United States Air and Radiation EPA420-R-01-021 

Environmental Protection April 2001 

Agency M6.EVP.005 

Modeling Diurnal and 

Resting Loss 

Emissions from Vehicles 

Certified to the Enhanced 

Evaporative Standards

 Printed on Recycled Paper 
EPA420-R-01-021 

April 2001 

Modeling Diurnal and Resting Loss Emissions from
 

Vehicles Certified to the Enhanced Evaporative Standards
 

M6.EVP.005
 

Larry C. Landman
 

Assessment and Standards Division
 

Office of Transportation and Air Quality
 

U.S. Environmental Protection Agency
 

NOTICE
 

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

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

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

technical information and to inform the public of technical developments which
 

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

This report documents the method used in MOBILE6 for
 

estimating the resting loss and diurnal emissions from vehicles
 

certified to the enhanced evaporative standards (i.e., 1999 and
 

newer vehicles plus some 1996 through 1998).
 

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

1998. This current version is the final revision of that draft.
 

This final revision incorporates suggestions and comments
 

received from stakeholders during the 60-day review period and
 

from peer reviewers.
 

i
 
TABLE OF CONTENTS 

Page Number

 1.0 Introduction . . . . . . . . . . . . . . . . . . . . 1

 2.0 Data Sources . . . . . . . . . . . . . . . . . . . . 3

 3.0 Simulating Test Data from In-Use 1996 and
 

Newer Vehicles . . . . . . . . . . . . . . . . . . . 5

 4.0 Analysis . . . . . . . . . . . . . . . . . . . . . . 7

 4.1 Resting Loss Emissions . . . . . . . . . . . . 8

 4.1.1 Properly Functioning Vehicles. . . . . . 8

 4.1.2 Malfunctioning Vehicles. . . . . . . . . 9

 4.1.3 Gross Liquid Leakers . . . . . . . . . . 11

 4.2 Diurnal Emissions . . . . . . . . . . . . . . . 11

 4.2.1 Properly Functioning Vehicles. . . . . . 12

 4.2.1.1 Multi-Day Diurnal Emissions . . 13

 4.2.2 Malfunctioning Vehicles. . . . . . . . . 15

 4.2.3 Gross Liquid Leakers . . . . . . . . . . 17

 5.0 Distribution of ETP Vehicles. . . . . . . . . . . . 17

 5.1 Effects of Changing Durability Requirements. . 18

 5.2 Effects of On-Board Diagnostic Systems . . . . 19

 6.0 Other Types of Evaporative Emissions. . . . . . . . 19

 7.0 Evaporative Emissions Of Heavy-Duty Vehicles. . . . 20

 8.0 Effects of the ORVR Rule. . . . . . . . . . . . . . 21

 9.0 Effects of the Tier-2 Rule. . . . . . . . . . . . . 22
 

10.0 Summary . . . . . . . . . . . . . . . . . . . . . . 24
 

ii
 
TABLE OF CONTENTS (Continued) 

APPENDICES 

Page Number

 A. Certification Tests on 65 ETP Vehicles. . . . . . . 25

 B. CAP-2000 Tests on Six Mercedes ETP Vehicles . . . . 27

 C.	 Twenty-Five 1990-1995 Model Year Vehicles

 Passing Both the Purge and Pressure Tests . . . . 28

 D.	 Ten "ETP-Like" Vehicles with Multiple RTD Tests

 Passing Both the Purge and Pressure Tests . . . . 31

 E.	 Eight 1990-1995 Model Year Vehicles

 Failing (Only) the Purge Test . . . . . . . . . . 33

 F.	 Five 1990-1995 Model Year Vehicles

 Failing the Pressure Test . . . . . . . . . . . . 35

 G. Peer Review Comments from H. T. McAdams . . . . . . 36

 H. Peer Review Comments from Sandeep Kishan. . . . . . 49

 I. Comments from Stakeholders. . . . . . . . . . . . . 54
 

iii
 
 

 

 

________________________________ 

Modeling Diurnal and Resting Loss Emission
 

from Vehicles Certified to the Enhanced Evaporative Standards
 

Report Number M6.EVP.005
 

Larry C. Landman
 

U.S. EPA Assessment and Standards Division
 

1.0 INTRODUCTION
 

Evaporative emissions of hydrocarbons (HC) are a significant
 

portion of the total HC emissions estimated in the MOBILE model.
 

In two parallel reports (M6.EVP.001 and 002), the Environmental
 

Protection Agency (EPA) identified the methods that are being
 

used in MOBILE6 to estimate resting loss and diurnal emissions
 

from 1995 and older model year vehicles. These estimates are
 

based on the results of real-time diurnal (RTD) tests of in-use
 

vehicles in which the ambient temperature cycled over a 24-degree
 

Fahrenheit range to simulate in real-time the daily heating and
 

cooling that parked vehicles experience over a 24-hour period.
 

Beginning with the 1996 model year, manufacturers were
 

required to certify at least twenty percent of their vehicles
 

using a new "enhanced" evaporative testing procedure (ETP); that
 

percentage of ETP vehicles was required to increase from the
 

twenty percent in 1996 up to one hundred percent by 1999. The
 

actual phase-in of these ETP vehicles proceeded at a slightly
 

faster pace (based on EPA’s analysis of data from the Wisconsin
 

Inspection/Maintenance program for model years 1996-1999). The
 

phase-in rate required by the regulations* is given in Table 1
 

(below) along with the observed (actual) phase-in rate.
 

Table 1 

Phase-In of Vehicles with 

Enhanced Evaporative Controls 

Model Required * Observed 

Year Percentage Percentage 

1995 0% 0% 

1996 20% 30% 

1997 40% 55% 

1998 90% 90% 

1999 100% 100% 

*
 The percentages for the "required" phase-in were copied from 40 CFR 86.096-8 
-2-

EPA expects that these ETP vehicles will have evaporative
 emissions different than their pre-1996 (pre-ETP) counterparts
 (thus, requiring distinct estimates). This assumption is based
 on a number of changes that the manufacturers have implemented in
 order to meet the enhanced evaporative standards. These changes
 include, but are not limited to:
 

•	 "quick connects" that reduce the possibility of 

improper assembly when the vehicle is serviced, 

•	 advanced materials that are less permeable, less 

susceptible to puncture, and more durable (i.e., 

elastomeric materials used in hoses and connectors), 

•	 improvements made to the purge system (to enable the 

vehicles to pass both the running loss test and the 

multi-day diurnal test), 

•	 tethered gas caps, and 

•	 improved fractional-turn gas caps. 

Since these changes are expected to result in improved control of
 

evaporative emissions, EPA used in MOBILE5, a separate set of
 

estimates for both resting loss and diurnal emissions from these
 

vehicles.
 

In the original analyses that supported this rule, EPA
 

estimated that by requiring vehicles to meet these enhanced
 

evaporative standards the following would result:
 

•	 for those ETP vehicles with properly functioning 

evaporative control systems (i.e., vehicles passing 

both the purge test and the pressure test), full-day 

diurnal emissions would be reduced by 50 percent 

compared to the corresponding pre-ETP vehicles, 

•	 for those ETP vehicles with malfunctioning evaporative 

control systems (i.e., vehicles failing either the 

purge test or the pressure test), there would be no 

reduction (zero percent) of full-day diurnal emissions 

compared to the corresponding pre-ETP vehicles, and 

•	 for all ETP vehicles, resting loss emissions would be 

reduced by 75 percent compared to the corresponding 

pre-ETP vehicles. 

In the previous version of the MOBILE model (i.e., MOBILE5),
 EPA used these estimated reductions to characterize the diurnal
 and resting loss emissions of the ETP vehicles. EPA also used
 the required phase-in rate (middle column in Table 1) to describe
 the distribution of the ETP vehicles among the 1996-98 model year
 

vehicles in the in-use fleet.
 
-3-

The goal of the analyses in this report was to review and
 possibly replace those MOBILE5 hypotheses in the light of
 additional data. Implementing this goal involved determining the
 following three items:
 

(1) the percentage of ETP vehicles for each of the phase-in
 model years (1996-98),
 

(2) the emissions (resting loss and diurnal) of these ETP
 

vehicles (see Sections 4.1 and 4.2), and
 

(3) the percentage (by age) of vehicles with properly
 

functioning evaporative control systems (see Section 5).
 

The first of these three items was relatively straightforward.
 

EPA chose to use the observed phase-in rate (third column in
 

Table 1), rather than the rate specified in the regulations, to
 

describe the percent of ETP vehicles for the 1996-98 model years
 

in the in-use fleet.
 

The parallel analyses (report M6.EVP.001) of the diurnal and
 

resting loss emissions for pre-enhanced (i.e., pre-1996 model
 

year) vehicles are based on results of tests of actual in-use
 

vehicles. However, the analyses in this report generally are not
 

based on testing of actual in-use ETP vehicles because EPA has
 

very few test results on that segment of the in-use fleet. In
 

this report, EPA explores methods of estimating the resting loss
 

and diurnal emissions from these in-use 1996 and newer vehicles
 

based primarily on RTD testing of older (pre-ETP) but similar
 

vehicles.
 

Since many of the estimates (developed in the report) of
 

resting loss and diurnal emissions for the ETP vehicles in
 

MOBILE6 are based on pre-ETP vehicles, EPA will likely revisit
 

these estimates when sufficient test data on actual ETP vehicles
 

become available.
 

2.0 DATA SOURCES 

In the parallel analyses (report M6.EVP.001) on the pre-ETP
 

(i.e., 1995 and earlier model year) vehicles, EPA based its
 

estimates of resting loss and diurnal emissions on the results of
 

real-time diurnal (RTD) tests on 270 in-use vehicles. However,
 

at the time of this analysis on the 1996 and newer vehicles, EPA
 

had only two available sources of RTD test data on vehicles that
 

were certified to the new evaporative standards:
 

1) results of RTD testing used by the Air Resources Board 

(ARB) of California and by the EPA (30 and 35 vehicles, 

respectively) to certify new ETP vehicles (1996-97 model 

year) (see Appendix A) and 

2) results of RTD testing performed by Mercedes-Benz on six 

of its 1996 model year vehicles (at two years of age) as 
-4-

part of the proposed Compliance Assurance Program (CAP
 2000) (see Appendix B).
 

However, these test data (from these two sources) on the
 1996 and newer vehicles have three serious limitations:
 

•	 First, all of the 1996 and newer vehicles from these two 

sources had properly functioning evaporative control 

systems. Since it is likely that some similar in-use 

vehicles during the course of their useful life would 

develop malfunctions in their evaporative control 

systems, any analysis restricted to these data sets would 

be limited by not including test results on such 

malfunctioning in-use vehicles. 

•	 Secondly, all of these RTD tests were performed using a 

single test fuel with a Reid vapor pressure (RVP) of 9.0 

psi and using a single temperature cycle (72 to 96 

degrees Fahrenheit). Thus, using only these data, it 

would be not be possible to predict evaporative emissions 

at other combinations of temperature cycle and fuel 

volatility. 

•	 Finally, the RTD test data on all 65 of these vehicles 

were reported in the form of full-day (not hourly) 

emissions. However, EPA’s procedure of estimating the 

resting loss portion of the emissions requires the hourly 

RTD emissions (at least for hours 19 through 24); thus, 

EPA could not use these data to estimate resting loss 

emissions. 

To compensate for those significant limitations, EPA
 

supplemented those data with the results of RTD testing of older
 

vehicles (used in M6.EVP.001) that were not certified to the
 

enhanced evaporative standards. Two sources of those additional
 

RTD test results were:
 

3)	 RTD testing performed on 119 in-use 1971-95 model year
 

vehicles for EPA by its testing contractor and
 

4)	 RTD testing performed on 151 in-use 1971-91 model year
 

vehicles for the Coordinating Research Council (CRC).
 

Although none of those 270 in-use vehicles tested in the EPA
 or CRC programs (sources 3 and 4) had been certified to the new
 evaporative standards, the combined sample does include both:
 

•	 in-use vehicles with malfunctions in their evaporative 

control systems 

as well as
 

•	 vehicles for which the RTD test was performed over three 

different temperature cycles and using fuels with at 

least two different RVPs. 
 

-5-

Obviously, it would be inappropriate to use test data from all
 270 of those vehicles. Only a few of the newest vehicles in that
 sample are likely to be comparable to the actual ETP vehicles.
 (Section 3 deals with the selection of that sub-sample.)
 

In Section 3.0, EPA discusses how it used RTD test results
 from some of the older (i.e., 1990-95) vehicles (i.e., from
 sources 3 and 4) to compensate for the limitations of the test
 results on the 1996 and later vehicles.
 

3.0 SIMULATING TEST DATA FROM IN-USE 1996 AND NEWER VEHICLES 

The MOBILE model must be able to estimate the resting loss
 

and diurnal evaporative emissions from the 1996 and newer model
 

year ETP vehicles over a variety of daily temperature cycles and
 

with a variety of fuel RVPs. However, as noted in the preceding
 

section, the only test data on those vehicles available at the
 

time of this analysis are with a single combination of fuel
 

volatility (RVP of 9.0 psi) and daily temperature profile (i.e.,
 

ambient temperatures cycling between 72 and 96 degrees
 

Fahrenheit). EPA, therefore, used the results of RTD tests on
 

pre-ETP vehicles (i.e., model years 1990 through 1995) to
 

estimate the effects on the actual "base line" emissions (from
 

source 1) of different fuel volatility and different temperature
 

cycles on the resting loss and diurnal evaporative emissions of
 

the 1996 and newer vehicles.
 

For the purpose of characterizing the effects of varying the
 

fuel RVP and/or the temperature cycle, EPA will continue (from
 

the parallel analyses) the approach of dividing the in-use fleet
 

into the following four strata:
 

1)	 The first of these four strata consists of vehicles
 

having substantial leaks of liquid gasoline (as opposed
 

to simply vapor leaks); these vehicles were labeled
 

"gross liquid leakers."
 

EPA proposed (in M6.EVP.001) using as a definition for
 

such vehicles the requirement that the hourly resting
 

loss (at 72 degrees Fahrenheit) be at least 1.0 grams per
 hour of HC. EPA realizes this definition needs to be
 amended to include vehicles having substantial leaks that
 are apparent only when the engines are operating (e.g.,
 some fuel line leaks). (See parallel report number
 

M6.EVP.009 entitled "Evaporative Emissions of Gross
 

Liquid Leakers in MOBILE6.")
 

2)	 The second of these four strata consists of vehicles (not
 

"gross liquid leakers") that pass both the purge and
 

pressure tests (i.e., vehicles with properly functioning
 

evaporative control systems).
 
