4
Co-evolution of Technology and Emissions Standards

Emissions-control technology for mobile sources has developed in a series of interactive steps with the promulgation of emissions standards for new vehicles and engines and for fuel regulation. For light-duty vehicles, emissions-control hardware has changed greatly over the past 50 years to reflect the changes in emissions standards, vehicle design, fuel-efficiency standards, and technological capabilities. The efforts of motor vehicle manufacturers, the manufacturers of emissions controls and other equipment, the California Air Resources Board (CARB), and the U.S. Environmental Protection Agency (EPA) have made vehicles much cleaner and more durable. Although the relationship among the parties has not always been harmonious, it has produced benefits not only for the United States but also for the world. On-highway diesel vehicles have had emissions-control standards since the 1970s while EPA’s emissionscontrol activities have been authorized for nonroad sources only since the passage of the 1990 Clean Air Act (CAA) amendments.

This chapter discusses the basic elements of mobile-source emissions control, emphasizing the interaction of emissions-control research and new-equipment emissions standards. Chapters 6 and 7 contain a detailed discussion of several emissions standards developed by CARB and EPA. The descriptions of emissions controls in this report are not intended to be comprehensive. Heywood (1988) provided a comprehensive summary of the primary technical issues related to pollutant formation and control methods for light-duty vehicles before 1990. More recent publications discussing current research in combustion and pollution control are available from many publications, including those by the So-



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State and Federal Standards for Mobile-Source Emissions 4 Co-evolution of Technology and Emissions Standards Emissions-control technology for mobile sources has developed in a series of interactive steps with the promulgation of emissions standards for new vehicles and engines and for fuel regulation. For light-duty vehicles, emissions-control hardware has changed greatly over the past 50 years to reflect the changes in emissions standards, vehicle design, fuel-efficiency standards, and technological capabilities. The efforts of motor vehicle manufacturers, the manufacturers of emissions controls and other equipment, the California Air Resources Board (CARB), and the U.S. Environmental Protection Agency (EPA) have made vehicles much cleaner and more durable. Although the relationship among the parties has not always been harmonious, it has produced benefits not only for the United States but also for the world. On-highway diesel vehicles have had emissions-control standards since the 1970s while EPA’s emissionscontrol activities have been authorized for nonroad sources only since the passage of the 1990 Clean Air Act (CAA) amendments. This chapter discusses the basic elements of mobile-source emissions control, emphasizing the interaction of emissions-control research and new-equipment emissions standards. Chapters 6 and 7 contain a detailed discussion of several emissions standards developed by CARB and EPA. The descriptions of emissions controls in this report are not intended to be comprehensive. Heywood (1988) provided a comprehensive summary of the primary technical issues related to pollutant formation and control methods for light-duty vehicles before 1990. More recent publications discussing current research in combustion and pollution control are available from many publications, including those by the So-

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State and Federal Standards for Mobile-Source Emissions ciety of Automotive Engineers (SAE). In particular, the SAE Progress in Technology (Johnson 2002a,b; Johnson 2005a,b) series contains collections of SAE technical papers relating to specific technologies, including combustion and pollutant control in internal combustion engines. The Manufacturers of Emissions Control Association (www.meca.org) is another source for more comprehensive information on mobile-source emissions controls. TECHNOLOGY-FORCING STANDARDS A central concept of the standards-setting process for mobile-source emissions by EPA and CARB is “technology forcing.” The committee defines a technology forcing standard to be the establishment by a regulatory agency of a requirement to achieve an emissions limit, within a specified time frame, that can be reached through use of unspecified technology or technologies that have not yet been developed for widespread commercial applications and have been shown to be feasible on an experimental or pilot-demonstration basis. The use of technology-forcing standards in the United States contrasts with the standards-setting process in Europe where new emissions-control technologies on mobile sources are required only after they have succeeded in the U.S. market (Faiz et al. 1996). When controls on vehicle emissions were first considered in Los Angeles in the 1950s, requiring of devices that were not commercially available was not accepted as a viable approach. Los Angeles County Supervisor Kenneth Hahn was denied permission to require the installation of emissions-control devices on vehicles sold in the county by a county legal counsel opinion that stated, “Any such requirement would be “arbitrary, capricious, and void” until “a satisfactory device is perfected and available on the market” (Krier and Ursin 1977, p. 98, as cited in Lents et al. 2000, p. II-10). Lack of progress on air pollution control in California and elsewhere in the United States prompted Congress in the 1970 CAA amendments to feature ambient air quality standards with deadlines for their attainment and a technology-forcing program to reduce emissions from vehicles (Muskie 1990). Chief Senate sponsor, Senator Edmund Muskie, stated that the CAA was not “to be limited by what is or appears to be technologically or economically feasible” but “to establish what the public interest requires to protect the public health of persons” even if that means that “industries will be asked to do what

