Leaded petrol worked brilliantly in engines, and spread lead across the planet
Out of some 33,000 substances tested, a lead compound proved the best at stopping engine knock: one part per 1,300 of petrol was enough. Tetraethyllead let engines run hotter and harder for most of the 20th century, until a geologist dating the solar system noticed lead contamination nearly everywhere.
Tetraethyllead, or TEL, is a lead atom bonded to four ethyl groups, a colourless, sweet-smelling, syrupy liquid. First made by the German chemist Carl Jacob Löwig, it was blended into petrol from the 1920s. Higher octane fuel resists knock, the uncontrolled early combustion that damages engines, so manufacturers could raise compression and gain power and economy. It could also be patented, which made it profitable.
The chemistry is neat. Its carbon-lead bonds are weak, so in a hot cylinder TEL falls apart into lead and short-lived ethyl radicals. Knock begins with a so-called cool flame, a low-temperature reaction ahead of proper ignition, and the lead mops up the radicals feeding it. Leftover lead would foul the engine, so refiners added dichloroethane and dibromoethane, which turn it into volatile compounds that blow out through the exhaust and into the air. Sold as Ethyl Fluid, the mixture was dyed reddish to mark treated fuel. Lead also cushioned valves against tiny welds that wore their seats.
Aviation made it strategic. In 1935 IG Farben gained a licence so the new Luftwaffe could fly high on high-octane fuel, and in Britain an ICI plant at Holford Moss in Cheshire began production by September 1940. Manufacturing was awkward, converting only 25 per cent of the lead used, yet no better process emerged.
Harm was noticed early. Journals raised concern about airborne lead particles by 1924, and in the 1920s 17 workers at Ethyl, DuPont and Standard Oil died from exposure. A brief ban was lifted in 1929 after industry lobbying and a recommendation from the US Surgeon General. In the 1950s Clair Patterson, using lead to date the solar system, found contamination in the air, on surfaces and in human blood and hair, and campaigned against it from 1965. Most industrialised countries had phased TEL out of road fuel by the early 2000s, and engines switched to hardened valve seats.
Source: Tetraethyllead
