EPA-AA-IMS/81-2
Derivation of I/M Benefits for Post-1980
Light Duty Vehicles for Low Altitude,
Non-California Areas
Revised March, 1981
Dave Hughe s
I
NOTICE
This report does not necessarily represent final EPA decisions or positions.
It is intended to present a technical analysis of the issue using data which
are currently available. The purpose in the release of such reports is to
facilitate the exchange of technical information and to inform the public of
technical developments which may form the basis for a final EPA decision,
position or regulatory action.
Inspection and Maintenance Staff
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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I. Introduction..
As is widely recognized, the post-1980 model year fleet will be predominantly
composed of vehicles which employ what has become known as Three Way catalyst
technology. This technology incorporates a sophisticated microprocessor-based
engine control system which holds the air/fuel ratio very close to stoichi-
ometry, thereby allowing the Three Way catalyst to simultaneously convert
Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx) to
harmless by-products. This is in contrast to conventional technology vehicles
(Model Years 1975-1980) which rely on mechanical control of the air/fuel ratio
and whose catalysts are only designed to convert HC and CO. As could be
expected, such a significant shift in technology will have an impact on the
expected in-use emissions performance of these vehicles both with and without
an Inspection and Maintenance (I/M) program.
This report discusses the derivation of I/M benefits for the post-1980 Federal
fleet as contained in EPA's emission factor model MOBILE2*. As such, it
relies heavily on an earlier report which presented the derivation of in-use
emission factors (i.e. the emissions performance of the fleet without I/M) for
the post-1980 Federal fleet [I].** It is recommended that the major structur-
al points presented in that report be understood first, so that the analysis
presented in this report can be more easily understood. This analysis will,
however, briefly summarize how the in-use emission factors were derived.
Both this analysis and the emission factor analysis have relied on a modeling
approach which structurally represents the trends observed in the data, as
opposed to performing statistical analyses (e.g., linear regression with
mileage as the independent variable) on large data bases as has traditionally
been done for earlier model year fleets. This was done due to a lack of a
sufficiently large data base on vehicles with representative Three Way
catalyst systems. There is a substantial data base made up of Three Way
catalyst systems which was closely examined to discern patterns of in-use
performance and to examine the effectiveness of the various I/M short tests.
That data base is not large enough, however, nor does it contain a sufficient
mileage spread to enable the analyses required under the more traditional
approach.
* MOBILE2 is a computer program which models the emissions performance of
the entire vehicle fleet over time, both with and without I/M. Figures of
percent benefit due to I/M are applied to the without-I/M case in MOBILE2
in order to model the air quality impact of I/M.
** The numbers in brackets indicate references which will be listed at the
end of the report.
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The following sections will first present a brief description of the expected
in-use performance of the post-1980 fleet. Second, the methodology and
assumptions used in deriving I/M benefits for the post-1980 fleet will be
presented and discussed. Finally, issues related to the use of the benefits
contained in MOBILE2 will be discussed.
II. Expected In-Use Performance of the Post-1980 Fleet.
It is at this point that previous knowledge of the report which describes the
derivation of emission factors for the post-1980 fleet would be of greatest
advantage.[1] The following section will briefly summarize that derivation.
As such, it discusses the expected modes of failure as opposed to presenting
the emission factor equations themselves. A discussion of failure modes will
be much more helpful in understanding how the I/M benefits were derived rather
than presenting the emission factor equations.
The emission factor model first breaks the post-1980 fleet into two separate
technology types: those vehicles which employ the Three Way catalyst tech-
nology described above, and those vehicles which rely on more conventional
technology (oxidation catalysts, air pumps, engine modifications, EGR). This
second group is expected to make up only 7% of the post-1980 fleet. This is a
small fraction of the fleet, and its emission factors were derived differently
than for the rest of the fleet. It would have been difficult to model I/M
benefits for this group of vehicles because of the internal methodology used
in deriving their emission factors. Given the small size of this group of
vehicles, the added complexity that would have been necessary to include them
in the calculation of I/M benefits, and the small increase in accuracy which
would result, it was decided to ignore them in this analysis. Consequently,
the results derived for the Three Way technology type have been assumed to be
valid for these vehicles as well.
Of chief concern then is the group of vehicles equipped with Three Way
catalyst technology. This group of vehicles was entitled "Closed Loop
Vehicles" in the emission factor analysis, due to the nature of the
microprocessor-based control system they employ. This system relies on a
feedback signal from an oxygen sensor placed in the exhaust manifold which
tells the microprocessor whether the air/fuel ratio is rich or lean of stoich-
iometry. The microprocessor uses this information in conjunction with
information supplied by other sensors to adjust the air/fuel ratio at the
carburetor. The system is therefore based on a "closed loop" form of control,
with the signal from the oxygen sensor used to "close the loop". Three Way
catalyst vehicles will therefore be referred to as Closed Loop vehicles for
the rest of this analysis in order to parallel the terminology used in the
emission factor analysis.