-6-

3) The third of these four strata consists of vehicles (not 

"gross liquid leakers") that fail the pressure test 

(regardless of their performance on the purge test). 

4) The fourth of these four strata consists of vehicles (not 

"gross liquid leakers") that fail only the purge test. 

While neither the purge test nor the pressure test (which
 are each being used to determine the stratification) actually
 measures evaporative emissions, a failure of the vehicle on
 either test is indicative of potential malfunctions of the
 vehicle's evaporative control system. Additionally, the
 recruitment of the vehicles in the third data source was
 intentionally skewed to recruit a larger proportion of vehicles
 with potentially malfunctioning evaporative control systems
 (i.e., a stratified random recruitment). Therefore, the results
 of any analysis must be weighted to correctly represent the
 entire in-use fleet. Thus, the analyses will be stratified to
 match the recruitment process.
 

As discussed previously, it was necessary to make use of the
 RTD tests performed on older (1990-95 model year) vehicles to
 predict the effects on the evaporative emissions of changes to
 the temperature cycle or the fuel RVP. In order to make use of
 those RTD tests on some of those 270 vehicles, EPA made the
 following assumptions:
 

1)	 The 1996 and newer vehicles are expected to be port fuel
 injected (PFI); therefore, EPA chose the 1990 to 1995
 

model year vehicles that were equipped with PFI as
 

appropriate surrogates.
 

2)	 To simulate the ETP vehicles with properly functioning
 

evaporative control systems, we then selected a subset
 

(of those 1990-95 model year vehicles equipped with PFI)
 

that passed both the purge test and the pressure test.
 

The resulting 25 vehicles are listed in Appendix C.
 

EPA believes that not all the vehicles in this sample of
 

25 pre-ETP vehicles (Appendix C) are appropriate for
 

simulating the actual ETP vehicles. Examining the sample
 

of 65 actual ETP vehicles in Appendix A, we note that the
 

first-day diurnal emissions range between 0.340 and 1.675
 

grams, with a mean of 0.745 and a median of 0.635.
 

We then restricted those 25 vehicles in Appendix C to
 those having the first-day diurnal emissions of at most
 1.7 grams (using a fuel with an RVP of 9.0 over a 72-96
 temperature cycle), producing the seven vehicles listed
 in Appendix D (all with multiple tests). This sevenvehicle sub-sample has a mean full-day diurnal of 0.902
 grams and a median of 0.741. (While EPA used this sevenvehicle sample in its analyses, another analyst could
 

more closely approximate both the mean and median in
 

Appendix A by further restricting the first-day diurnal
 

emissions to no more than 1.0 grams instead of 1.7. The
 
-7-

resulting (smaller) five-vehicle sub-sample has a mean of
 0.726 and a median of 0.653. However, EPA believes that
 the advantages of the somewhat larger sample size
 outweigh the advantages of the slightly improved
 statistical fit.) An additional three vehicles can be
 added by applying that 1.7 gram limit to vehicles tested
 only on a fuel with an RVP of 6.8 psi (resulting in the
 total of 10 vehicles listed in Appendix D as being
 possible "ETP-like").
 

Since the goal of this analysis is to predict the resting
 loss and diurnal emissions over a range of temperature
 cycles and fuel RVPs, we limited our analyses to the
 seven vehicles in Appendix D that were tested over a
 range of temperature cycles.
 

3)	 EPA believes that the RTD emissions from malfunctioning
 enhanced evaporative control vehicles (i.e., vehicles
 that developed problems with their evaporative control
 systems) will be similar to the RTD emissions from the
 1990 to 1995 model year vehicles that also develop
 problems with their evaporative control systems. That
 is, those 1996 and newer model year vehicles that had
 failed either EPA's purge or pressure tests are expected
 to have evaporative emissions similar to those 1990 to
 1995 model year PFI vehicles that also failed the same
 test.
 

Thirteen such vehicles were identified in the combined
 

EPA/CRC sample (eight of them failing only the purge test
 

and the remaining five failing the pressure test). (See
 

Appendices E and F, respectively.) EPA used the RTD
 

tests on these 13 vehicles to estimate the temperature
 

and fuel RVP effects on resting loss and diurnal
 

emissions for ETP vehicles that have malfunctioning
 

evaporative control systems.
 

4.0 ANALYSIS 

As noted in two parallel reports (M6.EVP.001 and
 M6.EVP.002), EPA is using (in MOBILE6) the results of the RTD
 test to model two distinct mechanisms of evaporative emissions:
 

1)	 "Resting loss" emissions are always present, regardless
 

of vehicle activity, and are relatively weakly related to
 

the ambient temperature as opposed to diurnal emissions
 

that are related to the rise in temperature.
 

The earlier reports calculated the hourly resting loss
 

emissions to be the mean of the RTD emissions from hours
 

19 through 24 at the nominal temperature for hour 24.
 

This method permitted EPA to estimate the hourly resting
 

loss emissions at three distinct temperatures (60, 72,
 

and 82 degrees Fahrenheit). In those analyses, resting
 
 

 

-8-

loss emissions were determined to be independent of the
 RVP of the test fuel.
 

2) "Diurnal" emissions are the pressure-driven emissions
 resulting from the daily increase in temperature.
 

The diurnal emissions were calculated by first estimating
 the resting loss value for the ambient temperature at
 each hour of the 24-hour cycle, and then subtracting that
 temperature-adjusted resting loss estimate from the RTD
 hourly test results.
 

In those two parallel reports, this approach permitted EPA to use
 the RTD test results to analyze separately both the relatively
 constant resting loss emissions and the (pressure driven) diurnal
 emissions.
 

4.1 Resting Loss Emissions 

In the parallel analyses of the resting loss emissions of
 

the pre-ETP vehicles (report M6.EVP.001), EPA used regression
 

analyses of the resting loss emissions (at three temperatures) to
 

model the resting loss emissions. This approach was repeated in
 

the previous draft version of this report (i.e., the version
 

reviewed by our stakeholders and by two formal peer reviewers).
 

4.1.1 Resting Loss Emissions of Properly Functioning Vehicles 

EPA initially (i.e., in the previous draft version of this
 

report) selected from Appendix C the (averaged) three resting
 

loss emissions from the seven vehicles that had been tested over
 

three temperature cycles. This yielded the following table of
 

results.
 

Table 2 

Mean Hourly Resting Loss Emissions
 

For Seven "ETP-Like Vehicles" (grams/day)
 

Vehicle Temperature (degrees F) 

Number 60 72 82 

5032 0.0045 0.0070 0.0150 

5038 0.0033 0.0053 0.0093 

5046 0.0075 0.0100 0.0305 

5047 0.0050 0.0120 0.0150 

5066 0.0000 0.0050 0.0075 

5068 0.0060 0.0095 0.0235 

5081 0.0050 0.0040 0.0090 

Mean 0.0045 0.0075 0.0157 

Std. Dev. 0.0024 0.0030 0.0085 
 

-9-

In the previous analyses, EPA then performed regression
 

analyses to model those three mean resting loss emissions (grams/
 

hour) (from the preceding table) as a function of temperature.
 

After the draft was released (for comments), it was noted that:
 

1.	 The sample size is quite small.
 

2.	 The estimated hourly emissions are close to the limit
 

that the equipment can measure.
 

3.	 The vehicles are not true ETP vehicles, they only
 

simulate what we expect from ETP vehicles.
 

4.	 If we estimate the hourly resting loss of comparable
 

pre-ETP vehicles (using the equation from M6.EVP.001) and
 

then compare those estimates to the means in the
 

preceding table, we find that these means are reductions
 

of 75 to 85 percent of the pre-ETP estimates. (This is
 consistent with the final "bullet" on page 2, which is a
 

re-statement of the conclusion reached in the original
 

regulatory analysis.)
 

Base on these four points (especially the last two), EPA revised 

its approach to estimating the resting loss emissions from the 

properly functioning ETP vehicles. Rather than use the new 

equation derived in the earlier draft version of this report, EPA 

chose to simply apply the previously estimated reduction factor 

of 75 percent to the equation for the comparable pre-ETP vehicles 

(from M6.EVP.001). This produces equation (1) below: 

Hourly Resting Loss (grams/hr) = -0.035168 + [0.000703 * Temperature (°F)] (1) 

EPA uses (in MOBILE6) equation (1) to estimate the hourly resting 

loss emissions (in grams per hour) of that portion of the fleet 

of ETP vehicles with properly functioning evaporative control 

systems. 

Equation (1) predicts that the mean hourly resting loss 

emissions (for the fleet of 1996 and newer model year vehicles 

with properly functioning evaporative control systems) would be 

negative for all ambient temperatures below 50.1 degrees 

Fahrenheit. EPA will (in MOBILE6) assume, that for each of the 

hours of the day that those temperatures occur (i.e., hourly 

temperature < 50.1 F), the resting loss emissions will be set to 

zero grams. 

4.1.2 Resting Loss Emissions of Malfunctioning Vehicles 

To estimate the resting loss emissions from ETP vehicles
 

with malfunctioning evaporative control systems (i.e., those ETP
 

vehicles that would fail either the purge or the pressure test),
 

EPA followed the same three-step pattern that was used for ETP
 

vehicles with properly functioning evaporative control systems
 

(in Section 4.1.1). That is:
 
-10-

1.	 A sample of pre-ETP vehicles was identified that could
 simulate these ETP vehicles.
 

In Section 3.0, EPA proposed using five 1990-95 model
 year PFIs to represent the 1996 and newer model year
 vehicles that failed the pressure test (only four of
 which were tested over all three temperature cycles) and
 using eight vehicles to represent the 1996 and newer
 model year vehicles that failed the purge test (see
 Appendices E and F).
 

2.	 The means of the resting loss emissions were regressed
 against temperature using the three temperature points
 (60, 72, 82 F).
 

Resting loss data on the 12 vehicles that were tested
 over all three temperature cycles were combined (into a
 single stratum) and analyzed. The resulting equation was
 contained in the previous draft of this report (that was
 released for comments).
 

3.	 After the previous draft was released (for comments), it
 was noted that these means also could have been modeled
 simply as reductions of 75 to 85 percent of the pre-ETP
 estimates. (Again, this is consistent with the final
 "bullet" on page 2, which is a re-statement of the
 conclusion reached in the original regulatory analysis.)
 

Therefore, EPA revised its approach to estimating the 

resting loss emissions from the malfunctioning ETP vehicles. 

Rather than use the new equation derived in the earlier draft 

version of this report, EPA chose to simply apply the previously 

estimated reduction factor of 75 percent to the equation for the 

comparable pre-ETP vehicles (from M6.EVP.001). This produces 

equation (2) below: 

For ETP Vehicles that Fail the Pressure Test: 

Hourly Resting Loss (grams/hr) = -0.02731 + [0.000703 * Temperature (°F)] (2) 

EPA uses (in MOBILE6) equation (2) to estimate the hourly resting 

loss emissions (in grams per hour) of that portion of the fleet 

of ETP vehicles that fail the pressure test. Additionally, the 

scope (domain) of equation (1) was expanded to cover all ETP 

vehicles that pass the pressure test regardless of their 

performance on the purge test. 

Equation (2) predicts that the mean hourly resting loss 

emissions (for the fleet of 1996 and newer model year vehicles 

that fail the pressure test) would be negative for all ambient 

temperatures below 38.8 degrees Fahrenheit. This will not 

present a problem, because (using the analyses from earlier 

versions of the MOBILE model) EPA will (in MOBILE6) assume, that 

for each of the hour of the day that the temperature does not 

exceed 40, the hourly resting loss emissions will be set to zero 

grams. 
 

-11-

4.1.3 Resting Loss Emissions of "Gross Liquid Leakers" 

In a parallel report (M6.EVP.001), EPA proposed that, for
 

the pre-1996 vehicles classified as gross liquid leakers, the
 

resting loss emissions are virtually independent of temperature,
 

averaging 9.16 grams per hour. EPA will continue to use that
 

assumption for the 1996 and newer vehicles that were certified to
 

the enhanced evaporative standard. That is, the hourly resting
 

loss emissions of all "gross liquid leakers" will be set at 9.16
 

grams per hour regardless of vehicle type, or model year, or
 

ambient temperature.
 

4.2 Diurnal Emissions 

The pattern of the analyses of the diurnal emissions closely
 

paralleled the pattern that developed with the resting loss
 

emissions. That is:
 

1.	 A samples of pre-ETP vehicles were identified that could
 

simulate these ETP vehicles (Appendices D, E, and F).
 

2.	 The means of the diurnal emissions were regressed against
 

a variable (VP_Product) developed in report M6.EVP.001.
 

3.	 After the previous draft was released (for comments), it
 

was noted that these means could have been modeled simply
 

as reductions of the pre-ETP estimates. (See the final
 

"bullet" on page 2.)
 

In Section 4.1, we developed equations (1 and 2) that 

estimate the resting loss emissions for each temperature (in 

degrees Fahrenheit). Applying those equations to each hour of 

the full 24 hours of the RTD test, and then adding the 24 

"temperature corrected" hourly resting loss emissions produces 

the full day's total resting loss (in grams). Subtracting that 

quantity from each of the RTD test scores yields the estimated 

(full-day) diurnal emissions in Appendices C, D, E, and F. 

Two factors that significantly affect a vehicle's diurnal
 emissions (see M6.EVP.001 and M6.EVP.002) are:
 

•	 the Reid vapor pressure (RVP) of the test fuel and 

•	 the temperature cycle, as represented by the 

combination of the cycle's midpoint temperature and 

temperature range. 

In parallel reports (M6.EVP.001 and M6.EVP.002), we created a
 single parameter that incorporated both of those factors. That
 new parameter is based on the vapor pressure (VP) of the fuel.
 In those reports, we used both the RVP of the fuel and the
 

ambient temperature to estimate the vapor pressure curve. (The
 

RVP is the VP measured at 100 degrees Fahrenheit.) The VP was
 

then used to create that new parameter which was used as the
 
 

 

-12-

variable on which diurnal emissions were calculated. That new 

parameter is defined by the following formula, equation (3). 

VP_Product_Term = (VPHIGH - VPLOW) * (VPHIGH + VPLOW) / 2 (3)

 Where
 

VPHIGH is the VP (in kiloPascals) associated with the 

day's high temperature. 

VPLOW is the VP (in kiloPascals) associated with the 

day's low temperature. 

The analyses in those parallel reports modeled the diurnal
 emissions as functions either of that VP product term or powers
 of that VP product term.
 

