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Clean Air World

Vehicles and Fuels

Motor vehicles are a major source of air pollution worldwide. In many urban areas, motor vehicles collectively produce 50 to 90 percent of local air pollution, depending upon the pollutant. Vehicles can also produce a significant amount of the toxic or hazardous pollutants found in our air. Motor vehicles are typically divided into on-road and nonroad categories for regulatory purposes. Most nations set standards for both engines and fuels in order to reduce air pollution. In the U.S., only EPA and the State of California are permitted to establish new vehicle and fuel standards; other states may adopt California standards if they choose. In addition to engine and fuel characteristics, mobile source emissions are also affected by ambient conditions, driving behavior, and transportation system characteristics.

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Cars, Trucks and Buses

Automobiles, motorcycles, trucks, and buses are commonly referred to as “on-road” mobile sources. Automobiles and light-duty trucks are a major source of air pollution all over the world. Emissions from these vehicles come from the tailpipe. Gasoline powered vehicles also generate evaporative emissions from fuel tanks, out of the oil reservoir, and around engine seals. Gasoline refueling vapors are also a significant source of emissions. Most cars and light-duty trucks are fueled by gasoline, and generate large quantities of volatile organic compounds (VOCs), nitrogen oxides (NOx), carbon monoxide (CO), and carbon dioxide (CO2) emissions. Motorcycles represent a large part of the vehicle fleet in developing countries. Two-stroke motorcycles are especially polluting and can emit more air pollution than a small fleet of modern automobiles. Most heavy-duty trucks and buses are powered by diesel fuel, which can generate significant amounts of NOx and sulfur oxide (SOx) emissions (especially in areas with high-sulfur content fuels), as well as potentially cancer-causing particulate matter. Emission controls for modern gasoline vehicles are capable of reducing vehicle emissions by more than 95 percent compared to uncontrolled carbureted vehicles. Diesel vehicle controls have also provided substantial reductions, especially for particulate matter (PM), although further NOx reductions require highly advanced engine technologies or retrofit of aftertreatment devices.

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Gasoline and diesel fuels are complex mixtures of many different chemicals. The precise combination of chemicals determines key fuel properties such as energy content, volatility (i.e., ability to vaporize), and the fuel’s ability to ignite and burn in the engine. In turn, the various fuel properties effect vehicle emissions, performance, and fuel cost. Fuel producers have developed different gasoline and diesel formulations designed for specific vehicle and engine technologies to provide adequate vehicle performance and decreased emissions at a reasonable cost. In fact, a vehicle and its fuel should be viewed as an integrated system, with fuel properties designed to match specific engine technologies, and vice versa. Fuel standards can also be designed to control specific pollutants. Depending upon the air quality conditions in a particular local area, fuel properties can be adjusted to reduce CO, hydrocarbon, NOx, or even PM emissions from vehicles. Some areas change their fuel formulations on a seasonal basis to address wintertime CO and summertime ozone problems. In many instances adopting new fuel standards can bring about immediate, cost-effective emission reductions, without making changes to an area’s vehicle fleet. Other fuel changes may be designed for the introduction of new, cleaner vehicles over the long-term. Alternatives to traditional fuels include compressed natural gas, biodiesel, ethanol, liquefied natural gas, methanol and propane. Hydrogen has been identified as a potential “fuel of the future,” with little to no net emissions. The advent of fuel cells as a potentially viable power source for vehicles has further raised the interest in hydrogen as a fuel.

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Other Engines and Equipment

Offroad mobile sources are defined as motorized equipment that is portable or self-propelled, but not certified for operation on roadways. Typical offroad equipment includes construction and farm equipment, airplanes, ships, locomotives, lawn and garden equipment, mobile generators and pumps, among many others. Increases in air traffic and shipping, along with construction activities, have resulted in significant emissions from nonroad sources in recent years. As emission controls on automobiles, trucks, and buses become more prevalent, the relative amount of air emissions generated by nonroad sources is becoming more significant. In general, smaller, lighter equipment is dominated by gasoline engines, while larger equipment relies heavily on diesel engines. In most cases offroad equipment is not centrally registered. In addition, offroad equipment operation profiles can vary widely depending upon the specific application and operator. For these reasons, engine populations, use patterns, and resulting emissions from these sources are much more uncertain than for on-road sources.

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