Knocking - Characteristics of hydrocarbons
The metallic sound throughout working of an internal combustion engine is known as knocking.
"As much the compression greater will be efficiency of engine." The fuel along with minimum (least amount) knocking characteristic is all the time preferred.
The trend to knock falls off in the given sequence: Straight chain alkanes > branched chain alkanes > olefins > cyclo alkanes > aromatic hydrocarbons.