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FOR LIBRARY USE ONLY NOX Emission Controls for Heavy-Duty Vehicles: Toward Meeting a 1986 Standard Final Report of the Motor Vehicle Nitrogen Oxides Standard Committee Assembly of Engineering National Research Council NATIONAL ACADEMY PRESS NAS'NAE Washington, D.C. 1981 DfA-,1981 LIBRARY
e, / NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the Committee responsible for the report were chosen for their special competences and with regard to appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Research Council was established by the National Academy of Sciences in l9l6 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and of advising the federal government. The Council operates in accordance with general policies determined by the Academy under the authority of its congressional charter of l863, which establishes the Academy as a private, nonprofit, self-governing membership corporation. The Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in the conduct of their services to the government, the public, and the scientific and engineering communities. It is admini- stered jointly by both Academies and the Institute of Medicine. The National Academy of Engineering and the Institute of Medicine were established in l964 and l970, respectively, under the charter of the National Academy of Sciences. The study culminating with this report was performed under contract number 68-0l-6l88 between the U.S. Environmental Protection Agency and the National Academy of Sciences. Library of Congress Catalog Number 8l-85575 International Standard Book Number 0-309-03226-1 Available from NATIONAL ACADEMY PRESS 2l0l Constitution Avenue, N.W. Washington, D.C. 204l8 Printed in the United States of America
PREFACE In late l980 the U.S. Environmental Protection Agency (EPA), as part of its rulemaking in establishing emission standards for heavy-duty vehicles called for in the Clean Mr Act as amended l977 (42 USC 740l et seq.) requested that the National Research Council study the technological feasibility of meeting a more stringent standard for nitrogen oxides (NOX) emissions by l986. The Assembly of Engineering established the Vehicle Nitrogen Oxides Standard Committee to carry out the requested study. The committee investigated NOX emission control technologies and the impacts of those technologies on heavy-duty engines, cost, efficiency, performance, and emissions other than NOX. In pursuing its charge, the committee solicited information from interested parties in the United States, Europe, and Japan. Committee members and staff visited the EPA Emissions Control Laboratory in Ann Arbor, Michigan, and the headquarters of the General Motors Corporation and the Ford Motor Company in Detroit. Several domestic and foreign engine manufacturers made presentations to the committee at its January l98l meeting in Chicago. (Appendix A summarizes the committee's contacts with outside sources of information.) In its search for data, the committee mailed a questionnaire (Appendix B) to domestic and foreign engine and parts manufacturers. In addition, contacts were made with foreign manufacturers and other companies at the February l98l Meeting of the Society of Automotive Engineers. EPA, the U.S. Departments of Energy and Transportation, the California Air Resources Board, and government and private research laboratories provided additional data. The policy background of the study underwent continual change. EPA had previously determined that a l.7-gram per brake horsepower hour duty NOX emissions would be required by the Clean Air Act. The agency was expected to publish a notice of proposed rulemaking on this issue in late l980. Instead, it published an advance notice of proposed rulemaking, in the January l9, l98l, Federal Register, in which it stated the opinion that diesel engines could not attain the l.7-g/bhp-h standard. The hearing dates specified in the advance notice were postponed from mid-summer to late fall. iii
The installation of a new administration in the White House and a change of leadership in EPA provided further changes in the public policy arena. In April l98l the White House distributed a document outlining President Regan's proposed "Actions to Help the U.S. Auto Industry." On April l3, l98l, EPA published in the Federal Register its plan for implementing the President's program. In that plan EPA announced several changes affecting emission regulations for heavy-duty vehicles, including its intent to promulgate an NOX standard based on the NOX emission level that can be achieved by diesel engines. The target date for a notice of proposed implementation is June l982. While EPA and the Executive Branch are instituting changes in regulatory policy