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