Octane and cetane are often talked about in similar terms, as equivalent properties for gasoline and diesel fuel, respectively. In the past, the Bell Performance Blog talked about octane value and what it does in gas-powered engines. Today we turn our attention to diesel fuel and its measure of ignition quality, cetane rating.
While they are categorically similar (as both are expressions of combustion quality), octane and cetane values actually work in opposite ways. A 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 gives some of the same benefits people normally associate with high-octane gasoline, even though they accomplish this by opposite methods. High cetane value normally results in 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 (hence, a shorter ignition delay). All this translates into a smoother-running, better-performing engine with more power and fewer harmful emissions.
Cetane value also impacts how well the diesel engine starts in cold weather. There is no spark plug to fire up the 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 compression 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.
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.
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:
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 vehicle manufacturer.
If it is 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.