United States Air and Radiation EPA420-P-99-026 

Environmental Protection July 1999 

Agency M6.EVP.001 

Evaluating Resting Loss 

and Diurnal Evaporative 

Emissions Using Real 

Time Diurnal Tests

 Printed on Recycled Paper 
EPA420-P-99-026 

July 1999 

Evaluating Resting Loss and Diurnal Evaporative Emissions
 

Using Real Time Diurnal Tests
 

M6.EVP.001
 

Assessment and Modeling Division
 

Office of Mobile Sources
 

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.
 
EPA420-P-99-026
 

- Draft ­

Evaluating Resting Loss and Diurnal
 

Evaporative Emissions Using RTD Tests
 

Larry C. Landman 

Document Number M6.EVP.001 

July 1, 1999 

U.S. EPA
 

Assessment and Modeling Division
 

National Vehicle Fuel and Emissions Laboratory
 

2000 Traverwood Drive
 

Ann Arbor, Michigan 48105-2425
 

734-214-4939 (fax)
 

mobile@epa.gov
 

NOTICE
 

These reports do not necessarily represent final EPA
 

decisions or positions. They are intended to present
 

technical analysis of issues using data which are currently
 

available. The purpose in release of these reports is to
 

facilitate the exchange of technical information and to
 

inform the public of technical developments which may form
 

the basis for a final EPA decision, position or regulatory
 

action.
 
ABSTRACT
 

This report is a revision of a draft that was released for
 

stakeholder review on October 8, 1997. The report numbering
 

convention was changed since the release of that earlier draft
 

which carried the document number M6.RTD.001. Subsequent versions
 

of that earlier draft (including this version) will all carry the
 

document number M6.EVP.001 (i.e., the "RTD" was changed to "EVP").
 

All versions of this report are entitled "Evaluating Resting Loss
 

and Diurnal Evaporative Emissions Using RTD Tests."
 

This document reports both on the methodology used to analyze
 

the data from real-time diurnal (RTD) tests on 270 vehicles and on
 

the results obtained from those analyses. The purpose of the
 

analysis is to develop a proposal for a model of the diurnal and
 

resting loss emissions of the in-use fleet. This revised draft
 

report incorporates suggestions received from stakeholders during
 

the 60-day review period.
 

i
 
TABLE OF CONTENTS
 

Page Number

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


 2.0 Vehicle Sample . . . . . . . . . . . . . . . . . 2


 3.0 Vehicle Testing. . . . . . . . . . . . . . . . . 4


 4.0 Weighting the EPA Data . . . . . . . . . . . . . 5


 5.0 Test Parameters . . . . . . . . . . . . . . . . 7


 6.0 Consolidating Vehicle Parameters for 24-Hour RTD . . 9


 6.1 Comparing TBI and PFI Vehicles . . . . . . . 10


 6.2 Comparing Carbureted and FI Vehicles . . . . . 11


 6.3 Comparing Cars and Trucks . . . . . . . . . 14


 6.4 Summarizing Stratification Parameters . . . . 17
 

6.5 Evaluating Untested Strata. . . . . . . . . . 18


 7.0 Evaporative Emissions Represented by the RTD . . . 18


 7.1 Resting Loss Emissions . . . . . . . . . . . 19


 7.2 Diurnal Emissions . . . . . . . . . . . . . 20


 7.3 Separating Out Gross Liquid Leakers . . . . . 21


 8.0 Characterizing Resting Loss Emissions . . . . . . . 22


 9.0 Characterizing 24-Hour Diurnal Emissions . . . . . 26
 

10.0 Gross Liquid Leakers . . . . . . . . . . . . . . 30
 

10.1 Frequency of Gross Liquid Leakers. . . . . . . 30
 

10.2 Magnitude of Emissions from Gross Liquid Leakers 32
 

10.3 Effects of Vapor Pressure Changes on Leakers. . 34
 

11.0 Other Topics . . . . . . . . . . . . . . . . . . 35
 

11.1 Temperature Ranges . . . . . . . . . . . . . 35
 

11.2 Heavy-Duty Vehicles . . . . . . . . . . . . 36
 

11.3 High Altitude Emissions . . . . . . . . . . . 36
 

11.4 Motorcycles . . . . . . . . . . . . . . . . 36
 

11.5 Pre-Control Vehicles . . . . . . . . . . . . 38
 

11.6 Duration of Diurnal Soak Period . . . . . . . 40
 

11.7 1996 and Newer Model Year Vehicles . . . . . . 40
 

ii
 
TABLE OF CONTENTS (Continued) 

