Octane, Gasoline and Engine Knock
The Octane rating on Gasoline had always been a confusing thing for me. Until I learned what it actually meant, it just seemed like this arbitrary measure and that higher octane fuel was ‘better’.
To understand Octane I first need to talk about Gasoline a bit. Gasoline is a liquid mixture of many different types of hydrocarbons, each with different properties:
Continuous-chain Alkanes burn unevenly or too quickly.
Branch Chain Alkanes burn evenly or more slowly
Aromatic Compounds burn evenly or more slowly.
It now makes sense that various types of hydrocarbons in gasoline need to be balanced for the optimal performance as fuel.
What is Octane?
Octane is related in that it is a measure of gasoline that determines its performance in one area in particular; its anti-knock characteristics. Engine knock is a loud sound that can be produced by combustion of fuel earlier than it should occur; it basically burns too quickly and burns incompletely yielding ‘gunk’ that can mess up the engine. Only put high octane fuel in your engine if it is safe to do so; consult your vehicles manual or company to find what type of fuel they recommend.
Octane simply is a reference to two hydrocarbons and their properties in a combustion engine. Heptane is designated an Octane Number of 0 (remember Continuous-chain Alkanes burn very quickly); while Isooctane is given an Octane rating of 100 (Also note that Branch Chain Alkanes burn more slowly). Isooctane is also known as 2,2,4-Trimethylpentane.
Gasoline is placed in a test engine and the compression ratio is increased until knocking occurs. This compression ratio is related to a particular mixture of Isooctane and Heptane causing knocking at that compression ratio.
The Octane number is the numerical value given to gasoline that has the same equivalent anti-knock characteristics as that same amount of Isooctane/Heptane mixture. For example Octane 89 fuel is gasoline that has the same anti-knock characteristics as a mixture containing 89% Isooctane and 11% Heptane.
Additives are added to Gasoline to increase the Octane number. Some additives and their representative Octane numbers are listed below:
Methyl t-butyl ether (MTBE): 116
High performance vehicles and race cars require high octane fuels to perform optimally with their powerful engines. Its also why some race cars use methanol and other alcohol based fuels (look at the Octane rating above!).
Sources: CHEM 2409 (Organic Chemistry I) notes from BCIT.