Both octane number and cetane number are numerical measures reflecting the ignition characteristics of engine fuels. But they are different, and the differences are manifold. Here are the main points of difference between octane number and cetane number.
- To begin with, the basic difference: octane number is a fuel rating valid for gasoline (or petrol), whereas cetane number is a rating for diesel fuel.
- Octane number reflects gasoline’s ability to resist auto-ignition (also called pre-ignition, self-ignition, pinging, detonation, or knocking) under high pressures and temperatures, while cetane number reflects just the reverse, the ability of diesel to auto-ignite or self-ignite under compression at relatively lower temperatures.
- The higher the octane number, the better the anti-knock property and hence the stability of fuel, as desired for the spark ignition system in gasoline engines. The higher the cetane number, the shorter the ignition delay time and hence the better the ignition/combustion quality of fuel, as desired for the no-spark, compression ignition system in diesel engines. (Both translate to better and more powerful engine performance, smoother running and lesser harmful emissions. Different engine designs may require different recommended fuel ratings.)
- Octane number describes how poor the ignition characteristics (but how good anti-knock characteristics) are, while cetane number describes how good the ignition characteristic are.
- Octane number derives its name from a highly ignition-resistant liquid hydrocarbon (2,2,4-trimethylpentane) named isooctane, which is assigned an octane number of 100 as a standard against which gasoline fuels are rated. Cetane number, on the other hand, derives its name from a liquid hydrocarbon named cetane, which ignites readily under compression and hence is assigned a cetane number of 100 as a standard against which compression ignition fuels like diesel and biodiesel are rated.
- Chemically, octane number of a gasoline fuel is given by the percentage of isooctane in a mixture of isooctane (octane number 100) and n-heptane (octane number 0) that would yield the same anti-knock properties as the given fuel. Cetane number, on the other hand, is given for a diesel fuel by the percentage of cetane in a mixture of cetane (cetane number 100) and isocetane (cetane number 15) that would give the same ignition characteristics as the given fuel.
That is all about the difference between octane number and cetane number.