United States Air and Radiation EPA420-R-01-021
Environmental Protection April 2001
Agency M6.EVP005
vvEPA Modeling Diurnal and
Resting Loss
Emissions from Vehicles
Certified to the Enhanced
Evaporative Standards
> Printed on Recycled Paper
-------
EPA420-R-01-021
April 2001
to the
M6.EVP.005
Larry C. Landman
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended, to present technical analysis of issues 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.
-------
ABSTRACT
This report documents the method used in MOBILE6 for
estimating the resting loss and diurnal emissions from vehicles
certified to the enhanced evaporative standards (i.e., 1999 and
newer vehicles plus some 1996 through 1998).
This report was originally released (as a draft) in November
1998. This current version is the final revision of that draft.
This final revision incorporates suggestions and comments
received from stakeholders during the 60-day review period and
from peer reviewers.
-------
TABLE OF CONTENTS
Page Number
1.0 Introduction 1
2.0 Data Sources 3
3.0 Simulating Test Data from In-Use 1996 and
Newer Vehicles 5
4.0 Analysis 7
4.1 Resting Loss Emissions 8
4.1.1 Properly Functioning Vehicles 8
4.1.2 Malfunctioning Vehicles 9
4.1.3 Gross Liquid Leakers 11
4.2 Diurnal Emissions 11
4.2.1 Properly Functioning Vehicles 12
4.2.1.1 Multi-Day Diurnal Emissions . . 13
4.2.2 Malfunctioning Vehicles 15
4.2.3 Gross Liquid Leakers 17
5.0 Distribution of ETP Vehicles 17
5.1 Effects of Changing Durability Requirements. . 18
5.2 Effects of On-Board Diagnostic Systems .... 19
6.0 Other Types of Evaporative Emissions 19
7.0 Evaporative Emissions Of Heavy-Duty Vehicles. ... 20
8.0 Effects of the ORVR Rule 21
9.0 Effects of the Tier-2 Rule 22
10.0 Summary 24
11
-------
TABLE OF CONTENTS (Continued)
APPENDICES
Page Number
A. Certification Tests on 65 ETP Vehicles 25
B. CAP-2000 Tests on Six Mercedes ETP Vehicles .... 27
C. Twenty-Five 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests .... 28
D. Ten "ETP-Like" Vehicles with Multiple RTD Tests
Passing Both the Purge and Pressure Tests .... 31
E. Eight 1990-1995 Model Year Vehicles
Failing (Only) the Purge Test 33
F. Five 1990-1995 Model Year Vehicles
Failing the Pressure Test 35
G. Peer Review Comments from H. T. McAdams 36
H. Peer Review Comments from Sandeep Kishan 49
I. Comments from Stakeholders 54
111
-------
Modeling Diurnal and Resting Loss Emission
from Vehicles Certified to the Enhanced Evaporative Standards
Report Number M6.EVP.005
Larry C. Landman
U.S. EPA Assessment and Standards Division
1.0 INTRODUCTION
Evaporative emissions of hydrocarbons (HC) are a significant
portion of the total HC emissions estimated in the MOBILE model.
In two parallel reports (M6.EVP.001 and 002), the Environmental
Protection Agency (EPA) identified the methods that are being
used in MOBILE6 to estimate resting loss and diurnal emissions
from 1995 and older model year vehicles. These estimates are
based on the results of real-time diurnal (RTD) tests of in-use
vehicles in which the ambient temperature cycled over a 24-degree
Fahrenheit range to simulate in real-time the daily heating and
cooling that parked vehicles experience over a 24-hour period.
Beginning with the 1996 model year, manufacturers were
required to certify at least twenty percent of their vehicles
using a new "enhanced" evaporative testing procedure (ETP); that
percentage of ETP vehicles was required to increase from the
twenty percent in 1996 up to one hundred percent by 1999. The
actual phase-in of these ETP vehicles proceeded at a slightly
faster pace (based on EPA's analysis of data from the Wisconsin
Inspection/Maintenance program for model years 1996-1999). The
phase-in rate required by the regulations* is given in Table 1
(below) along with the observed (actual) phase-in rate.
Table 1
Phase-In of Vehicles with
Enhanced Evaporative Controls
Model Required * Observed
Year Percentage Percentage
1995 0% 0%
1996 20% 30%
1997 40% 55%
1998 90% 90%
1999 100% 100%
The percentages for the "required" phase-in were copied from 40 CFR 86.096-8
-------
-2-
EPA expects that these ETP vehicles will have evaporative
emissions different than their pre-1996 (pre-ETP) counterparts
(thus, requiring distinct estimates). This assumption is based
on a number of changes that the manufacturers have implemented in
order to meet the enhanced evaporative standards. These changes
include, but are not limited to:
• "quick connects" that reduce the possibility of
improper assembly when the vehicle is serviced,
• advanced materials that are less permeable, less
susceptible to puncture, and more durable (i.e.,
elastomeric materials used in hoses and connectors),
• improvements made to the purge system (to enable the
vehicles to pass both the running loss test and the
multi-day diurnal test),
• tethered gas caps, and
• improved fractional-turn gas caps.
Since these changes are expected to result in improved control of
evaporative emissions, EPA used in MOBILES, a separate set of
estimates for both resting loss and diurnal emissions from these
vehicles.
In the original analyses that supported this rule, EPA
estimated that by requiring vehicles to meet these enhanced
evaporative standards the following would result:
• for those ETP vehicles with properly functioning
evaporative control systems (i.e., vehicles passing
both the purge test and the pressure test), full-day
diurnal emissions would be reduced by 50 percent
compared to the corresponding pre-ETP vehicles,
• for those ETP vehicles with malfunctioning evaporative
control systems (i.e., vehicles failing either the
purge test or the pressure test), there would be no
reduction (zero percent) of full-day diurnal emissions
compared to the corresponding pre-ETP vehicles, and
• for all ETP vehicles, resting loss emissions would be
reduced by 75 percent compared to the corresponding
pre-ETP vehicles.
In the previous version of the MOBILE model (i.e., MOBILES),
EPA used these estimated reductions to characterize the diurnal
and resting loss emissions of the ETP vehicles. EPA also used
the required phase-in rate (middle column in Table 1) to describe
the distribution of the ETP vehicles among the 1996-98 model year
vehicles in the in-use fleet.
-------
-3-
The goal of the analyses in this report was to review and
possibly replace those MOBILES hypotheses in the light of
additional data. Implementing this goal involved determining the
following three items:
(1) the percentage of ETP vehicles for each of the phase-in
model years (1996-98),
(2) the emissions (resting loss and diurnal) of these ETP
vehicles (see Sections 4.1 and 4.2), and
(3) the percentage (by age) of vehicles with properly
functioning evaporative control systems (see Section 5).
The first of these three items was relatively straightforward.
EPA chose to use the observed phase-in rate (third column in
Table 1), rather than the rate specified in the regulations, to
describe the percent of ETP vehicles for the 1996-98 model years
in the in-use fleet.
The parallel analyses (report M6.EVP.001) of the diurnal and
resting loss emissions for pre-enhanced (i.e., pre-1996 model
year) vehicles are based on results of tests of actual in-use
vehicles. However, the analyses in this report generally are not
based on testing of actual in-use ETP vehicles because EPA has
very few test results on that segment of the in-use fleet. In
this report, EPA explores methods of estimating the resting loss
and diurnal emissions from these in-use 1996 and newer vehicles
based primarily on RTD testing of older (pre-ETP) but similar
vehicles.
Since many of the estimates (developed in the report) of
resting loss and diurnal emissions for the ETP vehicles in
MOBILE6 are based on pre-ETP vehicles, EPA will likely revisit
these estimates when sufficient test data on actual ETP vehicles
become available.
2.0 DATA SOURCES
In the parallel analyses (report M6.EVP.001) on the pre-ETP
(i.e., 1995 and earlier model year) vehicles, EPA based its
estimates of resting loss and diurnal emissions on the results of
real-time diurnal (RTD) tests on 270 in-use vehicles. However,
at the time of this analysis on the 1996 and newer vehicles, EPA
had only two available sources of RTD test data on vehicles that
were certified to the new evaporative standards:
1) results of RTD testing used by the Air Resources Board
(ARE) of California and by the EPA (30 and 35 vehicles,
respectively) to certify new ETP vehicles (1996-97 model
year) (see Appendix A) and
2) results of RTD testing performed by Mercedes-Benz on six
of its 1996 model year vehicles (at two years of age) as
-------
-4-
part of the proposed Compliance Assurance Program (CAP
2000) (see Appendix B).
However, these test data (from these two sources) on the
1996 and newer vehicles have three serious limitations:
• First, all of the 1996 and newer vehicles from these two
sources had properly functioning evaporative control
systems. Since it is likely that some similar in-use
vehicles during the course of their useful life would
develop malfunctions in their evaporative control
systems, any analysis restricted to these data sets would
be limited by not including test results on such
malfunctioning in-use vehicles.
• Secondly, all of these RTD tests were performed using a
single test fuel with a Reid vapor pressure (RVP) of 9.0
psi and using a single temperature cycle (72 to 96
degrees Fahrenheit). Thus, using only these data, it
would be not be possible to predict evaporative emissions
at other combinations of temperature cycle and fuel
volatility.
• Finally, the RTD test data on all 65 of these vehicles
were reported in the form of full-day (not hourly)
emissions. However, EPA's procedure of estimating the
resting loss portion of the emissions requires the hourly
RTD emissions (at least for hours 19 through 24); thus,
EPA could not use these data to estimate resting loss
emissions.
To compensate for those significant limitations, EPA
supplemented those data with the results of RTD testing of older
vehicles (used in M6.EVP.001) that were not certified to the
enhanced evaporative standards. Two sources of those additional
RTD test results were:
3) RTD testing performed on 119 in-use 1971-95 model year
vehicles for EPA by its testing contractor and
4) RTD testing performed on 151 in-use 1971-91 model year
vehicles for the Coordinating Research Council (CRC).
Although none of those 270 in-use vehicles tested in the EPA
or CRC programs (sources 3 and 4) had been certified to the new
evaporative standards, the combined sample does include both:
• in-use vehicles with malfunctions in their evaporative
control systems
as well as
• vehicles for which the RTD test was performed over three
different temperature cycles and using fuels with at
least two different RVPs.
-------
-5-
Obviously, it would be inappropriate to use test data from all
270 of those vehicles. Only a few of the newest vehicles in that
sample are likely to be comparable to the actual ETP vehicles.
(Section 3 deals with the selection of that sub-sample.)
In Section 3.0, EPA discusses how it used RTD test results
from some of the older (i.e., 1990-95) vehicles (i.e., from
sources 3 and 4) to compensate for the limitations of the test
results on the 1996 and later vehicles.
3.0 SIMULATING TEST DATA FROM IN-USE 1996 AND NEWER VEHICLES
The MOBILE model must be able to estimate the resting loss
and diurnal evaporative emissions from the 1996 and newer model
year ETP vehicles over a variety of daily temperature cycles and
with a variety of fuel RVPs. However, as noted in the preceding
section, the only test data on those vehicles available at the
time of this analysis are with a single combination of fuel
volatility (RVP of 9.0 psi) and daily temperature profile (i.e.,
ambient temperatures cycling between 72 and 96 degrees
Fahrenheit). EPA, therefore, used the results of RTD tests on
pre-ETP vehicles (i.e., model years 1990 through 1995) to
estimate the effects on the actual "base line" emissions (from
source 1) of different fuel volatility and different temperature
cycles on the resting loss and diurnal evaporative emissions of
the 1996 and newer vehicles.
For the purpose of characterizing the effects of varying the
fuel RVP and/or the temperature cycle, EPA will continue (from
the parallel analyses) the approach of dividing the in-use fleet
into the following four strata:
1) The first of these four strata consists of vehicles
having substantial leaks of liquid gasoline (as opposed
to simply vapor leaks); these vehicles were labeled
"gross liquid leakers."
EPA proposed (in M6.EVP.001) using as a definition for
such vehicles the requirement that the hourly resting
loss (at 72 degrees Fahrenheit) be at least 1.0 grams per
hour of HC. EPA realizes this definition needs to be
amended to include vehicles having substantial leaks that
are apparent only when the engines are operating (e.g.,
some fuel line leaks). (See parallel report number
M6.EVP.009 entitled "Evaporative Emissions of Gross
Liquid Leakers in MOBILE6.")
2) The second of these four strata consists of vehicles (not
"gross liquid leakers") that pass both the purge and
pressure tests (i.e., vehicles with properly functioning
evaporative control systems).
-------
-6-
3) The third of these four strata consists of vehicles (not
"gross liquid leakers") that fail the pressure test
(regardless of their performance on the purge test).
4) The fourth of these four strata consists of vehicles (not
"gross liquid leakers") that fail only the purge test.
