<P> MTBE is manufactured via the chemical reaction of methanol and isobutylene . Methanol is derived from natural gas, and isobutylene is derived from butane obtained from crude oil or natural gas, thus MTBE is derived from fossil fuels . In the United States, it was produced in very large quantities (more than 200,000 barrels (32,000 m) per day in 1999) during its use as a fuel additive . </P> <P> MTBE is used as a fuel component in fuel for gasoline engines . It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline . </P> <P> In the US it has been used in gasoline at low levels since 1979, replacing tetraethyl lead as an antiknock (octane rating) additive to prevent engine knocking . Oxygenates also help gasoline burn more completely, reducing tailpipe emissions and dilute or displace gasoline components such as aromatics (e.g., benzene). Before the introduction of other oxygenates and octane enhancers, refiners chose MTBE for its blending characteristics and low cost . </P> <P> Other oxygenates are available as additives for gasoline including ethanol and other ethers such as ETBE . </P>

Why is mtbe used as a gasoline additive