We have already seen how iso-octane is an excellent fuel that is highly resistant to detonation due to its compact, branched structure. Therefore scientists created the octane rating, where iso-octane scores 100. At the other end of the scale is a fuel that scores 0 (zero), and the fuel chosen for this end of the scale was heptane (C7H16). Heptane is highly susceptible to detonation as it consists of 7 carbon atoms in a long, flimsy string. By burning the fuel in a special engine, chemists can measure its anti-knock properties and assign it an octane number relative to mixtures of heptane and iso-octane. There are a couple of different ways to calculate octane number, but they are outside the scope of this guide. Normal unleaded gasoline has an octane rating of around 90 to 95 depending on where you live and the exact method used to calculate the rating. (Fuel that scores 90 octane in North America would be considered roughly 95 octane in Europe due to the different calculation methods.) Gasoline with an octane rating of 90 has the same anti-knock qualities as a mixture of 90% iso-octane and 10% heptane. Octane ratings higher than 100 are possible, as some fuels have better anti-knock properties than 100% iso-octane.

You might wonder what the octane rating of actual octane (known as n-octane) is, as it is even longer and flimsier than heptane. The answer is, bizarre as it sounds, that n-octane has an octane rating of -10 (yes, minus ten!), and is therefore an extremely low-octane fuel. In fact, n-octane has such atrocious anti-knock properties that it is present in pump gasoline only in small amounts. (Gasoline isn't a simple mixture of iso-octane and heptane, but is a carefully blended mixture of various compounds.)

Once scientists had created the octane rating, they could use it to test all fuels, regardless of what they actually contain. In other words, the octane rating is a measure of anti-knock properties, not of the octane content or energy content of a fuel. For instance, ethanol (C2H5OH) has excellent anti-knock properties and has an octane rating in excess of 100. However, ethanol contains less energy per gallon than iso-octane. LPG also contains less energy than gasoline, but is an excellent fuel with an octane rating greater than 100. Similarly, it is possible for two fuels to contain virtually identical amounts of energy but have different octane ratings (n-octane and iso-octane are an excellent example of this as they contain almost identical amounts of energy but there is an enormous difference between their octane ratings).

There is no point using fuel with a higher octane rating than the engine was designed for. So long as the fuel doesn't cause engine knock, it's good enough. Some cars are optimised for higher octane fuel, but can run on slightly lower octane fuel by electronically detecting knock and delaying the spark timing, at the expense of power output and economy. You should use fuel with the octane rating recommended by the car manufacturer. Flex fuel cars that can run on either gasoline or an alternative fuel such as ethanol or LPG must make a compromise, as an engine designed to run on high-octane alternative fuels would knock when running on normal gasoline. Modern, turbocharged flex-fuel cars can alter the boost from the turbocharger to suit the fuel being used, giving good power and performance on both fuels.