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State and Federal Standards for Mobile-Source Emissions TABLE 4-1 Major Control Technologies of New Light-Duty Vehicles Sold in the United States Model Year Noncatalyst Oxidizing Catalyst Three-Way Catalyst and/or Fuel Injection 1974 100% — — 1975 16.7 80.7 2.4 1976 16.2 81.7 2.1 1977 10.8 85.3 3.9 1978 8.4 88.0 3.6 1979 8.1 89.6 2.2 1980 0.0 94.4 4.6 1981 0.0 31.0 69.0 Sources: Bresnahan and Yao 1985; Gerard and Lave 2002. seems to be impossible at the present time.” (Committee Report Compiled for the Senate Committee on Public Works by the Library of Congress, Ser. No. 93-18, p. 227, 1974, 116 Cong. Rec. 32901-32902 [1970]). A central component of the CAA was the requirement that new-vehicle emissions standards would be reduced by 90% for carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HCs) by 1975, although it was not clear in 1970 how such a reduction would be achieved. The act allowed for any type of technology to be used. Although the date to attain that level of reductions was subsequently delayed, the approach resulted in manufacturers introducing the two-way catalyst to control HCs and CO in 1975 and the three-way catalyst to control HCs, CO, and NOx in 1981. Table 4-1 shows the penetration of the catalyst into the light-duty vehicle fleet. Using the 1970 requirements for light-duty vehicles, Gerard and Lave (2005) discuss some factors critical to implementing technology-forcing policies. Technology-forcing standards are not used only for light-duty vehicles. For example, the 1990 CAA amendments recommend that technology-forcing emissions standards be adopted for evaporative emissions and nonroad engines. Section 202(k) requires EPA to develop standards for evaporative emissions control that “shall take effect as expeditiously as possible and shall require the greatest degree of emission reduction achievable by means reasonably expected to be available for production during any model year to which the regulations apply, giving appropriate consideration to fuel volatility, and to cost, energy, and safety factors.” Section 213 of the 1990 CAA amendments requires that nonroad emissions standards, which at that time were uncontrolled, “achieve the great-

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State and Federal Standards for Mobile-Source Emissions est degree of emission reduction achievable through the application of technology which the Administrator determines will be available for the engines or vehicles to which such standards apply, giving appropriate consideration to the cost of applying such technology within the period of time available to manufacturers and to noise, energy, and safety factors associated with the application of such technology. In determining what degree of reduction will be available, the Administrator shall first consider standards equivalent in stringency to standards for comparable motor vehicles or engines (if any) regulated under section 202.” TECHNOLOGIES TO CONTROL LIGHT-DUTY-VEHICLE EMISSIONS Emissions-control hardware on motor vehicles can be grouped into three basic types: engine and exhaust controls, evaporative controls, and diagnostic controls. Brief descriptions of these emissions-control technologies are given below. A more complete description of control technology may be found in many sources, including Heywood (1988), Mondt (2000), and NRC (2001). In addition, Chapter 6 contains a discussion of emissions standards adopted since 1990, including the California low emissions vehicle (LEV) standards and the federal standards, and the technologies used to meet those standards. Box 4-1 describes the process of certification and enforcement of emissions standards for light-duty cars and trucks. Engine and Exhaust Controls Table 3-3 in Chapter 3 displays a chronology of federal and California exhaust emissions standards for new passenger vehicles. Allowable emissions for light-duty trucks have been reduced at a similar pace, although emissions standards for these vehicles were not as low as those for passenger vehicles until recently. Engine emissions are regulated by performance-based standards (as opposed to technology-based standards),1 but the standards typically result in certain classes of hardware 1   A performance-based standard is generally technology-neutral and sets an upper limit for emissions coming from a source; a technology-based standard dictates the specific control technology. See Glossary for complete definitions.

