Anti-Knocking Compounds, Octane Number and Cetane Number

Knocking: Knocking is a sharp metallic sound given from the cylinders of spark ignition engines resulting a huge waste of energy. This decreases the power and increases the wear and tear of the engine.

Antiknock compounds: The compounds which are added to the gasoline to eliminate knocking are known as antiknock compounds. Lead tetraethyl [Pb(C₂H₅)₄] is a good antiknock. It is added up to 0.01% in the gasoline. It dissociates into ethyl radicals which stops knocking.

Use of gasoline in an engine: In internal combustion engine (within an automobile) the down stroke of the piston draws a mixture of gasoline vapour and air into the cylinder. During the upward stroke this mixture is compressed into a small volume. This compressed mixture is ignited by a spark from a spark plug producing CO₂ and H₂O. As result of the combustion of gasoline air considerable power is generated which pushes the piston and engine operates.

Octane number: The percentage of iso-octane in the mixture of iso-octane and n-heptane which produces the same knocking as the gasoline under examination. If we say that the octane number of a fuel is 70, it means the knocking property of the given fuel is the same as a mixture of 70% is octane and 30% n-heptane.

Cetane number: Cetane number of a given sample is the percentage of cetane in a cetane (n-hexadecane C₁₆H₂₄) and α-methyl naphthalene mixture which compares in ignition quality with the given sample. If the cetane number of a diesel fuel is 90, it means that it has same ignition as a mixture of 90% cetane and 10% α-methyl naphthalene.