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Watson AY, Bates RR, Kennedy D, editors. Air Pollution, the Automobile, and Public Health. Washington (DC): National Academies Press (US); 1988.

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Air Pollution, the Automobile, and Public Health.

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Automotive Emissions

JOHN H.JOHNSON

Michigan Technological University

Pollution from Automobiles—Problems and Solutions

Concern about the automobile as a source of air pollution has been expressed periodically, but national concern was first evidenced in the 1960s when California established the first new car emission standards. The scientific basis of this effort is the pioneering atmospheric chemistry research of A.J.Haägen-Smit, who showed that photochemical reactions among hydrocarbons (HC) and nitrogen oxides (NO x ) produce the many secondary pollutants that reduce visibility and cause eye and nose irritation in the Los Angeles area.

This paper reviews our current knowledge of automotive emissions, including standards, control technology, fuel economy, fuels and additives, in-use emissions, measurement methods for unregulated pollutants, and models for predicting future automotive emissions. Fuel economy is included because achieving high fuel economy and low emissions together makes the engineering effort more difficult. Emissions and fuel economy are interrelated because both are influenced by the engine combustion system design. In practice, the stringency of emission standards determines the importance of this interrelationship. After current knowledge in each area has been reviewed, important gaps in our knowledge are identified and research needed to fill these gaps is described.

Emissions Standards and Control Approaches

Evolving emission standards have resulted in three levels of stringency, and in turn, three types of control technology. Figure 1 describes the technologies applied in each of the three phases and the general time periods in which they were applied to cars, light-duty trucks, and heavy-duty trucks (Ford Motor Co. 1985a). The percent reduction in the HC, carbon monoxide (CO), and NO x emissions are also shown.

Figure 1.. Major phases in the reduction of automotive emissions.

Figure 1.

Major phases in the reduction of automotive emissions. (Adapted with permission from Ford Motor Co. 1985a.)

Air/fuel (A/F) ratio, which is controlled by the carburetor or fuel injection system, is the most important variable in determining emissions and in applying catalyst technology. Figure 2 (Heinen 1980) is a plot of NO x , HC, and CO concentrations in the exhaust versus A/F ratio for a typical gasoline engine. It is impossible to achieve the low emissions demanded by federal standards by A/F ratio control alone since the concentrations of the three pollutants are not minimums at the same A/F ratio. In fact, when CO and HC concentrations are a minimum, at an A/F ratio of around 16:1, NO x production is close to a maximum. Also shown is the A/F ratio for maximum power (13.5:1) and maximum fuel economy (17:1). The region where A/F ratio exceeds 17.5:1 is the lean burn region where misfires can occur along with slow flame speeds, causing increased HC concentration. The A/F ratio effects are used in all phases of control. The stoichiometric ratio of 14.7:1 is necessary in the Phase III control using three-way catalysts since the A/F ratio must be in a narrow window within ± 0.05 of the stoichiometric ratio to achieve high HC, CO, and NO x control efficiencies simultaneously. Exploring the lean burn region is an important area of research and development because of the potential of improved fuel economy and adequate emission control with only an oxidation catalyst.

Figure 2.. Concentrations of HC, CO, and NO x emissions as a function of air/fuel ratio in a typical gasoline engine.

Figure 2.

Concentrations of HC, CO, and NO x emissions as a function of air/fuel ratio in a typical gasoline engine. (Adapted with permission from Heinen 1980, and the Society of Automotive Engineers, Inc.)

Fuel Economy

Federal regulations also mandate automotive fuel economy. The period from 1968 to 1974 resulted in primary emphasis on emission control with loss of fuel economy from lower compression ratios, changes in spark timing, A/F ratio and axle ratio changes, and exhaust gas recirculation. Figure 3 (Heavenrich et al. 1986) shows the U.S. fleet combined, city, and highway fuel economy data for each model year since 1974. The figure also includes information on average vehicle weight. The fuel economy from 1977 to 1980 improved almost exactly in proportion to the decreasing weight of vehicles. If the data in figure 3 were normalized to the 1978 weight mix, it would show that fuel economy improvements leveled out in 1982. With the introduction of the oxidation catalytic converter in 1975, improved fuel economy and reduced emissions occurred simultaneously. Further emission reduction with simultaneous fuel economy improvement continues through application of new technology, especially computer engine control.

Figure 3.. U.

Figure 3.

U.S. fleet fuel economy and average vehicle weight by model year. (Adapted with permission from Heavenrich et al. 1986, and the Society of Automotive Engineers, Inc.)

In-Use Passenger Car Emissions

The in-use emissions from passenger cars exceed the new car standards mandated by law. Nonetheless, emissions continue to decrease in spite of high tampering rates and fuel switching (that is, using leaded fuel in engines developed to run on unleaded fuel). From field surveys in 14 cities, Greco (1985) found the overall tampering rates and catalyst tampering rates shown in figure 4.

Figure 4.. Overall and catalyst tampering rates by vehicle model year, based on 1984 survey.

Figure 4.

Overall and catalyst tampering rates by vehicle model year, based on 1984 survey. (Adapted from Greco 1985.)

Figure 5 shows EPA emission factors data as analyzed by General Motors Corp. (1985a). The measured emission concentrations for various model years are compared to the standards that were in effect during those years. The measured NO x concentrations follow the standards fairly well. Although the measured HC and CO concentrations are higher than the standards, the difference between the actual emissions and the standards appears to be narrowing (although the ratio is not decreasing) as improved technology, more frequent inspection and maintenance, and better training of mechanics has occurred. Even though the overall trend of emissions is down, a few vehicles have high emission levels, as shown in figure 6, probably because of electronic problems rather than catalyst removal or misfueling problems.

Figure 5.. Average vehicle lifetime HC, CO, and NO x emissions compared with standards (STD), by model year for all industry passenger vehicles.

Figure 5.

Average vehicle lifetime HC, CO, and NO x emissions compared with standards (STD), by model year for all industry passenger vehicles. (Adapted with permission from General Motors Corp. 1985a.)

Figure 6.. Scatter plots of CO emissions from 703 1981 model-year federal cars.

Figure 6.

Scatter plots of CO emissions from 703 1981 model-year federal cars. (Adapted with permission from General Motors Corp. 1985a.)

Emissions Regulations

In the 1960s, motor vehicles were identified as one of the primary sources of air pollutants in urban areas. Emission standards for passenger cars were first imposed in California in 1965. These were followed by U.S. federal standards in 1968. The 1970 Clean Air Act further imposed stringent HC, CO, and NO x reductions for 1975 and 1976. These reductions were subsequently delayed and changed by the 1974 Energy and Environmental Coordination Act and the 1977 Clean Air Act Amendments. Recognition of the motor vehicle as a major source of pollutants has spread to other countries, of which many have imposed diverse standards and test procedures reflecting various degrees of stringency. The differences have come about because of different regulatory philosophies and air quality goals, in combination with concerns about the conflicting goal of improved fuel efficiency (Barnes and Donohue 1985).

Emission Test Procedures

Passenger Cars. Emissions come principally from three automotive sources: the exhaust, the fuel system (evaporative), and crankcase ventilation gases. To give the standard (maximum allowable level of emission in grams per mile) operational meaning, two major aspects must be defined: the driving cycle and the emissions sampling method. Driving cycles are discussed below and sampling methods will be covered in a later section.

Regulations require exhaust emission measurements during the operation of the vehicle (or engine) on a dynamometer during a driving cycle that simulates vehicle road operation. The approach to driving cycles by various regulatory authorities represent two basic philosophies. According to the first, the driving cycle is made up of a series of repetitions of a composite of various vehicle operating conditions representative of typical driving modes. The European Economic Community and Japanese cycles reflect this philosophy. According to the second, the composite of driving modes is an actual simulation of a road route. The United States, Canada, Australia, Sweden, and Switzerland all use a version of the federal test procedure (FTP). The FTP cycle is divided into a “transient” portion and a “stabilized” portion with a total cycle time of 1,372 sec, a driving distance of 7.5 miles, and an average speed of 19.7 miles per hour (mph). Two such cycles are run: one with the vehicle at an ambient temperature of 16–30°C before start (“cold” cycle), and one with the engine control system hot (“hot” cycle) after a 10-min shutdown after running the cold cycle.

Trucks. Many of the light-duty trucks intended primarily for the carrying of goods are also capable of use as passenger vehicles. The gross vehicle weight for light-duty trucks in the United States is less than 8,500 Ib; trucks heavier than 8,500 Ib are classified as heavy-duty vehicles. The driving-cycle philosophies for the light commercial vehicles follow those for passenger cars. For heavy commercial vehicles, engine dynamometers are used, not chassis dynamometers; that is, the engine rather than the vehicle is certified. The new (effective 1985) U.S. transient test procedure for heavy-duty vehicles combines the two philosophies just described in that the cycle is made up in a random way from actual driving cycle data. The use of this cycle replaces the 13-mode steady-state cycle in use since 1973 in California and since 1974 nationally (U.S. Environmental Protection Agency 1972).