While neither the purge test nor the pressure test (which
are each being used to determine the stratification) actually
measures evaporative emissions, a failure of the vehicle on
either test is indicative of potential malfunctions of the
vehicle's evaporative control system. Additionally, the
recruitment of the vehicles in the third data source was
intentionally skewed to recruit a larger proportion of vehicles
with potentially malfunctioning evaporative control systems
(i.e., a stratified random recruitment). Therefore, the results
of any analysis must be weighted to correctly represent the
entire in-use fleet. Thus, the analyses will be stratified to
match the recruitment process.
As discussed previously, it was necessary to make use of the
RTD tests performed on older (1990-95 model year) vehicles to
predict the effects on the evaporative emissions of changes to
the temperature cycle or the fuel RVP. In order to make use of
those RTD tests on some of those 270 vehicles, EPA made the
following assumptions:
1) The 1996 and newer vehicles are expected to be port fuel
injected (PFI); therefore, EPA chose the 1990 to 1995
model year vehicles that were equipped with PFI as
appropriate surrogates.
2) To simulate the ETP vehicles with properly functioning
evaporative control systems, we then selected a subset
(of those 1990-95 model year vehicles equipped with PFI)
that passed both the purge test and the pressure test.
The resulting 25 vehicles are listed in Appendix C.
EPA believes that not all the vehicles in this sample of
25 pre-ETP vehicles (Appendix C) are appropriate for
simulating the actual ETP vehicles. Examining the sample
of 65 actual ETP vehicles in Appendix A, we note that the
first-day diurnal emissions range between 0.340 and 1.675
grams, with a mean of 0.745 and a median of 0.635.
We then restricted those 25 vehicles in Appendix C to
those having the first-day diurnal emissions of at most
1.7 grams (using a fuel with an RVP of 9.0 over a 72-96
temperature cycle), producing the seven vehicles listed
in Appendix D (all with multiple tests). This seven-
vehicle sub-sample has a mean full-day diurnal of 0.902
grams and a median of 0.741. (While EPA used this seven-
vehicle sample in its analyses, another analyst could
more closely approximate both the mean and median in
Appendix A by further restricting the first-day diurnal
emissions to no more than 1.0 grams instead of 1.7. The
-------
-7-
resulting (smaller) five-vehicle sub-sample has a mean of
0.726 and a median of 0.653. However, EPA believes that
the advantages of the somewhat larger sample size
outweigh the advantages of the slightly improved
statistical fit.) An additional three vehicles can be
added by applying that 1.7 gram limit to vehicles tested
only on a fuel with an RVP of 6.8 psi (resulting in the
total of 10 vehicles listed in Appendix D as being
possible "ETP-like").
Since the goal of this analysis is to predict the resting
loss and diurnal emissions over a range of temperature
cycles and fuel RVPs, we limited our analyses to the
seven vehicles in Appendix D that were tested over a
range of temperature cycles.
3) EPA believes that the RTD emissions from malfunctioning
enhanced evaporative control vehicles (i.e., vehicles
that developed problems with their evaporative control
systems) will be similar to the RTD emissions from the
1990 to 1995 model year vehicles that also develop
problems with their evaporative control systems. That
is, those 1996 and newer model year vehicles that had
failed either EPA's purge or pressure tests are expected
to have evaporative emissions similar to those 1990 to
1995 model year PFI vehicles that also failed the same
test.
Thirteen such vehicles were identified in the combined
EPA/CRC sample (eight of them failing only the purge test
and the remaining five failing the pressure test). (See
Appendices E and F, respectively.) EPA used the RTD
tests on these 13 vehicles to estimate the temperature
and fuel RVP effects on resting loss and diurnal
emissions for ETP vehicles that have malfunctioning
evaporative control systems.
4.0 ANALYSIS
As noted in two parallel reports (M6.EVP.001 and
M6.EVP.002), EPA is using (in MOBILE6) the results of the RTD
test to model two distinct mechanisms of evaporative emissions:
1) "Resting loss" emissions are always present, regardless
of vehicle activity, and are relatively weakly related to
the ambient temperature as opposed to diurnal emissions
that are related to the rise in temperature.
The earlier reports calculated the hourly resting loss
emissions to be the mean of the RTD emissions from hours
19 through 24 at the nominal temperature for hour 24.
This method permitted EPA to estimate the hourly resting
loss emissions at three distinct temperatures (60, 72,
and 82 degrees Fahrenheit). In those analyses, resting
-------
-8-
loss emissions were determined to be independent of the
RVP of the test fuel.
2) "Diurnal" emissions are the pressure-driven emissions
resulting from the daily increase in temperature.
The diurnal emissions were calculated by first estimating
the resting loss value for the ambient temperature at
each hour of the 24-hour cycle, and then subtracting that
temperature-adjusted resting loss estimate from the RTD
hourly test results.
In those two parallel reports, this approach permitted EPA to use
the RTD test results to analyze separately both the relatively
constant resting loss emissions and the (pressure driven) diurnal
emissions.
4.1 Resting Loss Emissions
In the parallel analyses of the resting loss emissions of
the pre-ETP vehicles (report M6.EVP.001), EPA used regression
analyses of the resting loss emissions (at three temperatures) to
model the resting loss emissions. This approach was repeated in
the previous draft version of this report (i.e., the version
reviewed by our stakeholders and by two formal peer reviewers).
4.1.1 Resting Loss Emissions of Properly Functioning Vehicles
EPA initially (i.e., in the previous draft version of this
report) selected from Appendix C the (averaged) three resting
loss emissions from the seven vehicles that had been tested over
three temperature cycles. This yielded the following table of
results.
Table 2
Mean Hourly Resting Loss Emissions
For Seven "ETP-Like Vehicles" (grams/day)
Vehicle
Number
5032
5038
5046
5047
5066
5068
5081
Mean
Std. Dev.
Temp<
60
0.0045
0.0033
0.0075
0.0050
0.0000
0.0060
0.0050
0.0045
0.0024
srature (degr
72
0.0070
0.0053
0.0100
0.0120
0.0050
0.0095
0.0040
0.0075
0.0030
ees F)
82
0.0150
0.0093
0.0305
0.0150
0.0075
0.0235
0.0090
0.0157
0.0085
-------
-9-
In the previous analyses, EPA then performed regression
analyses to model those three mean resting loss emissions (grams/
hour) (from the preceding table) as a function of temperature.
After the draft was released (for comments), it was noted that:
1. The sample size is quite small.
2. The estimated hourly emissions are close to the limit
that the equipment can measure.
3. The vehicles are not true ETP vehicles, they only
simulate what we expect from ETP vehicles.
4. If we estimate the hourly resting loss of comparable
pre-ETP vehicles (using the equation from M6.EVP.001) and
then compare those estimates to the means in the
preceding table, we find that these means are reductions
of 75 to 85 percent of the pre-ETP estimates. (This is
consistent with the final "bullet" on page 2, which is a
re-statement of the conclusion reached in the original
regulatory analysis.)
Base on these four points (especially the last two), EPA revised
its approach to estimating the resting loss emissions from the
properly functioning ETP vehicles. Rather than use the new
equation derived in the earlier draft version of this report, EPA
chose to simply apply the previously estimated reduction factor
of 75 percent to the equation for the comparable pre-ETP vehicles
(from M6.EVP.001) . This produces equation (1) below:
Hourly Resting Loss (grams/hr) = -0.035168 + [0.000703 * Temperature (°F)] (1)
EPA uses (in MOBILE6) equation (1) to estimate the hourly resting
loss emissions (in grams per hour) of that portion of the fleet
of ETP vehicles with properly functioning evaporative control
systems.
Equation (1) predicts that the mean hourly resting loss
emissions (for the fleet of 1996 and newer model year vehicles
with properly functioning evaporative control systems) would be
negative for all ambient temperatures below 50.1 degrees
Fahrenheit. EPA will (in MOBILE6) assume, that for each of the
hours of the day that those temperatures occur (i.e., hourly
temperature < 50.1 F), the resting loss emissions will be set to
zero grams.
4.1.2 Resting Loss Emissions of Malfunctioning Vehicles
To estimate the resting loss emissions from ETP vehicles
with malfunctioning evaporative control systems (i.e., those ETP
vehicles that would fail either the purge or the pressure test),
EPA followed the same three-step pattern that was used for ETP
vehicles with properly functioning evaporative control systems
(in Section 4.1.1). That is:
-------
-10-
1. A sample of pre-ETP vehicles was identified that could
simulate these ETP vehicles.
In Section 3.0, EPA proposed using five 1990-95 model
year PFIs to represent the 1996 and newer model year
vehicles that failed the pressure test (only four of
which were tested over all three temperature cycles) and
using eight vehicles to represent the 1996 and newer
model year vehicles that failed the purge test (see
Appendices E and F).
2. The means of the resting loss emissions were regressed
against temperature using the three temperature points
(60, 72, 82 F).
Resting loss data on the 12 vehicles that were tested
over all three temperature cycles were combined (into a
single stratum) and analyzed. The resulting equation was
contained in the previous draft of this report (that was
released for comments).
3. After the previous draft was released (for comments), it
was noted that these means also could have been modeled
simply as reductions of 75 to 85 percent of the pre-ETP
estimates. (Again, this is consistent with the final
"bullet" on page 2, which is a re-statement of the
conclusion reached in the original regulatory analysis.)
Therefore, EPA revised its approach to estimating the
resting loss emissions from the malfunctioning ETP vehicles.
Rather than use the new equation derived in the earlier draft
version of this report, EPA chose to simply apply the previously
estimated reduction factor of 75 percent to the equation for the
comparable pre-ETP vehicles (from M6.EVP.001). This produces
equation (2) below:
For ETP Vehicles that Fail the Pressure Test:
Hourly Resting Loss (grams/hr) = -0.02731 + [0.000703 * Temperature (°F)] (2)
EPA uses (in MOBILE6) equation (2) to estimate the hourly resting
loss emissions (in grams per hour) of that portion of the fleet
of ETP vehicles that fail the pressure test. Additionally, the
scope (domain) of equation (1) was expanded to cover all ETP
vehicles that pass the pressure test regardless of their
performance on the purge test.
Equation (2) predicts that the mean hourly resting loss
emissions (for the fleet of 1996 and newer model year vehicles
that fail the pressure test) would be negative for all ambient
temperatures below 38.8 degrees Fahrenheit. This will not
present a problem, because (using the analyses from earlier
versions of the MOBILE model) EPA will (in MOBILE6) assume, that
for each of the hour of the day that the temperature does not
exceed 40, the hourly resting loss emissions will be set to zero
grams.
-------
-11-
4.1.3 Resting Loss Emissions of "Gross Liquid Leakers"
In a parallel report (M6.EVP.001), EPA proposed that, for
the pre-1996 vehicles classified as gross liquid leakers, the
resting loss emissions are virtually independent of temperature,
averaging 9.16 grams per hour. EPA will continue to use that
assumption for the 1996 and newer vehicles that were certified to
the enhanced evaporative standard. That is, the hourly resting
loss emissions of all "gross liquid leakers" will be set at 9.16
grams per hour regardless of vehicle type, or model year, or
ambient temperature.
4.2 Diurnal Emissions
The pattern of the analyses of the diurnal emissions closely
paralleled the pattern that developed with the resting loss
emissions. That is:
1. A samples of pre-ETP vehicles were identified that could
simulate these ETP vehicles (Appendices D, E, and F).
2. The means of the diurnal emissions were regressed against
a variable (VP_Product) developed in report M6.EVP.001.
3. After the previous draft was released (for comments), it
was noted that these means could have been modeled simply
as reductions of the pre-ETP estimates. (See the final
"bullet" on page 2.)
In Section 4.1, we developed equations (1 and 2) that
estimate the resting loss emissions for each temperature (in
degrees Fahrenheit). Applying those equations to each hour of
the full 24 hours of the RTD test, and then adding the 24
"temperature corrected" hourly resting loss emissions produces
the full day's total resting loss (in grams). Subtracting that
quantity from each of the RTD test scores yields the estimated
(full-day) diurnal emissions in Appendices C, D, E, and F.
Two factors that significantly affect a vehicle's diurnal
emissions (see M6.EVP.001 and M6.EVP.002) are:
• the Reid vapor pressure (RVP) of the test fuel and
• the temperature cycle, as represented by the
combination of the cycle's midpoint temperature and
temperature range.
In parallel reports (M6.EVP.001 and M6.EVP.002), we created a
single parameter that incorporated both of those factors. That
new parameter is based on the vapor pressure (VP) of the fuel.
In those reports, we used both the RVP of the fuel and the
ambient temperature to estimate the vapor pressure curve. (The
RVP is the VP measured at 100 degrees Fahrenheit.) The VP was
then used to create that new parameter which was used as the
-------
-12-
variable on which diurnal emissions were calculated. That new
parameter is defined by the following formula, equation (3).
