The U.S. Environmental Protection Agency describes alternative fuels as those that are used in place of gasoline or diesel. Dozens of alternative fuels exist and new ideas are constantly explored, but the U.S. Department of Energy lists six types of alternative fuel that are already in production or in development as of 2014: biodiesel, electricity, ethanol, hydrogen, natural gas and propane.
According to the U.S. Department of Energy, biodiesel, ethanol, electricity, propane and natural gas are already available in some American markets for use in vehicles. Biodiesel is a renewable fuel source made from vegetable oils, animal fats and cooking grease that would otherwise be considered waste. Ethanol appears as an additive in gasoline to reduce emissions and is made from corn and other plants. Electricity powers plug-in vehicles, as well as hybrids that use a combination of traditional fuel and self-contained electrical generators. Propane and natural gas have long been used as an alternative fuel, but they have seen a resurgence in use for light and medium-duty fleet vehicles, such as police cars and school buses, due to existing delivery infrastructure and reduced emissions.
The U.S. Department of Energy reports that hydrogen is one of the most promising alternative fuels. Hydrogen fuel is potentially emissions-free, leaving only water vapor and oxygen as by-products. As of 2014, hydrogen fuel is not cost-effective enough to allow for widespread use.