most automobiles employ the four-stroke operation of the Otto cycle engine.A major engineering concern is to control the burning of the gasoline-air mixture inside each cylinder to obtain a smooth expansion of the gas mixture. If the mixture burns too rapidly,the piston receives a hard jerk rather than a smooth strong push.This action produces a "knocking" or "pinging" sound,as well as a decrease in efficiency in the conversion of combustion energy to mechanical energy.It turns out that straight-chain hydrocarbons have the greatest tendency to produce knocking,whereas the branched-chain and aromatic hydrocarbons give the desired smooth push.