When gasoline is ignited in an automobile engine, it has a tendency to explode. The result is a violent force against the piston head and what is described as an audible engine 'knock'. When the gasoline reacts with oxygen at a very fast rate (like in an explosion) the force produced against the piston head within the engine is violent, uncontrolled, and inefficient. To control the rate of this reaction and to ensure complete combustion of the fuel, certain chemical compounds are added. The ability of a fuel to resist engine knock is measured on a scale known as the octane rating.
Originally, branched chain hydrocarbons like iso-octane were substituted for linear hydrocarbons like n-heptane.
Branched chain hydrocarbons have a slower rate of combustion due to their increased surface area. Recently compounds termed 'oxygenated hydrocarbons' like methyl tert-butyl ether (MTBE) were introduced.
Oxygenated hydrocarbons reduce the combustion rate and improve the octane rating my supplying an added oxygen source. Unfortunately, MTBE use is controversial due to its toxicity and potential as a ground water pollutant.
