What Do Octane Ratings Mean? Or, What Are Those Numbers on Fuel Pumps?
Posted Monday, Dec 03, 2018
Have you ever pulled up to the gas pump and found yourself wondering, “Should I buy the less expensive 87 Regular or the 93 Premium? I mean, c'mon- what do octane ratings mean?” If so, you’re not alone. After all, if one fuel is regular and one is premium, shouldn’t the premium be much better for your car? Well, it all depends.
First of all, placing the word Premium on one grade of gas is a bit misleading, and I can’t help but feel the fuel companies are misleading on purpose. It is human nature to assume that something that is premium is better, and the companies are banking on your being willing to pay more for the “better fuel.” However, it’s only a better fuel if your car is designed to use it exclusively; otherwise, it’s extra money for nothing.
To understand this concept, we first have to look at what those numbers at the pump mean. If you read the label, you’ll notice that those numbers refer to the fuel’s octane rating, which, simply put, is the fuel’s resistance to detonation. In other words, a fuel rated 93 is more difficult to detonate than one rated 87- that's the simple answer to the question, "What do octane ratings mean?" To be sure, we're going to look a bit closer. You’ll also notice, on some labels, an equation written thus: (R + M)/2. In several countries, the octane rating used is the Research Octane Number (RON), which is derived by running a fuel through an engine with variable compression (more on compression later). Another type of testing uses a similar test, but heats the fuel and runs the variable-compression engine at a higher RPM. This standard is the Motor Octane Number (MON). In North America, the fuel companies rate their gasoline by averaging the two (RON and MON); hence, they add the R and the M and divide by two, or (R + M)/2. No matter which system you use, though, you’re still rating the fuel’s resistance to detonation when under compression, so let’s look at those terms.
As a piston moves toward the bottom of the cylinder during its intake stroke, it intakes a fuel-air mixture. When the piston is at the bottom of its stroke, it has pulled in an amount of fuel-air equivalent to the total volumetric capacity of the cylinder. As the piston moves upward, it compresses this mixture until it has reached the top of its compression stroke. For example, one cylinder in a 2017 Toyota Corolla with a 1.8 liter engine contains 27.43 cubic inches of fuel-air mixture when the piston is at the bottom. However, since this engine has a compression ratio of 10:1, the fuel-air mixture is compressed to occupy only 2.743 cubic inches by the time the piston reaches the top of the stroke. During the compression stroke, the resistance to detonation comes into play.
Ordinarily, once the fuel is compressed, the spark plug ignites the mixture, which quickly burns and expands to a great volume and thus drives the piston back down during the power stroke. However, the compressing of the fuel raises its temperature significantly, sometimes to the point of causing the mixture to detonate before it reaches the top of its compression stroke. But remember, octane rating is a reference to a vehicle’s resistance to detonate. So, if your vehicle has a compression ratio of around 9.2:1, fuel with an octane rating of 87 will not detonate during compression; hence, it will ignite and burn as designed. However, if you had a high performance car, like the 2017 Mustang Shelby GT350 with its 12.0:1 compression ratio, 87 octane fuel would probably detonate while the piston is still moving upward in its compression stroke.
Obviously, this violent explosion during the compression stroke tries to force the piston back down as it’s continuing its movement upward to the top of the stroke. You’ll hear what is commonly referred to as knocking, but more importantly, you’ll be subjecting your engine to tremendous forces at tremendously incorrect times; you could very well end up with cracked pistons or bent connecting rods (although some engines offer an electronics package to fight against this).
So, to avoid this knocking, do not fill your tank with regular if your owner’s manual recommends premium. But what about going the other direction? Will you get more performance or mileage out of a car rated for 87 if you run 93 through it? Nope. A gallon of gasoline has the capacity to release approximately 114,000 BTUs (British Thermal Units) no matter the octane rating. The reason cars that require premium tend to have more power or fuel economy than those using regular is because they compress the fuel-air mixture to a greater degree. The engine makes more power, not the fuel; the premium just allows the engine to compress to a greater amount without causing detonation. Even one of the major oil companies states on its website that a vehicle designed to run on Regular 87 will gain no power nor fuel economy by running on Premium 93.
So, the bottom line is to fill your tank with fuel of the octane rating recommended by your owner’s manual; using fuel of a different grade than this recommendation will cost you extra- either at the pump or in costly repairs.
Now that you have answered the question, "What do octane ratings mean," I recommend reading these articles concerning various numbers related to your car: What Do Those Numbers on Motor Oil Cans Mean? and Understanding Tire Numbers and Letters.
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