Apple #205: Gasoline Grades

I got some gas for my car tonight (or "petrol" for you folks from the UK). Filled up the tank, paid $2.11 a gallon. Not too bad a price -- for today, anyway.

As I was pumping the gas, I wondered, what exactly is the difference between Regular and Mid-grade and Premium? How much better is Premium, really?



Pumps like these, ready for all three grades of gasoline, are the norm.
(These particular pumps are available for purchase from Ken-Co)

• Grades of gasoline are defined in terms of the performance-grade of octane present in the gasoline. There are actually different types of octanes in various batches of gasoline, so to speak. The worst performing octane is given a zero rating, while the best is given a 100. The grades of gasoline reflect the range of performance levels of the octanes present in the gas:
  
• Regular (a.k.a. conventional) gas has an octane rating of 85 to just shy of 88
• Midgrade (oxygenated) gas has an octane rating of 88 to 90
• Premium (reformulated) gas has an octane rating greater than 90

• The higher the octane rating, the less likely your engine is to knock.

Which begs the question, what's engine knocking, and why should I care?

• Once upon a time, car engines had a hard time regulating the amount of fuel versus air that was going into the engine if the temperature dropped, or the humidity went up, and so on. Sometimes too much gas was released, which soaked the carbon deposits just outside the engine cylinder. Those gas-soaked carbon deposits then ignited, but they were igniting outside the cylinder, where you don't that to happen, and this premature ignition made a popping or knocking sound. Not only was it an unpleasant noise, it also meant your car wasn't getting the most bang from the fuel that it could, and it was damaging the engine besides.

This is a piston that's been essentially shot through, all because of engine knock.
(Photo from Lawrence Livermore National Laboratory)

• However, this problem was true of cars made before the mid-1980s. Since then, cars have been built with computerized fuel injectors that work to control the mix of fuel and air going into the engine, regardless of the weather. In most cases, cars made today won't have a problem with engine knock, no matter what grade of gasoline you buy.

• Some engines are made expressly for one grade of gasoline (my dad had a Cadillac for a while, and it required only Premium gas. Expensive!). If this is true of your car, you'd better use the grade that's called for.

Here's what it comes down to: grades of gasoline will affect how rapidly your car can reach higher speeds, or its horsepower. You won't get better mileage if you use a higher grade of gasoline.

So, in essence, the importance of gasoline grades is all but obsolete. And all those times I bought higher grades of gas, thinking I was doing a good thing for my car, I was just another sucker on the vine.

These decades, what matters more is the quality of gasoline you purchase. In other words, it matters which gas station you go to, not what you buy once you're there.

• The primary differentiating factor among brands of gasoline is the detergent they add. Detergents help to remove deposits that build up in the works. Every type of gasoline sold in the U.S. has to meet standards set by the Environmental Protection Agency for acceptable levels and types of detergent.
  

• Gas sold by lesser-known gas companies tends to have a different type of detergent. It is approved by the EPA, but it's of a lesser standard and was approved expressly so that some companies could sell their gasoline more cheaply. In other words, these detergents don't work as well, but they keep the total cost of the gas down. Most car folks warn people away from buying gas at cheap-o, off-brand gas stations.

• Name-brand companies like Mobil and Chevron and BP and so on add the better-perfoming detergents. Many have done this since before the EPA started requiring it.
  

Gas companies have been adding detergents for a long time
(Photo from Rare Ads)

• Of the well-known gas companies, Shell adds the highest amount of detergents. The company claims that this is extra-helpful to your car, but people who test these sorts of things say that more detergent doesn't necessarily make as much difference as the type of detergent does.

• Various gasoline makers add other things besides just detergents. They each use a slightly different recipe. They don't like to say just what that recipe is, but they all like to say theirs is the best. However, I couldn't find many objective poeple saying one name-brand type of gasoline is particularly better than another.

• If you buy the same brand of gasoline on a regular basis, it's a good idea to switch it up once in a while. The same way you need to switch your shampoo to wash away detergent build-up, changing the brand of gasoline means you're putting in a different formulation of detergents that will wash away your old brand's excess detergent and maybe a few deposits that the old gas might have missed.

Sources
Energy Information Administration, Definitions of Gasoline Grades
About.com, Chemistry, Before You Buy Gasoline or Petrol
About.com, Chemistry, How is Gasoline Made? What Are Octane Ratings?
Don't Waste Your Money, Gasoline Grades
Edmunds.com, Do You Really Need Premium?
Environmental Protection Agency, Gasoline Detergent Additives Enforcement and Recordkeeping Requirements, October 1997
Environmental Protection Agency, PA EPA Gets Gasoline Certification Program, 06/28/96.
American Petroleum Institute, Gasoline - Is It All the Same? What about Octane?
eHow, How to Purchase the Right Gasoline