on emission control, Congress is holding hearings (as of mid-l98l) on the Clean Air Act and its l977 amendments. Emission standards for heavy-duty vehicles are one of the issues that will be discussed in these hearings. Some of the regulatory changes proposed by EPA have to do with engine certification test procedures, which have a bearing on our study. We have noted these where it is appropriate. Our proposed changes in policy do not affect the committee's findings, because its main focus was on the available technology and the impacts of this technology's use. The committee held its first meeting on December 5, l980, and completed its draft report by the first week of May l98l. In order to carry out its full charge with the time and resources available, the committee devoted most of its time to its main charge of technology assessment. The committee noted the previous work of the NRC's forthcoming light-duty diesel study, which made a detailed examination of health and environmental impacts on the dollar value of the benefits of controls. Consequently, our committee did not study these topics in detail. We do, however, discuss them in the context of heavy-duty engine regulations. Thus, the committee completed the tasks listed in the contract with EPA by focusing mainly on an appraisal of control technology, the area of expertise of most committee members. We sought to provide an independent analysis of the scientific, technical and cost issues raised by NOX emission controls for heavy-duty vehicles of model year l986. The committee noted several policy questions that arose from the technical findings in this study; these questions should be considered in a rulemaking procedure, but in the committee's judgment their answers require policy decisions beyond the scope of the committee's charge. One committee member disagreed with this judgment and has attached a statement of his views as Appendix E. iv
ACKNOWLEDGMENTS This report was prepared by the Motor Vehicle Nitrogen Oxides Standard Com- mittee under the chairmanship of Laurence S. Caretto. The committee itself is solely responsible for the report's findings and conclusions, but a number of others made important contributions to the committee's deliberations. The committee was fortunate in having the able counsel of two advisors, William J. Lux, Manager, Product Engineering Center, John Deere Dubuque Works, and Russell P. Sherwin, Professor of Pathology, School of Medicine, Department of Pathology, University of Southern California. The committee extends its thanks to those employees of the heavy-duty vehicle manufacturing industry who served as liaison to the committee or discussed the issues with committee members. They provided briefings, data, and materials that helped the committee understand the research and produ- ction issues confronting the industry. Thanks are also given to the personnel of the Environmental Protection Agency who supplied the committee with materials and data. Their generous co- operation helped the committee to understand the difficulties and complexities of implementing the relevant sections of the Clean Air Act as amended l977 (42 USC 740l et seq.). The committee also received assistance from other federal agencies, most notably the Departments of Transportation and Energy. The final preparation of the report was guided by comments from anonymous reviewers designated by the Assembly of Engineering, under the direction of its Executive Director, David C. Hazen. The committee is indebted to Duncan M. Brown, who served as editor to the committee. The director of the study was Dennis F. Miller. Special recognition is due Vivian Scott, administrative assistant to the committee, and to Juliet W. Shiflet and Julia W. Torrence, secretaries, who served the committee with dedication.
MOTOR VEHICLE NITROGEN OXIDES STANDARD COMMITTEE LAURENCE S. CARETTO, (Chairman), Professor, Department of Mechanical and Chemical Engineering, California State University, Northridge ROY E. ALBERT, Professor, Institute of Environmental Medicine, New York University Medical Center GEORGE R. HEATON, Jr., Research Associate and Program Manager, Center for Policy Alternatives, Massachusetts Institute of Technology JOHN B. HEYWOOD, Professor, Department of Mechanical Engineering, Massachusetts Institute of Technology CHARLES H. KRUGER, Jr., Professor, Department of Mechanic? Engineering, Stanford University EDWIN S. MILLS, Professor, Department of Economics, Princeton University PHILLIP S. MYERS, Professor, Mechanical Engineering University of Wisconsin-Madison HENRY K. NEWHALL, Manager, Fuels Division, Chevron Research Company DENNIS F. MILLER, Study Director VIVIAN SCOTT, Administrative Assistant JULIET SHIFLET, Secretary JULIA TORRENCE, Secretary vi
