Octane

Octane Number

The anti-knock property “octane” is one of the most important characteristics of gasoline. Knocking occurs during the combustion of the air-fuel mixture, when the temperature and pressure of the un-burnt portion ahead of the flame front has been raised to the point of spontaneous ignition.

When this point is reached, combustion occurs instantly at the end charge, giving rise to high-pressure waves, which can damage the walls of the combustion chamber that produces a “knocking” noise. 

This phenomenon is affected by the chemical composition of the fuel and premature or spontaneous detonation is accompanied by loss off engine power.

Heat is wasted by having to flow through the piston rings, thereby causing the rings to stick due to carbonisation of the oil. 

In order to predict whether a particular grade of fuel will give satisfactory performance in a particular engine, the anti knock values have to be pre determined. This is done using standard ASTM-CFR laboratory test engines.

Testing

Samples are rated against two known octane standard reference fuels of simple composition and therefore reproducible, with particularly good and particularly poor resistance to knock, respectively. 

After much development work, iso-octane (2, 2,4-trimethylpentane) and n-heptane were selected with arbitrary octane rating of 100 and 0 respectively. 

The octane number of a sample fuel is defined as the volumetric content of iso-octane in the reference fuel blend that matches the identical knock behaviour under specified test conditions. 

RON and MON

Gasoline’s are octane rated in the under two different CFR engine conditions referred to as Research Octane Number (RON) and Motor Octane Number (MON).  

Research octane numbers are generally higher than those obtained by the Motor Method and the difference between the two ratings is known as the “sensitivity” of the fuel. 

​Fuel Sensitivity

The “sensitivity” of low octane fuels is usually small, but with high-octane fuels it varies greatly according to fuel composition.

For most commercial blends it is between 7 and 12 octane numbers in the 90 to 100 Research Octane Number range. 

​The actual octane performance of a gasoline on the road is sometimes referred to as; “road index number”. This is referred to and marketed in the United States and is calculated from both RON and MON results added and then divided by two. 

Legislation

For quality reasons both RON and MON have to comply with min and max octane specifications stipulated by its appropriate country’s legislation.