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The emissions performance of Closed Loop vehicles was modeled by designating
three distinct categories of emissions performance for HC and CO.* A data
base of 218 in-use, Closed Loop vehicles was primarily relied upon to deter-
mine the incidence and the emission levels of the various categories. This
data base is made up of the most representative and advanced Closed Loop
technology vehicles for which data are currently available. (See Section II.
of the emission factor analysis for a more complete discussion.[1])
The first category of vehicles represents those vehicles which have lost
microprocessor control of the engine and are operating in a full-rich mode.
This situation could result from a number of possible scenarios including
microprocessor failure, tampering, sensor failures and disconnection of
electrical wires. The failure scenarios possible will differ among the
various manufacturers, but in all cases the end result is very high HC and CO
emission levels. This category of vehicles 'is called the Primary category.
This name was given because the vehicles in the Primary category, while not
initially very numerous, contribute the majority of the fleet's in-use CO
emissions and a large share of the in-use HC emissions. The emission levels
attributed to the Primary category were based on data from ten in-use, low
mileage Closed Loop vehicles from the data base which were observed to have a
Primary category failure. The average emission levels of these vehicles were
HC = 3.85 g/mi, CO = 108.0 g/mi at an average mileage of 9,163 miles. The
Primary category was given an initial size of 3% of the fleet at zero miles
and a growth rate of 2% per 10,000 miles. That is, as the fleet ages, more
and more vehicles will experience the types of failures described above. (See
Sections IV.A.2.a. and IV.A.3.a of the emission factor analysis for a more
complete discussion.[1])
The second category of vehicles represents those vehicles which have experi-
enced misfueling (that is, those vehicles equipped with catalytic converters
which have been fueled with leaded gasoline). The Misfueling category was
modeled to make up 8% of the fleet based on the most recent EPA observations
of misfueling in the field. [2] The HC and CO emission levels associated with
vehicles in the misfueling category were derived from recent EPA misfueling
test programs. They are well below the emission levels of Primary category
vehicles but substantially above the emission levels of the remainder of the
fleet. (See Sections IV.A.2.C and IV.A.3.C of the emission factor analysis
for a more complete discussion.fi])
The third category of vehicles basically represents the remainder of the
fleet. As such it includes both well-maintained vehicles and vehicles which
have been tampered with or have suffered component wear or component failure,
but not of the magnitude which would lead to the Primary category. The
emission levels associated with this category were based on data from over 190
in-use, low—mileage Closed Loop vehicles from among the data base described
above. This category is referred to as the Secondary category. (See Sections
IV.A.2.b and IV.A.3.b of the emission factor analysis for a more complete
discussion.[1])
The NOx analysis was performed separately and will be briefly discussed in
the next section.
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The fleet therefore is broken down into three categories of vehicles for the
HC and CO analysis, each category with a distinct emissions performance. Each
category has a unique zero mile emission level and there is a common deterior-
ation rate used for all the categories for a given pollutant. The deteri-
oration rates were developed for the Secondary category and then adopted for
the Primary and Misfueling categories. This was done due to a lack of enough
separate data for the Primary and Misfueling categories to be able to predict
a unique deterioration rate for those categories. As could be expected from
the preceding discussion, it is chiefly the number of vehicles entering the
Primary category with its very high HC and CO emission levels, and not the
deterioration rates, which is of foremost concern from an I/M perspective.
The following section will present how the I/M benefits were derived.
III. Methodology Used in Determining I/M Benefits.
Basically two separate phenomenon were accounted for in calculating I/M
benefits for the post-1980 fleet: the identification of Primary category
vehicles and the identification of vehicles experiencing severe ignition and
misfire problems.
One issue that needs to be addressed before proceeding has to do with which
model year case of the emission factor analysis was used to determine I/M
benefits.[1] Those readers familiar with the emission factor analysis will
note that three separate cases were modeled for the post-1980 fleet: the 1981
model year fleet, the 1982 model year fleet and the 1983 and beyond model year
fleet. Separate analyses were required for 1981 and 1982 due to the presence
of the Clean Air Act Section 202(b)(5) waiver fleet: those cars that received
a waiver of the CO standard from 3.4 g/tni to 7.0 g/mi in 1981 and 1982. 1981
and 1982 are also the model years when the benefits which accrue from the
Parameter Adjustment regulations (44 F.R. 2960) are phased into the model.
