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Ever Wonder About Those Gas Pump Numbers?

I don’t always drive a car, but when I do, I fill up with octane 87 gas. I just follow what my car manual says, but I wonder what difference these octane numbers make?


Gasoline is a cocktail of chemicals. The main ingredients are the hydrocarbons, octane and heptane, which are named for the number of carbon atoms they contain (eight and seven). The hydrocarbons can vary in the particular arrangement of the atoms as well. When hydrocarbons and oxygen are burned, you get carbon dioxide and water. I used to think of carbon dioxide as plant food, but nowadays people recognize it as a greenhouse gas, contributing to climate change.


The octane number on a gas pump is an anti-knock index. The octane number differs between Europe and North America. In Europe, only the RON is used. In North America, the RON and MON are averaged. RON, Research Octane Number, is based on measurements taken from a test engine running at about 600 rpm. MON, Motor Octane Number, is based on a test engine running at 900 rpm. An octane number of 87 should behave like a mixture of 87% octane and 13% heptane. However, the actual proportion may differ, if other chemicals like ethanol have been added to adjust the effective octane number. The concoctions differ between companies and also across the border.


The octane number indicates the resistance of the fuel to self-igniting. When the piston in a four-stroke engine compresses the air and gas mixture, it’s not supposed to burn until the spark plug sparks. High-performance engines compress the mixture more to yield more power. They require a fuel with a higher octane number to operate efficiently. A lower octane fuel could spontaneously ignite and result in engine knocking which could damage the engine. These days, computerized fuel injectors can adjust to different fuels and reduce the chance of spontaneous ignition. If you use a high-octane fuel with a regular engine, you just pay more money without any benefit to performance (because your engine compression doesn’t change), although some people seem to think otherwise.

So, after all that, I am back to where I started. Good thing mental journeys don’t use up gas.