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State and Federal Standards for Mobile-Source Emissions being applied for compliance. The first nationwide emissions-reduction requirements were mandated for model-year 1968 and consisted of crankcase and engine controls. As shown in Table 4-2, these standards led to the use of positive crankcase ventilation (PCV) valves, higher air- Box 4-1 Certification of Light-Duty Vehicles to Emissions Standards Certification and enforcement of mobile-source emissions standards ensure their effective implementation. The general process of certification and enforcement is described here for light-duty cars and trucks. The requirements for light-duty-vehicle certification for EPA and CARB are similar because of collaborative efforts between the agencies and industry in the 1990s aimed at harmonizing EPA and CARB procedures to the greatest extent possible, streamlining the process for manufacturers, and shifting the focus to in-use testing of vehicles. These efforts resulted in EPA’s adoption of the Compliance Assurance Program 2000 (CAP2000) described further in its 1998 notice of proposed rule-making (NPRM) (63 Fed. Reg. 39654 [1998]) and 1999 Final Rule (64 Fed. Reg. 23906 [1999]), and CARB’s adoption of the elements of the CAP2000 program with its LEV II rule-making. Certification and enforcement occur in two phases: pre-production certification and in-use testing (enforcement), which are described in the following paragraphs based on information in EPA’s Final Rule (64 FR 23906) and CARB’s LEV II staff paper “Initial Statement of Reasons” (CARB 1998a ). A separate issue relates to the enforcement of sales of California-certified vehicles in California and states that have adopted those standards. Such enforcement is connected to vehicle registration, since California-certified vehicles can be identified by the vehicle identification number (VIN). The committee did not investigate sales enforcement practices. Before to selling a vehicle in the California or national market, manufacturers must submit to CARB or EPA, respectively, test-data that demonstrate the vehicle will meet corresponding emissions standards. Data must be submitted yearly for each engine family (group of vehicles with the same engine and emissions-control characteristics). The agencies then approve the vehicle for sale by issuing a certificate of conformity (EPA) or executive order (CARB). Preproduction testing must demonstrate that vehicles will comply with emissions standards at the end of their useful life, which is 120,000 mi for present-day light-duty vehicles. Compliance is demonstrated by determining the deterioration characteristics of emissions-control

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State and Federal Standards for Mobile-Source Emissions equipment (durability demonstration) and then showing that production vehicles will meet emissions standards using this same equipment and configuration (certification testing). Manufacturers have some flexibility in durability demonstrations, although their methods must be approved by the agencies. Durability demonstrations are performed on one prototype representing a broad group of vehicles with similar deterioration characteristics (durability groups) rather than on each engine family or vehicle model. Bench-aging of catalysts is one method for testing durability of emissions-control systems, as opposed to accumulating full useful-life mileage on an actual vehicle. Once the useful-life emissions of an emissions control system/configuration have been determined, it must be shown to result in emissions below the certification levels on production vehicles. One prototype vehicle from each test group (vehicles of the same emissions rates and certification levels) is used to demonstrate compliance, for example, by attaching a bench-aged catalyst and measuring the emissions according the defined federal test procedure (FTP). California and EPA certify vehicles in this same framework, but manufacturers must certify separately with each agency because certification categories are different in the California and federal programs (see Chapter 6 for more information on the LEV II and Tier 2 programs). Furthermore, there are differences between CARB and EPA in test procedures—for example, the ambient temperatures at which vehicles must be certified. In-use testing comprises the second part of the certification and enforcement program. Both agencies procure vehicles and test their emissions in-use to detect those that might not be achieving emissions levels at their certification levels. Manufacturers are also required to procure vehicles and test them in-use for emissions at low (10,000 mi), medium (50,000 mi), and high mileage (75,000-105,000 mi). These vehicles are tested as is; they are not screened for proper maintenance. If the vehicles do not meet emissions requirements, the manufacturers must test other in-use vehicles of this model to ascertain whether the emissions-control equipment functions properly. Testing data are provided to the agencies to help them target other vehicle models that might not comply with emissions standards. If in-use testing shows that certain vehicle models are not meeting their certification levels, remedial action may be required. CARB and EPA may order a vehicle recall at the manufacturers’ expense. A secondary benefit of in-use testing is that it provides EPA and CARB with in-use emissions data to be used in accurately assessing and predicting vehicle emissions. The in-use testing data also helps manufacturers to evaluate how well their durability demonstrations simulate actual deterioration.

