Labs’ engine research aims high: Reduce US oil imports 30 percent while achieving ultra-low emissions
Among the hardest and most challenging problems facing the national labs are the issues of energy security and environmental quality. Ground transportation consumes the largest share of oil in the US, and to meet the demand, oil imports have reached the highest levels in history.
To help reduce this dependence on foreign oil, promising new combustion strategies for efficient, clean engines are being explored at the Combustion Research Facility (CRF) in Center 8300 through a $6 million engine-combustion research program. The work is funded 90 percent by DOE’s Office of
FreedomCAR and Vehicle Technologies (OFCVT) and 10 percent by private industry.
“Our research is providing the science base needed by industry to develop higher efficiency, emission-compliant engines,” says Dennis Siebers, who manages Engine Combustion Dept. 8362. “There’s a significant potential for improving the fuel efficiency of engines while simultaneously reducing their pollutant emissions.” Moreover, he added, “Such improvements in fuel efficiency will contribute to a direct reduction in greenhouse gas [CO2] emissions.”
To achieve these goals, the engine research at the CRF is focusing on new combustion strategies that will allow high-efficiency, clean engines. Also included is research on fuels for these engines from both traditional and alternative sources. The new combustion strategies fall into a class being referred to as low-temperature combustion (LTC). In simple terms, LTC is combustion under conditions that are fuel-lean enough (or sufficiently dilute with recirculated exhaust gas) to avoid soot formation and the high combustion temperatures that lead to significant nitrogen oxide (NOx) formation.
Unique capabilities, new strategies
Sandia has been conducting engine-combustion research in collaboration with industry for more than 25 years. The research has led to a suite of advanced optical-diagnostic tools for analyzing the combustion in an operating engine, and to the advancement of predictive computer models. This research has impacted industry’s design and development process, contributing significantly to the efficiency and emissions improvements of engines that are currently in production. As Dennis summarizes, “We bring capabilities that are unique in the world for helping industry develop new combustion strategies for high-efficiency engines.”
Patrick Flynn, former vice president of research at the country’s largest diesel engine manufacturer, Cummins, Inc., comments: “I feel that these tools provided by the CRF will play an ever-increasing role in engine design evolution.” The application of these tools and the expertise of CRF researchers, three of whom have been elected fellows of the Society of Automotive Engineers, are central to the new research efforts on LTC.
The low-temperature combustion research at the CRF is being conducted as part of a broader DOE program. Because of its established reputation, Sandia was recently tasked by DOE OFCVT to create and lead a memorandum of understanding (MOU) surrounding the overall research efforts. The MOU involves five national labs (Sandia, Lawrence Livermore, Los Alamos, Oak Ridge, and Argonne) and 10 engine manufacturers (Cummins, General Motors, Ford, DaimlerChrysler, Caterpillar, Detroit Diesel, International Truck, Mack/Volvo, John Deere, and General Electric). The research is conducted in collaboration with several universities (Stanford, MIT, University of California at Berkeley, University of Wisconsin, University of Michigan, Pennsylvania State University, University of Illinois, and Wayne State University).
50 percent better mpg by 2012?
The DOE low-temperature combustion program covered by the MOU targets a 50 percent improvement in fuel efficiency in the light-duty sector (automobiles, SUVs, and pickups) by 2012 and a 30 percent improvement in heavy-duty trucks by 2013. With complete market penetration, these efficiency improvements would reduce US oil use by 4 million barrels per day or oil imports by one-third from their present levels. The improvements would also translate directly to a 9 percent reduction in the total US greenhouse gas emissions. Even greater reductions in oil use are possible through further improvements in engine efficiency and through the use of these high-efficiency engines in hybrid-electric
“Two factors have made the low-temperature combustion techniques practical to consider now: the advent of onboard computers and electronic fuel injection,” Dennis says. “These allow for real-time control of potentially unstable combustion conditions that can arise with the advanced strategies. It’s possible that cycle-by-cycle, or even cylinder-by-cylinder control will be necessary to implement low-temperature combustion,” he says. “This dictates the need for a fairly comprehensive understanding of the in-cylinder processes.” As a national laboratory tackling tough technical problems, Sandia is playing a vital role.