Page Number 

APPENDICES


 A. Temperature Cycles . . . . . . . . . . . . . . . 41


 B. Vapor Pressure . . . . . . . . . . . . . . . . . 42
 

C. Mean Emissions by Strata. . . . . . . . . . . . . 44
 

D. Modeling Hourly Resting Loss Emissions . . . . . . 47
 

E. Modeling 24-Hour Diurnal Emissions . . . . . . . . 48
 

F. Regression Tables of Diurnal Emissions . . . . . . 50
 

iii
 
 

*** DDDD rrrr aaaa ffff tttt ***
 

Evaluating Resting Loss and Diurnal
 

Evaporative Emissions Using RTD Tests
 

Report Number M6.EVP.001
 

Larry C. Landman
 

U.S. EPA Assessment and Modeling Division
 

1.0 Introduction
 

In previous versions of the highway vehicle emission factor
 

model (MOBILE), the estimates of the emissions resulting from the
 

daily rise of the ambient air temperature were based on a one-hour
 

test (adjusted to simulate an 8-hour test) in which the heating
 

process was accelerated. As part of the MOBILE model revision, an
 

effort has been undertaken to use the recently developed 72-hour
 

real-time diurnal (RTD) test (or a shortened version) to more
 

accurately estimate those temperature driven (i.e., diurnal)
 

emissions, as well as the resting loss emissions.
 

In the RTD test, the ambient temperatures gradually cycle
 

over a 24 degree Fahrenheit range during the course of each 24
 

hour period as illustrated in Figure 1-1:
 

Figure 1-1
 

Nominal RTD Temperature Cycle
 

(Temperatures Cycling Between 72° and 96° F)
 

100° 

90° 

80° 

70° 

Time (hours) 

Hourly Temperature (°F) 

0 6 12 18 24 

-2- DRAFT
 

July 13, 1999
 

The three hourly temperature cycles used in this study are given
 

in Appendix A. These three temperature cycles are parallel (i.e.,
 

identical hourly increases/decreases). Each temperature cycle
 

peaks at hour nine (i.e., at 3PM). The most rapid increase in
 

temperatures occurs during the fourth hour. For RTD tests that
 

exceed 24 hours (i.e., 33, 38, or 72 hours), the cycle is simply
 

repeated.
 

This document reports both on the methodology used to analyze
 

the data from these RTD tests and on the results obtained from
 

those analyses.
 

2.0 Vehicle Sample
 

In this analysis, EPA used real-time diurnal (RTD) test data
 

from two sources:
 

1) from five (5) individual testing programs (i.e., work 

assignments) performed for EPA by its contractor, and 

2) from a testing program performed for the Coordinating 

Research Council (CRC). 

The RTD testing performed for EPA was done by its testing
 

contractor (Automotive Testing Laboratories) over the course of
 

five (5) work assignments from 1994 through 1996 (performed under
 

three different EPA contracts). A total of 119 light-duty
 

vehicles (LDVs) and light-duty trucks (LDTs) were tested in these
 

programs. In the following table (Table 2-1), the distribution of
 

those 119 test vehicles is given:
 

1) by work assignment number,
 

2) by vehicle type (LDV versus LDT),
 

3) by model year range, and
 

4) by fuel metering system


 - carbureted (Carb)
 

- port fuel injected (PFI)
 

- throttle body injection (TBI).
 
-3­ DRAFT 

Table 2-1 

July 13, 1999 

Distribution of EPA Test Fleet 

Work 

Assignment No.

2-09 

Vehicle 

Type

LDV 

Model Year 

Range 

80-85 

86-95 

Fuel Metering 

Carb PFI TBI 

5 2 0 

7 15 10 

1-05 LDV 80-85 

86-95 

3 

1 

4 

24 

3 

12 

LDT 86-95 0 0 2 

0-05 LDV 71-77 

78-79 

3 

1 

0 

0 

0 

0 

80-85 

86-95 

5 

0 

0 

0 

0 

0 

0-07 LDV 86-95 0 5 1 

0-11 LDT 71-77 

78-79 

2 

0 

0 

0 

0 

0 

80-85 

86-95 

5 

0 

0 

5 

0 

4 

The recruitment method used for most of the vehicles in the
 

EPA sample was designed to recruit a larger number of vehicles
 

that had potential problems with their evaporative control
 

systems. Specifically, two tests of the integrity of each
 

vehicle's evaporative control system (a purge test and a pressure
 

test) were used to screen the candidate vehicles. This resulted,
 

among the newer vehicles, in a larger proportion of the test
 

vehicles failing either a purge test or pressure test (but not
 

both) than did the corresponding vehicles in the in-use fleet.
 

