4
ENVIRONMENTAL ISSUES

In 1990 Congress enacted and the President signed the Clean Air Act amendments imposing, among other elements, new federal regulations on automotive emissions (P.L. 101-549). The law is intended to reduce ground-level ozone and carbon monoxide (CO) in areas in the United States that do not meet ambient air quality standards. As will be discussed, this statute and its associated regulations will limit potential fuel economy improvements by adding weight to vehicles and impeding the use of existing and emerging technologies for improving fuel economy. This chapter examines briefly the nature of automotive emissions, trends in automotive emissions control and air quality, current and prospective clean air standards, the need to reduce emissions further, and the direct and indirect effects of emissions controls on fuel economy.

THE NATURE OF AUTOMOTIVE EMISSIONS

Air pollution from motor vehicles arises from evaporative emissions from the fueling systems of volatile organic compounds (VOCs) that include hydrocarbons (HCs) and from postcombustion chemical compounds that leave the engine through the exhaust (tailpipe) system and the crankcase. In engines using unleaded gasoline, the compounds in the exhaust are typically HCs, CO, and oxides of nitrogen (NOx); in diesel engines, they include particulates that are related to smoke; and in engines using alternative fuels such as methanol, they include such VOC compounds as formaldehyde. Exhaust emissions are a function of engine operation—for example, compression ratio, spark timing, air/fuel ratio (A/F), and postengine treatment for purposes of control. Hydrocarbons in the exhaust are incompletely burned or unburned fuel and oil. Carbon monoxide is formed in the combustion process and is always present in small quantities in the exhaust regardless of the air/fuel ratio. The greater proportion of fuel there is in the air/fuel mixture, the more CO is produced. Oxides of nitrogen are formed during the combustion process, increase with peak combustion temperature, and are also a function of the air/fuel ratio.



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Automotive Fuel Economy: How Far Should We Go? 4 ENVIRONMENTAL ISSUES In 1990 Congress enacted and the President signed the Clean Air Act amendments imposing, among other elements, new federal regulations on automotive emissions (P.L. 101-549). The law is intended to reduce ground-level ozone and carbon monoxide (CO) in areas in the United States that do not meet ambient air quality standards. As will be discussed, this statute and its associated regulations will limit potential fuel economy improvements by adding weight to vehicles and impeding the use of existing and emerging technologies for improving fuel economy. This chapter examines briefly the nature of automotive emissions, trends in automotive emissions control and air quality, current and prospective clean air standards, the need to reduce emissions further, and the direct and indirect effects of emissions controls on fuel economy. THE NATURE OF AUTOMOTIVE EMISSIONS Air pollution from motor vehicles arises from evaporative emissions from the fueling systems of volatile organic compounds (VOCs) that include hydrocarbons (HCs) and from postcombustion chemical compounds that leave the engine through the exhaust (tailpipe) system and the crankcase. In engines using unleaded gasoline, the compounds in the exhaust are typically HCs, CO, and oxides of nitrogen (NOx); in diesel engines, they include particulates that are related to smoke; and in engines using alternative fuels such as methanol, they include such VOC compounds as formaldehyde. Exhaust emissions are a function of engine operation—for example, compression ratio, spark timing, air/fuel ratio (A/F), and postengine treatment for purposes of control. Hydrocarbons in the exhaust are incompletely burned or unburned fuel and oil. Carbon monoxide is formed in the combustion process and is always present in small quantities in the exhaust regardless of the air/fuel ratio. The greater proportion of fuel there is in the air/fuel mixture, the more CO is produced. Oxides of nitrogen are formed during the combustion process, increase with peak combustion temperature, and are also a function of the air/fuel ratio.

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Automotive Fuel Economy: How Far Should We Go? Automotive pollutants, directly and indirectly, have adverse health effects and their discharge into the atmosphere has been subject to regulatory control for over two decades.1 Exhaust emissions can be limited by a variety of means. Exhaust gases that escape past the piston rings into the crankcase are drawn back into the engine using a positive crankcase ventilation system, and the unburned HCs are combusted. Emissions released through the exhaust pipe are controlled in virtually all vehicles today by three-way catalytic converters in the exhaust system and by electronic controls on gasoline-powered engines. The introduction in 1978 of the three-way catalyst marked a major stride in emissions control technology because it enabled the limitation of NOx, HC, and CO emissions to levels in compliance with current standards.2 Hydrocarbon emissions from the fuel system that occur while the car is in operation or while parked are controlled using a carbon canister that adsorbs the vapors. Such controls have been in use since 1975. Hydrocarbon losses during refueling, when vapor is displaced from the fuel tank by the entering liquid, can be controlled either by returning vapor from the vehicle to the service station tank (referred to as Stage 2 control) or by using a larger carbon canister on the vehicle that traps the fuel vapors. Stage 2 controls on fuel pumps in service stations are already used in some jurisdictions (e.g., California and Washington, D.C.); on-board control is contemplated, but it is currently not required. Carbon dioxide (CO2), a greenhouse gas, is a common and necessary by-product of any engine that burns carbon-based fuels, but its potential effect on global climate is a cause for increasing international concern.3 Recent studies have addressed the issue of possible global warming from projected increases in the concentrations of greenhouse gases—CO2, methane, nitrous oxide (N2O), and chlorofluorocarbons (CFCs)—in the atmosphere (National Academy of Sciences, 1991; OTA, 1991a).4 The 1   Carbon monoxide displaces oxygen from hemoglobin in the blood stream and is considered a causative or contributing factor in a number of health problems. Photochemical oxidants (smog) that include ground-level ozone are formed in the atmosphere in the photochemical reaction of VOCs and NOx emitted from the tailpipes of vehicles. Oxides of nitrogen are considered hazardous, and ozone is a debilitating irritant, especially of the respiratory system. It has been estimated that several hundred million incidents per year of respiratory symptoms might be avoided if all areas of the United States were in compliance with federal ground-level ozone standards, at a savings of $0.5 to $4 billion annually (Office of Technology Assessment [OTA], 1989). Animal toxicologic and human epidemiologic studies suggest that ozone plays a role in the initiation of respiratory disease processes. In addition, short-term health effects are associated with ozone exposure, including impairment of lung function. Many health professionals believe that lifetime exposure to high levels of ozone may result in premature aging of the lungs, but no reliable estimate of the effect on life expectancy is available. 2   In this system, platinum and rhodium are used in the front part of the converter to reduce NOx, and palladium and platinum are used in the rear part to oxidize HCs and CO. It is called a three-way catalyst because it controls HC, CO, and NOx. 3   The United States is the world's largest contributor of CO2 emissions from anthropogenic sources—one-quarter of the world's total (National Academy of Sciences, 1991). 4   The effects of N2O are distinct from the effects of NOx, which by common usage refers to nitric oxide (NO) and nitrogen dioxide (NO2). Nitrous oxide is a greenhouse gas; NOx is active in the photochemical reaction that produces ozone. Greenhouse gas emissions from human activities represent the following percentages of total