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State and Federal Standards for Mobile-Source Emissions TABLE 4-2 Summary of Emissions Controls Used to Meet Federal Standards for Light-Duty Vehicles Engine Emissions Controls • Crankcase controls (1961 to present) —PCV used to recycle hydrocarbons that leak past the piston rings into the crankcase back into the combustion process. • Engine adjustments (1968 to present) —Primary control consisted of modifications to mixture strength and spark timing as well as EGR. • Oxidizing catalysts (1975 to present) —Lean mixtures and oxidization catalysts were used for HC and CO control. EGR was used to control NOx. Air-pump or pulse-air valves were incorporated. • Unleaded gasoline phase-in (enabling technology for catalysts) (1975 to present) • Closed loop three-way catalysts (1981 to present) —Precise mixture control and three-way catalysts control HCs, CO, and NOx; >90% removal of each. Electronic Controls and Onboard Diagnostic (OBD) Systems • Onboard computers, oxygen sensors (1981 to present) • Preregulatory OBD Systems (1981 to 1993) —GM and Ford had OBD systems starting on 1981 models. • OBDI (1994 to 1995) • OBDII (1996 to present) Evaporative Emissions Controls • Early trap test technology (1971 to 1977) —Tank and carburetor bowl were vented to a small carbon canister. • Early sealed housing for evaporative determination (SHED) test technology (1978 to 1995) —Material in the detail seals on the carburetor is claimed for reduced permeation and increased purge. • Enhanced evaporative emissions controls (1996 to present) —Three-day diurnals, measuring running losses, high-temperature hot soaks, and 10-year life required larger canisters and more permeation control. Refueling controls were added to cars starting in 1998. Source: NRC 2001.

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State and Federal Standards for Mobile-Source Emissions to-fuel ratios, spark-timing modifications, and external exhaust-gas recirculation (EGR). As described in Chapter 3, these standards were modeled after standards mandated for California vehicles a few years earlier. Emissions-control systems used on the precatalyst vehicles of 1968-1974 relied on combustion controls, and some aspects of these controls compromised the performance of the engine, resulting in degraded drivability and fuel economy and promoting the practice of tampering with the emissions-control system (Bresnahan and Yao 1985; NRC 2001). In contrast, EGR systems developed during this precatalyst period retained most of the vehicle performance while lowering tailpipe NOx. Such systems are still in use today. Figure 4-1 shows a diagram of a modern EGR system. Higher combustion temperatures result in greater NOx formation. The EGR valve acts to recirculate exhaust gas in controlled amounts to dilute the air-fuel mixture, which lowers the peak combustion temperature and NOx formation. EGR is also used to reduce NOx emissions from diesel engines. Exhaust emissions-control devices consist of catalytic converters and air injection systems. The first generation of catalytic converters added to model-year 1975 vehicles promoted the oxidation of HCs and CO by passing the exhaust over a bed containing small amounts of platinum, palladium, and rhodium. These were known as two-way or oxidation catalysts. A critical regulatory requirement for technology was the use of unleaded gasoline since the combustion by-products of tetra-ethyllead, an additive used to increase gasoline octane quality, were found to reduce catalyst conversion efficiency. Stricter model-year 1981 standards provided another challenge. A development known as the three-way catalyst, which provides control of NOx in addition to CO and HCs, came into use and continues to be a central component of emissions controls. Figure 4-2 shows a diagram of a three-way catalyst. Another key technological development needed for the three-way catalyst is the adoption of electronic controls to tightly maintain the air-to-fuel ratio2 through the metering of fuel. Over the years since the three-way catalyst was first introduced, important im- 2   The air-to-fuel ratio is the ratio of the weight of air to gasoline entering the intake in a gasoline engine. The ideal ratio for complete combustion is 14.7. This ratio is called the stoichiometric air-to-fuel ratio. Air-to-fuel ratios less than 14.7 are called “rich” and contain excess fuel for complete combustion; air-to-fuel ratios greater than 14.7 are called “lean” and contain more air than is required for complete combustion.

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-1 Exhaust gas recirculation. Source: UTI 2005. Reprinted with permission; copyright 2005, Isuzu Motors, Japan. FIGURE 4-2 Schematic of a three-way catalyst. Source: MECA 2003. Reprinted with permission; copyright 2004, Manufacturers of Emissions Control Equipment.

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-3 Schematic of closed-loop controls. Source: Kyocera 2006. Reprinted with permission from Kyocera; copyright 2006, Kyocera International Inc. provements have been made in catalyst efficiency and life and exhaust gas sensors. The scope of improvement has allowed the three-way system to achieve the progressively stricter California and federal standards. Electronic Controls and Onboard Diagnostics Electronic controls on light-duty vehicles involve the use of onboard computers, sometimes known as engine-control units, and oxygen sensors and enable the adoption of closed-loop fuel control. Closed-loop control consists of the use of oxygen sensors in the exhaust and fuel-rate adjustment capability in the carburetor or fuel injection system. Figure 4-3 shows a schematic representation of the closed-loop system. The front (or upstream) oxygen sensor monitors the efficiency of combustion in the engine and allows the creation of a self-adjusting fuel metering system that maintains the air-to-fuel ratio within a very narrow range. As shown in Figure 4-4, the ability to decrease NOx, CO, and HCs simultaneously in a three-way catalyst depends on the accurate control of the air-to-fuel