VP_Product_Term = (VPHIGH - VPLOW) * (VPH|GH + VPLOW) /2 (3)
Where
is the VP (in kiloPascals) associated with the
day's high temperature.
is the VP (in kiloPascals) associated with the
day's low temperature.
The analyses in those parallel reports modeled the diurnal
emissions as functions either of that VP product term or powers
of that VP product term.
4.2.1 Diurnal Emissions of Properly Functioning Vehicles
Appendix D identifies 10 pre-ETP vehicles whose RTD
emissions suggests that they might be representative of ETP
vehicles with properly functioning evaporative control systems
(i.e., passing both the purge and pressure tests). Averaging the
45 test results on those 10 vehicles produces the following table
(including both standard deviations and 90 percent confidence
intervals for each of the nine values).
Table 3
Mean Diurnal Emissions of 10 Possible "ETP-Like" Vehicles
(grams / day)
Fuel
RVP
(psi)
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP
Product
Term
322
489
684
375
567
789
655
969
1,324
Count
2
3
3
4
8
5
7
7
6
Mean
Diurnal
(grams)
0.4740
0.3220
0.6487
0.1210
0.4398
0.7096
0.1727
0.5533
2.9615
Standard
Deviation
(grams)
0.0792
0.1497
0.2844
0.0579
0.1615
0.2133
0.0799
0.3419
3.0172
90% Conf. Interval
0.3819
0.1798
0.3786
0.0734
0.3458
0.5527
0.1230
0.3407
0.9353
0.5661
0.4642
0.9187
0.1686
0.5337
0.8665
0.2224
0.7659
4.9877
After initially modeling these values as a function of that
VP_Product term, it was noted that they could be modeled simply
as a reduction of estimated pre-ETP diurnal emissions (from
report M6.EVP.001). The exact magnitude of that reduction was
more difficult to determine. Comparing these nine mean diurnals
-------
-13-
with the corresponding estimates for the pre-ETP vehicles, it was
noted that these means represent reductions ranging from 48 to 90
percent from the predicted pre-ETP vehicles.
Since we are uncertain how representative these 10 pre-ETP
vehicles are of the actual in-use ETP vehicles, EPA decided to
retain the earlier, more conservative estimate (based on
engineering analyses). That is the full-day diurnal emissions of
these ETP vehicles (with properly functioning evaporative control
systems) will be estimated as being 50 percent reductions of the
corresponding pre-ETP vehicles. This produces equation (4) below:
Full-Day Diurnal (grams) = 0.19415 + [0.00252 * Sqr of VP_Product / 1,000] (4)
In MOBILE6, EPA uses equation (4) to estimate the 24-hour
diurnal emissions of all ETP vehicles with properly functioning
evaporative control systems with the following two modifications:
1) Regardless of the increase in ambient temperatures, there
are no diurnal emissions until the ambient temperature
exceeds 40°F. (This assumption was used consistently for
all evaporative emissions in MOBILES.)
For a temperature cycle in which the daily low temperature
is below 40° F, EPA calculate the diurnals emissions for
that day as an interrupted diurnal (see M6.EVP.002) that
begins once the ambient temperature reaches 40° F.
2) The 24-hour diurnal emissions will be zero grams for any
temperature cycle in which the diurnal temperature range is
zero degrees Fahrenheit (i.e., a constant temperature
throughout the entire day).
For temperature cycles in which the diurnal temperature
range is between zero and ten degrees Fahrenheit, the 24-
hour diurnal emissions will be a linear interpolation
between the predicted value for the ten-degree cycle (with
the appropriate RVP) and zero grams.
4.2.1.1 Multi-Day Diurnal Emissions of Properly Functioning Vehicles
In a parallel report (M6.EVP.003, entitled "Evaluating
Multiple Day Diurnal Evaporative Emissions Using RTD Tests"), EPA
develops equations for estimating the RTD emissions of the second
and third days of a multi-day diurnal test based on several
factors, including the reciprocal of the diurnal emissions of the
first day. Those analyses were based on the 270-vehicle sample
(all pre-ETP vehicles) described in Section 2.0 of this report.
The estimate (from that parallel report) of the ratio of the day-
2 diurnal to the day-1 diurnal for fuel-injected vehicles that
pass both the purge and pressure tests is given by the formula:
-------
-14-
Ratio = 0.74 + [ 47.48 - ( 0.70 * Mid-Point_Temp )
+ ( 0.12 * Weathered_RVP * Mid-Point_Temp )
- ( 8.11 * Weathered_RVP )] / Full-Day_Diurnal
Applying this formula to the data in Appendix A (with Mid-Point
Temperature = 84, RVP = 9.0, and full-day diurnal = 0.745)
produces a ratio (of day-2 to day-1) of 9.344. That is, the
predicted second day diurnal would be an unrealistically high
value of 6.96 grams (far higher than any of the values in
Appendix A). A similar problem exists in using the equations
from M6.EVP.003 to estimate the third day diurnals from these
vehicles. From a mathematical standpoint, this problem results
from dividing by the extremely low diurnal emissions associated
with this single stratum.
To obtain more realistic estimates of the second and third
day diurnals from ETP vehicles with properly functioning
evaporative control systems, EPA examined the RTD test results in
Appendix A. Most (56 out of 65) of those actual ETP vehicles
exhibited a decrease in RTD emissions from the first day to the
second day, and the same number exhibited a decrease from the
second day to the third day. (The decrease in RTD emissions was
small, averaging four to ten percent.) Performing regression
analyses on the 65 RTD tests in Appendix A, we obtained the
following two tables. (Table 4 contains the statistics of the
linear regression of the second day RTD to the first day RTD, and
Table 5 contains the statistics of the linear regression of the
third day to the second day.
Table 4
Regression of Day-2 versus Day-1 RTD Emissions
(65 Certification ETP Vehicles)
Dependent variable is:
No Selector
R squared = 91 .0% R squared (adjusted) = 90
s = 0.0905 with 65 - 2 = 63 degrees of freedom
Source
Regression
Residual
Variable
Constant
Day_1_of_3
9%
Sum of Squares df Mean Square
5.24310
0.516051
Coefficient
-0.013504
0.924233
1
63
s.e. of Coeff
0.0294
0.0365
5.24310
0.008191
t-ratio
-0.459
25.3
Day_2_of_3
F-ratio
640
prob
0.6479
< 0.0001
-------
-15-
Table 5
Regression of Day-3 versus Day-2 RTD Emissions
(65 Certification ETP Vehicles)
Dependent variable is:
No Selector
R squared = 93.7% R squared (adjusted) = 93
s = 0.0770 with 65 - 2 = 63 degrees of freedom
Source
Regression
Residual
Variable
Constant
Day_2_of_3
.6%
Sum of Squares df Mean Square
5.57703
0.373598
Coefficient
-0.016523
0.984061
1
63
s.e. of Coeff
0.0237
0.0321
5.57703
0.005930
t-ratio
-0.698
30.7
Day_3_of_3
F-ratio
940
prob
0.4876
< 0.0001
Based on this limited analysis, EPA will estimate (in
MOBILE6) the second and subsequent day of diurnal emissions to be
unchanged from the first day for this stratum of ETP vehicles
with properly functioning evaporative control systems.
4.2.2 Diurnal Emissions of Malfunctioning Vehicles
Appendices E and F identify eight pre-ETP vehicles failing
only the purge test and five pre-ETP vehicles failing the
pressure test that they might be representative of ETP vehicles
with malfunctioning evaporative control systems. Averaging the
test results on those vehicles produce Tables 6 and 7,
respectively.
-------
-16-
Table 6
Mean Diurnal Emissions of Eight 1990-1995 PFI Vehicles
Failing ONLY the Purge Test
Fuel
RVP
(psi)
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP
Product
Term
322
489
684
375
567
789
655
969
1,324
Count
2
3
3
5
7
7
8
8
5
Mean
Diurnal
(grams)
0.6565
1.1607
2.5117
1.3934
2.4154
7.5069
3.2950
6.3963
17.6246
Standard
Deviation
(grams)
0.9284
1 .4806
3.2896
2.3378
2.5391
7.8215
4.8373
7.5931
6.1136
90% Conf. Interval
0
0
0
0
0.8368
2.6438
0.4817
1.9801
13.1271
1.7364
2.5669
5.6360
3.1133
3.9941
12.3699
6.1083
10.8124
22.1221
Table 7
Mean Diurnal Emissions of Five 1990-1995 PFI Vehicles
Failing the Pressure Test
Fuel
RVP
(psi)
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP
Product
Term
322
489
684
375
567
789
655
969
1,324
Count
1
1
1
4
5
4
4
4
4
Mean
Diurnal
(grams)
3.8810
11.4250
18.8320
7.7988
7.9950
16.6288
10.5270
21.0090
37.3610
Standard
Deviation
(grams)
N/A
N/A
N/A
5.6994
7.3946
11.9879
9.1762
14.6606
24.6794
90% Conf. Interval
N/A
N/A
N/A
3.1110
2.5550
6.7687
2.9796
8.9506
17.0622
N/A
N/A
N/A
12.4865
13.4350
26.4888
18.0744
33.0674
57.6598
After initially modeling these two tables of values as
functions of the VP_Product term, it was noted that the equations
already developed (in report M6.EVP.001) as estimates of the
diurnal emissions from pre-ETP vehicles (with malfunctioning
evaporative control systems) accurately model these two sets of
data. Therefore, EPA is using (in MOBILE6) the following two
equations from (M6.EVP.001):
-------
-17-
For ETP Vehicles that Fail ONLY the Purge Test:
Full-Day Diurnal (grams) = 3.25800 + [0.00941 * Sqr of VP_Product / 1,000] (5)
For ETP Vehicles that Fail the Pressure Test:
Full-Day Diurnal (grams) = 0.47846 + [0.01497 * Sqr of VP_Product / 1,000] (6)
4.2.3 Diurnal Emissions of "Gross Liquid Leakers"
In a parallel report (Section 5 of report number
M6.EVP.002), EPA proposed estimating the mean of the diurnal
emissions for each temperature cycle of the vehicles classified
as "gross liquid leakers" using equation (7) , below. That
equation predicts diurnal emissions as a function of a single
variable, the diurnal temperature range (i.e., the daily high
temperature minus the daily low temperature):
For ETP Vehicles That Have Gross Liquid Leaks:
Full-Day Diurnal (grams) = 20.058 + [3.343* Diurnal_Temperature_Range ] (7)
EPA will continue to use equation (7) to estimate the mean
24-hour diurnal emissions of all gross liquid leakers regardless
of model year whenever the diurnal temperature range is at least
10 degrees Fahrenheit. The 24-hour diurnal emissions will be
zero grams for any temperature cycle in which the diurnal
temperature range is zero degrees Fahrenheit (i.e., a constant
temperature throughout the entire day). For temperature cycles
in which the diurnal temperature range is between zero and ten
degrees Fahrenheit, the 24-hour diurnal emissions will be a
linear interpolation of the predicted value for the ten-degree
cycle (i.e., 53.49 grams) and zero grams.
5.0 DISTRIBUTION OF ETP VEHICLES
In a parallel report (M6.EVP.006, entitled "Estimating
Weighting Factors for Evaporative Emissions in MOBILE6"), EPA
developed estimates for the distribution of the pre-ETP (i.e.,
pre-1996) vehicles for the following four strata (identified in
Section 3.0):
1) "gross liquid leakers," or simply GLLs (i.e., vehicles
having substantial leaks of liquid gasoline as opposed to
simply vapor leaks),
2) non-GLLs that pass both the purge and pressure tests
(i.e., vehicles with properly functioning evaporative
control systems),
-------
-18-
3) non-GLLs that fail the pressure test (regardless of their
performance on the purge test), and
4) non-GLLs that fail only the purge test.
At each age (where "age" equals "current year" minus "model
year"), the sum of the four strata must equal 100 percent. For
each of the first three of the four strata (of pre-ETP vehicles),
an equation was developed that estimated the fraction of the
vehicle population contained within that stratum for each "age"
(where "age" equals 0, 1, 2, . . ., 24). The fourth stratum
(non-ETPs failing only purge) is simply the remainder (i.e., 100
percent minus the sum of the other three strata). Two factors
are expected to alter that distribution of the pre-ETP vehicles:
• the increased durability requirements of the ETP rule
and
• the presence of an on-board diagnostic (OBD) system.
5.1 Effects of Changing Durability Requirements
The ETP rules require an increase in the durability of the
evaporative control systems of the newer vehicles. Specifically,
the ETP vehicles are required to meet the evaporative standards
for ten years (100,000 miles) instead of five years (50,000
miles). EPA expects that this doubling of the durability
requirement will affect the distribution of those four strata.
Until in-use data on the ETP vehicles become available, EPA
will assume in MOBILE6 that the doubling of the durability
requirement will result in reducing the failure rates to that of
vehicles half the age. For example, the failure rates (on the
purge test, pressure test, or liquid leak criterion) observed on
the pre-ETP vehicles at the age of eight years would not occur on
the ETP vehicles until twice that age (i.e., 16 years).
