United States Air and Radiation EPA420-R-01-023 

Environmental Protection April 2001 

Agency M6.EVP.008 

Estimating Running Loss 

Evaporative Emissions in 

MOBILE6

 Printed on Recycled Paper 
EPA420-R-01-023 

April 2001 

Estimating Running Loss Evaporative
 

Emissions in MOBILE6
 

M6.EVP.008
 

Larry C. Landman
 

Assessment and Standards Division
 

Office of Transportation and Air Quality
 

U.S. Environmental Protection Agency
 

NOTICE
 

This technical report does not necessarily represent final EPA decisions or positions.
 

It is intended to present technical analysis of issues using data which are currently available.
 

The purpose in the release of such reports is to facilitate the exchange of
 

technical information and to inform the public of technical developments which
 

may form the basis for a final EPA decision, position, or regulatory action.
 
ABSTRACT 

This report documents the method used in MOBILE6 for
 

estimating the running loss emissions from vehicles.
 

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 incorporates
 

the effects of "gross liquid leakers" (see report M6.EVP.009)
 

with the MOBILE5 running loss estimates, and then verifies that
 

this approach is consistent with the results of recent running
 

loss testing (while the MOBILE5 estimates alone are not).
 

This report was originally released (as a draft) in June
 

1999. 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.
 
Estimating Running Loss Evaporative
 

Emissions in MOBILE6
 

Report Number M6.EVP.008
 

Larry C. Landman
 

U.S. EPA Assessment and Standards 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.). In MOBILE6, the running
 loss emissions are calculated separately for each hour of the
 day, based on the vehicle activity for that hour. The hourly
 emissions are then weighted together (to form a daily composite
 value) proportional to the number of miles driven each hour.
 

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

•	 The New York City Cycle (NYCC) features low speed stopand-go traffic conditions with an average speed of 7.1 

mph. Details on this cycle can be found on EPA's website 

(at http://www.epa.gov/oms/emisslab/methods/nycccol.txt). 

•	 The EPA Urban Dynamometer Driving Schedule (UDDS) is 

commonly called the "LA-4" or "the city test" and 

represents city driving conditions. It is used for 

light-duty vehicle testing and has an average speed of 

19.6 mph. Details on this cycle can be found on EPA's
 

website (at http://www.epa.gov/oms/emisslab/methods/uddscol.txt).
 

•	 The Highway Fuel Economy Driving Schedule (HWFET or HFET) 

represents highway driving conditions under 60 mph with 

an average speed of 47.9 mph. Details on this cycle can 

be found on EPA's website (at 

http://www.epa.gov/oms/emisslab/methods/hwycol.txt). 

The duration of the running loss test is approximately one hour
 for each of those three driving cycles. 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).
 
 

________________________________ 

-2-

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 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.l 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 compared these
 new data to the MOBILE5 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). [1]* 

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. The 

following summer (1998), CRC conducted a testing program in which 

* The numbers in brackets refer to the references in Section 6 (page 8).
 
 

-3-

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). [2] 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. A summary of the results from those two programs 

are given below in Table 1. Within each age range, the mean 

running loss test emissions were calculated as well as the 90 

percent confidence intervals. 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 

Mean Runing 90 Percent 

CRC Md Yr Age Sample Loss Confidence Interval 

Project Range (years) Size (gram/mile) 

E-35 Pre-80 21.984 61 2.3044 0.9730 3.6358 

80-85 13.744 39 1.3800 0.5745 2.1855 

86-91 8.340 50 0.4678 0.1497 0.7859 

E-41** 92-97 4.320 50 0.3351 0.0901 0.5801 

**	 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 cycle used in E-35. 

3.0 MOBILE5 PREDICTIONS OF RUNNING LOSS EMISSIONS 

The MOBILE5 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. 

MOBILE5 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) MOBILE5 predictions were produced for 

the running loss emissions. Since the CRC testing measured all 

evaporative emissions that occurred during the test, those 

results (in Table 1) include both running loss and resting loss 

emissions. Therefore, resting loss emissions (from reference [3]) 

were calculated and added to the re-weighted MOBILE5 estimates. 

Since most of the CRC testing was performed during the
 summer of 1997, two separate MOBILE5 runs were necessary (one on
 

January 1, 1997 and the second on January 1, 1998). The two
 
_______________________________ 

-4-

MOBILE5 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 below in Table 2.
 

