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  1. 1. F E B RUA RY 2 0 1 1 Communication 16 H E A LT H E F F E C T S I N S T I T U T EThe Future of Vehicle Fuels and Technologies:Anticipating Health Benefits and ChallengesHEI Special Committee on Emerging Technologies
  2. 2. The Future of Vehicle Fuels andTechnologies: Anticipating Health Benefits and Challenges HEI Special Committee on Emerging Technologies Communication 16 Health Effects Institute Boston, Massachusetts Trusted Science · Cleaner Air · Better Health
  3. 3. Publishing history: The Web version of this document was posted at www.healtheffects.org in February 2011.Citation for document: HEI Special Committee on Emerging Technologies. 2011. The Future of Vehicle Fuels and Technologies: Anticipating Health Benefits and Challenges. Communication 16. Health Effects Institute, Boston, MA.© 2011 Health Effects Institute, Boston, Mass., U.S.A. Asterisk Typographics, Barre,Vt., Compositor. Printed byRecycled Paper Printing, Boston, Mass. Library of Congress Catalog Number for the HEI Report Series: WA 754 R432. Cover paper: made with at least 55% recycled content, of which at least 30% is post-consumer waste; free of acid andelemental chlorine.Text paper: made with 100% post-consumer waste recycled content; acid free; no chlorine used inprocessing.The book is printed with soy-based inks and is of permanent archival quality.
  4. 4. CONTENTSAbout HEI v NOTE TO READERS: Each chapter in this reportContributors vii describes specific technologies or fuels, the likelihood of theirCHAPTER 1. INTRODUCTION 1 use, their emissions, life-cycle Context 1 issues, and regulatory issues. Technologies 2 Chapter 6 provides a concise summary and the Committee’s Fuels 2 conclusions.CHAPTER 2. NEW TECHNOLOGIES 3 Engine Modifications 3 Gasoline Direct-Injection Engines 3 Gasoline Direct Injection — Stoichiometric (Homogeneous) 4 Gasoline Direct Injection — Lean-Burn (Stratified) 4 Turbocharging and Downsizing Gasoline Engines 5 High-Efficiency Dilute Gasoline Engine 5 Homogeneous Charge Compression Ignition 6 Low Temperature Diesel Combustion 7 Exhaust Aftertreatment 7 Diesel Particle Filters 8 DPFs Using Passive Regeneration 8 DPFs Using Active Regeneration 9 Aftermarket and Retrofit Systems 10 Selective Catalytic Reduction and Ammonia Slip Catalyst 11 NOx Adsorber Catalyst Technology 13 Sidebar: Impact of Vehicle Technologies on Climate 14CHAPTER 3. ELECTRIC DRIVE TECHNOLOGIES 15 Hybrid Electric Vehicles 15 Plug-In Hybrid Electric Vehicles 16 Battery Electric Vehicles 16 Fuel-Cell Vehicles 17 Questions Related to Fuels for Electric Drive Technologies 18 Electricity as a Fuel 18 Fuel-Cell Vehicles 18CHAPTER 4. NEW FUELS 19 Spark-Ignition Fuels 19 Gasoline 19 Ethanol 19 Other Alcohols 21 Natural Gas 21 Liquefied Petroleum Gas 22 Compression Ignition Fuels 22 Petroleum Diesel 22 Fischer-Tropsch Diesel 22 Biodiesel 23 Alcohols and Ethers 24
  5. 5. Communication 16 Source-Related Issues Concerning Fuels 24 Oil Sands 24 Oil Shale 25 Coal Gasification 25 Cross-Cutting Issues 25 Climate Impact and Life-Cycle Analysis for Fuels 25 Fuel Additives 26CHAPTER 5. OTHER EMISSIONS ISSUES 28 Brake Wear 28 Tire Wear 29 Lubricating Oil 30CHAPTER 6. SUMMARY AND CONCLUSIONS 31 Internal Combustion Engine Technologies 32 Gasoline Direct-Injection Technology 32 Selective Catalyst Reduction 32 Electric Drive Technology 32 Fuels for Internal Combustion Engines 34 Use of Ethanol in Gasoline Will Increase 34 Use of Fatty Acid Esters in Diesel Fuel (Biodiesel) Will Increase 35 Environmental Issues Related to the Source of Fuels Will Be Important 35 Use of Metallic Additives in Fuels Is a Continuing Concern 35REFERENCES 35ABBREVIATIONS AND OTHER TERMS 41ACKNOWLEDGMENTS 43HEI Board, Committees, and Staff 45
  6. 6. ABOUT HEI The Health Effects Institute is a nonprofit corporation chartered in 1980 as an independentresearch organization to provide high-quality, impartial, and relevant science on the effects of airpollution on health. To accomplish its mission, the institute • Identifies the highest-priority areas for health effects research; • Competitively funds and oversees research projects; • Provides intensive independent review of HEI-supported studies and related research; • Integrates HEI’s research results with those of other institutions into broader evaluations; and • Communicates the results of HEI’s research and analyses to public and private decision makers. HEI receives half of its core funds from the U.S. Environmental Protection Agency and halffrom the worldwide motor vehicle industry. Frequently, other public and private organizations inthe United States and around the world also support major projects or certain researchprograms. This project, in particular, was partially supported by the Federal HighwayAdministration. HEI has funded more than 280 research projects in North America, Europe, Asia, and LatinAmerica, the results of which have informed decisions regarding carbon monoxide, air toxics,nitrogen oxides, diesel exhaust, ozone, particulate matter, and other pollutants. These resultshave appeared in the peer-reviewed literature and in more than 200 comprehensive reportspublished by HEI. HEI’s independent Board of Directors consists of leaders in science and policy who arecommitted to fostering the public–private partnership that is central to the organization. TheHealth Research Committee solicits input from HEI sponsors and other stakeholders and workswith scientific staff to develop a Five-Year Strategic Plan, select research projects for funding, andoversee their conduct. The Health Review Committee, which has no role in selecting oroverseeing studies, works with staff to evaluate and interpret the results of funded studies andrelated research. All project results and accompanying comments by the Health Review Committee are widelydisseminated through HEI’s Web site (www.healtheffects.org), printed reports, newsletters andother publications, annual conferences, and presentations to legislative bodies and public agencies. v
  7. 7. C O N T R I B U TO R SHEI Special Committee on Emerging TechnologiesAlan Lloyd, Cochair, President, International Council David Kittelson, Professor of Mechanical Engineering,on Clean Transportation University of MinnesotaChristine Vujovich, Cochair, Former Vice President C. Andy Miller, Atmospheric Protection Branch,Marketing and Environmental Policy, Cummins Inc. National Risk Management Research Laboratory,Thomas Cackette, Chair, New Technologies U.S. Environmental Protection AgencySubcommittee, Chief Deputy Executive Officer, Norbert Pelz, Former Senior Manager, Fuels andCalifornia Air Resources Board Lubricants, DaimlerChrysler AGAlbert Hochhauser, Chair, Fuels Subcommittee, Kathryn Sargeant, Director, Air Toxics Center, Office ofFormer Senior Engineering Advisor, ExxonMobil Transportation and Air Quality, U.S. EnvironmentalResearch and Engineering Company Protection AgencySteven Cadle, Principal Scientist, Air Improvement Robert Sawyer, Class of 1935 Professor of EnergyResource; formerly at General Motors Research and Emeritus, University of California–Berkeley, and VisitingDevelopment Center Professor of Energy and Environment, UniversityWayne Eckerle, Vice President, Corporate Research College Londonand Technology, Cummins Technical Center Dennis Schuetzle, President, Renewable EnergyHelmut Greim, Former Director, Institute of Institute InternationalToxicology and Environmental Hygiene, and Chair of Tom Stricker, Vice President, Toyota MotorToxicology, Technical University of Munich North AmericaJohn Heywood, Director, Sloan Automotive Michael Walsh, International consultant on fuels andLaboratory, and Sun Jae Professor of Mechanical transportation emissionsEngineering, Massachusetts Institute of Technology Michael Wang, Senior Scientist and Group Leader,Roland Hwang, Vehicles Policy Director, Energy Systems Assessment Section, Center for TransportationProgram, Natural Resources Defense Council Research, Argonne National LaboratoryPeer ReviewersKathryn Clay, Alliance for Automobile Manufacturers S. Kent Hoekman, Desert Research InstituteJames Eberhardt, Office of Vehicle Technologies, Timothy Johnson, Corning Inc.U.S. Department of Energy Noboru Oba, Nissan Motor CompanyDouglas Lawson, National Renewable Energy Jeremy Martin, Union of Concerned ScientistsLaboratory Ichiro Sakai, American Honda Motor CompanyAxel Friedrich, Consultant; formerly at the German Daniel Sperling, University of California–DavisFederal Environment Agency (UBA)John German, International Council on CleanTransportationHEI Project ManagerRashid Shaikh, Director of ScienceHEI Publications StaffAsterisk Typographics, Composition Fred Howe, Consulting ProofreaderSuzanne Gabriel, Editorial Assistant Susan T. Landry, Consulting Science EditorBarbara Gale, Director of Publications Flannery Carey McDermott, Editorial AssistantHope Green, Editorial Assistant vii