Modifying the equations (by replacing "AGE" with "AGE/2")
for the pre-ETP vehicles in the parallel report (M6.EVP.006)
produces the following three equations to predict the
distributions (at each age) for the ETP strata:
Rate of Gross Liquid Leakers on the RTD Test for the ETP Vehicles:
0.08902
GLL
1 +414.613*exp(-0.1842*AGE)
Failure Rate on Pressure Test of ETP Vehicles:
0.6045
17.733*exp[-0.003405*(AGEA2)]
(1 - GLL )
-------
-19-
Rate of Passing Both for ETP Vehicles:
0.7200
/
~ V1 ' 1 +13.40*
exp[-0.003625*(AGEA2)]
5.2 Effects of On-Board Diagnostic (OBD) Systems
The majority of the light-duty vehicles have been equipped
with on-board diagnostic (OBD) systems since the early 1980's.
The latest generation of these systems (OBD II) is designed to
warn the driver when a malfunctioning component is likely to
cause high (exhaust or evaporative) emissions.
The factors expected to determine the effect of the presence
of OBD on the evaporative emissions of the ETP vehicles are:
• the ability of OBD to identify malfunctions that result
in high evaporative emissions and
• the response of the driver/owner to that warning.
The "response of the driver/owner to that warning" is most likely
dependent the manufacturer's warrantee and the presence of an
Inspection / Maintenance (I/M) program. These factors are
explored in detail in parallel reports (section 3.4.2 of report
M6.EXH.009, entitled "Determination of CO Basic Emission Rates,
OBD and I/M Effects for Tier 1 and Later LDVs and LDTs").
6.0 OTHER TYPES OF EVAPORATIVE EMISSIONS
Two other types of evaporative emissions (in addition to the
resting loss and diurnal emissions) are affected by the ETP
requirements. These are the hot soak emissions and the running
loss emissions.
Hot Soak emissions are the evaporative emissions produced
after the vehicle has been driven. These emissions can also be
thought of as "trip end" emissions. They result from the fact
that the vehicle, engine, fuel delivery system including tank,
are all well above ambient temperatures after all but the very
shortest trips. In MOBILE6, EPA assumes the following effects of
the ETP requirements on hot soak emissions:
• no effect on vehicles classified as "gross liquid
leakers,"
• a reduction (compared to the pre-ETP vehicles) of 50
percent on LDGVs with properly functioning evaporative
control systems (i.e., vehicles that pass both the
purge and pressure tests), and
• a smaller reduction on vehicles with malfunctioning
evaporative control systems (i.e., vehicles that fail
either the purge or pressure tests).
-------
-20-
Th is "smaller reduction" on vehicles with malfunctioning
evaporative control systems depends upon the ambient temperature.
The reduction is 30 percent for ambient temperatures of at least
95 degrees Fahrenheit. The reduction decreases (linearly) to
zero at temperatures of 65 degrees Fahrenheit or colder.
Therefore, the reduction (as a percentage) is given by the
following formula:
Reduction = Temperature - 65, where
the "Reduction" is "capped" by zero and 30 (percents) .
Similar to the hot soak emissions, the running loss
evaporative emissions, which are produced during periods of
vehicle operation (that is, driving or idling), are also affected
by the ETP requirements:
• no effect on running loss emissions for vehicles
classified as "gross liquid leakers,"
• a reduction (compared to the pre-ETP vehicles) of 80
percent on LDGVs with properly functioning evaporative
control systems (i.e., vehicles that pass both the
purge and pressure tests), and
• a smaller reduction on vehicles with malfunctioning
evaporative control systems (i.e., vehicles that fail
either the purge or pressure tests).
This "smaller reduction" on running loss emissions for
vehicles with malfunctioning evaporative control is identical to
the corresponding reduction in hot soak emissions for these same
vehicles.
7.0 EVAPORATIVE EMISSIONS OF HEAVY-DUTY VEHICLES
EPA did not conduct RTD testing of the heavy-duty gasoline-
fueled vehicles (HDGVs). In MOBILE6, EPA estimates the
evaporative emissions of these untested vehicle types
proportional to their emission standards. (This is the same
approach used in earlier versions of MOBILE.)
For the HDGVs from 8,501 pounds gross vehicle weight rating
(GVWR) up through 14,000 pounds (i.e., HDGV classes lib and 3),
the ETP standard for the combined RTD and hot soak tests is 3.0
grams (as compared to the 2.0 grams for the LDGTs). Therefore,
in MOBILE6, this ratio (i.e., 1.5 = 3.0 / 2.0) is applied to the
applicable LDGT evaporative emissions (i.e., hot soak emissions,
resting loss emissions, and diurnal emissions) to estimate the
corresponding evaporative emissions for these HDGVs that are not
GLLs. (We are assuming that the average emissions of the GLLs
are not affected by the ETP requirements.)
-------
-21-
Similarly, for the HDGVs over 14,000 pounds (i.e., HDGV
classes 4 through 8b and busses), since the combined RTD and hot
soak tests is 4.0 grams, a multiplier of 2.0 (i.e., 2.0 = 4.0 /
2.0) is applied to the applicable LDGT evaporative emissions from
non-GLLs.
8.0 EFFECTS OF THE ORVR RULES
"Refueling Emissions" are the evaporative emissions produced
while the vehicle is being refueled and gasoline vapors are
forced out as liquid gasoline takes their place. The refueling
emissions are basically the average displaced vapor (5.26 grams
of HC) per gallon of dispensed fuel, plus a small amount for
spillage (0.31 grams). These refueling emissions can be reduced
with the use of Onboard Refueling Vapor Recovery (ORVR) systems.
The phase-in rates (percents of vehicles) required by the
regulations are given in the following table (Table 8). (The
ORVR regulations for light-duty cars and trucks were issued April
6, 1994 (59 FR 16262) . The ORVR regulations for HD Class 2b
vehicles were issued October 6, 2000 (65 FR 59924) as part of the
"2004 Heavy-Duty" rule.)
Table 8
Phase-In of ORVR Systems
(Required Percentages by Vehicle Class and Model Year)
Model
Year
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
LDGVs
0%
40%
80%
100%
100%
100%
100%
100%
100%
100%
LDGTs
GVWR Up
To 6,000
0%
0%
0%
0%
40%
80%
100%
100%
100%
100%
LDGTs
6,001 Up
To 8,500
0%
0%
0%
0%
0%
0%
0%
40%
80%
100%
HDGTs
8,501 Up
To 10,000
0%
0%
0%
0%
0%
0%
0%
0%
80%
100%
For light-duty cars and trucks, the ORVR effectiveness was
assumed (in MOBILES) to reduce the portion of refueling emissions
that does not include spillage by 98 percent, and to reduce the
spillage by 50 percent. We will continue that assumption for
-------
-22-
MOBILE6. New for MOBILE6, is the extension of that assumption to
the HD Class 2b vehicles, phasing-in with the 2005 model year.
Table 8 indicates that the phase-in for the HD Class 2b
vehicles assumes zero percent for the 2004 model year. Actually,
the regulations permit, for the 2004 model year, an optional
phase-in of up to 40 percent. However, since the 2004 phase-in
is only "optional," we will assume a value of zero until actual
vehicle counts are available.
9.0 EFFECTS OF THE TIER-2 RULE
Beginning with the 2004 model year, manufacturers will be
required to certify at least twenty-five percent of their
gasoline-fueled passenger cars (LDVs) to the new Tier-2
standards; that percentage of Tier-2 vehicles will increase to
one hundred percent within a few additional years. A similar
phase-in will be required of the light-duty gasoline-fueled
trucks (LDGTs) and the heavy-duty gasoline-fueled trucks (HDGTs)
in class 2b (6,001 through 8,500 pounds GVWR). The 2007 Heavy-
Duty rule extended this to all of the other heavy-duty and bus
classes. The phase-in rates (percents of vehicles) required by
the regulations are given in the following table (Table 9).
Table 9
Phase-In of Tier-2 Vehicles
(Required Percentages by Vehicle Class and Model Year)
Model
Year
2003
2004
2005
2006
2007
2008
2009
LDGVs
0%
25%
50%
75%
100%
100%
100%
LDGTs
GVWR Up
To 6,000
0%
25%
50%
75%
100%
100%
100%
LDGTs
6,001 Up
To 8,500
0%
0%
0%
0%
0%
50%
100%
ALL HDGTs
GVWR
Over 8,500
0%
0%
0%
0%
0%
50%
100%
Concurrent with the phase-in of the new (more stringent)
Tier-2 evaporative requirements will be the phase-in by
California of its even more stringent LEV II evaporative
standards. The evaporative standards for both the Tier-2 and LEV
II programs are given in Table 10 (on the following page).
-------
-23-
Table 10
Evaporative Standards Under Tier-2 and LEV II
(grams/test over 3-day diurnal + hot soak)
Vehicle Class
LDV
LDT-1
LDT-2
LDT-3 & 4
Current (ETP)
2.0
Tier 2
0.95
0.95
0.95
1.2
LEV II
0.5
0.65
0.65
0.95
As explained in a parallel report (report number M6.EXH.007,
entitled "Accounting for the Tier 2 and Heavy-Duty 2005/2007
Requirements in MOBILE6"), the vehicle manufacturers have stated
that rather than producing separate systems for California and
the rest of the country, they will produce single federal systems
that also comply with the more stringent California standards.
Therefore, in MOBILE6, EPA assumes the evaporative emissions will
be based on the LEV II standards.
Thus, in MOBILE6, EPA assumes the following effects of the
Tier-2 requirements on diurnal, resting loss, and hot soak
emissions:
• no effect on vehicles classified as "gross liquid
leakers,"
• no effect on vehicles with malfunctioning evaporative
control systems (i.e., vehicles that fail either the
purge or pressure tests),
• a reduction (compared to ETP vehicles) of 75 percent on
all LDVs with properly functioning evaporative control
systems (i.e., vehicles that pass both the purge and
pressure tests) ,
• a reduction (compared to ETP vehicles) of 67.5 percent
on LDGTs up to 6,000 pounds (GVWR) (i.e., LDT-1 and
LDT-2) with properly functioning evaporative control
systems (i.e., vehicles that pass both the purge and
pressure tests) ,
• a reduction (compared to ETP vehicles) of 52.5 percent
on all LDGTs with GVWR from 6,001 to 8,500 pounds
(i.e., LDT-3 and LDT-4) and with properly functioning
evaporative control systems (i.e., vehicles that pass
both the purge and pressure tests),
• for HDGTs with GVWR up to 14,000 pounds and with
properly functioning evaporative control systems (i.e.,
vehicles that pass both the purge and pressure tests),
emissions will be 1.474 times the corresponding
emissions of the Tier-2 LDGTs with GVWR from 6,001 to
-------
-24-
8,500 (i.e., proportional to the certification
standards), and
for HDGTs with GVWR over 14,000 pounds and with
properly functioning evaporative control systems (i.e.,
vehicles that pass both the purge and pressure tests),
emissions will be 2.000 times the corresponding
emissions of the Tier-2 LDGTs with GVWR from 6,001 to
8,500 (i.e., proportional to the certification
standards).
10.0 SUMMARY
For most of the 1996 and newer model year vehicles that were
certified to the enhanced evaporative testing procedure (ETP),
EPA will model (in MOBILE6) the resting loss and diurnal
emissions similar to what was done in the previous version of the
MOBILE model (i.e., MOBILES). That is:
• For those ETP vehicles with properly functioning
evaporative control systems (i.e., vehicles passing
both the purge test and the pressure test), full-day
diurnal emissions will be reduced by 50 percent
compared to the corresponding pre-ETP vehicles.
• For those ETP vehicles with malfunctioning evaporative
control systems (i.e., vehicles failing either the
purge test or the pressure test), there will be no
reduction (zero percent) of full-day diurnal emissions
compared to the corresponding pre-ETP vehicles.
• For all ETP vehicles, resting loss emissions will be
reduced by 75 percent compared to the corresponding
pre-ETP vehicles.
New to MOBILE6 are:
• The emissions of "Gross Liquid Leakers" will be
unaffected by the ETP requirements.
• The assumption of increased durability will reduce the
predicted number of higher emitting vehicles.
• The presence of an OBD II system will reduce the number
of higher emitting vehicles (depending upon the
manufacturer's warrantee and I/M programs).
• The Tier-2 requirements will reduce the emissions of
gasoline-fueled cars and trucks (up to 14,000 pounds
GVWR) that have properly functioning evaporative
control systems.
