Evaluating vehicle emissions controls
ORNL researchers are developing specialized supercomputer
software tools to simulate the transformation of harmful
compounds in lean-burn engine exhaust into harmless emissions.
A major stumbling block to putting 80-mile-per-gallon cars on the road within this decade
is the lack of effective emissions controls for lean-burn engines. Lean-burn engines,
whether diesel- or gasoline-fueled, are designed to carry out combustion with an excess
of air. Such combustion achieves increased energy efficiency and reduced emissions of
greenhouse gases, such as carbon dioxide. If lean-burn engines could be used for
passenger cars, the Department of Energy estimates that fuel economy increases of over
30% could be readily achieved. Such improved efficiencies would be quite a step
forward in the move to reduce U.S. dependency on foreign oil. However, in spite of
higher efficiency, lean-burn emissions continue to be a problem because cleanup
technologies are not available for lean exhaust. Conventional catalytic converters are
unable to simultaneously reduce the nitrogen oxides, carbon monoxide, hydrocarbons, and
particulates from lean-burn engines to required levels. Thus, the development of new
exhaust cleanup technologies is critical.
ORNL's Bill Shelton and Stuart Daw are developing supercomputer software for
simulating the physics and chemistry of lean exhaust cleanup devices. Most of the
promising cleanup technologies involve complex chemical reactions between the gaseous
exhaust species and special solid-phase catalytic materials coated on ceramic substrates.
Up to now, the level of complexity involved has restricted the development of new
cleanup systems to the construction and testing of experimental prototypes. This empirical
approach proved adequate in previous decades for developing the automotive catalysts
used today, but it is simply too slow and costly for current needs.
In focusing on detailed simulations of the underlying
physical processes of cleanup devices, Shelton and
Daw are joining a new generation of researchers
who are trying to apply the power of
high-performance computing to go beyond
empiricism. Specific goals of the ORNL researchers
include the simultaneous description of atomic-scale
interactions on the surface with models for heat and
mass transport between the surface and gas. By
accurately modeling the dynamics of cleanup
devices at multiple scales, it is expected that
development of new technology can be greatly
"Simulations based on fundamental physics and
chemistry can reveal previously unanticipated approaches for formulating the catalytic
materials, and better ways to link them with the engine exhaust can be identified and
exploited," Shelton says. "Realistically, some degree of empiricism will always be
necessary, but even then accurate simulations can be used to more effectively plan and
ORNL is emerging as a leader in this field because of its experience in the experimental
evaluation of emissions control devices, its considerable expertise in fundamental surface
physics and chemistry, and its world-class facilities for high-performance computing.
"Through workshops and direct collaborations, we have been bringing together a broad
range of experts from national labs, universities, auto makers, emissions control
manufacturers, and engine companies who are extremely interested in addressing the
problems of lean-exhaust simulation," says Daw. "We hope to help DOE's Office of
Transportation Technologies set priorities for research and construct a coordinated
approach to overcoming this hurdle in developing a clean, efficient car."
"By combining models for the dominant physical processes at multiple scales, we are
obtaining previously unavailable insights into the coupling of local surface
transformations and chemical reactions with global heat and mass flow through the
devices," Shelton adds. "This approach could lead to more innovative solutions for
improving performance of emissions controls. For example, the multi-scale approach is
crucial to understanding the durability of the catalytic material—that is, how long the
catalyst will function before it must be replaced. Our goal is to understand how global
heat flow in the gas and ceramic substrate produces coarsening of catalyst nanoparticles.
This information would be used to determine options for reducing coarsening to delay
degradation in catalyst performance."
Shelton and Daw emphasize that their long-term objective is to produce simulations of
lean-exhaust cleanup that are directly relevant to realistic driving conditions. The
availability of simulations at this level will allow industry and DOE to calculate
cost-benefit ratios for different lean-exhaust-cleanup technologies so they can make more