What Is Octane?


95 octane fuel

What is Octane? We hear a lot about Octane in petrol and how a higher Octane rating means better performance in the majority of petrol powered engines, but why is this the case? To have a clear understanding of why high Octane fuel is better than otherwise we need to look into the structure of Octane and understand some of the terms used in relation to it.

What is Octane: The Octane Rating of Petrol

Virtually all petrol stations offer a variety of fuels with different Octane ratings, and the numbers usually range between the high 80s and the mid 90s. This number is the resistance of the fuel to self-igniting. Self ignition of fuel is bad because it may burn before the piston is in the correct position to benefit from the explosion of the fuel, which would reduce the efficiency of the engine. It is this reason – the timing of the ignition of the fuel – that makes higher Octane petrol a better fuel than lower rated petrol. It has nothing to do with the heat energy stored in the fuel, the products of the reaction, or anything else.

This number is based on the behaviour of a mixture of Octane and Heptane which is the other major component of standard fuel. For example, a mixture of 90% Octane and 10% Heptane has a standard resistance to spontaneously bursting into flame. That resistance is given the value “90”. If the amount of Octane is increased to 95% and the Heptane reduced to 5%, the resistance of the fuel is rated as “95”.

This effectively means that the rating of petrol is a good guide to the percentage of Octane in it. However this is not always the case. Fuels may contain other chemicals but as long as the overall mixture has the same resistance to self igniting as, say, the 90/10 mix of Octane/Heptane, that fuel can be labeled “90” Octane rating.

What is Octane: Two Different Octane Ratings

what is octane The rating for a fuel mixture can be determined under research conditions or under more real-life conditions. The Research Octane Number (RON) is obtained by testing a fuel mixture under very controlled conditions, which give the best results for the fuel. This is the number commonly used on petrol bowsers in Australia and Europe.

Other countries such as the United States use a number obtained from a test engine that is put under more realistic conditions than the engine used to obtain the RON. This results in a lower rating as stresses on the engine, variations in the fuel heat as it enters the pistons, jolting of the engine and many other factors give a lower number than the RON. This is called the Motor Octane Number, or MON. The image to the right shows a bowser with Octane ratings given as an average of the RON and MON values of the fuels supplied.

What is Octane: Physical Structure

Octane is a Hydrocarbon. Hydrocarbons are made out of only Carbon and Hydrogen atoms, as the name suggests. The family of hydrocarbons to which Octane belongs is called the alkane series. Octane is the eighth molecule in the family. It has 8 Carbon atoms all single bonded to each other with the 18 Hydrogens surrounding the carbon chain and occupying all the unused bonding sites on the Carbon atoms. The straight chain version of Octane can be drawn as shown below. The first image shows the pairs of outer shell electrons for each atom as obtained in the electron dot diagram for both Carbon and Hydrogen, with each Carbon having 8 outer shell electrons and each Hydrogen having two.

The second image, and all subsequent images, show these pairs of electrons as lines joining the atoms.

research octane number

These molecules are not flat in real life but are three dimensional. This is due to the electrons in the bonds pushing away from each other. We can draw the actual structure of Octane in 3D by following the vsepr rules:

vsepr octane

What is Octane: Isomers

There is more than one way to arrange the atoms in the formula C8H18. These different arrangements, of which the straight chain shown above is only one, are called isomers. All these rearrangements have the same chemical formula but have slightly different chemical properties. There are in fact 18 possible structures that can be made, several of which are shown below. Typically the Octane component of fuel is made of a mixture of these compounds. For this reason the Octane is more correctly called iso-Octane as it is a mixture of many C8H18 isomers.

Note that these isomer structures are given in the flat 2D perspective to make understanding the similarities easy. All these different structures are of course 3 dimensional in reality.

octane isomers

What is Octane: Where Does It Come From?

Octane and its isomers are found in crude oil along with many other hydrocarbons of all sizes. The vehicle fuel grade hydrocarbons are separated from the other components through the fractional distillation of crude oil which uses heat to separate the different length chains according to their boiling points.

What Happens When Octane Burns?

As with all hydrocarbons, Octane and its family of isomers burn with Oxygen to produce Carbon Dioxide and water which is the case when burning fossil fuels of any kind. This is assuming there is sufficient oxygen to react with all of the fuel. If this is the case, the following complete reaction will occur:

octane burning

If there is not enough Oxygen present then other products will also result including Carbon Monoxide and unburnt hydrocarbon fractions. Both these products are far more harmful both to people and to the environment than Carbon Dioxide.