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-4 Catalyst conversion efficiency as a function of air-to-fuel (A/F) mixture ratio. Source: Adapted from Canale et al. 1978. Reprinted with permission; copyright 1978, SAE International. ratio. The rear (or downstream) oxygen sensor monitors the efficiency of the catalytic converter. Onboard computers are also able to control ignition timing, transmission gear changes, and in a few engines, even valve timing. Onboard diagnostic (OBD) systems are incorporated into the computers of vehicles to monitor the performance of the emissions controls. OBD hardware and software do not directly control emissions but are a vital part of emissions-control systems by monitoring various engine functions, including the emissions-control system. Some manufacturers incorporated OBD on a voluntary basis in model-year 1981 to help with the service and reliability of their vehicles (Grimm et al. 1980; Gumbleton and Bowler 1982). The OBD system is made up of the sensors and actuators used to monitor specific components as well as the diagnostic software in the onboard computer. California regulators recognized the potential of the OBD system, expanded the scope, and required it on new

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State and Federal Standards for Mobile-Source Emissions vehicles starting with a 1988 model-year phase-in. California and EPA expanded the scope and coverage of diagnostics with the OBD II regulations, which were phased in beginning with the model-year 1994 vehicles. All light-duty vehicles built after 1996 (with a few exceptions) are equipped with the OBD II system. OBD II periodically checks many emissions-control functions, including the following components: catalysts, oxygen sensors, evaporative canister purge system, fuel-tank leak check, misfire detection, and onboard computers. As indicated in Figure 4-3, the rear oxygen sensor is required as a part of the OBD II—OBD I did not require the monitoring of the performance of the catalytic converter. Cold-Start Emissions Control HC and CO emissions are higher during starting and the first few minutes of vehicle operation. Under cold-start conditions, the engine computer commands the fuel injectors to add excess fuel to the intake air to ensure that enough fuel evaporates to yield a flammable mixture in the engine. A typical engine-computer strategy injects excess fuel during the first engine start using a fixed fueling schedule to reach idling conditions. This open-loop operation, before the catalyst reaches peak efficiency, can continue for several minutes at low ambient temperatures. During this period, the engines in properly operating modern vehicles have the highest emissions rates of CO, air toxics, and unburned HCs. Typical heat-up times under mild ambient conditions (70-80F) can be about 1 minute (min) for modern catalysts and even as short as a few seconds for modern close-coupled catalysts (catalysts close to the engine). When ambient temperatures are 20F or lower, however, catalyst and engine warm-up times can exceed 5 min (Sierra Research 1999). The long warm-up time can result in a substantial increase in emissions, as shown in Figures 4-5 and 4-6. In some locations, cold-start emissions can also be a large fraction of the emissions inventory. For example, in Fairbanks, Alaska, winter cold-start and initial-idle emissions contributed an estimated 45% of overall on-road CO emissions (NRC 2002a). Because of the importance of cold-start CO emissions, new cars and the lightest category of light-duty trucks (LDT1) have been required since 1994 to meet a CO limit of 10 g/mi on certification tests conducted at 20°F. To meet stricter emissions standards, including the cold-start CO standards and federal Tier 2 and California LEV II standards, vehicle

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-5 Average CO emissions for a 111-vehicle test sample taken in Fairbanks and Anchorage, Alaska, under temperatures ranging from 34° F to 14° F (“94 & Newer” applies to model years 1994-1998). Source: Sierra Research 2000. Reprinted with permission; copyright 2000, Sierra Research, Inc. FIGURE 4-6 Average HC emissions for a 111-vehicle test sample taken in Fairbanks and Anchorage, Alaska, under temperatures ranging from 34° F to 14° F (“94 & Newer” applies to model years 1994-1998). Source: Sierra Research 2000. Reprinted with permission; copyright 2000, Sierra Research, Inc.

