Research Octane Number (RON)
The most common type of octane rating worldwide is the
Research Octane Number (
RON). RON is determined by running the fuel in a test
engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion.Motors converting heat energy into motion are referred to as engines, which come in many types. A common type is a heat engine such as an internal combustion engine which typically burns a fuel with air and...
with a variable
compression ratio
Compression ratio
The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.
Motor Octane Number (MON)
There is another type of octane rating, called
Motor Octane Number (
MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable
ignition timing
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in most petrol engines and diesel engines used to power the majority of motor vehicles, but they are also used in many other applications...
to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.
Anti-Knock Index (AKI)
In most countries, including Australia and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States and some other countries, like Brazil, the headline number is the average of the RON and the MON, called the
Anti-Knock Index (
AKI, and often written on pumps as
(R+M)/2). It may also sometimes be called the
Pump Octane Number (
PON).
Difference between RON and AKI
Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel. See the table in the following section for a comparison.
Observed Road Octane Number (RdON)
The final type of octane rating, called
Observed Road Octane Number (
RdON), is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed in the 1920s and is still reliable today. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.