Table 2 

Re-Weighted MOBILE5 Predictions of Fleet Running Loss 

(At CRC Test Conditions) 

Predicted Predicted Predicted 

Age Run Loss Age Run Loss Age Run Loss 

(years) (g/mi) (years) (g/mi) (years) (g/mi) 

0 0.1028 8 0.1975 16 0.5434 

1 0.1220 9 0.2199 17 0.5836 

2 0.1421 10 0.2498 18 0.6193 

3 0.1456 11 0.2863 19 0.6409 

4 0.1507 12 0.3334 20 0.6554 

5 0.1576 13 0.3850 21 0.6648 

6 0.1672 14 0.4403 22 0.6706 

7 0.1800 15 0.4945 23 0.6742 

24 0.6759 

The comparison (between the data in Tables 1 and 2) is
 illustrated by the following graph (Figure 1).
 

Even the most cursory comparison between the average running 

loss emissions in Table 1 and the re-weighted MOBILE5 predicted 

running loss emissions in Table 2 (or simply between the data in 

Figure 1) suggests that not only do the predicted values 

underestimate the observed mean values, but they also do not even 

fall within those rather large 90 percent confidence intervals. 

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" (GLLs) (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 MOBILE5, it is questionable whether any of the test vehicles 

would meet EPA's definition of a GLL.* In the upcoming section 

(Section 4.0), we will consider the effect of adding the 

emissions from the GLLs to the (preceding) MOBILE5 estimate. 

*	 The possible absence of "gross liquid leakers" in the data set used for
 

MOBILE5 is not unreasonable considering the relatively small number of
 such vehicles in the in-use fleet.
 
 

 

-5-

Figure 1
 

Comparing Re-Weighted MOBILE5 Predictions with
 

CRC Running Loss Emissions

Resting Loss (grams / mile) 

4

3 

2 

1 

0 

CRC Means 

90% Conf. Interval 

M5 Estimate 

0 6 12 18 24 

 Vehicle Age (years) 

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 

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 2 by including the estimated running loss 

emissions of the "gross liquid leakers" (from reference [5]). The 

revised values are in Table 3 (on the following page). 

When we compared the CRC running loss test results (from
 Table 1) with these MOBILE5 predictions that were modified to
 include the effects of GLLs (from Table 3), we obtained the graph
 in Figure 2 (on the following page).
 
 

 

-6-

Table 3 

Re-Weighted MOBILE5 Predictions of Fleet Running Loss Including GLLs 

(At CRC Test Conditions) 

Predicted Predicted Predicted 

Age Run Loss Age Run Loss Age Run Loss 

(years) (g/mi) (years) (g/mi) (years) (g/mi) 

0 0.1170 8 0.3877 16 1.4559 

1 0.1411 9 0.4778 17 1.5509 

2 0.1697 10 0.5940 18 1.6335 

3 0.1821 11 0.7292 19 1.6846 

4 0.2014 12 0.8843 20 1.7210 

5 0.2279 13 1.0426 21 1.7447 

6 0.2642 14 1.1978 22 1.7609 

7 0.3163 15 1.3365 23 1.7727 

24 1.7788 

Figure 2
 

Comparing CRC Running Loss Emissions with
 

Re-Weighted MOBILE5 Predictions Including GLLs

Resting Loss (grams / mile) 

0 

1 

2 

3 

4

CRC Means 

90% Conf. Interval 

M5 Estimate 

plus GLLs 

0 6 12 18 24 

 Vehicle Age (years) 

A visual examination of Figure 2 (or of Tables 2 and 3)
 

indicates that for vehicles up through the age of 11 years, the
 

re-weighted MOBILE5 predictions are excellent estimates of the
 

mean CRC results (i.e., within 0.15 grams per mile). And, even
 
 

-7-

though the difference grows to almost 0.58 grams per mile for the
 

oldest vehicles:
 

•	 From a statistical standpoint, those larger differences 

are actually relatively small, less than 10 percent of a 

standard deviation. 

And,
 

•	 The differences between the CRC averages and the 

predicted results are the largest 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 of age, 

thereby reducing the effect of any potential offset. 

5.0 CONCLUSIONS 

EPA proposes, for MOBILE6, to use the MOBILE5 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 336.78 grams per hour 

(divided by the average speed to obtain units of "grams per 

mile"). 