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State and Federal Standards for Mobile-Source Emissions manufacturers and suppliers focused much effort on reducing cold-start emissions. This has been accomplished with low thermal inertia manifolds, ignition timing changes, and leaner air-fuel mixtures during cold start along with low-heat-capacity catalysts with higher geometric surface area and improved catalytic layer design. Evaporative Emissions Control Evaporative emissions are the HCs that escape from the vehicle that do not come from the tailpipe. Originally, losses due to the evaporation of residual fuel in the fuel metering system and diurnal fuel tank losses were of concern. Running losses (evaporative emissions during vehicle operation) also have been found to contribute to vehicle fuel emissions. Evaporative emissions are evaluated using the Sealed Housing Evaporative Determination (SHED) test developed by General Motors. The uncertainties in estimating the evaporative contribution to mobile-source HC emissions are large. Evaporative emissions were first controlled nationwide in model-year 1971. Residual gasoline fuel vapors in the carburetor and fuel tank were routed to a small (about 1 liter) container of activated carbon for temporary storage and eventual use by the engine. Figure 4-7 shows the design of a carbon canister. The basic design of evaporative control hardware has not changed much since 1971, but as standards on evaporative emissions became more stringent with the conclusion of running losses, control effectiveness increased greatly due to improvements in materials, understanding, and measurement techniques. For example, Babik (2005) reported that General Motors would use less-permeable materials to construct fuel tanks, reduce the number of connections in the fuel lines, increase the size and efficiency of the carbon canister, and make other modifications to meet new California evaporative emissions standards. Fuel Composition and Emissions-Control Technologies Fuel composition and quality are intrinsic in the design, development, and performance of vehicle systems to meet the tailpipe and evaporative emissions regulations. Some regulations can be considered stand-alone programs, such as those for reformulated fuels or oxygenated fuels. These are outside the scope of this report. Other properties affect

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-7 Cross-section of a carbon canister. Source: TTM 2004. Reprinted with permission; copyright 2004, Tightrope Technologies Motors, Inc. the operation of emissions-control equipment. An important example is the removal of lead from gasoline. The removal of lead from gasoline, beginning in 1975 and completed by 1992, enabled the widespread adoption of catalysts. Other advantages came with the adoption of catalysts and the use of unleaded gasoline, including increased spark-plug and engine life, longer exhaust-system life, and extended oil-change intervals. In addition, the health effects of airborne lead were reduced. The sulfur content of fuel has also been recognized to adversely affect the performance of catalyst technology (MECA 1998). This was a key finding of the Auto-Oil Project (Benson et al. 1991), an industry-funded multiyear multicompany research initiative that helped to bring about the introduction of reformulated gasoline. Among this project’s conclusions was that reducing sulfur concentrations from 450 to 50 ppm would result in over a 10% decrease in CO and HC exhaust emissions in 1990 model year vehicles. Although the impacts of sulfur are not as severe as the impacts of lead in gasoline, sulfur in fuel is converted during combustion to various sulfur-containing compounds that react with the catalyst surface and in-

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State and Federal Standards for Mobile-Source Emissions hibit the removal of other target pollutants. To address concerns about the increased sensitivity of the newer technology in vehicles to sulfur poisoning, EPA included new fuel standards that require refiners to meet an average sulfur concentration of 30 ppm beginning by January 1, 2006, in its Tier 2 rule (65 Fed. Reg. 6697 [2000]). Low-sulfur fuel provided by the Tier 2 regulation is expected to improve catalyst performance of many on-road light-duty vehicles by up to 30% for HCs, CO, and NOx. Meeting an average sulfur content of 30 ppm has been required in California gasoline since 1996. TECHNOLOGIES TO CONTROL HEAVY-DUTY-VEHICLE EMISSIONS Current federal regulations do not require certification of complete heavy-duty diesel vehicles, requiring instead certification of only the engines. This is because of the difficulty in devising per mile limits for the broad range of vehicles covered and the difficulty in developing a practical chassis dynamometer test. Consequently, the basic standards are expressed in grams per brake horsepower-hour (g/bhp·hr) instead of grams per mile, the unit used for cars and light trucks. Early regulations on heavy-duty diesel engines began in 1968 when the National Air Pollution Control Administration, the agency that set emissions standards before the founding of EPA, issued regulations to limit visible smoke emissions from diesel engines used in on-road trucks and buses. The technology used to achieve less smoke involved increasing the air-to-fuel ratio by turbocharging, intercooling (cooling the engine intake air to lower NOx emissions), and on some models, limiting the fuel rate on acceleration and adjusting the engine timing. On-road heavy-duty diesel vehicle emissions standards were implemented in California in 1973 and the rest of the United States in 1974, the standards were harmonized in 1988 (Lloyd and Cackette 2001). It was not until the 1977 CAA amendments that technology-forcing requirements for diesel particulates and NOx were adopted, calling for heavy-duty diesel engines to achieve the greatest emissions reduction achievable consistent with consideration of costs, technology feasibility, and other factors. These standards became more stringent throughout the 1990s, including more stringent particulate matter standards for urban buses. Diesel engine and combustion technology during 1988-2000 was vastly improved, more than was achieved in the first 100 years of the diesel engine. Through