The differences between the various model year cases are not great. The 1983
and beyond case was used as the base case in determining I/M benefits. This
was because the 1983 and beyond case is the most representative case for the
overall timeframe for which I/M benefits need to be calculated. It would have
added a significant measure of complexity without an appreciable gain in
accuracy to account for the small differences in the L981 and 1982 model years.
A. Identification of Primary Category Vehicles.
The only significant issue involved here is what percentage of Primary
category vehicles can be expected to be identified by the various I/M short
tests. Since Primary category vehicles emit such high levels of HC and CO and
indeed contribute the majority of the overall fleet's in-use CO emissions and
a large share of the fleet's HC emissions, it is reasonable to expect that an
I/M short test be capable of identifying most of these vehicles. This is
especially true if excess emissions (i.e. emissions above the Federal
standards) are considered instead of total emissions. Primary category
vehicles contribute an overwhelming share of the excess emissions, which are
of chief concern from an I/M perspective. The ability of the various short
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tests to identify Primary category vehicles was quantified by examining short
test data from the ten in-use Primary category vehicles mentioned earlier.
Table III.A.I. presents the results of applying inspection cutpoints to the
short test data. The individual data from each of the 10 vehicles is
presented in Appendix A.
Table III.A.I
Ability of the Various Short Tests to Identify Primary Category Vehicles
Applicable Percent of Primary Category
Short Test Cutpoints Vehicles Identified*
Idle(N) 1.2% CO, 220 ppm HC 50%
2500 rpm/Idle(N) 1.2% CO, 220 ppm HC 60%
Loaded Two Mode 1.2% CO, 220 ppm HC 70%
(30 mph/Idle(N))
At first inspection, Table III.A.I suggests definite incremental benefits in
going from the Idle(N) test to the 2500 rpm/Idle(N) test to the Loaded Two
Mode test. A closer inspection, however, reveals that making a distinction
between the 2500 rpm/Idle(N) and Loaded Two Mode tests on the basis of the
data currently on hand is unwarranted. As can be seen in Attachment A, one
vehicle, the second 1979 Mercury Marquis, "passed" the 2500 rpm/Idle(N) by
only 0.02% CO. Given the fact that this one vehicle was only a very margin-
ally "passing" vehicle and given the fact that previous examinations and
technical judgement do not support a significant distinction between the 2500
rpm/Idle(N) test and the Loaded Two Mode test, EPA has decided to associate a
70% Identification Rate of Primary category vehicles with the 2500 rpm/Idle(N)
test. That is, the Loaded Two Mode and 2500 rpm/Idle(N) tests are judged to
be equally capable of identifying Primary category vehicles - at a 70%
Identification Rate. The selection of an Identification Rate to be associated
with a given short test is significant in terms of the emission benefits which
result from the model. The significance of the Identification Rate of Primary
category vehicles will become more apparent as the report progresses. In
summary, given the limited data available, it is not warranted to make
distinctions of such consequence where previous experience and technical
judgement do not support the distinction.
* This quantity, the percent of Primary category vehicles identified by a
given short test, is referred to as the Identification Rate in the "User's
Guide to the Mobile Source Emissions Model: MOBILE2". Section 2.2.4 of that
report discusses the Identification Rate.
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50% was used as the default value within MOBILE2 for the Identification Rate
of Primary category vehicles in the calculation of I/M benefits. As can be
seen in Table III.A.I. this is the Identification Rate associated with the
basic Idle(N) test. Section 4 of this report will deal with the inner work-
ings of MOBILE2 and the use of alternate Identification Rates in greater
detail.
Identification Rates of 60%, 70%, 80% and 90% were also evaluated and
incorporated into MOBILE2 to provide flexibility for those states and
localities who choose to use the Loaded Two Mode test or the Idle(N)/2500 rpm
test, and also to provide flexibility for modifications if the addition of
more data at a later point changes the percent of Primary category vehicles
identified by the various short tests.
Those Primary category vehicles which are modeled as being identified in the
I/M process are all assumed to be repaired. The emission levels of the
repaired vehicles were modeled by assuming 8% of the repaired vehicles enter
the Misfueling category and adopt the Misfueling category's emission levels.
The rest are assumed to adopt the emission levels of the Secondary category.
Eight percent are modeled to enter the Misfueling category since the overall
misfueling rate of 8% is assumed to occur uniformly throughout the fleet.