Emission Standards

United States. Emissions standards and test procedures in the United States have changed significantly since the first automobile emission standards were imposed in California in 1966 (see table 1) (General Motors Corp. 1986). Light-duty truck standards are somewhat higher than the car standards because of the differences in weight.

Table 1.. Motor Vehicle Emission Standards in the United States.

Table 1.

Motor Vehicle Emission Standards in the United States.

The U.S. passenger car regulations require that the vehicle comply with the emission standards for five years or 50,000 miles, whichever occurs first. Certification testing of prototype vehicles for 50,000 miles of use is based on the Automobile Manufacturers' Association (AMA) 40.7-mile durability cycle. The cycle consists of numerous stops, acceleration, and high/ medium-speed driving (maximum of 55 mph) (U.S. Environmental Protection Agency 1973).

Europe. The European Economic Community, an inter-Europe regulatory body, has announced future model standards for passenger cars based on three engine size (displacement) categories. Large-car (>2-liter engine displacement) standards are roughly equivalent to current U.S. standards although there is no valid correlation between the distinct U.S. and European emission test cycles. Standards for medium cars. (1.4–2.0 liters) are considered to fall in the Phase I/Phase II range shown in figure 1. Requirements for small-car levels (<1.4 liters) are comparable to Phase I requirements. The standards include diesels; however, large diesel cars are only required to meet medium-car levels.

Japan. Catalyst forcing standards currently in effect for passenger cars are 0.25 HC/2.1 CO/0.25 NO x g/km for the unique 10-mode hot start and 7.0 HC/60 CO/4.4 NO x g/test for the 11-mode cold-start test procedures. These standards are generally considered to be equivalent to current U.S. California levels (Ford Motor Co. 1985a).

U.S. Fuel Economy Standards

There have been passenger car and light-truck fuel economy standards since 1978 and 1979, respectively. The manufacturers are required to conduct passenger car fuel economy tests according to the U.S. Environmental Protection Agency (EPA) urban or “city” driving cycle—the FTP for emission testing described earlier. The EPA also has a suburban or “highway” cycle that includes a significant amount of simulated highway driving. A combined fuel economy number based on these two tests is published by the EPA and the U.S. Department of Energy and used by manufacturers in their sales literature.

Manufacturers each have to meet the Corporate Average Fuel Economy (CAFE) standards for their sales-weighted fleet. Car standards started at 18 miles per gallon (mpg) in 1978, went to 27.5 mpg in 1985, but were reduced to 26 mpg by the U.S. Department of Transportation for 1986–1988.

Vehicle and Emission Control System Technology

The technology used for emission control in cars changed rapidly in the 1970s as the automotive industry spent considerable research and development funds to meet the stringent emission standards originally set by the 1970 and 1977 Clean Air Act Amendments. This technology is now being optimized to reduce the product cost associated with emission controls while improving the in-use durability of the emission control systems. Heavy-duty gasoline-powered vehicles have used this technology as allowable emissions have progressively decreased.

Control technology is being developed to meet proposed standards and anticipated changes in fuels. Proposed 1988, 1991, and 1994 particulate standards require new control systems for heavy-duty diesels. For the United States to become less dependent on imported petroleum fuels, there is interest in using methanol in passenger cars and diesel-fueled buses. There are continued efforts to develop stratified-charge engines for passenger cars because of their potential for better fuel economy at equivalent emissions. There is also a demand for development of direct-injection diesels that give 15 percent better fuel economy than prechamber or swirl-chamber engines with equivalent or better emissions. An additional demand exists for an adiabatic diesel engine (more precisely, a low-heat-rejection engine) that would have improved fuel economy and lower emissions with a simpler cooling system, particularly for vehicles in the heavy-duty class.

Spark-Ignition Gasoline-Powered Vehicles

During the past 15 years, emissions have been significantly lowered by improved design of the engine and fuel system while still achieving the high fuel economy demanded by the federal standards and the consumer market. These reductions have come about by A/F ratio control, cylinder-to-cylinder distribution of air and fuel, choke operation, combustion chamber design, fuel injection, exhaust gas recirculation (EGR), ignition systems, spark timing, valve timing, and many additional design details. The computer scheduling of spark timing, EGR, A/F ratio, and transmission gear ratio as a function of engine operating conditions are done very precisely with sensors and actuators. This scheduling is referred to as the engine calibration. With all of this technology, vehicles still do not meet HC/CO/NO x standards of 0.4/3.4/1 g/mi without aftertreatment devices. The period after 1983 has seen better optimization of systems and removing of components to reduce costs, but nevertheless, catalysts are still necessary.

Catalyst Control Systems. Meeting the 1975 HC/CO standards of 1.5/15 g/mi and at the same time increasing the fuel economy was achieved through the broad introduction of the oxidizing catalytic converter. The catalyst is cold (16–30°C) at the start of the FTP cycle and must warm up to 250–300°C before oxidation of CO and HC occurs. The time required for this is a function of catalyst design and position but can be from 20 to 120 sec. The HC emitted during this period can be one-fourth to three-fourths of the allowable limit (Hilliard and Springer 1984). The amount of NO x emitted during the cold start is only about 10 percent of the allowable limit.

The time period from 1975 to 1984 saw increased fuel economy and improved emission control through exploitation of the high HC and CO removal efficiency of the oxidizing catalytic converter, so that the engine calibration could be optimized for efficiency. Progress was made by decreasing the cold-start engine-out HC and CO emissions, by achieving faster converter light-off, by reducing heat loss from the exhaust system, and by reducing the deterioration of catalyst performance with cumulative driving distance (Amann 1985).

Reducing combustion temperature by spark retard and/or diluting the incoming mixture with EGR provided NO x control during the time period from 1973 to 1980. The 1981 standards stipulate no more than 1 g/mi NO x , which could not be achieved either by EGR or engine design and calibration. Two additional catalytic approaches have gained widespread application along with the microprocessor control system, to provide the necessary control: the “three-way” catalyst and the “dual” catalyst.

Three-way catalysts are capable, within a narrow range of exhaust stoichiometry, of simultaneously decreasing NO x , HC, and CO, as shown in figure 7. Within a narrow range of values of the A/F (approximately ±0.05 from the optimum), all three emissions are decreased with a reasonably high efficiency. An oxygen sensor is used in the exhaust in conjunction with a microprocessor to make this technology feasible.

Figure 7.. Conversion efficiency characteristics of a three-way catalyst.

Figure 7.

Conversion efficiency characteristics of a three-way catalyst. (Adapted with permission from Amann 1985.)

In a dual catalyst, two catalysts are used in series—a three-way catalyst followed by an oxidizing catalyst. Air is injected into the exhaust gas between the two catalysts to provide the oxygen necessary for the oxidizing catalyst to operate efficiently. Once more, precise A/F ratio control is required to make the three-way catalyst function. During the cold-start portion of the FTP cycle, the air supply to the oxidizing catalyst can be diverted to the exhaust ports to add oxygen to the combustion products of the rich start-up mixture for faster catalyst light-off and to achieve higher HC and CO control efficiencies in the three-way catalyst. The dual-bed converter is more complex than the single-bed three-way catalyst, because it requires an extensive air management system.

To maintain A/F ratio control within the narrow window, closed-loop control (feedback control of fuel delivery on oxygen level in exhaust) was introduced on many cars in 1981. The schematic of a typical system is shown in figure 8 (Amann 1985). The key element in the closed-loop system is the oxygen sensor inserted in the exhaust pipe ahead of the catalyst. It measures exhaust oxygen concentration and signals an electronic controller to adjust fuel rate continuously so that the mixture is maintained at the stoichiometric ratio.

Figure 8.. System for closed-loop control of A/F ratio.

Figure 8.

System for closed-loop control of A/F ratio. The oxygen sensor inserted in the exhaust pipe ahead of the catalyst measures oxygen concentration and signals the electronic controller to adjust fuel rate continuously. (Adapted with permission from Amann (more...)

Current Control Approaches. Since 1983 the number of engines with some type of fuel injection has grown drastically, but carburetors are still used on many engines. No particular trend in emission systems is evident except for the use of heated oxygen sensors to initiate closed-loop operation faster and more predictably and to maintain it during long idling periods. The heated sensors also deteriorate less with extended mileage (Way 1985). Most cars use closed-loop control with a three-way catalyst; many also have an oxidation catalyst that is a dual catalyst and one of three air supply systems (pulse air, air pump, or programmed pump).

Lean-Burn Combustion Systems. An important engine emission control system under development is the lean combustion system. This system uses a closed-loop microprocessor in conjunction with lean mixture sensor and an oxidation catalyst. This alternate emission control approach achieves good fuel economy (potential 10–15 percent improvement) and also meets the emission standards by operating beyond 22:1 A/F where NO x emission is low enough to meet the 1-g/mi standard. In this lean operating region, the engine needs a different sensor design to provide feedback, and also a highly turbulent fast-burn combustion system so that slow flame speed and misfires do not cause emissions and driveability problems. Toyota has developed and marketed such a system in Japan but not yet in the United States (Kimbara et al. 1985).

