Cetane Number

Cetane number, also called cetane rating, is a measure of the combustion quality of diesel fuel during compression ignition.  It indicates how readily the fuel will ignite under the conditions in a diesel engine.  Specifically, a higher cetane number means the fuel ignites more easily, which leads to smoother engine operation, less engine noise, and reduced emissions.

Cetane is a chemical compound found naturally in diesel, and it ignites easily under pressure.  Because of its high flammability, it serves as the industry standard measure for evaluating fuel combustion quality.  Think of it like octane ratings on gasoline. The difference is that, while a good octane rating means a fuel doesn’t ignite too soon, a good cetane number means a fuel ignites as soon as possible.  This is because of a fundamental difference in the way gasoline and diesel powered engines operate.  In a gas engine, fuel is compressed then ignited with a spark plug.  A diesel engine, however, doesn’t have a spark plug combustion occurs through compression alone.

The scale for measuring cetane ranges from 0 to 100, and the higher the number, the better.  Modern highway diesel engines tend to require fuel with a cetane number ranging from 45 to 55.  Typically, regular no. 2 diesel has a cetane number of 40 to 42, while Cenex Premium Diesel has a cetane number of 47 to 52.

Breakdown of What a Cetane Number Represents

Higher cetane numbers (50-55 or above)  -  The fuel ignites quickly after being injected into the combustion chamber, which results in efficient combustion, reduced knocking, and lower emissions.
Lower cetane numbers (below 40)  -  A longer ignition delay, where the fuel takes longer to ignite, leading to more unburned fuel, increased knocking, and potentially more emissions.

Typical Cetane Numbers

Diesel engines  -  Usually require fuel with a cetane number of at least 45-50.
High-speed diesel engines  -  Benefit from cetane numbers above 50, ensuring smooth ignition and lower emissions.
Biodiesel  -  Tends to have higher cetane numbers (around 50 or higher), improving ignition but sometimes with different energy outputs than traditional diesel.

Cetane index is a calculated value sometimes used as an estimate of the cetane number, especially when a full cetane test is impractical.