### Question
How is the "cetane rating" defined?


### Answer
The term **cetane rating** is defined in *15 U.S.C. § 2821* as follows -

> a measure, as indicated by a cetane index or cetane number, of the ignition quality of diesel fuel oil and of the influence of the diesel fuel oil on combustion roughness

#### Citation:
  - *15 U.S.C. § 2821*
  - Heading: **CHAPTER 55-PETROLEUM MARKETING PRACTICES > SUBCHAPTER II-OCTANE DISCLOSURE**