4.2.1 Diurnal Emissions of Properly Functioning Vehicles 

Appendix D identifies 10 pre-ETP vehicles whose RTD
 emissions suggests that they might be representative of ETP
 vehicles with properly functioning evaporative control systems
 (i.e., passing both the purge and pressure tests). Averaging the
 45 test results on those 10 vehicles produces the following table
 (including both standard deviations and 90 percent confidence
 intervals for each of the nine values).
 

Table 3 

Mean Diurnal Emissions of 10 Possible “ETP-Like” Vehicles 

(grams / day) 

Fuel Temp VP Mean Standard 

RVP Cycle Product Diurnal Deviation 

_(psi)_ __(°F)__ _Term_ Count (grams) (grams) 90% Conf. Interval 

6.3 60 - 84 322 2 0.4740 0.0792 0.3819 0.5661 

6.3 72 - 96 489 3 0.3220 0.1497 0.1798 0.4642 

6.3 82 - 106 684 3 0.6487 0.2844 0.3786 0.9187 

6.8 60 - 84 375 4 0.1210 0.0579 0.0734 0.1686 

6.8 72 - 96 567 8 0.4398 0.1615 0.3458 0.5337 

6.8 82 - 106 789 5 0.7096 0.2133 0.5527 0.8665 

9.0 60 - 84 655 7 0.1727 0.0799 0.1230 0.2224 

9.0 72 - 96 969 7 0.5533 0.3419 0.3407 0.7659 

9.0 82 - 106 1,324 6 2.9615 3.0172 0.9353 4.9877 

After initially modeling these values as a function of that
 VP_Product term, it was noted that they could be modeled simply
 as a reduction of estimated pre-ETP diurnal emissions (from
 report M6.EVP.001). The exact magnitude of that reduction was
 more difficult to determine. Comparing these nine mean diurnals
 
 

-13-

with the corresponding estimates for the pre-ETP vehicles, it was
 noted that these means represent reductions ranging from 48 to 90
 percent from the predicted pre-ETP vehicles.
 

Since we are uncertain how representative these 10 pre-ETP 

vehicles are of the actual in-use ETP vehicles, EPA decided to 

retain the earlier, more conservative estimate (based on 

engineering analyses). That is the full-day diurnal emissions of 

these ETP vehicles (with properly functioning evaporative control 

systems) will be estimated as being 50 percent reductions of the 

corresponding pre-ETP vehicles. This produces equation (4) below: 

Full-Day Diurnal (grams) = 0.19415 + [ 0.00252 * Sqr of VP_Product / 1,000 ] (4) 

In MOBILE6, EPA uses equation (4) to estimate the 24-hour 

diurnal emissions of all ETP vehicles with properly functioning 

evaporative control systems with the following two modifications: 

1)	 Regardless of the increase in ambient temperatures, there
 are no diurnal emissions until the ambient temperature
 exceeds 40°F. (This assumption was used consistently for
 all evaporative emissions in MOBILE5.)
 

For a temperature cycle in which the daily low temperature
 is below 40o F, EPA calculate the diurnals emissions for
 that day as an interrupted diurnal (see M6.EVP.002) that
 begins once the ambient temperature reaches 40o F.
 

2)	 The 24-hour diurnal emissions will be zero grams for any
 

temperature cycle in which the diurnal temperature range is
 

zero degrees Fahrenheit (i.e., a constant temperature
 

throughout the entire day).
 

For temperature cycles in which the diurnal temperature
 

range is between zero and ten degrees Fahrenheit, the 24-

hour diurnal emissions will be a linear interpolation
 

between the predicted value for the ten-degree cycle (with
 

the appropriate RVP) and zero grams.
 

4.2.1.1 Multi-Day Diurnal Emissions of Properly Functioning Vehicles 

In a parallel report (M6.EVP.003, entitled "Evaluating
 

Multiple Day Diurnal Evaporative Emissions Using RTD Tests"), EPA
 

develops equations for estimating the RTD emissions of the second
 

and third days of a multi-day diurnal test based on several
 

factors, including the reciprocal of the diurnal emissions of the
 first day. Those analyses were based on the 270-vehicle sample
 (all pre-ETP vehicles) described in Section 2.0 of this report.
 The estimate (from that parallel report) of the ratio of the day- 2 diurnal to the day-1 diurnal for fuel-injected vehicles that
 pass both the purge and pressure tests is given by the formula:
 
-14-

Ratio = 0.74 + [ 47.48 - ( 0.70 * Mid-Point_Temp )

 + ( 0.12 * Weathered_RVP * Mid-Point_Temp )

 - ( 8.11 * Weathered_RVP )] / Full-Day_Diurnal 

Applying this formula to the data in Appendix A (with Mid-Point
 

Temperature = 84, RVP = 9.0, and full-day diurnal = 0.745)
 

produces a ratio (of day-2 to day-1) of 9.344. That is, the
 

predicted second day diurnal would be an unrealistically high
 

value of 6.96 grams (far higher than any of the values in
 

Appendix A). A similar problem exists in using the equations
 

from M6.EVP.003 to estimate the third day diurnals from these
 

vehicles. From a mathematical standpoint, this problem results
 

from dividing by the extremely low diurnal emissions associated
 

with this single stratum.
 

To obtain more realistic estimates of the second and third
 

day diurnals from ETP vehicles with properly functioning
 

evaporative control systems, EPA examined the RTD test results in
 

Appendix A. Most (56 out of 65) of those actual ETP vehicles
 

exhibited a decrease in RTD emissions from the first day to the
 

second day, and the same number exhibited a decrease from the
 

second day to the third day. (The decrease in RTD emissions was
 

small, averaging four to ten percent.) Performing regression
 

analyses on the 65 RTD tests in Appendix A, we obtained the
 

following two tables. (Table 4 contains the statistics of the
 

linear regression of the second day RTD to the first day RTD, and
 

Table 5 contains the statistics of the linear regression of the
 

third day to the second day.
 

Table 4 

Regression of Day-2 versus Day-1 RTD Emissions 

(65 Certification ETP Vehicles) 

Dependent variable is: Day_2_of_3 

No Selector 

R squared = 91.0% R squared (adjusted) = 90.9% 

s = 0.0905 with 65 - 2 = 63 degrees of freedom 

Source 

Regression 

Residual

Sum of Squares 

5.24310 

0.516051 

df 

1 

63

Mean Square 

5.24310 

0.008191 

F-ratio 

640 

Variable 

Constant 

Day_1_of_3

Coefficient 

-0.013504 

0.924233 

s.e. of Coeff 

0.0294

0.0365 

t-ratio 

-0.459

25.3 

prob 

0.6479 

< 0.0001 
 

-15-

Table 5
 

Regression of Day-3 versus Day-2 RTD Emissions
 

(65 Certification ETP Vehicles)
 

Dependent variable is: Day_3_of_3 

No Selector 

R squared = 93.7% R squared (adjusted) = 93.6% 

s = 0.0770 with 65 - 2 = 63 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

5.57703 

0.373598 

df 

1 

63 

Mean Square 

5.57703 

0.005930 

F-ratio 

940 

Variable 

Constant 

Day_2_of_3 

Coefficient 

-0.016523 

0.984061 

s.e. of Coeff 

0.0237 

0.0321 

t-ratio 

-0.698 

30.7 

prob 

0.4876 

< 0.0001 

Based on this limited analysis, EPA will estimate (in
 

MOBILE6) the second and subsequent day of diurnal emissions to be
 

unchanged from the first day for this stratum of ETP vehicles
 

with properly functioning evaporative control systems.
 

4.2.2 Diurnal Emissions of Malfunctioning Vehicles 

Appendices E and F identify eight pre-ETP vehicles failing
 

only the purge test and five pre-ETP vehicles failing the
 

pressure test that they might be representative of ETP vehicles
 

with malfunctioning evaporative control systems. Averaging the
 

test results on those vehicles produce Tables 6 and 7,
 respectively.
 
-16-

Table 6
 

Mean Diurnal Emissions of Eight 1990-1995 PFI Vehicles
 

Failing ONLY the Purge Test
 

Fuel Temp VP Mean Standard 

RVP Cycle Product Diurnal Deviation 

_(psi)_ __(°F)__ _Term_ Count (grams) (grams) 90% Conf. Interval 

6.3 60 - 84 322 2 0.6565 0.9284 0 1.7364 

6.3 72 - 96 489 3 1.1607 1.4806 0 2.5669 

6.3 82 - 106 684 3 2.5117 3.2896 0 5.6360 

6.8 60 - 84 375 5 1.3934 2.3378 0 3.1133 

6.8 72 - 96 567 7 2.4154 2.5391 0.8368 3.9941 

6.8 82 - 106 789 7 7.5069 7.8215 2.6438 12.3699 

9.0 60 - 84 655 8 3.2950 4.8373 0.4817 6.1083 

9.0 72 - 96 969 8 6.3963 7.5931 1.9801 10.8124 

9.0 82 - 106 1,324 5 17.6246 6.1136 13.1271 22.1221 

Table 7
 

Mean Diurnal Emissions of Five 1990-1995 PFI Vehicles
 

Failing the Pressure Test
 

Fuel Temp VP Mean Standard 

RVP Cycle Product Diurnal Deviation 

_(psi)_ __(°F)__ _Term_ Count (grams) (grams) 90% Conf. Interval 

6.3 60 - 84 322 1 3.8810 N/A N/A N/A 

6.3 72 - 96 489 1 11.4250 N/A N/A N/A 

6.3 82 - 106 684 1 18.8320 N/A N/A N/A 

6.8 60 - 84 375 4 7.7988 5.6994 3.1110 12.4865 

6.8 72 - 96 567 5 7.9950 7.3946 2.5550 13.4350 

6.8 82 - 106 789 4 16.6288 11.9879 6.7687 26.4888 

9.0 60 - 84 655 4 10.5270 9.1762 2.9796 18.0744 

9.0 72 - 96 969 4 21.0090 14.6606 8.9506 33.0674 

9.0 82 - 106 1,324 4 37.3610 24.6794 17.0622 57.6598 

After initially modeling these two tables of values as
 

functions of the VP_Product term, it was noted that the equations
 

already developed (in report M6.EVP.001) as estimates of the
 

diurnal emissions from pre-ETP vehicles (with malfunctioning
 

evaporative control systems) accurately model these two sets of
 

data. Therefore, EPA is using (in MOBILE6) the following two
 

equations from (M6.EVP.001):
 
 

 

-17-

For ETP Vehicles that Fail ONLY the Purge Test: 

Full-Day Diurnal (grams) = 3.25800 + [ 0.00941 * Sqr of VP_Product / 1,000 ] (5) 

For ETP Vehicles that Fail the Pressure Test: 

Full-Day Diurnal (grams) = 0.47846 + [ 0.01497 * Sqr of VP_Product / 1,000 ] (6) 

4.2.3 Diurnal Emissions of "Gross Liquid Leakers" 

In a parallel report (Section 5 of report number 

M6.EVP.002), EPA proposed estimating the mean of the diurnal 

emissions for each temperature cycle of the vehicles classified 

as "gross liquid leakers" using equation (7), below. That 

equation predicts diurnal emissions as a function of a single 

variable, the diurnal temperature range (i.e., the daily high 

temperature minus the daily low temperature): 

For ETP Vehicles That Have Gross Liquid Leaks: 

Full-Day Diurnal (grams) = 20.058 + [3.343* Diurnal_Temperature_Range ] (7) 

EPA will continue to use equation (7) to estimate the mean 

24-hour diurnal emissions of all gross liquid leakers regardless 

of model year whenever the diurnal temperature range is at least 

10 degrees Fahrenheit. The 24-hour diurnal emissions will be 

zero grams for any temperature cycle in which the diurnal 

temperature range is zero degrees Fahrenheit (i.e., a constant 

temperature throughout the entire day). For temperature cycles 

in which the diurnal temperature range is between zero and ten 

degrees Fahrenheit, the 24-hour diurnal emissions will be a 

linear interpolation of the predicted value for the ten-degree 

cycle (i.e., 53.49 grams) and zero grams. 

5.0 DISTRIBUTION OF ETP VEHICLES 

In a parallel report (M6.EVP.006, entitled "Estimating
 

Weighting Factors for Evaporative Emissions in MOBILE6"), EPA
 

developed estimates for the distribution of the pre-ETP (i.e.,
 

pre-1996) vehicles for the following four strata (identified in
 

Section 3.0):
 

1)	 "gross liquid leakers," or simply GLLs (i.e., vehicles
 

having substantial leaks of liquid gasoline as opposed to
 

simply vapor leaks),
 

2)	 non-GLLs that pass both the purge and pressure tests
 (i.e., vehicles with properly functioning evaporative
 control systems),
 
 

 

-18-

3)	 non-GLLs that fail the pressure test (regardless of their
 performance on the purge test), and
 

4)	 non-GLLs that fail only the purge test.
 

At each age (where "age" equals "current year" minus "model
 year"), the sum of the four strata must equal 100 percent. For
 each of the first three of the four strata (of pre-ETP vehicles),
 an equation was developed that estimated the fraction of the
 vehicle population contained within that stratum for each "age"
 (where "age" equals 0, 1, 2, . . ., 24). The fourth stratum
 (non-ETPs failing only purge) is simply the remainder (i.e., 100
 percent minus the sum of the other three strata). Two factors
 are expected to alter that distribution of the pre-ETP vehicles:
 

•	 the increased durability requirements of the ETP rule 

and 

•	 the presence of an on-board diagnostic (OBD) system. 

5.1 Effects of Changing Durability Requirements 

The ETP rules require an increase in the durability of the
 evaporative control systems of the newer vehicles. Specifically,
 the ETP vehicles are required to meet the evaporative standards
 for ten years (100,000 miles) instead of five years (50,000
 miles). EPA expects that this doubling of the durability
 requirement will affect the distribution of those four strata.
 

Until in-use data on the ETP vehicles become available, EPA
 

will assume in MOBILE6 that the doubling of the durability
 

requirement will result in reducing the failure rates to that of
 

vehicles half the age. For example, the failure rates (on the
 

purge test, pressure test, or liquid leak criterion) observed on
 

the pre-ETP vehicles at the age of eight years would not occur on
 

the ETP vehicles until twice that age (i.e., 16 years).
 

Modifying the equations (by replacing "AGE" with "AGE/2") 

for the pre-ETP vehicles in the parallel report (M6.EVP.006) 

produces the following three equations to predict the 

distributions (at each age) for the ETP strata: 

Rate of Gross Liquid Leakers on the RTD Test for the ETP Vehicles: 

0.08902 GLL = 1 + 414.613*exp(-0.1842*AGE) 

Failure Rate on Pressure Test of ETP Vehicles: 

0.6045 = * ( 1 - GLL ) 1 + 17.733*exp[-0.003405*(AGE^2)]
 

 

 

-19-

Rate of Passing Both for ETP Vehicles: 

0.7200 = ( 1 - ) * ( 1 - GLL ) 1 + 13.40*exp[-0.003625*(AGE^2)] 

5.2 Effects of On-Board Diagnostic (OBD) Systems 

The majority of the light-duty vehicles have been equipped
 

with on-board diagnostic (OBD) systems since the early 1980’s.
 

