Just what is this octane, anyway?

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The octane rating, like what you see on a gas pump, is the amount of octane contained in the gasoline...sort of. Image from Wikimedia Commons.

The octane rating, like what you see on a gas pump, is the amount of octane contained in the gasoline…sort of. Image from Wikimedia Commons.

Octane is known to have something to do with gasoline, but not everyone understands what it is. The quick version is that it’s a component of gasoline, but not the only one, and yes the octane number has something to do with how much is in every gallon.

For starters, octane is a hydrocarbon

Before we get to octane ratings, some science! Octane is a hydrocarbon, a group of compounds composed of…well, hydrogen and carbon. Each molecule of contains eight atoms of carbon and eighteen of hydrogen, or C8H18. Likewise, the compound (and major gasoline component) heptane has seven atoms of carbon, pentane five, etc.

Hydrocarbons, along with oxygen, are what make a fuel a fuel. The reason is that when heat (and thus energy) is applied to them, energy is released when the bonds between carbon and hydrogen atoms (and between carbon atoms) are broken during combustion. The byproduct of this is typically gases created by the fusion of carbon and oxygen (such as carbon dioxide) and hydrogen and oxygen. (You know what that is.)

Thus, a greater amount of energy is created by combusting hydrocarbons than was used to combust them. Ideal, right?

It’s a gas, gas, gas

The octane number, like those by Regular and Premium, relates to the amount per unit of volume of gasoline compared to the amount of heptane. To explain that further, octane and heptane aren’t the only compounds present, but the octane rating is the amount of it relative to the amount of heptane in the overall mixture.

In other words, a mixture of 90 percent octane (or rather iso-octane, the isotope best suited to gasoline due to a high number of carbon to hydrogen bonds, which release more energy when combusted) to 10 percent heptane would have an octane number of 90, like what you see on the pump.

That said…

Except that isn’t entirely the case, since one of the properties of octane (and specifically iso-octane) is that it doesn’t ignite before it’s supposed to. (That causes engine knocking.) Under tightly controlled lab conditions, a parcel of gasoline with a perfect 90:10 ratio of octane to heptane will only knock a certain amount. What you see at the pump is actually a parcel of gasoline that has been tested and found to behave just like a 90:10 (or 88:12, or whatever) mixture.

It might not actually be 90:10; it may be more or less. Likewise, the energy content doesn’t actually depend on the octane number; octane numbers merely assert the propensity of the fuel for not prematurely combusting.

But hey – that happens to everyone, once in a while right? Not me though. (Yes you. ed.)

Further reading

If you’d like to find out more about octane, one need not, as the philosopher Burton observed, take my word for it. Check out these links for more reading:

Wikipedia, Fountain of All Internet Knowledge



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