The grade of gasoline

Betsy and Billy are on their way to church (they're running late!) when Billy discovers that the car is almost out of gas. Billy pulls into the nearest service station, which, as luck would have it, is overflowing with customers. Billy pulls in behind a small compact car, whose owner is filling it with regular gas. When he's finished, Billy inches his large, luxury sedan up to the pump and readies to fill its tank with super premium. Betsy decides to oversee the operation. She notices that the gas pump is the kind that has one delivery nozzle for three different grades of gasoline. Puzzled, she wonders if there is any regular gas left in the hose, and, if so, whether or not it will affect their car since it is recommended that they only use super premium. She momentarily considers asking Billy, but then decides to skip it since they’re already late. While these two motor off to their Sunday obligation, let's get Mr. Whizard to resolve Betsy's concern.

When we need gasoline for our vehicle and pull up to our local service station, we make a decision on what grade of gasoline we use. This decision is generally based on what the owner's manual recommends. Most service stations in the U.S. carry three grades of gasoline -- regular (87 octane), mid-grade (89 octane), and premium (92 or 93 octane).

The octane rating posted at the pump is actually the average of the motor octane number (MON) and research octane number (RON). The MON determines the octane value of the gasoline with the engine under load conditions while the RON determines the octane no-load condition.

Although using the octane grade specified in the owner's manual is the best practice, there are times when some engines (even new ones) will "ping" using regular gasoline, and a higher octane fuel may be necessary. This happened to me back in the early '80s when I had a vehicle that was supposed to operate on 87 octane, but wouldn't drive safely. I switched to 89 octane, and it ran fine. A higher octane fuel may also be justified when operating a larger vehicle or pulling a heavy boat or trailer which puts a heavier load on the engine.

Some people feel that using a higher octane will improve vehicle performance. This is not necessarily true and is usually a waste of money. However, there are a number of cases reported where a vehicle may have operated on 87 octane for a number of years and over a period of time develops combustion deposits that affect combustion and the octane required for the vehicle. This usually results in poor performance and in many cases can be alleviated by switching to a higher octane gasoline. This is called octane requirement increase (ORI).

But back to Betsy's concern about the residual regular grade in the filling hose before one fills up with premium gasoline. Assuming a 10-foot filling hose with an internal diameter of 3/4 inch, the amount of regular gasoline would be 0.229 gallons (V = pr²h, or 3.14 x (0.375 in.)² x 120 in. x 0.00433 gal/in.³). Assuming a 20-gallon tank and a fill up with a premium of 93 octane (with the hose filled with 87 octane at the start of the fillup), the effect is best described by the equations below:

Considering the precision of the octane test procedure is 0.2 octane numbers and the dilution with 87 octane reduces the octane in the tank from 93 to 92.9, Betsy would be better off worrying about getting to church on time than the effect of octane in her luxury car.

This month's Mr. Whizard is Harry Dietzmann, director of the Petroleum Products Research Department in the Fuels and Lubricants Research Division (08). An SwRI employee for more than 33 years, Dietzmann is a chemist by training whose activities include national fuel surveys, and gasoline and diesel lubricant, gear, automatic transmission fluid, and other automotive fluid qualification testing.

The Lighter Side SwRI Home

March 25, 2013