It may be possible to introduce this type of system into the U.S. market, but durability and driveability under hot and cold conditions need to be examined further (Kimbara et al. 1985). The other important technological limit might be that lean burn could be restricted to cars under 2,500– 3,000 lb because NO x generally increases with vehicle weight.

Diesel-Powered Passenger Cars: Particulate Control

There has been a major research and development effort during the past seven years to develop aftertreatment devices for diesel passenger cars to meet the federal 0.2 g/mi standard first proposed for 1985 and later put off until 1987. California has a 0.2-g/mi standard that was initiated in 1986, and will be lowered 0.08 g/mi in 1989. A number of prototype systems have been built and field tested to meet the 0.2 g/mi standard. Mercedes-Benz (Abtoff et al. 1985) introduced a catalytic trap oxidizer in 1985, in conjunction with careful modification of the engine (in particular, the turbocharger). The system meets and is certified to the 1986 California standards and has been sold in the 11 western states. Volkswagen has developed a prototype system that uses a Corning ceramic particulate filter in conjunction with Lubrizol 8220 manganese (Mn) additive. The additive consists of nonstoichiometric Mn fatty acid salts dissolved in naphtha, which is metered from a separate fuel-additive storage tank on the vehicle (lifetime filling) and mixed with the fuel (Wiedemann and Neumann 1985). Emissions of Mn oxide of all valence states, as well as MnSO4, may occur. Data suggest that most of the Mn residue is in the form of sulfate.

General Motors has also tested a system, shown in figure 9, with on-board tank-blending, additive dispensing, and ceramic fiber trap (Simon and Stark 1985). This system uses pressure and engine speed to provide a measure of particulate loading for triggering the glow plug igniters for regeneration. Simon and Stark (1985) investigated three different additives: cerium (0.13 g Ce/liter), manganese (0.07 g Mn/liter), and cerium plus manganese (0.07 g (Ce+ Mn)/liter). Their tests showed that vehicles equipped with properly tuned 4.3-liter engines and operated using a fuel additive would not, on a production basis, be able to meet the 1987 federal emissions standards at sea level or at altitude. Equipped with particulate traps, however, the vehicles would probably meet the 1987 federal standards and might, with further engine tailoring, be able to meet the 1989 California standards on a production basis.

Figure 9.. Diesel particulate trapping system utilizing a ceramic fiber trap, a fuel additive, glow plug igniters, and exhaust backpressure regeneration controls.

Figure 9.

Diesel particulate trapping system utilizing a ceramic fiber trap, a fuel additive, glow plug igniters, and exhaust backpressure regeneration controls. (Adapted with permission from Simon and Stark 1985, and the Society of Automotive Engineers, Inc.) (more...)

Diesel-Powered Heavy-Duty Vehicles

Diesel-powered heavy-duty vehicles use direct-injection turbocharged engines of two-cycle as well as four-cycle design. Diesel engines are designed for a commercial market and hence durability, reliability, and fuel economy drive their development. The approaches enforced to date to meet the standards for particulates, HCs, and NO x have involved improved turbochargers, intercooling, improved fuel systems and nozzles, and electronic fuel injection control. To reduce NO x emissions and improve fuel economy, some manufacturers use heat exchangers to lower air inlet temperature. The industry believes that the 1988 standards can be met with advanced electronic fuel systems and possibly with mechanical fuel systems with electronic-governing, air-to-air intercoolers (or low-flow radiators) and improved turbochargers, but that the 1991 particulate standard of 0.25 g/brake horsepower (bhp)-hr will need trap technology. EPA standards have emphasized particulate control rather than NO x control. There is the feeling that the 1988 standards will result in some loss of fuel economy. GM-Detroit Diesel Allison Division (DDAD) has also decided to remove their 2-cycle engines from the on-highway market because of the disadvantages of this engine under these tight emissions constraints. This is now being reevaluated under the new Detroit Diesel Corp.

In-Use Vehicle and Engine Characteristics

How vehicles and engines perform in the hands of the operator ultimately determines their emissions and, in turn, their impact on air quality. This section examines the emission characteristics of gasoline- and diesel-powered passenger cars and trucks as actually used by owners. The effects of field environmental conditions such as temperature, tampering (removal of and changes in components), or misfueling on emissions are discussed.

Gasoline-Powered Passenger Cars and Trucks

To develop an understanding of the in-use characteristics of gasoline-powered passenger cars, it is important to know whether tampering and misfueling with leaded fuel occur. Misfueling has a twofold impact on the environment: increased lead (Pb) emissions and increased regulated HC, CO, and NO x emissions due to poisoning of the catalyst. Tampering has a direct effect on the regulated emissions and can also affect the unregulated emissions. In this section, the latest tampering data gathered by the EPA and the Motor Vehicle Manufacturers' Association (MVMA) are examined first. This is followed by data showing the emissions and fuel economy of vehicles in use.

Tampering and Misfueling. The latest EPA report on tampering is based on a survey of 4,426 light-duty vehicles conducted in 14 cities between April and October 1984 (Greco 1985). These inspections were performed with the consent of the vehicle owners and therefore may underestimate tampering rates. Four categories were used to summarize the condition of the inspected vehicles:

1.

Tampered—at least one control device removed or rendered inoperative;

2.

Arguably tampered—possible but not clear-cut tampering;

3.

Malfunctioning;

4.

Okay—all control devices present and apparently operating properly.

Greco's overall survey averages of vehicle condition were as follows: tampered, 22 percent; arguably tampered, 29 percent; malfunctioning, 4 percent; okay, 46 percent. The rates for tampering with selected components and the rates of fuel switching are shown in table 2. These results have not been weighted to compensate for inspection and maintenance program representation and probably underestimate the actual nationwide rates. The tampering rates for catalytic converters and filler inlet restrictors (the insert in the fuel tank neck that prevents insertion of the larger leaded fuel nozzle) have increased steadily since 1978, whereas the rates for other components have fluctuated. The increasing tampering rates for catalytic converters and inlet restrictors may be partly due to the increasing age of the vehicles surveyed. In addition, the presence of inspection and maintenance programs affected tampering rates. The catalyst was removed in 3 percent of the vehicles in areas with mandatory inspection and maintenance programs and in 11 percent of the vehicles in areas having no programs.

Table 2.. Tampering Prevalence in Light-Duty Vehicles for Critical Emission Control Components, April-October 1984.

Table 2.

Tampering Prevalence in Light-Duty Vehicles for Critical Emission Control Components, April-October 1984.

Removing the catalytic converter increases HC and CO emissions by an average of 475 percent and 425 percent, respectively (U.S. Environmental Protection Agency 1983). For vehicles equipped with three-way catalysts, substantial increases in NO x emissions would also be expected to occur. Tampering with the EGR system can increase NO x emissions by an average of 175 percent (Greco 1985).

Fuel switching, defined as the presence of a tampered fuel filter inlet restrictor, a positive Plumbtesmo tailpipe test, or a gasoline Pb concentration of more than 0.05 g/gal, was found in 14 percent of the unleaded gasoline-powered vehicles, in the 1984 survey (see table 2). Regional distribution in the prevalence of misfueling is shown in table 3. The impact of fuel switching on emissions depends upon its duration and certain vehicle characteristics, but emission increases of 475 percent for HCs and 425 percent for CO can easily occur (Greco 1985).

Table 3.. Incidence of Misfueling in Large Urban and Nonurban Areas, 1981–1982.

Table 3.

Incidence of Misfueling in Large Urban and Nonurban Areas, 1981–1982.

The tampering rate for light-duty trucks was equal to or higher than that for automobiles in every tampering category, as shown in table 2. The difference in prevalence of catalytic converter tampering is particularly striking—nearly three times as prevalent in light-duty trucks as in passenger cars (14 percent versus 5 percent) (Greco 1985).

To confirm the EPA tampering and misfueling data, the MVMA recently studied catalyst removal and defeat of the fuel filler restrictor. The vehicles used in the MVMA survey were a sample of 1975–84 model year cars and light-duty trucks from scrapyards and impoundment areas in 10 cities (Motor Vehicles Manufacturers' Association 1985; Survey Data Research 1985).

The MVMA study sampled 1,865 vehicles, allowing the following conclusions to be reached to a 95 percent confidence level by Survey Data Research (1985):

1.

Nationwide, 8.3% of all the vehicles in the sample were found to have their catalytic converters removed. This removal rate is significantly higher among older (i.e., 1975–1978) model year passenger cars and light-duty trucks.

2.

The rate of fuel filler neck restrictor tampering on a national basis (7.3%) is slightly lower than the rate of catalytic converter removal (8.3%). Again, this tampering rate is higher among older (i.e., 1975–1979) model year cars and light-duty trucks.

