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Cleaner and Greener

Researchers combine technologies to increase automotive fuel efficiency.

Spinning wheels on a "chassis dynamometer" treadmill, Larry Moore "drives" a 2007 Swedish car that runs in place on an alternative fuel. While conducting an emissions study at the National Transportation Research Center located between Oak Ridge and Knoxville, Tenn., he intently watches a laptop screen outside the windshield, as if playing a computer game. This flex-fuel vehicle (FFV) is highly instrumented and, under the hood, resembles a patient having heart surgery.

 

This Saab BioPower flex-fuel car that ORNL imported from Sweden was first driven on a Maryville, Ohio, track to break it in for ORNL's
emissions studies.
This Saab BioPower flex-fuel car that ORNL imported from Sweden was first driven on a Maryville, Ohio, track to break it in for ORNL's emissions studies.
 

The Saab 9-5 BioPower car project at NTRC was highlighted in a speech given by Alexander Karsner, U.S. Department of Energy Assistant Secretary for Energy Efficiency and Renewable Energy at the Washington, D.C., Auto Show 2007 in January. Karsner recognized Oak Ridge National Laboratory's important research to understand the potential performance and fuel economy gains of FFVs. The BioPower, now on the market in Europe but not available in the United States, is the first FFV optimized for ethanol.

Because ethanol has lower energy content than gasoline, tank mileage of typical FFVs drops by 30% when they run on E85, a blend that is 15% gasoline and 85% ethanol. While FFVs sold in the United States are optimized for gasoline and are largely "ethanol tolerant," Saab stresses that the BioPower is optimized for ethanol use, produces more power on E85 than on gasoline and does not lose as much tank mileage as American FFVs, which are being improved to help address President George Bush's ambitious goal of reducing America's gasoline consumption by 20% in 10 years.

"My hat's off to Saab," says Brian West, a researcher with ORNL's Fuels, Engines and Emissions Research Center (FEERC) at NTRC who collects and evaluates data on the car's performance. "The BioPower is very clean on both fuels—gasoline and ethanol. Saab engineers did not sacrifice fuel economy or emissions to get the added performance from ethanol. This finding is important because while U.S.-legal FFVs are emissions certified on both gasoline and E85, the European regulations do not require certification on E85. We really did not know how clean the Saab would be on E85."

The Saab BioPower produces 180 horsepower with ethanol but only 150 horsepower with gasoline. According to West, by accelerating from 40 to 70 miles per hour in third gear, the car using E85 would be two seconds faster than the same model fueled by gasoline. In practical terms, this difference puts the gasoline car almost two car lengths behind on a 500-foot freeway on ramp.

"The improvement provides a clear incentive for people to choose ethanol over gasoline as the fuel for this car," FEERC Director Ron Graves says, noting that about 10% of the gasoline sold in the United States is premium gasoline even though many cars filled with this more expensive fuel do not need it. "Some people will pay extra for a perceived or real performance benefit." Graves says that engineers hope to exploit ethanol's desirable properties—higher octane number and latent heat of vaporization—to improve tank mileage of ethanol cars like the Saab BioPower.

Clean diesel engines

Under the U.S. Partnership for a New Generation of Vehicles (PNGV) program in the 1990s, the goal of the Big Three U.S. auto manufacturers, with support of DOE's national laboratories, was to demonstrate clean, efficient vehicles. The program envisioned combining an electric motor with a diesel engine to make a hybrid, family-sized sedan that could achieve 80 miles per gallon. The diesel engine uses at least 30% less petroleum-derived fuel per mile than today's internal combustion gasoline engines.

PNGV evolved into the FreedomCAR and Fuel Partnership, which emphasizes developing, instead of a specific vehicle, technologies from which automakers can choose. The FreedomCAR and Vehicle Technologies program funds research at ORNL and other labs to remove technical barriers that limit the use of these technologies.

The diesel engine, because of its fuel efficiency, continues to be of great interest to automakers. About half of the new cars sold in Europe have diesel engines. In the United States the main technical barrier to market penetration of diesel engines in light-duty vehicles such as sport utility vehicles, vans and sedans, has been the inability of this technology to meet Environmental Protection Agency emissions standards. The primary obstacles are nitrogen oxides (NOx), which contribute to acid rain and smog, and particulate matter, which threatens respiratory health.

The most elegant exhaust treatment method for diesel engines is the lean NOx trap (LNT), an absorber-based exhaust aftertreatment system that stores NOx as nitrates during lean operation. The NOx is stripped off and chemically reduced when the engine transitions periodically to a brief, rich combustion mode. The problem is that sulfur in diesel fuel exhaust occupies the NOx storage sites, rendering the aftertreatment ineffective. The partial solution was a recent ruling by EPA that requires oil refineries to reduce the concentration of sulfur in diesel fuel from 500 parts per million to 15 ppm. Despite lower sulfur in the fuel, LNTs still suffer from sulfur poisoning, so research is focused on mitigating this problem.

EPA's ruling cited data contributed by ORNL researchers. In 1999 West and Scott Sluder conducted transient driving experiments using a Mercedes A170 vehicle they equipped with a prototype LNT.

