United States        Air and Radiation      EPA420-P-99-Q24
            Environmental Protection               June 1999
            Agency                     M6.EVP.008
vvEPA     Estimating Running
            Loss Evaporative
            Emissions in MOBILE6
                                 > Printed on Recycled Paper

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                                     EPA420-P-98-024

                     -  Draft  -


    Estimating Running Loss Evaporative
             Emissions in MOBILE6

                 Larry C.  Landman

           Document Number  M6.EVP.008
                    June 28,  1999
                       U.S. EPA
           Assessment and Modeling  Division
     National  Vehicle Fuel  and Emissions  Laboratory
                2000  Traverwood Drive
           Ann  Arbor,  Michigan  48105-2425
                 734-214-4939  (fax)
                   mobile@epa.gov
                        NOTICE


These reports do not  necessarily  represent  final  EPA
decisions  or  positions.   They are  intended  to  present
technical analysis of  issues using data which are currently
available.  The purpose in release of these  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
     In earlier versions of EPA's MOBILE model, running loss
emissions (defined as evaporative hydrocarbons that are emitted
when the vehicle is in operation) were calculated as functions of
ambient temperature, fuel volatility, driving cycle, and vehicle
parameters (i.e.,  fuel delivery system, model year ranges, and
functionality of the evaporative control system).   This report is
not a complete re-analysis of the older data used in those
previous versions of MOBILE.  Rather, this report examines the
effects of "gross liquid leakers" (see report M6.EVP.009), and
then compares recent running loss test results with the result of
combining the estimated emissions of those leaking vehicles with
the MOBILES running loss estimates.

     Please note that EPA is seeking any input from stakeholders
and reviewers that might aid us in modeling any aspect of running
loss evaporative emissions.

     Comments on this report and its proposed use in MOBILE6
should be sent to the attention of Larry Landman.   Comments may be
submitted electronically to mobile@epa.gov, or by fax to  (734)
214-4939, or by mail to "MOBILE6 Review Comments", US EPA
Assessment and Modeling Division, 2000 Traverwood Drive, Ann
Arbor,  MI  48105.   Electronic submission of comments is preferred.
In your comments,  please note clearly the document that you are
commenting on, including the report title and the code number
listed.  Please be sure to include your name, address,
affiliation,  and any other pertinent information.

          This document is being released  and posted. Comments
will be accepted for sixty (60) days.  EPA will then review and
consider all comments received and will provide a summary of those
comments, and how we are responding to them.

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                        *** DRAFT ***
            Estimating  Running  Loss  Evaporative
                     Emissions  in  MOBILE6
                  Report Number  M6.EVP.008

                         Larry C.  Landman
             U.S.  EPA Assessment and Modeling Division
1 .0  Introduction

     Running loss emissions are defined as  evaporative
hydrocarbons that are emitted when  the  vehicle is in operation.
Since the MOBILE4 computer model, the US Environmental Protection
Agency (EPA) has estimated running  loss emissions based on
analyses of testing performed by one of its contractors
 (Automotive Testing Laboratories, Inc.).

     The test programs were designed to test  in-use vehicles with
three different driving cycles:

       a low speed cycle  (known as the New York City Cycle [NYCC])
        with an average  speed  of 7.1 mph,

       a Federal Testing  Procedure (FTP) LA-4 driving cycle with
        an average  speed of  19.6 mph,  and

       a high speed cycle (or Highway  Fuel Economy Test [HFET]).
        with an average  speed  of 47.9 mph.

The duration of the running loss test is approximately one hour
for each of those three driving cycle.   Therefore, the NYC driving
cycle is repeated six times (6 bags), the two portions of the LA-4
cycle are repeated three times (6 bags), and the HFET driving
cycle is repeated five times (5  bags).