3.

Both catalytic converter removal and fuel filler neck restrictor tampering rates are substantially lower in the sample of Inspection/Mainte nance area locations than the sample of Non-Inspection/Maintenance area locations.

As a result of this study, MVMA is now confident that the much more detailed EPA studies, covering in addition such components as the air pump, EGR system, the positive crankcase ventilation (PCV) system, the evaporative emissions control system, and others, are yielding results that are reasonably representative of the in-use fleet (Motor Vehicles Manufacturers' Association 1985).

Effects of Tampering on Emissions. Recently, the Automobile Club of Southern California conducted a test program using its 1981 fleet vehicles (General Motors, Buick, and Pontiac vehicles) in an effort to better understand the effect of system component failures. The primary objective of the program was to determine the degree to which fuel economy, exhaust emissions, horsepower, and driveability are affected by disabling key components of a computer-controlled system; a secondary objective was to establish a method of accurately and efficiently identifying vehicles with disabled components (Jones et al. 1982).

Jones and coworkers (1982) found that disabling the coolant temperature sensor, the throttle position sensor, or the mixture control solenoid has a major effect on vehicle performance. Disconnection of the coolant temperature sensor increased HC emissions an average of 549 percent and CO emissions an average of 1,120 percent over baseline emission levels; disconnection of the throttle position sensor increased HCs by 1,195 percent and CO by 3,113 percent; and disconnection of the mixture control solenoid increased HCs by 1,293 percent and CO by 3,438 percent. Each of these disablings is the disconnection of a single electrical connector (Jones et al. 1982).

■ Recommendation 1. Tampering and Misfueling. Tampering and misfueling statistics are fairly well developed but their effect on emissions is not as well known. Therefore, work should be done to better characterize the effect of tampering and misfueling on emissions from vehicles and to better assess their effect on ambient pollutant concentrations.

Diesel-Powered Passenger Cars

The available in-use data are much more limited for diesel passenger cars than for gasoline-powered cars for two reasons: first, there are far fewer of them, and second, most of the diesel cars that are in use were sold between 1979 and 1983 so only a small proportion of them are more than seven years old.

Hyde and coworkers (1982) drew the following conclusions about the relation between cumulative mileage and rate of emissions from a sample of 20 in-use lightduty diesel vehicles from General Motors, Volkswagen, and Mercedes-Benz.

1.

Particulate emissions do not show a mileage-related deterioration (increase) in the Volkswagen group and the Mercedes-Benz group, but show a large deterioration in the General Motors group because of a large increase in extract emissions.

2.

Federal Test Procedure HC emissions do not show a mileage-related deterioration in the Volkswagen and Mercedes-Benz groups, but show a deterioration in the General Motors group.

3.

FTP CO emissions show a deterioration in the General Motors and Volkswagen groups but not in the Mercedes-Benz group.

4.

FTP NO x emissions show a decrease with accumulated mileage in the General Motors and Mercedes-Benz groups but no change for the Volkswagen group.

Diesel-Powered Trucks

There are limited data on diesel engines in operational use although there are some recent laboratory data obtained by the various manufacturers in an EPA/Engine Manufacturers Association (EMA) in-use emission factor test program for heavy-duty diesels. As part of this emission factor testing effort, the following classifications were included in the sample of 30 engines: tampered engines, poorly maintained engines, and rebuilt engines. The engines were tested on FTP 13-mode steady-state and FTP Heavy Duty transient cycle.

Figure 10.. CO air quality and emission factor trend as calculated from three computer models and as measured from the base year 1973.

Figure 10.

CO air quality and emission factor trend as calculated from three computer models and as measured from the base year 1973. CO measurements were averaged from 50 U.S. stations and from 16 U.S. stations reporting the highest 8-hr yearly concentrations. (more...)

The EMA reached the following tentative conclusions from this program (General Motors Corp. 1985b):

1.

The in-use control of gaseous emissions from heavy-duty diesel engine from the 1979–80 model year is quite good.

2.

Tampering and poor maintenance do not result in excessive gaseous emissions.

3.

The lab-to-lab variability of transient emission test results of unburnt HCs as well as particulates needs to be improved.

■ Recommendation 2. Diesel Particulate Emissions and Control. There is a need for continued research on particulate control technology, including the regeneration systems, to reduce the cost and complexity of these systems and the associated fuel economy penalties. Work needs to continue with various additives, substrate materials, regeneration systems, and controls to develop optimum systems that are able to decrease the diesel particulate emissions to the levels of 0.1 g/bhp-hr for heavy-duty diesels and 0.08 g/mi for light-duty vehicles required in California. In conjunction with this research there is a need to measure the metal species and the size distribution of the particles coming from diesel particulate traps.

Models for Predicting Future Emissions

Computer models are used for predicting future emissions from in-use vehicles. The EPA publishes the vehicle emissions model most used at present (U.S. Environmental Protection Agency 1985). The highway source data are based on MOBILE3, a computer program issued by the EPA in June 1984 and recently updated (U.S. Environmental Protection Agency 1985). Figure 10 shows the predicted trends of various models compared to actual air quality data for CO. The curve for percent reduction of CO predicted by MOBILE2 does not correspond to the curves generated by air quality measurements from 50 U.S. stations or the 16 highest U.S. stations (General Motors Corp. 1985a).

There are large differences between General Motors' analysis of the actual emissions data and the EPA MOBILE3 emission factor data beyond 50,000 miles, with MOBILE3 being the higher (General Motors Corp. 1985a). General Motors attributes the difference to EPA's choosing too high a bhp-hr/mi constant for gasoline-as well as diesel-powered heavy-duty vehicles.

In addition, General Motors is concerned that the evaporative submodel in MOBILE3 uses a value of 11.5 psi for Reid Vapor Pressure (RVP) for gasoline volatility whereas the national average is 10.5 psi. In fact, the RVP varies with the season of the year in different parts of the country as formulations matched to seasonal conditions are refined and delivered to the pumps. It is unlikely that the model will ever give good results if a single RVP number is used to represent evaporation characteristics in all places at all seasons. Instead, the United States should be subdivided into the American Society for Testing Materials (ASTM) class regions to allow for seasonal changes in RVP. The model would then use RVP values that are representative of the season and region of the country. Other problems include estimating the number of trips per day for an average vehicle, identifying an appropriate ambient temperature, and understanding the effect of fuel aging.

Furthermore, a new approach using a proportion of vehicles in each model year with emission rates in each of a number of incremental ranges, that is, a distribution for emission rates within each model year, should be developed for modeling emission rates. The model needs to account for the few high-emission vehicles as well.

The evaporative emissions submodel needs additional work so that it better simulates the actual field fuel and control system effects, because the actual and test fuels have different vapor pressures. Regional and seasonal differences in RVP should be incorporated in the model along with the effects of alcohols.

■ Recommendation 3. Evaporative Emission Model. An improved vehicle evaporative emissions model should be developed that is valid over various types of operating conditions for a variety of ambient temperatures. At the same time, changes should be made in the EPA test procedure to obtain the data necessary to properly design and size the evaporative system for the high-temperature soak situation, and data should be sought that can be used in EPA's MOBILES computer model for other use patterns of cars.

Fuels and Fuel Additives

Trends in Gasoline Fuel Properties

The EPA limited the use of Pb in gasoline to 0.5 g/gal after July 1, 1985, and to 0.1 g/gal after January 1, 1986. This has increased refineries' interest in the use of alternative low-cost octane boosters, particularly light alkanes and methanol and/or ethanol alcohols blended with gasoline.

Figure 11 shows the recent upward trend in the RVP that has resulted from these industry trends (Ford Motor Co. 1985b). Through 1980, the average RVP of the fuels sampled stayed reasonably close to the specification of the certification fuel (9.0 psi RVP). The increase since 1980 results from the petroleum industry's use of more light stock, such as butanes, in the gasoline. Historically, the petroleum industry has favored adding light HCs to gasoline for economic reasons. Large quantities of butane, a volatile HC, are produced during the refining of crude oil and natural gas. Butane has a high research octane number (about 94), and it is a good substitute for Pb in gasoline blending. The addition of butane increases the “front end” volatility of a gasoline. High fuel volatility increases automotive evaporative emissions and increases vapor losses from fuel tanks by displacement during refueling (Stebar et al. 1985).

Figure 11.. Trends of gasoline RVP averaged by class of cities.

Figure 11.

Trends of gasoline RVP averaged by class of cities. Classification of cities by the ASTM D439 is based on weather conditions and geographical location. (Adapted with permission from Ford Motor Co. 1985b.)

Stebar and coworkers (1985) analyzed a large data base (267 cars from 1978 to 1985 with 141 from 1981) to develop figure 12. The figure illustrates the importance of different HC emission routes and the contributions via individual routes to total vehicle HC emissions for carbureted as well as fuel-injected cars. They observed that:

Figure 12.. Contribution to total HC emissions from various routes and processes as function of gasoline RVP.

