Is high-octane gas bad for the environment?

Illuminating answers to environmental questions.
Jan. 15 2008 7:35 AM

The Premium Premium

Is high-octane gas bad for the environment?

Feul nozzle

I'm pretty enamored of my car, so I try to do right by her by using premium gasoline. But is 93-octane gas significantly worse for the environment than run-of-the-mill unleaded?

As a native Angeleno whose maternal grandmother owned a 1965 Mustang, the Lantern totally understands where you're coming from. When you're head over heels about 3,400 pounds worth of steel, glass, and soft Corinthian leather, it's tempting to opt for premium gas in order to show your love. But unless your vehicle's owner manual specifically instructs you to use high-octane fuel, fight that urge.

A gasoline's octane rating is not a measure of its inherent energy. Instead, the number reflects only how well a gas resists premature detonation in an engine. When a mixture of air and fuel is forced into an engine's cylinders, it's not supposed to ignite until the spark plugs do their bit. But sometimes the air-fuel mixture doesn't cooperate and burns on its own. That results in violent rattling, which, at its worst, can shred your car's innards in a matter of weeks. To eliminate that engine knock, it's advisable to use a higher-octane, slower-burning gas.

Engine knock used to be a big problem during the heyday of the carburetor. But since fuel injectors started becoming the norm in the 1980s, it's become rarer and rarer. Fuel-injection engines are typically designed to handle a specific grade of gasoline, usually the least expensive 87-octane "regular."

If your car is built for 87- or 89-octane gas—check the owner's manual—upgrading to premium won't do you any good. In fact, it could even harm both your car and the environment, since more unburned gas will get into the emissions system and interfere with its ability to prevent noxious discharge. If you've been using high-octane gas and your tailpipe exhaust smells of sulfur, then there's a good chance it's because the car wasn't built to handle premium.

On the other hand, if your manufacturer recommends that you use high-octane gas, heed that advice. You most likely have a high-performance car with a high compression ratio, which means the engine puts the air-fuel mixture under a lot of stress. Premium gas is often necessary to prevent knocking under such circumstances and is thus key to preserving your car's health. Ten percent of the energy a car uses in its lifetime is expended during its production, so the greenest decision is often the one that keeps your vehicle on the road for as long as possible. (According to the American Petroleum Institute, an industry trade group, 19 percent of vehicles require premium—a figure that strikes the Lantern as rather high.)

Some hypermilers contend that they've achieved better gas mileage by using premium, but their claims probably can't hold up to scientific scrutiny. According to Tadeusz W. Patzek, an environmental-engineering professor at the University of California-Berkeley, the main determinant of miles-per-gallon potential is a fuel's heating value. And there is simply no significant difference between premium and regular in terms of heating value.

Patzek also states that a car's per-mile carbon dioxide emissions shouldn't be affected by the octane rating of its fuel. But emissions on the production end are another matter—a gas is usually made premium by the addition of oxygenates, or hydrocarbons that contain one or more oxygen atoms. Making a gallon of premium gas thus consumes more energy than making a gallon of regular.

But how much more? Oil companies are pretty secretive about their particular formulas. The best guess the Lantern could find is that if all oil companies reduced the octane rating of their finest premiums from 93 to 92, it would increase the nation's gasoline production efficiency by roughly 2 percent—or, in laymen's terms, it would enable us to squeeze another 182,000 barrels of usable gasoline out of our crude oil supply each day. Over a full year, the reduction would save us about 143.1 million barrels of oil annually—enough to satisfy our national oil demand for about seven days.

Big Oil won't be making any such octane-rating reductions soon, since it makes enormous profits by persuading drivers to fork over extra cash for higher octane numbers—whether they need to or not. An optimist might argue that the premium you pay for premium is actually a net positive for the environment, since oil companies can use that money to research biofuels or fuel cells. But do they? The Lantern suspects that no small amount of that 93-octane lucre goes toward Ferraris and filet mignons instead of switchgrass.

Is there an environmental quandary that's been keeping you up at night? Send it to, and check this space every Tuesday.



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