Differences between Octane Number and Cetane Number

Cetane number (diesel fuel) and octane number (gasoline) both measure the tendency of the fuel to ignite spontaneously. In the cetane number scale, high values represent fuels that ignite readily and, therefore, perform better in a diesel engine. In the octane number scale, high values represent fuels that resist spontaneous ignition and, therefore, have less tendency to knock in a gasoline engine. Because both scales were developed so that higher numbers represent higher quality for the respective use, high cetane number fuels have low octane numbers, and vice versa.

Octane Number Cetane NumberThe Octane Number of a gasoline is defined as the volume percent of isooctane (2,2,4-trimethylpentane, an isomer of octane) in a blend of isooctane and n-heptane. An 87-octane gasoline, for example, has the same octane rating as a mixture of 87 vol-% isooctane and 13 vol-% n-heptane. This does not mean, however, that the gasoline actually should contain these chemicals in these proportions. It simply means that it has the same autoignition resistance as the described mixture.

The cetane number of a fuel is defined as the volume percent of n-hexadecane in a blend of n-hexadecane and 1-methylnaphthalene that gives the same ignition delay period as the test sample. For example, a fuel with a cetane number of 40 will perform the same in the engine as a blend of 40% n-hexadecane and 60% 1-methylnaphthalene. Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils.

The octane Number is a measure of the auto ignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It's a measure of anti-detonation of a gasoline or fuel.

The Cetane Number is a measure of the combustion quality of diesel fuel under compression.

The higher the octane number the better is the resistance of Gasoline to combustion prematurely, known as Knocking.

The Higher Cetane number means that the Diesel will ignite readily and, therefore, perform better in a diesel engine.

Isooctane is given a maximum octane number, i.e. 100.N-heptane is given a minimum octane number, i.e. 0.

N-hexadecane is given a maximum octane number, i.e. 100.1-methylnaphthalene is given a minimum octane number, i.e. 0.

Octane numbers are only used between 0 and 100. The fuels having better anti-knocking property than isooctane are rated in other scales like octane Performance.

Cetane numbers are only used for the relatively light distillate diesel oils. For heavy (residual) fuel oil two other scales are used CCAI and CII.

Octane number is actually a measure of a fuel's autoignition (prematurely) delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular internal combustion engine, higher octane fuels will have larger ignition delay periods than lower octane fuels.

Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels.