How to Measure the Octane in Gasoline Containing Ethanol

by Joey Espinosa

Most car engines utilize gasoline, which is a mixture of hydrocarbon compounds. If the octane rating for the gasoline is too low, a “knocking” noise can occur when the engine operates. When a high-octane gasoline is used, knocking will not occur; however, high-octane gasoline is more expensive than low-octane gasoline. For standardization, two compounds that are important components in gasoline are assigned octane numbers, and the octane number for all other compounds are determined by empirical testing. By definition, iso-octane (2,2,4-trimethylpentane) has an octane number of 100, while n-heptane is assigned the rating of 0.

Measuring the octane rating in gasoline containing ethanol requires either running a test engine under specific conditions or knowing the octane rating of the starting gasoline without ethanol added. For most people, the latter route is most available.


Find the octane ratings of the components that will be blended together. For the gasoline, this must be known from the supplier or researched. This example assumes that the starting gasoline, referred to as Compound A, has an octane rating of 85, but that rating can vary. Pure ethanol, referred to as Compound B in this example, has a known octane number of 113.


Determine the ratio of the blend. For example, if the blend is 5 parts of Compound A and 3 parts of Compound B, the final ratio is 5-to-3.


Convert the ratio of each component into a decimal. For each compound, the decimal will be the number of parts of that compound divided by the total number of parts. Continuing the previous example, Compound A has a decimal of 5 / (5+3) = 0.625, and Compound B has a decimal of 3 / (5+3) = 0.375.


Calculate the total octane rating. Multiply the decimal by the octane rating, and then add the ratings together. If Compound A in the example has an octane rating of 85, the fractional rating for the component is 85 * 0.625 = 53.1 (rounded off). If Compound B has an octane rating of 113 (as in pure ethanol), for example, its fractional rating is 113 + 0.375 = 42.4. Therefore, the total octane rating for the example is 53.1 + 42.4 = 92.5.

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About the Author

Joey Espinosa has been writing since 1997, having been published in the chemistry journals "Heterocyclic Communications" and the "Journal of Chemical Education," and the ministry newsletter of the Hope Network. He has a Bachelor of Science in biochemistry and a Master of Science in chemistry from Furman University.

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