Octane

• Hydrocarbons are chains of carbon atoms with attached hydrogen atoms. The chain with eight carbon atoms is called octane. Gasoline is a combination of octanes and other, shorter hydrocarbon chains. The octane rating of gasoline is the percentage of octanes in the mixture. The higher the octane rating, the smoother the engine will run.
  
  

Diesel Octane

• Diesel octane is often called "cetane." The requirements for diesel fuel are different, because it is not ignited with a spark. Diesel fuel explodes when it is compressed past a certain point. Because of this method of ignition, diesel fuel can be used with a much lower octane rating. A low cetane rating results in engine knocks, rough running and excess emissions.
  
  

"40-Octane" Diesel

• The lowest cetane rating of diesel fuel that can be legally sold in the United States is 40 octane, or 40 cetane. When a truck tank is marked "40 octane," it means that the truck can operate with any diesel fuel that is legal to sell in the United States.