Natural gas is an odorless, gaseous mixture of hydrocarbons—predominantly methane (CH4). It accounts for about a quarter of the energy used in the United States. About one-third goes to residential and commercial uses, such as heating and cooking; one-third to industrial uses; and one-third to electric power production. Although natural gas is a clean-burning alternative fuel that has long been used to power natural gas vehicles, only about one-tenth of 1% is used for transportation fuel.
The vast majority of natural gas in the United States is considered a fossil fuel because it is made from sources formed over millions of years by the action of heat and pressure on organic materials. Alternatively, renewable natural gas (RNG), also known as biomethane, is produced from organic materials—such as waste from landfills and livestock—through anaerobic digestion. RNG qualifies as an advanced biofuel under the Renewable Fuel Standard.
Because RNG is chemically identical to fossil-derived conventional natural gas, it can use the existing natural gas distribution system and must be compressed or liquefied for use in vehicles.
CNG and LNG as Transportation Fuels
Two forms of natural gas are currently used in vehicles: compressed natural gas (CNG) and liquefied natural gas (LNG). Both are domestically produced, relatively low priced, and commercially available. Considered alternative fuels under the Energy Policy Act of 1992, CNG and LNG are sold in units of gasoline or diesel gallon equivalents (GGEs or DGEs) based on the energy content of a gallon of gasoline or diesel fuel.
Compressed Natural Gas
CNG is produced by compressing natural gas to less than 1% of its volume at standard atmospheric pressure. To provide adequate driving range, CNG is stored onboard a vehicle in a compressed gaseous state within cylinders at a pressure of 3,000 to 3,600 pounds per square inch.
CNG is used in light-, medium-, and heavy-duty applications. A CNG-powered vehicle gets about the same fuel economy as a conventional gasoline vehicle on a GGE basis. A GGE equals about 5.66 pounds of CNG.
Liquefied Natural Gas
Liquefied natural gas, or LNG, is natural gas in its liquid form. LNG is produced by purifying natural gas and super-cooling it to -260°F to turn it into a liquid. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of the compounds found in the fuel. The remaining natural gas is primarily methane with small amounts of other hydrocarbons.
Because of LNG’s relatively high production cost as well as the need to store it in expensive cryogenic tanks, the fuel’s widespread use in commercial applications has been limited. LNG must be kept at cold temperatures and is stored in double-walled, vacuum-insulated pressure vessels. LNG is suitable for trucks that require longer ranges because liquid is more dense than gas (CNG) and, therefore, more energy can be stored by volume in a given tank. LNG is typically used in medium- and heavy-duty vehicles. A GGE equals about 1.5 gallons of LNG.
In 2013, the United States imported about 33% of the petroleum it consumed, and transportation accounted for more than 70% of total U.S. petroleum consumption. With much of the world’s petroleum reserves located in politically volatile countries, the United States is vulnerable to supply disruptions. However, because U.S. natural gas reserves are abundant, this alternative fuel can be domestically produced and used to offset the petroleum currently being imported for transportation use.
Also known as liquefied petroleum gas (LPG) or propane autogas, propane is a clean-burning, high-energy alternative fuel that’s been used for decades to power light-, medium- and heavy-duty propane vehicles.
Propane is a three-carbon alkane gas (C3H8). It is stored under pressure inside a tank and is a colorless, odorless liquid. As pressure is released, the liquid propane vaporizes and turns into gas that is used in combustion. An odorant, ethyl mercaptan, is added for leak detection. (See fuel properties.)
Propane has a high octane rating, making it an excellent choice for spark-ignited internal combustion engines. It presents no threat to soil, surface water, or groundwater. Propane is produced as a by-product of natural gas processing and crude oil refining. It accounts for about 2% of the energy used in the United States. Of that, less than 2% is used for transportation fuel. Its main uses include home and water heating, cooking and refrigerating food, clothes drying, powering farm and industrial equipment. Rural areas without natural gas service commonly rely on propane as a residential energy source. The chemical industry uses propane as a raw material for making plastics and other compounds.
Propane as an Alternative Fuel
Interest in propane as an alternative transportation fuel stems mainly from its domestic availability, high-energy density, clean-burning qualities, and its relatively low cost. It is the world’s third most common transportation fuel and is considered an alternative fuel under the Energy Policy Act of 1992.
Propane autogas is specified as HD-5 propane and is a mixture of propane with smaller amounts of other gases. According to the Gas Processors Association’s HD-5 specification for propane, it must consist of at least 90% propane, no more than 5% propylene, and 5% other gases, primarily butane and butylene. (See fuel properties.)
Propane is stored onboard a vehicle in a tank pressurized to about 150 pounds per square inch—about twice the pressure of an inflated truck tire. Under this pressure, propane becomes a liquid with an energy density 270 times greater than the gaseous form. Propane has a higher octane rating than gasoline, which prevents engine knock. However, it has a lower Btu rating than gasoline, so it takes more fuel to drive the same distance. Propane’s clean burning characteristics allow the engine to have increased service life.