EPA excluded from its sample all those vehicles that failed both
 

the purge and pressure tests. Any analyses performed on the EPA
 

data must, therefore, account for this intentional bias toward
 

problem vehicles. (See Section 4.0.)
 

It is important to note that neither the purge test nor the
 

pressure test is a perfect identifier of vehicles that have
 

problems with their evaporative control systems. While vehicles
 

that passed both the purge test and the pressure test had, on
 

average, lower RTD emissions than similar vehicles that failed
 

either or both tests, there was a wide overlap on the RTD
 

emissions of the vehicles that passed both tests with the RTD
 

emissions of similar vehicles that failed one or both of those
 

tests. The size of the overlap varied with the strata (see
 

Section 6.4). But, on average, the cleanest (i.e., vehicles with
 

the lowest RTD results) one-fourth of the vehicles failing the
 

purge and/or pressure test(s) had lower RTD test results than the
 

dirtiest (i.e., highest RTD results) similar vehicles that passed
 

both the purge and pressure tests. In fact, the vehicle that had
 

the highest RTD emissions (other than the seven gross liquid
 

leakers discussed in section 7.3) was one that passed both tests.
 
________________________________ 

-4- DRAFT
 

July 13, 1999
 

The CRC program* involved performing RTD tests on a random
 

sample of 151 vehicles (mostly LDTs) during 1996. The
 

distribution of those 151 vehicles (by vehicle type, model year
 

range, and fuel metering system) is given in the following table:
 

Table 2-2
 

Distribution of CRC Test Fleet
 

Vehicle

 Type 

Car 

Model Year

 Range 

71-77 

Carb 

38 

PFI 

0 

TBI 

0 

Truck 71-77 13 0 0 

Truck 80-85 47 2 1 

Truck 86-91 7 24 19 

3.0 Vehicle Testing
 

The testing in the EPA study consisted of performing one or
 

more RTD tests on each vehicle in its "as-received" condition with
 

the exception that the tank fuel was replaced with specified
 

fuels. (To restore the vehicle to its "as-received" condition for
 

subsequent tests, the canister was conditioned to return it to
 

approximately the condition it was in prior to the first test.)
 

Up to three temperature cycles were used. (In addition to the
 

standard 72°-96° F cycle, 60°-84° and 82°-106° cycles were also
 

used.) Similarly, up to four different fuel volatilities were
 

specified; specifically, fuels having nominal Reid vapor pressure
 

(RVP) of 6.3, 6.7, 6.9, and 9.0 pounds per square inch (psi).
 

Since the actual RVP used in a given test may vary slightly from
 

the specified target RVP, EPA felt that tests performed using the
 

6.7 or 6.9 psi RVP fuel could all be treated as equivalent to
 

tests performed using a fuel with a nominal RVP of 6.8 psi.
 

The testing in the CRC study consisted of performing a single
 

RTD test on each vehicle in its "as-received" condition. Each
 

test used the standard temperature profile (i.e., temperatures
 

cycling between 72° and 96° F) and was performed using the fuel
 

already in each vehicle's fuel tank (typically having an RVP which
 

ranged from 6.7 to 7.0 psi). EPA felt these tests could also be
 

treated as equivalent to tests performed using a fuel with a
 

nominal RVP of 6.8 psi.
 

For the purpose of the following analyses, we treated all
 

testing performed using fuels with RVPs from 6.7 through 7.0 as if
 

they were all performed using a fuel with a nominal RVP of 6.8
 

*
 D. McClement, J. Dueck, B. Hall, "Measurements of Diurnal Emissions from
 

In-Use Vehicles, CRC Project E-9", Prepared for the Coordinating Research
 

Council, Inc. by Automotive Testing Laboratories, Inc., June 19, 1998.
 
________________________________ 

-5- DRAFT
 

July 13, 1999
 

psi. Thus, all the EPA testing performed using fuels with nominal
 

RVPs of either 6.7 or 6.9 will be combined and then used with all
 

of the CRC tests.
 