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Automotive Fuel Economy: How Far Should We Go? U.S. fleet of light trucks and automobiles accounts for about one-fifth of the total U.S. CO2 emissions, and any improvements in fuel economy, assuming an equal number of miles traveled, would lead to concomitant reductions in CO2 emissions (National Research Council, 1990; OTA, 1991). If, for example, the fuel consumption of the vehicle fleet decreased by 10 percent, total U.S. emissions of CO2 would decline by about 2 percent.5 Chlorofluorocarbons are used as the working fluid in automotive air-conditioning systems. Approximately 0.6 million metric tons of CFCs are released each year. They are potent greenhouse gases and will eventually be eliminated. General Motors is planning the complete removal of CFCs from its new-car fleet by 1995, and other manufacturers are expected to adopt similar schedules.6 AUTOMOTIVE EMISSIONS CONTROL AND AIR QUALITY: A BRIEF HISTORY Concern with air quality, including ground-level ozone, increased during the 1960s and led to federal and state motor vehicle standards for emissions of HCs, CO, and NOx. Enabling legislation was passed in California in 1965 and at the federal level in the Air Quality Act of 1967 and the Clean Air Act amendments of 1970, 1971, and 1977 (P.L. 91-604, P.L. 93-319, P.L. 95-95; Johnson, 1988; Rau and Wooten, 1980; Shiller, 1990). The federal legislation included authorization for the U.S. Environmental Protection Agency (EPA) to regulate mobile and stationary sources of pollution, establish National Ambient Air Quality Standards (NAAQS), require states to submit implementation plans to meet standards, and issue compliance orders or bring suits against offenders. By 1988, emissions per vehicle mile traveled (VMT) for new cars and light trucks had been decreased by roughly 90 percent from the uncontrolled level. Total VMT, however, increased 2.3 percent annually during the 1970s and 1980s, thereby offsetting some of the improvement. Consequently, vehicles remain a significant source of pollution in most major urban areas. In 1987, cars and light trucks accounted for 30 percent of the U.S. emissions of VOCs and NOx and 60 percent of CO emissions (Atkinson et al., 1990). Today, the ambient standard for CO emissions is being exceeded in about 60 urban areas, and approximately 115 million Americans live in 98     emissions on a CO2-equivalent basis: CO2, 66.3; methane, 20.4; CFCs, 9.7; and N20, 3.6 (National Academy of Sciences, 1991). 5   Such a reduction would not necessarily affect automobile emissions of HC, CO, and NOx because the regulations are expressed in grams per mile driven and are not related to the amount of fuel consumed. Increased fuel economy could nonetheless reduce total HC emissions because, if less gasoline is needed, the systemwide emissions of HC would be reduced (DeLuchi et al., 1991). 6   Limitations on the most effective refrigerants, CFCs, have already been endorsed for objectives unrelated to global warming, namely, protection of stratospheric ozone levels. Action to accelerate the scheduled phase-out of the CFCs is now under consideration by the Congress (New York Times, February 7, 1992).

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Automotive Fuel Economy: How Far Should We Go? areas (in 1990) that do not meet federal ozone standards (ozone nonattainment areas) (Atkinson et al., 1990; National Research Council, 1991). Further, emissions of VOCs are expected to increase in the 1990s if additional controls are not imposed. The problem is exacerbated by deterioration and modification (i.e., tampering) of vehicle emissions control devices during use.7 The percentage of older cars on the road is also increasing, which reduces the rate of reduction in automotive emissions. The proportion of the car fleet that is 12 years old or older rose from 11.9 percent in 1980 to 20.8 percent in 1990, and the proportion in the light-truck fleet grew from 18.4 to 27.6 percent (Motor Vehicle Manufacturers Association, 1991). Older vehicles account for a disproportionate share of emissions, even though they are driven fewer miles per year than new cars. For example, although pre-1981 vintage vehicles accounted for only about 35 percent of VMT in 1988, they accounted for about 70 percent of HC emissions, 75 percent of CO2 emissions, and 68 percent of NOx emissions.8 Congressional and public support for efforts to improve air quality has been strong and led to passage of the Clean Air Act amendments of 1990. These amendments address ambient air quality levels and, in particular, the ozone nonattainment problem through more stringent controls on automotive emissions, requirements for alternative clean fuels, controls on industrial facilities, and other measures. California's regulations will be more stringent than the federal ones, and under the Clean Air Act amendments they can be adopted by states that are not in compliance with ambient air quality standards. STANDARDS IN THE 1990 CLEAN AIR ACT AMENDMENTS Federal Standards The Clean Air Act amendments of 1990 established stringent tailpipe emissions standards for nonmethane hydrocarbons (NMHC),9 CO, NOx, and particulates for passenger cars and light trucks of 6,000 pounds gross vehicle weight (GVW) rating or 7   In 1988, results of a field test in Los Angeles that measured pollutant concentrations in air flowing out of the Van Nuys road tunnel suggested that vehicle emissions were two to four times higher than levels predicted by the vehicle-emissions inventory models (Ingalls et al., 1989; National Research Council, 1991). 8   The disparity in emissions increases with vehicle age both because older cars may have had more lenient emissions requirements at the time of manufacture and the control equipment deteriorates over time. Automobiles that are at least 15 years old accounted for less than 8 percent of VMT in 1988, yet they accounted for about 23 percent of HC emissions, 22 percent of CO2 emissions, and 18 percent of NO x emissions (Atkinson et al., 1990). On the other hand, longer vehicle life, while it adds to both direct fuel use and emissions, reduces waste materials requiring disposal and reduces energy consumption and other impacts associated with car production. 9   Hydrocarbon standards exclude methane, which does not contribute to the formation of ozone or urban smog. This exclusion becomes relevant in considering the problem of global climate change because methane is a greenhouse gas.