In 2013, the United States imported about 33% of the petroleum it consumed and transportation accounted for more than 70% of total U.S. petroleum consumption. With much of the worldwide petroleum reserves located in politically volatile countries, the United States is vulnerable to supply disruptions.
Fueling vehicles with propane is one way to diversify U.S. transportation fuels. The vast majority of propane consumed in the United States is produced here and distributed via an established infrastructure. Using propane vehicles instead of conventional vehicles increases U.S. energy security.
Electricity is considered an alternative fuel under the Energy Policy Act of 1992. Electricity can be produced from a variety of energy sources, including oil, coal, nuclear energy, hydropower, natural gas, wind energy, solar energy, and stored hydrogen. Plug-in vehicles are capable of drawing electricity from off-board electrical power sources (generally the electricity grid) and storing it in batteries. Though not yet widely available, fuel cell vehicles use hydrogen to generate electricity onboard the vehicle.
Powering Vehicles with Electricity
In plug-in electric vehicles, onboard rechargeable batteries store energy to power electric motors. Vehicles that run only on electricity produce no tailpipe emissions. But there are emissions associated with the production of most of the country’s electricity.
Fueling plug-in vehicles with electricity is currently cost effective compared to gasoline, especially if drivers take advantage of off-peak utility rates offered by many utilities. Electricity costs can vary by region, type of generation, time of use, and access point. Learn about factors affecting electricity prices from the U.S. Energy Information Administration.
Electric Charging Stations
Many plug-in vehicle owners will do the majority of their charging at home (or at fleet facilities, in the case of fleets). Some employers offer access to charging at the workplace. In many states, plug-in vehicle drivers also have access to public charging stations at libraries, shopping centers, hospitals, and businesses. Charging infrastructure is rapidly expanding, providing drivers with the convenience, range, and confidence to meet more of their transportation needs with plug-in vehicles.
In 2013, the United States imported about 33% of the petroleum it consumed, and transportation was responsible for nearly three-quarters of total U.S. petroleum consumption. With much of the world’s petroleum reserves located in politically volatile countries, the United States is vulnerable to price spikes and supply disruptions.
Using hybrid and plug-in electric vehicles instead of conventional vehicles can help reduce U.S. reliance on imported petroleum and increase energy security. Hybrid electric vehicles (HEVs) typically use less fuel than similar conventional vehicles, because they employ electric-drive technologies to boost efficiency. Plug-in hybrid electric vehicles (PHEVs) and all-electric vehicles (EVs) are both capable of using off-board sources of electricity, and almost all U.S. electricity is produced from domestic coal, nuclear energy, natural gas, and renewable resources.
Ethanol is a renewable fuel made from various plant materials collectively known as “biomass.” More than 95% of U.S. gasoline contains ethanol, typically E10 (10% ethanol, 90% gasoline), to oxygenate the fuel and reduce air pollution.
Ethanol is also available as E85, or high-level ethanol blends. This fuel can be used in flexible fuel vehicles, which can run on high-level ethanol blends, gasoline, or any blend of these. Another blend, E15, has been approved for use in newer vehicles, and is slowing becoming available.
There are several steps involved in making ethanol available as a vehicle fuel:
- Biomass feedstocks are grown, collected and transported to an ethanol production facility
- Ethanol is produced from feedstocks at a production facility and then transported to a blender/fuel supplier
- Ethanol is mixed with gasoline by the blender/fuel supplier to make E10, E15 or E85, and distributed to fueling stations
Ethanol as a vehicle fuel is not a new concept. Henry Ford and other early automakers suspected it would be the world’s primary fuel before gasoline became so readily available. Today, researchers agree ethanol could substantially offset our nation’s petroleum use. In fact, studies have estimated that ethanol and other biofuels could replace 30% or more of U.S. gasoline demand by 2030.
Ethanol (CH3CH2OH) is a clear, colorless liquid. It is also known as ethyl alcohol, grain alcohol, and EtOH. (See Fuel Properties search.) Ethanol has the same chemical formula regardless of whether it is produced from starch- and sugar-based feedstocks, such as corn grain (as it primarily is in the United States), sugar cane (as it primarily is in Brazil), or from cellulosic feedstocks (such as wood chips or crop residues).
Ethanol has a higher octane number than gasoline, providing premium blending properties. Minimum octane number requirements prevent engine knocking and ensure drivability. Low-octane gasoline is blended with 10% ethanol to attain the standard 87 octane requirement. Ethanol is the main component in high-level ethanol blends. (See E85 Specification to learn more.)
Ethanol contains less energy per gallon than gasoline, to varying degrees, depending on the volume percentage of ethanol in the high-level blend. Per gallon, ethanol contains about 30% less energy than gasoline. E85 contains about 25% less energy than gasoline.