4.0 Weighting the EPA Data
 

To correct for the intentional sampling bias toward "problem"
 

vehicles in the EPA testing programs (described in Section 2.0),
 

we first determined the number of vehicles in each stratum in both
 

the recruited sample and the in-use fleet.
 

Examining the purge/pressure data gathered in the I/M lanes
 

in Arizona and Indiana, we found 16,637 as-received vehicles for
 

which successful purge and pressure tests were performed. (These
 

tested were conducted at the Phoenix, Arizona I/M lane from June
 

1992 through August 1994 and at the Hammond, Indiana I/M lane from
 

January 1990 through February 1995.)
 

Modeling those preceding distributions with smooth (i.e.,
 

logistic growth) curves as functions of vehicle age* produced the
 

distributions in Table 4-1. A full discussion of this process is
 

given in Document Number M6.EVP.006, entitled "Estimating
 

Weighting Factors for Evaporative Emissions in MOBILE6."
 

The predicted purge failure rates (i.e., the sum of columns
 

two and three in the above table) closely approximates those used
 

in the MOBILE5 model for vehicles up to 12 years of age. The
 

predicted pressure failure rates (i.e., the sum of columns three
 

and four) also closely approximates those used in the MOBILE5
 

model for vehicles up to 12 years of age. Any differences between
 

the estimates used in MOBILE5 and those in Table 4-1 should not
 

affect the analyses in this report. A detailed analysis of the
 

failure rates on the purge and pressure tests (and, hence on the
 

appropriate weighting factors) is presented in document number
 

M6.EVP.006.
 

This approach assumes that the purge/pressure results are
 

functions only of age (i.e., independent of vehicle type, fuel
 

metering system, model year, etc.). To use these distribution
 

estimates within a given stratum (e.g., 1980-85 carbureted LDVs),
 

we determined the numbers of vehicles in each of the
 

purge/pressure categories that we would expect to find in a
 

randomly selected sample of the in-use fleet. We then calculated
 

the ratio of those expected category sizes to the number of
 

vehicles actually recruited and tested within each of those four
 

categories. Those ratios then became the weighting factors for
 

the analysis of that stratum.
 

*
 Vehicle age was estimated by first subtracting the model year from the
 

test year, and then adjusting so that the final value represents the age
 

at January first (which is the standard date for the MOBILE model).
 
-6- DRAFT
 

July 13, 1999
 

Table 4-1
 

Predicted Distribution of Purge/Pressure Results
 

(By Vehicle Age -- Independent of Model Year)


