elevation & chemistry

[Portia]

In high-altitude areas, like Colorado and Utah, fuels with an octane rating of 85 are sold because the atmosphere is at a lower pressure than at sea level. The compression pressure in a naturally aspirated engine will therefore be lower than at sea level, meaning a more "volatile" fuel can be used safely in higher altitudes (i.e., an engine that normally runs on an octane rating of 87 at sea level will run just fine on an octane rating of 85 at higher altitudes).

This is amazing! Makes so much sense (similar to boiling point elevation, I assume), and yet, here I am, learning something by reading (gasp!) the Internet. Makes me want to geek out and find the exact stoichiometric ratios at 4,000 and 5,280 and 0 feet elevation . . .

Kudos, Democritus. This is what keeps the board interesting.

Fun fact: I live at 600 feet in Western Washington, and I worked the last 4 months at sea level. Twice it dumped considerable more snow here than there: in November, about 8 inches (would have been respectable even in Utah) compared to 2-3; just a couple of weeks ago, 2-3, compared to zero (a weird situation to call in a no-show: "well, there is snow on the ground everywhere here, no way to get up and down the hill, no buses . . ." I swear, I know it's probably just raining there!).

I definitely miss my thin mountain air. Going up Mt. Rainier, etc. is so refreshing!

[Tao]

A big thing with octane is the image that has been advertised is it is "higher power" fuel (or, less often I think, higher mpg fuel). If your car isn't using a high-performance engine, paying for the slower burning, high-octane, fuel isn't going to do anything more than cost you more per gallon.

[Democritus]

I'm glad you found my answer interesting!

Just to clarify, the stoichiometric ratio of air to gasoline in combustion is unaffected by pressure. The lower density of high altitude air, however, could result in insufficient air for complete combustion, since "naturally aspirated engines" simply take in whatever air is available. Oxygen sensors in modern vehicles will notify an engine's fuel injection system to deliver less fuel, accordingly, to allow for complete combustion at high altitudes.

It's all a matter of activation energy, really. High octane fuels (premium and above) require more energy before combustion occurs (achieved by higher compression in an engine's cylinders before the spark plug activates) than lower octane fuels (your regular unleaded). Most car engines have a set compression ratio, so, as Tao said, filling your car with a higher octane than your manufacturer recommends is just burning money. Now, if your manufacturer recommends premium fuel, your engine has a higher compression ratio than most vehicles, so it needs fuel with a higher octane rating (i.e., less volatile fuel with a higher activation energy). This results in a more powerful engine. So, high octane fuel is really used for more powerful engines, not so much that the fuel contains more energy.