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

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                                                                          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.

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                            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.

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                       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

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                  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

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            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

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                               -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.

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     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

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                               -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.

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                               -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).

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                               -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

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                               -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

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                                -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

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                                -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:

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                               -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.

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                               -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

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                               -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

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                               -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 40F.   (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:

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                               -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

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                                -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.

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                                -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):

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                                -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),

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                               -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 )

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                               -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).

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                               -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.)

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                               -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

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                               -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

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                               -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

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                               -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

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                               -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.

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                               -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 40F  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.

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                               -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

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                               -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

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                               -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

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                               -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.

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                               -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.

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                          -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.

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                               -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

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                          -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.

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                               -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.

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