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.