A fuel’s octane rating is a measurement of how stable the fuel is when heat is introduced. Octane rating is a major consideration when deciding whether a fuel is suitable for a particular application. For example, in an engine, we want the fuel to ignite when the spar plug fires and not before, even under high pressure and temperatures.
Once the fuel is ignited, it must burn slowly and smoothly, even though heat and pressure are building up while the burns occurs. The unburned fuel should be ignited by the traveling flame front, not by some other source of ignition, such as carbon deposits or the heat from the expanding gasses. A fuel’s octane rating is known as a measurement of the fuel’s anti-knock properties.
A fuel higher octane rating can be subjected to a more severe combustion environment before spontaneous or abnormal combustion occurs. To understand how two gasoline samples can be different, even though they have the same octane rating, we need to know how octane rating is determined.
A universal method of determining the octane rating of a fuel sample was developed. The rating you see on the pump at a fuel dock is known as the pump octane number. The small print on the pump has a rating formula. The rating is determined by the R+M/2 method. This number is the average of the research octane reading and the motor octane rating.
The Research Octane Rating is a measure of a fuel’s anti-knock properties under a light load or part throttle conditions. During this test, combustion heat easily dissipated.
The Motor Octane Rating is a measure of a fuel's anti-knock properties under a heavy load or full throttle condition, when heat buildup is at maximum.