What You Should Know About the Diesel Fuel Cetane

Cetane is a colorless, liquid hydrocarbon (a molecule from the alkane series) that ignites easily under compression. For this reason, it was given a base rating of 100 and is used as the standard measure of the performance of compression ignition fuels, such as diesel and biodiesel. All the sundry hydrocarbon constituents of diesel fuel are measured and indexed against cetane's base-100 rating.

What Is Cetane Number?

Similar to the octane number rating that's applied to gasoline to rate its ignition stability, the cetane number is the rating assigned to diesel fuel to rate its combustion quality. While gasoline's octane number signifies its ability to resist auto-ignition (also referred to as pre-ignition, knocking, pinging, or detonation), diesel's cetane number is a measure of the fuel's delay of ignition time (the amount of time between the injection of fuel into the combustion chamber and the actual start of combustion of the fuel charge).

Because diesel engines rely on compression ignition (no spark), the fuel must be able to auto-ignite—and generally, the quicker the better. A higher cetane number means a shorter ignition delay time and more complete combustion of the fuel charge in the combustion chamber. This, of course, translates into a smoother running, better performing engine with more power and fewer harmful emissions.

How Does the Cetane Number Test Work?

The process for determining the true cetane rating of fuel requires either the use of precisely controlled test engines and procedures or fuel analysis that relies on exacting instruments and conditions. Since using dedicated engines and processes or instruments for real fuel tests is painstaking, expensive, and time-consuming, many diesel fuel formulators use a "calculated" method to determine cetane numbers. Two common tests are ASTM D976 and ASTM 4737 that use fuel density and boiling/evaporation points to arrive at cetane ratings.

How Does Cetane Number Affect Engine Performance?

Just as there are no advantages to using gasoline with a higher octane rating than recommended for a specific engine by its manufacturer, using diesel fuel with a higher cetane rating than required for a particular diesel engine design yields no bonuses either. Cetane number requirements depend mainly on engine design, size, ​speed of operation, and load variations—and to a slightly lesser extent, atmospheric conditions. Conversely, running a diesel engine on fuel with a lower-than-recommended cetane number can result in rough operation (noise and vibration), low power output, excessive deposits and wear, and hard starting.​

Cetane Numbers of Various Diesel Fuels

Normal modern highway diesel engines run best with a fuel rated between 45 and 55. The following list of cetane numbers denotes varying grades and types of compression ignition diesel fuels:

• Regular diesel: 48
• Premium diesel: 55
• Biodiesel (B100): 55
• Biodiesel blend (B20): 50
• Synthetic diesel: 55

It's crucial to find a service station that markets fuel of the cetane number recommended by your vehicle manufacturer. Always look for an appropriate label affixed to the diesel fuel pump that states both the fuel type and its cetane number.