Octane number

Every brand of gasoline has an octane rating or number which indicates its ability to resist knocking. When the numbers were first developed, the researchers found that normal heptane (a hydrocarbon) had almost no ability to resist knocking so they gave it an octane number of zero. Then they found that isooctane (also a hydrocarbon) was very resistant to knocking so they gave it the octane number of 100. To measure a particular sample of gasoline they discovered when it began to create detonation. Then they mixed isooctane to heptane to find out what percentage of isooctane created the same results as the sample of gasoline. In United States, there are two methods for determining the octane number depending upon operating conditions. The Research Octane Number (RON) is obtained when conditions are somewhat mild. The Motor Octane Number (MON) is obtained when conditions are somewhat severe and give a much lower number than the RON. The numbers on the gasoline pumps are usually an average of the RON and MON. Usually the pump number is about four numbers less than the RON. Thus if the automobile manufacturer recommends using gasoline with a RON of 91 or more, it would be acceptable to use pump gas rating of 87. The pump number is the anti-knock index which is half the sum of the RON and the MON. See motor octane number, research octane number