  8. 8. COMMUNICATION 16The Future of Vehicle Fuels and Technologies: Anticipating HealthBenefits and ChallengesHEI Special Committee on Emerging Technologies priority areas for future research to inform regulatory andCHAPTER 1. INTRODUCTION other decisions during the next 10 years. Driven by a need for energy independence, increased In general, the Committee has avoided quantifying thefuel efficiency, and concerns about climate change and impact on fuel efficiency of the technologies and fuelsreduction of air pollutant emissions, the development of included in this report or to delineate their benefits innew fuels and technologies for the transportation sector is terms of greenhouse gases (GHGs). The rationale for thismoving forward at an unprecedented pace in the United choice is that technologies and fuels may be used on differ-States and other parts of the world. The mission of the ent vehicle platforms by different manufacturers, with orHealth Effects Institute (HEI*) is to understand the health without other processes, to reduce fuel consumption andconsequences of exposure to emissions from vehicles and improve efficiency, and the engines may be tested underother sources in the environment. Therefore, HEI decided different conditions. The Committee has mostly relied onto assess the nature and pace of new fuels and technolo- qualitative statements. The purpose of this report, thus, isgies, along with potential unintended consequences of to elucidate, as well as possible, how each technology maytheir use. To do this effectively, HEI established in 2009 a affect emissions, environmental quality, GHG emissions,Special Committee on Emerging Technologies (SCET) com- and, in turn, what effect it may have on health in a qualita-posed of leading experts in a range of disciplines relevant tive fashion.to assessing new fuels and technologies. Although the main scope of this report is the study of SCET produced The Future of Vehicle Fuels and Tech- emerging technologies and fuels in the United States, andnologies: Anticipating Health Benefits and Challenges to to some extent, Europe and Japan, the Committee has also addressed technologies and fuels in the developing coun-provide HEI with a compilation of those automotive tech- tries wherever appropriate.nologies and fuels that are likely to be commercially avail-able within the next 10 years in the United States at a levelof market share that could result in population exposure. CONTEXTThe report highlights expected changes in emissions and The technology of motor vehicles, and especially that ofother effects from the use of each technology and fuel exam- the powertrains, continues to improve and change (Heywoodined. The primary audience for the report is HEI’s Research and Welling 2009). Tailpipe emissions have been drasticallyCommittee, which will review the projections and trends reduced in response to stringent emission regulations; how-in the use of these technologies and fuels and identify the ever, there continues to be a compelling need for attain-potential health implications that may arise from these ment of regulatory standards in many parts of the Uniteddevelopments. This assessment of fuels, technology, emis- States (U.S. Environmental Protection Agency [EPA] 2010a),sions, and potential effects on health is designed to be a which frequently include reductions in vehicles’ emissions.key resource in guiding HEI’s decisions to determine the After a hiatus of about 20 years, average fuel consumption of vehicles sold in the United States has begun to improve, and new regulations limiting carbon dioxide (CO2) emis- sions and increasing fuel economy requirements — alongWork of the HEI Special Committee on Emerging Technologies was sup-ported with funding from the United States Environmental Protection with concerns about supplies and cost of petroleum — willAgency (Assistance Award CR–83234701) and motor vehicle manufactur- accelerate this trend (U.S. EPA and National Highway Traf-ers. Support for the preparation and publication of this Communicationwas provided by the Federal Highway Administration (Grant DTFH61-09- fic Safety Administration 2010). These regulatory require-G-00010). This report has not been subjected to peer or administrative ments, along with the rising cost of fuel and concernsreview by any of the sponsors and may not necessarily reflect their views,and no official endorsement should be inferred. about supplies, will provide pressure to increase the extent* A list of abbreviations and other terms appears at the end of this report. and pace of changes in technology and fuels.Health Effects Institute Communication 16 © 2011 1
  9. 9. The Future of Vehicle Fuels and Technologies The discussion in this report has implicitly assumed Where appropriate, the Committee has also made com-that, broadly speaking, new technologies are suitable for ments on the relative costs of technologies, although thisnew fuels and vice versa. However, the problem of match- discussion is cost qualitative, like those of most of theing engines and fuels with each other is receiving much other factors considered in the report (for more quantita-attention. Questions are being raised, such as: How far tive estimates, see National Research Council [NRC] 2010).should the engines be optimized for the use of a specific In assessing future powertrain and aftertreatment develop-fuel? And, how far should a fuel’s composition be opti- ments, the Committee has used today’s mainstream gaso-mized for use with a specific engine technology? In other line and diesel engine technologies as baselines.words, greater attention to the fuel–engine system should This report describes the technologies that hold the mostbe paid. Though important, consideration of such issues promise for significantly improving the efficiency of thesewas beyond the scope of this report. baseline gasoline and diesel engines as well as the more promising alternatives to the internal combustion engineTECHNOLOGIES technologies that are close to or already in limited produc- Emission controls in gasoline engines and associated tion. The prospects for these new technologies and anycatalyst technologies for the reduction of hydrocarbons, related emission concerns — chiefly how they might con-carbon monoxide (CO), and oxides of nitrogen (NOx) are tribute to air pollution problems — are the primary focus.highly effective and continue to improve. Technologies for Because of the special nature of electric-drive vehicles,the reduction of particulate matter (PM) emissions from die- which use special technologies allowing them to use elec-sel engines are maturing rapidly. Technologies for reduc- tricity as the source of power, electric vehicles (EVs) areing NOx from diesel engines are becoming available and discussed in a separate chapter that bridges considerationsare being continuously improved. related to technologies and fuels. A variety of factors govern the output of emissions froma vehicle, including the powertrain and emissions controltechnologies, the engine’s average operating efficiency, the FUELSvehicle’s weight, size, and relative performance capability After considering the existing and upcoming technolo-(for example, acceleration and speed), along with the fuel gies, the Committee focused on fuels. This report providesused as the source of power, and driver behavior and driv- HEI with a “roadmap” of fuel use and related issues thating conditions. Vehicle-based technologies and fuels also are projected for the next 10 years. Significant changes instrongly affect the vehicle’s fuel (or energy) consumption the mix of transportation fuels in the marketplace areand hence its GHGs. They also have a concomitant impact anticipated; the factors driving such changes include:on emissions of criteria and toxic air pollutants. • The need to reduce GHGs In this report, the Committee examines the more prom-ising options for future powertrains in on-road vehicles • The rate of introduction of new automotive technology(which dominate transportation’s contribution to urban air • The need for improvements in ambient air qualitypollution and significantly affect global GHG emissions). • The need for reduced reliance on imported petroleumAs much as possible, the Committee has provided a brief and petroleum productsdescription of the technologies, commented on their likeli- • The rate of utilization of resources from agricultural andhood of use and the time frame, assessed their emissions forestry sectorsand other potential impacts, and discussed any specific • The availability of new sources such as natural gas fromregulatory issues. The powertrain developments consid- shaleered include mainstream gasoline and diesel engines and • The price of petroleum and petroleum productsthe anticipated changes in their component technolo-gies, as well as hybrids, plug-in electric hybrid vehicles Use of ethanol is increasing, mostly blended with gaso-(PHEVs), battery-powered electric vehicles (BEVs) and line; there is also some use as E85 (85% ethanol with 15%fuel-cell vehicles (FCVs). gasoline) in flexible-fuel vehicles. Other fuels that could be The Committee also evaluated anticipated changes in produced from biomass are being explored. BEVs andexhaust-aftertreatment technologies — that is, particle filters PHEVs are entering the market. If production volumes ofand catalysts. These assessments are intended to provide such vehicles grow to become a significant part of totalguidance as to the relative attractiveness of the different sales, then the use of electricity as a major fuel in transpor-powertrain and aftertreatment technologies and the impact tation will become important. The use of natural gas as athey are likely to have on fuel consumption and GHG source of energy is on the increase, mostly for power gen-emissions, as well as on toxic and unregulated pollutants. eration, chemical production, and home heating. It is used2