Figure 12.

Contribution to total HC emissions from various routes and processes as function of gasoline RVP. Evaporative emissions are derived from nonoperating vehicles parked overnight (diurnal); recently turned off, nonoperating vehicles (hot soak); and vehicles (more...)

1.

Evaporative emissions (primarily diurnal losses) are the major contributor to the increase in vehicle HC emissions with increase in RVP.

2.

Hot soak emissions (particularly with carbureted cars) are a larger contributor to HC emissions than are diurnal losses.

3.

Refueling and exhaust HC emissions have low sensitivity to changes in RVP.

4.

Exhaust emissions are the largest contributor to total HC emissions and constitute about the same proportion of the total for both carbureted and fuel-injected cars. At 12 RVP, exhaust emissions represent about half of total HC emissions for both types of engines.

Furey (1985) measured the vapor pressures and distillation characteristics of a large number of gasoline/alcohol and gasoline/ether fuel blends. In that study, the maximum increase in RVP above that of gasoline ranged from 0.2 psi for tert-butyl alcohol to 3.4 psi for methanol. As little as 0.25 percent methanol, ethanol, and Oxinol™ 50 (a 1:1 mixture of methanol and gasoline-grade tert-butyl alcohol) was found to produce measurable increases in RVP.

The EPA estimates that the difference in volatility between the certification fuel and commercial gasoline is responsible for about half of the evaporative emissions from late-model light-duty vehicles and that this trend will continue, as shown in figure 13, if no action is taken to change it (Schwarz 1985).

Figure 13.. Predicted trend of the fraction of vehicles meeting evaporative emission standards and the reasons why the remaining fraction does not meet the standards.

Figure 13.

Predicted trend of the fraction of vehicles meeting evaporative emission standards and the reasons why the remaining fraction does not meet the standards. Because the RVP of commercial gasoline is different from the RVP of certification fuel, a significant (more...)

The Coordinating Research Council-Air Pollution Research Advisory Committee (CRC-APRAC) is also investigating another important gasoline fuel issue—benzene emissions. Their preliminary findings from testing specially blended fuels in five late-model cars with three-way catalysts show that the benzene fraction of exhaust HCs increases with increasing benzene content and aromaticity. In refueling and evaporative emissions the benzene fraction increases with benzene content but not with aromaticity (Coordinating Research Council-Air Pollution Research Advisory Committee 1985).

Fuel Usage Trends

Although total gasoline usage has moved slowly upward during the past three years (from 6.5 million barrels per day (MMB/D) to 6.8 MMB/D by 1985), this trend may be temporary. The U.S. Department of Energy (1985) projects that gasoline demand will turn downward in the balance of the 1980s and remain flat in the early 1990s as shown in figure 14. By 1995, total gasoline consumption is projected to be 6.1 MMB/D (8.1 percent below 1983 levels). This number could be somewhat higher if the U.S. Department of Transportation establishes the post-1988 fuel economy standards at 26 mpg.

Figure 14.. Projected motor fuel consumption by fuel type.

Figure 14.

Projected motor fuel consumption by fuel type. (Adapted from the U.S. Department of Energy 1985.)

Total diesel highway fuel demand will continue to grow over the next two decades primarily because of increased use of diesel engines in heavy-duty vehicles. Total highway diesel fuel usage is projected to rise 30 percent from 1.0 MMB/D in 1982 to 1.3 MMB/D in 1995, as shown in figure 14. Figure 15 shows the breakdown of projected fuel usage by application including off-highway usage (U.S. Department of Energy 1985).

Image

Figure

Figure 15 . Projected motor fuel consumption by vehicle type. (Adapted from the U.S. Department of Energy 1985.)

Methanol-Fueled Vehicles

From an energy perspective, methanol is one of the most promising long-term alternative fuels for motor vehicles. It can be made from natural gas now and from coal later. One major practical problem is that motor vehicle consumption for a fuel has to reach 10 percent of the present market to create an economically viable free market (Society of Automotive Engineers/U.S. Department of Energy 1985). For the use of methanol to become widespread, it should be competitive in price with gasoline. Gasoline prices would probably have to exceed $1.50/gal (1985 dollars) for a significant period of time to provide the necessary confidence for investors in methanol processing facilities and car buyers (Sobey 1985). The EPA is, overall, encouraging the use of methanol as outlined by Gray (1985).

Spark-Ignition Engines for Passenger Cars. The technology for methanol-fueled vehicles exists and demonstration fleets have been tested. A summary of the emission results obtained to date for the 1983 Ford Escort fleet is given by Nichols and Norbeck (1985). Overall the vehicles averaged 6,800 miles with a range of 3,100 to 20,300 miles. The average formaldehyde emission rate varied from 54 mg/mi to 79 mg/mi and accounted for 7.0 to 8.8 percent of the reactive HC mass on a mole-of-carbon basis. The formaldehyde as a percent of reactive HC in the exhaust for any individual vehicle ranged between 5.0 and 17.9 percent.

A recent EPA summary of methanol emissions data has been documented by Alson (1985). The HC emissions are largely methanol, and the aldehydes are nearly all formaldehyde. Figure 16 is a summary of formaldehyde emission data (using the FTP) comparing methanol-, diesel-, and gasoline-powered vehicles, the latter with three-way catalysts, oxidation catalysts, and no catalyst (Alson 1985). Methanol-powered vehicles have higher formaldehyde emissions than diesel-powered or catalyst-controlled gasoline-powered vehicles.

Figure 16.. Comparison of formaldehyde emissions from methanol-, diesel-, and gasoline-powered vehicles, the latter with three-way catalysts, oxidation catalysts, and no catalyst.

Figure 16.

Comparison of formaldehyde emissions from methanol-, diesel-, and gasoline-powered vehicles, the latter with three-way catalysts, oxidation catalysts, and no catalyst. (Adapted from Alson 1985.)

Ford has recently discussed the concept of a methanol/gasoline flexible fuel system that would accept either methanol or gasoline. An electronic fuel-injected Escort was modified to use an optical fuel sensor for determining the methanol/gasoline mixture ratio. The sensor output is continuously processed by the electronic engine controller which optimizes fuel quantity and spark timing in response to the methanol/gasoline mixture ratio. This system was tested on a 1983 Escort-Lynx having a 1.6-liter electronic fuel injection engine in a production vehicle. The vehicle used the production engine compression ratio of 9.0, while the fuel tank, fuel filter, and the electric fuel pump were replaced with parts that methanol would not corrode. Figure 17 shows a schematic of the system in the vehicle (Wineland 1985).

Figure 17.. Schematic of Ford Escort modified to use a flexible fuel system installation.

Figure 17.

Schematic of Ford Escort modified to use a flexible fuel system installation. Electronic engine controller (EEC); electronic fuel injection (EFI); exhaust gas oxygen (EGO). (Adapted with permission from Wineland 1985, and the Ford Motor Co.)

Spark-Assisted, Compression-Ignition and Stratified-Charge, Spark-Ignition Buses. Methanol is considered to be a good choice as an alternative fuel for buses for several reasons. First, buses are usually a fleet operation so that methanol fuel distribution should be significantly easier than in the consumer market. Second, the particulate and odor emissions are less than those of the diesel engines it would replace. Third, the use of methanol should improve the reactivity of the exhaust, although the methanol and formaldehyde emissions could be a problem if control systems are not properly developed and maintained.

Lipari and Keski-Hynnila (1985) studied the effect of a catalyst on formaldehyde emissions of a methanol-fueled, two-stroke diesel bus engine and found that even with this catalyst, emissions were still higher than those from conventional diesels in the steady-state 13-mode cycle. Additional data are needed for the transient FTP cycle and for light-load low-temperature operation, since the production of formaldehyde across the catalyst could occur under certain operating conditions.

Areas in Need of Additional Research. Methods for accurately measuring HC emissions from methanol-fueled vehicles are lacking. The HC unburnt fuel of a neat (100 percent) methanol vehicle is basically methanol. Measurements of HC and formaldehyde concentrations have not been developed yet that can show how high the individual excursions are under acceleration, deceleration, and other transient conditions. There is a particular lack of data taken under light-load or idling conditions, especially of operation at low temperatures. Data show that the NO x concentration can increase as exhaust from methanol-fueled vehicles passes across a catalyst. Further work needs to be done to understand this effect and to control it properly in the field or determine if it is merely a measurement problem. There is a need to study the worst-case dispersion situations outlined by Harvey et al. (1984) using these new methanol and formaldehyde concentration data. Similar experimental field studies with real-time instrumentation should also be gathered so as to ensure that methanol technology is safe in the hands of the consumer. In these latter studies, misfueling and tampering should be monitored and their effects measured, for we know they occur in gasoline-powered vehicles.

