Everything you need to know about gasoline octane numbers
95 RON, 86 MON, 87 AKI... What is the meaning of the octane numbers, which are usually between 80 and 100, that are displayed next to the names of the gasolines at the pump?
What is octane?
The gasoline used to propel our vehicles is made up of several components, 20 to 30% of which are alkanes. This family of hydrocarbons includes octane, a saturated hydrocarbon whose chemical formula is C8H18.
If it has a number as part of its name, it is because octane is characterized by its extremely high resistance to spontaneous combustion, also known as spontaneous ignition. The octane number therefore expresses the gasoline’s ability to resist spontaneous ignition (in a spark-ignition engine). High octane gasoline will not ignite (or will rarely ignite) in the combustion chamber until the spark plug has produced a spark, thus maintaining the performance and durability of the engine.
How are octane numbers calculated?
To determine the octane number of a gasoline, its resistance to spontaneous combustion must be compared with the resistance of a reference fuel composed of a mixture of iso-octane and heptane. Isomer of octane, or iso-octane, has been assigned an octane number of 100, unlike heptane, which is especially “explosive” and has an octane number of 0.
This means that in terms of self-ignition, a gasoline with an octane number of 95 will react in the same way as a reference fuel composed of 95% iso-octane and 5% heptane.
But the octane number is only a comparative scale. Some fuels, including those used in competition, may have numbers higher than 100.
To prevent knocking
When spontaneous combustion occurs, it causes a detonation whose distinctive sound is referred to as knocking. These uncontrolled explosions subject the engine to unusual and harmful thermal and mechanical stresses, and cause fouling, the emission of pollutants and cracks in engine components, which threaten the longevity of the engine over time.
The engine should therefore be fueled with a gasoline of the octane number specified by the manufacturer. The rule is simple: choose an octane number equal to or higher than the octane number for which your vehicle’s engine was designed.
Classifications around the world
There are three kinds of octane numbers used around the world:
- The Research Octane Number (RON) is the most common, especially in Europe. It represents fuel behavior at low speeds and when accelerating.
- The Motor Octane Number (MON) represents fuel behavior at high speeds and high loads.
- The Anti-Knock Index (AKI) is used on the American continent and is the average of the RON and MON.
The number and variety of octane numbers available at the pump varies from country to country. While service stations in Ireland and Italy (RON 95) only offer one number, those in China have been able to offer up to seven different grades.
In general, grades range from RON 80 to RON 100 throughout the world, or from AKI 81 to AKI 91 in America.