The latest generation of these systems (OBD II) is designed to
 

warn the driver when a malfunctioning component is likely to
 

cause high (exhaust or evaporative) emissions.
 

The factors expected to determine the effect of the presence
 

of OBD on the evaporative emissions of the ETP vehicles are:
 

•	 the ability of OBD to identify malfunctions that result 

in high evaporative emissions and 

•	 the response of the driver/owner to that warning. 

The "response of the driver/owner to that warning" is most likely
 

dependent the manufacturer’s warrantee and the presence of an
 

Inspection / Maintenance (I/M) program. These factors are
 

explored in detail in parallel reports (section 3.4.2 of report
 

M6.EXH.009, entitled "Determination of CO Basic Emission Rates,
 

OBD and I/M Effects for Tier 1 and Later LDVs and LDTs").
 

6.0 OTHER TYPES OF EVAPORATIVE EMISSIONS 

Two other types of evaporative emissions (in addition to the
 

resting loss and diurnal emissions) are affected by the ETP
 

requirements. These are the hot soak emissions and the running
 

loss emissions.
 

Hot Soak emissions are the evaporative emissions produced
 

after the vehicle has been driven. These emissions can also be
 

thought of as "trip end" emissions. They result from the fact
 

that the vehicle, engine, fuel delivery system including tank,
 

are all well above ambient temperatures after all but the very
 

shortest trips. In MOBILE6, EPA assumes the following effects of
 

the ETP requirements on hot soak emissions:
 

•	 no effect on vehicles classified as "gross liquid 

leakers," 

•	 a reduction (compared to the pre-ETP vehicles) of 50 

percent on LDGVs with properly functioning evaporative 

control systems (i.e., vehicles that pass both the 

purge and pressure tests), and 

•	 a smaller reduction on vehicles with malfunctioning 

evaporative control systems (i.e., vehicles that fail 

either the purge or pressure tests). 
 

-20-

This "smaller reduction" on vehicles with malfunctioning
 evaporative control systems depends upon the ambient temperature.
 The reduction is 30 percent for ambient temperatures of at least
 95 degrees Fahrenheit. The reduction decreases (linearly) to
 zero at temperatures of 65 degrees Fahrenheit or colder.
 Therefore, the reduction (as a percentage) is given by the
 following formula:
 

Reduction = Temperature - 65, where 

the "Reduction" is "capped" by zero and 30 (percents). 

Similar to the hot soak emissions, the running loss
 

evaporative emissions, which are produced during periods of
 

vehicle operation (that is, driving or idling), are also affected
 

by the ETP requirements:
 

•	 no effect on running loss emissions for vehicles 

classified as "gross liquid leakers," 

•	 a reduction (compared to the pre-ETP vehicles) of 80 

percent on LDGVs with properly functioning evaporative 

control systems (i.e., vehicles that pass both the 

purge and pressure tests), and 

•	 a smaller reduction on vehicles with malfunctioning 

evaporative control systems (i.e., vehicles that fail 

either the purge or pressure tests). 

This "smaller reduction" on running loss emissions for
 

vehicles with malfunctioning evaporative control is identical to
 

the corresponding reduction in hot soak emissions for these same
 

vehicles.
 

7.0 EVAPORATIVE EMISSIONS OF HEAVY-DUTY VEHICLES 

EPA did not conduct RTD testing of the heavy-duty gasolinefueled vehicles (HDGVs). In MOBILE6, EPA estimates the
 evaporative emissions of these untested vehicle types
 proportional to their emission standards. (This is the same
 approach used in earlier versions of MOBILE.)
 

For the HDGVs from 8,501 pounds gross vehicle weight rating
 

(GVWR) up through 14,000 pounds (i.e., HDGV classes IIb and 3),
 

the ETP standard for the combined RTD and hot soak tests is 3.0
 

grams (as compared to the 2.0 grams for the LDGTs). Therefore,
 

in MOBILE6, this ratio (i.e., 1.5 = 3.0 / 2.0) is applied to the
 

applicable LDGT evaporative emissions (i.e., hot soak emissions,
 

resting loss emissions, and diurnal emissions) to estimate the
 

corresponding evaporative emissions for these HDGVs that are not
 

GLLs. (We are assuming that the average emissions of the GLLs
 

are not affected by the ETP requirements.)
 
 

 

 

-21-

Similarly, for the HDGVs over 14,000 pounds (i.e., HDGV
 classes 4 through 8b and busses), since the combined RTD and hot
 soak tests is 4.0 grams, a multiplier of 2.0 (i.e., 2.0 = 4.0 /
 2.0) is applied to the applicable LDGT evaporative emissions from
 non-GLLs.
 

8.0 EFFECTS OF THE ORVR RULES 

"Refueling Emissions" are the evaporative emissions produced
 while the vehicle is being refueled and gasoline vapors are
 forced out as liquid gasoline takes their place. The refueling
 emissions are basically the average displaced vapor (5.26 grams
 of HC) per gallon of dispensed fuel, plus a small amount for
 spillage (0.31 grams). These refueling emissions can be reduced
 with the use of Onboard Refueling Vapor Recovery (ORVR) systems.
 

The phase-in rates (percents of vehicles) required by the 

regulations are given in the following table (Table 8). (The 

ORVR regulations for light-duty cars and trucks were issued April 

6, 1994 (59 FR 16262). The ORVR regulations for HD Class 2b 

vehicles were issued October 6, 2000 (65 FR 59924) as part of the 

"2004 Heavy-Duty" rule.) 

Table 8 

Phase-In of ORVR Systems 

(Required Percentages by Vehicle Class and Model Year) 

LDGTs LDGTs HDGTs 

Model GVWR Up 6,001 Up 8,501 Up 

Year LDGVs To 6,000 To 8,500 To 10,000 

1997 0% 0% 0% 0% 

1998 40% 0% 0% 0% 

1999 80% 0% 0% 0% 

2000 100% 0% 0% 0% 

2001 100% 40% 0% 0% 

2002 100% 80% 0% 0% 

2003 100% 100% 0% 0% 

2004 100% 100% 40% 0% 

2005 100% 100% 80% 80% 

2006 100% 100% 100% 100% 

For light-duty cars and trucks, the ORVR effectiveness was
 

assumed (in MOBILE5) to reduce the portion of refueling emissions
 

that does not include spillage by 98 percent, and to reduce the
 

spillage by 50 percent. We will continue that assumption for
 
 

-22-

MOBILE6. New for MOBILE6, is the extension of that assumption to
 the HD Class 2b vehicles, phasing-in with the 2005 model year.
 

Table 8 indicates that the phase-in for the HD Class 2b
 vehicles assumes zero percent for the 2004 model year. Actually,
 the regulations permit, for the 2004 model year, an optional
 phase-in of up to 40 percent. However, since the 2004 phase-in
 is only "optional," we will assume a value of zero until actual
 vehicle counts are available.
 

9.0 EFFECTS OF THE TIER-2 RULE 

Beginning with the 2004 model year, manufacturers will be
 required to certify at least twenty-five percent of their
 gasoline-fueled passenger cars (LDVs) to the new Tier-2
 standards; that percentage of Tier-2 vehicles will increase to
 one hundred percent within a few additional years. A similar
 phase-in will be required of the light-duty gasoline-fueled
 trucks (LDGTs) and the heavy-duty gasoline-fueled trucks (HDGTs)
 in class 2b (6,001 through 8,500 pounds GVWR). The 2007 HeavyDuty rule extended this to all of the other heavy-duty and bus
 classes. The phase-in rates (percents of vehicles) required by
 the regulations are given in the following table (Table 9).
 

Table 9 

Phase-In of Tier-2 Vehicles 

(Required Percentages by Vehicle Class and Model Year) 

Model 

Year 

2003 

2004 

LDGVs 

0% 

25% 

LDGTs 

GVWR Up 

To 6,000 

0% 

25% 

LDGTs 

6,001 Up 

To 8,500 

0% 

0% 

ALL HDGTs 

GVWR 

Over 8,500 

0% 

0% 

2005 

2006 

50% 

75% 

50% 

75% 

0% 

0% 

0% 

0% 

2007 

2008 

2009 

100% 

100% 

100% 

100% 

100% 

100% 

0% 

50% 

100% 

0% 

50% 

100% 

Concurrent with the phase-in of the new (more stringent)
 

Tier-2 evaporative requirements will be the phase-in by
 

California of its even more stringent LEV II evaporative
 

standards. The evaporative standards for both the Tier-2 and LEV
 II programs are given in Table 10 (on the following page).
 
-23-

Table 10 

Evaporative Standards Under Tier-2 and LEV II 

(grams/test over 3-day diurnal + hot soak) 

Vehicle Class 

LDV 

Current (ETP) Tier 2 

0.95 

LEV II 

0.5 

LDT-1 2.0 0.95 0.65 

LDT-2 0.95 0.65 

LDT-3 & 4 1.2 0.95 

As explained in a parallel report (report number M6.EXH.007,
 

entitled "Accounting for the Tier 2 and Heavy-Duty 2005/2007
 Requirements in MOBILE6"), the vehicle manufacturers have stated
 

that rather than producing separate systems for California and
 

the rest of the country, they will produce single federal systems
 

that also comply with the more stringent California standards.
 

Therefore, in MOBILE6, EPA assumes the evaporative emissions will
 

be based on the LEV II standards.
 

Thus, in MOBILE6, EPA assumes the following effects of the
 

Tier-2 requirements on diurnal, resting loss, and hot soak
 

emissions:
 

•	 no effect on vehicles classified as "gross liquid 

leakers," 

•	 no effect on vehicles with malfunctioning evaporative 

control systems (i.e., vehicles that fail either the 

purge or pressure tests), 

•	 a reduction (compared to ETP vehicles) of 75 percent on 

all LDVs with properly functioning evaporative control 

systems (i.e., vehicles that pass both the purge and 

pressure tests), 

•	 a reduction (compared to ETP vehicles) of 67.5 percent 

on LDGTs up to 6,000 pounds (GVWR) (i.e., LDT-1 and 

LDT-2) with properly functioning evaporative control 

systems (i.e., vehicles that pass both the purge and 

pressure tests), 

•	 a reduction (compared to ETP vehicles) of 52.5 percent 

on all LDGTs with GVWR from 6,001 to 8,500 pounds 

(i.e., LDT-3 and LDT-4) and with properly functioning 

evaporative control systems (i.e., vehicles that pass 

both the purge and pressure tests), 

•	 for HDGTs with GVWR up to 14,000 pounds and with 

properly functioning evaporative control systems (i.e., 

vehicles that pass both the purge and pressure tests), 

emissions will be 1.474 times the corresponding 

emissions of the Tier-2 LDGTs with GVWR from 6,001 to 
-24-

8,500 (i.e., proportional to the certification
 standards), and
 

•	 for HDGTs with GVWR over 14,000 pounds and with 

properly functioning evaporative control systems (i.e., 

vehicles that pass both the purge and pressure tests), 

emissions will be 2.000 times the corresponding 

emissions of the Tier-2 LDGTs with GVWR from 6,001 to 

8,500 (i.e., proportional to the certification 

standards). 

10.0 SUMMARY 

For most of the 1996 and newer model year vehicles that were
 

certified to the enhanced evaporative testing procedure (ETP),
 

EPA will model (in MOBILE6) the resting loss and diurnal
 

emissions similar to what was done in the previous version of the
 

MOBILE model (i.e., MOBILE5). That is:
 

•	 For those ETP vehicles with properly functioning 

evaporative control systems (i.e., vehicles passing 

both the purge test and the pressure test), full-day 

diurnal emissions will be reduced by 50 percent 

compared to the corresponding pre-ETP vehicles. 

•	 For those ETP vehicles with malfunctioning evaporative 

control systems (i.e., vehicles failing either the 

purge test or the pressure test), there will be no 

reduction (zero percent) of full-day diurnal emissions 

compared to the corresponding pre-ETP vehicles. 

•	 For all ETP vehicles, resting loss emissions will be 

reduced by 75 percent compared to the corresponding 

pre-ETP vehicles. 

New to MOBILE6 are:
 

•	 The emissions of "Gross Liquid Leakers" will be 

unaffected by the ETP requirements. 

•	 The assumption of increased durability will reduce the 

predicted number of higher emitting vehicles. 

•	 The presence of an OBD II system will reduce the number 

of higher emitting vehicles (depending upon the 

manufacturer’s warrantee and I/M programs). 

•	 The Tier-2 requirements will reduce the emissions of 

gasoline-fueled cars and trucks (up to 14,000 pounds 

GVWR) that have properly functioning evaporative 

control systems. 
-25-

Appendix A
 

Certification Tests on 65 ETP Vehicles
 

30 Vehicles for California Certification

Source 

CERT-ARB 

CERT-ARB 

CERT-ARB 

Make 

FORD 

FORD 

FORD 

 ----- RTD Test ----

(grams per day) 

Day 1 Day 2 Day 3 

0.824 0.765 0.711 

0.415 0.385 0.355 

0.680 0.420 0.420 

Hot 

Soak 

(g/hr) 

0.154 

0.130 

0.160 

CERT-ARB 

CERT-ARB 

CERT-ARB 

FORD 

FORD 

GM 

0.415 

1.675 

1.100 

0.430 

1.710 

1.105 

0.420 

1.765 

1.225 

0.095 

0.140 

0.130 

CERT-ARB 

CERT-ARB 

CERT-ARB 

GM 

GM 

GM 

1.356 

1.150 

0.757 

1.147 

1.040 

0.645 

1.064 

0.847 

0.620 

0.220 

0.087 

0.131 

CERT-ARB 

CERT-ARB 

CERT-ARB 

MITSUB 

CHRYS 

CHRYS 

0.742 

0.810 

0.719 

0.545 

0.785 

0.688 

0.544 

0.806 

0.680 

0.207 

0.137 

0.070 

CERT-ARB 

CERT-ARB 

CERT-ARB 

TOYOTA 

TOYOTA 

TOYOTA 

0.680 

0.530 

0.520 

0.662 

0.430 

0.450 

0.666 

0.420 

0.430 

0.100 

0.060 

0.070 

CERT-ARB 

CERT-ARB 

CERT-ARB 

TOYOTA 

HONDA 

HONDA 

0.610 

0.360 

0.460 

0.500 

0.320 

0.370 

0.480 

0.300 

0.240 

0.150 

0.090 

0.110 

CERT-ARB 

CERT-ARB 

CERT-ARB 

HONDA 

HONDA 

MAZDA 

0.410 

0.490 

0.625 

0.350 

0.420 

0.740 

0.350 

0.400 

0.778 

0.121 

0.160 

0.120 

CERT-ARB 

CERT-ARB 

CERT-ARB 

MAZDA 

MAZDA 

MAZDA 

0.635 

0.548 

0.500 

0.584 

0.508 

0.530 

0.597 

0.499 

0.420 

0.100 

0.250 

0.120 

CERT-ARB 

CERT-ARB 

CERT-ARB 

NISSAN 

NISSAN 

VOLKS 

0.518 

0.549 

0.734 

0.463 

0.460 

0.625 

0.467 

0.483 

0.607 

0.130 

0.072 

0.177 

CERT-ARB 

CERT-ARB 

CERT-ARB 

VOLKS 

VOLKS 

ISUZU 

0.960 

1.250 

1.235 

0.870 

1.040 

1.115 

0.830 

0.978 

1.046 

0.280 

0.250 

0.355 
 

-26-

Appendix A (Continued) 

Certification Tests on 65 ETP Vehicles 

36 Vehicles for EPA Certification

Source 

CERT-EPA 

CERT-EPA 

CERT-EPA 

Make 

FORD 

FORD 

FORD 

 ----- RTD Test ----

(grams per day) 

Day 1 Day 2 Day 3 

1.039 0.860 0.791 

1.060 0.828 0.719 

1.210 1.172 1.139 

Hot 

Soak 

(g/hr) 

0.177 

0.190 

N.A. 

