Last month, the Bell Performance Blog talked about octane value and what it really does in gas-powered engines. Today we turn our attention to diesel fuel and its measure of ignition quality, cetane.
While gasoline's octane number signifies its ability to resist auto-ignition (also referred to as pre-ignition, engine knocking, pinging, or detonation), the cetane number of diesel fuel 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.
Recall that diesel engines are called compression ignition engines because they don't have a spark plug; they rely on compressing the fuel and increasing its temperature through compression until the fuel ignites on its own. Because diesels rely on compression ignition (no spark), the fuel must be able to auto-ignite well --and generally, the quicker the better.
A higher cetane number in the fuel means some of the same kind of benefits people normally associate with high octane gasoline. A shorter ignition delay time and, generally speaking, more complete combustion of the fuel charge in the combustion chamber (which means less black smoke emissions during start-up and operation). The higher the cetane number, the more easily the fuel can be ignited. This, of course, translates into a smoother running, better performing engine with more power and fewer harmful emissions.
Cetane value also relates to how well the diesel engine starts in cold weather. There is no spark plug to fire up ignition, so the diesel engine has to have the fuel warm up in temperature before it will ignite. Larger diesel trucks may turn over for a minute or two before the constant cycle of compresson and release warms the fuel up enough that it reaches the point that it ignites on its own. Diesel fuel with a higher cetane fuel ignites more easily and makes the diesel easier to start in cold weather. Trucks especially know this to be true.
Beyond this fact, 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.
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 the cetane numbers of 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
How do you know the cetane rating of the diesel fuel you're buying? 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 diesel fuel of the cetane number recommended by the veicle manufacturer.
If its hard for you to get diesel fuel with a cetane number sufficient for your needs, a dedicated cetane improver like Super-Tane should be all you need. If your diesel engine is a high mileage engine that has seen its performance go down over time, you may just need a good detergent diesel fuel treatment like Dee-Zol. Cleaning the combustion chamber offers a net cetane reduction in the engine's requirement, since diesel auto-ignition is related to the amount of space in the combustion chamber.
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This post was published on February 16, 2012 and was updated on March 13, 2020.