Reducing emissions a challenge
In addition to reducing fuel consumption, the new LTC engine concepts are being driven by the need to reduce pollutant emissions. Stringent new emission regulations call for a factor of 10 reduction in soot and NOx by 2010. “Those regulations are really challenging,” notes John Dec (8362), who is working on clean combustion concepts for high efficiency engines in one of Sandia’s eight engine labs, adding that meeting the current emission regulations on high-efficiency diesel engines “took 20 years and a lot of work.”
Fairly good aftertreatment options exist for controlling soot from high efficiency diesel engines, but NOx aftertreatment for diesel exhaust is difficult. This is because the exhaust contains excess oxygen, which makes conventional automotive catalytic converters ineffective. Special “lean-NOx” catalysts have been demonstrated, but they have reliability problems and are expensive, sometimes costing as much as the engine itself.
“You’d like to take care of the NOx problem at its source,” John said, “and that means lowering the combustion temperature.”
To accomplish this, John’s research centers on a concept that combines some of the advantages of gasoline engines (which have premixed fuel and air with no soot emissions) and diesel engines (which have high efficiencies due to their high compression ratio and lack of throttling losses). The concept, homogeneous charge compression ignition (HCCI), has been known for some time but the operating range was very limited, and the technical challenges could not be overcome without modern computerized controls.
John’s research is conducted in both a conventional, “all-metal” engine used for performance and emissions measurements, and a second engine with quartz windows to allow laser diagnostics to be used to probe the combustion chamber, illuminating various aspects of the in-cylinder processes.
Although much work is still required to perfect the concept, it is efficient and has low emissions. “Market penetration,” John says, “could take several years, but the potential fuel savings are tremendous.”
An approach that has the potential for more rapid market penetration is being explored by Paul Miles (8362). Paul is studying modifications to standard diesel combustion that result in low-temperature combustion in automotive-size diesel engines, greatly reducing NOx and particulate emissions.
Paul is investigating fuel spray and fuel-air mixing to understand in-cylinder geometries that enhance the combustion completeness, and to provide data for the development of computational tools for engine design by colleagues at Los Alamos National Laboratory and the University of Wisconsin. “The fuel injection, mixing, and combustion processes in engines are so complicated, and the physical processes are so convoluted,” Paul says, “you’re not going to design and optimize advanced combustion systems for these engines other than by computer.”
Fuels a focus too
Another part of the research effort is on fuels, especially fuels that enable the full potential of low-temperature combustion. One aspect to be sorted out is what the most appropriate fuel might be. Since gasoline and diesel engines have been around some 100 years, those fuels are now highly optimized for current engine designs, but there is no reason to expect they are ideal for low-temperature combustion.
Another aspect is how to accommodate the changing nature of the feedstocks for fuels. In the future, bio-derived fuels and fuels from heavier crude oils, oil sands, and potentially shale oil will play an increasing role.
Fuels are a specialty in the engine lab of Chuck Mueller (8362), who is studying fuel effects on low-temperature combustion strategies. Chuck began studying oxygenates in 1997 as a prospective way to reduce soot, and more recently to see if they can enable low-temperature combustion technologies. Experiments in his lab have already shown a drop of two orders of magnitude in pollutant emissions with no loss of fuel economy. “It’s really pretty revolutionary,” he says. “You’d think all the breakthroughs would have been made by now, but this is a rich field.”
“There are still many hurdles to overcome in order to make combustion efficient, clean and practical,” he says, “and emissions restrictions typically involve trade-offs between cost and performance. The concepts themselves may be relatively simple,” Chuck adds, “but implementing them will be challenging.”