One of the ironies of automobile research: as diesel engines become more fuel efficient, reducing their emissions becomes more challenging.
Better efficiency means that more of the energy in diesel fuel is being used to move the vehicle and less is escaping out the tailpipe in the form of heat. While this is undoubtedly good, it presents a challenge for emissions reduction.
Current diesel catalytic converters, which use chemical processes to remove exhaust gas pollutants or transform them into something less harmful, are built to work between 200 and 300 degrees Celsius. The highly efficient diesel engines being developed now can put out exhaust at 150 degrees Celsius or lower. Emission controls for these new engines, then, must be re-worked in order treat this lower-temperature gas and meet environmental regulations.
The National Science Foundation (NSF) and the U.S. Department of Energy (DOE) have tapped into a team of researchers from University of Houston Cullen College of Engineering and Oak Ridge National Laboratory to do exactly that.
The researchers received a three-year, $1.2 million grant jointly awarded by the two agencies. They are led by Bill Epling, associate professor in the Cullen College's chemical and biomolecular engineering department. His UH collaborators, all members of that same department, are Vemuri Balakotaiah, professor and Hugh Roy and Lillie Cranz Cullen Distinguished University Chair; Lars Grabow, assistant professor; Mike Harold, department chair and M.D. Anderson Professor; and Dan Luss, Cullen Professor of Engineering. Their Oak Ridge collaborator is Jim Parks, emissions and catalysis research group leader.
The team's work is based on one fact about catalytic converters that has been neglected by researchers and businesses in the emissions reduction field. During operation, the properties of the exhaust gas and the converter itself change from one spot to the next. The temperature of the converter shifts, for example, and the exact mix of exhaust gas pollutants changes. Since catalytic converters rely on sensitive chemical processes, their efficiency at reducing pollution rises and falls from one spot to the next as conditions change.
The group's idea? Develop catalytic converters that account for and even make use of these changes.
"I want to tailor the design of this catalyst to take advantage of gradients that always exist inside the catalytic converter. Why is the catalyst at the front of the reactor the same as at the back? Except for manufacturing purposes, there's no reason," Epling said.
Epling and his partners will not focus on creating entirely new catalyst materials. Instead, they will explore the best way to use existing catalysts like platinum and palladium. This will include altering the exact ratios of the different materials – more platinum at one end, more palladium at another, for instance – and altering the catalyst's thickness at different points in the converter.
The Cullen College's chemical and biomolecular engineering department is uniquely suited for this project, said Epling. In recent years it has added three researchers to what was already an outstanding reaction engineering team. With a total of six investigators in this group, the department now houses expertise in practically every area of catalysis and reaction engineering.
"We can do simulations of molecules reacting with a surface up to simulations of a full-scale reactor," Epling said. "The experimentalists can get feedback from those modelers and redesign the catalyst to take advantage of what's happening inside the catalytic converter."
While this work will initially focus on catalytic converters for large diesel vehicles like semi trucks and buses, their findings can easily be scaled to standard passenger vehicles, Epling said. Given that diesels are already far more fuel-efficient than gasoline engines, improving their operation can only help the environment.
"There's a huge stigma to diesel – they're dirty, they're polluters – but they're clean now. If you see a new truck on the road with black stuff coming out, that's because it's out of tune," said Epling. "It would be good if the next generation of diesel engines are fuel efficient enough that people notice how much they've changed from a few decades ago and realize that they can actually be the most efficient and environmentally-friendly choice."