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Automotive Fuel Economy: How Far Should We Go? less (see Table 4-1) (P.L. 101-549). The first set of requirements (Tier I) are to be passenger cars and light trucks of 6,000 pounds gross vehicle weight (GVW) rating or less (see Table 4-1) (P.L. 101-549). The first set of requirements (Tier I) are to be phased in beginning with model year (MY) 1994, and 100 percent compliance is to be achieved by MY 1996. By the end of 1999, the EPA will determine the need, cost, and feasibility of additional standards (Tier II) for vehicles produced for MY 2004 and thereafter. The Clean Air Act amendments also require the EPA administrator to issue regulations to reduce evaporative HC emissions from all gasoline-fueled vehicles and to limit the volatility of gasoline. In addition, the EPA administrator is to issue standards requiring on-board diagnostics to detect emissions-system malfunctions and impose controls on vehicle-refueling emissions by light-duty vehicles. California Standards California, as noted, has established emissions standards that are more stringent than the federal standards. California's standards impose emissions levels for five categories of vehicles: (1) conventional vehicles (CVs); (2) transitional low-emission vehicles (TLEVs); (3) low-emission vehicles (LEVs); (4) ultra-low emission vehicles (ULEVs); and (5) zero-emission vehicles (ZEV's) (see Table 4-2). A gradual phase-in of these increasingly "clean" automobiles is planned as manufacturers produce the TABLE 4-1 Passenger-Car Emissions Standards (grams per mile), Clean Air Act Amendments of 1990     Gasoline Engines Diesel Engines Standard NMHC CO NOx PM NOx Current (1991) 0.41T 3.4 1.00 0.20 1.0 Tier I 0.25 (0.31) 3.4 (4.2) 0.4 (0.6) 0.08 (0.10) 1.0 (1.25) Tier II (0.125) (1.7) (0.2) (0.08) (0.2) NOTES: NMHC = nonmethane hydrocarbons; T = total hydrocarbons; CO = carbon monoxide; NOx = oxides of nitrogen; PM = particulate matter. Standards are for 5 years/50,000 miles or (10 years/100,000 miles) and for up to 3,750 pounds loaded vehicle weight. Tier I standards must be achieved by MY 1996; Tier II, if imposed, would apply to MY 2004 and beyond.

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Automotive Fuel Economy: How Far Should We Go? TABLE 4-2 California's Passenger-Car Emissions Standards (grams per mile)   Gasoline Engines Diesel Engines Vehicle Class NMOG CO NOx PM 93 base 0.25 (0.31) 3.4 (4.2) 0.4 (0.08) TLEV 0.125 (0.166) 3.4 (4.2) 0.4 (0.6) (0.08) LEV 0.075 (0.090) 3.4 (4.2) 0.2 (0.3) (0.08) ULEV 0.040 (0.055) 1.7 (2.1) 0.2 (0.3) (0.04) ZEV* 0 0 0 0 NOTES: NMOG = nonmethane organic gases; CO = carbon monoxide; NO x = oxides of nitrogen; PM = particulate matter; TLEV = transitional low-emission vehicle; LEV = low-emission vehicle; ULEV = ultra LEV; ZEV = zero-emission vehicle. Standards are for 5 years/50,000 miles or (10 years/100,000 miles) and for up to 3,750 pounds loaded vehicle weight. For 1993 base, NMOG = nonmethane hydrocarbons only. * In 1998, 2 percent of manufacturers' sales must be ZEVs and in 2003, 10 percent. vehicles in four phases during the 1990s and on to 2003 (Chang et al., 1991). To meet California's goal to reduce nonmethane organic gases (NMOG), the manufacturers will have to achieve a decreasing fleet-average standard for these emissions using a combination of the vehicles identified above.10 Under the provisions of the Clean Air Act amendments, the 31 other nonattainment states may adopt California's standards, and a number of states intend to do so.11 The states that have taken action or are considering it represent about half of the new-car market in the United States. Given the problems associated with marketing cars with different standards in different states, all automobiles may be 10   Nonmethane organic gases are equivalent to nonmethane hydrocarbons. For passenger cars and light-duty trucks whose loaded vehicle weight (LVW; curb weight plus 300 pounds) is less than or equal to 3,750 pounds, the standard will evolve from 0.390 grams per mile (gpm) for NMOG in MY 1992 (compliance with the fleet average would be required in MY 1994) to 0.062 gpm in MY 2003. For light-duty trucks greater than or equal to 3,751 pounds LVW, the standard will move from 0.5 gpm in MY 1992 to 0.093 gpm in MY 2003. For NOx, emissions levels of 0.2 gpm are prescribed for LEVs and ULEVs. 11   Delaware, the District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, and Virginia have signed an agreement to adopt the California standards (New York Times, October 30, 1991).