Therefore 8% of the vehicles in the Primary category have also experienced
misfueling. When these vehicles are repaired, the engine control system is
returned to normal operation but the catalyst (and oxygen sensor) will still
be damaged due to misfueling. Thus, these vehicles should be assigned
emission levels which reflect a history of misfueling.
Primary category vehicles which are not identified by the I/M short test at a
given inspection are assumed to also not be identified at all subsequent
inspections. For example, the 50% of Primary category vehicles not identified
at the first round of inspections in the default case will thereafter always
remain in the fleet as Primary category vehicles. This reflects the assump-
tion that some Primary category failure modes will have low emissions at the
operating points tested by the given short test and are not capable of ever
being identified by that short test. This assumption is conservative (results
in lower benefits from the I/M program) relative to the possible competing
assumption that some Primary vehicles (50% in the default case) are able to
pass a given inspection due to a random process which occurs independently at
each inspection. No evidence exists to support or contradict this less
conservative assumption.
Those Primary category vehicles which are identified and repaired in the I/M
process are subsequently assumed to be eligible to reenter the Primary
category at the same rate as other vehicles in the fleet. These vehicles are
not exempt from ever entering the Primary category again. This is because of
the number and variety of scenarios which can lead to Primary category
operation as well as due to the effects of change of ownership in conjunction
with tampering by -a new owner.
It should also be noted at this point that the likelihood of a vehicle
entering the Primary category is modeled as slowly increasing as a vehicle
accumulates mileage. That is, as a vehicle ages, it is more likely to
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experience a Primary category failure, due to increased component wear and
decreased incentive for the owner to properly maintain his vehicle. This was
modeled by relying on the Primary category growth rate used in the emission
factor analysis. A "constant" 2%/10,000 mile growth rate was used in that
analysis, but since the 2% is coming from an ever smaller group of non-Primary
category vehicles, the assumption of a constant rate implies an increasing
likelihood of Primary category failure as a vehicle ages. For example, at
zero miles, 97% of the fleet is not in the Primary category and out of this
group 2% of the total fleet enters the Primary category over the first 10,000
miles. At 100,000 miles, only 77% of the fleet is not in the Primary
category, yet the same number of vehicles, 2% of the total fleet, enters the
Primary category over the next 10,000 miles. Thus, the emission factor
analysis models an increasing likelihood for a vehicle to enter the Primary
category as it ages. The algorithm which describes this tendency was used to
determine the growth of the Primary category between inspection points for the
calculation of I/M benefits.
The failure rate of Primary category vehicles (not to be confused with the
Identification Rate) will be low, especially after the first years of an I/M
program's implementation. This is due to the fact that the growth rate of the
Primary category is modeled to be only 2%/10,000 miles. There are therefore
simply not that many Primary category vehicles to identify, although identify-
ing the ones present in the field results in substantial emission benefits.
Table IV.A.I includes an example of the expected failure rate of Primary
category vehicles.
The identification of Primary category vehicles in the I/M process accounts
for all of the CO benefits contained in MOBILE2 and roughly half of the HC
benefits. The HC benefits resulting from the identification of Primary
category vehicles cannot be presented exactly since they were calculated in
conjunction with the HC benefits which result from the identification of
vehicles with ignition/misfire problems. A summary of total benefits will be
presented later. To give the reader an indication of the magnitude of the
benefits resulting from the identification of Primary category vehicles, for
the default case (50% Identification Rate corresponding to use of the basic
Idle(N) test) the CO benefits are approximately 30% after the first year of
program implementation and the HC benefits are approximately 15%. These
figures of percent benefit represent the percent of emissions reduction
attributed to I/M as compared to the base case (without I/M).
NOx Penalty Issue.
The reader may wonder if there should not be a NOx "penalty" associated with
the HC/CO "benefits" which accrue from the repair of Primary category
vehicles. This would be because of the very low NOx emission levels which
result when a vehicle is in the Primary category's rich mode of operation.