CERT-EPA 

CERT-EPA 

CERT-EPA 

FORD 

FORD 

GM 

0.832 

1.135 

0.589 

1.127 

0.984 

0.485 

1.230 

0.878 

0.485 

0.134 

0.211 

0.168 

CERT-EPA 

CERT-EPA 

CERT-EPA 

GM 

GM 

GM 

N.A. 

0.764 

0.669 

N.A. 

0.644 

0.560 

N.A. 

0.666 

0.521 

0.172 

0.266 

0.208 

CERT-EPA 

CERT-EPA 

CERT-EPA 

GM 

HONDA 

HONDA 

0.440 

0.610 

0.817 

0.300 

0.559 

0.754 

0.330 

0.541 

0.796 

0.070 

0.070 

0.100 

CERT-EPA 

CERT-EPA 

CERT-EPA 

HONDA 

MAZDA 

MAZDA 

0.813 

0.588 

0.567 

0.636 

0.703 

0.500 

0.353 

0.580 

0.528 

0.460 

0.080 

0.130 

CERT-EPA 

CERT-EPA 

CERT-EPA 

MAZDA 

MAZDA 

TOYOTA 

0.600 

0.490 

0.600 

0.530 

0.500 

0.530 

0.510 

0.200 

0.510 

0.310 

0.120 

0.310 

CERT-EPA 

CERT-EPA 

CERT-EPA 

TOYOTA 

TOYOTA 

TOYOTA 

0.390 

0.350 

0.340 

0.320 

0.340 

0.260 

0.310 

0.370 

0.250 

0.030 

0.030 

0.070 

CERT-EPA 

CERT-EPA 

CERT-EPA 

NISSAN 

NISSAN 

NISSAN 

0.600 

0.491 

0.594 

0.430 

0.456 

0.572 

0.410 

0.436 

0.553 

0.110 

0.057 

0.096 

CERT-EPA 

CERT-EPA 

CERT-EPA 

NISSAN 

VOLKS 

VOLKS 

0.463 

0.402 

0.626 

0.417 

0.396 

0.533 

0.425 

0.424 

0.534 

0.107 

0.041 

0.109 

CERT-EPA 

CERT-EPA 

CERT-EPA 

VOLKS 

VOLKS 

CHRYSLER 

0.670 

0.960 

1.100 

0.630 

0.870 

1.245 

0.610 

0.830 

1.275 

0.190 

0.280 

0.355 

CERT-EPA 

CERT-EPA 

CERT-EPA 

CHRYSLER 

CHRYSLER 

CHRYSLER 

1.310 

1.469 

0.741 

1.230 

1.181 

0.686 

1.080 

1.004 

0.825 

0.230 

0.140 

0.170 

CERT-EPA 

CERT-EPA 

CERT-EPA 

ISUZU 

ISUZU 

ISUZU 

0.847 

0.594 

1.375 

0.779 

0.538 

1.200 

0.749 

0.531 

1.243 

0.448 

0.187 

N.A. 
-27-

Appendix B
 

CAP-2000 Data on Six 1996 Model Year Mercedes ETP Vehicles
 

Model 

S420 

Test Date 

11/26/97 

Odometer 

(miles) 

46,846 

Total HC 

Diurnal + 

Hot Soak 

(grams*) 

0.4687 

Hot Soak 

(grams*) 

0.0807 

2-Day Diurnal 

(grams*) 

0.388 

S500 12/12/97 31,447 0.4336 0.0736 0.360 

S500 01/09/98 38,099 0.4229 0.0839 0.339 

S420 01/20/98 29,997 0.7171 0.1101 0.607 

S420 02/06/98 26,606 0.4317 0.0867 0.345 

S420 02/27/98 42,870 0.6935 0.1255 0.568 

Mean: 0.4345 

* The units "grams" are somewhat inconsistent.
 

"Grams" on the Hot Soak test refers to grams
 per test. Since each test is one hour in
 duration, this is equivalent to grams per
 hour.
 

"Grams" on the Diurnal (RTD) test refers to
 

grams per day.
 

"Grams" in the "Total" column are the sum of
 

the grams per hour on the hot soak and the
 

grams per day on the diurnal tests. This
 

"mixed" unit is the basis of the standard
 

used for the ETP certification.
 
 

-28-

Appendix C 

Twenty-Five 1990-1995 Model Year Vehicles
 

Passing Both the Purge and Pressure Tests
 

Vehicle 

_No._ 

4912 

Fuel 

RVP 

(psi)

6.8 

6.8 

 Temp 

Cycle 

_(°F)_ 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

567 

789 

RTD 

(gr/day) 

0.980 

5.120 

Resting 

Loss 

(gr/hour)* 

0.012 

0.102 

Daily Rst 

Loss 

(gr/day) 

0.428 

2.588 

Diurnal 

(gr/day) 

0.552 

2.532 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

1.930 

3.350 

-0.005 

0.045 

0.020 

1.220 

1.910 

2.130 

4923 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

0.670 

4.480 

0.000 

0.048 

0.140 

1.292 

0.530 

3.188 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

1.710 

2.550 

0.018 

0.032 

0.572 

0.908 

1.138 

1.642 

4928 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

4.830 

8.230 

0.065 

0.142 

1.700 

3.548 

3.130 

4.682 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

4.170 

4.370 

0.045 

0.058 

1.220 

1.532 

2.950 

2.838 

4932 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

1.700 

2.850 

0.017 

0.037 

0.548 

1.028 

1.152 

1.822 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

1.490 

2.080 

0.023 

0.017 

0.692 

0.548 

0.798 

1.532 

5032 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.374 

0.772 

1.231 

0.004 

0.006 

0.012 

0.236 

0.284 

0.428 

0.138 

0.488 

0.803 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.473 

0.741 

2.433 

0.005 

0.008 

0.018 

0.260 

0.332 

0.572 

0.213 

0.409 

1.861 

5038 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

0.615 

1.011 

0.005 

0.007 

0.260 

0.308 

0.355 

0.703 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.441 

1.302 

4.366 

0.002 

0.004 

0.006 

0.188 

0.236 

0.284 

0.253 

1.066 

4.082 

* "Hourly Resting Loss" emissions are calculated at the
 

lowest temperature of each cycle.
 
-29-

Appendix C (Continued) 

Twenty-Five 1990-1995 Model Year Vehicles
 

Passing Both the Purge and Pressure Tests
 

Vehicle 

_No._ 

5046 

Fuel 

RVP 

(psi)

6.8 

6.8 

6.8 

 Temp 

Cycle 

_(°F)_ 

60 - 84 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

375 

567 

789 

RTD 

(gr/day) 

0.439 

0.565 

1.498 

Resting 

Loss 

(gr/hour)* 

0.011 

0.007 

0.020 

Daily Rst 

Loss 

(gr/day) 

0.404 

0.308 

0.620 

Diurnal 

(gr/day) 

0.035 

0.257 

0.878 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.360 

0.971 

9.716 

0.004 

0.013 

0.041 

0.236 

0.452 

1.124 

0.124 

0.519 

8.592 

5047 9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.366 

0.653 

0.906 

0.005 

0.012 

0.015 

0.260 

0.428 

0.500 

0.106 

0.225 

0.406 

5052 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

3.502 

4.273 

8.937 

0.032 

0.071 

0.114 

0.908 

1.844 

2.876 

2.594 

2.429 

6.061 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

2.966 

5.853 

11.820 

0.039 

0.106 

0.205 

1.076 

2.684 

5.060 

1.890 

3.169 

6.760 

5066 6.3 

6.3 

6.3 

60 - 84 

72 - 96 

82 - 106 

322 

489 

684 

0.390 

0.351 

0.605 

-0.007 

0.001 

0.006 

-0.028 

0.164 

0.284 

0.418 

0.187 

0.321 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.295 

0.397 

0.581 

0.000 

0.003 

0.004 

0.140 

0.212 

0.236 

0.155 

0.185 

0.345 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.281 

0.626 

1.936 

-0.001 

0.007 

0.011 

0.116 

0.308 

0.404 

0.165 

0.318 

1.532 

5068 6.3 

6.3 

6.3 

60 - 84 

72 - 96 

82 - 106 

322 

489 

684 

0.814 

0.580 

1.150 

0.006 

0.006 

0.009 

0.284 

0.284 

0.356 

0.530 

0.296 

0.794 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.368 

0.839 

1.391 

0.003 

0.009 

0.018 

0.212 

0.356 

0.572 

0.156 

0.483 

0.819 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.638 

1.385 

2.132 

0.009 

0.010 

0.029 

0.356 

0.380 

0.836 

0.282 

1.005 

1.296 
-30-

Appendix C (Continued) 

Twenty-Five 1990-1995 Model Year Vehicles
 

Passing Both the Purge and Pressure Tests
 

Vehicle 

_No._ 

5081 

Fuel 

RVP 

(psi)

6.3 

6.3 

 Temp 

Cycle 

_(°F)_ 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

489 

684 

RTD 

(gr/day) 

0.647 

1.187 

Resting 

Loss 

(gr/hour)* 

0.001 

0.009 

Daily Rst 

Loss 

(gr/day) 

0.164 

0.356 

Diurnal 

(gr/day) 

0.483 

0.831 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

0.326 

0.639 

0.005 

0.007 

0.260 

0.308 

0.066 

0.331 

9009 6.8 72 - 96 567 35.565 0.095 2.420 33.145 

9026 6.8 72 - 96 567 1.755 0.031 0.884 0.871 

9028 6.8 72 - 96 567 16.984 0.024 0.716 16.268 

9033 6.8 72 - 96 567 0.879 0.003 0.212 0.667 

9038 6.8 72 - 96 567 5.818 0.106 2.684 3.134 

9040 6.8 72 - 96 567 0.810 0.006 0.284 0.526 

9048 6.8 72 - 96 567 9.443 0.228 5.612 3.831 

9056 6.8 72 - 96 567 3.095 0.046 1.244 1.851 

9059 6.8 72 - 96 567 1.009 0.013 0.452 0.557 

9088 6.8 72 - 96 567 2.750 0.023 0.692 2.058 

9135 6.8 72 - 96 567 1.591 0.012 0.428 1.163 

9141 6.8 72 - 96 567 10.328 0.209 5.156 5.172 

9143 6.8 72 - 96 567 7.904 0.070 1.820 6.084 
 

-31-

Appendix D 

Ten Possible “ETP-Like” 1990-1995 Model Year Vehicles
 

Passing Both the Purge and Pressure Tests
 

Tested Over Multiple Cycles
 

(Subset of Appendix C)
 

Vehicle 

_No._ 

5032 

Fuel 

RVP 

(psi)

6.8 

6.8 

6.8 

 Temp 

Cycle 

_(°F)_ 

60 - 84 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

375 

567 

789 

RTD 

(gr/day) 

0.374 

0.772 

1.231 

Resting 

Loss 

(gr/hour)* 

0.004 

0.006 

0.012 

Daily Rst 

Loss 

(gr/day) 

0.236 

0.284 

0.428 

Diurnal 

(gr/day) 

0.138 

0.488 

0.803 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.473 

0.741 

2.433 

0.005 

0.008 

0.018 

0.260 

0.332 

0.572 

0.213 

0.409 

1.861 

5038 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

0.615 

1.011 

0.005 

0.007 

0.26 

0.308 

0.355 

0.703 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.441 

1.302 

4.366 

0.002 

0.004 

0.006 

0.188 

0.236 

0.284 

0.253 

1.066 

4.082 

5046 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.439 

0.565 

1.498 

0.011 

0.007 

0.020 

0.404 

0.308 

0.620 

0.035 

0.257 

0.878 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.360 

0.971 

9.716 

0.004 

0.013 

0.041 

0.236 

0.452 

1.124 

0.124 

0.519 

8.592 

5047 9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.366 

0.653 

0.906 

0.005 

0.012 

0.015 

0.260 

0.428 

0.500 

0.106 

0.225 

0.406 

* “Hourly Resting Loss” emissions are calculated at the lowest 

temperature of each cycle. 
 

