What is Knocking
The objectionable metallic sound produced during the working of an internal combustion engine is known as knocking. It leads to wastage of fuel.
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Factors effect Knocking.
Knocking depends upon the composition of the fuel, it falls off in the following order : Straight chain paraffins > Branched chain paraffins > Olefins > Aromatic hydrocarbons
Knocking may also be prevented or minimised by adding small amounts of certain compounds known as anti-knock compounds, e.g. tetraethyl lead (TEL) [(C2H5)4Pb] Such petrol is known as leaded petrol or ethyl petrol.
Tetraethyl lead is used as a mixture of TEL (63%), ethylene bromide (26%), ethylene chloride (9%) and methylene blue (2%). About 0.5-0.6 ml of this mixture is added to 1 litre of petrol.
In the cylinder of combustion engine, TEL decomposes to produce radicals which combine with the radicals produced due to irregular combustion. As a result, the reaction chains are broken and smooth burning of fuel occurs. This prevents knocking 
This lead is deposited in the cylinder which is removed by ethylene bromide to form lead bromide volatile at the ignition temperature,
However, use of TEL in petrol is facing a serious problem of lead pollution. To avoid this, a new compound cyclopentadienyl manganese carbonyl (called as AK-33-X) is used, in developed countries, as an ant knocking compound.
Frequently Asked Questions
Knocking is when fuel burns unevenly in an engine, causing noise and damage.
Knocking happens when fuel-air mix ignites too early in the engine.
Cracking breaks large molecules into smaller ones; knocking is uneven fuel burning in engines.
Knocking is uneven burning in engines; anti-knocking agents reduce this problem.