Premium gasoline must be called “premium” because it is better for your automobile. After all, one of that adjective’s definitions is “a high value or a value in excess of that normally or usually expected,” according to Merriam-Webster’s Collegiate Dictionary. But is that common assumption safe? The answer to this question lies in the process of refining gasoline from oil, the dynamics of the typical internal-combustion engine and another definition of “premium”—this one from its noun form: “a sum over and above a regular price paid chiefly as an inducement or incentive.”
High Octane, Low Knock
All gasoline is a brew of many different hydrocarbon molecules, ranging from heptane to decane and beyond. The hydrocarbon clearly identified on the pump—and the one many consumers associate with gasoline quality—is octane: eight carbon atoms and 18 hydrogens. The familiar octane number, though, is not a measure of the percentage of octane in the gas but a measure of how that gasoline compares with a pure mixture of octane and heptane. At special laboratories across the globe, chemists concoct such reference fuels and compare gasoline varieties to them in a one-cylinder engine following a standard protocol. “The American Society of Testing and Materials has this thick document on how you determine octane rating with this specialized one-cylinder engine,” explains Joseph Shepherd, a mechanical engineer at the California Institute of Technology. “The higher the number, the harder it is to have knock.”
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Premium Fights Knock
“Knock”—an unregulated explosion in a chamber designed for highly regulated combustion—is the bane of an internal-combustion engine. During the four-stroke cycle of a typical car motor, the piston drops in the cylinder, allowing it to fill with a mixture of gasoline and air. The piston then moves up again, compressing the fuel mix; when it reaches the top, the spark plug ignites the explosive vapor, driving the piston down again. As the piston returns to the top of the cylinder, it expels what remains of the spent fuel through the exhaust valves and the entire process starts again. Knock occurs when the compression of the fuel-and-air mixture alone, and not the spark plug, sets off an explosion.
Each hydrocarbon molecule in gasoline behaves differently under pressure, but octane best resists the temptation to explode. “You rate the gasoline on how it knocks compared with this reference mixture,” explains William H. Green, a chemist at the Massachusetts Institute of Technology. “Ones that don't knock very much are the premium.” That is, they behave in an engine as if they have a high proportion of octane, even if they don't.
The Rub: Today's Engines Are Knock-Free
Most modern cars are designed to employ a specific compression ratio, a measure of how much room is available to the fuel when the piston is at the bottom and the top of the cylinder. This compression ratio—typically around 8 to 1—tolerates lower octane fuels (such as 87-octane regular gasoline) without knocking. “The compression ratio is fixed by the designer of the engine,” Green says. “The regular fuel will burn properly and the premium fuel will burn properly, and therefore there is no reason you should pay the extra money.” High-performance engines in some sports cars or engines in older, heavier automobiles, however, can boast much higher compression ratios and so require higher octane gasoline.
Such high compression ratios could be turned to efficiency rather than speed, Green notes, especially if put into the engines of lighter cars. And other automotive fuels, such as ethanol, can offer high octane ratings. But for standard cars on the road today, purchasing premium gasoline is simply paying a premium for a fuel that delivers no added benefits. “If you think you need it,” Green says, “you’re being very eccentric.”
