Early automotive engines ‘knocked' whenever poor quality gasoline was used. In 1921, Thomas Midgley Jr. used tetraethyl lead as antiknock additive in gasoline to make engines run smoother and quieter. By 1926, an octane rating was introduced to measure the quality of gasoline (compression tolerance). The use of lead additives was discontinued in the 1970s because of environmental concerns.
Today, small amount of chemicals (alcohols, ethers) are added to gasoline to improve octane rating, enhance gasoline performance (metal deactivators), and reduce engine friction and wear to extend engine life (detergents). Seasonal chemical additives are used in some areas for geographical concerns, such as the addition of methanol to prevent freezing of fuel line.
Lead tetraethyl molecule