Octane Ratings

Octane is a hydrocarbon molecule, specifically an alkane with the chemical formula C8H18. It is a colorless liquid that is a component of petroleum. The term “octane” is commonly associated with the octane rating of fuels, as I mentioned earlier, which measures a fuel’s resistance to knocking in internal combustion engines.

In the context of the octane rating, the term is somewhat of a misnomer. The octane rating is not directly related to the concentration or presence of octane (C8H18) in the fuel. Instead, it refers to the fuel’s ability to resist knocking compared to a mixture of iso-octane (2,2,4-trimethylpentane) and heptane. Iso-octane is assigned an octane rating of 100, indicating excellent resistance to knocking, while heptane is assigned an octane rating of 0, indicating a high likelihood of knocking.

So, when we say a fuel has an octane rating of, for example, 91, it means that the fuel’s knocking characteristics are equivalent to a mixture of 91% iso-octane and 9% heptane. The higher the octane rating, the greater the resistance to knocking.

The octane rating is expressed as a numerical value, with two commonly used methods: Research Octane Number (RON) and Motor Octane Number (MON). The average of these two values is often referred to as the Anti-Knock Index (AKI) or Pump Octane Number (PON), which is the number typically seen at gas stations.

A higher octane rating indicates a greater resistance to knocking. High-performance engines and those with higher compression ratios often require fuels with higher octane ratings to prevent knocking and optimize performance. On the other hand, engines with lower compression ratios may not benefit as much from higher-octane fuels.

In practical terms, consumers often see octane ratings such as 87, 89, and 91 at gas stations. These numbers represent the AKI or PON and indicate the fuel’s ability to resist knocking. For instance, 87-octane fuel is more prone to knocking than 91-octane fuel.

It’s important to note that using a higher-octane fuel than recommended for an engine does not generally provide additional performance benefits and may not be cost-effective. Modern engine management systems are designed to adjust parameters to accommodate the octane rating specified by the manufacturer.