Cetane number or CN is to diesel fuel what octane rating is to gasoline. It is a measure of the fuel's combustion quality.

Cetane is an alkane molecule that ignites very easily under compression, so it was assigned a rating of 100. All the hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. So, the cetane number measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components.

A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.

To measure cetane number properly is rather difficult (requires actually running the fuel in a special diesel engine called Cooperative Fuel Research(CFR) engine...and there are not many of these about...under standard test conditions). For most practical purposes people control quality using the Cetane Index, which is a calculated number based on the density and distillation range of the oil. There are various versions of this, depending on whether you use Metric or Imperial units: and how many distillation points are used. These days most oil companies use the '4-point method'.

There is very little actual cetane in diesel fuel. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage to keep raising the CN past 50. After that point the fuel's performance hits a plateau.

Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel has additives to improve CN & lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersant, and other additives depending on geographical and seasonal needs.