CONTENTS EXECUTIVE SUMMARY x l INTRODUCTION l THE HEAVY-DUTY FLEET AND ITS USE PATTERNS 2 HEAVY-DUTY EMISSIONS REGULATIONS 3 Test Procedures 3 Proposed Emissions Standards 8 UNCONTROLLED ENGINE EMISSIONS AND CURRENT STANDARDS 8 CONTRIBUTION OF HEAVY-DUTY ENGINES TO TOTAL EMISSIONS l4 REGULATORY OPTIONS UNDER THE CLEAN AIR ACT l8 REFERENCES 20 2 HEAVY-DUTY DIESEL ENGINES DESCRIPTION OF THE PROBLEM 2l Diesel Combustion and Emissions 2l Diesel Engine Trade-offs 23 CONTROL TECHNIQUES 24 Retarding Injection Timing 24 Varying the Shape of the Rate-of-Injection Curve 24 Turbocharging and Charge Cooling 26 Modifying Engine Designs 26 Exhaust Gas Recirculation (EGR) 26 Catalytic Controls 26 Water Injection 27 Turbocompounding 27 Insulating Engines Thermally 27 Trapping Particulates 27 Using Alternative Fuels 28 Use of Alternative Engines 28 l986 AVAILABILITY OF CONTROL TECHNIQUES 28 The System Problem 28 AVAILABLE DATA 29 ANALYSIS OF DIESEL NOX EMISSION CONTROL DATA 33 SUMMARY 44 REFERENCES 46 vii
3 HEAVY-DUTY GASOLINE ENGINES 47 INTRODUCTION 47 Control Techniques 47 EFFECTS OF ENGINE MODIFICATIONS ON .EMISSIONS AND FUEL ECONOMY 48 Air/Fuel Ratio 48 Spark Timing 48 Exhaust Gas Recirculation 48 Valve Timing 50 Combustion Chamber Redesign 50 Compression Ratio Changes 50 Fuel System Modifications 50 PERFORMANCE OF CURRENT HEAVY-DUTY GASOLINE ENGINES 50 Examples of Current-Production Low-NOx Engines 5l EMISSIONS DATA FOR PROTOTYPE l986 CONTROL SYSTEMS 57 Noncatalytic NOx Controls 57 Catalytic Control of N^ Emissions 60 New Engine, Fresh Catalyst Emissions Data 63 Catalyst Durability 66 Overall Assessment of Catalytic NOx Control 70 CONCLUSIONS 70 REFERENCES 72 4 ENVIRONMENTAL HEALTH EFFECTS 73 BIOMEDICAL EFFECTS OF NOx 73 HEALTH ASPECTS OF POLLUTANT TRADE-OFFS 75 SUMMARY AND CONCLUSIONS 76 REFERENCES 77 5 CONTROL COSTS AND OTHER REGULATORY QUESTIONS 79 CONTROL COSTS 79 INDIRECT COSTS OF NOX CONTROL 8l REGULATORY ISSUES 85 Differences Between Gasoline and Diesel Engines 85 Vehicle Size Considerations 86 Emissions Averaging 87 Test Procedures 88 Regulations and Technological Feasibility 89 TECHNOLOGY BEYOND l986 90 Improved Fuel Controls 9l Catalyst Technology 9l Exhaust Gas Recirculation 9l Particulate Traps 92 Alternative Fuels and Engines 92 CONCLUSIONS 94 REFERENCES 95 viii
APPENDIX A: SUMMARY OF COMMITTEE CONTACTS 97 APPENDIX B: LETTER AND QUESTIONNAIRE 99 APPENDIX C: CERTIFICATION TEST CYCLES FOR HEAVY-DUTY ENGINES l04 APPENDIX D: GLOSSARY OF TECHNICAL TERMS ll0 APPENDIX E: STATEMENT OF GEORGE R. HEATON, JR ll2 ix
EXECUTIVE SUMMARY Heavy-duty vehicles are extremely varied in size, operating conditions, and vehicle type; the category includes trucks, buses, large vans, and recreational vehicles. Gasoline engines, with relatively high fuel costs and low capital costs, dominate the lighter end of the category, powering most vehicles with gross vehicle weight ratings between 8,500 and 26,000 pounds. Diesel engines dominate the heavier end, with gross vehicle weight ratings exceeding 26,000 pounds. It has been estimated that heavy-duty vehicles contribute about l0 percent of the nation's total emissions of nitrogen oxides (NOX). In the California South Coast Air Basin, their contribution to total NOX emissions is estimated to be about l8 percent. Both the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board predict that, because of reductions in NOX emissions from other sources, the relative importance of NOX emissions from heavy-duty vehicles will increase over the next decade. The Clean Air Act as amended l977 (42 USC 740l et seq.) required EPA to promulgate an emission standard for NOX, effective in the l986 model year, that represents a 75-percent reduction in emissions from a baseline cor- responding to the uncontrolled NOX emissions of heavy-duty gasoline engines. Compliance will be determined by using the new heavy-duty "transient" test procedure, which replaces the old, "steady-state" procedure. EPA has determined that the baseline NOX emission level of uncontrolled heavy-duty gasoline engines, measured by the transient test procedure, is 6.8 grams per brake horsepower-hour (g/bhp-h). Thus the statutory 75-percent reduction in emissions would result in a standard of l.7 g/bhp-h. Heavy-duty diesel engines without emission controls have NOX emission levels one and one-half to three times as high as the uncontrolled gasoline engine baseline. The percentage reduction required for diesel engines to meet the same standard as gasoline engines is therefore greater than 75 per- cent, at between 83 and 92 percent. Because of EPA's uncertainty about whether the l.7-g/bhp-h standard can be met by production engines in l986, the agency has recently suggested, for l986, an interim standard of about 4 g/bhp-h.