  10. 10. HEI Communication 16on a fairly limited basis in the United States for transporta- that — at a constant vehicle weight and performance —tion, and although the prospects for expanded use in the continuous improvements in the conventional internalUnited States are not clear, the use of natural gas in trans- combustion engine can lead to a reduction of 30% to 50%portation in Europe and Asia is increasing. The use of in fuel consumption of light-duty vehicles (LDVs) over thehydrogen as a fuel for transportation may well emerge in next 20 to 30 years (Bandivadekar et al. 2008; NRC 2010).the next 5 to 10 years. The supply of gasoline and diesel Such improvements will come from steady advances infrom oil sands and heavy oils (crude oil with high vis- weight reductions (made possible by the use of new mate-cosity and high hydrogen-to-carbon ratio) is steadily rials and innovative designs), better aerodynamics, elec-increasing, but because of the higher energy demands that tronic controls, and other changes (such as variable valveextraction and production require, the associated GHG timing, transmissions with more gears, and cylinder de-emissions are likely to be higher, too, than those from fuels activation). The use of biofuels, powertrain electrification,derived from petroleum. The environmental impact of and hydrogen-fuel cells will lead to improvements in theextraction is also a cause for concern. average fuel consumption across the fleet, as well. Improve- As noted above, the Committee took a broad view and ments in traffic management and other social policies couldincluded nontraditional fuels such as electricity in the yield added benefits.report. We recognize that indirect factors may affect fuel The extent to which the various technologies and fuelschoices. Such indirect factors include changes in emis- will penetrate the market in the future — and thus the ulti-sions that may result from the use of different sources to mate benefit they will provide — will depend on a varietyproduce fuel. For instance, heavy crude may generate more of factors, including technology cost, fuel cost, availabilityemissions than conventional crude oil in the processing of required infrastructure, government policy, customersteps, but this source will produce fuel that is equivalent to preferences, and more. The Committee attempted to quali-conventional fuels. Recognizing the challenges inherent in tatively assess the likelihood of use of each option, recog-understanding this complex area, the Committee also looked nizing there are uncertainties in any future projection.at fuels from the perspective of life-cycle analysis and GHG Also, the Committee focused mostly on technologies thatemissions for a broader determination of the impact of fuel are likely to have an effect on emission characteristics.choices on these issues. Finally, indirect impact on land Extensive discussions of all the technologies, their costs,use from widespread use of biofuels continues to be an and their impacts have recently been published (NRCarea of intense interest and uncertainty; whereas the Com- 2010; U.S. EPA 2008).mittee recognized these issues, it was beyond the scope ofthis report to perform a detailed evaluation. ENGINE MODIFICATIONS Throughout this analysis, the Committee relied largelyon detailed assessments done by others, and used such Gasoline Direct-Injection Enginespublished data as well as its own judgment to reach a con- The dominant technology used to control the fuel flowclusion as to the expected evolution of these promising in gasoline engines has been port injection; however, thetechnologies and fuels. direct injection of fuel into the cylinders of gasoline engines is increasingly being used because it improves fuel efficiency and performance. Although the gasoline direct-CHAPTER 2. NEW TECHNOLOGIES injection (GDI) system is more expensive than the port- The need to improve fuel economy and the need to injection system, it provides better control of the air-to-reduce both GHG emissions and other pollutants are driv- fuel ratio, especially while starting an engine and duringing major changes in the automotive sector, and this trend warm-up. Another important feature of the GDI is that itwill continue during the next decades. State and federal allows the use of a higher engine compression ratio, madegovernments as well as international organizations have possible because of cooling of the in-cylinder air charge asmandated improvements along these lines. The internal the direct-injected fuel spray evaporates. Because of the lesscombustion engine will continue to improve and remain complete mixing of fuel vapor and air, however, the partic-the dominant technology for the next two or so decades, ulate emissions of the engine increase, including the num-although we are likely to see an increase in powertrain ber of ultrafine particles (UFPs; particles that are less thanelectrification and the use of nonpetroleum fuels within 100 nm in diameter). The timing of fuel injection is impor-this period. tant, therefore, because earlier fuel injection provides bet- How much room is left to improve the efficiency of con- ter mixing and lower PM emissions. Direct-fuel injectionventional internal combustion engines? It appears likely also enables effective turbocharging and engine downsizing 3
  11. 11. The Future of Vehicle Fuels and Technologies(discussed below). Two different approaches have been Currently, there are no standards for UFPs, although thisdeveloped to use the direct-fuel injection concept: stoi- is an area of potential health concern. Perhaps the mostchiometric (homogeneous) and lean burn (stratified). far-reaching expression of concern regarding UFPs is in Europe, where a standard based on the number of particlesGasoline Direct Injection — Stoichiometric will be phased in for all diesel vehicles starting in 2011(Homogeneous) and will be fully in place by 2013. This standard is being implemented less because of specific health questions andDescription of the Technology In a GDI-stoichiometric more to ensure that diesel-exhaust particle filters (DPFs)engine, fuel is injected at high pressure directly into the are installed on all diesel vehicles. A particle number stan-combustion chamber instead of upstream of the intake dard will also be extended to all gasoline engines, startingvalve. Direct injection provides better fuel vaporization, in 2014 and with full implementation by 2015 (DieselNetmore flexibility as to when the fuel is injected, and a more 2010). For gasoline-powered, lean-burn GDI vehicles tostable combustion event. The heat of vaporization of the meet such a standard, auto manufacturers may need to em-fuel lowers the charge temperature, which reduces engineknock and allows for higher compression ratio and higher ploy a particulate trap, as discussed below. The Californiaintake pressures with reduced levels of enrichment. GDI is Air Resources Board, in the context of proposed rules toparticularly effective in improving fuel efficiency when implement its Low-Emission Vehicle (specifically LEV III)combined with turbocharging and engine downsizing. The regulations for light-duty and medium-duty vehicles, isstoichiometric combustion also allows efficient use of the currently considering an optional particle number stan-well-developed three-way catalyst to treat emissions; this dard that would be based on Europe’s Particulate Measure-is an advantage in comparison to lean-burn direct injection, ment Programme (PMP) and would include solid particleswhich requires more complex aftertreatment to reduce PM down to 23 nm in size (California Air Resources Boardand NOx emissions. 2010a). The PMP measurement method accounts for only solid particles; in contrast, the particle mass standardsLikelihood of Use and Time Frame Because of the diffi- include both solid and volatile particles. There is an on-culties presented by lean-burn combustion (discussed going debate about what the lower size cutoff should bebelow), automotive companies are preferentially adopting and whether volatile particles should be counted.the use of GDI-stoichiometric. Such engines — in combina-tion with turbocharging and engine downsizing — are cur- Gasoline Direct Injection — Lean-Burn (Stratified)rently in use in automotive applications around the globe, Description of the Technology Lean-burn (stratified-with most major manufacturers offering them. In view of the charge) GDI allows operation with excess air in the cylin-increasing pressure for improvements in fuel efficiency and der chamber, reducing the amount of intake throttling,reduction in CO2 emissions, the use of GDI-stoichiometric thus reducing pumping losses and fuel consumption. Intechnology is likely to become widespread during the the lean-burn mode, fuel is injected near the spark plugcoming decade. during the compression stroke to create a stratified chargeEmissions of Potential Concern The major concern aris- near the spark plug (U.S. EPA 2008). Under certain operat-ing from the use of GDI-stoichiometric engines is higher ing conditions, the air-to-fuel ratio can be as high as 20:1 toemissions of PM, both total mass and UFPs, although 40:1 (as compared with 14.7:1 for stoichiometric combus-such emissions are less than those from lean-burn engines. tion). The advantages of lean-burn GDI technology areUFPs emitted from GDI-stoichiometric engines have not reduced pumping losses and reduced heat losses (thebeen well characterized, though research in this area is excess air reduces combustion temperature, which in turncurrently underway. reduces heat loss to the cooling and exhaust systems). Lean-burn combustion, when combined with engine down-Life-Cycle Issues None. sizing and turbocharging, can result in useful improve- ment in fuel economy above what GDI-stoichiometricSpecific Regulatory Issues The regulatory issues raised technology can achieve.by the use of GDI-stoichiometric engines are the increasedmass of PM (which can be reduced by refinements in fuel Likelihood of Use and Time Frame Lean-burn GDI en-injection that minimize contact with the combustion cham- gines began to appear in the mid-1990s, primarily in Japanber walls and valves) and the production of UFPs. Control and Europe, and continue to be used today on a limitedof emissions of NOx is not a concern because of the stoi- basis. In areas with stringent NOx emission requirements,chiometric mixture and use of a three-way catalyst. aftertreatment costs are higher than for stoichiometric4