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 

0.06 Based on Running Loss Testing = 1 + 120 * exp[-0.4 * AGE] 

Although this analysis concentrates on light-duty vehicles,
 

this approach shall be used in MOBILE6 for all vehicle types.
 
-8-

6.0 REFERENCES
 

1) D. McClement, "Measurement of Running Loss Emissions from InUse 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, April 2001. 

4) Larry Landman, "Estimating Weighting Factors for Evaporative 

Emissions in MOBILE6," Report numbered M6.EVP.006, April 

2001. 

5) Larry Landman, "Evaporative Emissions of Gross Liquid Leakers 

in MOBILE6," Report numbered M6.EVP.009, April 2001. 
-9-

Appendix A 

Response to Peer Review Comments from Sandeep Kishan 

This report was formally peer reviewed by one peer reviewer
 (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.
 

************************************
 

This memorandum provides peer review comments on two EPA
 documents: "Estimating Running Loss Evaporative Emissions in
 

MOBILE6," Document No. M6.EVP.008, June 28, 1999, and
 

"Evaporative Emissions of Gross Liquid Leakers in MOBILE6,"
 

Report Number M6.EVP.009, June 30, 1999. Both of these are draft
 

reports.
 

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.
 

Document No. M6.EVP.008 (June 28, 1999)
 

We have the following questions, comments, and recommendations on
 

this draft report. For each item we give the page number and
 

paragraph that the comment refers to, if it is a specific
 

comment.
 

Overall this report was clearly written and the general
 

methodology seems alright. We do not have any recommendations of
 

any alternate datasets. It seems to us that the more serious
 

problem with the report is that we are not convinced that the
 

MOBILE5 predictions adequately describe the new CRC E-35 and E-41
 

data. In the comments below, we make suggestions which would
 

help clarify this comparison to the reader.
 

1.	 Page 1, Section 1.0 – We agree with the general methodology
 

used for data collection in past studies. That is, we agree
 

that the running loss emissions do not need to be tested at
 

combinations of temperatures and volatilities that are
 

either both low or both high.
 

EPA, of course, agrees with its own methodology.
 
-10-

2.	 Page 4, Paragraph 1 – The report suggests a number of
 

possible explanations for the differences between
 

corresponding values in Table 1 and Table 2. A short
 

paragraph listing and very briefly discussing alternative
 

explanations would give credence to the conclusion that
 

gross liquid leakers is the most likely explanation.
 

It was not EPA's intention to develop several possible
 

explanations as to why the CRC running loss results were
 

substantially higher than the predictions from MOBILE5.
 

Rather, EPA's goal was to find (and test) a reasonable
 

hypothesis. This report concludes that the sum of the
 

running loss emissions attributable to "gross liquid
 

leakers" (GLLs) and the MOBILE5 estimates of running loss
 

emissions from the non-GLLs is a close approximation of the
 

actual results found in the CRC sample.
 

3.	 Page 4, Paragraph 1 – A comparison of the Table 2 averaged
 

results from the CRC projects with the Table 2 MOBILE5
 

predictions by age is a weak comparison. In general,
 

whenever averages are used in data analysis, information is
 

lost. We suggest that instead, a tougher, and therefore
 

more revealing, comparison should be made by comparing the
 

individual running loss minus resting loss values from the
 

CRC studies with the MOBILE5 predictions by vehicle age.
 

This could be conveniently done in a plot of running loss
 

versus vehicle age with the CRC data points on the plot and
 

the MOBILE5 curve on the plot. If it is possible to make
 

such a plot with the CRC data, the result avoids the loss of
 

information produced by taking averages. Also, if the plot
 

were made in this way, it would not be necessary to delete
 suspected gross liquid leaker running loss values from the
 plot. These points would merely be points with high running
 loss values and could be highlighted as those which are
 suspected of being gross liquid leakers.
 

The CRC data will be provided (in a spreadsheet) with this
 report. So, the users may create their own scatter plots if
 they desire. The plots in this report have been revised to
 include the (90 percent) confidence intervals at each point
 rather than the full scatter plot of all the data.
 

4.	 Page 5, Table 3 – The values for mean age in Table 3 are
 exactly the same as the values in Table 1. Presumably, if
 six vehicles have been removed to produce Table 3 the mean
 ages will be different.
 

The reviewer is correct about that error. However, based on
 some of the comments from this reviewer, EPA decided to
 change the approach / emphasis (not the actual analysis or
 
-11-

conclusions) to improve clarity. One of the results of that
 

change was the dropping of that table from this revision.
 

