ORNL Review: Toward a Cleaner Diesel Vehicle

Toward a Cleaner Diesel Vehicle

At the University of Tennessee at Knoxville, a diesel car spins its wheels on a treadmill called a chassis dynamometer. It looks like a patient having heart surgery. It is highly instrumented to provide on-line measurements of its engine's speed, power, and fuel use and the ability of its exhaust treatment system to remove harmful constituents.

Researchers in ORNL's Engineering Technology Division (ETD) take measurements on this car, as well as on engines at the four stationary engine dynamometers at DOE's Advanced Propulsion Technology Center (APTC). These dynamometers will eventually be moved from this user facility at ORNL to the National Transportation Research Center, which will also acquire chassis dynamometers. ORNL research on these machines is guiding the development of effective emissions control systems for next-generation vehicles.

The heart of the lean, clean car of the future proposed by the U.S. Partnership for a New Generation of Vehicles (PNGV) is likely to be a compression-ignition, direct-injection diesel engine that uses 40% less fuel per mile than do today's typical gasoline-burning cars. If the diesel engine is combined with an electric motor in a hybrid car, it could come close to meeting the PNGV goal of 80 miles per gallon for a family-sized sedan.

Unfortunately, the lean-burn operation of diesel engines is incompatible with today's catalytic converters used to eliminate 90% of the nitrogen oxides (NO_x) in gasoline car exhaust. In addition to producing NO_x, which contributes to acid rain and smog (which, in turn, creates a greenhouse effect), diesel engines also emit particulate matter-airborne soot particles that may be hazardous to humans inhaling them because they are small enough to reach the lungs. To meet PNGV goals and the tough emissions standards mandated for 2006 by the Environmental Protection Agency (EPA), new exhaust treatment systems are being developed for diesel engines by catalyst companies and DOE national laboratories. The APTC, led by Ron Graves, Ralph McGill, and others in ETD, is playing a key role in evaluating these emissions control systems to help improve their effectiveness.

"We use an engine dynamometer to determine how well the engine, fuel system, and emissions control system work together," McGill says. "We measure the engine's speed and load, the fuel system's air-fuel ratio, and the concentration of constituents in the exhaust before and after treatment by the emissions control system. We are trying to determine and optimize the efficiency of the catalytic converter in reducing emissions."

Brian West checks a Mercedes A170 diesel engine, which is being tested to determine the efficiency of its emissions control system in removing nitrogen oxides and particulate matter from its exhaust. The engine is run on an ORNL stationary engine dynamometer, which applies the same resistance to the engine as the road would if the engine were connected to wheels. (Top photo enhanced by Gail Sweeden.)

"We are now conducting dynamometer experiments on seven engines—one gasoline and six diesel engines, plus two vehicles," Graves says. "This effort demonstrates the high level of interest in the diesel engine today and the challenge of solving the emissions problems with those engines." The ETD researchers are working with auto makers and diesel engine manufacturers through seven cooperative research and development agreements. Important results have emerged from these collaborations.

"We have developed methods and instruments to measure faster and more accurately the concentrations of a broad range of exhaust constituents," says Graves. "These constituents include nitrogen oxides, particulate matter, sulfur oxides, carbon monoxide, and hydrocarbons.

"We showed that advanced diesel vehicles could achieve 2006 emissions standards. We did some clever engineering to create a highly effective emissions control strategy for a diesel car. We determined the right mixture of hydrogen and carbon monoxide from unburned diesel fuel that could regenerate the NOx adsorber and simulated this exhaust mixture with bottled gas. This mixture is injected at precise intervals and reacts with the nitrogen oxides, converting them to nitrogen, carbon dioxide, and water vapor."

Bill Partridge prepares to use a mass spectrometer to analyze the effectiveness of diesel fuel hydrocarbons in regenerating a catalyst used to remove nitrogen oxides from engine emissions. (Photo by Curtis Boles and enhanced by LeJean Hardin.)

John Storey and others in ETD developed an electrostatic method of capturing diesel particulates from engine exhaust so their structure and makeup can be studied. Doug Blom in the Metals and Ceramics Division is studying these samples of particulate matter using the Hitachi HF-2000 transmission electron microscope. He has found that the structure of these particles ranges from noncrystalline to semicrystalline—the atoms are lined up in layers that are oriented in different directions. Pete Reilly of ORNL's Chemical and Analytical Sciences Division has developed a laser-based ion trap mass spectrometer that can be used to determine the composition of particles measuring 1 to 100 nanometers in real time.

"If the particles we measure are mostly sulfuric acid, then we must rethink whether they represent a health risk or are harmlessly diluted by water in the lungs," Graves says. "If they are a health problem, it could go away after fuel sulfur is lowered to meet EPA limits." (See An Emissions Mission: Solving the Sulfur Problem.) More precise information on the makeup of particulate matter could steer scientists to a better understanding of its health effects.

Computational visualization of temperature contours and flow streamlines

In this computational visualization by ORNL's Engineering Technology and Computer Science and Mathematics divisions (courtesy of Ross Toedte), the temperature contours and flow streamlines represent the combined effects of exhaust gas flow, heat transport, and chemical reactions in a typical automotive catalytic converter during a cold start. Cold start performance is critical to reducing harmful emissions from automobiles.

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