  12. 12. HEI Communication 16engines, and PM controls may be necessary in the future. The abnormal phenomenon of knock in gasoline-engineUse of gasoline with very low sulfur is also required due to combustion limits both the compression ratio of the enginethe adverse effect of sulfur on lean-exhaust aftertreatment and the extent to which it can be boosted or turbocharged.systems. These considerations are likely to limit the wide- GDI technology reduces the impact of these constraintsspread use of this technology, and currently no manu- on turbocharged engine operation. The injection of fuelfacturer in the United States has plans to introduce this directly into the cylinder cools the in-cylinder air chargetechnology. To the extent that sulfur limits in gasoline fuel as the gasoline spray evaporates when the fuel drops moveare reduced, or more sulfur-tolerant lean NOx catalytic con- through this air. This evaporative cooling effect offsets theverters are developed, use of this technology could expand onset of knock (which is caused by excessively high tem-in the future. However, because of the higher PM emissions peratures of the unburned mixture during combustion)from such engines, a particle trap may also be required. and allows higher boost. As a result, the low compres- sion ratio of traditional turbocharged engine designs canEmissions of Potential Concern With the lean-burn GDI be substantially increased. Thus, as discussed above, tur-engine, as with the GDI-stoichiometric engine, PM mass bochargers are being widely used, along with GDI andand particulate numbers increase as compared with con- engine downsizing, to enhance fuel efficiency of gasoline-ventional gasoline engines. Due to excess air in the com- powered vehicles.bustion chamber, the exhaust from lean-burn GDI enginesis lean, an environment in which the conventional three- Likelihood of Use and Time Frame The use of turbo-way catalyst does not work well; therefore, emission con- chargers for gasoline engines — in combination with enginetrol requires the use of a lean-NOx catalytic converter or downsizing — is increasing rapidly to increase fuel effi-similar technology (Tashiro et al. 2001), adding to the over- ciency, especially with GDI technology. Turbochargers haveall cost. Also, because the fuel is added later during the been used on gasoline engines around the world for manycombustion cycle, there is less time for mixing of fuel and years. Recent refinements in turbochargers, including vari-air, increasing the emissions of PM above those from a able geometry, improved materials, and other factors, haveGDI-stoichiometric engine. increased the reliability and performance of these units over those of just a decade ago (U.S. EPA 2008). In theLife-Cycle Issues None. future, it is likely that most turbocharged gasoline engines will use direct-fuel injection.Specific Regulatory Issues As mentioned above, com-pared with emissions from GDI-stoichiometric engines, Emissions of Potential Concern The engine-out pollutantlean-burn GDI engines can produce higher NOx emissions, emissions levels in turbocharged engines may be some-which must be controlled by the use of a lean-NOx cata- what higher than for a standard gasoline engine, and thelytic converter. Such catalytic converters can be poisoned thermal loading on the exhaust catalyst system can beby sulfur in the exhaust. Therefore, gasoline with very low higher. These problems can be resolved, although there aresulfur levels, at or below 10 to 15 parts per million (ppm), some concerns regarding cold starting with turbocharging,is needed to achieve low NOx control. Gasoline sulfur lim- under the proposed LEV III standards in California. Still,its vary throughout the world; in the United States, the no critical concerns related to emissions and the use of tur-currently allowed levels are 30 ppm (average) and 80 ppm bocharging are expected.(maximum). Reductions in the sulfur level of fuel wouldgreatly facilitate deployment of lean-burn GDI engines. Life-Cycle Issues None.Issues related to control of PM from GDI engines are dis-cussed above in the section about GDI-stoichiometric Specific Regulatory Issues None.engines and apply to lean-burn GDI engines as well. High-Efficiency Dilute Gasoline EngineTurbocharging and Downsizing Gasoline Engines Description of the Technology In view of the increasinglyDescription of the Technology In a turbocharged engine, tighter emission standards for diesel engines, some of itsthe turbocharger compressor increases the density of the efficiency and cost advantages are being compromised.air entering the engine cylinders. Exhaust gases flowing Therefore, there has been interest in exploring ways to usethrough the turbocharger turbine drive this compressor. gasoline engine technologies for heavy-duty applicationsThus, more fuel can be burned in a given size engine, (Alger et al. 2005), but more recently, attention has shiftedincreasing its torque and power. The engine can then be to applications in the light- and medium-duty market. Indownsized (and the maximum speed reduced). high-efficiency dilute gasoline engine (HEDGE) technology, 5
  13. 13. The Future of Vehicle Fuels and Technologiesexhaust gas recirculation (EGR) is used to reduce throttling Life-Cycle Issues None.losses and to mitigate engine knock. EGR is used to controlthe amount of air inducted into the combustion chambers Specific Regulatory Issues See above under Emissions ofand a stoichiometric mixture is created. The EGR extends Potential Concern.the knock margin, which allows more advanced timing and Homogeneous Charge Compression Ignitionimproved fuel economy. Finally, by using a base enginewith high peak cylinder pressure capability, the engine can Description of the Technology Homogeneous charge com-be downsized and down-speeded — which also reduces pression ignition (HCCI) is an engine combustion processfuel consumption. Furthermore, as with lean-burn GDI, the that has potential for improving the efficiency of inter-dilute operation achieved with EGR reduces combustion nal combustion engines while reducing pollutants in thetemperature and heat losses. HEDGE technology integrates exhaust. HCCI may be considered a special case of lowmostly existing technologies that allow a gasoline-fueled temperature combustion, which is discussed below. A veryengine to operate at torque-per-liter and thermal-efficiency lean fuel–vapor air mixture, usually hotter than in stan-levels comparable to modern diesel engines, but at a sub- dard gasoline engines, is compressed in the engine cylindersstantially lower engine and aftertreatment cost. Devel- to a sufficiently high temperature to cause spontaneousopment of cost-effective ignition systems, component ignition. Alternatively, a dilute fuel, air, and residual gasdurability, exhaust-catalyst technology, and sensor tech- plus EGR mixture can be used. The combustion of thesenology is the present focus of research in this area. extremely lean or dilute mixtures in an internal combus- tion engine produces low NOx emissions with the poten-Likelihood of Use and Time Frame Modern gasoline en- tial for higher efficiency. The fuel and air mixture has to begine development continues at a rapid pace, especially well mixed in HCCI engines for efficient, controlled auto-with the introduction of GDI engine technology in the last ignition. Also, the compression-ignited combustion pro-5 years. There is also a trend toward using highly diluted cess starts at multiple points, which is inherently difficultlean-burn GDI engines. Given the current pace of gasoline to control (while compression ignition is also used in atechnology advancement and the potential benefits of conventional diesel engine, the timing of the ignition isHEDGE, there is interest in this technology for the light- controlled by injecting fuel into already compressed and hot air, leading to a rapid initiation of combustion; withand medium-duty markets, as noted. Early versions of HCCI, air and fuel are premixed).“HEDGE-light” can be found in the third-generation ToyotaPrius, currently on the market. The use of HEDGE technol- Another challenge has to be overcome as well: achievingogy is likely to expand during the next few years. spontaneous (or compression) ignition requires higher tem- peratures at the end of compression, which can lead to aEmissions of Potential Concern Conventional HEDGE main combustion event that occurs too fast. The high pres-technology is presumed to operate under stoichiometric sure and temperature can result in engine damage or accel- erated wear because they exceed the engine’s mechanical ca-conditions using the conventional, durable three-way cat- pacity. With HCCI and its more recent rendition — partiallyalyst that is used almost universally with gasoline engines. premixed compression ignition — the air and fuel are usu-Therefore, regulated emissions are expected to meet recent ally partially premixed (and stratified); this helps to con-California Air Research Board LEV III standards or better. trol both the ignition event and the burn rate. There is alsoParticulate and other nonregulated emissions are not ex- a problem of deposits in the combustion chamber of HCCIpected to be significantly different from those of gasoline engines. Further, this novel combustion process cannot yetengines available today. Two studies (Mohr et al. 2003; be employed at high or very low engine power levels, so itAlger et al. 2010) suggest that particle number emissions needs to be combined with standard spark-ignited enginemay be an issue relative to more stringent standards under operation at these higher and lower engine load conditions,discussion in the European Union (EU). Thus, particulate thus reducing some of its benefits. Some of the difficultiesfiltration may be necessary for future gasoline engines, noted above with the HCCI approach are related to thewhether they employ a HEDGE approach or more conven- chemical composition and combustion properties of currenttional gasoline architecture. There are also some remaining fuels; it is possible that the most appropriate fuel for HCCIconcerns regarding cold-start emissions and combustion engines will be different from today’s gasoline and dieselstability (emissions). The emissions profile from highly fuels, possibly raising emissions and regulatory issues.diluted, stoichiometric engines, however, is expected to bedifferent in that such engines are not likely to produce very Likelihood of Use and Time Frame HCCI is currently inmuch NOx and PM, although they may produce increased the development stage with continuing research, prototypelevels of hydrocarbons and aldehydes. engines, and a limited number of concept vehicles. The6
