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2 Vehicle Emissions-Control Technologies Inspection and maintenance (~/M) PROGRAMS were created to ensure that motor vehicle emissions-control systems operate properly throughout the life- time ofthe vehicle. They do so by identifying vehicles with higher then allow- able emissions and requiring them to be repaired or removed from the fleet. Therefore, it is important to understand the functions ofthe basic components of motor vehicle emissions-conkol systems. As outlined in Chapter I, emis- sions-conko] hardware has changed over time to reflect changing emissions standards as well as changes in vehicle design, fuel efficiency standards, and technological capabilities. Emissions controls can be grouped into three basic types: engine, evaporative, and diagnostics. Each of these can be further divided as follows (years of introduction in parentheses): Engine Emission Controls · Engine Adjustments (l 968 to ~974) Primary control consisted of modifications to mixture strength and spark timing. · Oxidizing Catalysts (1975 to 1980) Lean mixtures and oxidization catalysts were used for hydrocar- bons (HC) and carbon monoxide (CO) control. Exhaust gas recirculation (EGR) was used to control nitrogen oxides (NOx). · Closed-Loop Three-Way Catalysts (1981 to current) Precise mixture control and three-way catalysts control HC, CO, and NOx 46
Vehicle Emissions-Control Technologies 47 Evaporative Emission Controls · Early Trap Test Technology (1971 to 1977) Tank and carburetor bowl were vented to a small carbon canister. · Early seated housing for evaporative determination (SHED) Test Technology (1978 to 1995) Material in the detail seals on the carburetor are claimed for re- duced permeation and increased purge. · Enhanced Evaporative Emissions Controls (1996 to 2003) ~ . 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. On-Boar1 Diagnostic Systems (OBEY · Preregulatory Systems (1981 to 1987) GM and Ford had OBD systems starting on 1981 models. · OBDI (1988 to 1995) · OBDIT (1996 and beyond) These are described in greater detail below. OVERVIEW The first emissions-reduction requirements were mandated nationwide for model-year ~ 968, and they consisted of crankcase and engine controls. The emissions regulations were written as performance-based standards (as op- posed to technology-based) but resulted in the application of certain classes of hardware for compliance. First-generation catalytic converters (two-way catalytic converters) were added in 1975 to provide significant after-treatment reductions of HC and CO. The enablers were the development of catalyst technology and the nationwide availability of lead-free fuel. The next major innovation, which enabled improved emissions and perfor- mance, was the development of computer controls on vehicles. On-board computers permitted the adoption of closed-Ioop fuel control and three-way catalytic converters (able to provide after-treatment control of NOX in addition to CO and HC) in model-year 1981. Closed-loop fuel control consists of the
48 Evaluating Vehicle Emissions I/M Programs aciclition of an air-to-fuel ration sensor in the exhaust anc} fuel rate adjustment capabilityin the carburetor or filet injection system. The air/fuel sensorpro- vicles system feedback and allows the air/fuel ratio to be adjuster! to very precise values. The ability to simultaneously recluce NOx, CO, anc! HC in a three-way catalyst depencis on accurate control of the air/fuel ratio. In con- trast, earTieropen-Ioop systems clicinot have these feecibackmechanisms anc! clepenclec} on the initial calibration anc! adjustment of the carburetor or fuel injection settings. Open-Ioop systems are sensitive to atmospheric pressure, temperature, fuel properties, and wear and, therefore, suffer from variable . . . emissions c unrig use. Evaporative emissions were first controllec! nationwicle2 in moclel-year ~ 97 ~ . Carburetor and fuel-tank vapors were router! to a small (about ~ liter) container of activated carbon for temporary storage anc! eventual use by the engine. Basic evaporative control hardware has not changec!much since then, but control effectiveness has increaser! greatly as materials, unclerstanding, anc! measurement techniques have improved. OBD hardware ant! software clo not directly control emissions but are a vital part of contemporary emissions-controT systems. OBD systems monitor and control venous engine Unctions, including the emissions-con~oT system, and help to diagnose emissions-control problems so that repairs will be more timely and effective. Some manufacturers incorporated OBD on a voluntary basis in model-year ~ 98 ~ . The most recent version of OBD, known as OBDIT, is required on all model-year ~ 996 and newer vehicles. ENGINE CONTROLS The conbo} of engine emissions canbe segregated into crankcase controls, combustion controls, and exhaust ailcer-~eahnent. Crankcase condoms contain The air/fuel ratio is the ratio by weight of air to gasoline entering the intake in a gasoline engine. The ideal ratio for complete combustion is 14.7 parts of air to 1 part fuel. Air/fuel ratios less than 14.7 are termed rich and contain excess fuel for complete combustion; air/fuel ratios greater than 14.7 are termed lean and contain more air than is required for complete combustion. 2California typically has required controls 1 or more years before the federal requirement.