Low NOx levels result in this situation due to two parallel phenomenon: the
low NOx levels which are always produced by an engine during rich modes of
operation and the increased NOx conversion efficiency of the Three Way
catalyst during rich operation. The emission factor analysis accounted for
the low NOx levels of Primary category vehicles by creating a separate NOx
category, the Low category, which parallels the growth of the Primary category
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8
(See Section V.A. of the emission factor analysis for a complete dis-
cussionfl]). It stands to reason that when Primary/Low category vehicles are
repaired, the low NOx emission levels will rise as the HC/CO emission levels
fall. Thus, a theoretical NOx "penalty" results from I/M. This issue was
examined in a similar fashion as was done in developing the HC/CO credits for
identifying Primary category vehicles. A model was constructed wherein Low
category vehicles were identified and repaired. A comparison was then made
between the with and without I/M cases. The results of that analysis showed
only a very small NOx penalty. In a "worst-case" scenario which set the upper
bound for a possible NOx penalty, the "percent penalty" was only 2% at 50,000
miles and 2.7% at 100,000 miles. The reason why the NOx "penalty" is so small
whereas the HC/CO benefits are fairly large (15%/30%) has to do with the
difference between the category emission levels for HC/CO as compared to NOx,
and the difference between the contribution of the category deterioration
rates for HC/CO and NOx. For HC and especially for CO, the gap between the
emission levels of the Primary category and the other categories is much
larger than between the NOx Low category and the other NOx categories. Thus
there is less difference for NOx before and after repair when the repaired
vehicles enter a new category and therefore a small "penalty" results. Also
for NOx the deterioration rates of the various categories play a much larger
role in determining the fleetwide composite. Therefore shifts between
categories are much less significant. Given the small size of the NOx penalty
and realizing that identifying so small a percentage would be overstepping the
inherent accuracy of the model, the NOx penalty issue was ignored.
B. Identification of Vehicles With Ignition/Misfire Problems.
Ignition and misfire problems have historically been the cause of a signifi-
cant portion of excess HC emissions. While a modeling approach such as has
been used in this analysis catlnot accurately account for the emissions impact
of every possible vehicle malfunction, ignition/misfire problems were judged
to have a significant enough impact on in-use HC emissions that they should be
accounted for. Post-1980 vehicles can be expected to be free of many histor-
ical maladjustments (e.g. idle mixture) due to technology changes but will
still be prone to ignition/misfire problems. Post-1980 vehicles will still
experience shorting and cracking of ignition wires, spark plug failures,
cracking of distributor elements etc., all of which will lead to excessive HC
emissions. These types of failures are readily identifiable by the various
I/M short tests since they usually result in greatly increased HC emissions at
all modes of operation.
A final introductory point which needs to be made is that ignition/misfire
problems are assumed to occur in proportion to vehicle age. That is, as a
vehicle ages, it has a greater likelihood of experiencing an ignition/misfire
problem. This point is significant in that it largely determined which data
could best be used to examine the I/M benefit resulting from the identifica-
tion of vehicles with ignition/misfire problems. In particular, data confined
to small mileage intervals are not suitable.
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Me thodology.
There is an important difference in the way benefits were calculated for the
identification of vehicles with ignition/misfire problems as opposed to the
identification of Primary category vehicles. The presence of Primary category
vehicles (i.e. vehicles with a failure of the computer control system with a
resultant rich mode of operation) among the fleet was clearly delineated in
the emission factor analysis through the establishment of a category. The
category grows with time (in the absence of I/M) and its size and therefore
its effect can be calculated at any point by knowing the category growth
rate. The calculation of I/M benefits for these vehicles was therefore fairly
straightforward, since the presence of these vehicles in the fleet is so
clearly delineated.
The emission factor analysis did not account for the impact of vehicles with
ignition/misfire problems by creating a corresponding "ignition/misfire"
category, however. Rather, it accounted for the impact of these vehicles
implicitly through the HC zero-mile emission levels assigned to the Secondary
and Misfueling categories and the HC deterioration rate used for all
categories.* Repair of ignition/misfire problems in an I/M program is modeled
by adjusting deterioration rates, rather than by adjusting the distribution of
vehicles among categories. The number of vehicles failing the I/M test due to
misfires and their HC emission levels have not been modeled directly; only
their effect on the average emissions of all vehicles in a given category
(Primary, Secondary, or Misfueled) appears in the model.
The Volvo/Saab fleet described in Section II.D. of the emission factor
analysis played the major role in determining the I/M benefits resulting from
the identification of vehicles with ignition/misfire problems[1], This fleet
contains 104 1978/1979 Volvos and Saabs which all met the criteria established
to screen out those vehicles with maladjustments which can be expected to be
* The HC zero-mile emission level assigned to the Secondary category was based
on data from 191 representative, in-use vehicles. One of these, a 1980
General Motors six cylinder Citation with 12,879 miles, had its number 1
cylinder shorted in the distributor cap. FTP HC emissions from this vehicle
were 4.8 g/mi. This one vehicle contributed 8% of the average HC emission
level of the 191 vehicles in the Secondary category data base. The impact of
this vehicle was also carried over into the Misfueling category since the
zero-mile emission level of the Misfueling category was arrived at by applying
a multiplicative factor to that of the Secondary category..
