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Tetra-ethyl leadTetra-ethyl lead (TEL) is a viscous, colorless, toxic liquid organometallic chemical compound, formula (CH2CH3)4Pb, that was used as a additive in gasoline (petrol). TEL is damaging to the environment, because lead is toxic to humans and other organisms. It was used to reduce premature ignition (or "knocking) in the engine and to increase the octane rating. The lead deposited on the valve seats reduced wear on them, and the increase in octane rating allowed the use of higher compression ratios for greater efficiency and power. Its use in gasoline is banned or discouraged in most Western nations.
The production of harder metals for valves and valve-seats, a reduction in the compression ratio demanded and substitution with other compounds that reduce pre-detonation helped to lessen the need for TEL. The introduction of the catalytic converter (which tetra-ethyl lead caused to clog up) and a reduced tolerance for hazardous waste caused the cessation of TEL use in Western countries in the latter part of the 20th century.
It was discovered as an effective anti-knocking agent by Thomas Midgley in 1921, working under Charles Kettering at General Motors Research. Due to its extreme toxicity, many early reseachers of TEL became ill (including Midgley himself), and dozens died. In 1924, DuPont and General Motors created the Ethyl Gasoline Corporation to produce and market TEL. In the US in 1972 the EPA launched an initiative to phase out leaded gasoline, which caused Ethyl Corp. to sue the EPA. The EPA won, so in 1976 the phase out began and was completed by 1986. A 1994 study indicated that the concentration of lead in blood dropped 78% from 1978 to 1991.
TEL is produced by reacting ethyl chloride with a sodium-lead alloy. TEL has a very weak carbon-lead bond, and under the heat of an internal combustion engine it decomposes into lead and ethyl radicals, propagating combustion by radical reactions. When TEL burns, it produces lead and many lead compounds (including lead oxide), which would quickly build up and destroy an engine. That is why scavengers such as ethylene dibromide and ethylene dichloride are used, which form volatile lead bromide and lead chloride respectively.
Many vehicles produced before the use of leaded gasoline became discouraged require modification to a greater or lesser extent to run successfully on unleaded gasoline. Valve seat wear problems can be fitted by installing new hardened valve seats, which can be done by a competent automotive machine shop. Many higher-performance cars of that period used high compression ratios that required the high-octane fuels of the period (100 octane was readily available) in order to avoid engine knocking. A major engine rebuild is required to reduce the compression ratio to one that is compatible with currently available gasoline ratings (generally by the use of dished pistons) and this also reduces power.
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