It is interesting reading about the different types of fuels available across the world and like many people I wanted to find out what Research Octane Numbers are. For example, when filling your car, the bowser will have a selection of numbers such as 91, 95, 98 etc.
To make things slightly more difficult, there is also a MON (Motor Octane Number) and a R+M/2 Octane Number which is also called an AKI (Anti Knock Index). This last one is the average of the RON and MON. I will talk about the RON and will delve into the past to explain how these figures are derived.
What is an Octane Number?
Simply put, this is the amount of resistance that a fuel needs to detonate. The higher the number, the less likely it is that a particular fuel will detonate in an engine. The number is compared to a standard fuel (not necessarily petrol).
There are two machines that do the testing. Both the Motor Octane Machine and the Research Octane Machine, were made in the 1930’s and were designed to test for octane numbers in the 0-100 range. Any number above 100 is an extrapolation and is usually reserved for competition fuels that are illegal for normal road use. Both of these machines are dinosaurs and are not adequate for today’s high technology fuels or engines, they are there to simply test the octane of fuels.
The machines are one-cylinder engines that have an adjustable head on them that can move up or down to increase or lower the compression ratio while the engine is running. The Motor and Research machines are the same in this respect, but they differ in several other characteristics. The Motor Octane Machine is run at a higher RPM, hotter temperature and with more timing, and will put more stress on any fuel and more accurately represents a racing engine as opposed a pure road engine.
The Research Octane Machine will always produce a higher number for the obvious reason that it does not put the same amount of stress on the fuel sample. It is the RON that is displayed at fuel stations and defines the performance of the fuel.
Most performance cars are tuned to use 98 or higher fuels with family cars and economy cars tuned for 91 or 95 depending on the country of sale.