■ Recommendation 4. Formaldehyde Measurements. Real-time measurements of formaldehyde concentration should be performed under transient and extreme conditions such as acceleration and deceleration, low temperature, light load, and extended idling with restricted ventilation. This research work should be done with and without catalysts since worst-case conditions in the field will occur with catalysts removed. Similar measurements should also be made on bus engines.

Trends in Diesel Fuel Properties

Recent trends in diesel fuel properties have an adverse effect on particulate emissions. They make it harder to meet stringent particulate emission standards for cars and trucks (0.2 g/mi in 1987 for cars and 0.6 g/bhp-hr in 1988, 0.25 g/bhp-hr in 1991 for trucks) because the EPA certification is based on typical in-use fuels. An automotive quality No. 2 diesel fuel with low sulfur and low aromatics is necessary if low particulate emissions are to be achieved (Weaver et al. 1986). Two important fuel characteristics affecting diesel engine emissions have been deteriorating in recent years: the cetane number has been falling and 90 percent boiling point has been rising, as shown in figure 18 (Wade and Jones 1984).

Figure 18.. Historical trends of diesel fuel properties.

Figure 18.

Historical trends of diesel fuel properties. Curve represents data from a DOE survey of type T-T diesel fuel. Open circles represent data from a MVMA survey of No. 2 diesel fuel. (Adapted with permission from Wade and Jones 1984, and the Society of Automotive (more...)

■ Recommendation 5. Automotive Quality No. 2 Diesel Fuel. An automotive quality No. 2 diesel fuel with low sulfur and low aromatics is necessary if low particulate emissions are to be achieved. Research should be undertaken in cooperation with the automotive and petroleum industries to decide on effective and economical cetane number, sulfur and aromatic content, and 90 percent boiling point temperature specification limits for automotive quality No. 2 diesel fuel, and to formulate fuels that meet the specifications. The need for this research becomes more urgent as diesel fuel usage continues to increase. Improved emissions and more control options require a quality low-sulfur diesel fuel.

Refueling Emissions

The basic source of HC emissions associated with the vehicle refueling process is the vapors contained in vehicle fuel tanks that are displaced by gasoline during refueling operations. However, additional emissions are associated with vehicle refueling operations as the result of “breathing losses” from underground storage tanks at gasoline service stations. Stage I (delivery of gasoline to station) vapor recovery is approximately 95 percent efficient (Austin and Rubenstein 1985).

At Stage II (dispensing of gasoline to vehicle fuel tanks) vapor recovery, gasoline vapors are collected at the vehicle fillpipe opening using a nozzle spout. The nozzle is also equipped with a vapor passage in the body of the nozzle that connects the annular space between the spout and the boot to the vapor space in the underground storage tank (Austin and Rubenstein 1985). A Stage II system reduces fillpipe emissions by 85–95 percent. Such systems are being used successfully in California.

The automotive industry, the petroleum industry, and the EPA are debating whether refueling losses should be controlled by onboard vehicle systems or by Stage II systems (Austin and Rubenstein 1985; Schwarz 1985). By use of onboard control systems, vapors displaced from the vehicle tank are vented to an enlarged canister where they are absorbed and subsequently purged into the engine (Austin and Rubenstein 1985; Schwarz 1985). A separate canister to control refueling emissions or an enlarged evaporative canister could be used.

In a further detailed analysis of Stage II and onboard control, Austin and Rubenstein (1985) reached the following general conclusion: “the implementation of Stage II controls is a clearly superior alternative to the onboard control concept.” Their specific reasons for this conclusion were:

1.

Stage II controls have been proven in California and they can achieve about 85 percent control.

2.

Stage II controls are the more cost-effective, that is, $0.21/lb HC are reduced versus $0.66 to $2.25/lb HC depending on whether the EPA's or Ford's cost estimate is used for onboard control. The onboard systems can be made more cost-effective with additional evaporative emissions control.

3.

Stage II controls give better short-term control because of lead time, and vehicle turnover due to replacement, among others as shown by Austin and Rubenstein (1985).

■ Recommendation 6. Evaporative Emissions Control. Some combination of field RVP control along with a test fuel with typical RVP (or calculation corrections for RVP differences) should be developed. Controlling RVP in motor gasoline, an approach successfully applied in California, is needed generally for controlling field evaporative emissions. The question of whether car manufacturers should be testing with a worst-case RVP test fuel or a typical fuel needs further study.

Additives

Additives are used to improve engine performance and durability and to ensure that fuel specifications and quality are maintained during transport and storage. They are an integral part of today's fuels. Tupa and Doren (1984) discuss in great detail the specific functions and benefits of additives, typical use levels, and test methods for evaluation. Generic types of additives and their uses are shown in table 4 along with general levels of additive treatment for the various types of additives. The variety of chemical compounds used in gasolines today are listed in table 5 and table 6 (Tupa and Doren 1984).

Table 4.. General Fuel Additive Classification and Typical Bulk Treatment Ranges.

Table 4.

General Fuel Additive Classification and Typical Bulk Treatment Ranges.

Table 5.. Chemicals Typically Used for Gasoline Additives.

Table 5.

Chemicals Typically Used for Gasoline Additives.

Table 6.. Chemicals Typically Used as Diesel Additives.

Table 6.

Chemicals Typically Used as Diesel Additives.

How additives may affect control technologies needs additional research. Knowledge of the size distribution of particles from diesel particulate traps and the metal species they contain. Data on operation with as well as without the working traps are needed since tampering of control devices can occur in the field. The effect of the additive compounds that plug the trap pores also needs further study.

■ Recommendation 7. Diesel Fuel Additives. Data should be obtained about the size distribution of particles in diesel exhaust and about the metal species they contain, with and without a particulate trap, with a diesel fuel containing a typical additive under consideration for production use. Data on the HCs bound to the particles and the vapor-phase HCs should also be obtained.

Methods for Measuring the Unregulated Pollutants*

The unregulated pollutants in automotive exhaust have been measured with varying degrees of sophistication for the past 20 years. Interest in a particular pollutant varies as studies of its potential health effects are reported; benzo[a]pyrene represents a good example. Since most of the unregulated pollutants are present only in small amounts in exhaust (in the parts-per million [ppm] range or less) and very small amounts in the ambient air (in the parts-per-billion [ppb] range or less) their amount or concentration is measured only with great difficulty and usually at high expense. There is little doubt that there is a need to balance the degree of difficulty, the cost, and the sensitivity against the real value the procedure produces in assessing health effects or engine performance.

Sampling

Since most of these pollutants are found at low concentrations, nearly all methods of analysis call for collecting a sample over an extended time interval and concentrating it before analysis. Samples are frequently collected by the use of impingers, filters (Evans 1980; Perez et al. 1980; Gorse and Salmeen 1982; Gross et al. 1982; Fox 1985), and solid sorbents (Hampton et al. 1982; Fox 1985).

Sampling of the exhaust may be more important in determining the value of the analysis than the actual measurement itself. For example, there is every reason to believe that during the sampling of particulates on a filter, chemical reactions take place between the organic compounds in the particulates and gaseous or aerosol compounds such as nitric acid (HNO3), NO2, and sulfuric acid. These reactions produce the so-called “artifacts of sampling” that are of concern to all who work in this field (Lee et al. 1980; Perez et al. 1980; Pierson et al. 1980; Gorse and Salmeen 1982; Herr et al. 1982; Risby and Lestz 1983). One of the possible sampling artifacts of greatest concern is the formation of the biologically active compounds nitropyrene and nitrobenz[a]pyrene from the reaction of NO2 with the relatively innocuous compounds pyrene and benzo[a]pyrene, respectively (Gibson et al. 1980; Schuetzle et al. 1980). Other artifacts of concern are the formation of HNO3 and sulfuric acid on the surface of the sampling material. The effects of artifact formation can be minimized by reducing the length of time a filter is exposed to the exhaust stream to the minimum required to collect a suitable sample, by using inert materials for filter construction, and by cooling and diluting the exhaust stream prior to sample collection.

Analytical Methods

Not every analytical method used for characterizing emissions from spark-ignition engines is applicable for analysis of emisbecause of interference from combustion products found in compression-ignition engines. Diesel engines produce higher levsions from compression-ignition engines, els of particulates, NO x , sulfur oxides (SO x ) and certain HCs, all of which can interfere with one or more of the analyses that are commonly used on spark-ignition engine emissions. Some real-time monitoring techniques based on the absorption of light fail when applied to diesel exhaust analysis, either because of scattering of light by suspended particulates or absorption of light by aromatic HCs present in the gaseous phase. Electrochemical methods are affected because particulates foul the membranes and electrode surfaces used in the measuring cells. Applying some of the methods used for continuous monitoring of chemical species in ambient air is even more difficult when one considers that spark-ignition as well as compression-ignition engines generate interfering species that affect the sensitivity, accuracy, and repeatability of the analyses.

