Additives in petrol that increase the octane number of petrol by increasing anti-knocking property of petrol are known as known as dope or doping agents. The main function of doping agents in petrol is reduced the combustion rate of the air petrol mixture. The doping agents undergo thermal degradation to form free radicals which reduce combustion rate and increase octane number. Earlier tetraethyl lead (TEL) was predominantly used. Although as the deposit leads in the cylinder and on the spark plug is removed as lead bromide along with exhaust gases. Therefore, nowadays various organic compounds containing oxygen are used as anti-knocking agents to increase anti-knocking properties of the petrol and is known as unleaded petrol.
However, in case of diesel engine knocking is due to the slow combustion rate air diesel mixture initiated due to compression of air. In diesel oils, the doping agents like is amyl nitrate, ethyl nitrite, acetone peroxide etc are used. They provide nascent oxygen at a higher temperature to increase combustion rate and cetane number.