-------
-25-
Appendix A
Certification Tests on 65 ETP Vehicles
30 Vehicles for California Certification
Source
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
CERT-ARB
Make
FORD
FORD
FORD
FORD
FORD
GM
GM
GM
GM
MITSUB
CHRYS
CHRYS
TOYOTA
TOYOTA
TOYOTA
TOYOTA
HONDA
HONDA
HONDA
HONDA
MAZDA
MAZDA
MAZDA
MAZDA
NISSAN
NISSAN
VOLKS
VOLKS
VOLKS
ISUZU
(grc
Dav1
0.824
0.415
0.680
0.415
1.675
1.100
1.356
1.150
0.757
0.742
0.810
0.719
0.680
0.530
0.520
0.610
0.360
0.460
0.410
0.490
0.625
0.635
0.548
0.500
0.518
0.549
0.734
0.960
1.250
1.235
RTD Test
ims per dc
Day 2
0.765
0.385
0.420
0.430
1.710
1.105
1.147
1.040
0.645
0.545
0.785
0.688
0.662
0.430
0.450
0.500
0.320
0.370
0.350
0.420
0.740
0.584
0.508
0.530
0.463
0.460
0.625
0.870
1.040
1.115
»y)
Day 3
0.711
0.355
0.420
0.420
1.765
1.225
1.064
0.847
0.620
0.544
0.806
0.680
0.666
0.420
0.430
0.480
0.300
0.240
0.350
0.400
0.778
0.597
0.499
0.420
0.467
0.483
0.607
0.830
0.978
1.046
Hot
Soak
(q/hr)
0.154
0.130
0.160
0.095
0.140
0.130
0.220
0.087
0.131
0.207
0.137
0.070
0.100
0.060
0.070
0.150
0.090
0.110
0.121
0.160
0.120
0.100
0.250
0.120
0.130
0.072
0.177
0.280
0.250
0.355
-------
-26-
AppendixA (Continued)
Certification Tests on 65 ETP Vehicles
36 Vehicles for EPA Certification
Source
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
CERT-EPA
Make
FORD
FORD
FORD
FORD
FORD
GM
GM
GM
GM
GM
HONDA
HONDA
HONDA
MAZDA
MAZDA
MAZDA
MAZDA
TOYOTA
TOYOTA
TOYOTA
TOYOTA
NISSAN
NISSAN
NISSAN
NISSAN
VOLKS
VOLKS
VOLKS
VOLKS
CHRYSLER
CHRYSLER
CHRYSLER
CHRYSLER
ISUZU
ISUZU
ISUZU
(gr*
Day1
1.039
1.060
1.210
0.832
1.135
0.589
N.A.
0.764
0.669
0.440
0.610
0.817
0.813
0.588
0.567
0.600
0.490
0.600
0.390
0.350
0.340
0.600
0.491
0.594
0.463
0.402
0.626
0.670
0.960
1.100
1.310
1.469
0.741
0.847
0.594
1.375
RTD Test
ims per da
Day 2
0.860
0.828
1.172
1.127
0.984
0.485
N.A.
0.644
0.560
0.300
0.559
0.754
0.636
0.703
0.500
0.530
0.500
0.530
0.320
0.340
0.260
0.430
0.456
0.572
0.417
0.396
0.533
0.630
0.870
1.245
1.230
1.181
0.686
0.779
0.538
1.200
>y)
Day 3
0.791
0.719
1.139
1.230
0.878
0.485
N.A.
0.666
0.521
0.330
0.541
0.796
0.353
0.580
0.528
0.510
0.200
0.510
0.310
0.370
0.250
0.410
0.436
0.553
0.425
0.424
0.534
0.610
0.830
1.275
1.080
1.004
0.825
0.749
0.531
1.243
Hot
Soak
(g/hr)
0.177
0.190
N.A.
0.134
0.211
0.168
0.172
0.266
0.208
0.070
0.070
0.100
0.460
0.080
0.130
0.310
0.120
0.310
0.030
0.030
0.070
0.110
0.057
0.096
0.107
0.041
0.109
0.190
0.280
0.355
0.230
0.140
0.170
0.448
0.187
N.A.
-------
-27-
Appendix B
CAP-2000 Data on Six 1996 Model Year Mercedes ETP Vehicles
Model
S420
S500
S500
S420
S420
S420
Test Date
11/26/97
12/12/97
01/09/98
01/20/98
02/06/98
02/27/98
Odometer
(miles)
46,846
31,447
38,099
29,997
26,606
42,870
Total HC
Diurnal +
Hot Soak
(grams*)
0.4687
0.4336
0.4229
0.7171
0.4317
0.6935
Hot Soak
(grams*)
0.0807
0.0736
0.0839
0.1101
0.0867
0.1255
Mean:
2-Day Diurnal
(grams*)
0.388
0.360
0.339
0.607
0.345
0.568
0.4345
The units "grams" are somewhat inconsistent.
"Grams" on the Hot Soak test refers to grams
per test. Since each test is one hour in
duration, this is equivalent to grams per
hour.
"Grams" on the Diurnal (RTD) test refers to
grams per day.
"Grams" in the "Total" column are the sum of
the grams per hour on the hot soak and the
grams per day on the diurnal tests. This
"mixed" unit is the basis of the standard
used for the ETP certification.
-------
-28-
Appendix C
Twenty-Five 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests
Vehicle
No.
4912
4923
4928
4932
5032
5038
Fuel
RVP
(psi)
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
72-96
82-106
60-84
72-96
72-96
82-106
60-84
72-96
72-96
82-106
60-84
72-96
72-96
82-106
60-84
72-96
60-84
72-96
82-106
60-84
72-96
82-106
72-96
82-106
60-84
72-96
82-106
VP Product
Term
(kPaA2)
567
789
655
969
567
789
655
969
567
789
655
969
567
789
655
969
375
567
789
655
969
1,324
567
789
655
969
1,324
RTD
(gr/dav)
0.980
5.120
1.930
3.350
0.670
4.480
1.710
2.550
4.830
8.230
4.170
4.370
1.700
2.850
1.490
2.080
0.374
0.772
1.231
0.473
0.741
2.433
0.615
1.011
0.441
1.302
4.366
Resting
Loss
(gr/hour)*
0.012
0.102
-0.005
0.045
0.000
0.048
0.018
0.032
0.065
0.142
0.045
0.058
0.017
0.037
0.023
0.017
0.004
0.006
0.012
0.005
0.008
0.018
0.005
0.007
0.002
0.004
0.006
Daily Rst
Loss
(gr/dav)
0.428
2.588
0.020
1.220
0.140
1.292
0.572
0.908
1.700
3.548
1.220
1.532
0.548
1.028
0.692
0.548
0.236
0.284
0.428
0.260
0.332
0.572
0.260
0.308
0.188
0.236
0.284
Diurnal
(gr/dav)
0.552
2.532
1.910
2.130
0.530
3.188
1.138
1.642
3.130
4.682
2.950
2.838
1.152
1.822
0.798
1.532
0.138
0.488
0.803
0.213
0.409
1.861
0.355
0.703
0.253
1.066
4.082
"Hourly Resting Loss" emissions are calculated at the
lowest temperature of each cycle.
-------
-29-
Appendix C (Continued)
Twenty-Five 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests
Vehicle
No.
5046
5047
5052
5066
5068
Fuel
RVP
(psi)
6.8
6.8
6.8
9.0
9.0
9.0
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
(kPaA2)
375
567
789
655
969
1,324
655
969
1,324
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
RTD
(qr/dav)
0.439
0.565
1.498
0.360
0.971
9.716
0.366
0.653
0.906
3.502
4.273
8.937
2.966
5.853
11.820
0.390
0.351
0.605
0.295
0.397
0.581
0.281
0.626
1.936
0.814
0.580
1.150
0.368
0.839
1.391
0.638
1.385
2.132
Resting
Loss
(qr/hour)*
0.011
0.007
0.020
0.004
0.013
0.041
0.005
0.012
0.015
0.032
0.071
0.114
0.039
0.106
0.205
-0.007
0.001
0.006
0.000
0.003
0.004
-0.001
0.007
0.011
0.006
0.006
0.009
0.003
0.009
0.018
0.009
0.010
0.029
Daily Rst
Loss
(qr/dav)
0.404
0.308
0.620
0.236
0.452
1.124
0.260
0.428
0.500
0.908
1.844
2.876
1.076
2.684
5.060
-0.028
0.164
0.284
0.140
0.212
0.236
0.116
0.308
0.404
0.284
0.284
0.356
0.212
0.356
0.572
0.356
0.380
0.836
Diurnal
(qr/dav)
0.035
0.257
0.878
0.124
0.519
8.592
0.106
0.225
0.406
2.594
2.429
6.061
1.890
3.169
6.760
0.418
0.187
0.321
0.155
0.185
0.345
0.165
0.318
1.532
0.530
0.296
0.794
0.156
0.483
0.819
0.282
1.005
1.296
-------
-30-
Appendix C (Continued)
Twenty-Five 1990
Passing Both the
•1995 Model Year Vehicles
Purge and Pressure Tests
Vehicle
No.
5081
9009
9026
9028
9033
9038
9040
9048
9056
9059
9088
9135
9141
9143
Fuel
RVP
(psi)
6.3
6.3
9.0
9.0
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
Temp
Cycle
72-96
82-106
60-84
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
72-96
VP Product
Term
(kPaA2)
489
684
655
969
567
567
567
567
567
567
567
567
567
567
567
567
567
RTD
(gr/dav)
0.647
1.187
0.326
0.639
35.565
1.755
16.984
0.879
5.818
0.810
9.443
3.095
1.009
2.750
1.591
10.328
7.904
Resting
Loss
(gr/hour)*
0.001
0.009
0.005
0.007
0.095
0.031
0.024
0.003
0.106
0.006
0.228
0.046
0.013
0.023
0.012
0.209
0.070
Daily Rst
Loss
(gr/dav)
0.164
0.356
0.260
0.308
2.420
0.884
0.716
0.212
2.684
0.284
5.612
1.244
0.452
0.692
0.428
5.156
1.820
Diurnal
(gr/dav)
0.483
0.831
0.066
0.331
33.145
0.871
16.268
0.667
3.134
0.526
3.831
1.851
0.557
2.058
1.163
5.172
6.084
-------
-31-
Appendix D
Ten Possible "ETP-Like" 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests
Tested Over Multiple Cycles
(Subset of Appendix C)
Vehicle
No.
5032
5038
5046
5047
Fuel
RVP
(psi)
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
9.0
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
(kPaA2)
375
567
789
655
969
1,324
567
789
655
969
1,324
375
567
789
655
969
1,324
655
969
1,324
RTD
(gr/day)
0.374
0.772
1.231
0.473
0.741
2.433
0.615
1.011
0.441
1.302
4.366
0.439
0.565
1.498
0.360
0.971
9.716
0.366
0.653
0.906
Resting
Loss
(gr/hour)*
0.004
0.006
0.012
0.005
0.008
0.018
0.005
0.007
0.002
0.004
0.006
0.011
0.007
0.020
0.004
0.013
0.041
0.005
0.012
0.015
Daily Rst
Loss
(gr/day)
0.236
0.284
0.428
0.260
0.332
0.572
0.26
0.308
0.188
0.236
0.284
0.404
0.308
0.620
0.236
0.452
1.124
0.260
0.428
0.500
Diurnal
(gr/day)
0.138
0.488
0.803
0.213
0.409
1.861
0.355
0.703
0.253
1.066
4.082
0.035
0.257
0.878
0.124
0.519
8.592
0.106
0.225
0.406
"Hourly Resting Loss" emissions are calculated at the lowest
temperature of each cycle.
-------
-32-
Appendix D (Continued)
Ten Possible "ETP-Like" 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests
Tested Over Multiple Cycles
(Subset of Appendix C)
Vehicle
No.
5066
5068
5081
9033
9040
9059
Fuel
RVP
(psi)
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
9.0
9.0
6.8
6.8
6.8
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
72-96
82-106
60-84
72-96
72-96
72-96
72-96
VP Product
Term
(kPaA2)
322
489
684
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
489
684
655
969
567
567
567
RTD
(gr/day)
0.390
0.351
0.605
0.295
0.397
0.581
0.281
0.626
1.936
0.814
0.580
1.150
0.368
0.839
1.391
0.638
1.385
2.132
0.647
1.187
0.326
0.639
0.879
0.810
1.009
Resting
Loss
(gr/hour)*
-0.007
0.001
0.006
0.000
0.003
0.004
-0.001
0.007
0.011
0.006
0.006
0.009
0.003
0.009
0.018
0.009
0.010
0.029
0.001
0.009
0.005
0.007
0.003
0.006
0.013
Daily Rst
Loss
(gr/day)
-0.028
0.164
0.284
0.140
0.212
0.236
0.116
0.308
0.404
0.284
0.284
0.356
0.212
0.356
0.572
0.356
0.380
0.836
0.164
0.356
0.260
0.308
0.212
0.284
0.452
Diurnal
(gr/day)
0.418
0.187
0.321
0.155
0.185
0.345
0.165
0.318
1.532
0.530
0.296
0.794
0.156
0.483
0.819
0.282
1.005
1.296
0.483
0.831
0.066
0.331
0.667
0.526
0.557
"Hourly Resting Loss" emissions are calculated at the lowest
temperature of each cycle.