Vehicle 

Age 

0 

1 

--- Res

Fail Purge 

Pass Pressure 

1.49% 

1.86% 

ults on Purge 

Fail Purge 

Fail Pressure 

0.05% 

0.08% 

and Pressure 

Pass Purge 

Fail Pressure 

1.38% 

1.79% 

Tests --­

Pass Purge 

Pass Pressure 

97.1% 

96.3% 

2 

3 

2.30% 

2.82% 

0.14% 

0.23% 

2.30% 

2.96% 

95.3% 

94.0% 

4 

5 

3.43% 

4.13% 

0.36% 

0.55% 

3.77% 

4.79% 

92.4% 

90.5% 

6 

7 

4.91% 

5.76% 

0.82% 

1.20% 

6.03% 

7.53% 

88.2% 

85.5% 

8 

9 

6.66% 

7.59% 

1.72% 

2.40% 

9.30% 

11.34% 

82.3% 

78.7% 

10 

11 

8.51% 

9.40% 

3.26% 

4.32% 

13.64% 

16.14% 

74.6% 

70.1% 

1 2 

1 3 

10.24% 

11.01% 

5.57% 

6.99% 

18.78% 

21.47% 

65.4% 

60.5% 

1 4 

1 5 

11.69% 

12.28% 

8.53% 

10.14% 

24.09% 

26.54% 

55.7% 

51.0% 

1 6 

1 7 

12.79% 

13.22% 

11.77% 

13.35% 

28.73% 

30.61% 

46.7% 

42.8% 

1 8 

1 9 

13.57% 

13.86% 

14.86% 

16.26% 

32.15% 

33.36% 

39.4% 

36.5% 

2 0 

2 1 

14.10% 

14.28% 

17.55% 

18.71% 

34.25% 

34.86% 

34.1% 

32.1% 

2 2 

2 3 

14.44% 

14.56% 

19.77% 

20.73% 

35.23% 

35.40% 

30.6% 

29.3% 

2 4 

2 5 

14.65% 

14.73% 

21.61% 

22.41% 

35.42% 

35.31% 

28.3% 

27.6% 

NOTE: Since no vehicles in the EPA testing programs were
 

recruited from among those that failed both the purge and the
 

pressure tests (the third column in the preceding table), EPA used
 

the data from the CRC program to characterize the RTD emissions of
 

that category. Since (as Table 4-1 indicates) this stratum is
 

quite small for newer vehicles, its exclusion had a most a slight
 

affect on the estimate of fleet emissions of those newer vehicles.
 

(See Section 6.5.)
 
-7- DRAFT
 

July 13, 1999
 

5.0 Test Parameters
 

Since emissions from vehicles classified as gross liquid
 

leakers (vehicles identified as having substantial leaks of liquid
 

gasoline, as opposed to simply vapor leaks) are characterized
 

separately from those of the remaining vehicles, the analyses in
 

this section were also performed with those vehicles omitted (see
 

section 7.3).
 

There are three testing parameters in the EPA programs that
 

could affect the RTD test results. Those are:
 

1) the RVP of the test fuel,
 

2) the temperature cycle, and
 

3) the site from which each vehicle was recruited.
 

Since it is well known that both the ambient temperature and
 

the fuel volatility will affect evaporative emissions, these two
 

parameters were automatically included in the calculations. All
 

of the analyses that used tests performed with fuels ranging from
 

6.7 to 7.0 psi RVP were conducted assuming the nominal RVP to be
 

6.8 psi, as noted previously.
 

The question of whether the "site" variable is significant
 

was raised because EPA's testing contractor (ATL) recruited
 

vehicles from two different parts of the country. Twenty-two (22)
 

vehicles were recruited from and tested in Indiana; the remaining
 

97 vehicles were recruited from and tested in Arizona. Since the
 

higher temperatures in Arizona might have resulted in higher
 

canister loadings for those as-received vehicles, we compared the
 

24-hour RTD results (weighted to correct for recruitment bias) of
 

the 1986 and newer PFI LDVs tested at both sites (Figure 5-1) and
 

of the 1986 and newer TBI LDVs tested at both sites (Figure 5-2).
 

All of these 24-hour RTD emissions were obtained using 6.7-6.9 psi
 

RVP fuel over the 72°-96° F cycle.
 

Despite the small sample sizes in the Indiana data (only six
 

PFIs and four TBIs), the closeness of the distribution curves is
 

compelling and suggests that there is no reason to treat the test
 

data separately. Therefore, the "site" parameter was dropped from
 

the remaining analyses.
 
24-Hour RTD Emissions (grams of HC) 24-Hour RTD Emissions (grams of HC)

-8- DRAFT
 

July 13, 1999
 

Figure 5-1
 

Weighted Cumulative Distributions at Two Sites
 

RTD Emissions of the 1986 and Newer PFIs
 

40 

30 

20 

10 

0 

PFIs in INDIANA 

PFIs in ARIZONA 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

Figure 5-2
 

Weighted Cumulative Distributions at Two Sites
 

RTD Emissions of the 1986 and Newer TBIs
 

40 

30 

20 

10 

0 

TBIs in INDIANA 

TBIs in ARIZONA 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 
-9- DRAFT
 

July 13, 1999
 

6.0 Consolidating Vehicle Parameters for 24-Hour RTD
 

Since emissions from vehicles classified as gross liquid
 

leakers (see section 7.3) are characterized separately from those
 

of the remaining vehicles, the analyses discussed in this section
 

were also performed with those vehicles omitted.
 

When analyzing exhaust emissions, we note that some vehicle
 

technologies (sometimes identified by model year ranges) have
 

distinct exhaust emission characteristics. Before beginning the
 

primary analysis of these evaporative emissions, we examined the
 

data to determine if analogous technology groupings exist for the
 

RTD test results. Specifically, it was necessary to determine:
 

1) whether tests results from different model year ranges (i.e., 

1981-85 and 1986-93) can be combined, 

2) whether tests results from port fuel-injected vehicles (PFIs) 

can be combined with throttle body injected vehicles (TBIs) 

into a single stratum of fuel-injected vehicles, 

3) whether tests results from carbureted vehicles can be 

combined with fuel-injected vehicles, and 

4) whether tests results from cars and trucks can be combined 

(despite the differences in fuel tank size). 

