What are Gasoline Octane Ratings

by : Bob Jent

You have probably noticed that most gas stations provide the option between different "octane ratings" of gasoline for vehicles. Octane ratings are the numbers that typically correspond with the regular, mid-grade and premium nomenclature that often tend to perpetuate confusion. Contrary to these suggestive designations, a higher octane, "supreme" rating does not necessarily mean that the gasoline is simply better for your vehicle. Rather, different octane ratings are available to accommodate different vehicles with engines that require gasoline to meet certain compression requirements to function properly and avoid damage to the engine.

To understand octane ratings, first understanding the basic operation of an internal combustion engine is helpful. You have probably heard vehicles being classified as having four, six or eight cylinder engines. A cylinder is a fundamental component of an internal combustion engine, and each cylinder has a cylindrical shaped metal piston that moves within it. An intake valve opens to allow a mixture of air and gasoline into a cylinder in one stroke, which is called the intake stroke. Then, the piston moves to compress the air and gas mixture within the cylinder during what is called the compression stroke. A spark plug is designed to ignite the compressed air and gas mixture, creating an explosion. The fuel in the vehicle must be able to withstand the pressure exerted on it during the compression stroke, or else it can spontaneously ignite instead of being ignited by the spark plug as intended.

The amount of compression that fuel can withstand before it spontaneously ignites is determined by the fuel's octane rating. Ignition should be initiated by a spark from the spark plug, as ignition from compression can cause damage to an engine. High octane gasoline can withstand high compression, while lower octane gasoline is capable of withstanding lower levels of compression without spontaneously igniting.

The type of engine in a vehicle determines the octane rating of the gasoline that the vehicle requires to obtain adequate compression. The term octane refers to eight hydrocarbons in a chain, and heptane refers to a chain of seven carbons. Gasoline that is given an eighty-seven octane rating is made up of 87 percent octane, with the remaining 13 percent comprised of heptane. Likewise, gasoline designated as 93 octane is made up of 93 percent octane, and is generally recommended for high performance vehicles. Using the octane rating designated for your vehicle is recommended to prevent engine problems. The recommended octane rating is usually specified in the vehicle owner's manual as well as inside the fuel door of the vehicle.