Clean fuels as a concept lacks clear definition, though the term generally refers to blends or substitutes for gasoline fuels, including compressed natural gas, methanol, ethanol, liquefied gas, and even electric power from renewable sources. Clean fuels as a possible alternative to straight fossil fuels have primarily emerged as a lower pollution alternative in the transportation sector. The U.S. federal tax code, for instance, defines “clean fuels” [Page 73]that are potentially eligible for rebates or tax breaks for motor vehicles designed to use one of the following fuels:

• Natural gas
• Liquefied natural gas (LNG)
• Liquefied petroleum gas (LPG)
• Hydrogen
• Electricity (e.g., some gasoline/electric hybrids)
• Any other fuel that is at least 85 percent alcohol or ether (e.g., E85)