Petrol is primarily characterised by its octane number which defines its ability to burn efficiently and produce more energy per unit. A low octane petrol particle blast inside the engine instead of burning from spark this is called knocking as it produces knocking sound in the engine, to counter this an anti-knocking agent is added to petrol. Earlier Lead was used as an anti-knocking agent but it is highly toxic ,also the researchers who experimented lead with petrol died unnatural death. In later stages when effects of lead toxicity started showing up lead was replaced by benzene compounds as anti-knocking agents this is know as Unleaded petrol. Benzene is carcinogenic and started showing its ill effects after few years especially in densely populated cities. Govts have reduced the concentration of benzene in petrol thereafter and emphasised on implementation of Euro iv standards.