The HC deterioration rate used for the various categories is also assumed to
include the effects of vehicles with ignition/misfire problems. That is, part
of the deterioration rate is attributed to the increasing presence of vehicles
with ignition/misfire problems and correspondingly high HC emission levels.
These problems are assumed to occur more frequently at higher mileage, thereby
adding a "misfire" component to the deterioration rate.
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10
prevented by the advent of EPA's "Parameter Adjustment" regulations (44 F.R.
2960). The Volvo/Saab fleet was used due to its having a wider mileage spread
than the other Closed Loop vehicle fleets. The presence of a wide mileage
spread was considered necessary in order to better see the effects of the
increasing likelihood of vehicles to experience ignition/misfire problems as
they accumulate mileage. The calculation of I/M benefits was done in the
following manner. First, the data from the Volvo/Saab fleet was put through
an I/M simulation where vehicles were identified by only using an HC cutpoint
of 200 ppm. The I/M short test used as the basis of evaluation was the
Idle(N)/2500 rpm test where emissions are measured and evaluated at both
Idle(N) and 2500 rpm. A cutpoint of 200 ppm HC was used. Putting the fleet
of 104 Volvos and Saabs through the I/M simulation resulted in 2 vehicles
being identified as failures. The mean HC emissions of these two vehicles was
4.18 g/mi at an average mileage of 15,365 miles. One vehicle seemed to be
experiencing intermittent misfire and the other had a failed spark plug. Both
of these cars would have only been identified using the HC cutpoint. That is,
they would not have been failed by a corresponding CO cutpoint of 1% CO. They
were classic HC-only failures.
The second methodological step in determining ignition/misfire benefits
involved developing a deterioration rate for the various categories without
the presence of vehicles with ignition/misfire problems. That is, assuming
that an I/M process would identify these vehicles as requiring maintenance,
what would the HC deterioration rate be without their presence. This was done
by calculating the percent reduction due to removing the two vehicles
described above. This percent reduction was then applied to the HC emission
factor equation developed for the Secondary category of Closed Loop vehicles:
HC = 0.23 g/mi + 0.12 g/mi/10,000 miles x (Miles/10,000) (See Section
IV.A.2.b. of the emission factor analysis [1]).* The percent reduction was
applied at the Volvo/Saab fleetwide average mileage of 13,786 miles. This
determined one point on the new line. A second point on the line was chosen
to be the zero mile level of the HC emission factor equation for the Secondary
category: 0.23 g/mi. This was done even though this point reflects the
presence of a vehicle with ignition/misfire problems as discussed above (the
Citation with a cylinder shorted in the distributor cap), due to the fact that
the I/M process would not begin to identify these vehicles until their first
inspection at one year of age (approximately 14,000 miles).
Table III.B.I. presents the pertinent values used in the above development and
Figure III.B.I. presents a graphical illustration of the concept.
* Since the Volvo fleet most closely resembles a fleet of Secondary category
vehicles, the new HC deterioration rate was calculated using the Secondary
category's emission performance as a base. The new rate was then applied to
the Primary and Misfueling categories as well. Primary and Misfueling
category vehicles are expected to experience ignition/misfire problems at the
same rate and with the same proportional effect as Secondary category vehicles.
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u
1.0 J-
0,54-
Figure 3EC.&..1.
13,784
/o,ooo
HO, 000
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12
Table III.B.I.
Values Used in the Development of an HC
Deterioration Rate Without the Presence
of Vehicles with Ignition/Misfire Problems
Average HC emissions of total
Volvo/Saab fleet = 0.462 g/mi (n=104)
Average fleetwide mileage = 13,786 miles (n=104)
Average HC emissions without
vehicles with ignition/misfire
problems = 0.389 g/mi (n=102)
Percent reduction = (0.462 - 0.389/0.462)xlOO = 15.8%
Emission factor equation of HC = 0.23 g/mi + (0.12 g/mi/10,OOP miles)*
Secondary category (miles/10,000)
Closed Loop vehicles[1]
HC emission level from the above
equation at 13,786 miles:
HC = 0.23 g/mi + (0.12) * (13,786/10,000) = 0.395 g/mi
HC emission at 13,786 miles
with 15.8% reduction:
HC = 0.395 - (0.158X0.395) = 0.333 g/mi
New Emission Factor Equation accounting for the identification
of vehicles with ignition/misfire problems:
a. Zero-Mile = 0.23 g/mi
b. Deterioration Rate = 0.33 g/mi - 0.23 g/mi = 0.07 g/mi/10,OOP miles
13,786 miles
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13
The reason why a regression analysis was not performed on the Volvo/Saab fleet
after removing the vehicles with ignition/misfire problems can best be under-
stood by considering the development of deterioration rates for the Secondary
category as presented in Section IV.A.2.b. of the emission factor ana-
lysis [1]. The reader will note that although a regression analysis of the
complete Volvo/Saab fleet was performed in that development (after deletion of
some cars for parameter maladjustment) , the result was not applied for the HC
case. Instead, the result for CO (the ratio between in-use performance and
Certification performance) was used to replace the HC results because the
latter was judged to be unreliable.