  14. 14. HEI Communication 16applications of HCCI have been explored for both gasoline injection, or even preinjection, of a small fraction of theand diesel engines, although the interest is greater for diesel total fuel is being used to prime the combustion of the main(see section on low-temperature diesel combustion below). fuel injection pulse so that the main combustion processThe current prognosis of this engine technology is that unless occurs faster, is thus more robust, and can be started laterthe problems and tradeoffs outlined above are resolved, im- (after the piston has reached its top center position). Thus,provements in mainstream spark-ignition engine efficiency parts of the overall combustion occur under lower temper-and emissions control will render the benefits marginal. ature conditions and with a better mixed fuel. Such ap- proaches are continuing to be examined and developed,Emissions of Potential Concern NOx emissions with HCCI and are starting to be used in production engines.combustion are substantially lower than in standard gaso-line engines. Still, the spontaneous ignition of very lean or Likelihood of Use and Time Frame It seems unlikely thatdilute mixtures is not as complete as the traditional spark- fully implemented low-temperature diesel combustion tech-ignited flame propagation combustion process, so the emis- nology will be commercialized within the next decade. Insions of hydrocarbons, aldehydes and CO are higher. It is the shorter term, partial use of low-temperature dieselanticipated that catalysts in the exhaust system will be combustion will provide, at the very least, an opportunityneeded to meet the levels being set for future regulations for modest reduction of the cost of the total engine com-for emissions levels. With lean-engine operation, and low bustion system.exhaust gas temperatures, the effectiveness of the NOxcatalyst is significantly lower than that of current three- Emissions of Potential Concern The lower chamber tem-way catalyst performance with stoichiometric mixtures. perature reduces exhaust temperature and catalyst effi-While PM emissions from homogeneous HCCI engines are ciency, resulting in higher hydrocarbon, aldehyde and COnot likely to be a concern, it is not known whether PM emissions; PM emissions are generally quite low. In theemissions may be produced from stratified versions. short term, low-temperature diesel combustion will be used only in a narrow engine operating window, where DPFs andLife-Cycle Issues None. NOx-reduction systems are likely to be used; the impact on levels of vehicle emissions is likely to be modest underSpecific Regulatory Issues None. such conditions. However, there are concerns about cold starts and, when the low-temperature diesel combustion isLow Temperature Diesel Combustion used over a wider operating range, further characterization of emissions will be necessary.Description of the Technology The problem of emis-sions of NOx and PM in diesel engines arises from the Life-Cycle Issues None.engine’s diffusion-flame combustion process. High fuel–airmixture temperatures at the end of compression are re- Specific Regulatory Issues None.quired to achieve rapid, spontaneous ignition. These high-compression temperatures result in high burned-gas tem- EXHAUST AFTERTREATMENTperatures within the stoichiometric diffusion flame thatforms around each individual diesel fuel spray, resulting Technology for the control of emissions from conven-in high rates of nitric oxide (NO) formation. The very rich tional port-fuel injection gasoline engines is mature and hasmixture within each diesel fuel spray is at high temperature been stable for some time. In combination with clean fuelsand pressure, which results in high rates of soot formation. (low sulfur and no lead additive), and with exhaust-oxygenIf combustion in the diesel engine could occur at lower sensors, catalyst efficiencies of 99.7% for hydrocarbonstemperatures and with better mixed fuel–air mixtures (less and 99.5% for NOx are being achieved using the three-wayrich or even leaner than stoichiometric mixtures), then catalyst converters; such vehicles can meet the very strin-both the rates of NOx and soot formation would be signif- gent super ultra-low emission vehicle standards, as pro-icantly lower. This approach is called low-temperature posed by the California Air Resources Board. As discusseddiesel combustion. above, some of the emerging engine technologies highlight To date, a full realization of low-temperature combustion the need for development of improved catalysts; activein a practical diesel engine has not been achieved. With research is underway to develop such catalysts.multiple fuel injections in each engine cycle — which is The picture for emissions control from diesel-powerednow feasible with piezoelectric fuel injectors and common vehicles has evolved rapidly during the last decade. Therail fuel injection systems — and EGR, the standard diesel major problem in the past has been emissions of highcombustion process is shifting in this direction. An early levels of soot. Soot emissions have been largely addressed 7
  15. 15. The Future of Vehicle Fuels and Technologiesby the use of efficient particulate traps and ultra low-sulfur catalysts are used in passive DPFs to reduce the soot-diesel fuel. The catalysts used on some traps, however, oxidation temperature. The Johnson Matthey trap — fromincrease the levels of nitrogen dioxide (NO2) markedly, a leading manufacturer — or, as it is known, the continu-although total NOx remains essentially the same. Reduc- ously regenerating trap (CRT) (Allansson et al. 2002), posi-tion of NOx emissions has lagged behind control of diesel tions a NO2-generating, precious metal-oxidizing catalyst,soot and several strategies are now being used to control generally platinum, upstream of the uncatalyzed DPF; thisNOx, most notably the selective catalytic reduction (SCR) catalyst converts NO in the exhaust to NO2. NO2 is a strongsystems that are beginning to be introduced in the heavy- oxidizing agent that reacts rapidly with soot at tempera-duty diesel market in the United States, starting in 2010. tures between about 250 and 450 C. In applications whereThe U.S. EPA requires that the DPF-SCR systems be dura- the exhaust temperature is above 250 C a significant frac-ble for 435,000 miles or 10 years or 22,000 hours, with tion of the time, e.g., in over-the-road trucks, the CRT doessevere penalties for noncompliance. As a result, emissions exactly what its name implies. A variant, called the cata-from diesel engines now entering the U.S. market are sub- lytic continuously regenerating trap (CCRT), has a catalyticstantially lower than those from older diesel engines. coating; a rare earth metal such as platinum is added to the DPF itself, which facilitates regeneration and helps over-Diesel Particle Filters come problems with the lower temperature limit. Another There are many types of DPFs but, in the United States, version of a passive system is exemplified by the Engel-the so-called wall-flow filter is by far the most common filter hard DPX; Engelhard is another major manufacturer of fil-for transportation applications. It consists of a honeycomb- ters. This is a catalyzed DPF, which also converts NO tolike ceramic structure with alternate passages blocked (Fino NO2 to oxidize the soot, but in this case the NO2 formation2007; Johnson 2008). Wall-flow filters are extremely effec- takes place within the filter itself rather than in an up-tive at removing diesel PM, with removal efficiencies typi- stream catalyst; an advantage of this system is that it needscally well over 95%. There are a variety of other types of smaller amounts of the precious metal catalyst. Because offilter media; these include ceramic foams, sintered metal, concerns about NO2 emissions, new low-NO2 versionsand those made of wound, knit, or braided fibers. All of of CRTs — such as the advanced CCRT — are now availablethese filters have qualitatively similar emission perfor- that meet the California 2009 requirements for unreactedmances and differ mainly in durability, cost, and packag- NO2 emissions (the so-called NO2 slip) (Johnson Mattheying. Disposable, low-temperature fibrous paper filters that Emission Control Technologies 2009).can only be used with cooled exhaust have been used in Another way to achieve passive regeneration is to use aniche applications such as underground mining and are fuel-borne catalyst. In this case a metallic catalyst, typi-not considered here. cally some combination of cerium, strontium, and iron is High-efficiency particle filters can quickly become loaded added to the fuel. During the combustion process, catalyticwith soot particles, which must be removed to prevent metallic nanoparticles become intimately mixed with theplugging. Removal is done by oxidizing the collected soot soot, making it much more reactive so that oxidation canparticles in place in a process called regeneration. Temper- take place at temperatures below 400 C (Vincent et al.atures of 600 C or more must be reached to oxidize soot in 1999; Vincent and Richards 2000). This technology hasengine exhaust, but such high temperatures are never been around for about 10 years and has seen some level ofreached in normal engine