Cetane number and rating of CI fuels

Cetane number

The cetane number (CN) of a diesel fuel is the percentage by volume of cetane (n-hexadecane, C$_{16}$H$_{34}$) in a mixture with alpha-methylnaphthalene that gives the same ignition delay as the fuel under test. Cetane (ignites very easily) has CN = 100 and alpha-methylnaphthalene (ignites poorly) has CN = 0. In current practice heptamethylnonane (HMN, CN = 15) is often used in place of alpha-methylnaphthalene, so that $\text{CN} = \%\ \text{cetane} + 0.15 \times \%\ \text{HMN}$.

Rating of CI fuels

• The fuel is tested in a CFR diesel engine at 900 rpm with fixed injection timing (13 degrees before TDC).
• The compression ratio is changed until ignition starts exactly at TDC, i.e. the ignition delay is 13 degrees.
• Reference blends are run to find the blend with the same delay at the same compression ratio.
• Another quick estimate is the diesel index from aniline point and API gravity: $\text{DI} = \text{aniline point}(^\circ\text{F}) \times \text{API}/100$.

Normal diesel has CN 40-55. A high CN means a short ignition delay.

Octane number versus cetane number

Effect on diesel knock

A low CN gives a long ignition delay. A large amount of fuel collects before burning and then burns suddenly, giving a very rapid pressure rise and the harsh knock. A high-CN fuel has a short delay, smooth combustion, easier starting and less noise.