[text from the DOE's official site] The Energy Policy Act (EPAct 1992) of 1992 aims to reduce U.S. dependence on imported petroleum and improve air quality by addressing all aspects of energy supply and demand, including alternative fuels, renewable energy, and energy efficiency.
EPAct 1992 encourages the use of alternative fuels through voluntary and regulatory activities and approaches carried out by the U.S. Department of Energy (DOE). Voluntary activities are implemented through Clean Cities, and EPAct 1992 regulations require that federal and state and alternative fuel provider fleets build an inventory of alternative fuel vehicles.
EPAct 1992 also defines "alternative fuels" as methanol, ethanol, and other alcohols; blends of 85% or more of alcohol with gasoline (E85); natural gas and liquid fuels domestically produced from natural gas; liquefied petroleum gas; hydrogen; electricity; biodiesel (B100); coal-derived liquid fuels; fuels, other than alcohol, derived from biological materials; and P-Series fuels, which were added to the definition in 1999.
Under EPAct 1992, DOE has the authority to add more alternative fuels to the list of authorized alternative fuels if certain criteria are met. You can read the full text of the act here.