How alternative fuels reduce emissions
Researchers from West Virginia University recently measured and compared the emissions of a new generation of buses. It was found that buses running on compressed natural gas (CNG) emitted significantly less exhaust fumes than diesel-powered buses. They found 92 percent fewer dust particles and 33 percent less nitrogen in the exhaust gases from CNG buses. For the past eight years, professors, staff and students in the engineering department at West Virginia University have been researching the use of alternative fuels in all types of vehicles.
After studying emissions from buses and heavy-duty trucks in 32 cities, researchers have discovered that alternative fuels can make a significant contribution to reducing particulate matter and other pollutants in urban areas. The results of the study were published in Environmental Science and Technology, the journal of the American Chemical Society.
Compared to all other fuels, natural gas has the lowest particle emissions. Scientists found that methanol, ethanol and biodiesel blends also produced far fewer emissions than the conventional diesel fuel used in buses and heavy-duty trucks. City bus and heavy-duty truck engines are the main contributors to urban air pollution and smog.
"There is increased interest in alternative fuels for these vehicles," said Donald Lyons, chair of the Department of Mechanical and Aerospace Engineering and director of the National Center for Alternative Fuels at West Virginia University. “In particular, the Environmental Protection Agency's (EPA) emissions certification standards for city buses and heavy-duty trucks, which will go into effect [in the US] in 1998, underscores the need to reduce nitrogen emissions. The data from our study is used by transport association directors across the country when making decisions about reducing emissions,” he said.
In their study of more than 800 vehicles from 32 transit agencies in 17 states in the US and in Mexico City - the city with one of the most polluted air in the world - the researchers used two transportable emissions test laboratories. "This is the first time emissions from modern alternative fuel buses have been measured in the field," said Lyons. “This information will help mass transit operators understand the benefits of lower emissions achieved by using natural gas and other alternative fuels. As concerns about air pollution are widespread around the world, this data is useful for transport associations and government agencies worldwide.”
The test buses were mounted on a dynamometer test stand that simulates routine use. Normal vehicular traffic was generated by mounting the test vehicle on rollers that are part of the dynamometer. The vehicle's drive axle was connected to flywheels, which simulate the vehicles' inertia, and to gauges, which measure the power output. Another unit contains exhaust gas analyzers, a data acquisition system and a control system. This device measures the concentrations of the individual components in vehicle exhaust gases without any time delay.
In this study, a specific driving cycle – the CBD cycle – was used for all test buses. The model is designed to provide a general representation of public transportation in a downtown business district. It consists of 14 identical segments. Each segment includes ten seconds of acceleration, 18.5 seconds of driving at a cruising speed of 35 kilometers per hour, 4.5 seconds of deceleration and seven seconds of idling. The total distance traveled was three kilometers.
The main component of natural gas is methane. Methane has the lowest molecular weight and the simplest structure of all fuels studied. The result is that little unburned or only partially oxidized hydrocarbons are produced as particle emissions. This also explains why natural gas has the lowest particulate emissions.
The study demonstrated that methanol fuels also produce the lowest amount of nitrogen oxide emissions - less than half the emissions of natural gas and diesel. Ethanol fuels produce the second lowest amount of nitrogen oxide emissions. Natural gas and diesel fuels produce about the same amounts of nitrogen.
Recent tests in Atlanta and Flint showed that nitrogen emissions from a new generation of natural gas powered engines are lower than from diesel powered vehicles.
The study also demonstrated that hydrocarbon emissions from natural gas are slightly higher than those from diesel fuel. However, the researchers point out that 90 percent of hydrocarbon emissions from natural gas contain unburned methane, which is believed to be non-reactive in the formation of ozone in the atmosphere.
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