The end product of this portion of the development is a deterioration rate
which represents the average HC deterioration of post-1980 vehicles without
the presence of vehicles with ignition/misfire problems. This deterioration
rate is applied to all three categories of vehicles (Primary, Secondary,
Misfueling) since ignition/misfire problems are assumed to have a similar
impact on all three.
The third and final methodological step is fairly straight-forward. Simply
put, at the first inspection point after the I/M process is initiated, the HC
emission performance of the various categories is assumed to drop down to the
line representing the category without the presence of vehicles with ignition/
misfire problems. That is, I/M is assumed to identify those vehicles and to
cause them to be repaired. The emissions performance of each of the three
categories of vehicles in the fleet drop down to a new line for the category.
The three categories can of course be simply weighted together to give a fleet
composite.
Following the inspection, each category readopts its previous deterioration
rate (0.12 g/mi/10,000 miles). This is because after the I/M inspection,
vehicles can be expected to develop new ignition/misfire problems at the same
rate of occurrence as before the inspection. At subsequent inspections
however, each category will once again drop down to the line which represents
the category's HC emissions without ignition/misfire problems. Figure
III.B.2. graphically illustrates this concept for the first three inspec-
tions. It examines what is happening for the Secondary category only.
Exactly parallel scenarios are modeled for the Primary and Misfueling
categories.
The I/M benefits which accrue from identifying vehicles with ignition/ misfire
problems account for roughly half of the HC benefits contained in MOBILE2. CO
benefits for this phenomenon were not addressed since it is overwhelmingly a
HC phenomenon.
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IV. Post-1980 Model Year I/M Benefits as Contained in MOBILE2.
A. Introduction.
The preceding sections presented the assumptions and methodologies used to
model the impact of an Inspection/Maintenance program on the post-1980 fleet's
emissions. These assumptions and methodological steps were incorporated into
a computer model which tracked the fleet through an Inspection/Maintenance
program over time. This was done in order to calculate actual figures of the
percent benefit due to I/M (i.e. the percent reduction in HC/CO emission
levels due to I/M). A detailed discussion of the computer model will not be
given here, although a copy of the computer program is attached. (Appendix
B). The computer program simply incorporates the methodology developed in
this report.
The output of the program is a series of large matrices. Each matrix contains
the benefits which are modeled to accrue for a given Identification Rate of
Primary category vehicles. Each matrix has nineteen rows of benefits. These
rows contain the benefits which are modeled to accrue for an I/M program
starting-up in any of the nineteen calendar years following a given model
year. The computer program is based on an assumption that the maximum life of
a vehicle is 20 years. Therefore as the start-up of an I/M program comes
later and later than the introduction of a given model year, the number of
years for which I/M benefits are calculated steadily decreases. For example,
for the 1981 model year fleet, if the I/M program begins in January of 1983,
I/M benefits need to be calculated for the following 18 years. If the I/M
program doesn't begin until January of 1990, however, 1981 model year vehicles
would only be active participants of the program for 11 years. There would
therefore only be a need to calculate benefits for 11 years in that case.
The output of the computer program presented in Appendix B. then, is a series
of matrices which provide the HC and CO benefits due to I/M under a wide
variety of scenarios. These scenarios are determined by the Identification
Rate of Primary category vehicles and the year of I/M program start-up in
relation to the model year. Table IV.A.I presents an example of the benefits
stored in one of the matrices. This example gives the benefits resulting from
using the default Identification Rate of 50% and with the I/M program
starting-up in the first year of a model year. The total array of benefits as
contained in MOBILE2 will not be presented due to its size. Table IV.A.I also
presents the failure rate of Primary category vehicles resulting from the
above scenario. It should be noted that this failure rate does not include
the identification of vehicles with ignition/misfire vehicles. These vehicles
can be expected to contribute between 2% and 6% to an I/M program's failure
rate, especially as given model year fleets age.
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Table IV.A.I.