     The running loss emissions test programs were designed to
collect data at four levels of fuel volatility  (7.0, 9.0, 10.4,
11.7 psi in Reid Vapor Pressure [RVP])  and  at three levels of
ambient temperature  (80, 95, and 105 F).   Not  all vehicles were
tested for all combinations of fuel RVPs and ambient temperatures,
however.  There was usually no testing  at extreme conditions, such
as the combinations of high RVP fuel and high ambient temperature
 (e.g.,  11.7 psi/105 F), and low RVP fuel  and low ambient
temperature  (e.g.,  7.0 psi/80 F),  because  of  their less likely
occurrences in the real world.  Also, if the running loss emission

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                                -2-                           DRAFT
results from a test vehicle were low  (less than 0.5 grams) at
certain fuel and temperature combination  (for example,  9.0 psi/95
F),  it was assumed that at the combinations of lower fuel
volatility and/or lower ambient temperatures  (i.e., 7.0 psi/95  F,
9.0 psi/80 F, and, 7.0 psi/80 F),  this  vehicle  would have
emissions at a similarly low level.  Therefore,  to save resources,
the vehicle was not tested for the combinations of lower fuel
volatility and lower ambient temperatures.  Further, there have
been no tests on 11.7 psi RVP fuel shortly after the issuance of
MOBILE4 in 1989.

     In MOBILE4 model, when the test data were not available at
certain combinations of fuel volatility and ambient temperature,
the gram per mile (g/mi)  running loss emissions were estimated
from a variable called "True Vapor Pressure (TVP)."  In the
MOBILE4.1 model,  this TVP was used to correlate with the running
loss emissions from failed vehicles.  These TVPs by bag are
expressed as functions of fuel volatility and fuel tank
temperature.  The TVP values were calculated for all combinations
of fuel volatility (7.0,  9.0,  10.4, and 11.7 psi RVP)  and tank
temperature profiles  (with the initial tank temperatures at 80,
87,  95, and 105 F).

     In recent years, industry sources have performed running loss
testing programs in which random samples of in-use vehicles were
tested (see Section 2).   In this analysis, we will compare these
new data to the MOBILES predictions to determine whether changes
need to be made for MOBILE6.
2.0  New Running Loss  Test  Data

     During the summer of 1997, running loss tests were performed
on 150 vehicles as part of a testing program (project number E-35)
conducted for the Coordinating Research Council  (CRC).  The
running loss emissions for these vehicles were measured over a
single LA-4 driving cycle, using tank fuel  (RVP about 6.8 psi),
and ambient temperature  about 95 degrees Fahrenheit. [1]*   The
following summer  (1998),  CRC conducted a testing program in which
running loss tests were performed on 50 late-model year vehicles
(1992 through 1997, with a mean age of 4.5 years) (project number
E-41).  These 50 newer vehicles were again tested using tank fuel
(RVP about 6.8 psi) and with an ambient temperature of about 95
degrees Fahrenheit; however, a longer driving cycle was used
consisting of an LA-4 followed by two NYCC cycles followed by a
second LA-4. [2] A summary of  the  results  from  those two programs
are given in Table 1  (on the following page).   Within each age
range, the mean running loss test emissions was adjusted by
subtracting the estimated resting loss emissions  (calculated as in
reference [3])  for  each fuel  delivery system, model year range,  and
   The numbers  in brackets refer to the references  in Section 6 (page 7).

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                                 -3-
DRAFT
then separately for vehicles that  failed the pressure  test* and
those that  passed the pressure  test.   In Table 1, the  value  "Mean
Age" was calculated by subtracting the model year from the test
year (either  1997 or  1998) .
                               Table 1
                  Summary of CRC Running Loss Testing


CRC
Project
E-35


E-41**


Md Yr
Range
Pre-80
80-85
86-91
92-97

Mean
Age
(years)
21.984
13.744
8.340
4.320


Sample
Size
61
39
50
50
Run Loss
Minus
Rst Loss
(g ra m/m i le)
2.2951
1.3467
0.4514
0.3220

Std Dev
Run Loss
(g ram/m i le)
6.2995
2.9659
1.3099
1.0533
      The running loss results of the vehicles tested in Project E-41 are based on a
      longer driving cycle but at a slower average speed than the E-35 cycle.
3.0  MOBILES  Predictions  of  Running Loss Emissions

     The  MOBILES model was run to generate predictions of the
running loss  emissions in the CRC project  E-35,  that is:

      the  ambient temperature was  set equal to 95 F,
      the  driving cycle was set to a single LA-4,  and
      the  fuel RVP was set to 6.8 psi.

