Explain knocking in diesel engine and catane number

In a diesel engine, if the combustion rate of air diesel mixture initiated due to compression of air becomes very slow then a large amount of fuel collected in the cylinder. This accumulated fuel burns violently and none uniformly producing loud sound this is known as diesel knock.

• The knocking tendency of diesel increase with an increase in compactness of the fuel molecule but in case of petrol it is opposite.
• As the branching in hydrocarbons increase knocking in diesel engine increase (cetane has cetane number=100 and ∝-methyl naphthalene has cetane number =0).
• Longer the length of the hydrocarbon better is its knocking characteristics in case of a diesel engine.
• Alkane has more knocking property in case of a diesel engine as compared to the corresponding alkane.
• Alcohols have lesser knocking characteristics in diesel engine.
• Aromatics have greater knocking tendency in diesel engine.

Thus, in case of a diesel engine, it can be summarized as

• Straight chain paraffin – knocking increases downwards.
• Cycloparaffin – Ant knocking property decreases downwards cetane number decrease downwards.
• Alkene -Inition delay increase
• Branched chain paraffin -Ignition quality decrease
• Aromatic compound