-------
-33-
Appendix E
Eight 1990-1995 Model Year PFI Vehicles
Failing (Only) the Purge Test
Vehicle
No.
4925
4933
5004
5035
5040
Fuel
RVP
(psi)
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
72-96
82-106
60-84
72-96
72-96
82-106
60-84
72-96
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
(kPaA2)
567
789
655
969
567
789
655
969
375
567
789
655
969
1,324
375
567
789
655
969
1,324
375
567
789
655
969
1,324
RTD
(gr/dav)
4.170
4.450
2.170
3.830
10.750
18.670
7.170
12.120
0.989
1.673
2.924
1.025
5.440
20.391
5.593
5.869
22.973
14.493
24.068
24.872
0.667
1.143
6.961
1.065
2.930
20.658
Resting
Loss
(gr/hour)*
0.063
0.080
0.035
0.058
0.145
0.352
0.137
0.228
0.003
0.023
0.031
0.015
0.018
0.047
-0.016
0.016
-0.033
0.015
0.032
0.040
0.003
0.010
-0.013
-0.003
0.012
-0.008
Daily Rst
Loss
(gr/dav)
1.949
2.357
1.277
1.829
3.917
8.885
3.725
5.909
0.509
0.989
1.181
0.797
0.869
1.565
0.053
0.821
-0.355
0.797
1.205
1.397
0.509
0.677
0.125
0.365
0.725
0.245
Diurnal
(gr/dav)
2.221
2.093
0.893
2.001
6.833
9.785
3.445
6.211
0.480
0.684
1.743
0.228
4.571
18.826
5.540
5.048
23.328
13.696
22.863
23.475
0.158
0.466
6.836
0.700
2.205
20.413
-------
-34-
Appendix E (Continued)
Vehicle
No.
5069
5070
5087
Fuel
RVP
(psi)
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
9.0
9.0
Temp
Cycle
(°F)
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
72-96
82-106
60-84
72-96
VP Product
Term
(kPaA2)
322
489
684
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
489
684
655
969
RTD
(qr/dav)
1.774
3.593
6.810
1.322
1.953
9.565
7.082
12.372
20.430
0.351
0.690
1.209
0.375
0.745
1.176
0.416
1.381
9.141
1.830
2.435
1.478
2.533
Resting
Loss
(qr/hour)*
0.001
0.012
0.003
0.004
0.011
0.039
-0.017
0.007
0.080
0.002
0.001
0.016
0.002
-0.004
0.007
0.003
0.019
0.057
0.042
0.048
0.029
0.043
Daily Rst
Loss
(qr/dav)
0.461
0.725
0.509
0.533
0.701
1.373
0.029
0.605
2.357
0.485
0.461
0.821
0.485
0.341
0.605
0.509
0.893
1.805
1.445
1.589
1.133
1.469
Diurnal
(qr/dav)
1.313
2.868
6.301
0.789
1.252
8.192
7.053
11.767
18.073
0.000
0.229
0.388
0.000
0.404
0.571
0.000
0.488
7.336
0.385
0.846
0.345
1.064
-------
-35-
Appendix F
Five 1990-1995 Model Year Vehicles
Failing the Pressure Test
Vehicle
No.
4937
5008
5021
5044
5067
Fuel
RVP
(psi)
6.8
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp
Cycle
(°F)
72-96
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
(kPaA2)
567
375
567
789
655
969
1,324
375
567
789
655
969
1,324
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
RTD
(gr/dav)
3.330
12.853
17.632
29.663
19.811
35.202
57.174
7.789
15.477
23.810
17.246
24.840
41.963
0.286
0.523
0.706
0.467
0.494
1.914
5.206
13.206
21.981
12.128
8.644
18.697
7.106
29.697
50.741
Resting
Loss
(gr/hour)*
0.028
-0.018
0.013
0.054
-0.002
0.038
0.014
0.004
0.029
0.065
-0.003
0.038
-0.034
0.004
0.011
0.014
0.011
0.007
0.005
0.037
0.056
0.113
0.025
0.070
0.062
0.036
0.107
0.040
Daily Rst
Loss
(gr/dav)
1.109
0.005
0.749
1.733
0.389
1.349
0.773
0.533
1.133
1.997
0.365
1.349
-0.379
0.533
0.701
0.773
0.701
0.605
0.557
1.325
1.781
3.149
1.037
2.117
1.925
1.301
3.005
1.397
Diurnal
(gr/dav)
2.221
12.848
16.883
27.930
19.422
33.853
56.401
7.256
14.344
21.813
16.881
23.491
42.342
0.000
0.000
0.000
0.000
0.000
1.357
3.881
11.425
18.832
11.091
6.527
16.772
5.805
26.692
49.344
-------
-36-
Appendix G
Response to Peer Review Comments from H. T. McAdams
This report was formally peer reviewed by two peer reviewers
(H. T. McAdams and Sandeep Kishan). In this appendix, comments
from H. T. McAdams are reproduced in plain text, and EPA's
responses to those comments are interspersed in indented italics.
Comments from the other peer reviewer appear in the following
appendix (Appendix H).
It is important to note that this final version of the
report has changed substantially from the draft version that
Professor McAdams reviewed. In that earlier version, the
goal was to develop equations that would predict the diurnal
and resting loss emissions of these ETP vehicles. In the
interim between these versions of this report, we realized
that the predicted results (from that draft) were comparable
to the MOBILES predictions. Therefore, in this final
version, our goal changed and became the testing and
validation of those MOBILES predictions.
This change of direction resulted in many of Professor
McAdams' comments no longer being applicable. However, all
of his comments were considered, and those that were still
applicable were incorporated.
************************************
Modeling Diurnal and Resting Loss Emissions from Vehicles
Certified to the Enhanced Evaporative Standards
Report Number M6.EVP.005
By
Larry C. Landman
Assessment and Modeling Division
U.S. EPA Office of Mobile Sources
Review and Comments By H. T. McAdams
1.0 REPORT CLARITY
Reporting the results of this study presents more than the usual
challenge to the report writer. It is necessary to make a great
many statements that are highly conditional, and this fact leads
to sentences that sometimes are lengthy and complicated. For
example, consider the following excerpt:
-------
-37-
EPA believes that the RTD emissions from malfunctioning
enhanced evaporative control vehicles (i.e., vehicles that
developed problems with their evaporative control systems)
will be similar to the RTD emissions from the 1990 to 1995
model year vehicles that also develop problems with their
evaporative control systems. That is, those 1996 and newer
model year vehicles that had failed either EPA's purge or
pressure tests are expected to have evaporative emissions
similar to those 1990 to 1995 model year PFI vehicles that
also failed the same test.
It is evident that the author is doing his best to keep the
record straight, something not easy to do when making a statement
with so many qualifiers. The problem is aggravated by the
necessary wordiness of such designations as "emissions from
malfunctioning enhanced evaporative control vehicles" and
"emissions from the 1990 to 1995 model year vehicles that also
develop problems with their evaporative control systems." The
added "That is ..." phrases, intended to clinch the matter, tend
only to further confuse. It might be useful to insert a simple
"word table" or Venn diagram to show explicitly how the various
vehicle classes are related and then to find a simple but
descriptive name for each category. An example pertaining to the
vapor pressure product term will be found later in this review.
The above comments are based on several years experience as
technical writer and editor for Cornell Aeronautical Laboratory,
now Calspan Corporation, Buffalo, NY. Though stylistic and
grammatical editing are not considered to be within the scope of
this review, it is believed that the report could benefit from
further attention to these concerns.
2.0 OVERALL METHODOLOGY
Every scientific discipline has its own investigative and
statistical parochialism. Some of the "soft sciences," like
psychology and sociology, tend to thrive on correlation analysis
and nonparametric statistics. The engineering sciences, on the
other hand, find statistical approaches like regression analysis
and formal tests of significance more to their liking. Neither
scientific group can be faulted for their choice, but both would
gain from cross fertilization of ideas and procedures.
The methodology employed in this report is generally consistent
with the methodology followed by EPA in developing their Complex
Model for Reformulated Gasoline (RFG). Heavy emphasis was put on
regression analysis and the strict application of statistical
tests of significance. From participation in that effort, this
reviewer learned much that is applicable to Landman's study of
evaporative emissions. This includes visualizing the nature of
the curves, surfaces or hypersurfaces represented by the model,
evaluating confidence bounds for the regression function, and
estimating the relative importance of terms in the equation.
-------
-38-
Perhaps one of the most neglected aspects of statistical analysis
is the subject of the power of a test of significance. Often
failing to reject a null hypothesis amounts to saying that, under
the prevailing circumstances, the test simply did not have
sufficient power to do so. Either the sample size was not
adequate, or the data had too much dispersion, or the
significance level was set too high to be appropriate for the
situation at hand. More emphasis needs to be put on the reasons
for using certain significance tests and on whether a given
significance level, such as the commonly used 0.05, is
appropriate for the present application.
This report is subject to many of the above complaints.
Nevertheless, it meets essentially all criteria for a valid
scientific study according to present views and standards. The
plea here is that statistical principles should be applied
thoughtfully rather than automatically and that there may be
value in sometimes breaking from the crowd. In what follows,
specific examples taken from the report will be used to
illustrate some of the above contentions.
3.0 APPROPRIATENESS OF DATASETS SELECTED
The datasets available for modeling diurnal and resting loss
emissions are far from ideal. As pointed out above, precision
limits for estimating these emissions rest heavily on the amount
and quality of the data. By quality is meant data that is not
subject to bias and is not so "noisy" that it precludes all but
the most evident conclusions. If the data is so bad that it
leaves us with little that we did not already know, then it
clearly contributes little information.
There is probably no perfect set of data. However, application of
the principles of sampling and experiment design can do much to
move us toward that goal. In particular, they can help us
estimate the sample size required to provide estimates that we
can live with. They can help to prevent the confounding of the
effects of two or more variables. And, finally, they can help
optimize our data by providing the most information for the least
amount of experimental effort. The role played by factorial
experiments is well known for its capabilities toward this end,
and there are even more efficient designs applicable in unique
circumstances.
One of the shortcomings of the datasets used in the modeling of
evaporative emissions is the limited number of vehicles. Though
evaporative emissions are probably subject to less vehicle-to-
vehicle variation than are exhaust emissions, it is highly
desirable to remove vehicle effects from the effects of fuel
properties and temperature cycles wherever possible. The data are
not structured well to achieve this end, but in at least one
situation, to be demonstrated later, vehicle differences can be
removed with a very beneficial effect.
When data are sparse, it is even more imperative than usual to
extract the most information from the limited amount of data
-------
-39-
available. An example, drawn from the data in this report, will
be given in the next section, Data Analysis and Statistical
Methodology.
Typically in this report a candidate regression model is fitted
to the data and the coefficients are then tested for
"significance." Those that fail the test are then dropped from
the equation, the result being the same as if they were assigned
the value zero. Still, by virtue of the principle of least
squares, the "most likely" value of the coefficient is the one
that was computed. This seeming impasse needs to be examined
thoughtfully before arbitrarily rejecting the coefficient at 0.05
significance -- or 0.10, or 0.01 or 0.001, especially when the
test is based on a small sample.
The obligation of the analyst, therefore, does not stop here. It
is just as important to know the error bounds for the so-called
significant coefficients as it is to know that some coefficients
can seemingly be ignored. It is even more important to know error
bounds for emission estimates computed by the regression
equation. It should be kept in mind, too, that the precision of
the estimates is not constant for all values of the predictor
variables. It should be an obligation of the analyst to tell us
how good the estimates are near the center of the sample space as
well as how bad the estimates are near the edges of the sample
space. The analyst can not just report significance and then
"look the other way" when error bounds are so wide that emission
estimates are essentially useless.
The present report does not provide this information, but it is
admitted that it is not customarily to do so. Therefore, the
author can not be faulted. To perform the necessary computations
and prepare the required displays may not be practical under the
constraints of the present report. That does not preclude,
however, a broader look at the characteristics of estimates in
future studies.
Error bounds in the form of 90 percent confidence intervals
have been added to several of the tables (Tables 3, 6, and
7). Additionally, two tables of regression statistics have
been added (Tables 4 and 5).
4.0 DATA ANALYSIS AND STATISTICAL METHODOLOGY
It has been said that regression analysis is the most widely used
and most widely misused of all statistical methods. Though an
evident hyperbole, the statement contains an element of truth.
Couched in the framework of General Linear Model (GLM),
regression has wide appeal in a great variety of applications.
The truth is, however, that regression analysis is not a
universal solvent and is not without its shortcomings and
pitfalls. In what follows, we examine Landman's analysis in the
light of these considerations and suggest, wherever indicated, an
alternative approach.
