An octane rating is a designation that refers to the detonation resistance of a particular fuel mixture. There are two main octane rating systems, and a third is an average of the first two. The ratings are determined in test engines by first checking the detonation characteristics of a fuel mixture and then comparing them to various ratios of iso-octane and heptane. A fuel mixture with an octane rating of 80 would have similar detonation characteristics to a mixture of 80% iso-octane and 20% heptane. In some circumstances, a fuel may have a rating higher than 100, since some substances have better detonation resistance than pure iso-octane.
In a typical internal combustion engine, the air/fuel mixture is intended to ignite and burn in a controlled process known as deflagration. Detonation occurs when the air/fuel mixture ignites in an uncontrolled manner outside of the flame front that begins at the spark plug. The process of detonation is sometimes referred to as knocking or pinging, and it can cause increased cylinder pressure and severe engine damage. Higher compression engines are both more efficient and more likely to cause detonation, so it can be necessary for fuel to have anti-knocking characteristics.
To create a benchmark that fuels could be measured by, iso-octane was chosen due to its detonation resistance. Since heptane increases the likelihood of detonation, this substance is typically mixed in during testing. The two main testing methods to determine octane rating are represented by the research octane number (RON) and motor octane number (MON). A fuel's RON is determined by burning a fuel under controlled conditions in a motor that has a variable compression at 600 revolutions per minute (RPM). Speeds of over 900 RPM are used to obtain the MON, which also uses a preheated air/fuel mixture and variable ignition timing.
The third type of octane rating is the anti-knock index (AKI), which is the average of RON and MON for a particular fuel. Each of these numbers indicates how a fuel compares in detonation resistance to a mixture of iso-octane and heptane, and can be useful in predicting how it will react in modern, high compression engines. Many engines have a minimum suggested octane rating for fuel, and anything lower may result in detonation and damage to the engine.
Octane ratings refer only to anti-knocking qualities and do not necessarily indicate energy capacity or anything else. Despite its excellent anti-knocking properties, iso-octane is actually lower in energy content than many other fuels. The power that is often associated with high octane ratings typically relates to high compression. Modern engines with high compression ratios tend to be more powerful, and they require high octane fuel in order to avoid detonation. Even if the fuel itself has a relatively low energy content in relationship to its volume, such as gasoline mixed with ethanol, a high compression ratio engine may still produce exceptional power.