An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating.
The mixture known as gasoline, when used in high compression internal combustion
engines, has a tendency to ignite early (pre-ignition or detonation) causing a damaging "engine knocking" (also called "pinging" or "pinking") noise. Early research into this effect was led by
A.H. Gibson and Harry Ricardo in England and Thomas
Midgley and Thomas Boyd in the United States. 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.
The most popular additive was tetra-ethyl lead. However, with the discovery of the
environmental and health damage caused by the lead, and the incompatibility of lead with catalytic converters found on virtually all US automobiles since 1975, this practice began to wane
in the 1980s. Most countries are phasing out leaded fuel; different additives have replaced the lead compounds. The most popular
additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).
The typical antiknock agents in use are:
Tetra-ethyl lead
In the U.S., where lead was blended with gasoline (primarily to boost octane levels) since the early 1920s, standards to phase
out leaded gasoline were first implemented in 1973. In 1995, leaded fuel accounted for only 0.6 % of total gasoline sales and
less than 2,000 tons of lead per year. From January 1, 1996, the
Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. Possession and
use of leaded gasoline in a regular on-road vehicle now carries a maximum $10,000 fine 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
until 2008 [citation needed]. The ban on leaded gasoline
led to thousands of tons of lead not being released in the air by automobiles. Similar bans in other countries have resulted in
lowering levels of lead in people's bloodstreams.[1]
[2]
A side effect of the lead additives was protection of the valve seats from erosion. Many
classic cars' engines have needed modification to use lead-free fuels since leaded fuels
became unavailable. However, "Lead substitute" products are also produced and can sometimes be found at auto parts stores.
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.
In some parts of South America, Asia and the
Middle East, leaded gasoline is still in use. Leaded gasoline was phased out in
sub-Saharan Africa with effect from 1 January, 2006. A growing number of countries
have drawn up plans to ban leaded gasoline in the near future.
To avoid deposits of lead inside the engine, lead scavengers are added to the gasoline together with tetra-ethyl lead. The
most common ones are:
MMT
Methylcyclopentadienyl manganese tricarbonyl (MMT) has
been used for many years in Canada and recently in Australia
to boost octane. It also helps old cars designed for leaded fuel run on unleaded fuel without need for additives to prevent valve
problems.
US Federal sources state that MMT is suspected to be a powerful neurotoxin and respiratory toxin, and a large Canadian study concluded that
MMT impairs the effectiveness of automobile emission controls and increases pollution from motor vehicles.
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