-32-

Appendix D (Continued) 

Ten Possible “ETP-Like” 1990-1995 Model Year Vehicles
 

Passing Both the Purge and Pressure Tests
 

Tested Over Multiple Cycles
 

(Subset of Appendix C)
 

Vehicle 

_No._ 

5066 

Fuel 

RVP 

(psi)

6.3 

6.3 

6.3 

 Temp 

Cycle 

_(°F)_ 

60 - 84 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

322 

489 

684 

RTD 

(gr/day) 

0.390 

0.351 

0.605 

Resting 

Loss 

(gr/hour)* 

-0.007 

0.001 

0.006 

Daily Rst 

Loss 

(gr/day) 

-0.028 

0.164 

0.284 

Diurnal 

(gr/day) 

0.418 

0.187 

0.321 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.295 

0.397 

0.581 

0.000 

0.003 

0.004 

0.140 

0.212 

0.236 

0.155 

0.185 

0.345 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.281 

0.626 

1.936 

-0.001 

0.007 

0.011 

0.116 

0.308 

0.404 

0.165 

0.318 

1.532 

5068 6.3 

6.3 

6.3 

60 - 84 

72 - 96 

82 - 106 

322 

489 

684 

0.814 

0.580 

1.150 

0.006 

0.006 

0.009 

0.284 

0.284 

0.356 

0.530 

0.296 

0.794 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.368 

0.839 

1.391 

0.003 

0.009 

0.018 

0.212 

0.356 

0.572 

0.156 

0.483 

0.819 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.638 

1.385 

2.132 

0.009 

0.010 

0.029 

0.356 

0.380 

0.836 

0.282 

1.005 

1.296 

5081 6.3 

6.3 

72 - 96 

82 - 106 

489 

684 

0.647 

1.187 

0.001 

0.009 

0.164 

0.356 

0.483 

0.831 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

0.326 

0.639 

0.005 

0.007 

0.260 

0.308 

0.066 

0.331 

9033 6.8 72 - 96 567 0.879 0.003 0.212 0.667 

9040 6.8 72 - 96 567 0.810 0.006 0.284 0.526 

9059 6.8 72 - 96 567 1.009 0.013 0.452 0.557 

* “Hourly Resting Loss” emissions are calculated at the lowest 

temperature of each cycle. 
-33-

Appendix E 

Eight 1990-1995 Model Year PFI Vehicles
 

Failing (Only) the Purge Test
 

Vehicle 

_No._ 

4925 

Fuel 

RVP 

(psi)

6.8 

6.8 

 Temp 

Cycle 

_(°F)_ 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

567 

789 

RTD 

(gr/day) 

4.170 

4.450 

Resting 

Loss 

(gr/hour)* 

0.063 

0.080 

Daily Rst 

Loss 

(gr/day) 

1.949 

2.357 

Diurnal 

(gr/day) 

2.221 

2.093 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

2.170 

3.830 

0.035 

0.058 

1.277 

1.829 

0.893 

2.001 

4933 6.8 

6.8 

72 - 96 

82 - 106 

567 

789 

10.750 

18.670 

0.145 

0.352 

3.917 

8.885 

6.833 

9.785 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

7.170 

12.120 

0.137 

0.228 

3.725 

5.909 

3.445 

6.211 

5004 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.989 

1.673 

2.924 

0.003 

0.023 

0.031 

0.509 

0.989 

1.181 

0.480 

0.684 

1.743 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

1.025 

5.440 

20.391 

0.015 

0.018 

0.047 

0.797 

0.869 

1.565 

0.228 

4.571 

18.826 

5035 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

5.593 

5.869 

22.973 

-0.016 

0.016 

-0.033 

0.053 

0.821 

-0.355 

5.540 

5.048 

23.328 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

14.493 

24.068 

24.872 

0.015 

0.032 

0.040 

0.797 

1.205 

1.397 

13.696 

22.863 

23.475 

5040 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.667 

1.143 

6.961 

0.003 

0.010 

-0.013 

0.509 

0.677 

0.125 

0.158 

0.466 

6.836 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

1.065 

2.930 

20.658 

-0.003 

0.012 

-0.008 

0.365 

0.725 

0.245 

0.700 

2.205 

20.413 
-34-

Appendix E (Continued) 

Vehicle 

_No._ 

5069 

Fuel 

RVP 

(psi)

6.3 

6.3 

6.3 

 Temp 

Cycle 

_(°F)_ 

60 - 84 

72 - 96 

82 - 106 

VP Product 

Term 

_(kPa^2)_ 

322 

489 

684 

RTD 

(gr/day) 

1.774 

3.593 

6.810 

Resting 

Loss 

(gr/hour)* 

0.001 

0.012 

0.003 

Daily Rst 

Loss 

(gr/day) 

0.461 

0.725 

0.509 

Diurnal 

(gr/day) 

1.313 

2.868 

6.301 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

1.322 

1.953 

9.565 

0.004 

0.011 

0.039 

0.533 

0.701 

1.373 

0.789 

1.252 

8.192 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

7.082 

12.372 

20.430 

-0.017 

0.007 

0.080 

0.029 

0.605 

2.357 

7.053 

11.767 

18.073 

5070 6.3 

6.3 

6.3 

60 - 84 

72 - 96 

82 - 106 

322 

489 

684 

0.351 

0.690 

1.209 

0.002 

0.001 

0.016 

0.485 

0.461 

0.821 

0.000 

0.229 

0.388 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.375 

0.745 

1.176 

0.002 

-0.004 

0.007 

0.485 

0.341 

0.605 

0.000 

0.404 

0.571 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.416 

1.381 

9.141 

0.003 

0.019 

0.057 

0.509 

0.893 

1.805 

0.000 

0.488 

7.336 

5087 6.3 

6.3 

72 - 96 

82 - 106 

489 

684 

1.830 

2.435 

0.042 

0.048 

1.445 

1.589 

0.385 

0.846 

9.0 

9.0 

60 - 84 

72 - 96 

655 

969 

1.478 

2.533 

0.029 

0.043 

1.133 

1.469 

0.345 

1.064 
-35-

Appendix F 

Five 1990-1995 Model Year Vehicles
 

Failing the Pressure Test
 

Vehicle 

_No._ 

4937 

Fuel 

RVP 

(psi)

6.8 

 Temp 

Cycle 

_(°F)_ 

72 - 96 

VP Product 

Term 

_(kPa^2)_ 

567 

RTD 

(gr/day) 

3.330 

Resting 

Loss 

(gr/hour)* 

0.028 

Daily Rst 

Loss 

(gr/day) 

1.109 

Diurnal 

(gr/day) 

2.221 

5008 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

12.853 

17.632 

29.663 

-0.018 

0.013 

0.054 

0.005 

0.749 

1.733 

12.848 

16.883 

27.930 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

19.811 

35.202 

57.174 

-0.002 

0.038 

0.014 

0.389 

1.349 

0.773 

19.422 

33.853 

56.401 

5021 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

7.789 

15.477 

23.810 

0.004 

0.029 

0.065 

0.533 

1.133 

1.997 

7.256 

14.344 

21.813 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

17.246 

24.840 

41.963 

-0.003 

0.038 

-0.034 

0.365 

1.349 

-0.379 

16.881 

23.491 

42.342 

5044 6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

0.286 

0.523 

0.706 

0.004 

0.011 

0.014 

0.533 

0.701 

0.773 

0.000 

0.000 

0.000 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

0.467 

0.494 

1.914 

0.011 

0.007 

0.005 

0.701 

0.605 

0.557 

0.000 

0.000 

1.357 

5067 6.3 

6.3 

6.3 

60 - 84 

72 - 96 

82 - 106 

322 

489 

684 

5.206 

13.206 

21.981 

0.037 

0.056 

0.113 

1.325 

1.781 

3.149 

3.881 

11.425 

18.832 

6.8 

6.8 

6.8 

60 - 84 

72 - 96 

82 - 106 

375 

567 

789 

12.128 

8.644 

18.697 

0.025 

0.070 

0.062 

1.037 

2.117 

1.925 

11.091 

6.527 

16.772 

9.0 

9.0 

9.0 

60 - 84 

72 - 96 

82 - 106 

655 

969 

1,324 

7.106 

29.697 

50.741 

0.036 

0.107 

0.040 

1.301 

3.005 

1.397 

5.805 

26.692 

49.344 
-36-

Appendix G 

Response to Peer Review Comments from H. T. McAdams
 

This report was formally peer reviewed by two peer reviewers
 

(H. T. McAdams and Sandeep Kishan). In this appendix, comments
 

from H. T. McAdams are reproduced in plain text, and EPA’s
 

responses to those comments are interspersed in indented italics.
 

Comments from the other peer reviewer appear in the following
 

appendix (Appendix H).
 

It is important to note that this final version of the
 

report has changed substantially from the draft version that
 

Professor McAdams reviewed. In that earlier version, the
 

goal was to develop equations that would predict the diurnal
 

and resting loss emissions of these ETP vehicles. In the
 

interim between these versions of this report, we realized
 

that the predicted results (from that draft) were comparable
 

to the MOBILE5 predictions. Therefore, in this final
 

version, our goal changed and became the testing and
 

validation of those MOBILE5 predictions.
 

This change of direction resulted in many of Professor
 

McAdams’ comments no longer being applicable. However, all
 

of his comments were considered, and those that were still
 

applicable were incorporated.
 

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

Modeling Diurnal and Resting Loss Emissions from Vehicles
 

Certified to the Enhanced Evaporative Standards
 

Report Number M6.EVP.005
 

By
 

Larry C. Landman
 

Assessment and Modeling Division
 

U.S. EPA Office of Mobile Sources
 

Review and Comments By H. T. McAdams
 

1.0 REPORT CLARITY
 

Reporting the results of this study presents more than the usual
 

challenge to the report writer. It is necessary to make a great
 

many statements that are highly conditional, and this fact leads
 to sentences that sometimes are lengthy and complicated. For
 example, consider the following excerpt:
 
-37-

EPA believes that the RTD emissions from malfunctioning
 enhanced evaporative control vehicles (i.e., vehicles that
 developed problems with their evaporative control systems)
 will be similar to the RTD emissions from the 1990 to 1995
 model year vehicles that also develop problems with their
 evaporative control systems. That is, those 1996 and newer
 model year vehicles that had failed either EPA's purge or
 pressure tests are expected to have evaporative emissions
 similar to those 1990 to 1995 model year PFI vehicles that
 also failed the same test.
 

It is evident that the author is doing his best to keep the
 record straight, something not easy to do when making a statement
 with so many qualifiers. The problem is aggravated by the
 necessary wordiness of such designations as "emissions from
 malfunctioning enhanced evaporative control vehicles" and
 "emissions from the 1990 to 1995 model year vehicles that also
 develop problems with their evaporative control systems." The
 added "That is ..." phrases, intended to clinch the matter, tend
 only to further confuse. It might be useful to insert a simple
 "word table" or Venn diagram to show explicitly how the various
 vehicle classes are related and then to find a simple but
 descriptive name for each category. An example pertaining to the
 vapor pressure product term will be found later in this review.
 

The above comments are based on several years experience as
 technical writer and editor for Cornell Aeronautical Laboratory,
 now Calspan Corporation, Buffalo, NY. Though stylistic and
 

grammatical editing are not considered to be within the scope of
 

this review, it is believed that the report could benefit from
 

further attention to these concerns.
 

2.0 OVERALL METHODOLOGY
 

Every scientific discipline has its own investigative and
 

statistical parochialism. Some of the "soft sciences," like
 

psychology and sociology, tend to thrive on correlation analysis
 

and nonparametric statistics. The engineering sciences, on the
 

other hand, find statistical approaches like regression analysis
 

and formal tests of significance more to their liking. Neither
 

scientific group can be faulted for their choice, but both would
 

gain from cross fertilization of ideas and procedures.
 

The methodology employed in this report is generally consistent
 with the methodology followed by EPA in developing their Complex
 Model for Reformulated Gasoline (RFG). Heavy emphasis was put on
 regression analysis and the strict application of statistical
 tests of significance. From participation in that effort, this
 reviewer learned much that is applicable to Landman's study of
 evaporative emissions. This includes visualizing the nature of
 the curves, surfaces or hypersurfaces represented by the model,
 evaluating confidence bounds for the regression function, and
 

estimating the relative importance of terms in the equation.
 
-38-

Perhaps one of the most neglected aspects of statistical analysis
 is the subject of the power of a test of significance. Often
 failing to reject a null hypothesis amounts to saying that, under
 the prevailing circumstances, the test simply did not have
 sufficient power to do so. Either the sample size was not
 adequate, or the data had too much dispersion, or the
 significance level was set too high to be appropriate for the
 situation at hand. More emphasis needs to be put on the reasons
 for using certain significance tests and on whether a given
 significance level, such as the commonly used 0.05, is
 appropriate for the present application.
 

This report is subject to many of the above complaints.
 Nevertheless, it meets essentially all criteria for a valid
 scientific study according to present views and standards. The
 plea here is that statistical principles should be applied
 thoughtfully rather than automatically and that there may be
 value in sometimes breaking from the crowd. In what follows,
 specific examples taken from the report will be used to
 illustrate some of the above contentions.
 

3.0 APPROPRIATENESS OF DATASETS SELECTED
 

The datasets available for modeling diurnal and resting loss
 emissions are far from ideal. As pointed out above, precision
 limits for estimating these emissions rest heavily on the amount
 and quality of the data. By quality is meant data that is not
 subject to bias and is not so "noisy" that it precludes all but
 the most evident conclusions. If the data is so bad that it
 

leaves us with little that we did not already know, then it
 

clearly contributes little information.
 

There is probably no perfect set of data. However, application of
 

the principles of sampling and experiment design can do much to
 

move us toward that goal. In particular, they can help us
 

estimate the sample size required to provide estimates that we
 

can live with. They can help to prevent the confounding of the
 

effects of two or more variables. And, finally, they can help
 

optimize our data by providing the most information for the least
 

amount of experimental effort. The role played by factorial
 

experiments is well known for its capabilities toward this end,
 

and there are even more efficient designs applicable in unique
 

circumstances.
 

One of the shortcomings of the datasets used in the modeling of
 evaporative emissions is the limited number of vehicles. Though
 evaporative emissions are probably subject to less vehicle-to- vehicle variation than are exhaust emissions, it is highly
 desirable to remove vehicle effects from the effects of fuel
 properties and temperature cycles wherever possible. The data are
 not structured well to achieve this end, but in at least one
 situation, to be demonstrated later, vehicle differences can be
 removed with a very beneficial effect.
 

When data are sparse, it is even more imperative than usual to
 

extract the most information from the limited amount of data
 
-39-

available. An example, drawn from the data in this report, will
 be given in the next section, Data Analysis and Statistical
 Methodology.
 

Typically in this report a candidate regression model is fitted
 to the data and the coefficients are then tested for
 "significance." Those that fail the test are then dropped from
 the equation, the result being the same as if they were assigned
 the value zero. Still, by virtue of the principle of least
 squares, the "most likely" value of the coefficient is the one
 that was computed. This seeming impasse needs to be examined
 thoughtfully before arbitrarily rejecting the coefficient at 0.05
 significance -- or 0.10, or 0.01 or 0.001, especially when the
 test is based on a small sample.
 

The obligation of the analyst, therefore, does not stop here. It
 is just as important to know the error bounds for the so-called
 significant coefficients as it is to know that some coefficients
 can seemingly be ignored. It is even more important to know error
 bounds for emission estimates computed by the regression
 equation. It should be kept in mind, too, that the precision of
 the estimates is not constant for all values of the predictor
 variables. It should be an obligation of the analyst to tell us
 how good the estimates are near the center of the sample space as
 well as how bad the estimates are near the edges of the sample
 space. The analyst can not just report significance and then
 "look the other way" when error bounds are so wide that emission
 estimates are essentially useless.
 