Vehicle Emissions-Co7~trol Technologies 49 the "blow-by" gases3 within the engine and use the engine's vacuum to recycle them back into the combustion process. Combustion controls modify engine hardware and engine settings to reduce the amount of unburned combustion gases and thus Tower emissions. Examples include EGR and variable spark- t~ming. Exhaust after-treatment (i.e., catalytic converters) reduces atmospheric emissions by treating the exhaust stream to eliminate pollutants before they exit the tailpipe. Crankcase Emissions Controls Crankcase emissions controls are an important pert ofthe emissions-con- troT system intended to eliminate blow-by emissions. Positive crankcase venti- lation (PCV) was the first emissions-conkoT system used on cars. It was introduced first on some California models in ~ 96 ~ and nationally on a large number of models in 1963 . PCV was required by California on 1964 and later vehicles. Uncontrolled blow-by emissions have been estimated to be 4. ~ g of HC per mire, whichis 100 times the tailpipe HC emissions allowed under the California ultralow-emissions vehicles requirements. Figure 2- ~ illustrates the operation of the PCV. A small fraction of com- bustion gases leek pest the piston rings and collect in the crankcase. Precon- tro! vehicles vented these gases directly to the atmosphere. The PCV system uses the engine's intake vacuum to draw these gases back into the combustion process. All the pipes and hoses must be present for this system to work. Combustion Emissions Controls The combustion emission controls, used on the precatalyst vehicles in 1968 through ~ 974, involved proper engine mechanical operation, lean (excess air) air/fuel combustion ratios, spark modifications, and external exhaust gas recirculation. Good engine condition is required for emissions control. Poor engine condition that has excessive blow-by gases, inadequate spark for com- bustion, and vacuum leaks can defeat all other attempts at emissions reductions 3Blow-by gases are hydrocarbons that leak past the piston rings into the crank- case during the combustion and exhaust process.
