Antiknock agent

An antiknock agent is added to gasoline. It reduces engine knocking and increase the fuel's octane rating.
Gasoline is a fuel mixture. High compression internal combustion engines place gasoline under great pressure. The pressure can cause the gasoline to ignite early ("pre-ignition" or "detonation") causing a damaging knocking sound, also called "pinging" or "pinking". 
Originally, organic compounds with lead were added to petrol to prevent knocking: "tetraethyllead" (TEL). This is now banned because the lead affects people's health. Various alternatives have been tried. 2,2,4-"trimethylpentane", an isomer of octane, is one example.