-------
-40-
To say that a model is linear is simply to say that the response
vector is a linear combination of a set of basis vectors. The
basis vectors themselves, however, may be as "nonlinear" as they
please and are often just the terms of a polynomial: 1, x, x2,
x3, ... xn. The analyst's task is to determine coefficients for
these terms so as to minimize the sum of squares of the
residuals. He must also somehow select the terms to be included
in such a way that the data are neither "underfitted" nor
"overfitted." It is here that he resorts to R2 and to statistical
tests of significance for each of the regression coefficients.
Although R2 is widely used as a measure of the efficacy of a
regression model, it can be misleading. Moreover, it may not be
realized that any number of models can be constructed to give
exactly the same R2 and even exactly the same residuals point by
point. Viewed in this light, the fact that there seems to be a
good fit according to R2 is not necessarily a cause for
rejoicing.
It must be kept in mind that R2 is a function of the residuals
only at points where we have data and can tell us nothing about
the response at points where we have no data. Unless we know how
the function performs over its entire domain of definition, some
of these functions, even the one we have selected, may oscillate
radically between points at which we do have data. That is why it
is important to know something about the geometry or
hypergeometry of a regression equation before relying on it to
interpolate between data points and sometimes to extrapolate
beyond them.
Caution and common sense need to be exercised when evaluating
regression models, whether by R2, t-tests of the regression
coefficients or other means. That becomes clear if we examine
Table 3 of the report and its accompanying text.
Randman notes that for each temperature cycle, diurnal emissions
increase with fuel volatility and that for a given fuel, diurnal
emissions increase as the temperature cycle increases. He might
also have noted that the effect of temperature cycle on emissions
is greater for the more volatile fuel, a fact that seems
consistent with physical reasoning. Together, these three
observations present an almost classic instance of a two-factor,
factorial experiment in which response depends on both factors
and their interaction.
The use of the VP_Product term was proposed (in report
M6.EVP.001), in part, because it incorporates both of these
two factors. Historically, it closely corresponds to the
uncontrolled diurnal index (UDI) used in earlier versions of
MOBILE but is easier to calculate.
It is true that when emissions are regressed directly on prod and
RVP, R-square is only 0.8593, whereas it is 0.9658 when log
emissions are regressed on the same two variables. However, as
shown below, R-square increases to 0.9000 for direct regression
when the interaction term prod*RVP is introduced.
-------
-41-
Without Interaction Term
Coefficient Std. Error t
Constant 1.6273 2.0537 0.7924
Prod 0.0046 0.0012 3.9844
RVP -0.4739 0.3192 1.4845
R-square = 0.8593
With Interaction Term
Constant
Prod
RVP
Interaction
Coefficient
7.7374
0.0046
-1.2253
0.0011
Std. Error
7.1024
0.0102
0.8964
0.0012
t
1.0894
0.4457
1.3669
0.9014
R-square = 0.9000
Note that when the interaction term is added, the apparent
significance of some of the other terms seem to decrease. The
point to be noted here is that whether a term shows up as
significant or not often depends on how many other terms are in
the equation. When a term is dropped, the sum of squares
associated with it are redistributed, partly to the error term
but not entirely. Part of the ge redistributed terms are said to
be aliased with other "significant" terms, and these aliases can
be explicitly computed. Likewise, when additional terms are added
to an existing equation, the order in which they are introduced
can make a large difference in their "significance." The method
of stepwise regression is an attempt to deal with this problem.
Consequently, it may not be wise to rule out certain terms on the
strength of their t-value alone.
Interaction terms were considered. Their use did provide an
improved "fit" at the tested values. However, at
intermediate values (i.e., RVPs between 7 and 9), the
resulting predictions were not consistent with known
responses. Therefore, EPA did not use them.
It also needs to be remembered that whether a term is called
significant or not strongly depends on the significance level.
Just because 0.05 is conventionally used does not make it
sacrosanct. Often we are really more concerned with the Type II
error than with the Type I and do not take advantage of the
trade-offs between the two. That concern is no more recognized
than in quality control and sampling acceptance plans. A sampling
-------
-42-
plan is designed to a specified consumer's risk and producer's
risk. The consumer has to be protected to minimize the risk that
he will accept a bad lot of material. But, the producer has to be
protected against the risk that he will have a perfectly good lot
rejected. A compromise that both can live with has to be found,
Similarly, in evaluating a regression coefficient, we need to
know the consequences of retaining a coefficient when its effect
really doesn't exist, but we also need to know the consequences
of dropping a coefficient when its effect really does exist. By
increasing the risk of a Type I error we can decrease the risk of
a Type II error. Also, ruling out a term by rejecting it at any
significance level says that that term is zero. However, we are
willing to accept a coefficient with an extremely wide confidence
interval and take no note of the fact. After all, the term is
[sic] significant.
Which is the most appropriate model, with or without the
logarithmic transformation of emission measurements?
Two factors bear on the answer to this question. One deals with
the error distribution, the other with whether the effects of the
pressure product term and RVP are additive or multiplicative.
If the variance of the observations that make up the mean diurnal
emissions for each of the six means are proportional to the
square of the means, then the log transformation may be
appropriate, because it tends to stabilize the variances. (A
constant variance is one of the requirements for regression.)
However, if the variance already was stable (i.e., constant for
all means), then the log transformation would tend to destabilize
the variance and possibly lead to a biased result.
The log transformation also has another useful property. Each of
the coefficients of the log model expresses the proportional, or
percent change in emissions associated with a unit change in prod
or RVP.
That variance increases as the level of emissions increases seems
plausible and could be examined by computing the variance for
each of the six categories. Also, whether effects are additive or
multiplicative could be examined by comparing successive
differences and successive ratios for the group means. At any
rate, the log model seems effective. [As a matter of editorial
note, the residual mean square in Table 3 should be 0.032764 not
0.32764.]
Another problem that may be encountered, as it was in the
development of the Complex Model for Reformulated Gasoline (RFC),
is the reversal of the sign of the slope (derivative) that
characterizes the effect of a variable on emissions. Such a
reversal can occur even when it is known from theory and
experience that the function is monotonic non-decreasing or non-
increasing. That is why one needs to know what the function looks
like, particularly whether it makes any turns that are "counter-
intuitive." For example, in the case of a quadratic, it is
-------
-43-
helpful to know where the zeros of the polynomial lie and even
more important to know where the zeros of the derivative function
lie.
Landman experienced the opposite of this effect in developing a
model for diurnal emissions. In Table 4, he uses an equation
containing only a constant and a cubic term. His rationale for
the choice, in his words, is as follows:
In Section 4.2.1, we noted that the diurnal emissions for
the vehicles with properly functioning evaporative control
systems were not a strictly increasing function of the VP product
term. However, for the vehicles that failed the pressure test,
the diurnal emissions increased as the VP product increased. We,
therefore, repeated the approach used in earlier analyses of
regressing the diurnal emission emissions against the cube of the
VP product term producing Table 4.
It is not clear how the lack of monotonicity leads to the
conclusion that a cubic function of the product term, referred to
hereinafter as prod, is most appropriate. It turns out that, over
the range of the data, prodl, prod2, prod3 and even prod4 are
highly correlated, the coefficient of correlation between pairs
of these variables ranging from 0.92 to 0.98. It follows, then,
that any of the prod functions might perform about as well as any
other. This fact becomes evident when other powers of the product
term are used as the basis of the model, as will be shown below.
There may be a more cogent reason, however, for the lack of
monotonicity. For a product term to give consistent results, it
is necessary for the product to exhibit reciprocity. Let us call
the two factors that make up the product xala and xa2a. All pairs
of xala and xa2a that map into a given value of prod should
produce the same effect on evaporative emissions. Otherwise,
inconsistencies may arise.
According to equation (3) of the Randman paper, xala is simply
the range of vapor pressure for the day, and xa2a is simply the
midrange of the day's vapor pressure. In the present notation,
the product term is defined as
prod = 1/2 [(xala - xa2a) * (xala + xa2a)]
or
prod = 1/2 (xala2 -xa2a2)
Consequently,
prod3 = 1/8 (xala2 - xa2a2)3
When this expression is expanded, it can be seen that powers of
vapor pressure as high as six will be encountered. More
importantly, for reciprocity to hold, the same emissions should
be associated with a given value of prod3, whether that value was
produced by - say - a short VP range combined with a high
midrange or long range combined with a low midrange. Table 4,
however, being based on the data in Appendix C, has other
-------
-44-
difficulties. One of the vehicles, #5044, shows zero diurnal
emissions for all conditions except one, and that one shows a
value far out of line with the same condition of the other
vehicles. It is no wonder that R2 is only 0.411! A look at a plot
of the residuals would make that fact quite clear, and it is for
reasons such as this that residuals should be examined to see if
there is any indication of "lack of fit" such as outliers or
trends.
Suppose, now, that we remove vehicle #5044 and recompute the
regression. Then we get:
Constant = 10.6634
Coefficient of prod3 = 0.0171
R2 = 0.87
It is clear that the one vehicle strongly biases the results. It
is also clear that different vehicles exhibit different responses
and that vehicle effects should be removed in the analysis if
possible.
Actually, the data are ideally suited for removing vehicle
effects by means of dummy variables. Below, regression results
are given for individual vehicles, as well as for meaningful
subsets of the data in Appendix C.
HOW VEHICLE EFFECTS MODIFY THE MODEL FOR DIURNAL EMISSIONS
Vehicle Set
All
All but #5044
Vehicle #5008
Vehicle #5021
Vehicle #5044
Vehicle #5067
Best choice*
Constant
7.9460
10.6634
14.7713
11.3437
-0.2064
5.8753
7.2744
Coef. of prod3
0.01300
0.0171
0.0186
0.0137
0.0006
0.0191
0.0171
R2
0.4106
0.8742
0.9735
0.9425
0.8745
0.9490
0.9422
*Vehicle #5044 omitted, vehicle effects removed
Note that by simply excluding one vehicle from the analysis and
removing the effects of vehicles we go from R2 of 0.41 to 0.94
and even up to 0.97 for individual vehicles.
Now that we have found a consistent set of data - namely, the set
of data with vehicle #5044 removed - let us try various powers of
the product term as predictor variable, as well as various
combinations of those powers. The resulting values of R2 are
listed below.
-------
-45-
INSENSITIVITY OF PRODn IN PREDICTING DIURNAL EMISSIONS
Power, n R-square
1 0.8421
2 0.8820
3 0.8742
1&2 0.8836
1&3 0.8821
2&3 0.8830
1&2&3 0.8848
As conjectured, it little matters what power of the product term
is used, or what combination of powers; the resulting prediction
capability, as judged by R2, seem to be about the same.
5.0 APPROPRIATENESS OF THE CONCLUSIONS
Though not called out explicitly, conclusions are found in the
Summary section of Landman's report. They can be listed, briefly,
as follows.
1. Separate estimates of diurnal and rest loss emissions
are given for "gross liquid leakers" and vehicles subject only to
vapor losses. Liquid losses per hour are estimated as a constant;
vapor losses are estimated by means of a relatively simple
empirical equation,
2. A major predictor variable, called "prod" in this
review, is the product of two vapor pressures and is said to take
into account both vapor pressure and temperature. For some
equations prod may be raised to the second or third power; it may
also be modified, in some cases, by additional variables, notably
RVP.
3 . Separate estimates are provided for various vehicle
strata, the strata being defined by whether the vehicle passed or
failed the purge and/or pressure tests.
4. Evaporative emissions are assumed to be zero for
temperatures below 40°F and for any temperature cycle in which
the temperature stays relatively constant. The term "constant" is
defined as not varying more than a few degrees from the mean
temperature.
How appropriate are these conclusions?
To put into perspective what is expected of the evaporative
emission estimates, let us consider the estimate for gross liquid
leakers. A single number is supposed to represent emissions from
vehicles of all ages, all places and all climates, all drivers
and all lifestyles ... and so on. How close can the estimate be
to "truth," when only a few vehicles are tested and the results
are scaled up according to the relative frequency of somewhat
arbitrarily designated "strata" that also had to be estimated.
-------
-46-
The answer is probably "Not very," but the report provides no
clue, however vacuous, of just how "very" is very.
These points are addressed in the parallel report devoted to
these vehicles with substantial leaks of liquid gasoline
(i.e., report number M6.EVP.009, entitled "Evaporative
Emissions of Gross Liquid Leakers in MOBILE6").
Now consider how much more complicated and subject to error are
the estimates provided by an empirical equation based on "an
educated guess" of what form that equation should take and what
predictor variables it should contain. The problem is aggravated
by the fact that the precision of the estimate varies widely over
the range of the predictor variables. Then add the uncertainty of
the strata weights and ... but, enough already!
EPA and Mr. Landman are to be commended for having the courage to
accept such a mammoth challenge, but their accomplishments would
gather infinitely more kudos if they could assure us that their
estimates are within 5% of real-world truth. Admittedly, to be
able to say how good their estimates are is an even more
difficult problem than to compute those estimates in the first
place. But ... unless we have some measure of how good the
estimates are, we might just as well not have computed them at
all!