operation. Therefore, other use in retrofit applications outside of the United States.means must be used to achieve regeneration. There are two However, there are concerns about metallic emissionstypes of regeneration processes, passive — where soot is resulting from introduction of metals into fuels, and fuel-oxidized at lower temperatures with the aid of an oxidation borne catalysts are not being pursued in the United States.catalyst — and active — where heat is added to reach ex-haust temperatures sufficient for soot combustion; within Likelihood of Use and Time Frame Use of various ver-each category a number of designs are available. sions of the continuously regenerating traps and catalyticDPFs Using Passive Regeneration DPFs has increased in heavy-duty retrofit applications over the past 10 years. As we will discuss, an active ver-Description of the Technology Passive DPFs are used sion of a catalyzed CRT or DPF is the most widely usedmainly in retrofit applications; essentially all DPFs in- system for heavy-duty diesel truck engines built in thestalled in new trucks use some form of active regeneration. United States since the 2007 model year. Passive regen-However, passive regeneration plays an extremely impor- eration DPFs are generally not suitable for use in light-tant role even in active systems because it is associated duty diesel applications because the exhaust temperaturewith little or no fuel-consumption penalty. Several types of in light-duty diesel vehicles is often lower than 250 C8
  16. 16. HEI Communication 16(however, all light-duty diesel vehicles sold in the United This fuel is oxidized over the catalyst, raising the exhaustStates since 2004 have active DPFs and meet or exceed the temperature sufficiently high to start regeneration.EPA Tier 2 standards). The Johnson Matthey company recently introduced a compact soot filter, an active DPF for European passengerEmissions of Potential Concern The emissions of con- cars (Twigg 2009). In this device, the oxidizing catalyst andcern from the DPFs that use NO2 to oxidize the soot are DPF are integrated and the device is mounted very close toNO2 and nanoparticles. The amount of NO2 formed in the turbocharger outlet. The combination of close couplingthese devices is considerably more than what is consumed to raise the operating temperature of the device and zonedduring soot oxidation and the excess NO2 is emitted from catalyst design eliminates the need for a fuel-borne cata-the tailpipe (Gense et al. 2006). If DPFs are used in a stand- lyst and decreases the fuel consumed during regeneration.alone system without additional de-NOx devices, then the The device may lead to some NO2 and sulfuric acid nano-emissions of NO2 may raise environmental and health con- particle emissions, but emissions from such systems havecerns. The catalysts that convert NO to NO2 are also effec- not been well characterized.tive at converting SO2 from fuel and lube oil combustion In the United States, the main application for DPFs is into sulfuric acid, which then nucleates to form nanoparti- trucks and buses because the light-duty diesel vehicle mar-cles as the exhaust dilutes in the atmosphere (Vaaraslahti ket is very small. Nearly all the vehicles sold in the Unitedet al. 2004; Grose et al. 2006). These systems also store sul- States since the 2007 model year use an active version offates and release them when exhaust temperature is high a catalyzed DPF system. These devices combine passive(Swanson et al. 2009). Such release could lead to emission regeneration with active regeneration whenever the pres-“hot spots” under certain traffic conditions. Sulfur in the sure drop across the DPF (or some other measure of filterfuel also degrades catalyst performance. Therefore, the use loading) indicates excessive soot buildup. These systemsof these systems was limited before ultra-low sulfur diesel also include a diesel oxidation catalyst (DOC), which isfuel became widely available. Fuel consumption penalties positioned upstream of the DPF. The DOC converts NO infrom increased back pressure are small, typically 1% to the exhaust to NO2, which plays a role in active regenera-2% or less. tion. Active regeneration is accomplished by injecting Metallic fuel additives that serve as fuel-borne catalysts are fuels into the exhaust stream; the combustion of fuel raisesused in some European vehicles and for some aftermarket the DOC temperature, leading to combustion of the accu-applications (Richards et al. 2006). The main emissions of mulated PM and soot.concern are metallic nanoparticles that could be releasedin the event of a filter failure. It is difficult to obtain Likelihood of Use and Time Frame Since the 2007 modelapproval from the EPA (U.S. EPA 2009) or the California year, all new diesel engines sold in the United States, bothAir Resources Board to use metallic fuel additives; how- heavy duty and light duty, are equipped with an activeever, other countries remain interested in uses of fuel- DPF. In Europe, active DPF for LDVs started to be used inborne catalysts. The use of metallic fuel additives appears 1999 and are widely used today.to cause only very small fuel consumption penalties. (Seefurther discussion in the section on fuels.) Emissions of Potential Concern The Advanced Collabora- tive Emissions Study (ACES) has been organized by theDPFs Using Active Regeneration Health Effects Institute. ACES has recently completed a detailed study characterizing the emissions under a vari-Description of the Technology All new diesel-powered ety of driving cycles from four 2007-compliant heavy-dutypassenger cars sold in Western countries use some form of diesels (HDDs) equipped with a DOC and DPF (differentactive regeneration (Twigg and Phillips 2009). Since Peugeot designs of devices were used by different manufacturers).introduced the first widely used active DPF system in The levels of criteria pollutants as well as about 300 un-2000, several million diesel passenger cars using this tech- regulated air pollutants in the emissions were measurednology have been sold. The system combines a silicon car- in the study (Coordinating Research Council 2009). Thebide wall-flow filter with an upstream oxidizing catalyst, investigators reported that emissions of PM, carbon mon-active fuel-injection control, and a cerium-based fuel-borne oxide, and nonmethane hydrocarbons were at least 90%catalyst (Salvat et al. 2000). Exhaust temperatures associ- below the EPA 2007 standard; average NOx emissions wereated with passenger car operation are usually too low to 10% below the standard. The engines were fitted withallow passive regeneration so the pressure drop across the either a DOC and a catalyzed DPF or only with a DPF withfilter is monitored; whenever it becomes excessive, addi- a means of active regeneration. The real-time particle-tional fuel is injected into the cylinder late in the cycle. number concentrations (diameter 5.6 to 30 nm) during 9
  17. 17. The Future of Vehicle Fuels and Technologiesregeneration were several orders of magnitude higher than these filters in actual use. The fuel-consumption penaltythe particle-number concentrations without active regen- associated with these systems depends upon the drivingeration; however, the authors point out, the average parti- cycle, with little or no penalty for highway driving wherecle-number emissions were about 90% lower than those regeneration is mainly passive. Low-temperature start-stopemitted with the use of 2004 technology. For the next phase operations lead to frequent regeneration and fuel consump-of the study, currently underway, ACES has launched an tion penalties of up to several percent.animal bioassay to study the health effects of inhalationexposure to emissions from one of the four engines tested Aftermarket and Retrofit Systemsearlier; the results will be available in 2013. Description of the Technology DPFs with passive regen- Even though the levels of pollutants are substantially re- eration like the CRT, CCRT, and catalytic DPF describedduced, the issue of the toxicologic potential of the remain- above are widely used around the world in retrofit applica-ing compounds remains an area of interest. For example, tions. The applications must be chosen carefully to ensurein a recent study, engines fitted with a variety of DPFs were that the operating cycle provides exhaust temperaturestested for the oxidative potential of PM in the exhaust (Bis- above 250 C a significant fraction of the time. Generallywas et al. 2009); oxidative potential is widely considered light-load and start-stop applications are inappropriate.to be an indicator of the potential to cause biological injury. Other types of DPF retrofit systems use off-line regenera-The DPF reduced the oxidative potential by 60% to 98% tion. With these systems the DPF is heated, generally elec-when expressed per vehicle distance traveled; when ex- trically but sometimes with a burner, to cause regeneration.pressed per unit mass, there were substantial differences Regeneration may occur with the device in place or with itamong the various devices. In the aforementioned ACES removed and installed in a separate heating appliance.study, total nitro-polycyclic aromatic hydrocarbon (nitro- There are other types of retrofit systems that use active, on-PAH) emissions from the 2007 engines were lowered by line regeneration systems with burners and sensors.81% when compared with 2004 engines, but nitro-PAH A DOC consists of a flow-through structure similar toemissions for the 16-hour cycle were not zero and were a that used on gasoline engine three-way catalysts, with afactor of two to three times higher than background levels. platinum or platinum-palladium catalyst. Its main advan-Nitrosamines were also detected in the emissions. tages are simplicity and low cost. DOCs oxidize NO, CO, As indicated above, passive