50 Evaluating Vehicle Emissions I/M Programs Fit - - In ~ And ~ ED Ink* i~ ~ Fat ~ ~ Fir ~ ~ rat vie_ _ ~ ~~ ~ P4.'sp`* F~. . 111._. _ ~ _^Z=~ '~\'~ I' '-~ \ ~ ~ x. _.~ -- ~ L He' Few is,, ~ __ . .~ . Ciao ~ l .J ~~S:~ i: I.. ~ Eli snip Mona ~ ~,. , ~ lo ~ ~~ ~ ~~.~ . f ~ ~ ~~ ~- ~ W~ F _ _ me, .,~,,~. 1~ - * FIGURE-2-1 Operation of positive crankcase ventilation. Source: NAPA Echlin 2001 a. Repunted by permission; copyright 2001, Dana Engine Controls. and create permanent damage to catalytic converters. A lean air/fuel mixture minimizes emissions of CO and HC. Air/fuel ratios with excess fuel produce more power at the expense of fuel economy and increased CO and HC emis- sions. External spark control devices, such as the spark advance control sole- noid, were added to reduce HC andNOx emissions curing the 1970s end early 1 980s. These parts were used to optimize the spark timing during the combus- tion cycle to simultaneously reduce HC and NOX emissions. EGR, shown in Figure 2-2, was a primary NOX control measure and has been used on the great majority ofeng~nesfroml973to today. Higher combustion temperatures result in greater NOX formation. Exhaust gas is recirculated in controlled ~ ~ _ ~ _ 1 ~ 1~ 1_ ~ 1 1 ~ ~ ~ 1 ~ , · amounts to ollute the com~usnon charge and lower the peak combustion tem- perature. In general, emissions-control systems used on the precatalystvehi- cles in 1968- ~ 974 relied completely on combustion controls and usually com- promised the performance ofthe engine, resulting in degraded dnveability and fuel economy, end promoting the practice oftampering wish, or defeating, the emissions-control system. Engine design modifications coupled with exhaust after-treatment devices represent the best method for meeting current and future emissions standards. It is difficult for engine design modifications alone to reduce simultaneously HC and NOX emissions. Exhaust After-Treatment After-treatment devices consist of catalytic converters and air-injection systems. The early catalytic converters promoted the oxidation of HC and
Vehicle Emissions-Control Technologies 51 .. ................... . . . . _ :~ ~ .. .... 8~_·~------- Per _ FIGURE 2-2 Process of exhaust gas recirculation. Source: NAPA Echlin 2001b. Re- pnnted by pennission; copyright 2001, Dana Engine Controls. CO by passing the exhaust over a bed containing small amounts of precious metals (platinum, palladium, and rhodium). Catalysts were typically mounted under the floor, at about the front seat position, although some were positioned in the engine compartment. Faster warm-up and, therefore, quicker catalytic converter operation were achieved with a location closer to the engine. A location further back in the vehicle gave better protection to the catalyst from damage due to high temperatures. The three-way catalytic converter introduced in 1981 and later model years looked very much like the previous oxidizing models but included NOx control. Figure 2-3 illustrates that if a 14.7: ~ (stoichiometnc) air/fuel ratio were maintained, the catalysts could effectively promote both the oxidation of HC and CO and the reduction of NOX. Simultaneous efficiencies of over 80% were possible under warmed-up operation. Closed-Ioop fuel control was re- quired for effective operation. Catalysts can be damaged by several mechanisms (Ihara et al. 1987~. They can be poisoned through an accumulation of deactivating matenals due to excessive oil consumption or the use of leaded fuel over a long period. They
52 Evaluating Vehicle Emissions I/M Programs NOX CO ~ 00 Best open >I 80 ~ area for 3-way ~) ' 60 ~ i~ ~ Lean A/F mixture 40 RichA/F 11 ~ ! Stoichio~tric a' 20 mixtu:7J ~ Ratio O 0 ~ Window 13 14 Air-to-Fuel Ratio 15 16 FIGURE 2-3 Catalyst conversion efficiency as a function of air/fuel (A/F) mixture ratio. Source: Adapted from Canale et al. (1978~. suffer thermal damage from high-Ioad misfire, when the mass rate of air/fuel mixture is high enough to produce a damaging temperature rise. The substrate can melt or fracture and pass out the tailpipe. Thermal damage can occur suddenly or over a Tong period. Air injection was and, to some extent, still is used both to promote HC and CO oxidation after the engine by adding air to the hot gases as they exit the exhaust port to augment the effectiveness ofthe catalytic converter. Engine- driven air pumps were common, with valves to supply or divert air as required. Another type of air injection system, the pulse air system, was sometimes used on four-cylinder engines. Check valves and appropriate plumbing alllowed the normal pulsations of the exhaust system to supply excess air to the exhaust. Electnc air pumps are becoming common on new applications, operating only for a briefperiod at start-up, while the catalyst is cold and not fully operational. EVAPORATIVE CONTROLS Evaporative emissions are the HCs that escape from the vehicle that do not come from the tailpipe. These emissions include fuel and vapor leaks from the tank and plumbing, refrigerant from the air-conditioning system, coolant leaks, tire emissions, and solvents in the vinyls and adhesives ofthe vehicle. Such sources contribute significantly to overall HC emissions. Evaporative
