#43: Give me fuel, give me fire…

The Question

Old Man Schenk says:
2005-08-02 11:02:35

Dear Shaun,

Sparked (pun intended) by your diesel fuel answer, I have another fuel-related question: octane. The commonly held belief seems to be that the higher octane rating a gasoline has, the more power you get from your engine (the is probably encouraged by the use of words like “super” and “premium” to describe higher octane gas). Other people have told me that octane is a measurement of temperature, not power, and that putting high octane fuel into some engines can actually reduce performance. SO, what is octane, what does it measure, and what does that mean for me the consumer?

The Answer

The octane number on the fuel you fill your car with measures the percentage of octane in the gasoline. So, an 87 octane fuel contains 87% octane and 13% heptane. (Octane is a hydrocarbon consisting of 8 carbon molecules chained together, heptane is 7).

What does it matter what the percentages are? Well, it comes down to one of my favorite topics – spontaneous combustion. It turn out that heptane will spontaneously combust rather easily when compressed. And since the piston of the engine is compressing the air/gasoline combination for ignition by a spark plug, having it combust BEFORE the spark plug goes off would be bad. (This is what causes engine knocking). Hence, the less heptane and more octane, the more your gasoline can be compressed without combusting. High performance engines compress the fuel a lot more, and so require a higher octane to prevent damaging engine knocking.

Unfortunately, higher octane ratings mean more money (it’s harder to refine) – so that’s why the Premium gas costs more. It probably won’t do too much more for your everyday engine. My Kia certainly doesn’t need it.

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