Ethanol Energy Balance
In the United States, ethanol is primarily produced from the starch in corn grain. Recent studies using updated data about corn production methods demonstrate a positive energy balance for corn ethanol, meaning that fuel production does not require more energy than the amount of energy contained in the fuel.
Cellulosic ethanol, which is produced from cellulosic feedstocks, is expected to improve the energy balance of ethanol, because cellulosic feedstocks are anticipated to require less fossil fuel energy to produce ethanol. Biomass used to power the process of converting non-food-based feedstocks into cellulosic ethanol is also expected to reduce the amount of fossil fuel energy used in production. Another potential benefit of cellulosic ethanol is that it results in lower levels of life cycle greenhouse gas emissions. (Find out more about emissions related to ethanol.)
For more information on the energy balance of ethanol, see the U.S. Department of Energy’s Bioenergy Technologies Office’s Ethanol Myths and Facts, and download the following documents.
- Ethanol – The Complete Energy Lifecycle Picture(PDF)
- 2008 Energy Balance for the Corn-Ethanol Industry(PDF)
- Argonne National Laboratory’s GREET Model
- DOE response to article, Use of U.S. Croplands for Biofuels Increases Greenhouse Gases through Emissions from Land Use Change(PDF)
- Life-Cycle Energy Use and Greenhouse Gas Emission Implications of Brazilian Sugarcane Ethanol Simulated with the GREET Model(PDF) (Abstract)
Depending heavily on foreign petroleum supplies puts the United States at risk for trade deficits and supply disruption. In 2005, 60% of petroleum products were imported, however, that was reduced to 33% in 2013 as a result of increased domestic crude supplies and ethanol production—imports would have reached 41% without ethanol (2014 Ethanol Industry Outlook). The Renewable Fuels Association’s 2013 Ethanol Industry Outlook(PDF)calculated that, from 2005 through 2012, ethanol increased from 1% to 10% of gasoline supply.
Biodiesel is a renewable, biodegradable fuel that can be manufactured domestically from vegetable oils, animal fats, or recycled restaurant grease. It is a cleaner-burning replacement for petroleum diesel fuel.
Biodiesel is a liquid fuel often referred to as B100 or neat biodiesel in its pure, unblended form. Like petroleum diesel, biodiesel is used to fuel compression-ignition engines, which run on petroleum diesel. See the table for biodiesel’s physical characteristics.
How well biodiesel performs in cold weather depends on the blend of biodiesel. The smaller the percentage of biodiesel in the blend, the better it performs in cold temperatures. Regular No. 2 diesel and B5 perform about the same in cold weather. Both biodiesel and No. 2 diesel have some compounds that crystallize in very cold temperatures. In winter weather, fuel blenders and suppliers combat crystallization by adding a cold flow improver. For the best cold weather performance, users should work with their fuel provider to ensure the blend is appropriate.
The United States imports about a third of its petroleum, two-thirds of which is used to fuel vehicles in the form of gasoline and diesel. Depending heavily on foreign petroleum supplies puts the United States at risk for trade deficits, supply disruption, and price changes. Biodiesel is produced in the U.S. and used in conventional diesel engines, directly substituting for or extending supplies of traditional petroleum diesel.
Renewable diesel is a fuel made from fats and oils, such as soybean oil or canola oil, and is processed to be chemically the same as petroleum diesel. It meets the ASTM D975 specification for petroleum in the United States and EN 590 in Europe. Renewable diesel can be used as a replacement fuel or blended with any amount of petroleum diesel. Nearly all domestically produced and imported renewable diesel is used in California due to economic benefits under the Low Carbon Fuel Standard.
Renewable diesel and biodiesel are not the same fuel. Renewable diesel, previously known as green diesel, is a hydrocarbon produced most often by hydrotreating and also via gasification, pyrolysis, and other biochemical and thermochemical technologies. It meets ASTM D975 specification for petroleum diesel. Biodiesel is a mono-alkyl ester produced via transesterification. Biodiesel meets ASTM D6751 and is approved for blending with petroleum diesel.
Renewable diesel offers many benefits, including:
- Engine and infrastructure compatibility—renewable diesel meets the conventional petroleum ASTM D975 specification allowing it to be used in existing infrastructure and diesel engines.
- Fewer emissions—an NREL study(PDF) found renewable diesel reduced both carbon dioxide and nitrogen oxide emissions when compared with petroleum diesel. California’s Low Carbon Fuel Standard Certified Carbon Intensities shows renewable diesel reduces carbon intensity on average by 65% when compared with petroleum diesel.
- More flexibility—Renewable diesel is a replacement for diesel and can be used to fully replace diesel or blended with any amount. Renewable diesel can be made from multiple feedstocks and at plants that also produce sustainable aviation fuel.