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State and Federal Standards for Mobile-Source Emissions model-year 2006, the standards were met primarily through engine-operation modifications, including fuel injection, electronic engine controls, combustion chamber design, air handling, and reduced oil consumption (MECA 1997). In particular, the standards were attained through improved diesel combustion systems using high-pressure electrically actuated fuel injectors. Variable injection timing and multiple injections during a combustion event were possible through computer control. EGR is also becoming popular on diesel engines as a means to reduce NOx. In early 2001, EPA issued new, more stringent regulations on emissions from heavy-duty vehicles (65 Fed. Reg. 59896 [2000]; 66 Fed. Reg. 1535 [2001]). These regulations tighten emissions standards and require a decrease in the fuel sulfur content, strategies similar to those adopted for light-duty vehicles. Low sulfur fuel is an important prerequisite for developing technologies to lower PM emissions. The regulations that will be phased in for 2007 to 2010 model-year vehicles will reduce PM and NOx emissions by at least 90% compared with current standards. The use of new exhaust emissions-control technologies will be required to meet the more stringent standards for diesel engines. These standards force examination of a range of technologies, including diesel particle filters (DPF), selective catalytic reduction (SCR) of NOx with ammonia, and NOx absorber catalysts. Figure 4-8 shows a diagram of an SCR system. Chapter 7 contains a discussion of these standards and the technologies that will be used to meet these standards. Gasoline heavy-duty engines, like those used in passenger vehicles, have benefited from the application of high-energy ignition (HEI), positive crankcase ventilation (PCV), exhaust gas recirculation (EGR), and oxidation catalyst technologies. Subsequently, more stringent heavy-duty gasoline engine regulations were met using a heavy-duty version of the three-way catalyst and closed-loop oxygen-sensor fuel-metering system proven in passenger cars. TECHNOLOGIES TO CONTROL NONROAD SOURCES FROM LAWNMOWERS TO LOCOMOTIVES As emissions from on-road sources were reduced, emissions from nonroad sources became a more important issue. Nonroad sources include the following engine categories:

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State and Federal Standards for Mobile-Source Emissions FIGURE 4-8 Schematic of selective catalytic reduction (SCR) system for NOx reduction. Source: Omnitek 2006. Reprinted with permission; copyright 2006, Omnitek Engineering Corporation. Compression-ignition-equipment engines (construction and mining, agriculture). Small spark-ignition engines (lawn mowers, chain saws) Large spark-ignition engines (forklifts, generators) Marine compression-ignition vessel engines (commercial, recreational inboard) Marine spark-ignition vessel engines (jet skis, personal watercraft) Recreational vehicles (all-terrain vehicles [ATVs], snowmobiles, motorcycles) The 1990 CAA amendments directed EPA to prepare a study of the scope and sources of nonroad emissions and to regulate them if they were found to make a substantial contribution to nonattainment of ozone or CO ambient air quality standards. The EPA report did not make a formal determination of a significant effect, but it contained an inventory

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State and Federal Standards for Mobile-Source Emissions of emissions from nonroad sources and concluded, “because nonroad sources are among the few remaining uncontrolled sources of pollution, their emissions appear large in comparison to the emissions from sources that are already subject to substantial emissions-control requirements” (EPA 1991). Space does not permit a full description of all regulatory actions by EPA and CARB. A few categories of such sources and their emissions-control technologies are provided. Chapter 7 contains a discussion of recent emissions standards for small nonroad spark-ignition engines and personal watercraft. Nonroad emissions are the new frontier for mobile-source emissions control for EPA and CARB. Table 4-3 lists some of the possible emissions-control technologies for nonroad source. The multiple engine types in nonroad sources and their variety of uses are likely to require multiple emissions standards. Different nonroad sources will need to have sets of standards and procedures for demonstrating compliance. Although some control technology, such as the oxidation catalyst, is well understood, the challenge will be in applying this technology to specific engine applications. Transfer of technology to farm and construction equipment from heavy-duty diesel and gasoline engines involves different duty cycles, environment, and durability needs. Nowhere is the coop- TABLE 4-3 Possible Emissions-Control Technologies for Nonroad Mobile Sources • Spark-Ignition Engines Fuel injection and feedback control systems Exhaust gas recirculation systems Three-way catalysts and advanced catalyst systems High-energy ignition Hybrid electric systems Advanced combustion system design and control • Compression-Ignition Engines Turbocharging Intercooling Cooled exhaust gas recirculation systems Oxidation catalysts Selective catalyst reduction system Lean NOx catalysts NOx storage catalysts Catalyzed particulate filters Hybrid electric systems