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Automotive Fuel Economy: How Far Should We Go? designed to meet the more stringent emissions standards if they become widely accepted, even though air quality conditions differ from region to region. This could mean that California's standards would control planning and decision making on future emissions control systems, not the federal requirements. Emission Standards and Technology Development An attractive technology for improving fuel economy involves the introduction of excess air (and/or exhaust gas recycling) during combustion—the lean-burn approach. For NOx control under lean-burn conditions using excess air, the current three-way catalyst is ineffective. A completely new catalyst is needed for NOx control under these conditions, and as discussed below, the outlook for such a development is uncertain. The tiered structure of the regulations may inhibit technology development, however. Manufacturers have no incentive to introduce technologies that meet intermediate levels of control because of the possibility that regulations could quickly make the new technologies obsolete. In addition, the new requirements for increased durability of emissions systems (from 50,000 to 100,000 miles) may also have a detrimental effect on the introduction of new technologies because it will be more difficult to obtain the required operating experience with the new technology to be certain of adequate performance. In addition, this requirement will require changes in current designs in order to exceed the standards during initial operation (e.g., by adding increased amounts of catalyst) to offset deteriorating performance. These changes will add cost and may impose increases in vehicle weight. In sum, control of NOx and HC emission to meet Tier II and California LEV and ULEV levels presents many substantial challenges to automotive manufacturers.12 As discussed below, compliance with the standards will make it difficult to introduce more fuel efficient vehicles. EMISSIONS CONTROL AND FUEL ECONOMY Current gasoline-engine catalytic systems approximate optimality as a result of 15 years of operating experience. Modern engines operate in a relatively efficient combustion region and ensure that 98 to 99 percent of the carbon in the fuel is converted to CO2 in the engine. In short, essentially all of the energy in the fuel is released by efficient combustion.13 12   Control of exhaust CO was not addressed because, in general, controls for exhaust HC also control for CO. 13   Some small percentage of the fuel is burned late in the cycle because of trapping in engine crevices, in the oil, and so forth. Late burning results in inefficient conversion of fuel energy into work.

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Automotive Fuel Economy: How Far Should We Go? Reducing emissions further will not improve fuel economy.14 In fact, emissions control reduce fuel economy by increasing vehicle weight15 and limiting the opportunities for improved fuel economy. Automotive manufacturers indicated in presentations to the committee (see Appendix F) that the 1996 federal Tier I emissions standards will increase weight by 45 to 66 pounds and reduce fuel economy by 1.5 to 1.8 percent. California's standards are expected to reduce fuel economy by an additional 1.5 to as much as about 4 percent.16 Control of Oxides of Nitrogen Automotive manufacturers will emphasize improving the performance of the three-way catalyst to meet future NOx standards until a new catalyst is available for control of NOx under lean-burn conditions (i.e., air/fuel ratio greater than stoichiometric).17 The Tier II NOx standard will be particularly difficult to meet; it is equivalent to about 95 percent control from precontrol levels.18 New catalyst systems will also be required to meet California's HC standard for LEVs and ULEVs, which will probably entail fuel economy penalties. The current three-way catalyst system can be modified to reduce NO x emissions further by using larger amounts of noble metals (particularly rhodium) in the catalytic converter,19 controlling the air-fuel ratio more precisely, introducing an electrically heated catalyst (or light-off catalyst with secondary air) to improve effectiveness during cold starts, and recycling exhaust gases to lower the peak temperature in the cylinders. In addition, further reduction of sulphur in gasoline would reduce the catalyst's light-off temperature and would allow the use of palladium and cerium in the catalytic system (Monroe et al., 1990). Even so, it is not certain that the level of NOx (0.2 gpm) 14   The mass of emissions from automobiles is directly proportional to VMT. Any actions that reduce VMT will generally reduce total emissions of HC, CO, NOx, and CO2. 15   Emissions-control equipment on 1991 models accounts for about 25 pounds in cars and 35 pounds in light trucks (Kelly Brown, Ford Motor Company, personal communication, December 1991). 16   In presentations to the committee, Honda estimated that Tier I standards will reduce economy by 1.5 percent and increase vehicle weight by 45 to 66 pounds and that California's standards will reduce fuel economy by an additional 1.5 percent. Ford estimated that the Tier I standards will reduce fuel economy by 0.5 miles per gallon (mpg) and increase weight by 20 pounds. Nissan estimated that the Tier I standards will reduce fuel economy by 1.8 percent and that California's LEV standard will lead to a 5.6 percent decline in fuel economy. 17   In stoichiometric operation, the air/fuel ratio is set so that the mixture includes precisely the amount of oxygen needed for complete combustion of the fuel. Under lean-burn conditions, excess oxygen (air) is added. 18   In addition, the new standards require 100,000 miles durability for emission-control equipment, compared with 50,000 miles currently. 19   One study indicates that roughly three times as much rhodium is required to achieve 0.4 gpm NOx as to reach 0.7 gpm (Sierra Research, 1988). The increased cost for the extra rhodium would be about $39 per car, assuming the cost of rhodium is $3,000 per ounce. The modification corresponds to a cost of $1,900 per ton of NOx removed, based on amortizing the $39 incremental catalyst cost over 10 years at 10 percent, 0.3 gpm incremental removal of NOx and 10,000 miles driven per vehicle-year.

