Understanding Octane Rating

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David S. Williams, P.E., CFEI

Have you ever noticed advertisements at gasoline pumps stating that one of the octane ratings is “high-performance” fuel? In my experience, this has led to much misunderstanding about octane ratings.

The first place to start when clearing the air with respect to a fuel’s octane rating is understanding its use. Internal combustion engines that use gasoline rely on compression of a fuel-air mixture and ignition with a spark (combustion). The idea is to have the pressure of combustion applied to the top of the engine’s piston at the optimum time. It is helpful to think of the combustion process like waves moving away from the point where ignition began, like waves moving away from a rock tossed into water. Make no mistake, combustion waves move very fast by comparison, but they are not instantaneous. A combustion wave expands in the cylinder pushing the piston down during much of its travel.


It is also important to know that there are multiple ways for a fuel-air mixture to ignite. When a cylinder is preparing to fire, fuel and air vapors have been drawn into the area above the piston. As the piston rises, the fuel-air mixture is compressed. The amount of compression varies in different engines. This is called “compression ratio” and is a way of describing the pressure and volume change the fuel-air mixture experiences. If you read my article on refrigeration, you know that the fuel-air mixture gets very hot as it is compressed. If the mixture gets hot enough, it will ignite without a spark, a process known as autoignition. Diesel engines use this method to ignite the mixture. Just remember that diesel fuel and gasoline are very different.

Perfect mixtures of substances don’t happen in the real world. Even in the air you are breathing there is a slight difference in how much oxygen and nitrogen there is from place to place in the room. For an internal combustion engine, the ideal situation would be for the fuel-air mixture to be the same everywhere in the cylinder. Although engine designs are very advanced these days, the fuel-air mixture still varies a small amount from place to place inside the cylinder. If the mixture is ignited due to heat and compression, and not due to the spark from the spark plug, a combustion wave could originate in a different place than at the spark plug and you could have two combustion waves. Imagine two rocks thrown into the water at the same time. Inside the cylinder, when the two combustion waves meet, very high local pressures can develop that can damage the engine. These are called knocks or pings.

There are test engines that have variable compression ratios. There are two gasoline formulas used in the test engines to create a scale. With one formula, the test engine will begin to knock with only a slight increase in compression ratio. While using the other formula, the compression ratio can be increased almost to its maximum before knocks begin. Keep in mind that these formulas are not intended to be used in consumer vehicles. They just establish the scale. Gasoline formulas produced by the likes of Shell or Chevron are used in the test engine and the compression ratio is increased until knocks begin. If the resulting compression ratio of the test is 85% of the full scale, the octane rating of the gasoline formula is 85. There are other test methods as well, but this generally describes what is typical. 

Octane Ratings

Average automobile engines today are designed to run on gasoline with at least an 85-octane rating. No matter what you do with that engine, if it is performing as intended, it won’t knock while burning 85-octane rating gasoline. If you use gasoline with a higher-octane rating, you won’t notice any difference and the overall efficiency of the engine won’t change. However, internal combustion engines can produce more power if they have higher compression ratios. If the average engine had its compression ratio increased, it would knock in some instances while burning 85-octane rating gasoline. If your car’s owner’s manual says you should use a higher-octane rating, then your car’s engine likely has a higher-than-average compression ratio. The extra cost for higher-octane rating gasoline does not make sense if you don’t need it to prevent knocking. For many engines, an octane rating of 85 is “high-performance” fuel.

Modern cars with advanced performance engines can adjust how the engine operates to take advantage of higher octane rating gasoline. I had a car that could do that. I tested three gasoline formulas with different octane ratings and compared the mileage. For that car, the overall cost was the same using gasoline with either a 92-octane rating or 87-octane rating. That meant I could use the higher octane rating gasoline, which resulted in more power when I needed it, without an increase in cost. If you like performance cars like I do, you’ll understand why I was very pleased!

About the Author

David S. Williams, P.E., CFEI is a consulting engineer with our Seattle-Tacoma Office. Mr. Williams provides consultation in the areas of mechanical component failure/fracture analysis, combustion systems, boilers, rotating power equipment, fluid flow control and waste gas handling systems, motor control, and sheet metal stamping equipment. You may contact David for your forensic engineering needs at dwilliams@edtengineers.com or (253) 345-5187.

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