Clean-Diesel Breakthrough: Simultaneous Decrease in Emissions of Both Particulates and Oxides of Nitrogen during Combustion

1999 R&D 100 Award Winner

• Description of the Technology
• How the Technology Works
  • Engine Operating Conditions
  • Oxygen-rich Air Supply
• What Problem Does This Technology Solve?
  • Environmental Problem--Transportation Pollution
  • Technical Problem--Barriers to Oxygen Enrichment
• Why is the Technology Important?
  • Technical Importance
  • Commercial Importance
• How Will the Technology Benefit the Average Consumer or the Public in General?
  • Better Quality of Life in Cities
  • Slower Global Warming
• What's the Current Status of the Technology?
• Who Sponsored this Research?
• References
• For More Information

Description of the Technology

Argonne National Laboratory has developed a technology that should allow diesel engines to operate more cleanly and efficiently. It was demonstrated in a locomotive diesel, but should apply to all types of diesel engines, including those in trucks, buses, heavy equipment, and cars. Practical and less expensive than alternative technologies, it could end a long-standing struggle with diesel pollution.

Argonne's researchers make the diesel fuel burn more completely and cleanly by adding extra oxygen to the engine’s air supply under specific, optimized engine conditions that we discovered this year. The oxygen-rich air is supplied by separating it from ambient air using membranes that act as a chemical filter. Argonne has collaborated with the chemical industry to optimize the membranes for this application.

Unlike past "oxygen-enrichment" techniques, our technology reduces visible smoke and minimizes production of both particulate matter (fine-particle pollution) and oxides of nitrogen (NO_x). The ability to decrease both particulate and NO_x emissions simultaneously during combustion is a major breakthrough that has eluded researchers since the 1970s.

Particulate matter has been shown to aggravate respiratory diseases, and it degrades visibility and begrimes buildings. Oxides of nitrogen are precursors to ozone, contribute significantly to smog, and may contribute to global warming. The U.S. Environmental Protection Agency (EPA) has recently enacted tough new standards on these two pollutants for trucks, buses, and locomotives. These industries are now scrambling to clean up their engines. Our new technology could put an end to that scramble for some time.

How the Technology Works

The breakthrough came when Argonne researchers tested a newcombination of three changes to engine operating conditions: (1) increased oxygen content in the engine air supply, (2) retarded timing of fuel injection, and (3) increased fuel flow rate (Figure 1). Argonne’s tests were the first to adjust all three parameters. Previous strategies had changed only one or two of these conditions.

This three-way optimization simultaneously reduces both particulate and NO_x emissions without sacrificing engine power. In fact, this technology even increases engine power and lowers fuel consumption.

Overall, this operating strategy is a practical, commercially viable solution to diesel pollution. Both timing and fuel flow can be adjusted easily. The crucial factor is a small, low-power, onboard supply of oxygen-enriched air. Working with an industrial partner, Argonne has also optimized an oxygen-production technology, based on advanced separation membranes, that’s small and cheap enough to make the new strategy practical for manufacturers. It should now be possible to make units that are about the same size as the air filter needed for a given engine.

Engine Operating Conditions. The fundamental principle of our strategy is oxygen- enriched combustion. Additional oxygen is added to the combustion air supply, which causes the fuel to burn more completely. The more complete the combustion is, the less smoke and soot are produced. This is not a new concept: oxygen-enrichment for both diesel and gasoline engines has been studied since the 1970s.

However, while those studies showed impressive reductions in particulate levels, NO_x levels stubbornlyincreased, no matter what researchers tried. Researchers at General Motors initially rejected this technology in the 1970s because of two problems: the lack of an onboard, economical oxygen supply unit and the accompanying high NO_x emissions (Wartinbee 1971).

Several researchers around the world, including engineers at Argonne, worked on these two challenges extensively for the last 20 years, with limited success. Argonne changed that by "thinking outside the box" in two ways: first, by using only a modest increase in oxygen level in the engine intake air; second, by combining that change with optimized fueling conditions. In all the previous efforts, no one had tried a slight enrichment and optimized fueling conditions. Many experimenters used engine air containing 25 to 35% oxygen (compared with the normal 21%). Argonne found the best results near 23%.

Results: The new emissions control strategy is one of the important results of a collaborative research program Argonne is conducting with the Electro-Motive Division (EMD) of General Motors and the Association of American Railroads to reduce emissions from locomotive engines. Because the test data are protected under an agreement with these partners, the complete results cannot be disclosed, but the data in Figure 2 have been approved for public release.

The combination of slightly oxygen-enriched (~23%) combustion air, retarded fuel injection timing, and increased fueling ratereduced particulate emissions by 60%, reduced NO_x emissions by 15%, and increased gross engine power by 18%, at engine full load (Figure 2). The engine power increases because more complete combustion means that more fuel can be burned in each stroke, releasing more total energy to power the cylinder. The NO_x levels decrease as an inherent result of the timing and fuel changes. These improvements have the potential to meet the EPA’s locomotive standards for 2004 (Tier 1) for NO_x and the 2005+ (Tier 2) standards for particulates.

