Every time you fill up the tank in your car or truck, you are asked to choose between three types of fuel, designated by three different numbers, each one greater than the last. While most people know that higher numbers equal more money per gallon, what they often don't know is what the difference between regular, plus, and supreme is in terms of fuel analysis. However, an explanation may leave consumers better informed the next time they find themselves at the pump.
What do the ratings mean?
The octane number is generated by a fuel analysis done to compare the gasoline to isooctane, which is pure. Isooctane gets a 100 rating and serves as the basis of comparison. You can think of the 87, 89, or 90 sort of like grades on a test.
The ratings are important because of the engine's compression ratio. This is measured by comparing the volume of the cylinder from the bottom most movement of the piston to the top most movement. The bigger the compression ratio, the greater the amount of heat generated per stroke.
The three numbers you see at the pump measure the gasoline's resistance to knock. But what is knock exactly?
Knock is something you don't want. The term refers to the sound that can be heard when the gasoline spontaneously explodes rather than burning like it's supposed to. This leads to a loss of power, and could cause engine damage.
Why do the numbers differ?
They differ because of the additives. We all know that gasoline is refined from oil. The substance that comes directly from the refinery process has an octane level of about 70. Additives like MTBE and ETBE are used to increase the rating of the fuel. Analysis is then performed with the mixture to make sure its components are pure and a rating is assigned.
MTBE is an organic compound used as an oxygenate. Its use is declining in the US because it has been found in the ground water. ETBE is also used as an oxygenate and offers many of the benefits of ethanol, but without inducing the evaporation of gas, which causes smog. ETBE also doesn't absorb moisture from the atmosphere, like ethanol does, which can get into engines.
What kind of vehicles require plus and supreme?
Motor vehicles with more horsepower need higher-octane gasoline. One way to increase the horsepower of the engine is to increase that all-important compression ratio. As you know by now, a higher compression ratio requires a gasoline with a higher number. This is what many manufacturers mean when they describe a vehicle as having a "high performance."
The ratings have nothing to do with miles per gallon, and they usually won't make your car run better or help your engine last longer. Unless you are specifically instructed to use the premium options, you really don't need to shell out the extra cash. All grades are treated to an all-important fuel analysis to ensure that they are free of contaminants and won't harm your engine.
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