Cetane vs Octane

Cetane and octane both relate to combustion, but they describe almost opposite fuel behaviors in different engine types. Mixing the terms leads to bad comparisons.

Octane for gasoline

Octane measures resistance to autoignition or knock in spark-ignition fuel under standardized conditions.

Cetane for diesel

Cetane describes how readily diesel fuel begins combustion after injection into hot compressed air under standardized conditions.

The numbers are not interchangeable

A fuel with a high octane number is not being graded on the same scale or property as a diesel fuel with a high cetane number.

Use the engine-specific specification

Gasoline vehicles need the proper octane grade. Diesel vehicles need diesel fuel that meets the required diesel standard. Do not use one rating to judge the other fuel.

Quick checklist

Use the product label, manufacturer directions, and the specific requirements of your situation. Compare the complete product or system rather than making a decision from one marketing phrase, ingredient, or number.