Cetane & Octane

Cetane is like a mirror image of octane

You have heard of octane rating. It’s on the pump when you buy gasoline. Octane and cetane are constituent parts of petroleum like butane, propane, hexane, pentane, and 100 other distillates that can be refined out of crude oil.

Octane is an actual petroleum product, but it is expensive to isolate. It dampens combustion. So, the higher the amount of octane in gasoline the slower gasoline combusts. Inside of a gasoline engine when the fuel/air mixture is compressed, it gets hot. When it reaches the combustion temperature it ignites. If this is not controlled, the fuel will ignite before the spark plug fires and cause the engine to “knock”. Therefore, refineries increase the amount of octane to control the point of combustion and make the engine run better. Because octane is expensive, they use other chemicals in place of octane, like tetraethel lead, to dampen engine knock. That’s why you have an “octane rating” rather than an actual octane value.

Cetane works the other way. A diesel engine does not compress a fuel/air mixture. It only compresses air. As the air is compressed it gets very hot. The fuel is injected as an atomized spray at the point when it is required to combust. Since there is no spark plug, cetane, being a lighter material and more easily ignited burns first and is used to ignite the other less volatile elements. So, cetane acts like a spark plug. A cetane rating of 40 is considered the minimum amount of cetane in diesel fuel to provide acceptable ignition.

In my experience, I have never had a fuel sample test below 40. Many fuels are higher.

NFPA 110, 2010 edition, 8.3.8 does not specify what tests should be run, only that they meet ASTM standards. Since a cetane test would add to the cost and is not specified, I test for the more common problems of microbial growth and water.


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