5. Page 5, bottom half –
 

[This material is now on the bottom half of page 6.]
 

At this point in the report, the discussion centers around
 

the significance of the differences between the MOBILE5
 

prediction curve and the CRC data values. There are two
 

problems with this analysis.
 

First, statistical tests of significance are usually made in
 

a space where the variance is relatively homogeneous and
 

normally distributed. The fact that the standard deviations
 

of the measured values in Tables 1 and 3 are one and a half
 

to three times the means indicates to us that the running
 

loss values have skewed distributions. Since automotive
 

emissions values typically are skewed approximately in a
 

log-normal fashion, we suggest that instead the test of
 

significance be based in log space. For us, this would most
 

convincingly be put forward by plotting the running loss
 

emissions on a log scale in the plot suggested by Comment 3.
 

The MOBILE5 curve should pass somewhere through the center
 

of the 200 CRC measurements.
 

As noted in our response to the third comment, the users may
 

create their own scatter plots (using a logarithm scale in
 

this case) to compare the results from the CRC sample with
 

the estimates from MOBILE5 and MOBILE6.
 

Second, to verify that no significant difference exists
 

between the measured CRC values and the MOBILE5 predicted
 

value, some sort of formal statistical test should be
 

performed. We suggest that a paired t-test be used to
 

compare the average of the residuals (measured CRC value –
 

predicted MOBILE5 value) for each car with the standard
 

deviation of the mean of the residuals. These calculations
 

should be done in log space, where we presume the variance
 

is homogeneous and normal. If the mean residual is found to
 

be not significantly different from zero, then it can be
 

concluded that the measurements and the predictions are the
 

same. If the mean residual is significantly different from
 

zero, then either the MOBILE5 model needs to be changed or
 

an explanation needs to be provided that the significant
 

difference is small and is of small practical importance.
 

The approach suggested by the reviewer is valid; however, it
 

is more extensive than what EPA is attempting. In fact, a
 

statistical analysis of the results at the age of 20 years
 

may find the difference to be statistically significant, but
 

the relatively small number of in-use vehicles at that age
 reduces the effect of that difference on the overall in-use
 
-12-

fleet (composite) running loss value to be insignificant for
 

practical purposes.
 

6.	 Page 6, Figure 1 – It appears to us that this figure
 

indicates that MOBILE5 under predicts the CRC values by
 

about 40%. Since the standard deviations in linear space of
 

the running loss values from the CRC studies are greater
 

than the means of those running loss emissions, any model
 

which has a curve between 0 and the means will be within one
 

standard deviation of the means.
 

The reviewer is correct. We have, therefore, dropped this
 

figure (graph) from this revision (as noted in the response
 

to the fourth comment) to improve clarity. The new Figure 2
 

(in this revision) avoids these problems and still conveys
 

the desired information.
 

7.	 Page 5, Paragraph 2 –
 

[This material is now on page 6.]
 

The word "excellent" is glaring in light of the
 

relationships shown in Figure 1. Regardless of the outcome
 

of any further analysis on this data in this report, we
 

suggest selecting a less enthusiastic word.
 

As noted in the response to the preceding comment, that
 

figure has been dropped (replaced) to improve clarity.
 

However, the word "excellent" has been retained, but it now
 

applies to the fit in the (new) Figure 2 (for vehicles under
 

the age of 11). We believe that its use is appropriate.
 

8.	 Page 6, Section 5.0 –
 

[This section ("Conclusions") is now on page 7.]
 

Based on our comments above, the report does not convince us
 

that there is no significant or important difference between
 

the CRC running loss values and the MOBILE5 running loss
 

predictions. Nevertheless, it could very well be that the
 

conclusions stated in Section 5.0 are correct.
 

The differences between the MOBILE5 estimates of running
 loss (plus resting loss) emissions and the results obtained
 by CRC in its recent testing programs are significant. We
 

believe that (for practical purposes) those differences are
 

explained (and eliminated) by including the emissions from
 the "gross liquid leakers" as illustrated in Figure 2 on
 

page 6 (of this revision) which was added to improve
 

clarity.
 
-13-

Appendix B
 

Response to Comments from Stakeholders
 

No comments were submitted in response to EPA’s posting a
 

draft of this report on the MOBILE6 website.