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Automotive Fuel Economy: How Far Should We Go? that is required by the new standards can be met, and in any event, the modifications would add weight and reduce fuel economy. In addition, no manufacturer is optimistic about meeting the Tier II NOx standard (0.2 gpm) in gasoline-powered systems, even with a new NOx catalyst. It is easier in general to meet the NOx standards with lighter weight vehicles than with heavier ones.20 Some improvement in NOx control may be achieved in heavier vehicles by using oversized engines with internal exhaust-gas recycling (EGR) as a means of reducing the peak cylinder temperature. Such a strategy, combined with the three-way catalyst, EGR, and electrical heating of the catalyst, may make it possible for the emissions of heavier vehicles to approach the Tier II standard. But such an approach will add weight, reduce efficiency, and increase cost. It is possible that the lowest levels of NOx control cannot be met with the current mix of vehicles. The Technical Challenge: Lean NOx Catalyst Substantial efforts are under way to develop NOx control technologies for lean-burn systems (gasoline or diesel). Researchers are exploring new catalytic compositions, in particular, zeolites containing copper and other metals (Hamada et al., 1990; Iwamoto, 1990; Iwamoto et al., 1991; Li and Hall, 1991; Montreuil and Gandhi, 1991; Sato et al., 1991). Systems that lower NOx emissions in the exhaust from the engine (so-called engine-out emissions) by 40 to 60 percent are available in laboratories. The catalysts are temperature sensitive, however, and under exhaust conditions typical of high-speed operation, they tend to deactivate, presumably because of the agglomeration of the active metal and deterioration of the zeolite. Thus, the systems require increased control of exhaust temperature. Moreover, the volume of catalyst required is large, which increases heat and back pressure in the exhaust system, which in turn lowers fuel economy. The new catalysts are also sensitive to oxygen and sulfur. Because of the oxygen sensitivity, these catalysts may be of limited value for diesel exhaust systems and ultra-lean gasoline engines (e.g., A/F of 22 to 25).21 The sulfur sensitivity could require a substantial reduction of the sulfur content of fuel—for example, to 30 parts per million or less. Moreover, it may be necessary to enrich the HC concentrations in the exhaust to enhance reduction activity, which makes it more difficult to meet the HC 20   This arises from the fact that, all else being equal, the volume of exhaust gas varies directly with the weight of the vehicle. Because the emissions standard is given in grams per mile, the permissible exhaust NOx concentration that satisfies a given standard is larger in the lighter vehicle. 21   Because diesel exhaust is generally at a lower temperature than gasoline-engine exhaust, the temperature control problem associated with the new catalysts could be ameliorated in diesel applications. However, there remains a problem with the particulates in diesel exhaust; they are likely to accumulate on the NOx catalyst, thereby lowering its effectiveness.

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Automotive Fuel Economy: How Far Should We Go? exhaust standards without an additional oxidation catalyst downstream of the NOx catalyst.22 One of the automotive manufacturers with an aggressive R&D program in lean NOx catalysts expressed optimism about developing a durable catalyst that will achieve a 50 percent reduction of the NOx in the engine-out exhaust. Based on information provided to the committee by some of the manufacturers, however, more than 75 percent catalytic reduction of the engine-out NOx is required to meet the Tier II standard of 0.2 gpm NOx. It appears that development of lean NOx catalytic control will be possible, but in view of the problems being encountered, it may not be possible to meet the lowest level of NOx (0.2 gpm) that has been promulgated in all vehicles. This could mean that the heaviest vehicles would not qualify and, under the worst case, would have to be discontinued. Because NOx control may be critical to achieving satisfactory levels of urban ozone, appropriate alternatives should be considered (National Research Council, 1991). Alternative NOx Control Strategies Cars and light-duty trucks emitted about 6 million tons of NOx in 1985. About 2.8 million tons were emitted from other transportation sources, 6.8 million from large utility boilers, 1.8 million from other large boilers, and 1.8 million from small stationary sources (EPA, 1989). Although cars and light trucks are significant contributors of NOx emissions, they are hardly the only culprits. Reductions in NOx emissions from stationary sources have not kept pace with those from mobile sources over the past two decades. In fact, much of the reduction from large stationary sources has been offset by growth in emissions from small stationary facilities (Atkinson et al., 1990). In new large plants, such as utility boilers, up to 90 percent reduction of the NOx emissions is possible using existing technology, and somewhat lower percentage reductions are possible in retrofitted applications and in smaller facilities.23 Existing standards, however, do not appear to require such significant reductions from stationary sources. In light of the difficulty of achieving further stringent NOx reductions in automobiles, perhaps reduction from stationary 22   Use of lean systems will also result in the oxidation of NO to NO2 in the exhaust system. As a result, at a given level of NOx, the NO2 concentration in the exhaust will be increased, perhaps increasing the ground-level concentration of NO2 (Li and Hall, 1991). Although NO2 is more toxic than NO, this change is not expected to be a problem because ambient levels of NO2 currently are satisfactory. However, higher NO2 levels at a given level of NOx could increase the ground-level concentration of ozone because NO2 is the photochemically active component, not NO. 23   Control is accomplished using catalytic or noncatalytic chemical treatment (Boer et al., 1990; Epperly et al., 1988; Sedman and Ellison, 1986). Costs compare quite favorably with those for incremental NO x removal in automobile sources (OTA, 1989), without taking into account any fuel economy penalty. Even higher levels of control in stationary sources should be possible using combinations of technology (combustion modification and chemical treatment).