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State and Federal Standards for Mobile-Source Emissions erative evolution of technology and standards more appropriate than that for nonroad sources. Nonroad Diesels and Locomotives As described in Chapter 3, locomotives and nonroad engines used in farm and construction equipment with engines smaller than 175 horsepower (hp) have been and continue to be exempt from state regulation. Locomotive engines operate on diesel fuel and can be either two- or four-stroke design engines. EPA has regulated locomotives nationwide since 2000. Engines smaller than 175 hp used in farm and construction equipment can be either gasoline or diesel powered and are primarily four-stroke design. Nearly all large farm and construction equipment is diesel powered and can share control technology with heavy-duty trucks. Spark-ignition engines in farm and construction applications may use versions of gasoline heavy-duty-truck emissions controls. Handheld Engine Applications Handheld engine applications can be two- and four-stroke gasoline-fueled engines. String trimmers, leaf blowers, and chain saws rely on the low weight, compact high power of two-stroke gasoline engines. As discussed in Chapter 7, two-stroke engines emit large amounts of unburned fuel and therefore emit more HC emissions than four-stroke engines (see also Boyle 2002). Substituting a larger, heavier four-stroke engine poses challenges in weight, cost, and product performance, yet many four-stroke engines are being developed with lower exhaust emissions. Some applications (for example, European chain saws) include some emissions-control technology. However, the capability of such control technology to achieve EPA and state chain saw emissions regulations and durability remains to be determined by manufacturers. Lawn Mowers Most lawn mowers are powered by four-stroke spark ignition engines, although a few are powered by electric motors. Most gasoline-powered lawn mowers have high HC and CO emissions during operation

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State and Federal Standards for Mobile-Source Emissions and high evaporative losses during refueling, operation, and storage. Catalyst use to reduce exhaust emissions presents some complications for this equipment. The heat release when HCs and CO are oxidized can result in high exhaust temperatures and hot catalytic-device surfaces. The application of catalytic exhaust systems can increase the risk of fire during operation, refueling, and storage. Improved four-stroke engine designs and catalytic-device shielding might reduce the risk of fires. New technologies, such as stratified scavenging, have been found to improve two-stroke engine emissions performance. These technologies create a separation of the exhaust flow and the intake flow by creating pressurized pockets that prevent the flow of fresh fuel charge into the combustion chamber until the exhaust cycle is complete. Unburned fuel (HCs) is thereby reduced. The most recent CARB standards for these engines, including fuel evaporative controls, are discussed more in Chapter 7. Watercraft and Related Two-Stroke Engine Applications Personal watercrafts, such as jet skis, are powered by two-stroke engines for their superior power-to-weight ratio. Lower emissions and direct-injected two- and four-stroke engines are being developed for these applications and for snowmobiles. Even though the four-stroke engine emits much less HC than a two-stroke engine, emissions-control equipment to reduce engine-out emissions of CO and HCs may be necessary to meet regulations. Chapter 7 contains more information on recent emissions standards for jet skis. CONCLUSIONS The basic elements of mobile-source emissions control result from the co-evolution of emissions-control research and the promulgation of vehicle and engine emissions standards. A central concept of the standards-setting process for mobile-source emissions is technology forcing. A technology-forcing emissions standard requires a new vehicle or engine to achieve an emissions limit through use of unspecified technology or technologies that have not yet been developed for widespread commercial applications. A review of emissions-control technologies emphasizes two general conclusions.

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State and Federal Standards for Mobile-Source Emissions The concept of technology forcing is central to the standards-setting process for CARB and EPA. It has been applied to a wide range of sources, including light-duty vehicles, on-road diesel engines, and nonroad engines, for the control of CO, HCs, and NOx. Over the almost 50 years of mobile-source emissions regulations, controls have evolved from the use of simple technologies to control light-duty vehicles to today’s sophisticated integration of engine, fuels, and emissions-control technologies to control emissions from an array of mobile sources. Compared with emissions rates of 1967 model-year light-duty vehicles, the rates of new, properly operating vehicles decreased by 95-99%.