knocking, in an internal-combustion engine, sharp sounds caused by premature combustion of part of the compressed air-fuel mixture in the cylinder.

Knocking is a phenomenon that occurs in a gasoline engine if the fuel used does not have a high enough octane rating.

Anti – knocking agents are chemical compounds added in the fuels that help in effective combustion.

An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs.

Examples are: 

• Methylcyclopentadienyl manganese tricarbonyl (MMT)
• Ferrocene.
• Iron pentacarbonyl.
• Toluene.
• Isooctane.

• Ethanol boosts the octane number of fuel, which helps prevent pre-ignition knock.

• Organic lead (tetraethyl lead) is used as an antiknock agent in gasoline and jet fuels.