The form of the predictive equations and the validity of the
predictor variables are subject to question on several counts.
The product term is particularly open to criticism. Probably it
has a factual or theoretical basis not known to this reviewer,
but if so, it is difficult to understand the indifference of this
term to what power it is raised to.
If test results had been obtained over a wider range of this
VP_Product term (e.g., using 11.0 RVP fuel over an 82-106
degree cycle), then the exponent used would seem less
"indifferent."
More important, perhaps, is the matter of reciprocity. For any
given value of prod, there is an infinitude of pairs of factors
that map into that value. For the prod function or powers of that
function, all pairs should yield the same evaporative emission
estimates.
In general, EPA chose the lowest power (exponent) that would
explain the observed results (i.e., the simplest
explanation).
There are two instances in which the proposed models exhibit a
step function. At temperatures below 40° F. emissions are taken
as zero. Though this estimate may be reasonable, "Nature abhors a
vacuum." Likewise, engineers and mathematicians are not
comfortable with discontinuities unless there is good reason for
those discontinuities to occur. A similar impasse is faced in
defining emissions to be constant if the range of the temperature
cycle is zero, or within a few degrees of zero. Means exist for
-------
-47-
smoothing these discontinuities and even, perhaps, for assuring
that the derivative of the function exhibits no discontinuities.
Using a smooth curve rather than abruptly setting the
emissions to zero is appealing. However, the actual
differences in the resulting fleet emissions are too small
to be meaningful.
Finally, we come to the practical matter of applying these models
to real-world situations. Inasmuch as some twenty or so equations
are provided, there must be means for selecting the one that is
uniquely appropriate for the particular problem at hand. This
fact seems to assume that the vehicle or vehicles under
consideration have already been purge and pressure tested as well
as examined for liquid leaks. So far as the use of the models for
compiling an emission inventory for the present fleet is
concerned, there would seem to be no problem. However, in future
applications, classification of vehicles would have to be a
precursor to application of the models.
Since the in-use fleet actually contains all of these
strata, all of these equations (as well as many others) are
used in MOBILE6. The resulting predictions are then
weighted together. (See parallel report number M6.EVP.006,
entitled "Estimating Weighting Factors for Evaporative
Emissions in MOBILE6.")
6.0 RECOMMENDATIONS FOR ANY ALTERNATE DATASETS AND/OR ANALYSES
The ideal dataset would be one in which vehicles are recruited in
accordance with a sampling plan and experiment design tailored to
the requirements of the moment. Some replication should be built
into the plan to allow estimation of errors attributed to
"unassignable causes." These are the errors that remain after we
have identified and estimated all the fixed effects that we could
think of. In addition, the design should be such that the
relative magnitude of those "fixed effects" are not confounded
with errors due to the unassignable causes.
The prod variable needs to be examined in depth, with regard to
reciprocity and correlation of successive powers, as well as
whether there are other variables that might better serve the
purpose. If there is a theoretical reason for using a term as
complex as prod3, it should be revealed.
A particular objective that should be the goal of any experiment
design is to choose variables and the levels of those variables
in such a way that they are orthogonal. This type of design
assures that the estimates of the effects of all variables are
completely independent of each other.
With regard to discontinuities in the models, means should
provided for "fairing the curve" so that it blends smoothly into
both the top and bottom of the step. A method for realizing such
-------
-48-
smoothing by means of exponential was suggested in work connected
with development of the Complex Model.
It is recommended that the above changes be incorporated in the
present report to whatever degree is practical within allowable
time and resource constraints. Although a complete assessment of
error limits is beyond the scope of the present report, there
does exist enough information to make a start on this very
important issue. Most statistical software gives as output the
standard error of regression coefficients and the standard error
of estimate at various points in the predictor space. It is
recommended that some effort be made in this direction, if only
to show the general magnitude of the errors. In future studies,
effort should be made toward continued refinement of the error
bounds.
1-20-99
htm
-------
-49-
Appendix H
Response to Peer Review Comments from Sandeep Kishan
This report was formally peer reviewed by two peer reviewers
(H. T. McAdams and Sandeep Kishan) . In this appendix, comments
from Sandeep Kishan are reproduced in plain text, and EPA's
responses to those comments are interspersed in indented italics.
Each of these comments refer to page numbers in the earlier draft
version (dated July 1, 1999) that do not necessarily match the
page numbers in this final version. Comments from the other peer
reviewer appear in the preceding appendix (Appendix G).
It is important to note that this final version of the
report has changed substantially from the draft version that
Sandeep Kishan reviewed. In that earlier version, the goal
was to develop equations that would predict the diurnal and
resting loss emissions of these ETP vehicles. In the interim
between these versions of this report, we realized that the
predicted results (from that draft) were comparable to the
MOBILES predictions. Therefore, in this final version, our
goal changed and became the testing and validation of those
MOBILES predictions.
This change of direction resulted in many of Sandeep
Kishan's comments no longer being applicable. However, all
of his comments were considered, and those that were still
applicable were incorporated.
************************************
This memorandum provides peer review comments on two EPA
documents: "Evaluating Resting Loss and Diurnal Evaporative
Emissions Using RTD Tests", Document No. M6.EVP.001, November 20,
1998 and "Modeling Diurnal and Resting Loss Emissions" Report
Number M6.EVP.005, October 1, 1998. Both of these are draft
reports.
The original peer review covered two of the MOBILE6
documents. Only the portion of that review pertaining to
this report is reproduced below in this appendix. The
remainder of the peer review is reproduced in report number
M6.EVP.001 (Appendix I of that report).
Overall, we think that the reports are good, and they present
some new data analysis techniques that are attractive. Since, in
the past, we have had to do similar data analyses and modeling
for evaporative emissions from vehicle test data, we can
appreciate many of the difficulties and data limitations you are
subject to. We hope the comments below help you with this
effort.
-------
-50-
Document No. M6.EVP.005 (October 1. 1998)
This report was clearly written and the stratification seems to
be appropriate for this analysis. We think that the dataset used
is discouraging but it may be that no alternate datasets can be
found for this purpose. Therefore, we think that it is important
to let the reader know that you are committed to revisiting these
relationships when new data does become available. We also have
the same concern with the regressions in this report as with
those already discussed for the previous report.
1. Page 3, end of Section 1.0 - It might be appropriate to
state that the models developed in this report are intended
to be a temporary patch for MOBILE6 until EPA or someone
else gets actual vehicle measurement data on the effects of
RVP, temperature, and purge and pressure status on the
evaporat ive emi s s ions.
A statement to that effect has been added.
Page 5, Section 3.0 - You are proposing to use 1990 to 1995
model year vehicle data to estimate the effects of
temperature, RVP, and purge and pressure status on trends in
the 1996 and 1997 vehicles. What evidence do you have that
the failure modes of 1996 and 1997's will be like the
failure modes of 1990 to 1995's? Are the materials,
connectors, etc., the same? Consider the five bulleted
items in Section 1.0; we think you need some discussion
about why these trends and these slightly older vehicles
would be similar to those in the 1996 and 1997 vehicles.
The reviewer is correct; the evidence that these vehicles
are comparable to the ETP vehicles is lacking. However,
until we obtain test data on in-use ETP vehicles, this data
set is the best we have to work with.
3. Page 10, Section 4.1.2, bottom of the page - During the
regression of estimated resting losses versus temperature
for different vehicles, it was found that the r for the
regression of resting losses versus temperature produced a
low r and a temperature coefficient that was not
statistically significant. Rather than averaging the
resting loss emissions for all 12 cars together, it would be
more appropriate to use a categorical variable for the
identity of the cars. This will produce a larger r2 since
the researcher recognizes that it's the differences among
the cars that produce most of the variability in the
dataset. The result will be a good estimate for the slope
on the temperature and, possibly, also make the temperature
coefficient statistically significant.
-------
-51-
With the changes made to this revision, this comment is no
longer applicable.
Page 10, Section 4.1.2, paragraph 2 - You had data
describing resting loss emissions for the two separate
strata - one where vehicles failed the pressure test and one
where vehicles failed the purge test. Why didn't you just
use those individual strata results to predict the
temperature effect on resting losses for those type of
malfunctions? The data values in Appendix D look reasonable
for the vehicles in those strata. Instead, these strata
were combined and then modeled. Why?
With the changes made to this revision, this comment is no
longer applicable.
Page 12, Section 4.2.1 - By performing the regression on
diurnal emissions on the average emissions of vehicles, data
from only five vehicles could be used. However, if instead,
the regression had been performed on the individual emission
values of the vehicles, 12 vehicle's data would supply
information to the regression about temperature and RVP
relationships. Thus, the choice of performing regressions
on averages rather than on individual values causes the
resulting model to lose information which could have been
provided by an additional seven vehicles. If the
regressions are performed in SAS, a class variable for
vehicle can be used to account for an unbalanced set of data
with respect to vapor pressure product and RVP. The
resulting coefficients for RVP and vapor pressure product
would be better estimates of the true relationships.
With the changes made to this revision, this comment is no
longer applicable; however, this suggestion will be used in
future analyses.
Page 15, Section 4.2.2 - In Appendix C, one vehicle also has
measurements at 6.3 psi fuel RVP. Why did you not use these
values in your regressions? If you use a class variable for
vehicle identification, the information from these three
additional measurements can be brought into the regression.
The data at 6.3 psi were incorporated in this latest
revision.
The use of the cube of the vapor pressure product in this
regression is troubling. What evidence do you have that the
cube is the appropriate transformation? It seems to us that
since a class variable for vehicle identification was not
used, it is unlikely that the cube transformation is
correct.
-------
-52-
With the changes made to this revision, this comment is no
longer applicable.
8. Page 27, Appendix C - Note that for five of the six diurnal
emissions calculated for vehicle Number 5044, the values are
zero. This is evidently because the estimated daily resting
loss was greater than the measured RTD grams. The zero
values for this vehicle were not mentioned in the text in
Section 4.2.2. How are these zero values handled in the
regression summarized in Table 4?
With the changes made to this revision, this comment is no
longer applicable.
9. Page 17, Section 4.2.3 - The analysis has available eight
vehicles to perform the regression. All eight vehicles
could be used in the regression instead of using only five
vehicles. Again, if class variables are used for the
identification of each variable, SAS can use all the
information to determine regression coefficients for the
input variables. The result would be better estimates of
the coefficients.
With the changes made to this revision, this comment is no
longer applicable; however, this suggestion will be used in
future analyses.
10. We would like to see some plots of the raw data versus the
values of input variables in the model or versus temperature
and RVP.
The reader can easily plot the data in this report if such
graphs are desirable.
11. Diurnal emissions for vehicles passing the purge and
pressure test were transformed to logs and then regressed
while vehicles that failed the purge and/or pressure tests
were regressed without taking the logs. What evidence do
you have for taking these different approaches? In general,
we would expect the log of the diurnal emissions to be a
better approach to take than the cube of the vapor pressure
product. A discussion of the engineering aspects of the
system under different purge/pressure result conditions
could lead to a resolution.
With the changes made to this revision, this comment is no
longer applicable.
12. It seems that this whole report is based on measurements
taken on the wrong model year vehicles. We presume that the
intent in doing this is to provide some sort of
-------
-53-
functionality in the MOBILE model for the 1996 to 1997 model
years using 1990 to 1995 vehicle data but only until the
data actually taken on 1996 and newer vehicles can be
obtained and analyzed. You might consider adding a
statement that says that when this new data does become
available, these models will be revisited.
A statement to that effect has been added to the end of
Section 1 (page 3). This comment is similar to this
reviewer's first comment.
-------
-54-
Appendix I
Response to Written Comments from Stakeholders
The following comment was submitted in response to EPA's
posting a draft of a related report (M6.IM.003) on the MOBILE6
website. The full text of this comment is posted on the MOBILE6
website.
Comment Number: 102
Name/Affiliation: David Lax / API
Date: January 25, 2000
Comment:
Under the heading of:
"Adjustments for Enhanced Evaporative Vehicles
"To account for the improved durability of enhanced evap
control systems, EPA reduced the baseline failure rates by
50%. They did this to both non-cap and cap failures. This
approach is appropriate for non-cap failures. (Although
some manufacturers went with % turn caps, e.g., Ford and
possibly some GMs.) As mentioned above, the big change in
cap technology occurred in the mid-80s with the switch to
screw-in caps, and this is not accounted for in EPA's
estimates."
EPA's Response:
In the report actually being commented on, the pressure
failures were divided into those related to the fuel cap and
those not involving the fuel cap. While that breakdown is
not used in this report, it is encouraging that API agrees
that it is appropriate to estimate the failure rate (on the
pressure test) of the ETP vehicles by reducing the failure
rate of the pre-ETP vehicles.
------- |