Oxygen-Rich Air Supply. The practical application of this breakthrough depends on a compact, continuous supply of oxygen-rich air to the engine. Carrying bottled oxygen has never been an option because of safety issues. Argonne researchers recognized in 1989 that membranes that separate air into oxygen and nitrogen could provide more oxygen for engine combustion air.

These devices produce oxygen in the following manner. A membrane unit is a bundle of hollow tubes made of porous material (generally poly sulfone) coated with a patented nonporous polymer (Figure 3). Pressurized air is passed across the tubes, as in a shell-and-tube heat exchanger, with the shell side held at low pressure. Under this pressure differential, the nonporous polymer separates the air into two streams, one rich in oxygen and one rich in nitrogen. This separation occurs as air molecules dissolve in the polymer,then diffuse across the support tube (a mechanism called selective permeation by solution-diffusion). Oxygen dissolves and diffuses faster than nitrogen, so the gas that emerges on the shell side is rich in oxygen. (As a result, the tube-side air ends up rich in nitrogen.)

In 1989, membranes were too large for use in vehicles and required too much power for air separation. Furthermore, such membranes were optimized to generate nitrogen-rich air for the chemical industry and food processing and storage, so simply adapting them for oxygen enrichment was not energy-efficient, practical, or economical.

In 1991, Argonne tested new, small membrane units from Compact Membrane Systems, Inc. (CMS), which manufactures membrane cartridges for many applications in the food, medical, and chemical industries. Though smaller, these membranes still required too much power and were still too large to fit under the hood of a car or truck. Argonne worked with CMS to optimize the membranes for diesel engine oxygen enrichment.

Argonne tested many prototype cartridges to find the optimum design (e.g., tube dimensions, polymer composition, and coating thickness) and optimum operating conditions (e.g., flow rates, pressure differential). The primary criteria were small size (and indirectly, lower cost) and minimal power requirements for given engine airflow demands.

Results. Through this collaboration, Argonne has made membranes that are about 10 times shorter than they were in 1989 and that use 60% less power (Figure 4). These advances greatly improve the practicality of the membranes, as do improvements in cost. Production costs have dropped because advances in fiber spinning and processing technology now allow manufacturers to mass produce such modules while using less polymer coating material—from 1990 to 1996, the cost per square foot dropped by 80%.

What Problem Does This Technology Solve?

Environmental Problem — Transportation Pollution. This new strategy for emissions control for diesel engines represents a significant step toward addressing public concern over air pollution from transportation. Particulate matter and NO_x emissions from diesel engines are a major source of urban air pollution. Particulate matter may contain organic compounds that have the potential to cause cancer or mutations. It also decreases visibility, by forming a haze, and coats buildings in black grime. Oxides of nitrogen contribute to the formation of ground-level ozone, smog, and acid rain. Ozone is the component of smog that irritates eyes and burns and damages lungs, aggravating many respiratory disorders. Acid rain changes the chemistry of bodies of water and damages plants.

Particulates are especially a cause for concern. Recent epidemiological studies have reported increased deaths associated with exposures to combustion-related fine particulate matter less than 2.5 micrometers (0.0001 inch) in size (Schwartz et al. 1996). Other studies found that the particulates emitted by diesel engines are concentrated in this fine-particle range (Kittleson 1998).

In 1996, highway diesels (light-duty vehicles, light-duty trucks, and heavy trucks) emitted 172,000 short tons of particulates under 10 micrometers in size (PM-10), or 62.8% of the total, and 1.93 million short tons of NO_x, or 27% of the total (ORNL 1998). Locomotives are also contributors: in 1990, 10% of all NO_x emissions from mobile sources were from locomotives, which released 888,860 tons (980,000 metric tons) of NO_x and 21,768 tons (24,000 metric tons) of particulate matter (EPA, 40 CFR Parts 85, 89, 92).

To make this more tangible, consider that a fully loaded, 80,000-pound Class 8 truck—a familiar diesel polluter—emits about 73 pounds of particulates and 2,900 pounds of NO_x a year, according to Argonne estimates. That’s the weight of a typical car—with a big bag of soot in the trunk as well. And there are about 2 million Class 8 trucks on U.S. roads today (ORNL 1998).

