An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs.
Notable early antiknock agents, especially tetraethyllead, added to gasoline included large amounts of toxic lead.
[1][2] The chemical was responsible for global negative impacts on health, and the phase out of leaded gasoline from the 1970s onward was reported by the United Nations Environmental Programme to be responsible for "$2.4 trillion in annual benefits, 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes.
The discovery that lead additives modified this behavior led to the widespread adoption of the practice in the 1920s and therefore more powerful higher compression engines.
From January 1, 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles in the United States.
However, fuel containing lead may continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines.
[citation needed] Gasoline, as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improve combustion, and to allow easier starting in cold climates.
Around 70% of the difference was accommodated by more advanced processes at the refinery stage, cracking other hydrocarbon products from the distillation stack to modify them into fuels that would blend gasoline closer the appropriate octane.
[9] MTBE's water pollution issues prompted plans for a phaseout, starting in 2000 with an EPA draft proposal, which was addressed several times at the state level in the years to follow, and eventually cemented in place federally with a 9-year phaseout in 2005's Energy Policy Act, with significant proportions of fuel ethanol designated as the replacement antiknock agent for the US automotive fuel system.
Congress' attempts to promote ethanol for its geopolitical use as a backstop on any attempts to limit the US' gasoline supply, and also its incentives to reward Iowan corn farmers, whose state political primaries hold a special place in the electoral system, escalated ethanol from an additive to be used as needed, then to a fixed blending proportion of 5%, and then 10%, which is today the most common US fuel blend.
It is hydrophilic, pulling water vapor out of moist air, and it also increases the level of free oxygen in the fuel significantly.
Automotive engines addressed this with the mandated shift over to ethanol-tolerant metals and seals, and with the use of smart electronic fuel injection, which has some flexibility to adjust combustion properties and timing.
Automotive engines did not see major issues because of these factors, and because automobiles in active use typically cycle through their gas tank in a matter of weeks.
A large Canadian study from 2002 (funded by automakers, who are against its use) concluded that MMT impairs the effectiveness of automobile emission controls and increases pollution from motor vehicles.
It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom.
Reflecting its symmetrical structure and charge neutrality, Fe(CO)5 is volatile; it is one of the most frequently encountered liquid metal complexes.
Fe(CO)5 is the archetypal fluxional molecule due to the rapid interchange of the axial and equatorial CO groups via the Berry mechanism on the NMR timescale.
[21][22] Toluene and benzene were used as octane rating boosters for aviation fuel by the Royal Air Force in the World War Two.
[23] The Allison V-1710 engine would not run with the RAF fuels as it required tetraethyl lead for lubrication of its valvetrain, but the Packard-built Merlins would.
Toluene has also been used as a coolant for its good heat transfer capabilities in sodium cold traps used in nuclear reactor system loops.
Properties of xylenes and ethylbenzene are nearly identical to toluene, with the latter advertised by a refinery as "component of high performance fuels".
In World War II, xylidine was an important antiknock agent in very high performance aviation gasolines.