Alternative fuels for pollution reduction

San Diego's natural gas busses are on the right track

provided by West Virginia University


est Virginia University researchers recently measured and compared the emissions from fleets of new generation transit buses in Atlanta, Georgia., and Flint, Michigan, and found that the buses fueled by compressed natural gas (CNG) exhibited considerably lower exhaust emissions than those fueled by diesel. The level of particulate matter in the exhaust was found to be 92 percent lower and nitrogen oxides 33 percent lower for CNG buses.

After testing emissions from buses and heavy-duty vehicles in 32 cities, researchers at WVU found that alternative fuels have a strong potential to reduce particulate matter and other pollutants in urban areas. Results of the study were published recently in Environmental Science and Technology, the journal of the American Chemical Society.


Anything but diesel


Natural gas has the lowest particulate matter emissions level compared to all the other fuels. The scientists also found that methanol, ethanol, and blends of biodiesel produced far few emissions than the traditional diesel fuels used in many buses and heavy trucks. Urban bus and heavy truck engines are a major source of city air pollution and smog.

"There is an increased interest in alternative fuels for these vehicles," said Donald Lyons, chairman of Mechanical and Aerospace Engineering and director of the National Center for Alternative Fuels at WVU. "Specifically, the EPA's Emissions Certification Standards for 1998 Urban Bus and Heavy Truck Engines place emphasis on reductions of nitrogen oxides emissions. The data from our study will be used by fleet managers everywhere when decisions are made on how to reduce emissions," he said.

In a study of over 800 vehicles located at 32 transit agencies in 17 states and in Mexico City a place with one of the highest levels of air pollution in the world the researchers used two transportable heavy-duty vehicle emissions testing laboratories to measure the emissions from vehicles at the actual operating sites of the vehicles being tested.


On-site testing


WVU has the only laboratory facility in the world that can be taken to the actual testing site where heavy-duty vehicles are in operation, said Lyons. Bringing vehicles to be tested to a fixed testing site would be prohibitively expensive and may not provide realistic test conditions, he said.

"This is the first time there has been emissions testing on modern alternative-fueled buses in actual field operation," Lyons said, "and this information will demonstrate to fleet owners the benefits of lower emissions that can be achieved by using natural gas and other alternative fuels. Because the concern for air pollution is world wide, fleet managers and government regulators throughout the whole world want this kind of data."

The buses to be tested were mounted on a dynamometer test bed, which allows them to be driven in place as if they were in normal service. Normal vehicle operation is simulated by placing the vehicle being tested on rollers that are part of the dynamometer. The drive axle of the vehicle is connected to flywheels that simulate vehicle inertia and to instrumentation that measures power output. Another unit, the instrumentation trailer, contains exhaust analyzers, a data acquisition system and a control system. This unit instantaneously measures concentrations of air pollution compound in the exhaust of the vehicle.

In this study, a special driving cycle - the central business district (CBD) cycle - was used for all buses tested. This pattern was developed as a general representation of transit vehicle operation in a downtown business district and consist of 14 identical segments. Each segment includes 10 seconds of acceleration, 18.5 seconds of 20 miles-per-hour cruise, 4.5 seconds of deceleration and seven seconds of idle. Total driving distance is two miles.


Emissions omissions


The major component of natural gas is methane, which has the lowest molecular weight and simplest structure of all the fuels in the study. This accounts for less unburned and partially oxidized hydrocarbons generated as particulate emissions and explains why natural gas has the lowest particulate matter emissions level.

Methanol fuel produces the lowest level of nitrogen oxide emissions less than half of that of natural gas and diesel, the study shows. Ethanol fuel provides the second lowest level of nitrogen oxide emissions. Natural gas and diesel fuels produce almost the same levels of nitrogen oxides.

The recent tests in Atlanta and Flint demonstrated that nitrogen oxide emissions from the new generation of natural gas engines could be lower than diesel. It is noteworthy that the engine control technology plays an important role in determining the levels of emissions. Researchers at WVU are pursuing a number of research projects, funded by engine manufactures, to develop improved engine technology.

Hydrocarbon emissions from natural gas are somewhat higher than that of diesel, the study shows. But the WVU researchers point out that 90 percent of the hydrocarbon emissions from natural gas is unburned methane, considered to be non-reactive in the formation of ozone in the atmosphere.

Extensive research in this area will continue, Lyons said, as engine manufacturers and WVU scientists work toward development of engines that have even lower emissions. WVU receives funding from the US Department of Energy, engine manufacturers, fuel suppliers and others to support the National Center for Alternative Fuels at WVU.

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