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Automotive Fuel Economy: How Far Should We Go? sources should be contemplated as an offset for relaxing the new controls on light-duty vehicles.24 In areas with the greatest ozone problems, NOx from stationary sources is a smaller percentage of total NOx emissions than the national average (OTA, 1989). As a result, a given percentage relaxation of the automotive NOx standard would require a disproportionately larger increase in NOx control from stationary sources to achieve the same impact on air quality in those areas. Before Tier II standards are adopted nationwide, a detailed assessment should be made to determine whether it is more practical on a region-by-region basis to achieve higher levels of NOx control from stationary sources than to achieve 95 percent control for mobile sources of NOx. Control of Hydrocarbons Reductions in VOCs of roughly 30 to 50 percent are thought to be necessary to bring most ozone nonattainment areas into compliance (Atkinson et al., 1990). Cars and light-duty vehicles account for about 30 percent of VOC emissions. Thus, the need for further control of automotive exhaust HCs is understandable. Heated Catalysts As noted earlier, the 0.075 gpm NMOG standard for LEVs in California is likely to result in added cost and vehicle weight and a consequent reduction in fuel economy, and the ULEV standard is even more stringent. An electrically heated catalyst may be required to meet California's HC standard of 0.075 gpm for LEVs, according to tests by the California Air Resources Board (1990). One manufacturer told the committee that such a catalyst would add up to 97 pounds of weight to a vehicle and reduce fuel economy by 3 percent. A study by the Automotive Consulting Group (1991) indicates, however, that an electrically heated catalyst would add 40 pounds, reduce fuel economy by about 0.9 mpg, and cost $822 to $1,045 per car. Efforts are under way to develop cost-effective approaches for controlling HC emissions.25 A recent announcement by Ford's research and engineering center at Dunton, Essex indicates, however, that a much simpler system that avoids electrical heating has good potential to be a strong competitor to other known methods for catalyst heating (Griffiths, 1991).26 It appears that less expensive systems will be developed, but California's standards will continue to present significant challenges. 24   The committee appreciates that such a strategy might require the modification of existing legislation. 25   For example, the catalyst might be heated initially by briefly igniting a mixture of fuel and air in an ''afterburner" slightly upstream of the catalyst. Fuel could be supplied by calibrating the engine to run with excess fuel. Air could be supplied by an electrically driven pump. Additionally, by suitable control of fuel to individual cylinders, a mixture with both excess air and excess fuel could be supplied to the catalyst. However, this approach would not be effective until the catalyst reached the initial light-off temperature. 26   It is not clear whether this development is directly applicable to cars meeting U.S. standards. The impact of this development on the manufacturers' ability to meet California's NMOG standard of 0.04 gpm for ULEVs is also unclear.

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Automotive Fuel Economy: How Far Should We Go? Gasoline Volatility The EPA administrator is expected to promulgate controls on gasoline volatility during the five-month summer period. Compared with 1985 levels, this would reduce VOCs in 1994 by 12 percent in ozone nonattainment areas and by 14 percent in attainment areas (OTA, 1989). In 1994, the VOC reduction would be 1.3 million tons in nonattainment areas and 3.3 million tons nationwide. In 1992, the EPA administrator is also expected to regulate gasoline vapor losses during refueling. These vapor losses are the last significant source of vehicle emissions that is largely uncontrolled. As noted, there are two alternative means for controlling vapor losses, control on the vehicle (on-board) and control at the service station (Stage 2 controls).27 In 1999, on-board control is projected to eliminate about 180,000 tons of VOCs annually in ozone nonattainment areas and 370,000 tons nationwide, a 1.6 percent reduction from the level in 1985 (OTA, 1989). Stage 2 systems could have a greater impact—up to 10 percent greater control if installed in all service stations (Sierra Research, 1988). Stage 2 controls are effective immediately regardless of the age of the car, whereas on-board controls become effective only with the turnover of the vehicle fleet. In addition, the weight of the on-board system (about 5 pounds for cars) is avoided, and fuel economy is thus improved incrementally.28 Also, Stage 2 controls provide the flexibility to use the system only where it is needed. Two recent studies indicate that Stage 2 control is at least as cost-effective (dollars per ton of HC emissions eliminated) as on-board control and may be significantly more cost-effective (OTA, 1989; Sierra Research, 1988). In the committee's view, recovery of refueling vapors at the service station should be given careful attention as an alternative to on-board control. Reformulated Gasolines Also in 1992, the EPA is expected to issue regulations calling for cleaner, reformulated gasolines in nine ozone nonattainment areas. Regulation of benzene, other aromatic HCs, heavy metals, and oxygenated HCs is expected. In anticipation of these regulations, the automotive manufacturers and the oil industry formed a cooperative program, the Auto/Oil Air Quality Improvement Research Program, to assess the effects of changes in gasoline composition on emissions. Results thus far indicate that HC emissions can be reduced in MY 1983-1985 and MY 1989 vehicles by 5 to 9 percent by including 15 percent of methyl tertiary butyl ether (MTBE, an 27   Control is achieved by use of a special nozzle on the gasoline pump that allows the capture of HCs that would otherwise be expelled during refueling. 28   The National Highway Traffic Safety Administration (1991) has also expressed concern that on-board vehicle vapor recovery systems may not be safe.

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Automotive Fuel Economy: How Far Should We Go? oxygenated HC) in gasoline.29 Use of MTBE will lower fuel economy (mpg) as a result of the lower volumetric heat content per gallon, but it would have no effect on the use of crude oil, assuming MTBE is made from natural gas. Changes in concentrations of aromatics, olefins, and heavier HCs have had variable effects on emissions. Also, one of the automotive manufacturers told the committee that, individually, these latter changes would lower fuel economy by up to 2.8 percent, largely because of the decrease in volumetric heat content of the fuel. These reductions are not additive, and estimates of the combined effects of changes in fuel composition that might be made are not yet available. There might be some offsetting savings in the operation of refineries making gasoline, but the matter requires further study. Sulfur in Gasoline The Auto/Oil program has also shown that reducing the sulfur content of gasoline from 466 to 49 parts per million reduces HC emissions by 16 percent, CO by 13 percent, and NOx by 9 percent in 10 MY 1989 cars (Auto/Oil Air Quality Improvement Research Program, 1991; Benson et al., 1991). Emissions prior to catalytic treatment were not affected by sulfur, so the change is due to improved performance of the three-way catalyst. Other data show that sulfur inhibits the conversion of NOx to ammonia, and presumably lower sulfur fuels would lead to higher ammonia emissions. Additional work is planned to define better the effect of sulfur concentration and compound type on emissions. Alternative Fuels Evaporative and exhaust HC emissions would also be reduced with the use of alternative fuels. Both methanol and compressed natural gas (CNG) are expected to be used in fleets of 10 or more vehicles to control emissions further. In addition, 29   The following results (all expressed in percent) have been reported by the Auto/Oil group (Hochhauser et al., 1991):   Model Year   1983-1985 1989 Change THC CO NOx THC CO NOx Aromatics: from 45 to 20% +13.6 -2.5 -11.0 -6.5 -13.3 +2.0 MTBE: from 0 to 15% -9.1 -14.1 -1.3 -5.5 -11.1 +1.4 Olefins: from 20 to 5% +5.7 -1.8 -6.7 +5.8 +1.5 -6.1 T90: from 360 to 280°F* -5.8 +13.6 +2.2 -21.7 +1.2 +4.9 * T90 refers to the temperature at which 90 percent of the fuel boils. It is related to the amount of heavier HCs in the fuel.

