Cetane Number or Cetane Rating

CETANE NUMBER OR CETANE RATING

Cetane number is introduced to express the knocking characteristics of diesel. Cetane (hexa decane) (C16 H34) has a very short ignition lag and hence its cetane number is taken as 100. On the other hand a-methyl naphthalene has a long ignition lag and hence its cetane number is taken as zero.

Definition

Thus, cetane number is defined as "the percentage of hexadecane present in a mixture of hexa decane and A-methyl naphthalene, which has the same ignition lag as the fuel under test".

The cetane number decreases in the following order.

Straight chain paraffins > Cycloparaffins > Olefins > Branched paraffins > Aromatics.

The cetane number of a diesel oil can be increased by adding additives called dopes.

Important dopes: Ethyl nitrate, Iso-amyl nitrate.

1. Comparison of gasoline oil and diesel oil

Gasoline Oil

1. Low boiling fraction of petroleum contains C₅ - C₉ hydrocarbons.

2. Fuel for SI engine.

3. Knocking tendency is measured in octane rating.

4. Knocking is due to premature ignition.

5. Anti-knocking is improved by the addition of TEL.

6. Its exhaust gases contain higher amount of pollutants.

7. More consumption, lower thermal efficiency.

Diesel Oil

1. High boiling fraction of petroleum contains C₁₅ - C₁₈ hydrocarbons.

2. Fuel for CI engine.

3. Knocking tendency is measured in cetane rating.

4. Knocking is due to ignition lag.

5. Anti-knocking is improved by doping with ethyl nitrate.

6. Its exhaust gases contain lesser amount of pollutants.

7. Less consumption, higher thermal efficiency.