In 1997, the EPA established tighter standards for heavy trucks, buses, and locomotives. For heavy trucks built in model year 2002, emissions of both particulates and NO_x must be 50% lower than in 1998. Buses are also subject to stricter standards. For locomotives, which had not been regulated previously, the agency adopted standards on NO_x, hydrocarbons, carbon monoxide, and particulate matter that apply to all new engines, all in-use locomotives, and any engines built after 1973 when they are remanufactured. The standards become more stringent with time — the lowest levels are targeted for 2005 and beyond. For line-haul locomotives operating today, meeting the 2005 standards will mean cutting NO_x emissions by 60% and particulate emissions by 41% (EPA, 40 CFR Parts 85,89, 92). For light-duty vehicles, the regulations are tighter yet.

Technical Problem — Barriers to Oxygen Enrichment. However, the new regulations create severe engineering problems for diesel engine manufacturers. For example, current truck engines barely meet the current standards (4 grams per brake horsepower-hour [g/bhp-h] of NO_x and 0.1 g/bhp-h of particulates). The task of getting a 50% reduction seems as difficult as squeezing water from a rock. In particular, the manufacturers are now caught in a bind between the regulations on particulates and the regulations on NO_x.

Conventional strategies for decreasing NO_x, such as exhaust gas recirculation and retarded injection timing, invariably increase particulates because of NO_x-particulates trade-offs inherent in diesel combustion (Figure 5). The converse is true for the particulate-reduction strategy of using oxygen-enriched engine air. Many studies using this approach reduced particulate levels by as much as 80%, but NO_x levels stubbornly increased — sometimes by as much as 200-400% — no matter what the researchers tried (Figure 5). These oxygen-enrichment scenarios would require aftertreatment for NO_x — adding complexity, inefficiencies, and cost. However, no practical and efficient NO_x aftertreatment was then — or is now — available.

Despite the success of oxygen enrichment for reducing particulates in all the previous studies, the accompanying higher NO_x emissions presented several barriers to its practical use:

• The extra NO_x cannot be removed by a conventional catalytic converter because
• diesel exhaust contains high levels of oxygen that interfere with the chemical reaction (reduction) that turns NO_x into nitrogen and oxygen, and
• particulates clog the catalyst, decreasing durability.
• Alternatives to catalytic after-treatment (e.g., nonthermal plasma, selective catalytic reduction of NO_x, and NO_x traps) are currently being investigated. However, these technologies are only in preliminary research stages and are projected to be expensive.
• All prior NO_x aftertreatment strategies resulted in a power and/or fuel penalty—their power demands more than offset the power boost from oxygen-enhanced combustion.

Why Is the Technology Important?

Argonne's new strategy is important because it broke the "NO_x barrier" to permit practical use of oxygen enrichment to decrease diesel emissions. We have identified a "window" of clean operation on a two-cylinder EMD 567 locomotive research diesel engine in the laboratory (Figure 5), and we believe this window is available in other diesel engines. Within that window, we obtain all the well-known benefits of oxygen enrichment, with none of its former penalties. Our efforts also helped remove power, size, and cost barriers to the use of air separation membranes, opening the way to practical devices for vehicles.

Technical Importance. Our three-way optimization with oxygen enrichment

• for the first time, simultaneously reduces both NO_x (by 15%) and particulates (by 60%),
• is an all-in-one, in-cylinder treatment that solves the emissions problem at the source,
• does not drain engine power (in fact, increases gross power by 18%), and
• improves fuel efficiency (2-10% improvement in brake-specific fuel consumption across the entire load range in a locomotive notch schedule).

Commercial Importance. With the availability of the advanced polymer membranes, our new strategy is practical for these reasons:

• It does not require manufacturers to redesign their engines, although because of the power boost, engines could become smaller.
• It could help locomotive operators in complying with future EPA standards.
• It offers flexibility, since there is a "window" within which adjustments can be made.
• It could be retrofitted on existing vehicles: like an air filter, it is a passive device in the intake, and it is fitted after the air filter.
• It uses a simple design and extremely durable materials and should pose minimal maintenance issues.
• It is relatively inexpensive: membranes of the size required for passenger vehicles are expected to cost $75 to $160 in mass production (prototypes are $1,500).

This combination of effectiveness, flexibility, and low cost will be very attractive for manufacturers and consumers. For comparison, proposed particulate traps for cars are projected to cost $200 plus 2 cents a gallon to operate. Special NO_x-treatment catalytic converters are projected to cost $300 plus periodic maintenance. Both would be needed on a vehicle, for a total initial cost of $500.

How Will the Technology Benefit the Average Consumer or the Public in General?