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Automotive Fuel Economy: How Far Should We Go? methanol-gasoline mixtures are likely to be used more generally.30 It appears feasible to modify light-duty vehicles to use 100 percent methanol and CNG and maintain performance that is generally comparable to that of gasoline-fueled vehicles. Control of Stationary Sources It is estimated that about 52 percent of VOCs emanate from stationary sources and about 48 percent from mobile sources in ozone nonattainment areas. Further, the percentages are roughly constant regardless of the ozone level (OTA, 1989). Organic solvent evaporation, surface coatings, and the petroleum and gas industries are the major stationary sources of VOCs. Clearly, opportunities to control these stationary sources should be carefully considered as an offset to more stringent HC standards for automobiles. Impact of Emissions Standards on Light Trucks In the past, emission standards have affected light trucks in much the same way as they have affected cars, and this trend is expected to continue. As noted earlier, MY 1991 cars have an average of 25 pounds of emissions-control hardware, compared with 35 pounds on light-duty trucks; and meeting the 1996 emissions standards will add another 20 pounds to cars and about 25 pounds to light trucks. These numbers do not include the hardware required to meet Tier II or California LEV standards. The future emissions standards for light trucks weighing up to 3,750 pounds (LVW)31 (27 percent of MY 1991 sales) are projected to be essentially the same as those for cars, considering both the federal (Tier I) and California (LEV) requirements (EPA, 1991). As a consequence, it will be possible to use emissions-control hardware that is similar to that on cars, as long as the performance characteristics important in light trucks can be maintained. Lean-burn operation with gasoline is less likely to be used on vehicles that carry substantial payloads because the higher load factor means that lean burn can only be used for a smaller portion of the operating time. Emissions standards for heavier trucks, which constituted over 70 percent of light-truck sales in 1991, have been adjusted to higher levels in consideration of vehicle 30   In the 1990s, it is estimated that the use of methanol-gasoline blends will effectively reduce the rates of evaporative and exhaust VOC emissions by 30 percent, compared with vehicles meeting current standards and operating on low-volatility fuel (vapor pressure of 9 pounds per square inch) (OTA, 1989). Over the longer term, there is the potential to reduce VOC emission rates by up to 90 percent, but improved engine design and catalysts that control formaldehyde emissions are required. The EPA has estimated that exhaust emissions from a dual-fueled vehicle while operating on CNG would effectively be reduced by 40 percent and evaporative emissions by 100 percent. However, the latter does not include the emission of methane due to fueling and distribution losses. Methane losses during pipeline transmission and distribution to end users are estimated to be 0.5 to 4 percent of the energy transported (Ho and Renner, 1991). Thus, the widespread use of CNG to fuel vehicles could have an adverse impact on global climate change if a significant fraction of vehicles were fueled with CNG. 31   LVW is loaded vehicle weight, which is curb weight plus 300 pounds.

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Automotive Fuel Economy: How Far Should We Go? weight and payload. For example, the base federal NOx standard for the MY 1994 is 0.4 gpm for trucks weighing up to 3,750 pounds (LVW) and 0.7 for those up to 5,750 pounds, both less than 6,000 pounds GVWR.32 Similarly, the California LEV standards for NOx are 0.2 and 0.4 gpm, respectively. The committee met several times with the automotive manufacturers, and at no time were truck emissions standards raised as presenting problems that were different from those for cars. It is likely that the Tier I exhaust emissions standards for trucks will be met largely by improving the performance of the three-way catalyst, as with cars. Lean-burn operation is a possibility at the lower end of the weight spectrum of light trucks, but as noted, it is less likely at the heavier end. Information presented elsewhere in this chapter raises doubts regarding the probability of meeting future emissions standards with the diesel engine. The prospects may be somewhat better in larger trucks (6,000 to 8,500 pounds GVWR) because of the somewhat less stringent NOx standards. In addition, heavier and more expensive control systems might be feasible on the larger, more expensive trucks. However, in general the emissions standards will increase the percentage of trucks that use gasoline engines because of the NOx and particulate emissions characteristics of diesel engines.33 Manufacturers told the committee that a 25 to 50 percent reduction in NOx from the current 1 gpm requirement might be achieved in diesels using combustion modification. They thought, however, that higher levels of control would require use of methanol fuel instead of diesel and even then were not optimistic about meeting the 0.2 gpm Tier II standard. Tier I standards call for a reduction of particulates from the current 0.2 to 0.08 gpm. While this appears possible, further development is required. Surveillance of Existing Vehicles One alternative to increasingly stringent emissions standards for new vehicles is surveillance and servicing of existing vehicles. The EPA has estimated that emissions reductions from a comprehensive inspection and maintenance program could exceed 30 percent for HC and 50 percent for CO and would be roughly 5 to 10 percent for NO x (Sierra Research, 1988). The emissions from a car with an ineffective catalyst can be equivalent roughly to those from 10 to 20 well-functioning cars or more. In short, the gains from improved surveillance and maintenance are large.34 A number of states currently monitor emissions from vehicles by measuring pollutants when the engine is idling. These programs will expand; the EPA has 32   GVWR is the maximum weight for which a vehicle is designed. 33   There are three approaches to controlling diesel particulates: higher fuel-injection pressure, oxidation catalysts, and particulate traps. These approaches can be combined and, in fact, the most cost-effective system currently utilizes higher pressure injection and an oxidation catalyst. 34   In addition to encouraging the repair of malfunctioning control systems, a surveillance program would discourage tampering with control systems. The retirement of noncomplying cars would also improve fuel economy because many would be replaced with new cars with higher fuel economy.

