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.
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
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
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emissions on a CO2-equivalent basis: CO2, 66.3; methane, 20.4; CFCs, 9.7; and N20, 3.6 (National Academy of Sciences, 1991). |
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
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
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
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
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)
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 NOxCatalyst
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
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 NOxControl 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
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.
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
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):
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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
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
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
-
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.
REFERENCES
Amann, C.A. 1990. Technical options for energy conservation and controlling environmental impact in highway vehicles. Energy and Environment in the 21st Century. Proceedings of conference held at Massachusetts Institute of Technology, Cambridge, Mass., March 26-28. Cambridge, Mass.: Energy Laboratory.
Atkinson, D., A. Cristofaro, and J. Kolb. 1990. Role of the automobile in urban air pollution. Energy and Environment in the 21st Century . Proceedings of conference held at Massachusetts Institute of Technology, Cambridge, Mass., March 26-28. Cambridge, Mass.: Energy Laboratory.
Auto/Oil Air Quality Improvement Research Program. 1991. Effects of Fuel Sulfur Levels on Mass Exhaust Emissions. Technical Bulletin No. 2. Dearborn, Mich.: Ford Motor Company.
The Automotive Consulting Group, Inc. 1991. The Electrically Heated Catalyst: A "Systems" Cost Analysis. Ann Arbor, Mich.
Benson, J.D., V. Burns, R.A. Gorse, A.M. Hochhauser, W.J. Koehl, L.J. Painter, and R.M. Reuter. 1991. Effects of Gasoline Sulfur Level on Mass Exhaust Emissions-Auto/Oil Air Quality Improvement Research Program. SAE Paper 912323. Warrendale, Pa.: Society of Automotive Engineers.
Boer, F.P., L.L. Hegedus, T.R. Gouker, and K.P. Zak. 1990. Controlling power plant NOxemissions. ChemTech 20(5):312.
California Air Resources Board. 1990. Low-Emission Vehicle Technical Support Document. Los Angeles.
Chang, T.T., R.H. Hammerle, S.M. Japor, and T.J. Saleem. 1991. Alternative transportation fuels and air quality. Environmental Science and Technology 25(7):1190-1197.
Congressional Quarterly. 1990. Clean Air Act amendments. For the Record 24 (November) 24:3934-3963.
DeLuchi, M.A. 1990. State-of-the-art assessment of emissions of greenhouse gases from the use of fossil and nonfossil fuels, with emphasis on alternative transportation fuels. University of California, Davis.
DeLuchi, M.A. 1991. Greenhouse gas emissions from the use of transportation fuels and electricity. Argonne National Laboratory, Ill. Photocopy.
DeLuchi, M.A., Q. Wang, and D.L. Greene. 1991. Motor vehicle fuel economy, the forgotten HC control strategy? Oak Ridge National Laboratory, Tenn. Photocopy.
Electric Power Research Institute. 1991. They're new! They're clean! They're electric! EPRI Journal April/May:4-15.
Epperly, W.R., J.E. Hofmann, J.H. O'Leary, and J.C. Sullivan. 1988. The NOx OUT(R) process for reduction of nitrogen oxides. Paper presented at the 81st A.P.C.A. Annual Meeting and Exhibition, Dallas, June 23.
Griffiths, J. 1991. Ford puts out a cleaner cat. Financial Times November 28.
Hamada, H., Y. Kintaichi, M. Sasaki, T. Ito, and M. Tabata. 1990. Highly selective reduction of nitrogen oxides with hydrocarbons over H-form zeolite catalysts in oxygen-rich atmospheres. Applied Catalysis 64:L1-L4.
Ho, S.P., and T.A. Renner. 1991. Global Warming Impact of Gasoline vs. Alternative Transportation Fuels. SAE Paper 901489. Warrendale, Pa.: Society of Automotive Engineers.