Vehicle Em~ssions-Cor~trol Technologies 53 Exhaust 289 Evap Exh.~ 163 Evap 89 - 0 100 200 300 400 soo Tons/Day 2000 Year 2010 FIGURE 2-4 Exhaust and evaporative emissions for the South Coast Air Basin (SCAB) in 2000 and 2010 using EMFAC2000 Version l.99h. emissions have become increasingly important as other sources have been controlled. Figure 2-4 shows model projections for the aggregate exhaust and evaporative emissions for the South Coast Air Basin for an average summer day in 2000 and 20 ~ 0. The most recent version of CaTifornia's mobile-source emissions model, EMFAC2000, was used to develop those estimates. The uncertainties in those projections are not reported by the mode} but are likely to be large. Evaporative emissions are estimated to be about one-third ofthe daily light-duty vehicle (LDV) HC in the South Coast Air Basin in 2000 and are estimated to be 45% of the total in 2010. Evaporative emissions can escape from a wide variety of places on the vehicle. The purpose of this section is to define some terms and illustrate where they might occur. The materials used and, to some extent, the design ofthe later fuel injection systems have resulted in a more durable system less prone to leaks than the carburetor models that they replaced. Fuel injection systems are first discussed, followed by the differences found in the older carburetor-equipped models. The fuel tank is usually located at the rear of the vehicle. A vapor volume is provided above the liquid, even when the tank is full, to allow for expansion and to help separate the liquid from the vapor. The fill neck (the pipe between
54 Evaluating Vehicle Emissions I/M Programs the gas cap and the tank) can be a separate component connected to the tank in one or more places with rubber hoses and clamps. The gas cap is a critical componentone that is removed and replaced repeatedly over the lifetime of the vehicle. A cap not replaced or one with a cracked seal results in uncon- trolled evaporative emissions of up to 20 g of HC in a single day. Fuel-injection vehicles typically have a fuel supply pump thatis mounted in the tank. The chassis fuel line, typically en 8- or e-millimeter-diameter tube, carries the pressurized fuel forward to the engine. The chassis line typically is made of steel and rigidly mounted to the underbody of the vehicle. Nylon has also been used for a number of years; it offers superior corrosion resis- tance and reduces the potential for fuel leaks. A serviceable fuel fitter is usually fitted in the supply line. The chassis line is connected to the fuel tank with a flexible hose for assembly, service, and isolation. A similar flexible connection is made to the engine at the front ofthe vehicle. Many engine fuel systems use an eng~ne-mounted pressure regulator and return excess fuel back to the tank through a duplicate chassis line. The return line is not at the supply pressure, but it is still pressurized. These fuel lines and connectors might be sources of liquid fuel leaks. Vapors from the filer tank are routed through a tank vent tube to a carbon canister for storage. The canister is required for the late ~ 990s models with on-board control of refueling vapors and may be close to the tank or located in the engine compartment. Vehicle motion can produce "slosh" in the tank, and liquid can be trapped in the vent unless provisions have been made to separate it. Some applications use special liquid/vapor separators to ensure that only vapor is routed to the carbon canister. The canister is rejuvenated or "purged" during engine operation by using the engine ' s vacuum to draw air through the carbon bed and to the engine. The canister has at least three connections: a tank vapor vent, a purge line, and an air-supply port. Engines with carburetors have all the above features and one more a line venting the carburetor to the carbon canister. The carburetor includes a reser- voir or "bowl" where fuel is stored before it is drawn through the carburetor metering systems into the combustion chamber. A carburetor typically has approximately 50 milliliters (3 ~ g) of fuel in the bowl. After engine shutdown, residual engine heat rises into the bowl, creating vapors and, under severe summer conditions, boiling away the fuel. Unless the carburetor bowl is prop- erly vented, these vapors can find their way into the environment.