Better Quality of Life in Cities. Diesel emissions silently affect all of us every minute of every day, through insidious, generalized pollution of the natural environment. But consider the millions of Americans who live in EPA "nonattainment areas" —30 million people in the 77 particulate nonattainment areas and 100 million people in the 38 ozone nonattainment areas (EPA 1998). For us, the effects are particularly acute — and we know exactly what they are:

• You hold your breath as a truck pulls away from a traffic light, belching black smoke.
• You live by a busy truck route or railyard, and your children grow up breathing diesel exhaust.
• You work at a loading dock or transit terminal, and your lungs burn with every breath.
• You work in an office building and breathe delivery-truck exhaust that gets into the building’s air-handling system.
• You can’t see the lovingly crafted detail on an old building because it’s coated in grime.
• You own an old building, and you pay a fortune for cleaning and restoration.
• You can’t see the skyline of the city that is your home — it’s a smudgy yellow blur.
• You don’t catch as many fish at your favorite lake, because it’s been contaminated with acid rain.
• Your grandfather with asthma can’t go for walks and stays cooped up in his apartment in mid-summer because of an "ozone alert."

Of course, we can’t yet eliminate diesel pollution altogether, and change may not come soon enough to help today’s grandparents, but our breakthrough technology certainly promises to make cities healthier and more pleasant places for their children and grandchildren.

Slower Global Warming. This innovation might also help those children and grandchildren stay cooler in the future, because it could help reduce emissions of carbon dioxide, a "greenhouse gas" involved in global warming. Here’s why: diesel engines are the most efficient internal combustion engines. Because they burn less fuel in the first place, they emit less carbon dioxide exhaust. If trucks and buses get cleaner, the general public may be more willing to consider diesel cars. Today’s diesel cars perform just as well as gasoline cars (in blind tests, drivers can’t tell the difference) but are 30% more fuel efficient. The new technology could bring that fuel efficiency to the vast automotive market, significantly reducing greenhouse gas emissions. Argonne estimates that compared with a gasoline car, a diesel car would emit 28% less greenhouse gas (over the total energy cycle, including refining and vehicle use). Of course, better fuel efficiency would also help save our petroleum resources and reduce our need to depend on foreign sources of oil.

What's the Current Status of the Technology?

Currently, diesel engine manufacturers, including EMD, are considering adopting the technology. Further engine refinements and appropriate packaging developed in the future could lead to practical implementation in locomotives, trucks, and cars.

The technology has been demonstrated for a research locomotive engine in the laboratory, but the concept should be equally applicable to heavy trucks, buses, and light-duty vehicles, such as cars and pickup trucks. In the United States in 1996, there were an estimated 2.5 million heavy trucks (over 26,000 pounds), 658,000 buses of all types, and 20,000 locomotives, traveling hundreds of millions of miles every year (ORNL 1998).

Many of these vehicles would be candidates for retrofitting with the new technology. Retrofitting is most important for locomotives, which are being regulated retroactively to 1973. In principle, retrofitting heavy trucks would also be feasible. After appropriate testing to find the operating "window" for specific engines, the technology could be integrated in both old and new vehicles. The general feasibility of air-separation membranes for passenger cars was demonstrated at Argonne for an application to reduce emissions from cold-starts in gasoline engines; however, additional testing would be needed to demonstrate membranes for diesel cars, trucks, buses, or locomotives.

Argonne has applied for three patents on the emissions control strategy and holds another four patents on the use of membrane-based oxygen and/or nitrogen enrichment for engine emissions control. The patents for the membrane coating materials are held by DuPont and licensed to CMS. In collaboration with Argonne, CMS is already making and selling prototype "cartridges" for use in nitrogen enrichment of air for heavy-duty diesel engine applications (to reduce NO_x only). Mass production of this cartridge is being considered. CMS believes scale-up to locomotive-size membranes using the new design will be straightforward, as they already produce similar units for applications requiring similar volumes and flow rates.

Who Sponsored This Research?

This research is being conducted under a cooperative research and development agreement (CRADA). Argonne's portion of the research is funded by the U.S. Department of Energy (DOE), Office of Science, Laboratory Technology Research Program; Argonne's research partners are the Association of American Railroads (Washington, DC) and General Motors Electro-Motive Division (LaGrange, Ill.). Compact Membrane Systems is located in Wilmington, Delaware. DOE's Office of Energy Efficiency and Renewable Energy, Transportation Technology, Advanced Automotive Technologies, has provided funding to Argonne to verify this concept for light-duty applications.

References

Code of Federal Regulations, EPA 40 CFR Parts 85, 89, and 92, February 11, 1997, "Emission Standards for Locomotives and Locomotive Engines: Proposed Rule."

EPA, 1998, "US Air Quality Nonattainment Areas," Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, effective date September 22, 1998, accessed October 14, 1998).

Kittleson, D.B., 1998, "Engines and Nanoparticles: A Review," J. Aerosol Science, 29(5/6):575-588.

ORNL, 1998, Transportation Energy Data Book: Edition 18, Oak Ridge National Laboratory, report number ORNL-6941.

Schwartz et al., 1996, "Is Daily Mortality Associated Specifically with Fine Particles?" J. Air & Waste Management Association, 46:927-939.

Wartinbee, Jr., W.J., 1971, "Emissions Study of Oxygen Enriched Air," SAE Paper No. 710606.

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