Hochhauser, A.M., J.D. Benson, V. Burns, R.A. Gorse, W.J. Koehl, L.J. Painter, B.H. Rippon, R.M. Reuter, and J.A. Rutherford. 1991. The Effect of Aromatics, MTBE, Olefins, and T90 on Mass Exhaust Emissions from Current and Older Vehicles. SAE Paper 912322. Warrendale, Pa.: Society of Automotive Engineers.
Ingalls, M.N., L.R. Smith, and R. E. Kirksey. 1989. Measurement of On-Road Vehicle Emission Factors in the California South Coast Air Basin. Vol. 1: Regulated Emissions. San Antonio, Texas: Southwest Research Institute.
Iwamoto, M. 1990. Catalytic decomposition of nitrogen monoxide. Studies in Surface Science and Catalysis 54:121-143.
Iwamoto, M., H. Yahiro, K. Tanda, N. Mizuno, and Y. Mine. 1991. Removal of nitrogen monoxide through a novel catalytic process. 1. Decomposition on excessively copper ion exchanged ZSM-5 zeolites. Journal of Physical Chemistry 93:3727-3730.
Johnson, J.H. 1988. Automotive emissions. In Air Pollution, the Automobile, and Public Health. The Health Effects Institute. Washington, D.C.: National Academy Press.
Khazzoom, J.D. 1991. A model of the auto manufacturers' target emission rate for new passenger vehicles. San Jose State University, Calif. Photocopy.
Li, Y., and W.K. Hall. 1991. Catalytic decomposition of nitric oxide over Cu-zeolites. Journal of Catalysis 129:202-215.
Monroe, D.R., M.H. Krueger, and D.J. Upton. 1990. The effect of sulfur on three-way catalysts. GMR-7135. General Motors Research Laboratories, Warren, Mich.
Montreuil, C.N., and H.S. Gandhi. 1991. Lean NOx catalyst. Ford Motor Company. Dearborn, Mich.
Motor Vehicle Manufacturers Association. 1991. Facts & Figures '91. Washington, D.C.
National Academy of Sciences. 1991. Policy Implications of Greenhouse Warming. Washington, D.C.: National Academy Press.
National Highway Traffic Safety Administration (NHTSA). 1991. An Assessment of the Safety of Onboard Refueling Vapor Recovery Systems. Prepared by Enforcement, NHTSA. Washington, D.C.
National Research Council. 1991. Rethinking the Ozone Problem in Urban and Regional Air Pollution. Washington, D.C.: National Academy Press.
Office of Technology Assessment (OTA), U.S. Congress. 1989. Catching Our Breath: Next Steps for Reducing Urban Ozone. Washington, D.C.: U.S. Government Printing Office.
Office of Technology Assessment (OTA), U.S. Congress. 1991a. Changing by Degrees: Steps to Reduce Greenhouse Gases. Washington, D.C.: U.S. Government Printing Office.
Office of Technology Assessment (OTA), U.S. Congress. 1991b. Improving Automobile Fuel Economy: New Standards New Approaches. Washington, D.C.: U.S. Government Printing Office
Quarles, J., and W.H. Lewis, Jr. 1990. The NEW Clean Air Act, A Guide to the Clean Air Program as Amended in 1990. Washington, D.C.: Morgan, Lewis and Bockius.
Rau, J.G., and D.C. Wooten. 1980. Environmental Impact Analysis Handbook . New York: McGraw-Hill.
Sato, S., Y. Yu-u, H. Yahiro, N. Mizuno, and M. Iwamoto. 1991. Cu-ZSM-5 zeolite as highly active catalyst for removal of nitrogen monoxide from emission of diesel engines. Applied Catalysis 70:L1-L5.
Sedman, C.B., and W. Ellison. 1986. German FGD/DeNOx experience. Proceedings of the Third Annual Pittsburgh Coal Conference. Sponsored by the University of Pittsburgh and the U.S. Department of Energy's Pittsburgh Energy Technology Center. Pittsburgh, Pa.
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.