The present report does not provide this information, but it is
 

admitted that it is not customarily to do so. Therefore, the
 

author can not be faulted. To perform the necessary computations
 

and prepare the required displays may not be practical under the
 

constraints of the present report. That does not preclude,
 

however, a broader look at the characteristics of estimates in
 

future studies.
 

Error bounds in the form of 90 percent confidence intervals
 

have been added to several of the tables (Tables 3, 6, and
 

7). Additionally, two tables of regression statistics have
 

been added (Tables 4 and 5).
 

4.0 DATA ANALYSIS AND STATISTICAL METHODOLOGY
 

It has been said that regression analysis is the most widely used
 

and most widely misused of all statistical methods. Though an
 

evident hyperbole, the statement contains an element of truth.
 Couched in the framework of General Linear Model (GLM),
 regression has wide appeal in a great variety of applications.
 

The truth is, however, that regression analysis is not a
 

universal solvent and is not without its shortcomings and
 

pitfalls. In what follows, we examine Landman's analysis in the
 

light of these considerations and suggest, wherever indicated, an
 alternative approach.
 
-40-

To say that a model is linear is simply to say that the response
 vector is a linear combination of a set of basis vectors. The
 basis vectors themselves, however, may be as "nonlinear" as they
 please and are often just the terms of a polynomial: 1, x, x2,
 x3, ... xn. The analyst's task is to determine coefficients for
 these terms so as to minimize the sum of squares of the
 residuals. He must also somehow select the terms to be included
 in such a way that the data are neither "underfitted" nor
 "overfitted." It is here that he resorts to R2 and to statistical
 tests of significance for each of the regression coefficients.
 

Although R2 is widely used as a measure of the efficacy of a
 regression model, it can be misleading. Moreover, it may not be
 realized that any number of models can be constructed to give
 exactly the same R2 and even exactly the same residuals point by
 point. Viewed in this light, the fact that there seems to be a
 good fit according to R2 is not necessarily a cause for
 rejoicing.
 

It must be kept in mind that R2 is a function of the residuals
 only at points where we have data and can tell us nothing about
 the response at points where we have no data. Unless we know how
 the function performs over its entire domain of definition, some
 of these functions, even the one we have selected, may oscillate
 radically between points at which we do have data. That is why it
 is important to know something about the geometry or
 hypergeometry of a regression equation before relying on it to
 interpolate between data points and sometimes to extrapolate
 beyond them.
 

Caution and common sense need to be exercised when evaluating
 

regression models, whether by R2, t-tests of the regression
 

coefficients or other means. That becomes clear if we examine
 

Table 3 of the report and its accompanying text.
 

Randman notes that for each temperature cycle, diurnal emissions
 

increase with fuel volatility and that for a given fuel, diurnal
 

emissions increase as the temperature cycle increases. He might
 

also have noted that the effect of temperature cycle on emissions
 

is greater for the more volatile fuel, a fact that seems
 

consistent with physical reasoning. Together, these three
 

observations present an almost classic instance of a two-factor,
 

factorial experiment in which response depends on both factors
 

and their interaction.
 

The use of the VP_Product term was proposed (in report
 M6.EVP.001), in part, because it incorporates both of these
 two factors. Historically, it closely corresponds to the
 uncontrolled diurnal index (UDI) used in earlier versions of
 MOBILE but is easier to calculate.
 

It is true that when emissions are regressed directly on prod and
 

RVP, R-square is only 0.8593, whereas it is 0.9658 when log
 

emissions are regressed on the same two variables. However, as
 

shown below, R-square increases to 0.9000 for direct regression
 

when the interaction term prod*RVP is introduced.
 
 

 

-41-

Without Interaction Term 

Coefficient Std. Error t _ 

Constant 1.6273 2.0537 0.7924 

Prod 0.0046 0.0012 3.9844 

RVP -0.4739 0.3192 1.4845

 R-square = 0.8593
 

With Interaction Term 

Coefficient Std. Error t _ 

Constant 7.7374 7.1024 1.0894 

Prod 0.0046 0.0102 0.4457 

RVP -1.2253 0.8964 1.3669 

Interaction 0.0011 0.0012 0.9014

 R-square = 0.9000
 

Note that when the interaction term is added, the apparent
 

significance of some of the other terms seem to decrease. The
 

point to be noted here is that whether a term shows up as
 

significant or not often depends on how many other terms are in
 

the equation. When a term is dropped, the sum of squares
 

associated with it are redistributed, partly to the error term
 

but not entirely. Part of the ge redistributed terms are said to
 

be aliased with other "significant" terms, and these aliases can
 

be explicitly computed. Likewise, when additional terms are added
 

to an existing equation, the order in which they are introduced
 can make a large difference in their "significance." The method
 

of stepwise regression is an attempt to deal with this problem.
 

Consequently, it may not be wise to rule out certain terms on the
 

strength of their t-value alone.
 

Interaction terms were considered. Their use did provide an
 

improved "fit" at the tested values. However, at
 

intermediate values (i.e., RVPs between 7 and 9), the
 

resulting predictions were not consistent with known
 

responses. Therefore, EPA did not use them.
 

It also needs to be remembered that whether a term is called
 

significant or not strongly depends on the significance level.
 

Just because 0.05 is conventionally used does not make it
 

sacrosanct. Often we are really more concerned with the Type II
 

error than with the Type I and do not take advantage of the
 

trade-offs between the two. That concern is no more recognized
 

than in quality control and sampling acceptance plans. A sampling
 
-42-

plan is designed to a specified consumer's risk and producer's
 risk. The consumer has to be protected to minimize the risk that
 he will accept a bad lot of material. But, the producer has to be
 protected against the risk that he will have a perfectly good lot
 rejected. A compromise that both can live with has to be found,
 

Similarly, in evaluating a regression coefficient, we need to
 know the consequences of retaining a coefficient when its effect
 really doesn't exist, but we also need to know the consequences
 of dropping a coefficient when its effect really does exist. By
 increasing the risk of a Type I error we can decrease the risk of
 a Type II error. Also, ruling out a term by rejecting it at any
 significance level says that that term is zero. However, we are
 willing to accept a coefficient with an extremely wide confidence
 interval and take no note of the fact. After all, the term is
 [sic] significant.
 

Which is the most appropriate model, with or without the
 logarithmic transformation of emission measurements?
 

Two factors bear on the answer to this question. One deals with
 the error distribution, the other with whether the effects of the
 pressure product term and RVP are additive or multiplicative.
 

If the variance of the observations that make up the mean diurnal
 emissions for each of the six means are proportional to the
 square of the means, then the log transformation may be
 appropriate, because it tends to stabilize the variances. (A
 constant variance is one of the requirements for regression.)
 

However, if the variance already was stable (i.e., constant for
 

all means), then the log transformation would tend to destabilize
 

the variance and possibly lead to a biased result.
 

The log transformation also has another useful property. Each of
 

the coefficients of the log model expresses the proportional, or
 

percent change in emissions associated with a unit change in prod
 

or RVP.
 

That variance increases as the level of emissions increases seems
 

plausible and could be examined by computing the variance for
 

each of the six categories. Also, whether effects are additive or
 

multiplicative could be examined by comparing successive
 

differences and successive ratios for the group means. At any
 

rate, the log model seems effective. [As a matter of editorial
 

note, the residual mean square in Table 3 should be 0.032764 not
 0.32764.]
 

Another problem that may be encountered, as it was in the
 development of the Complex Model for Reformulated Gasoline (RFG),
 is the reversal of the sign of the slope (derivative) that
 characterizes the effect of a variable on emissions. Such a
 reversal can occur even when it is known from theory and
 experience that the function is monotonic non-decreasing or nonincreasing. That is why one needs to know what the function looks
 

like, particularly whether it makes any turns that are "counterintuitive." For example, in the case of a quadratic, it is
 
-43-

helpful to know where the zeros of the polynomial lie and even
 more important to know where the zeros of the derivative function
 lie.
 

Landman experienced the opposite of this effect in developing a
 model for diurnal emissions. In Table 4, he uses an equation
 containing only a constant and a cubic term. His rationale for
 the choice, in his words, is as follows:
 

In Section 4.2.1, we noted that the diurnal emissions for
 the vehicles with properly functioning evaporative control
 systems were not a strictly increasing function of the VP product
 term. However, for the vehicles that failed the pressure test,
 the diurnal emissions increased as the VP product increased. We,
 therefore, repeated the approach used in earlier analyses of
 regressing the diurnal emission emissions against the cube of the
 VP product term producing Table 4.
 

It is not clear how the lack of monotonicity leads to the
 conclusion that a cubic function of the product term, referred to
 hereinafter as prod, is most appropriate. It turns out that, over
 the range of the data, prod1, prod2, prod3 and even prod4 are
 highly correlated, the coefficient of correlation between pairs
 of these variables ranging from 0.92 to 0.98. It follows, then,
 that any of the prod functions might perform about as well as any
 other. This fact becomes evident when other powers of the product
 term are used as the basis of the model, as will be shown below.
 

There may be a more cogent reason, however, for the lack of
 

monotonicity. For a product term to give consistent results, it
 

is necessary for the product to exhibit reciprocity. Let us call
 

the two factors that make up the product xä1ä and xä2ä. All pairs
 

of xä1ä and xä2ä that map into a given value of prod should
 

produce the same effect on evaporative emissions. Otherwise,
 

inconsistencies may arise.
 

According to equation (3) of the Randman paper, xä1ä is simply
 

the range of vapor pressure for the day, and xä2ä is simply the
 

midrange of the day's vapor pressure. In the present notation,
 

the product term is defined as

 prod = 1/2 [(xä1ä - xä2ä) * (xä1ä + xä2ä)]
 

or

 prod = 1/2 (xä1ä2 -xä2ä2)
 

Consequently,

 prod3 = 1/8 (xä1ä2 - xä2ä2)3
 

When this expression is expanded, it can be seen that powers of
 vapor pressure as high as six will be encountered. More
 importantly, for reciprocity to hold, the same emissions should
 be associated with a given value of prod3, whether that value was
 

produced by - say - a short VP range combined with a high
 

midrange or long range combined with a low midrange. Table 4,
 

however, being based on the data in Appendix C, has other
 
-44-

difficulties. One of the vehicles, #5044, shows zero diurnal
 emissions for all conditions except one, and that one shows a
 value far out of line with the same condition of the other
 vehicles. It is no wonder that R2 is only 0.411! A look at a plot
 of the residuals would make that fact quite clear, and it is for
 reasons such as this that residuals should be examined to see if
 there is any indication of "lack of fit" such as outliers or
 trends.
 

Suppose, now, that we remove vehicle #5044 and recompute the
 regression. Then we get:
 

Constant = 10.6634
 Coefficient of prod3 = 0.0171
 R2 = 0.87
 

It is clear that the one vehicle strongly biases the results. It
 is also clear that different vehicles exhibit different responses
 and that vehicle effects should be removed in the analysis if
 possible.
 

Actually, the data are ideally suited for removing vehicle
 effects by means of dummy variables. Below, regression results
 are given for individual vehicles, as well as for meaningful
 subsets of the data in Appendix C.

 HOW VEHICLE EFFECTS MODIFY THE MODEL FOR DIURNAL EMISSIONS
 

Vehicle Set Constant Coef. of prod3 _ R2 _ 

All 7.9460 0.01300 0.4106 

All but #5044 10.6634 0.0171 0.8742 

Vehicle #5008 14.7713 0.0186 0.9735 

Vehicle #5021 11.3437 0.0137 0.9425 

Vehicle #5044 -0.2064 0.0006 0.8745 

Vehicle #5067 5.8753 0.0191 0.9490 

Best choice* 7.2744 0.0171 0.9422 

*Vehicle #5044 omitted, vehicle effects removed
 

Note that by simply excluding one vehicle from the analysis and
 

removing the effects of vehicles we go from R2 of 0.41 to 0.94
 

and even up to 0.97 for individual vehicles.
 

Now that we have found a consistent set of data - namely, the set
 of data with vehicle #5044 removed - let us try various powers of
 the product term as predictor variable, as well as various
 combinations of those powers. The resulting values of R2 are
 listed below.
 
-45-

INSENSITIVITY OF PRODn IN PREDICTING DIURNAL EMISSIONS

 Power, n R-square

 1 0.8421 

2 0.8820

 3 0.8742 

1&2 0.8836 

1&3 0.8821 

2&3 0.8830 

1&2&3 0.8848 

As conjectured, it little matters what power of the product term
 is used, or what combination of powers; the resulting prediction
 capability, as judged by R2, seem to be about the same.
 

5.0 APPROPRIATENESS OF THE CONCLUSIONS
 

Though not called out explicitly, conclusions are found in the
 Summary section of Landman's report. They can be listed, briefly,
 as follows.

 1. Separate estimates of diurnal and rest loss emissions
 are given for "gross liquid leakers" and vehicles subject only to
 vapor losses. Liquid losses per hour are estimated as a constant;
 vapor losses are estimated by means of a relatively simple
 empirical equation,

 2. A major predictor variable, called "prod" in this
 

review, is the product of two vapor pressures and is said to take
 

into account both vapor pressure and temperature. For some
 

equations prod may be raised to the second or third power; it may
 

also be modified, in some cases, by additional variables, notably
 

RVP.

 3. Separate estimates are provided for various vehicle
 

strata, the strata being defined by whether the vehicle passed or
 

failed the purge and/or pressure tests.

 4. Evaporative emissions are assumed to be zero for
 

temperatures below 40oF and for any temperature cycle in which
 

the temperature stays relatively constant. The term "constant" is
 

defined as not varying more than a few degrees from the mean
 

temperature.
 

How appropriate are these conclusions?
 

To put into perspective what is expected of the evaporative
 emission estimates, let us consider the estimate for gross liquid
 leakers. A single number is supposed to represent emissions from
 vehicles of all ages, all places and all climates, all drivers
 and all lifestyles ... and so on. How close can the estimate be
 to "truth," when only a few vehicles are tested and the results
 

are scaled up according to the relative frequency of somewhat
 

arbitrarily designated "strata" that also had to be estimated.
 
-46-

The answer is probably "Not very," but the report provides no
 clue, however vacuous, of just how "very" is very.
 

These points are addressed in the parallel report devoted to
 these vehicles with substantial leaks of liquid gasoline
 (i.e., report number M6.EVP.009, entitled "Evaporative
 Emissions of Gross Liquid Leakers in MOBILE6").
 

