Gasoline and Octane Ratings

You get the same amount of energy from gasoline, regardless of its octane rating.
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Gasoline consists of a complex mixture of hydrocarbons. Most of these are alkanes with 4-10 carbon atoms per molecule. Smaller amounts of aromatic compounds are present. Alkenes and alkynes may also be present in gasoline.

Gasoline is most often produced by the fractional distillation of petroleum, also known as crude oil (it is also produced from coal and oil shale). The crude oil is separated according to different boiling points into fractions. This fractional distillation process yields approximately 250 mL of straight-run gasoline for each liter of crude oil. The yield of gasoline may be doubled by converting higher or lower boiling point fractions into hydrocarbons in the gasoline range. Two of the main processes used to perform this conversion is cracking and isomerization.

How Cracking Works

In cracking, high molecular weight fractions and catalysts are heated to the point where the carbon-carbon bonds break. Products of the reaction include alkenes and alkanes of lower molecular weight than were present in the original fraction. The alkanes from the cracking reaction are added to the straight-run gasoline to increase the gasoline yield from the crude oil. An example of a cracking reaction is:

alkane C13H28 (l) → alkane C8H18 (l) + alkene C2H4 (g) + alkene C3H6 (g)

How Isomerization Works

In the isomerization process, straight-chain alkanes are converted into branched-chain isomers, which burn more efficiently. For example, pentane and a catalyst may react to yield 2-methylbutane and 2,2-dimethylpropane. Also, some isomerization occurs during the cracking process, which increases the gasoline quality.

Octane Ratings and Engine Knock

In internal combustion engines, the compressed gasoline-air mixtures have a tendency to ignite prematurely rather than burning smoothly. This creates engine knock, a characteristic rattling or pinging sound in one or more cylinders. The octane number of gasoline is a measure of its resistance to knock. The octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with a little knock. On the other hand, heptane is given an octane rating of zero. It is an unbranched compound and knocks badly.

Straight-run gasoline has an octane number of about 70. In other words, straight-run gasoline has the same knocking properties as a mixture of 70% isooctane and 30% heptane. Cracking, isomerization and other processes can be used to increase the octane rating of gasoline to about 90. Anti-knock agents may be added to further increase the octane rating. Tetraethyl lead, Pb(C2H5)4, was one such agent, which was added to gas at the rate of up to 2.4 grams per gallon of gasoline. The switch to unleaded gasoline has required the addition of more expensive compounds, such as aromatics and highly branched alkanes, to maintain high octane numbers.

Gasoline pumps typically post octane numbers as an average of two different values. Often you may see the octane rating quoted as (R+M)/2. One value is the research octane number (RON), which is determined with a test engine running at a low speed of 600 rpm. The other value is the motor octane number (MON), which is determined with a test engine running at a higher speed of 900 rpm. If, for example, a gasoline has a RON of 98 and a MON of 90, then the posted octane number would be the average of the two values or 94.

High octane gasoline does not outperform regular octane gasoline in preventing engine deposits from forming, in removing them, or in cleaning the engine. However modern high octane fuels may contain additional detergents to help protect high compression engines. Consumers should select the lowest octane grade at which the car's engine runs without knocking. Occasional light knocking or pinging won't harm the engine and doesn't indicate a need for higher octane. On the other hand, a heavy or persistent knock may result in engine damage.

Additional Gasoline and Octane Ratings Reading

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Helmenstine, Anne Marie, Ph.D. "Gasoline and Octane Ratings." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Gasoline and Octane Ratings. Retrieved from Helmenstine, Anne Marie, Ph.D. "Gasoline and Octane Ratings." ThoughtCo. (accessed May 28, 2023).