MOBILES estimates were calculated for each model year within each
of the three  purge/pressure strata from reference [4].  Then,  using
the weighting factors from Appendix A of that reference, revised
(i.e., re-weighted)  MOBILES predictions were produced.

     Since  most of the CRC testing was performed during the summer
of 1997,  two  separate MOBILES runs were necessary (one at January
1, 1997 and the second at January 1,  1998).   The two MOBILES runs
were averaged together to estimate the running loss emissions of
the in-use  fleet (by vehicle age) measured during summer 1997.
Those predictions are given in Table  2 on  the following page.
 * While vehicles  in the CRC testing  programs were  not  recruited based  on
   their performance on the purge  and pressure tests  (as the EPA vehicles
   were),  those tests were performed on all of  the vehicles.

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                                 -4-
DRAFT
                               Table 2

          Re-Weighted  MOBILES Predictions of Fleet Running Loss
                       (At CRC Test Conditions)
Age
(years)
0
1
2
3
4
5
6
7
8
Predicted
Run Loss
(g/m i )
0.0974
0.1 152
0.1340
0.1374
0.1423
0.1490
0.1583
0.1707
0.1872
Age
(years)
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
Predicted
Run Loss
(g/m i )
0.2090
0.2372
0.2719
0.3149
0.3639
0.4160
0.4667
0.51 18
0.5482
Age
(years)
1 8
1 9
20
21
22
23
24

Predicted
Run Loss
(g/m i )
0.5751
0.5936
0.6054
0.6126
0.6167
0.6190
0.6198

     Even the most  cursory comparison between the average running
loss emissions in Table  1  and  the  re-weighted MOBILES predicted
running loss emissions in  Table  2  suggests that the predicted
values underestimate the observed  mean values.  This
underestimation is  most  significant  for vehicles over the age of
10 years.  There are a number  of possible explanations for those
differences; however, EPA  believes that the most likely
explanation is the  presence of vehicles identified as "gross
liquid leakers"  (see reference  [5]) in  the  CRC sample.

     In reference  [5], EPA  used the term  "gross  liquid leaker" to
identify vehicles having substantial  leaks of liquid gasoline, as
opposed to simply vapor  leaks.   In that report,  EPA stated that
the running loss emissions from  such  a vehicle tested over a
single LA-4 driving cycle  would  be at least 7.0 grams per mile.
When we examine the running loss test data used in the analysis
for MOBILES, it is  questionable  whether any of the test vehicles
would meet EPA's definition of a "gross liquid  leaker."*  In  the
upcoming section (Section  4.0),  we will consider the effect of
adding the emissions from  the  "gross  liquid leakers" to the
(above) MOBILES estimate.
4.0  Effect of "Gross  Liquid  Leakers" on  Running  Loss  Emissions

     In reference [5],  EPA defined for running  loss testing,
"gross liquid leakers" to  be  vehicles with both liquid leaks of
 * The possible absence of "gross  liquid leakers" in the  data set used  for
   MOBILES  is  not unreasonable considering the relatively small  number of
   such vehicles in the in-use fleet.