Vehicle Emissions-Control Technologies 55 OBD SYSTEMS The exhaust emissions standards (HC, 0.4 ~ g/mi; CO, 3 .4 g/mi; NOX, ~ .0 g/mi) that were required in the 1981 mode] year forced the adoption of computer-based closed-Ioop control systems. The digital computers used for these applications were able to monitor and actuate many vehicle system functions and signal the driver and repair technician if system problems were present. Computer-controlled cars are able to maintain near optimal air/fuel ratios, enabling the adoption of durable three-way catalytic converters. Many manufacturers used OBD systems to help with the service and reliability of their vehicles (Grimm et al. ~ 980; Gumbleton and Bowler ~ 982~. The OBD system is made up of the sensors and actuators used to monitor and modify specific components as well as the diagnostic software in the on-board com- puter. Such a system can communicate its findings to a service technician using diagnostic trouble codes stored in the computer's memory. California regulators recognized the potential ofthe OBD system, expanded the scope, and required it on new vehicles starting with a 1988 mode} year phase-in. California and the U. S. Environmental Protection Agency expanded the scope and coverage of diagnostics with the OBDII regulations, which were phased in beginning with the model-year ~ 994 vehicles. All LDVs built after ~ 996 (with a few exceptions) are equipped with the OBDIT system. OBDIlperiodically checks many emissions-control functions. The check includes monitoring the following emissions-control components: catalysts, oxygen sensors, evaporative canister purge system, fuel tank leak check, misfire detection, and on-board computers. If a problem is detected that could cause emissions to exceed 1.5 times the emissions standards, the OBDII system illuminates a malfunction indicator light (MTL), also known as the "cheek engine" light, on vehicle dashboards. Note that the MILilluminates if emissions coula1exceed ~ .5 times the certification standard end that OBDTI does not actually measure emissions. Some vehicles with illuminated MILs might not have emissions at that level yet, and some may have an intermittent problem. An issue with intermittent problems is that the MIL might illuminate once a problem is detected but remain on for a significant period oftime before the light is extinguished, and the stored code erased. Besides notifying the operator of a malfunction, the objective ofthe OBDTT system is to protect the catalytic converter from permanent damage from exposure to excessive emis- sions. Further developments of OBD technology, such as remote monitoring
56 Evaluating Vehicle Emissions I/M Programs or reporting, can provide alternatives to the traditional tailpipe tests used now to monitor vehicle emissions-control systems. OBDIT systems that can report vehicle status through remote monitoring or reporting are referred to as OBDIll systems. The I/M aspects of the OBD system are discussed later in this report. SUMMARY Emissions-controT systems have grown more comprehensive and have matured over the years as the technology and the public commitment to air quality have grown. The earliest tailpipe controls ( 1968-1974) required fre- quent adjustments and benefited only certain modes of vehicle operation. The need for periodic inspection and adjustments for the life of the vehicle was critical to maintaining emissions performance. Oxidation catalytic converter after-treatment systems ~ ~ 975- ~ 980) allowed major reductions in CO and HC emissions levels but still relied on engine adjustments and periodic inspections to ensure good performance. An important aspect of the periodic inspection was to ensure that the vehicle still had a functional, nontampered emissions- contro] system. Introduction of the computer-controlled exhaust feedback systems in 1981 created a new generation of self-adjusting systems. The three-way catalyst controlled NOx, but required stoichiometric operation to have sufficient CO for NOx reduction to occur. OBD systems were added later (by regulation in ~994) to signal the driver that there was a systemprob- lem and to help the service technician make the correct diagnosis and repair. OBD technology has the potential to change the role of I/M for new-technol- ogy vehicles.
