The transportation sector accounts for over one third of total U.S. energy-related carbon dioxide emissions due to the high percent usage of petroleum-based fuels.  Switching to cleaner fuels can have a significant positive impact on combating climate change, improving our nation's air quality, and attaining energy independence.  The Energy Policy Act of 1992 (EPAct) defines the following fuels as Alternative Fuels:

• Compressed Natural Gas
• Biodiesel
• Ethanol
• Hydrogen
• Electricity
• Propane

COMPRESSED NATURAL GAS (CNG)

Natural Gas is predominately made up of methane, and can come from natural gas reservoirs, landfill gas, and water/sewage treatment.  It is mostly produced in the US and is available throughout the country through a pipeline distribution system extending from wells to end-users.  There are two types of natural gas vehicles: dedicated vehicles, which run on pure natural gas, and bi-fuel vehicles, which can run on both natural gas and gasoline or diesel.  CNG contains only about 25% of the energy content of gasoline, so CNG vehicles require more frequent refueling and more storage capacity.

Relative to conventional gasoline, CNG emits 90-97% less carbon monoxide, 25% less carbon dioxide, 35-60% less nitrogen oxide, 50-75% less nonmethane hydrocarbon, almost no particulate matter, and fewer toxic and carcinogenic pollutants.

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-cng.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/fuels/natural_gas.html

BIODIESEL

Biodiesel is produced domestically from vegetable oils, animal fats, or recycled restaurant greases.  When pure and unblended (B100), biodisel is nontoxic, biodegradable, and renewable.  It can be used to fuel diesel engines in a cleaner manner, as it contains no sulfur and aromatics, and emits far less carbon monoxide, hydrocarbons, particulates, and air toxics than regular diesel.  When blended with petroleum diesel (i.e. B5, B20, which are 5% and 20% biodiesel, respectively), the fuel can be used in regular diesel engines without modification.  However when using pure biodiesel, some engine modifications may be required.  Biodiesel has a higher flashpoint and lower volatility than petroleum diesel, so it does not ignite as easily and also degrades quicker.  Higher percentage biodiesel fuels provide better engine performance, but a decrease of 2-8% in fuel economy. 

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-biodiesel.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/fuels/biodiesel.html

ETHANOL

Ethanol is produced domestically from the fermentation and distillation of starch crops that have been converted into simple sugars, such as corn, potatoes, wood, waste paper, wheat, and brewery waste.  In the United States, over 90% of ethanol comes from corn.  For transportation, ethanol is usually mixed with gasoline to create blends compatible with different engines that emit fewer harmful emissions. A common fuel is E10, which is a blend of 10% ethanol with 90% gasoline and works in any conventiona internal combustion vehicle.  Due to the low percentage of renewables, however, E10 is not considered an alternative fuel according to EPAct regulations.  When ethanol is present at 85% or higher, it is considered an alternative fuel.  Vehicles need to be modified to run on E85 or higher; such vehicles are called flexible fuel vehicles (FFVs).

Ethanol contains only 60% of the energy content of gasoline, so vehicles require more ethanol than gasoline to go the same distance.  However, unlike gasoline ethanol is renewable and emits 15% fewer ozone-forming volatile organic compounds, 40% less carbon monoxide, 20% fewer particulates, 10% less nitrogen oxide, and 80% less sulfate.

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-ethanol.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/ethanol/basics.html

HYDROGEN

Hydrogen can be used directly as a fuel (pure or mixed with natural gas) in internal combustion engines, or in fuel cell vehicles to produce electricity.  Fuel cells generate electrical energy out of chemical energy by combining hydrogen from fuel with oxygen from air.  Electrons are stripped off of the hydrogen molecules and forced to travel through an external circuit and then recombine with the hydrogen ions and oxygen molecules.  This electron flow in the external circuit forms an electrical current, which is sufficient to power a vehicle.  The only resulting emission is the water formed from the combination of hydrogen and oxygen.  Other emissions may be emitted from the production of hydrogen if it is extracted from fossil fuels, thus a more sustainable alternative fuel depends on producing hydrogen from renewable sources like wind or solar.

Hydrogen has a lower energy content than gasoline, so a larger amount would need to be stored in the vehicle in order to provide an adequate driving range.  Storage technologies for high-pressure tanks for compressed hydrogen, insulated tanks for liquid hydrogen, and chemical bonding of hydrogen with another material are currently under development.

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-fuelcells.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/fuels/hydrogen.html

ELECTRICITY

Electricity can be used to run electric vehicles and plug-in hybrid electric vehicles using power directly from the grid.  Electric vehicles (EVs) produce no tailpipe emissions; however they may indirectly cause harmful emissions depending on the way the power was initially produced (i.e. electricity produced from fossil fuels would have let out emissions during generation, while electricity produced from renewable sources such as wind and wolar would not have).  EV batteries can be recharged by plugging them in to any electric outlet, and per-mile fuel costs are generally less than gasoline but vary depending on location, time, and generation type.

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-electric.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/fuels/electricity.html

PROPANE

Most propane is produced domestically as a byproduct of natural gas processing and crude oil refining.  Propane vehicles can be either dedicated, which run on 100% propane, or bi-fuel, which are capable of running on both propane and gasoline.  Relative to conventional gasoline, propane emits far less carbon dioxide, carbon monoxide, and nonmethane hydrocarbons.  Exact estimates vary with engine design.

Propane in its gaseous form contains about 75% of the energy content of gasoline, which is one of the largest energy densities of all alternative fuels.  It is a gas at normal temperatures and pressures, and in this state, engines run more efficiently under low-speed, light-throttle conditions.  Liquid Propane Injection engines run on liquid propane and provide fuel economy more on par with gasoline.  They have performed well in terms of power, engine durability, and cold starting.

Link to EPA fact sheet: http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-propane.pdf

Link to AFDC fact sheet: http://www.afdc.energy.gov/afdc/fuels/propane.html