"We were the first to conduct a laboratory experiment to demonstrate the potential of LNT and other aftertreatment technologies to enable a diesel car to meet Tier 2 emissions standards," West says. "We demonstrated that the LNT helped lower NOx levels, and that the diesel particle filter effectively removed particulate matter. Using diesel fuel with different levels of sulfur, we also quantified the harmful effect of sulfur on catalysts and tailpipe emissions."

Assisting industry

The Dodge Ram rolled out at a 2007 auto show by Daimler Chrysler has a Cummins 6.7-liter diesel engine and NOx aftertreatment system that bear the marks of ORNL research. John Wall, vice president and chief technical officer of Cummins Inc., noted in a letter to ORNL's Bill Partridge "the significant contribution you and your FEERC colleagues have made to the research required to introduce this vehicle. The knowledge and tools developed in our cooperative research and development agreement were critical to the R&D efforts that culminated in the release of an aftertreatment technology that meets the 2010 environmental standards in 2007."

Wall credited the mass spectrometer system, called SpaciMS, and pioneered by Partridge, for "changing the way we think about tuning engine combustion." He added that this instrument and "the fluorescence lifetime thermometer your team developed helped us understand the changes in NOx adsorber catalysts as they aged, critical information for catalyst system design."

Wall also lauded Tom Watkins and colleagues at ORNL's High Temperature Materials Laboratory, who used computer modeling and images produced by the aberration-corrected scanning transmission electron microscope to predict the lifetime of catalysts and particulate filters, such as Corning's honeycomb-like cordierite material, used for diesel exhaust aftertreatment. Durability of aftertreatment technology is an issue because truck drivers expect these systems to last for half a million miles. One study showed that rhenium particles become less catalytically active when they coalesce into nano-sized "rafts" after exposure to diesel exhaust.

Another effective emissions control strategy for diesel engines is exhaust gas recirculation, a process in which a carefully controlled amount of combustion by-products is mixed with the incoming air. "Basically, the exhaust of fuel you burn is dumped back into the cylinder," says researcher Johney Green. "Exhaust gas recirculation reduces the temperature in the cylinder, resulting in less production of NOx. One challenge we must resolve is that this strategy actually produces more soot until a critical threshold is crossed, resulting in the simultaneous reduction of NOx and particulate matter emissions."

In May 2002 ORNL became the first DOE lab to publicize the discovery of low-temperature diesel combustion. The muted reaction at a Department of Energy program review meeting made ORNL researchers suspect that some automakers and diesel engine manufacturers may have known about the phenomenon but for competitive reasons chose to downplay their reaction.

"Getting this discovery out in the open has helped the diesel engine community move forward faster now that engineers are working on the problem in a noncompetitive way," Green says. ORNL's presentations, he adds, helped redirect the DOE diesel engine portfolio toward increased research on controlling and stabilizing what the agency terms "high-efficiency clean combustion."

Meanwhile, across the ORNL campus various groups are analyzing alternative fuels and materials for engines. Nuclear Science and Technology Division researchers are developing computer simulations of combustion of biofuels, such as B5, diesel fuel containing 5% biodiesel made from soybean oil. The Materials Science and Technology Division is seeking to improve the thermal efficiency of heavy-duty diesel engines in trucks, ranging from tractor trailers to large pickup trucks. Thermal efficiency is the percentage of a fuel's heat energy value that is converted to mechanical energy to power a diesel engine.

Program manager Ray Johnson says, "In 2006, 150-horsepower diesel engines for passenger cars had a 41.5% thermal efficiency whereas 400-horsepower diesel engines for trucks had a 45% thermal efficiency. The DOE thermal efficiency goals are 45% by 2010 for diesel cars and 55% by 2012 for trucks. With widespread implementation of new improvements, we could realize a fuel reduction of 20%."

Engine designers working for Cummins, Caterpillar and Detroit Diesel say they cannot achieve these goals without stronger, lighter, higher-temperature, lower-friction materials that ORNL researchers can characterize, synthesize and model. In a new cooperative program involving an industrial partner, advanced materials and components will be tested in a state-ofthe- art, heavy-duty diesel engine, allowing researchers to measure the durability of new, advanced materials tested in the engine and also the effects of new materials and components on the engine's performance and efficiency. This initiative builds on a current project in which ceramic and intermetallic valves were tested for hundreds of hours in a stationary natural gas engine and found to be more durable and corrosion-resistant than the standard steel valves.

Capturing waste heat

More than half of the energy value of fuel in current automobile engines is lost to the atmosphere as waste heat. DOE's Solid State Energy Conversion Technology Program is developing technology using thermoelectric modules for waste heat recovery in vehicles to improve thermal efficiency and engine performance. Such a system based on temperature differences not only could convert waste heat directly to electricity to operate the car's electrical accessories, such as pumps and compressors, but also could provide heating and cooling directly. However, the poor efficiency of current thermoelectric materials is a significant barrier to large-scale commercialization.

DOE has asked ORNL to lead a new, computational quantum theory-driven effort to develop practical, high-efficiency thermoelectric materials that would enable commercialization of automotive thermoelectric generators and greater market penetration of solid-state heating and cooling.

Should the initiative result in significant commercialization of thermoelectric modules with a modest 10% efficiency, the impact on America's fuel consumption, and the resulting impacts on security and the environment, would be of enormous and lasting value.—Carolyn Krause

Larry Moore

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