regeneration plays an impor- and hydrocarbons, but they do not oxidize PM because thetant role in the operation of active systems, so that the temperature of the exhaust is not high enough. Before DPFsso-called active systems share many characteristics of pas- became available, DOCs were widely used as the primarysive systems. Thus NO2 and sulfate nanoparticles remain emission-control system on diesel passenger cars and lightpotential emissions of concern. The ACES program showed trucks (California Code of Regulations 2007; Toussimis etthat nanoparticle emissions during the 16-hour test cycle al. 2000). DOCs are still a common retrofit technology forwere undetectable except during regeneration. Thus, the heavy-duty trucks and buses (U.S. EPA 1999; Californiapotential for exposure to nanoparticles is greatest under Air Resources Board 2000) and are used extensively inoperating conditions that lead to regeneration. The U.S. underground mines.emission standards for 2010 heavy-duty trucks strongly A variant on the DOC is the so-called partial-flow or openreduce NOx emissions, so even though NO2 may constitute filter (Heikkilä et al. 2009; Mayer et al. 2009). This type ofhalf or more of NOx emissions, the absolute levels are low. device combines elements of DOCs and DPFs by either For DPFs that are used in combination with fuel-borne using convoluted flow passages to direct a portion of thecatalysts that typically contain some combination of cerium, flow over the filtration medium or simply by having a filtra-strontium, and iron compounds, there is also a concern tion medium that has a very open structure. These filtersregarding emissions of metallic ash in case of a failed filter. are usually catalyzed to allow passive regeneration. Reports DPFs are quite effective in reducing the emissions of on the effectiveness of these devices are mixed, rangingPM — as well as a variety of other compounds, except NO2 from little or no PM mass removal efficiency (Mayer et al.— from diesel engines. However, some of the observations 2009) to nearly 80% (Heikkilä et al. 2009). The remainingdiscussed above also underscore the importance of ongo- PM is emitted and is an environmental and health concern.ing research and vigilance in monitoring emissions from Interest in these devices stems mainly because they areDPF devices. In Europe, DPFs used in LDVs have been very less likely than DOCs to become plugged and they reducestable and durable. Although the certification standards the levels of hydrocarbons by oxidation. Some modernfor the performance of DPFs for heavy-duty applications engines may have low enough engine-out PM emissions toare quite stringent in the United States, little information meet current standards with little or no aftertreatment; inis currently available on the durability and stability of such cases, partial-flow devices may be considered. The10
  18. 18. HEI Communication 16problem with such devices is that, like a DOC, they do not Specific Regulatory Issues Both California and the U.S.qualitatively change the nature of PM emissions, so that EPA now require retrofit filters to demonstrate that NO2emissions of concern would be similar to that of a DOC emissions are less than 20% of NOx emissions.except at slightly lower levels. These systems are alsoreported to be subject to particle blowoff under some oper- Selective Catalytic Reduction andating conditions. At present, the devices have limited use Ammonia Slip Catalystfor transportation applications, although there is interest Description of the Technology Because diesel enginesin their use for off-road applications to meet Tier 4 emis- operate under lean conditions, reduction of NOx to nitro-sion standards. gen gas is particularly challenging. Originally developed for stationary sources, selective catalytic reduction (SCR)Likelihood of Use and Time Frame Aftermarket retrofit technology is designed to reduce NOx emissions. A reduc-filter systems are being deployed on a fairly wide scale, tant, typically ammonia (NH3) or urea, is injected into thespurred by special programs and mandates from state and exhaust stream for the chemical conversion (Majewskifederal governments in the United States and Europe. Ret- 2005a,b). Urea serves as an alternative source of NH3rofit applications are available for many heavy-duty vehi- because it thermally decomposes to NH3.cles (HDVs) built during 1994 to pre-2007. For most on-highway motor vehicle applications, zeoliteEmissions of Potential Concern DPF and DOCs used in catalysts, using urea as the reductant, are the system ofretrofit applications have similar emissions to those choice. Zeolites are microporous crystalline aluminosili-described above though the precise composition depends cate minerals, found naturally or produced artificially, andon the catalyst(s) used; however, the emissions from retro- are often referred to as “molecular sieves.” By substitutingfit devices have been even less well characterized. Because other metals for the aluminum and silicon, these struc-DPF systems do not reduce NOx emissions, most diesel tures can be adapted for a wide variety of catalytic pur-engine manufacturers plan to use additional technologies poses. For motor vehicles, most manufactures have settledto control NOx emissions. NO2 emissions, currently not on the use of iron- and copper-exchanged zeolites or adirectly regulated, are a concern for catalyzed DPF sys- combination of the two. A more detailed description oftems, unless measures are taken to control NO2, as in the SCR technology can be found in Majewski (Majewskiadvanced CCRT. Storage and release of sulfates might also 2005a,b). Future developments in SCR technology may usebe a problem under some conditions; for example, under cerium, titanium, or tungsten to promote acidic zirconiahigh-load conditions, sulfate-based nucleation mode nano- SCR catalysts (Rohart 2008).particles can form in the aftertreatment system. It should If NH3 is not fully consumed in the catalytic process, itbe noted that these nanoparticles will not be measureable is emitted in the exhaust, a process referred to as ammoniaunder the EU’s PMP particle-number measurement proto- slip. An ammonia slip catalyst is commonly used to con-col. Finally, some retrofit DPFs contain vanadium pentox- trol such emissions. It is an oxidation catalyst, typicallyide, which is unstable at high temperatures and its vapors with a platinum-based formulation (Majewski 2005b). Thisare a cause for concern (see below). catalyst can lead to the formation of nitrous oxide (N2O), DOCs, used for retrofit and other applications, have a a GHG as well as an ozone-depleting gas (Havenith andmore limited impact on emissions. They lead to an increase Verbeek 1997; Ravishankara et al. 2009; Wuebbles 2009).in nitrogen dioxide emissions (Liu and Woo 2006; Majewski In view of the need to reduce CO2 emissions, another2009). Further, DOCs do not remove solid carbon or ash trend in this area should be noted: to improve efficiency,particles, though organic compounds adsorbed to the PM heavy-duty diesel engines are being calibrated to lower PMare combusted. In addition, unless low-sulfur fuel is used, levels and higher engine-out NOx levels. Therefore, highersulfate or sulfuric acid particles and nanoparticles are also efficiency de-NOx technologies will be required in the future,emissions of concern (Maricq et al. 2002). The DOC device increasing efficiency percentages from the low 90s in 2010is effective for removing CO, hydrocarbons, and organic to the high 90s, later in the decade. To achieve such highcarbon (a part of the PM) from emissions. In addition, efficiency, more urea will have to be injected into the SCR,because DOCs do not remove organic carbon particles which could lower the exhaust temperature. Given the widewhen cold, various schemes have been developed to store operating range of diesel engines, it is conceivable that sec-hydrocarbons during engine warm-up. There is no signifi- ondary emissions of urea-related by-products will increase.cant fuel-consumption penalty. Therefore, this area deserves continuing attention. Several alternatives to the use of urea are also being in-Life-Cycle Issues None. vestigated. One of these alternatives relies on the reduction 11
  19. 19. The Future of Vehicle Fuels and Technologiesof NOx by hydrocarbons (Parvulescu et al. 1998; Liu and Emissions of Potential Concern The introduction of ureaWoo 2006). The catalyst used to accomplish this is similar into the exhaust system represents a large departure fromto urea-SCR except that fuel or reformed fuel (a hydrogen conventional emission-control systems; our understandingand CO mixture) is used as a reducing agent instead of of the chemical reactions within the catalyst and result-urea. So far, hydrocarbon-SCR has not proved successful ing emission products is quite poor at this time. Studiesfor a number of reasons: the temperature window corre- with the urea-SCR system have indicated the presence ofsponding to high conversion efficiency is limited; the fuel nitrogen-containing compounds, including nitro-PAHs. Forconsumption penalty is large; and NOx emissions (Twigg example, various studies have indicated the presence of2007) are a concern. The combination of increased fuel nitrosamines (Farber and Harris 1984), nitromethane, nitro-consumption and the emissions of NOx — a potent GHG — propane, hydrogen cyanide, and increases in NOx (Sluderraises questions about the impact of this technology on cli- et al. 2005), and melamine (Hori and Oguchi 2004). Al-mate change. However the technology may be improving as though more systematic studies are needed before reachingTenneco and GE Transportation have recently announced any final conclusions, it should be noted that nitrogen-plans to jointly develop hydrocarbon-SCR technology for containing organic compounds are often toxic or