Editing Cetane number (section)

== Definition ==
The cetane number of a fuel is defined by finding a blend of [[cetane]] and [[heptamethylnonane|isocetane]] with the same ignition delay. Cetane has a cetane number defined to be 100, while isocetane's measured cetane number is 15, replacing the former reference fuel [[Alpha-methyl naphthalene|alpha-methylnaphthalene]], which was assigned a cetane number of 0. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane's 100 and isocetane's 15.<ref name=":0" />
:cetane number = % ''n''-cetane + 0.15(% isocetane)<ref name=":0" />

[[Cetane]] number is an inverse function of a fuel's ignition delay, the time period between the start of ignition and the first identifiable pressure increase during combustion 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. For heavy (residual) fuel oil two other scales are used, [[Calculated Carbon Aromaticity Index|CCAI]] and [[Calculated Ignition Index|CII]].