The possibility of substituting cleaner-burning alternative fuels for gasoline or diesel has drawn increasing attention during the last decade. The motives for this substitution include conservation of oil products and energy security, as well as the reduction or elimination of pollutant emissions.

Some alternative fuels do offer the potential for large, cost-effective reductions in pollutant emissions in specific cases. Care is necessary in evaluating the air-quality claims for alternative fuels. However, in many cases, the same or even greater emission reduction could be obtained with a conventional fuel, through the use of a more advanced emission control system. Which approach is the more cost-effective will depend on the relative costs of the conventional and the alternative fuel.

Alternative fuels include

• methanol (made from natural gas, coal or biomass)

• ethanol (made from grain)

• vegetable oils

• compressed natural gas (CNG)

  mainly composed of methane

• liquefied petroleum gas (LPG)

  composed of propane or butane

• electricity

• hydrogen

• synthetic liquid fuels derived from hydrogenation of coal

• and various fuel blends such as gasohol