What is Cetane Number? How is the Quality of Diesel improved?

Diesel

Diesel engine fuel are heavier portions of gas oil and lighter portions of residues remaining in the petroleum distillation.
Diesel fuel should contain maximum number of straight-chain hydrocarbon molecules.
In diesel engine, the fuel is exploded not by a spark but by the application of heat and pressure.
An ideal diesel fuel does not knock in the engine.
The diesel oil generally contains 85% of C and 12% of H . Its composition range is C₁₅ C₁₈ .
Its calorific value is about 11,000°kcal/kg.
Its density is about 0.86 to 0.95 .
A good diesel oil should have very little ash content, sulphur and aromatics.

Cetane number

Diesel fuels are rated by their cetane number.
Cetane number is nothing but the spontaneous ignition temperature of a particular diesel fuel.
Cetane, C₁₆H₃₄ , is the best diesel fuel which has the highest ignition temperature and it is given the ratio 100 .
α-methyl naphthalene is the diesel fuel which has the lowest ignition temperature and is given zero rating.
Mixture of these two forms a standard scale with which the spontaneous ignition temperature of an unknown diesel fuel can be found out.
If the cetane number of a diesel fuel is 40, it means that the diesel oil is a mixture of 40% of cetane and 60% of α-methyl naphthalene.

Chemical structure and knocking

The knocking tendency in CI-engines increases as follows:

n-alkane → iso-paraffins → olefins → naphthalenes → aromatics.

Improvement of cetane number

To increase the cetane number of a diesel fuel means increasing its spontaneous ignition temperature.
Diesel engines require a fuel of cetane number greater than 45.
The cetane number of a diesel fuel can be increased by the addition of suitable activities in small amounts which are also known as ‘pre-ignition dopes’.
Some of the additives used for this purpose are methyl alcohol, acetone, alkyl nitrates, diethyl ether, butadiene etc.

Diesel index

According to American Petroleum Institute Scale (API Scale), the quality of a diesel fuel is indicated by diesel index number. The higher the diesel index number, better is the diesel fuel.

Diesel

Difference between gasoline and diesel oil

S. No	Gasoline	Diesel oil
1.	Gasoline is used as a fuel for SI (spark – ignition) engines.	Diesel oil is used as a fuel for CI (compression ignition) engines.
2.	It is a low boiling fraction of petroleum (C₅ C₉hydrocarbons)	It is high boiling fraction of petroleum (C₁₅C₁₈hydrocarbons )
3.	Knocking is due to premature ignition	Knocking is due to ignition lag
4.	Its knocking tendency is measured is octane rating	Its knocking tendency is measured in cetane rating
5.	It has lower thermal efficiency and more fuel consumption	It has higher thermal efficiency and less fuel consumption
6.	Gasoline exhaust gases contain higher amounts of pollutants.	Diesel oil exhaust gases contain lesser amounts of pollutants.