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Medium- and heavy-duty trucks and buses account for approximately one quarter of U.S. highway fuel consumption. They also account for more than half of the highway emissions of nitrogen oxides (NOx) and particulate matter (PM10), air pollutants linked to environmental and health problems. Clearly, these vehicles represent an important opportunity for reducing petroleum consumption and emissions, with resulting energy security, environmental, and public health benefits. These benefits can be realized through use of alternative fuel and advanced technologies.
There are numerous alternative fuel and advanced technology options available for medium- and heavy-duty vehicles, many of which are the subject of NREL Fleet Test and Evaluation projects. This page briefly describes some of the options and provides links to more information.
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 The NREL Fleet Test and Evaluation team evaluated prototype liquefied natural gas waste transfer trucks operated by Norcal Waste Systems, Inc., such as the one pictured here. |
Alternative Fuels
The most widely used alternative fuels for medium- and heavy-duty vehicles are natural gas, propane, electricity, and biodiesel. Natural gas—in the form of compressed natural gas (CNG) and liquefied natural gas (LNG)—was used in more than 10% of U.S. transit buses in 2003, while propane and electricity powered a smaller fraction of the transit fleet. These fuels are also used in other applications, such as refuse trucks and shuttle buses. Biodiesel is a renewable fuel that can be manufactured from vegetable oils, animal fats, or recycled restaurant greases. Biodiesel use in medium- and heavy-duty vehicles is growing rapidly, in part because of the ability to use biodiesel/petroleum diesel blends in unmodified diesel engines. Gas-to-liquid (e.g., Fischer-Tropsch) fuels, which are created by converting gaseous fuels such as natural gas or biogas to liquid fuels, are emerging as another alternative fuel option. For more information, visit the NREL Nonpetroleum Based Fuels site and the Alternative Fuels and Advanced Vehicles Data Center.
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 The NREL Fleet Test and Evaluation team is evaluating IndyGo hybrid electric buses, such as the one pictured here. |
Hybrid Electric Propulsion
Hybrid electric vehicles combine a primary power source, an energy storage system, and an electric motor to achieve a combination of emission, fuel economy, and range benefits unattainable with any of these technologies alone. For example, hybrid electric buses operated by Indianapolis Public Transportation Corporation (IndyGo) feature a diesel-fueled Capstone MicroTurbine (primary power source), nickel-cadmium batteries (energy storage system), and an electric motor. Like many hybrid electric vehicles, these buses also feature regenerative braking, which converts energy produced during braking to electric power. For more information, visit the NREL Hybrid Electric and Fuel Cell Vehicles site and the Alternative Fuels and Advanced Vehicles Data Center Hybrid Electric Vehicles site.
Hydrogen and Fuel Cells
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 The NREL Fleet Test and Evaluation team is evaluating a hybrid electric fuel cell bus (pictured) at Hickam Air Force Base. |
Fuel cell vehicles powered by hydrogen are considered by many to be the future of transportation. A fuel cell typically uses the chemical energy of hydrogen to produce electricity and water, cleanly and efficiently. Fuel cells also can be used as the primary power source in a hybrid electric vehicle. Numerous medium- and heavy-duty fuel cell vehicle demonstrations are underway—visit the Fuel Cell Bus Club and the California Fuel Cell Partnership for examples. For more information, visit the Hydrogen, Fuel Cells and Infrastructure Technologies site and the Alternative Fuels and Advanced Vehicles Data Center Hydrogen Vehicles site.
Idle Reduction
Heavy-duty long-haul trucks consume more than 800 million gallons of fuel each year by idling overnight to heat and cool the cabin, generate electricity for amenities, operate a generator, and eliminate cold-starting problems. Alternatives to idling that don't involve using a truck's high-horsepower, high-torque engine to produce power could save fuel, reduce emissions, and cut operating costs. One idle reduction solution is to use efficient auxiliary power units (APUs) to meet power needs. Another solution is to make grid-based electricity available to trucks at truck stops. For more information, visit the Advanced Vehicle Testing Activity Idle Reduction page and the Alternative Fuels and Advanced Vehicles Data Center Idle Reduction site.
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 Caterpillar's MorElectric Technology, an APU-based idle reduction solution being evaluated by NREL's Fleet Test and Evaluation team (courtesy of Caterpillar). |
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