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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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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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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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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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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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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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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