Post-1980 I/M Benefits for.a 50% Identification Rate and Program
Start up in the First Year Of a Model Year
Primary Category
Benefit Year Failure Rate* HC Benefit* CO Benefit*
13 89
2 1 23 28
31 28 30
41 31 31
5 1 33 31
6 1 35 32
7.1 36 32
8 1 36 32
9 1 37 32
10 1 37 32
11 1 38 32
12 1 38 32
13 1 38 32
14 1 38 32
15 1 39 32
16 1 39 32
17 1 39 32
18 1 39 32
19 1 39 32
* Percent.
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B. MOBILE2.-
The complete matrices of benefits described above are stored in MOBILE2.
MOBILE2 has the capability to internally combine the I/M emission benefits
resulting from all model years of vehicles (before and after the 1981 model
year) and to arrive at a fleetwide figure of the percent benefit due to I/M in
any given calendar year. A detailed description of how that is done will not
be given in this report. Those readers interested in exactly how MOBILE2
applies I/M benefits to calculate a fleetwide figure can find a discussion of
the basic concepts in the report which describes the derivation of I/M
benefits for earlier model year fleets. (That report had not been released in
final form at the time when this report was released, so an EPA identification
number could not be referenced.)
C. The Default Case and Optional Cases.
The MOBILE2 default case for post-1980 vehicles includes a 50% identification
rate of Primary category vehicles and full benefits from identifying vehicles
with ignition/misfire problems. It is not possible for users of MOBILE2 to
modify the ignition/misfire benefits. All I/M programs are assumed to be
capable of identifying these vehicles. The rate of identification of Primary
category vehicles can be modified, however. MOBILE2 contains benefits result-
ing from 60%, 70%, 80%, and 90% Identification Rates in addition to the
default value of 50%. The 50% default value is currently associated with the
use of the Idle(N) test. Users of MOBILE2 who wish to use other Identifica-
tion Rates should be prepared to demonstrate that the rate they use can be
achieved with a specific short test procedure and specific inspection
standards. The Identification Rates are therefore related to specific short
tests. This is in contrast to earlier model year benefits as calculated by
MOBILE2 which vary based on stringency factors. Stringency factors are a
function of HC and CO cutpoints.
D. Mechanic Training.
There are not assumed to be any incremental benefits from mechanic training
for post-1980 vehicles. This is not because mechanic training is assumed to
be of no importance for the post-1980 fleet. Rather it is due to the expected
nature of repairs on these vehicles and their inability to pass an I/M retest
if they have not been correctly repaired. Since post-1980 vehicles rely on a
complex network of interactive electrical/mechanical components, the repair of
Primary category vehicles is expected to rely chiefly on correct system
diagnosis usually followed by component replacement or repair. Repairs are
therefore expected to be largely of a component nature as opposed to the
largely adjustment nature of repairs for more conventional technology vehicles
(e.g. idle mixture adjustment, timing adjustment). One effect of this will be
to make emission-related repairs much more of an all—or—none issue. It is
assumed that a Primary category vehicle will either be successfully diagnosed
and repaired with correspondingly low emissions (equal to Secondary or
Misfueled vehicles) or it will be incorrectly diagnosed and incorrectly
repaired with correspondingly very high emissions. An incorrectly repaired
vehicle is unlikely to be able to pass an I/M retest. A similar logic
prevails for repairs on vehicles with ignition/misfire problems. Repairs on
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these vehicles are also largely of a component nature and the emission results
of repair are also very much an all-or-none issue. Thus, there is not assumed
to be a benefit resulting from mechanic training for post-1980 vehicles since
a vehicle will not be able to pass the I/M test until the owner finds a
mechanic who can correctly repair his vehicle.*
It may be the case that as more data on post-1980 vehicles becomes available,
benefits for the identification and repair of Misfueling and Secondary
category vehicles emitting above the standard may be practical and justifi-
able. At this point the available data is insufficient to support such an
analysis and the effect is assumed to be small. Should this situation change,
and Secondary and Misfueling category vehicles are also shown to contribute
significant I/M benefits, mechanic training might also play more of a role for
the post-1980 fleet.
* EPA recommends that local I/M planners provide for and encourage mechanic
training for post-1980 vehicles. This is to ensure an adequate number of
trained mechanics to be able to handle the repairs generated by an I/M program
as well as to help ensure competitively-based pricing in the repair market-
place.
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References
1. "Derivation of 1981 and Later Light Duty Vehicle Emission Factors for Low
Altitude, Non-California Areas", EPA-AA-IMS/80-8.
2. EPA Internal Memorandum; August 2, 1979; from Benjamin R. Jackson, Deputy
Assistant Administrator, Mobile Source and Noise Enforcement, to all
Regional Administrators.
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