With lead and MTBE additives now phased out, spiking petrol with a small amount of ethanol raises its octane level enough to prevent the engine from "knocking" under load.
Previously, MTBE (methyl tertiary-butyl ether) was the oxygenate of choice, but fell out of favour in 2004 when it was found to contaminate ground water.
The etherification process for FCC gasoline is to react methanol with the isoolefines in FCC gasoline to form methyl tert-alkyl ethers on a resin type catalyst, so as to enhance the octane number.