Q. What exactly is accomplished in the refining process of gas that makes the price from regular to supreme (87 octane - 91) such a big difference?
- Jeff, Valrico
A. Refineries can use cheaper ingredients in "regular" blends. And premium costs more to distribute. But should you pay more for premium? This is a good time for some myth-busting.
As of this week, there's about a 30-cent difference between blends, according to the AAA Daily Fuel Gauge Report. A gallon of regular costs $3.576 in Tampa Bay, while mid-grade costs $3.735 and premium costs $3.847.
But there's little reason to buy premium or higher-octane gasoline unless your owner's manual specifically says the engine needs it. Higher octane numbers alone won't give a regular car better performance, mileage or horsepower.
On the other hand, putting regular gas in a car that needs premium gas will do serious damage.
On its own, the "Octane Rating" of 91 or 93 that comes with many premium fuels is not a measure of power or quality. It's a ratio of how the fuel is blended, like a 2 percent label on milk, said Patrick Kelly, a senior policy analyst with the American Petroleum Institute.
An engine works by squirting a mist of gasoline into a piston chamber until the piston rises up, squeezes the vapor, and a spark plug ignites an explosion that pushes the piston back down, turning the vehicle's drive mechanism.
Gasoline can tolerate only so much pressure before it wants to set itself on fire and explode, spark or no spark. If the explosion comes too soon in the cycle, the engine will "knock" and cause major damage.
Octane, from "octo," is a name for a fuel chemical with eight hydrocarbon atoms.
Octane handles pressure very well without self-exploding, but is relatively more expensive to make in a refinery. So refineries add other, less expensive fluids to come up with a final "regular" blend, like 87 percent octane, 13 percent something else.
That works fine in normal engines.
But those less expensive fluids (like heptane - seven carbon atoms ) can't handle high-pressure without self-exploding. If you put regular fuel in a high-pressure engine, like one with a turbocharger, and those less-expensive ingredients can self-explode too soon in the piston cycle, causing knocks. So refineries make a "premium" blend with more octane that tolerates higher pressures.
That's why high-pressure or "premium" gasoline works fine in a higher-pressure engine, but may not be worth the extra money in a lower-pressure regular engine.
To think of it another way, octane is like firewood, and cheaper additives are like fireworks. A blend of the two is OK in a regular engine that can handle the mix, but put too many cheaper fireworks in a high-pressure engine, and everything explodes too soon.
"A lot of cars require that mid-grade or premium gasoline," Kelly said. "But unless your car has something else wrong, it should not knock (with regular)."
Diesel fuel is even lower on the octane rating scale because it's designed to self-ignite in a diesel engine. No spark necessary. Still, diesel costs more than premium lately because demand for it is so high.
There are other reasons premium costs more.
Oil companies like BP, Shell or Chevron add special detergents and additives to their fuels, especially premium blends, to help set themselves apart in the market. Those ingredients can come with a higher price.
Also, Kelly said, supply and demand are at work with the cost of premium fuel.
Drivers buy far more regular fuel than premium, and that volume helps keep down the price of regular. Plus, the industry spends more to distribute less-popular premium through terminals, pipelines and tanks, and that adds to the final price.
As for which brand to chose, Kelly notes that different stations may sell different octane ratings under the name regular or premium, so watch the numbers more than the name.
Submit your question here or visit our Gas Prices page and retail writer Richard Mullins will get you the answer