biologi-diesel locomotives and some nonroad applications (Green cally active, making the presence of these compounds inCar Congress 2009). Additionally, the use of solid metal the emissions an area of potential health concern. How-amine chloride salts used as an NH3 storage system for the ever, there is also a concern that some of the detectedreplacement of aqueous urea is also being considered, since chemicals may be the result of artifacts of sample collec-such compounds have a high specific NH3 storage capacity tion. There is an additional issue in that the exhaust from(Fulks et al. 2009). SCR systems may contain novel nitro-organic compounds and possibly others for which appropriate sampling andLikelihood of Use and Time Frame A federal NOx stan- analytical methods are not available; thus, there is a needdard of 0.20 g/bhp-hr (grams per brake horsepower-hour) to develop analytic methods to detect and characterizehas been phased in for diesel engines between model years such compounds. A comprehensive study on emissions2007 and 2010. For the 2010 model year, most major HDD from 2010-compliant HDD engines is planned as a part ofmanufacturers (e.g., Cummins, Detroit Diesel, Mack, Pac- ACES. These engines will be equipped with DPF andcar, and Volvo) are using the urea-SCR system, thus the use devices designed to remove NOx, and the study will char-of SCR is expected to increase rapidly in the United States. acterize a large number of regulated and unregulated com-However, one manufacturer (Navistar) plans to use an EGR- pounds in the exhaust.or NH3-SCR system to meet the standard (Dixon 2009). A recent study in Europe raised questions about theSCR is currently being used in Europe and Japan for HDD effectiveness of SCR for reducing NOx emissions (Ligternikapplications, and more than half a million diesel trucks in et al. 2009). Seven trucks that met the Euro V emissionsEurope now use urea SCR and the numbers are rapidly standard were driven according to standardized procedures.increasing (Green Car Congress 2008). Further tightening Emissions of NOx were about three times higher than pre-of CO2 regulations will drive engine-efficiency improve- viously estimated for urban driving. Problems with ureaments further, resulting in higher NOx emissions and the injection during stop-and-go driving, where exhaust tem-need for more effective SCR systems. However, because such peratures are generally lower, have been reported (Xu et al.efforts will also lead to lower exhaust temperatures, the 2007; Zhan et al. 2010). At higher speeds, NOx emissionstask of developing better SCR systems will be challenging. were greatly reduced. However, this study’s report did not For LDVs, urea-SCR systems are already in use in Europe provide information about composition of the SCR catalystand the United States (e.g., in Mercedes-Benz, BMW, and employed in the trucks. Furthermore, it should be notedAudi vehicles). The market penetration for these diesel that there are significant differences between the emis-systems in the light-duty fleet may reach levels of 5% to sions standards and compliance requirements in the United15% in the United States (Tim Johnson, Director, Emerging States and those in Europe, with those in the United StatesTechnologies and Regulations, Corning Incorporated, 2009, being more stringent; these differences make it problem-personal communication). In Europe, the light-duty diesel atic to extend the Euro V data to the United States.fleet is expected to decrease, chiefly because of the shrink- There has been concern related to the use of copper-ing differential between the price of gasoline and diesel based zeolite catalysts used in some SCR systems. Copperfuels. Oxidation catalysts are currently being deployed in is an extremely potent catalyst for dioxin formation, andEurope and Japan, and very likely to be used in the United there is the possibility that SCR with copper may lead toStates, to control NH3 slip from SCR systems. A high mar- dioxin emissions (Heeb et al. 2007). A study performed byket penetration for this technology is expected. EPA investigators was presented at a recent Society of12
  20. 20. HEI Communication 16Automotive Engineers (SAE) meeting. Based on the test- Specific Regulatory Issues As mentioned above, SCRing to date of a 2007 heavy-duty diesel Cummins engine emission-control technology has been developed to meetequipped with a retrofitted and mildly aged copper/zeolite the federal 2010 emission requirements for NOx (0.20 g/SCR catalyst only (without urea injection), no statistically bhp-hr), which apply to virtually all on-road heavy-dutysignificant differences were seen between the engine-out diesel engines (e.g., line-haul trucks, buses, refuse trucks).dioxin/furan emissions and the SCR-out dioxin/furan emis- DPF-SCR devices are also being used on most new light-sions. Additional testing has yet to be conducted on the duty diesel vehicles. European heavy-duty diesel emissioncomparison of a complete catalyst system (i.e., a copper/ standards (Euro VI) will become effective between 2013 andzeolite SCR catalyst compared to an iron/zeolite SCR cata- 2014, and approach the 2010 U.S. emission standards; thelyst, with both including an oxidation catalyst, urea injec- European standards will, however, remain less stringent.tion, particulate filter, and NH3 slip catalyst) (Laroo 2010). Given that limited data exist on NH3 emissions fromCopper-iron-zeolite catalysts are desirable because they production-ready heavy-duty diesel vehicles equipped withhave the best performance in reducing NOx over a broad SCR, further study of this issue is warranted. At present,temperature range (Majewski 2005b). there are no federal limits on NH3 emissions for new vehi- Some early SCR catalyst formulations included vanadium cles or retrofit applications. The California Air Resourcespentoxide deposited on titanium dioxide, with tungsten Board has not yet set an NH3 standard for new vehicles,trioxide often added as a cocatalyst or promoter. However, but has put a limit on NH3 emissions from retrofit applica-one study reported vaporization of vanadium pentoxide tions, mandating that “the diesel emission control strategywhen heated to above 580 C; emission of vanadium pen- must not increase the emissions of NH3 to a level greatertoxide is of health concern. However, as noted above, most than 25 ppm by volume at the tailpipe on average over anyautomotive SCR systems rely on zeolite-based formulations, test cycle used to support the emission reduction claims”although some vanadium-based systems may be used for (California Code of Regulations 2007). It appears likelyretrofit applications and off-road applications. Vanadium that the EU will cap NH3 emissions to 10 ppm in the Eurocatalysts are cheaper and more sulfur-tolerant than the VI regulations for heavy-duty diesel vehicles.copper-iron-zeolite variants, but vanadium-based systemsare not stable at higher temperatures (Johnson 2010). NOx Adsorber Catalyst Technology Finally, there is also a concern that truck drivers may Description of the Technology The NOx adsorber catalystcontinue driving after urea in the on-board storage tank (NAC) is also commonly referred to as the lean-NOx trap,has been depleted. This issue remains controversial and is the NOx storage-reduction catalyst, and the NOx storagelikely to be addressed using engine cut-off switches and catalyst. It is predominantly used in diesel engines and is aregulatory enforcement actions. multifunctional, cyclically operated catalyst unit, with In summary, based on available information, it is antici- two major operation phases: trapping and regeneration.pated that the combination of DPF and urea-SCR systems During the trapping phase, the NAC removes NOx from thewill substantially reduce criteria pollutants in emissions exhaust by oxidizing and storing it on the catalyst, in thefrom diesel engines. Although emissions of toxic, unregu- form of a nitrate. This process occurs under excess oxygenlated species are expected to be low or very low, conclu- (lean) conditions typical for diesel exhaust. During thesions regarding their overall risk are difficult to draw at rich regeneration phase, for a few seconds every minute,this point because such exhaust has not been characterized NOx, is released from the catalyst and chemically reducedsystematically or in detail. Although the performance stan- to nitrogen by excess reductant, typically by injectingdards for diesel aftertreatment devices are very stringent in diesel fuel. The key catalytic functions inherent to a NOxthe United States, little information is available at this adsorber are oxidation, trapping, reductant transformation,time concerning the long-term performance and stability and reduction. To satisfy these requirements, a NAC mustof DPF-SCR devices. All these issues deserve further study. contain a capable reduction-oxidation component such as platinum and rhodium (as in a three-way catalyst) and aLife-Cycle Issues There are no known significant life- storage component with basic properties, such as alkali orcycle issues related to the deployment of urea-SCR tech- alkaline-earth metal compounds. The storage componentnology. Urea is used for many purposes in the chemical also traps sulfur oxide (SOx) species originating from fuelindustry. Some large-scale urea spillages have occurred, and lubricating oil, leading to a progressive decrease in theresulting in wildlife deaths. Accidental localized spills NOx-trapping capacity. The NAC is best used with veryassociated with SCR-related urea distribution and infra- low-sulfur fuel. Removal of SOx is accomplished under high-structure may occur, but are not expected to pose any sig- temperature, rich conditions (Epling et al. 2004), which cannificant health or environmental concerns. also damage the catalyst. NAC technology is used as a part of 13