What Is Octane and How Does It Work?

While everyone who drives has heard the word "octane", very few people understand the term or its relationship to fuel. So, what is octane? It's a hydrocarbon molecule that gives gasoline the ability to resist detonating prematurely in the engine, which causes engine knocking or pinging during combustion.

An octane rating measures the amount of the molecule that is in everyday gasoline. The higher the octane number, the greater the fuel's resistance to knocking. Since knocking can damage an engine, octane is very in-demand! In this article, we'll explore the history and uses of octane, helping you to understand its role in high-performance engines.

High Octane Fuel, Higher Compression Ratios

If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1.

Car Engine Image Gallery

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Lower-octane fuel (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a high-performance engine has a higher compression ratio and requires higher-octane fuels.

The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight — that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

Octane Offers Unparalleled Fuel Stability

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels.

For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well; you can compress it a lot and nothing happens.

Testing Octane in Vehicle Engines

Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). Fuel ignites spontaneously at a given compression level, and can only be used in engines that do not exceed that compression ratio.

Middle range octane fuel, with ratings around 89 to 90, serves as a middle ground, formulated for vehicles that benefit from a slight bump in performance and efficiency. Higher octane fuel, boasting ratings of 91 to 93 or higher, is engineered for high-performance engines with high compression ratios, turbochargers, or those specifically tuned to require premium gasoline.

The History of Octane

During WWI, it was discovered that you can add a chemical called tetraethyl lead (TEL) to gasoline and significantly improve its octane rating above the octane/heptane combination. Cheaper grades of gasoline could be made usable by adding TEL. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:

Lead clogs a catalytic converter and renders it inoperable within minutes.
The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).

When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline (known as AvGas), and octane ratings of 100 or more are commonly used in super-high-performance piston airplane engines. In the case of AvGas, 100 is the gasoline's performance rating, not the percentage of actual octane in the gas. The addition of TEL boosts the compression level of the gasoline — it doesn't add more octane.

Currently engineers are trying to develop airplane engines that can use unleaded gasoline. Jet engines, meanwhile, burn kerosene.

The Science Behind Octane Ratings

Octane ratings serve as a crucial benchmark for assessing fuel quality, directly impacting engine performance and longevity. At the core, these ratings measure a fuel's ability to resist premature detonation, also known as knocking, during the combustion process. A minimum octane rating will therefore offer less detonation resistance than a higher octane fuel.

High-performance engines, designed with higher compression ratios, demand fuels with elevated octane numbers to harness their full potential while safeguarding against internal damage. This resistance to knocking is quantified through various standard tests, resulting in the Research Octane Number (RON) and Motor Octane Number (MON), which reflect the fuel's stability under different operational conditions.

Understanding the science behind the motor octane rating system allows consumers to make informed choices, matching their engine speed with the appropriate fuel grade. This helps consumers optimize both performance and fuel economy.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.