The analytical methods used to measure concentration or amount of unregulated pollutants are summarized in table 7. It should be emphasized that these are the analytical methods presently used in laboratories where measurements are being made on a regular basis for judging engine performance. They meet current requirements but will not necessarily meet the requirements of the future.

Table 7.. Summary of Analytical Methods for Characterizing Unregulated Emissions from Spark-Ignition and Compression-Ignition Engines.

Table 7.

Summary of Analytical Methods for Characterizing Unregulated Emissions from Spark-Ignition and Compression-Ignition Engines.

We need to find out which of the unregulated pollutants must be measured to evaluate advanced technology for the control of emissions from gasoline as well as diesel engines. It would be folly to measure the concentrations of pollutants just because they are there. Present methods of analysis are so tedious, expensive, and unreproducible that unnecessary analyses are to be avoided whenever possible.

Areas in Need of Additional Research. Gaps in unregulated emission measurement methods center on the lack of real-time measurement methods that have the sensitivity required for producing results at moderate costs. If advanced emission control technology is to be studied with transient-cycle test protocols, these real-time techniques are necessary.

Real-time measurements based upon piezoelectric devices, tunable diode laser systems, thermal lens spectroscopy, long-path differential optical absorption spectroscopy, ultraviolet fluorescence spectroscopy, and differential absorption lidar have been reported by Fox (1985). The studies reported in these cases are normally of ambient air with no concern for interferences that may be present in gasoline and diesel exhaust. Pitts et al. (1984) reported the measurement of gaseous HNO3, NO2, formaldehyde, SO2, and benzaldehyde in the exhaust of light-duty vehicles, using an instrument that coupled a multiple reflection cell to a differential optical absorbtion spectrometer. The techniques hold promise for the future and should be explored in more extensive studies.

■ Recommendation 8. Emissions Measurement Methods. Studies should begin immediately with an evaluation of the best available emissions data on engines operating with and without emission control devices, to determine which of the unregulated pollutants really pose a potential threat to human health. Other unregulated pollutants might be added to this list if their concentrations reflect engine or emission control device performance. Next, every effort should be made to improve the analytical procedures presently used to measure the concentrations of those pollutants, to the point where they can be readily carried out by technicians. This may require that packaged sets of reagents and equipment be marketed for a specific analysis. For example, prepacked traps might be available for collecting gaseous HC prior to thermal desorption onto a gas chromatograph with a specified capillary column for the analysis of specific HCs at predetermined conditions.

Current Regulated and Unregulated Emissions

The main focus of this section is unregulated organic emissions, for significant data on regulated emissions and non-organic unregulated emissions from in-use vehicles have been presented already. MOBILE3, a computer model discussed earlier, is the best source of data about regulated emissions since the EPA analyzes all manufacturers' data and develops sales-weighted emission factors (U.S. Environmental Protection Agency 1985). Emission factors for regulated pollutants, based on California Air Resources Board (1980) data, are also available.

Regulated Emissions

Table 8 from the National Research Council (1983b) shows a summary of the regulated emissions from light-duty vehicles. Imposing the HC, CO, and NO x standards has resulted in 84, 79, and 56 percent reductions, respectively, in 50,000-mi emissions from gasoline-powered, sparkignition vehicles.

Table 8.. Exhaust Emission Rates for Light-Duty Gasoline-Powered Vehicles.

Table 8.

Exhaust Emission Rates for Light-Duty Gasoline-Powered Vehicles.

Unregulated Emissions

Gas-Phase Hydrocarbons. The components detected as gas-phase HCs are listed in table 9 (National Research Council 1983b). In another report, the National Research Council (1983a, appendix A) prepared an extensive list of vapor-phase compounds in both diesel- and gasoline-powered vehicles by reviewing 250 papers in the literature.

Table 9.. Unregulated Gaseous Hydrocarbons Emitted from Vehicles.

Table 9.

Unregulated Gaseous Hydrocarbons Emitted from Vehicles.

Diesel Exhaust Particulate . Diesel exhaust particulate material has been the subject of extensive study in the past five years. It is typically about 25 percent extractable into organic solvents, although different vehicles may have extractable fractions of 5–90 percent, depending to some extent on operating conditions. More than half the extractable material is aliphatic HC of 14–35 carbon atoms, alkyl-substituted benzenes, and derivatives of the polycyclic aromatic hydrocarbons (PAH) such as ketones, carboxaldehydes, acid anhydrides, hydroxy compounds, quinones, nitrates, and carboxylic acids. There are also heterocyclic compounds containing sulfur, nitrogen, and oxygen atoms within the aromatic ring. The alkyl-substituted PAHs and PAH derivatives tend to be more abundant than the parent PAH compound (National Research Council 1983b).

The particulate-extract in the high-performance liquid chromatograph (HPLC) eluent can be separated into nonpolar, moderately polar, and highly polar fractions. The fractions can then be further analyzed by gas chromatography/mass spectrometry (GC/MS). Table 10 lists the results of such an analysis of the nonpolar and moderately polar fractions of a particulate extract from an Oldsmobile diesel vehicle, including the approximate extract concentrations for this particular vehicle. The highly polar fraction has not been fully characterized. It contains the PAH carboxylic acids, acid anhydrides, and probably sulfonates and other highly polar species (National Research Council 1983b).

Table 10.. Qualitative Analysis of Nonpolar and Moderately Polar Fractions of Diesel Particulate Extract.

Table 10.

Qualitative Analysis of Nonpolar and Moderately Polar Fractions of Diesel Particulate Extract.

Most (75 percent) of the direct bacterial mutagenicity resides in the moderately polar fraction. The remaining direct mutagenicity is in the highly polar fraction. These aspects are discussed further in the National Research Council's report (1983b).

Over 50 chromatographic peaks of nitro-PAH compounds have been identified in diesel particulate extracts, as listed in table 11. The most abundant of the nitro-PAHs is 1-nitropyrene, ranging from 25 to 2,000 ppm in the vehicle extracts studies. The other nitro-PAHs are present at concentrations from below the ppm range to a few ppm. The nitropyrenes have been studied in greater detail than other PAH compounds. They are released in diesel and gasoline exhaust (according to particulate extracts) at rates of approximately 8.0 (diesel fuel), 0.30 (leaded gasoline), and 0.20 mg/mi (unleaded gasoline) (National Research Council 1983b).

Table 11.. Nitroarenes in Diesel Exhaust Particulate Extracts.

Table 11.

Nitroarenes in Diesel Exhaust Particulate Extracts.

1-Nitropyrene has been the only nitro-PAH detected in spark-ignition particulate extracts. Very low 1-nitropyrene particulate extract concentrations have been found recently in on-road heavy-duty diesel and light-duty spark-ignition vehicles (National Research Council 1983b).

Gasoline-Powered Vehicle Refueling Hydrocarbons. Williams (1985) has reported the concentration of HCs in the breathing zone of individuals during vehicle refueling. Gas chromatographic data for gasoline and the refueling vapor indicate that only the lower molecular weight, more volatile compounds are emitted. Williams concluded that:

1.

Vapor composition does not equal gasoline composition;

2.

Range of total HC concentrations varied widely with the environmental conditions, resulting in exposures from 8 to 3,000 ppmC; and

3.

Propane, butane, and pentane provide more than 80 percent of total exposure.

Areas in Need of Additional Research. To do efficient particulate control development work and to better understand emission characteristics, there is a need for a fast-response real-time particulate mass measurement instrument. The tapered element oscillating microbalance (TEOM) holds the most promise, but there is a gap between what is known about its principle of operation and the reality of making its use practical for measuring particulates.

The other instrument gap involves measuring methanol accurately. Formaldehyde has also been identified as a potentially important unregulated pollutant that needs careful real-time measurement and control because it is generally considered to be a carcinogen.

Measurement of particulate emissions from heavy-duty diesel engines using the EPA test procedures with dilution tunnels is inadequate. The current repeatability of measurements is poor. Barsic (1984) showed from a round-robin test that the root mean square of the 2-σ standard deviations were 76 percent of a 0.25 g/bhp-hr standard for six heavy-duty diesel engines tested in seven laboratories. For measurements intended to implement the 0.25 g/bhp-hr or the 0.1 g/bhp-hr standard, this variation is unacceptable.

It is uncertain whether particulate emission standards should be based on amount of total particulate matter, on which current standards are based, or amount of soluble organic component extracted from the particulates. The soluble organic component is the portion of the particulate that has been shown to be mutagenic and possibly carcinogenic (Claxton 1983), suggesting that future health-related regulations should be based on this fraction. Basing standards on the soluble organic component poses the problem of separating and quantifying the specific toxic components by one of the present methods—solvent extraction, vacuum sublimation, or thermogravimetric analysis. Variability associated with the separation methods and sampling condition affects the mass of the soluble fraction collected, compounding the previously stated measurement variability problem for the total particulate matter.

Present measurement methods for the collection of vapor-phase HCs from diesel engines do not collect all the compounds. Characterization of potentially toxic HCs is not possible if they cannot all be collected.