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                                -5-
DRAFT
gasoline and running loss test emissions of at least  7.0  grams per
mile.  Using that definition, we note that six  (6) of  the vehicles
in the CRC testing programs met those criteria.  We can then
revise Table 1 by omitting those "gross liquid leakers."  The
revised values are given below in Table 3:
                              Table 3

                 Summary of CRC Running Loss Testing
                Only Vehicles NOT "Gross Liquid  Leakers"
CRC
Project
E-35


E-41
Md Yr
Range
Pre-80
80-85
86-91
92-97
Mean
Age
(years)
21.984
13.744
8.340
4.320
Sample
Size
58
37
49
50
Run Loss
Minus
Rst Loss
(gram/mile)
1.1113
0.7128
0.2825
0.3220
Std Dev
Run Loss
(gram/mile)
1.5031
1.1103
0.5435
1.0533
     We then plotted, on the  same graph  (Figure  1  on the  following
page) both the re-weighted MOBILES estimates  (from Table  2)  and
"Non-Gross Liquid Leaker" mean emissions  (from Table 3).  A  visual
examination of Figure 1  (and Tables 2 and 3)  indicates  that  for
vehicles up through the age of 11 years, the  re-weighted  MOBILES
predictions are excellent estimates of the mean  CRC  results  (i.e.,
within 0.20 grams per mile).  And, even though that  difference
grew to almost 0.50 grams per mile for the oldest  vehicles:

       From a statistical standpoint, those differences are
        relatively small,  less than one-third of a standard
        deviation.
     And,
       The largest differences between the CRC averages and the
        predicted results occur in the portion of the in-use fleet
        that contributes the least to the total emissions due to
        the small number of in-use vehicles involved.  For example,
        fewer that one-tenth of the fleet is composed of vehicles
        older than 15 years,  thereby reducing the effect of any
        potential offset.

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                                 -6-
                                                     DRAFT
                               Figure 1

              Comparing  Re-Weighted  MOBILES Predictions  to
        CRC Running  Loss Emissions Excluding "Gross Liquid Leakers"
           3.0
       E
       5

       in
       in
       tn
       E
       LJ
       in
       in
       o
       o>
           2.0
1.0
              CRC Means
                           MOBILES
                                10       15

                            Vehicle Age   (years)
                                       2 0
2 5
5.0  Conclusions

     EPA proposes,  for MOBILE6, to use the MOBILES  model to
estimate the running loss emissions from that portion of the fleet
that does not contain vehicles that are "gross  liquid leakers."
For the portion of  the fleet composed  (entirely)  of vehicles that
are "gross liquid leakers," EPA proposes to use report M6.EVP.009
(i.e., reference  [5])  to both estimate  and weight the  emissions.
The mean running  loss emissions of "gross liquid leakers" was
estimated to be 17.65 grams per mile (less the  resting loss
emissions).

     In that same report, the estimated frequency of "gross liquid
leakers" in the in-use fleet (as a function of  the  vehicle's age)
is given by the equation:
   Rate of Gross Liquid Leakers

      Based on Running Loss Testing
                                      0.06
                                       1 + 1 20 * exp[-0.4 * A G E ]

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                                -7-                           DRAFT
6.0  References

1 ) D. McClement, "Measurement of Running Loss Emissions from In-
   Use Vehicles (CRC Project E-35)", CRC Report No. 611, Prepared
   for the Coordinating Research Council, Inc. by Automotive
   Testing Laboratories, Inc., February 1998.

2 ) D. McClement, "Real World Evaporative Testing of Late Model In-
   Use Vehicles, CRC Project E-41", Prepared for the Coordinating
   Research Council, Inc. by Automotive Testing Laboratories,
   Inc., December 17, 1998.

3 ) Larry Landman,  "Evaluating Resting Loss and Diurnal Evaporative
   Emissions Using RTD Tests," Report numbered M6.EVP.001, July
   1999.

4 ) Larry Landman,  "Estimating Weighting Factors for Evaporative
   Emissions in MOBILE6," Report numbered M6.EVP.006, June 1999.

5 ) Larry Landman,  "Evaporative Emissions of Gross Liquid Leakers
   in MOBILE6," Report numbered M6.EVP.009, June 1999.

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