Q: Please explain the RON and AKI fuel ratings.

A: Actually, there are three antiknock indexes. Gasoline engines can be prone to igniting the air/fuel mixture prematurely, whether it’s due to too much ignition advance, too much compression or a lean-burn condition. This premature ignition is called knock. A refiner can manufacture gas that is more resistant to knock, but at a higher cost.

The standard hydrocarbons used to determine an antiknock rating are octane and heptane, with octane molecules having eight carbon atoms and heptane molecules seven. Heptane is prone to knock; octane is not. Octane rating is the knock resistance of a sample of gasoline compared with a mixture of octane and heptane, so 95-octane fuel is as knock-resistant as a mix of 95 percent octane and 5 percent heptane.

“RON” stands for “Research Octane Number.” Fuels rated on this scale are used to operate a specialized engine with a variable compression ratio to determine the octane rating of each fuel experimentally.

“MON” stands for “Motor Octane Number,” which is measured in a similar fashion, but under conditions that resemble a running, modern engine: leaner mixtures and hotter cylinder-head temperatures. A fuel’s MON rating is usually eight to 10 points lower than its RON.

So what is AKI, the number displayed at the pump? It stands for “Anti-Knock Index,” and it’s the average of the other two numbers. Apparently EPA bureaucrats couldn’t decide which industry-standard rating to use, so AKI=(RON+MON)/2.

Q: The electric radiator fans on my 2002 Malibu quit working. I’ve checked, and all the fuses are good. The mini relays are OK too. I can jumper power to the fans and they work. Is there a thermostat or sensor that turns them on?

A: We can assume that the ground connection to the fans is good, if a single jumper wire to the fan’s positive 12-volt input makes them run. So that’s one thing out of the way.

Now grab your scan tool. Don’t have one? Head over to the auto-parts store — many times they will let you borrow one, free of charge, if you’re quick.

The onboard computer controls the fans, so make sure that the coolant temperature is high enough to activate those fans. The tool can then tell you if the computer has commanded the fans to run. If it has, you’ve missed something in the wiring. If not, there’s a bad sensor.