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Automotive Fuel Economy: How Far Should We Go? established guidelines that require state inspection and maintenance programs to achieve at least 25 percent reductions of HCs and CO. On-board diagnostic equipment is expected on MY 1994 cars to detect malfunctions that can cause vehicles to exceed emissions requirements, and this equipment offers new opportunities for ensuring adequate emissions performance by the fleet. The new on-board diagnostic equipment will substantially change the type of surveillance program that is required, however. INDIRECT IMPACTS Improvements in vehicle fuel economy will have important indirect environmental impacts. For example, replacing the cast iron and steel components of vehicles with lighter weight materials (e.g., aluminum, plastics, or composites) may reduce fuel consumption but would generate a different set of environmental impacts, as well as result in different kinds of indirect energy consumption. Depending on the energy intensities of the substitute materials, and the manufacturing processes employed, those indirect costs could offset to varying degrees the improvements in vehicle fuel economy. Although the committee has not done an extensive analysis of these effects, it appears that the production of a large automobile requires the energy equivalent of two to three years of gasoline consumption, with a corresponding set of emissions to the environment. (The energy consumed may be of forms that are not petroleum based.) The recycling of discarded vehicles to a greater extent than is the case today could reduce the net energy required to manufacture new vehicles and would conserve resources.35 However, such programs could limit the use of plastics and composites because of the special difficulties they pose in the recycling system. Fuel economy improvements reduce total HC emissions, considering emissions from the production, transportation, refining, and marketing of fuel (DeLuchi et al., 1991). But greenhouse gas emissions from the production of substitute materials, such as aluminum, could substantially offset decreases of those emissions achieved through improved fuel economy, although this would strongly depend on the energy source used for the production of the materials. Additionally, such indirect impacts should be important considerations in evaluating alternative-fuel or electric vehicles (Amann, 1990; Electric Power Research Institute, 1991). CONCLUSIONS The 1996 federal Tier I emissions standards can be met with a loss in fuel economy of roughly 0.5 mpg. The Tier II and California standard of 0.2 gpm of NOx may not be feasible for the current mix of automobiles and light trucks, and it is likely to be most difficult to meet for the largest vehicles. In addition, these standards are likely to foreclose   35   In Europe, the automobile companies have initiated a more intensive recycling program. A similar program is under consideration by the U.S. companies (see Automotive News, September 23, 1991).

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Automotive Fuel Economy: How Far Should We Go? broad application of promising current and emerging technologies for improving fuel economy, such as lean-burn engines, particularly for larger vehicles. Although the Tier II standard is yet to be adopted, the California standard is being implemented, and it is being seriously considered by states that collectively account for half of U.S. sales of cars and light trucks. In view of the difficulty of meeting these NOx standards, other approaches, such as increasing control of NOx from stationary sources, should be considered. It appears possible that a lean NOx catalytic system achieving roughly a 50 percent reduction in NOx with the required durability will be developed. Such a system might make it possible to meet Tier II and California LEV standards in the smallest cars and trucks, but the system is unlikely to achieve the control needed in the heaviest light-duty vehicles without an additional, substantial breakthrough in catalyst technology. Diesel-powered vehicles are unlikely to meet the Tier II and California LEV standards for NOx and particulates. Thus, the substantial fuel economy benefit of the diesel engine is unlikely to be available when these standards take effect. Lighter weight vehicles have the potential for lower NOx emissions and higher fuel economy and thus offer health advantages in improved air quality and lower CO2 emissions. As a result, trends toward lower emissions levels and higher fuel economy will favor lighter vehicles. In view of the importance of HC emissions in the formation of ground-level ozone, service station control of fueling emissions (Stage 2) seems to promise distinct advantages over on-board recovery. Once installed, such a system would be effective for all vehicles, not just new ones. In addition, the Stage 2 system can be applied only in areas where it is needed. In view of these benefits, service station control of refueling emissions should be given further consideration by the EPA. Emissions of HC, CO, and NOx from existing vehicles probably can be significantly reduced by lowering the sulfur content of gasoline. In addition, lowering sulfur content would reduce the light-off temperature and increase the flexibility to use other catalytic metals in new vehicles. The EPA should consider implementation of further sulfur reduction as part of its program on reformulated gasoline. Emissions of HC and CO from the current fleet could generally be reduced by adding MTBE to gasoline. This would reduce fuel economy in terms of miles per gallon, but it would not increase the use of petroleum, assuming the MTBE is made from natural gas. Other changes in gasoline composition have mixed effects. Surveillance and enforcement of emissions standards in the existing fleet may be an attractive alternative to increasingly stringent controls on new vehicles.  

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Automotive Fuel Economy: How Far Should We Go? Shiller, J.W. 1990. The automobile and the atmosphere. In Energy: Production, Consumption, and Consequences, J.L. Helm, ed. National Academy of Engineering. Washington, D.C.: National Academy Press. Sierra Research, Inc. 1988. The Feasibility and Costs of More Stringent Mobile Source Emission Controls. Prepared for the Office of Technology Assessment, U.S. Congress. Sacramento, Calif. U.S. Environmental Protection Agency (EPA). 1989. The 1985 NAPAP Emissions Inventory (version 2): Development of the Annual Data and Modelers' Tapes. Office of Research and Development. EPA-600/7-89-012a. Washington, D.C. U.S. Environmental Protection Agency (EPA). 1991. Implementation Strategy for the Clean Air Act Amendments of 1990. Office of Air and Radiation. Washington, D.C.