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

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