Fact or Fiction?; July 2007; Scientific American Magazine; by David Biello; 1 Page(s)

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."

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."