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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 fuel and biodiesel. All the sundry hydrocarbon constituents of diesel fuel are measured and indexed to cetane's base 100 rating.
What Is Cetane Number?
Similar to the octane number rating that is applied to gasoline to rate its ignition stability, 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 diesels 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 true cetane rating requires the use of precisely controlled test engines and procedures or fuel analysis with exacting instruments and conditions. Because 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. These two tests use fuel density and boiling/evaporation points to derive cetane ratings.
How Does Cetane Number Affect Engine Performance?
Just as there is no benefit to using gasoline with an octane rating higher than recommended for a specific engine by its manufacturer, using diesel fuel with a higher cetane rating than is required for a particular diesel engine design yields no bonuses. 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 diesels run best with a fuel rated between 45 and 55. Following is a list of cetane numbers 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
A label should be affixed to the pump that states both the fuel type and cetane number. It's important to find a station that dispenses fuel of the cetane number recommended by the vehicle manufacturer.