There is need to continue the development and use of advanced HPLC and GC/MS techniques in conjunction with separation methods to more accurately measure the amounts of key biologically active HCs in the particulate as well as the vapor phases. The nitroaromatics are important compounds whose concentrations in diesel exhaust with and without particulate traps should be measured more accurately.

There is a need to investigate and develop measurement methods that quantify diesel odor. Pioneering work was carried out in the late 1960s and early 1970s, but was dropped around 1978 because of the potential health effects of diesel particulate emissions. Diesel odor, along with particulates, is still the typical person's perception of the diesel pollutants that are of concern. There is a need to apply odor measurement methods to new engines used in light-duty and heavy-duty vehicles and advanced engines that use particulate traps or incorporate advanced high-temperature materials.

Refined organic compound measurement is particularly important to advance the development of low-heat-rejection (or commonly called adiabatic, as an ideal goal) diesel engines because their combustion chamber wall and gas temperatures will be higher. This elevated temperature will increase the amount of lubricating oil appearing as particulate emissions and has the potential of producing reactions between the HCs and oxygen/HNO3/NO x and other such gaseous mixtures to form toxic and biologically active species.

A particular need in unregulated pollutant characterization data for gasoline engines is additional nitrous acid (HNO2) data as an extension to Pitts et al. (1984). That paper showed higher levels of HNO2 from older (1974 and earlier) light-duty vehicles than from 1982 and newer cars that use three-way catalyst systems. The data show that even though the number of older cars is small, their HNO2 emission levels are so high that they may be the major source of all gaseous HNO2 from automotive emissions. HNO2 is a key precursor to photochemical air pollution and is also an inhalable nitrite.

There is little known about how and when the nitro-PAHs are formed in the exhaust system (or the dilution tunnel) of diesel engines. Flow reactor studies with the basic species—NO, NO2, CO, CO2, N2, O2, SO2, HCs—present in the exhaust, along with detailed engine studies that include the effects of the particles in the reactions, could help resolve this issue.

■ Recommendation 9. PAH Measurements. There is a need for a program of comparative measurements of PAHs in partial-exhaust sampling systems and in full-flow dilution tunnel systems, with measurements made in the atmosphere downwind from the plume, for the purpose of determining how well laboratory data reflect the true composition of emissions into the atmosphere.

■ Recommendation 10. Kinetics of Nitro-PAH Formation. Research is recommended to discover how and when nitro-PAHs are formed in the diesel engine exhaust system and dilution tunnel. This work can best be done by flow reactor studies of the basic gases in conjunction with detailed engine studies that include the actual HCs and particles.

■ Recommendation 11. Particulate Measurement Variability. Research is required to reduce the variability in heavy-duty diesel particulate measurements. Work needs to be undertaken to determine how to better control the parameters that influence this variability.

■ Recommendation 12. HC Characterization. There is a need for research on the complete characterization of particulate-phase and gas-phase HCs in diesel exhaust.

■ Recommendation 13. Diesel Odor. There is a need to investigate and develop analytical methods that quantify diesel odor. This research should take advantage of the knowledge gained in the past eight years about measuring particulate-bound and vapor-phase HCs.

■ Recommendation 14. Nitrous Acid. Additional data should be obtained about HNO2 emissions from older gasoline-powered vehicles. The literature shows high levels of HNO2 from older cars that may be contributing significantly to increased photochemical smog and direct effects.

Summary of Research Recommendations

HIGH PRIORITY

Based upon current information, the following research studies are most likely to yield useful data.
Recommendation 1
Tampering and Misfueling
Tampering and misfueling statistics are fairly well developed but their effect on emissions is not as well known. Therefore, work should be done to better characterize the effect of tampering and misfueling on emissions from vehicles and to better assess their effect on ambient pollutant concentrations.
Recommendation 2
Diesel Paniculate Emissions and Control
There is a need for continued research on particulate control technology, including the regeneration systems, to reduce the cost and complexity of these systems and the associated fuel economy penalties. Work needs to continue with various additives, substrate materials, regeneration systems, and controls to develop optimum systems that are able to decrease the diesel particulate emissions to the levels of 0.1 g/bhp-hr for heavy-duty diesels and 0.08 g/mi for light-duty vehicles required in California. In conjunction with this research there is a need to measure the metal species and the size distribution of the particles coming from diesel particulate traps.
Recommendation 4
Formaldehyde Measurements
Real-time measurements of formaldehyde concentration should be performed under transient and extreme conditions such as acceleration and deceleration, low temperature, light load, and extended idling with restricted ventilation. This research work should be done with and without catalysts since worst-case conditions in the field will occur with catalysts removed. Similar measurements should be made on bus engines.
Recommendation 7
Diesel Fuel Additives
Data should be obtained about the size distribution of particles in diesel exhaust and about the metal species they contain with and without a particulate trap, with a diesel fuel containing a typical additive under consideration for production use. Data on the HCs bound to the particles and the vapor-phase HCs should also be obtained.
Recommendation 10
Kinetics of Nitro-PAH Formation
Research is recommended to discover how and when nitro-PAHs are formed in the diesel engine exhaust system and dilution tunnel. This work can best be done by flow reactor studies of the basic gases in conjunction with detailed engine studies that include the actual HCs and particulates.
Recommendation 11
Paniculate Measurement Variability
Research is required to reduce the variability in heavy-duty diesel particulate measurements. Work needs to be undertaken to determine how to better control the parameters that influence this variability.
Recommendation 12
HC Characterization
There is a need for research on the complete characterization of particulate-phase and gas-phase HCs in diesel exhaust.
MEDIUM PRIORITY
Recommendation 3
Evaporative Emission Model
An improved vehicle evaporative emissions model should be developed that is valid over various types of operating conditions for a variety of ambient temperatures. At the same time, changes should be made in the EPA test procedure to obtain the data necessary to properly design and size the evaporative system for the high-temperature soak situation, and data should be sought that can be used in EPA's MOBILES computer model for other use patterns of cars.
Recommendation 5
Automotive Quality No. 2 Diesel Fuel
An automotive quality No. 2 diesel fuel with low sulfur and low aromatics is necessary if low particulate emissions are to be achieved. Research should be undertaken in cooperation with the automotive and petroleum industries to decide on effective and economical cetane number, sulfur and aromatic content, and 90 percent boiling point temperature specification limits for automotive quality No. 2 diesel fuel, and to formulate fuels that meet the specifications. The need for this research becomes more urgent as diesel fuel usage continues to increase. Improved emissions and more control options require low-sulfur diesel fuel.
Recommendation 6
Evaporative Emissions Control
Some combination of field RVP control along with a test fuel with typical RVP (or calculation corrections for RVP differences) should be developed. Controlling RVP in motor gasoline, an approach successfully applied in California, is needed generally for controlling field evaporative emissions. The question of whether car manufacturers should be testing with a worst-case RVP test fuel or a typical fuel needs further study.
Recommendation 8
Emissions Measurement Methods
Studies should begin immediately with an evaluation of the best available emissions data on engines operating with and without emission control devices, to determine which of the unregulated pollutants really pose a potential threat to human health. Other unregulated pollutants might be added to this list if their concentrations reflect engine or emission control device performance.
Next, every effort should be made to improve the analytical procedures presently used to measure the concentrations of those pollutants, to the point where they can be readily carried out by technicians. This may require that packaged sets of reagents and equipment be marketed for a specific analysis. For example, prepacked traps might be available for collecting gaseous HCs prior to thermal desorption onto a gas chromatograph with a specified capillary column for the analysis of specific HCs at predetermined conditions.
Recommendation 9
PAH measurements
There is a need for a program of comparative measurements of PAHs in partial-exhaust sampling systems and in full-flow dilution tunnel systems, with measurements made in the atmosphere downwind from the plume, for the purpose of determining how well laboratory data reflect the true composition of emissions into the atmosphere.
Recommendation 13
Diesel Odor
There is a need to investigate and develop analytical methods that quantify diesel odor. This research should take advantage of the knowledge gained in the past eight years about measuring particulate-bound and vapor-phase HCs.
Recommendation 14
Nitrous Acid
Additional data should be obtained about HNO2 emissions from older gasoline-powered vehicles. The literature shows high levels of HNO2 from older cars that may be contributing significantly to increased photochemical smog and direct effects.

Acknowledgment

I would like to thank Peter V.Woon for all of his assistance in the preparation of this paper.

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  • Footnotes

    *

    This section was written by David Leddy, Michigan Technological University.

    Air Pollution, the Automobile, and Public Health. © 1988 by the Health Effects Institute. National Academy Press, Washington, D.C.

    Correspondence should be addressed to John H.Johnson, Michigan Technological University, Department of Mechanical Engineering and Engineering Mechanics, Houghton, MI 49931.

    Copyright © 1988 by the Health Effects Institute.
    Bookshelf ID: NBK218144