EPA, in addition to proposing a new transient test cycle to replace existing steady-state test cycles, has proposed that the emission standards apply to engines until the ends of their useful lives (rather than until the end of a specified period of operation), and has introduced a new requirement that a specified fraction of production engines must meet the standard in an audit of production line engines. New engines, on the average, will need emission levels lower than that set by the numerical standard in order to comply with these requirements. Major Findings I. General Finding Data from heavy-duty vehicle engines with NC^ emission levels signifi- cantly below those of today's engines are limited. In addition, only a small fraction of these data were gathered using the transient test proce- dure. Consequently, there is major uncertainty about the level of NOX emission standard that can be achieved in the mid-l980s, and about the effects that a given standard will have on engine emissions of hydrocarbons, carbon monoxide, and particulates; fuel consumption; performance; and durability. II. Findings on Diesel Engines o Current 49-state production diesel engines have already achieved NOX reductions of 20 to 60 percent from the uncontrolled diesel NOX emission level; diesel engines sold in California have achieved 50- to 75-percent reductions in NOX emissions relative to the uncontrolled diesel levels. The additional control techniques available to meet the l986 NOX standard for heavy-duty diesels are modifications in fuel injection timing and fuel injection pressure, cooling of the intake air charge, and possibly exhaust gas recirculation (EGR). Electronic systems to control timing are expected to be on the market by l986, but whether they will have been available long enough to be incorporated widely in production engines is uncertain, as is the extent of their advantages in emissions control. Charge cooling using ambient air will be available, since it requires mainly straightforward hardware changes. The outlook for exhaust gas recirculation is not clear; it may be possible for some engine manufacturers to recirculate small amounts of exhaust gases in their engines without significant losses of durability and drive- ability, especially if electronic control systems are available. It appears unlikely that substantial amounts of exhaust gas can be recycled satisfactorily in all types of heavy-duty diesel engines. xl
With currently available emission control systems and those under development, lowering NOX emissions raises hydrocarbon and particulate emissions, increases fuel consumption, and decreases engine performance. (Engine-out particulate emissions might be controlled by particulate traps, but the availability of such devices in l986 is uncertain.) The trade-offs between NOx emissions and particulate and hydrocarbon emissions, and between NO^ emissions and fuel consumption for relatively new engines, have been estimated by the committee as follows : Engine-Out Emissions (g/bhpâh) Fuel Consumption NOX Particulates Hydrocarbons Penalty, percent 8 0.4-0.5 0.6-0.8 0 6 0.5-0.7 0.7-l.4 2.5-4 4 0.6-l.0 0.8-l.7 7-l2 These values are based on the new transient test cycle. For each level of emissions control, the fuel consumption penalty is defined as the increase in fuel consumption relative to the fuel consump- tion that an engine with the same technology would have at an NOX emission level of 8 g/bhp-h. o The NOX standards corresponding to the above low-mileage emission levels would be higher than the low-mileage levels, after allowance is made for deterioration in emission control over the engine's useful life, and for production line variability. o There are no data that show the technical feasibility of the heavy- duty diesel engine's meeting an NOX emission standard of l.7 g/bhp-h. III. Findings on Gasoline Engines o Control techniques available to meet the l986 heavy-duty engine NOX standard are based on engine modifications and the use of catalytic converters for exhaust treatment. Engine modifications include changes in air-fuel ratio, spark timing, and valve timing; redesign of combustion chambers, intake manifolds, and fuel metering systems; and use of EGR. Three-way catalyst systems, often with feedback- controlled fuel metering, can provide additional NOX reductions in the exhaust system. xii
o Without the use of catalysts in the exhaust system to reduce emis- sions of NOX, â NOX emission levels of about 5 g/bhp-h can be achieved with little or no effect on fuel consumption. â NOX levels of about 3 g/bhp-h can be achieved with a 3- to 7-percent increase in fuel consumption. â Attempts to engineer emission controls to achieve NOX levels lower than about 3 g/bhp-h without catalysts will increase fuel consumption and degrade engine performance substantially. o With three-way catalytic systems using fresh catalysts, NOX emission levels below the l.7-g/bhp-h standard mandated by the Clean Air Act as amended l977 (42 USC 740l et seq.) have been achieved by relatively new engines, with 4- to 7-percent increases in fuel consumption. No data on the durability of catalytic NOx control systems on heavy- duty gasoline engines are available. However, data on heavy-duty engine exhaust temperatures suggest the potential for significant deterioration of catalysts. Catalytic NOX control systems cannot be considered practical until adequate durability data be come ava ilable. IV. Other Findings o From a public health standpoint, it might be imprudent to suppress the emissions of NOX from heavy-duty vehicles at the cost of a substantial increase in the emission of particulates. The extent to which these two species affect human health requires further study, especially to assess the carcinogenic risk of diesel particulates. o For an increase in fuel consumption greater than about 2 percent, the most significant cost of further NOX control will be the cost of the increased fuel consumption. For larger fuel penalties, the increased fuel cost will be substantially greater than the initial cost of the control system. o This report has addressed technology available for use in l986. Beyond that time technology that is now uncertain can be further developed, and new technologies may emerge. These technological improvements will be influenced by regulatory requirements. xiii