**EPA420-R-01-23**

**Estimating Running Loss Evaporative**

**Emissions in MOBILE6**

**Larry C. Landman**

**Document Number M6.EVP.008**

**April 2001**

**U.S. EPA**

**Assessment and Standards 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.

**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]{.underline} 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 stop-and-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).

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).

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]      |        |         |        |         |         |         |
| {.under |        |         |        |         |         |         |
| line}** |        |         |        |         |         |         |
|         |        |         |        |         |         |         |
| ##      |        |         |        |         |         |         |
| Summary |        |         |        |         |         |         |
|  of CRC |        |         |        |         |         |         |
|  Runnin |        |         |        |         |         |         |
| g Loss  |        |         |        |         |         |         |
| Testing |        |         |        |         |         |         |
+---------+--------+---------+--------+---------+---------+---------+
|         |        | *       |        | **R     | #       |         |
|         |        | *Mean** |        | uning** | ### 90  |         |
|         |        |         |        |         | Percent |         |
|         |        |         |        |         |         |         |
|         |        |         |        |         | **Con   |         |
|         |        |         |        |         | fidence |         |
|         |        |         |        |         | Int     |         |
|         |        |         |        |         | erval** |         |
+---------+--------+---------+--------+---------+---------+---------+
| **CRC** | **Md   | **Age** | **Sa   | *       |         |         |
|         | Yr**   |         | mple** | *Loss** |         |         |
+---------+--------+---------+--------+---------+---------+---------+
| **[P    | **[Ra  | **[(    | **[S   | *       |         |         |
| roject] | nge]{. | years)] | ize]{. | *[(gram |         |         |
| {.under | underl | {.under | underl | /mile)] |         |         |
| line}** | ine}** | line}** | ine}** | {.under |         |         |
|         |        |         |        | line}** |         |         |
+---------+--------+---------+--------+---------+---------+---------+
| > 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     | 92-97  | 4.320   | 50     | 0.3351  | 0.0901  | 0.5801  |
| -41\*\* |        |         |        |         |         |         |
+---------+--------+---------+--------+---------+---------+---------+
| > *     |        |         |        |         |         |         |
| *\*\*** |        |         |        |         |         |         |
| > The   |        |         |        |         |         |         |
| >       |        |         |        |         |         |         |
| running |        |         |        |         |         |         |
| > loss  |        |         |        |         |         |         |
| >       |        |         |        |         |         |         |
| results |        |         |        |         |         |         |
| > of    |        |         |        |         |         |         |
| > the   |        |         |        |         |         |         |
| > v     |        |         |        |         |         |         |
| ehicles |        |         |        |         |         |         |
| >       |        |         |        |         |         |         |
|  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]{.underline} 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 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-W    |            |         |            |         |            |
| eighted |            |         |            |         |            |
| MOBILE5 |            |         |            |         |            |
| Pred    |            |         |            |         |            |
| ictions |            |         |            |         |            |
| of      |            |         |            |         |            |
| Fleet   |            |         |            |         |            |
| Running |            |         |            |         |            |
| Loss    |            |         |            |         |            |
|         |            |         |            |         |            |
| (At CRC |            |         |            |         |            |
| Test    |            |         |            |         |            |
| Cond    |            |         |            |         |            |
| itions) |            |         |            |         |            |
+---------+------------+---------+------------+---------+------------+
|         |            |         |            |         |            |
+---------+------------+---------+------------+---------+------------+
| **Age** | **P        | **Age** | **P        | **Age** | **P        |
|         | redicted** |         | redicted** |         | redicted** |
| **[(    |            | **[(    |            | **[(    |            |
| years)] | **Run      | years)] | **Run      | years)] | **Run      |
| {.under | Loss**     | {.under | Loss**     | {.under | Loss**     |
| line}** |            | line}** |            | line}** |            |
|         | **[(       |         | **[(       |         | **[(       |
|         | g/mi)]{.un |         | g/mi)]{.un |         | g/mi)]{.un |
|         | derline}** |         | derline}** |         | derline}** |
+---------+------------+---------+------------+---------+------------+
| 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.

**[Figure 1]{.underline}**

**Comparing Re-Weighted MOBILE5 Predictions with**

**CRC Running Loss Emissions**

![](media/image1.wmf){width="5.989583333333333in"
height="3.5833333333333335in"}

**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]{.underline} 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).

+---------+------------+---------+------------+---------+------------+
| **Table |            |         |            |         |            |
| 3**     |            |         |            |         |            |
|         |            |         |            |         |            |
| Re-W    |            |         |            |         |            |
| eighted |            |         |            |         |            |
| MOBILE5 |            |         |            |         |            |
| Pred    |            |         |            |         |            |
| ictions |            |         |            |         |            |
| of      |            |         |            |         |            |
| Fleet   |            |         |            |         |            |
| Running |            |         |            |         |            |
| Loss    |            |         |            |         |            |
| In      |            |         |            |         |            |
| cluding |            |         |            |         |            |
| GLLs    |            |         |            |         |            |
|         |            |         |            |         |            |
| (At CRC |            |         |            |         |            |
| Test    |            |         |            |         |            |
| Cond    |            |         |            |         |            |
| itions) |            |         |            |         |            |
+---------+------------+---------+------------+---------+------------+
|         |            |         |            |         |            |
+---------+------------+---------+------------+---------+------------+
| **Age** | **P        | **Age** | **P        | **Age** | **P        |
|         | redicted** |         | redicted** |         | redicted** |
| **[(    |            | **[(    |            | **[(    |            |
| years)] | **Run      | years)] | **Run      | years)] | **Run      |
| {.under | Loss**     | {.under | Loss**     | {.under | Loss**     |
| line}** |            | line}** |            | line}** |            |
|         | **[(       |         | **[(       |         | **[(       |
|         | g/mi)]{.un |         | g/mi)]{.un |         | g/mi)]{.un |
|         | derline}** |         | derline}** |         | derline}** |
+---------+------------+---------+------------+---------+------------+
| 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]{.underline}**

**Comparing CRC Running Loss Emissions with**

**Re-Weighted MOBILE5 Predictions Including GLLs**

![](media/image2.wmf){width="5.989583333333333in"
height="3.5833333333333335in"}

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 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**

**Based on Running Loss Testing =**

Although this analysis concentrates on light-duty vehicles, this
approach shall be used in MOBILE6 for all vehicle types.

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, 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.

**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)]{.underline}

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.*

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]{.underline} 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]{.underline} 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]{.underline} 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 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]{.underline} 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]{.underline} 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 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]{.underline}** 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.*

**Appendix B**

### Response to Comments from Stakeholders 

[No]{.underline} comments were submitted in response to EPA's posting a
draft of this report on the MOBILE6 website.