What does the cetane number indicate for diesel fuel?

The cetane number indicates the ignition quality of diesel fuel. More precisely, it reflects how readily the fuel ignites after being injected into hot, compressed air inside a diesel engine.

Cetane number is tied to ignition delay: the interval between injection and the start of combustion. A higher cetane number generally means a shorter delay, which can improve cold starting and make combustion quieter and smoother. A lower number can produce a longer delay, harsher combustion, and more difficult starting in some conditions.

The scale is based on reference fuels. Cetane, also called n-hexadecane, is assigned a value of 100, while isocetane is assigned 15. A test fuel is compared with blends of those references to determine its rating.

Cetane number is not the same as energy content, viscosity, or sulfur content. Those are separate fuel properties. It is also distinct from gasoline’s octane rating: octane measures resistance to unwanted autoignition, whereas cetane measures readiness to ignite under diesel-engine conditions.