Octane Is Control Not Power

Many people mistakenly equate "octane" with "power". In reality, higher-octane fuel is not more explosive or what not, but its power derives from more control.

Perhaps there are many out there who could do with a good dose of higher octane in their professions. Applying a little bit of power in a controlled direction is much better in the long run than applying lots and lots of power all over the place.

If knowledge is power, then wisdom is control. PaulCaswell

In complex systems where power is the result of composition, control is part of the power equation. But increasing control only increases power when the initial conditions are right. --WaldenMathews

Despite the HydroCarbon? name, what you see on the pump is actually the AntiKnockIndex?, in units called Octanes. The higher compression you put on your engine, the higher antiknock fuel it'll need to run right.

Unfortunately, everyone has latched onto octane as a quality indicator, and so now the gas at the 92 octane pump is probably a tiny bit better quality than the gas at the 87 octane pump.

Virtually all other performance characteristics are the same from the 80 octane gas they sell at high elevations out through 120+ octane gas from airports.

--KyleJerviss

I thought the issue what that the flash point of higher octane fuel was lower (higher?) such that it wouldn't spontaneously ignite under compression in the cylinder but rather would wait until the actual spark from the sparkplugs, thereby not screwing up engine timing.

-- AndyPierce

That's it exactly. The rating scale is defined from zero, which corresponds to n-heptane -- an energetic but notoriously "knocky" fuel -- to isooctane (more properly called 2,2,4 trimethylpentane), which is well-behaved and therefore assigned an "octane number" of one hundred.

Right. "Knock", or "pre-ignition" occur when pressure, not spark, make the mixture ignite. Octane is one of the hydrocarbons in the gasoline mix, and one with a relatively high resistance to pre-ignition. Hence, its name was adopted as the name of the anit-knock index. Octane itself is too expensive as an anti-knock additive, so other substances have been adopted. Fancy cars need high octane because their engines are designed for high compression and higher power. "High octane" also became associated with upper-end machinery, hence the association as higher quality gas.

Back to the main theme. If your car is knocking, it's not developing full power, and the cause is not due to reduced explosive power in the gasoline, it's due to a timing control problem. More "octane" means more control means more power. The rule holds only if your problem was pre-ignition. Otherwise higher octane won't help.

To add to the above: part of the tradeoff you get for the antiknock characteristics is lower fuel efficiency. That is, a gallon of high-octane fuel has less energy (fewer calories, joules, kilowatt-hours, what have you) than lower-octane fuel. Thus, putting high octane fuel into a low-compression engine is worse than useless--it lowers your mileage and your performance. You use it in a sports car because those engines get more power through higher compression.

This all makes sense. Think of diesel fuel. Diesel engines don't have spark plugs; they rely on compression ignition. Thus, this is less "antiknock" than even standard gasoline. You use this in trucks and construction equipment because diesel actually has more energy per gallon than gasoline.

To add to the above: part of the tradeoff you get for the antiknock characteristics is lower fuel efficiency. That is, a gallon of high-octane fuel has less energy (fewer calories, joules, kilowatt-hours, what have you) than lower-octane fuel.

Didn't know that, but it brings home powerfully and elegantly the truth of this page. Giving up raw power (lower calorie) for control (anti-knock) yields a net power increase, teaching that until control exists, increasing power is not the way. This would be a great lead-in to a discussion of CMM level two, but I'll control myself instead. -- WaldenMathews