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OCR for page 46
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
OCR for page 47
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
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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.
OCR for page 49
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.
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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
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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
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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
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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
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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
component—one 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.
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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
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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.
Representative terms from entire chapter:
evaporative emissions