Now consider how much more complicated and subject to error are
 the estimates provided by an empirical equation based on "an
 educated guess" of what form that equation should take and what
 predictor variables it should contain. The problem is aggravated
 by the fact that the precision of the estimate varies widely over
 the range of the predictor variables. Then add the uncertainty of
 the strata weights and ... but, enough already!
 

EPA and Mr. Landman are to be commended for having the courage to
 accept such a mammoth challenge, but their accomplishments would
 gather infinitely more kudos if they could assure us that their
 estimates are within 5% of real-world truth. Admittedly, to be
 able to say how good their estimates are is an even more
 difficult problem than to compute those estimates in the first
 place. But ... unless we have some measure of how good the
 estimates are, we might just as well not have computed them at
 all!
 

The form of the predictive equations and the validity of the
 predictor variables are subject to question on several counts.
 The product term is particularly open to criticism. Probably it
 

has a factual or theoretical basis not known to this reviewer,
 

but if so, it is difficult to understand the indifference of this
 

term to what power it is raised to.
 

If test results had been obtained over a wider range of this
 

VP_Product term (e.g., using 11.0 RVP fuel over an 82-106
 

degree cycle), then the exponent used would seem less
 

"indifferent."
 

More important, perhaps, is the matter of reciprocity. For any
 

given value of prod, there is an infinitude of pairs of factors
 

that map into that value. For the prod function or powers of that
 

function, all pairs should yield the same evaporative emission
 

estimates.
 

In general, EPA chose the lowest power (exponent) that would
 explain the observed results (i.e., the simplest
 explanation).
 

There are two instances in which the proposed models exhibit a
 step function. At temperatures below 40o F. emissions are taken
 as zero. Though this estimate may be reasonable, "Nature abhors a
 vacuum." Likewise, engineers and mathematicians are not
 comfortable with discontinuities unless there is good reason for
 

those discontinuities to occur. A similar impasse is faced in
 

defining emissions to be constant if the range of the temperature
 

cycle is zero, or within a few degrees of zero. Means exist for
 
-47-

smoothing these discontinuities and even, perhaps, for assuring
 that the derivative of the function exhibits no discontinuities.
 

Using a smooth curve rather than abruptly setting the
 emissions to zero is appealing. However, the actual
 differences in the resulting fleet emissions are too small
 to be meaningful.
 

Finally, we come to the practical matter of applying these models
 to real-world situations. Inasmuch as some twenty or so equations
 are provided, there must be means for selecting the one that is
 uniquely appropriate for the particular problem at hand. This
 fact seems to assume that the vehicle or vehicles under
 consideration have already been purge and pressure tested as well
 as examined for liquid leaks. So far as the use of the models for
 compiling an emission inventory for the present fleet is
 concerned, there would seem to be no problem. However, in future
 applications, classification of vehicles would have to be a
 precursor to application of the models.
 

Since the in-use fleet actually contains all of these
 

strata, all of these equations (as well as many others) are
 

used in MOBILE6. The resulting predictions are then
 

weighted together. (See parallel report number M6.EVP.006,
 

entitled "Estimating Weighting Factors for Evaporative
 

Emissions in MOBILE6.")
 

6.0 RECOMMENDATIONS FOR ANY ALTERNATE DATASETS AND/OR ANALYSES
 

The ideal dataset would be one in which vehicles are recruited in
 

accordance with a sampling plan and experiment design tailored to
 

the requirements of the moment. Some replication should be built
 

into the plan to allow estimation of errors attributed to
 

"unassignable causes." These are the errors that remain after we
 

have identified and estimated all the fixed effects that we could
 

think of. In addition, the design should be such that the
 

relative magnitude of those "fixed effects" are not confounded
 

with errors due to the unassignable causes.
 

The prod variable needs to be examined in depth, with regard to
 

reciprocity and correlation of successive powers, as well as
 

whether there are other variables that might better serve the
 

purpose. If there is a theoretical reason for using a term as
 

complex as prod3, it should be revealed.
 

A particular objective that should be the goal of any experiment
 

design is to choose variables and the levels of those variables
 

in such a way that they are orthogonal. This type of design
 

assures that the estimates of the effects of all variables are
 

completely independent of each other.
 

With regard to discontinuities in the models, means should
 provided for "fairing the curve" so that it blends smoothly into
 both the top and bottom of the step. A method for realizing such
 
-48-

smoothing by means of exponential was suggested in work connected
 with development of the Complex Model.
 

It is recommended that the above changes be incorporated in the
 present report to whatever degree is practical within allowable
 time and resource constraints. Although a complete assessment of
 error limits is beyond the scope of the present report, there
 does exist enough information to make a start on this very
 important issue. Most statistical software gives as output the
 standard error of regression coefficients and the standard error
 of estimate at various points in the predictor space. It is
 recommended that some effort be made in this direction, if only
 to show the general magnitude of the errors. In future studies,
 effort should be made toward continued refinement of the error
 bounds.
 

1-20-99
 htm
 
-49-

Appendix H 

Response to Peer Review Comments from Sandeep Kishan
 

This report was formally peer reviewed by two peer reviewers
 

(H. T. McAdams and Sandeep Kishan). In this appendix, comments
 

from Sandeep Kishan are reproduced in plain text, and EPA’s
 

responses to those comments are interspersed in indented italics.
 

Each of these comments refer to page numbers in the earlier draft
 

version (dated July 1, 1999) that do not necessarily match the
 

page numbers in this final version. Comments from the other peer
 

reviewer appear in the preceding appendix (Appendix G).
 

It is important to note that this final version of the
 

report has changed substantially from the draft version that
 

Sandeep Kishan reviewed. In that earlier version, the goal
 

was to develop equations that would predict the diurnal and
 

resting loss emissions of these ETP vehicles. In the interim
 

between these versions of this report, we realized that the
 

predicted results (from that draft) were comparable to the
 

MOBILE5 predictions. Therefore, in this final version, our
 

goal changed and became the testing and validation of those
 

MOBILE5 predictions.
 

This change of direction resulted in many of Sandeep
 

Kishan’s comments no longer being applicable. However, all
 

of his comments were considered, and those that were still
 applicable were incorporated.
 

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

This memorandum provides peer review comments on two EPA
 

documents: “Evaluating Resting Loss and Diurnal Evaporative
 

Emissions Using RTD Tests”, Document No. M6.EVP.001, November 20,
 

1998 and “Modeling Diurnal and Resting Loss Emissions” Report
 

Number M6.EVP.005, October 1, 1998. Both of these are draft
 

reports.
 

The original peer review covered two of the MOBILE6
 documents. Only the portion of that review pertaining to
 this report is reproduced below in this appendix. The
 remainder of the peer review is reproduced in report number
 M6.EVP.001 (Appendix I of that report).
 

Overall, we think that the reports are good, and they present
 some new data analysis techniques that are attractive. Since, in
 the past, we have had to do similar data analyses and modeling
 for evaporative emissions from vehicle test data, we can
 appreciate many of the difficulties and data limitations you are
 subject to. We hope the comments below help you with this
 effort.
 
-50-

Document No. M6.EVP.005 (October 1, 1998)
 

This report was clearly written and the stratification seems to
 be appropriate for this analysis. We think that the dataset used
 is discouraging but it may be that no alternate datasets can be
 found for this purpose. Therefore, we think that it is important
 to let the reader know that you are committed to revisiting these
 relationships when new data does become available. We also have
 the same concern with the regressions in this report as with
 those already discussed for the previous report.
 

1.	 Page 3, end of Section 1.0 – It might be appropriate to
 state that the models developed in this report are intended
 to be a temporary patch for MOBILE6 until EPA or someone
 else gets actual vehicle measurement data on the effects of
 RVP, temperature, and purge and pressure status on the
 evaporative emissions.
 

A statement to that effect has been added.
 

2.	 Page 5, Section 3.0 – You are proposing to use 1990 to 1995
 

model year vehicle data to estimate the effects of
 

temperature, RVP, and purge and pressure status on trends in
 

the 1996 and 1997 vehicles. What evidence do you have that
 

the failure modes of 1996 and 1997’s will be like the
 

failure modes of 1990 to 1995’s? Are the materials,
 

connectors, etc., the same? Consider the five bulleted
 

items in Section 1.0; we think you need some discussion
 

about why these trends and these slightly older vehicles
 

would be similar to those in the 1996 and 1997 vehicles.
 

The reviewer is correct; the evidence that these vehicles
 

are comparable to the ETP vehicles is lacking. However,
 

until we obtain test data on in-use ETP vehicles, this data
 

set is the best we have to work with.
 

3.	 Page 10, Section 4.1.2, bottom of the page – During the 

regression of estimated resting losses versus temperature 

for different vehicles, it was found that the r2

 for the 

regression of resting losses versus temperature produced a 

low r2 and a temperature coefficient that was not 

statistically significant. Rather than averaging the 

resting loss emissions for all 12 cars together, it would be 

more appropriate to use a categorical variable for the 

identity of the cars. This will produce a larger r2 since 

the researcher recognizes that it’s the differences among 

the cars that produce most of the variability in the 

dataset. The result will be a good estimate for the slope 

on the temperature and, possibly, also make the temperature 

coefficient statistically significant. 
-51-

With the changes made to this revision, this comment is no
 longer applicable.
 

4.	 Page 10, Section 4.1.2, paragraph 2 – You had data
 describing resting loss emissions for the two separate
 strata – one where vehicles failed the pressure test and one
 where vehicles failed the purge test. Why didn’t you just
 use those individual strata results to predict the
 temperature effect on resting losses for those type of
 malfunctions? The data values in Appendix D look reasonable
 for the vehicles in those strata. Instead, these strata
 were combined and then modeled. Why?
 

With the changes made to this revision, this comment is no
 longer applicable.
 

5.	 Page 12, Section 4.2.1 – By performing the regression on
 

diurnal emissions on the average emissions of vehicles, data
 

from only five vehicles could be used. However, if instead,
 

the regression had been performed on the individual emission
 

values of the vehicles, 12 vehicle’s data would supply
 

information to the regression about temperature and RVP
 

relationships. Thus, the choice of performing regressions
 

on averages rather than on individual values causes the
 

resulting model to lose information which could have been
 

provided by an additional seven vehicles. If the
 

regressions are performed in SAS, a class variable for
 

vehicle can be used to account for an unbalanced set of data
 

with respect to vapor pressure product and RVP. The
 

resulting coefficients for RVP and vapor pressure product
 

would be better estimates of the true relationships.
 

With the changes made to this revision, this comment is no
 

longer applicable; however, this suggestion will be used in
 

future analyses.
 

6.	 Page 15, Section 4.2.2 – In Appendix C, one vehicle also has
 

measurements at 6.3 psi fuel RVP. Why did you not use these
 

values in your regressions? If you use a class variable for
 

vehicle identification, the information from these three
 

additional measurements can be brought into the regression.
 

The data at 6.3 psi were incorporated in this latest
 revision.
 

7.	 The use of the cube of the vapor pressure product in this
 

regression is troubling. What evidence do you have that the
 

cube is the appropriate transformation? It seems to us that
 

since a class variable for vehicle identification was not
 

used, it is unlikely that the cube transformation is
 

correct.
 
-52-

With the changes made to this revision, this comment is no
 longer applicable.
 

8.	 Page 27, Appendix C – Note that for five of the six diurnal
 emissions calculated for vehicle Number 5044, the values are
 zero. This is evidently because the estimated daily resting
 loss was greater than the measured RTD grams. The zero
 values for this vehicle were not mentioned in the text in
 Section 4.2.2. How are these zero values handled in the
 regression summarized in Table 4?
 

With the changes made to this revision, this comment is no
 longer applicable.
 

9.	 Page 17, Section 4.2.3 – The analysis has available eight
 vehicles to perform the regression. All eight vehicles
 could be used in the regression instead of using only five
 vehicles. Again, if class variables are used for the
 identification of each variable, SAS can use all the
 information to determine regression coefficients for the
 input variables. The result would be better estimates of
 the coefficients.
 

With the changes made to this revision, this comment is no
 

longer applicable; however, this suggestion will be used in
 

future analyses.
 

10.	 We would like to see some plots of the raw data versus the
 

values of input variables in the model or versus temperature
 

and RVP.
 

The reader can easily plot the data in this report if such
 

graphs are desirable.
 

11.	 Diurnal emissions for vehicles passing the purge and
 

pressure test were transformed to logs and then regressed
 

while vehicles that failed the purge and/or pressure tests
 

were regressed without taking the logs. What evidence do
 

you have for taking these different approaches? In general,
 

we would expect the log of the diurnal emissions to be a
 

better approach to take than the cube of the vapor pressure
 product. A discussion of the engineering aspects of the
 system under different purge/pressure result conditions
 could lead to a resolution.
 

With the changes made to this revision, this comment is no
 

longer applicable.
 

12.	 It seems that this whole report is based on measurements
 

taken on the wrong model year vehicles. We presume that the
 

intent in doing this is to provide some sort of
 
-53-

functionality in the MOBILE model for the 1996 to 1997 model
 years using 1990 to 1995 vehicle data but only until the
 data actually taken on 1996 and newer vehicles can be
 obtained and analyzed. You might consider adding a
 statement that says that when this new data does become
 available, these models will be revisited.
 

A statement to that effect has been added to the end of
 Section 1 (page 3). This comment is similar to this
 reviewer’s first comment.
 
-54-

Appendix I 

Response to Written Comments from Stakeholders 

The following comment was submitted in response to EPA’s
 posting a draft of a related report (M6.IM.003) on the MOBILE6
 website. The full text of this comment is posted on the MOBILE6
 website.
 

Comment Number: 102 

Name / Affiliation: David Lax / API 

Date: January 25, 2000 

Comment: 

Under the heading of:
 

"Adjustments for Enhanced Evaporative Vehicles
 

"To account for the improved durability of enhanced evap
 

control systems, EPA reduced the baseline failure rates by
 

50%. They did this to both non-cap and cap failures. This
 

approach is appropriate for non-cap failures. (Although
 

some manufacturers went with 1⁄4 turn caps, e.g., Ford and
 

possibly some GMs.) As mentioned above, the big change in
 

cap technology occurred in the mid-80s with the switch to
 

screw-in caps, and this is not accounted for in EPA’s
 

estimates."
 

EPA’s Response: 

In the report actually being commented on, the pressure
 

failures were divided into those related to the fuel cap and
 

those not involving the fuel cap. While that breakdown is
 

not used in this report, it is encouraging that API agrees
 

that it is appropriate to estimate the failure rate (on the
 

pressure test) of the ETP vehicles by reducing the failure
 

rate of the pre-ETP vehicles.