How to Measure the Octane in Gasoline Containing Ethanol

Commercial gasoline receives an octane rating.

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.

Things You'll Need

• Starting gasoline's octane rating without ethanol added
• Calculator (optional)

Instructions

1. 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.

2. 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.

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.

4. 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.

Tips & Warnings

• In some countries, including the United States and Canada, the rating shown at a gas station is not the octane number but the anti-knock index (AKI). The AKI is determined by averaging the octane number results from two experimental tests. The test engines used in both experiments are similar, but the conditions are different.

References

• Fuel-Testers: Gasoline Octane and E10 Ethanol Blend Fuels
• FAQs.org: Gasoline FAQ--Anti-Knock Index
• Chevron: What is Gasoline?
• Chevron: Knocking Information
• "Principles of Environmental Chemistry;" Octane Ratings of Selected Hydrocarbons; James Girard; 2005
• Ethanol Producer's Database: Fuels, Burning Rate, Stoichiometry, Octane Number, Cetane Number