What are Anti knocking agent in fuel?

'Gasoline' has the potential to ignite too early ('detonate') in high compression combustion engines, producing a 'knocking' or 'ping' effect. The first compound discovered to remedy this effect was tetraethyllead (tetra-ethyl-lead, TEL). This anti-knocking agent therefore came into widespread use in petroleum, but use of this chemical began to be phased out after the toxicity of lead became known.

Thus other antiknock agents used instead of tetraethyllead are: ferrocene, iron pentacarbonyl, toluene, MMT (an organomagnesium compound) and several others.