We stratified the test vehicles using the following three (3)
 

model year ranges:
 

1) 1972 through 1979,
 

2) 1980 through 1985, and
 

3) 1986 through 1995.
 

Based on the assumption that changes to the EPA certification
 

requirements for evaporative emissions will result in changes to
 

vehicles' evaporative control systems, we separated the RTD
 

results on the pre-1980 vehicles from the results on the 1980 and
 

newer vehicles. (For the same reason, data from the 1996 and
 

newer model year vehicles will form a new stratum once we begin to
 

test those vehicles.) While a similar argument can be made for an
 

additional break at the 1978 model year point, we lacked the data
 

to separately analyze the 1978-79 model year vehicles. A second
 

break point was added between the 1985 and 1986 model years at the
 

recommendation of some of the automotive manufacturers who based
 

their suggestion on improvements in the control of evaporative
 

emissions. Therefore, this second break point was not based on
 

any changes in EPA test requirements or applicable standards nor
 

on any analysis of the results of the RTD tests.
 
-10- DRAFT
 

July 13, 1999
 

6.1 Comparing TBI and PFI Vehicles
 

To determine the appropriateness of combining the RTD test
 

results of PFIs with those of TBIs, we found two samples
 

containing otherwise similar vehicles:
 

1) 1986 and newer trucks in the CRC testing program (see Figure 

6-1) and 

2) 1986 and newer LDVs in the EPA testing program (see Figure 6­

2). 

In each of those two samples, the testing was performed over the
 

72°-96° temperature cycle using fuel with an RVP ranging from 6.7
 

to 7.0 psi. The similarity between PFI and TBI among the 1986 and
 

newer model year trucks in the CRC testing program is illustrated
 

in Figure 6-1.
 

Figure 6-1
 

Cumulative Distributions of PFIs and TBIs
 

RTD Emissions in the CRC Testing Program
 

24-Hour RTD Emissions (grams of HC)

40 

30 

20 

10 

0 

CRC 86-91 Truck TBI 

CRC 86-91 Truck PFI 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

Characterizing those two CRC samples yields: 

Sample Standard 

Size Median Mean Deviation 

1986-91 CRC 19 3.13 5.41 5.70 

Truck TBIs 

1986-91 CRC 24 2.05 5.85 7.87 

Truck PFIs 
-11- DRAFT
 

July 13, 1999
 

The similarity between PFI and TBI among the 1986 and newer model
 

year LDVs in the EPA testing program is illustrated in Figure 6-2.
 

Figure 6-2
 

Weighted Cumulative Distributions of PFIs and TBIs
 

RTD Emissions in the EPA Testing Program
 

24-Hour RTD Emissions (grams of HC)

60 

40 

20 

0 

EPA 86-95 LDV TBI 

EPA 86-95 LDV PFI 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

Both the distributions shown in Figure 6-2 and the
 

characterizations of those two EPA samples presented in the
 

following table have been weighted to correct for recruitment
 

bias. 

Sample 

Size Median Mean 

1986-95 EPA 21 4.52 9.84 

LDV TBIs 

1986-95 EPA 41 2.08 9.32 

LDV PFIs 

Based on the similarity of the cumulative distribution curves and
 

on the close fit of the means (in the strata illustrated in
 

Figures 6-1 and 6-2), the PFI and TBI strata were merged into a
 

single fuel-injected (FI) stratum for the remaining analyses.
 

6.2 Comparing Carbureted and Fuel Injected Vehicles
 

To determine whether test results from carbureted vehicles
 

can be combined with those from fuel injected vehicles, we
 
-12-	 DRAFT
 

July 13, 1999
 

identified the only four samples containing otherwise similar
 

vehicles:
 

1)	 in the CRC testing program, 43 1986 and newer FI trucks and 7
 

corresponding carbureted trucks (see Figure 6-3),
 

2)	 in the EPA testing program, 64 1986 and newer FI LDVs and 6
 

corresponding carbureted LDVs (see Figure 6-4),
 

3)	 in the CRC testing program, 3 1980-85 FI trucks and 46
 

corresponding carbureted trucks, and
 

4)	 in the EPA testing program, 6 1980-85 FI LDVs and 13
 

corresponding carbureted LDVs.
 

However, the two comparisons using the 1980-85 model year vehicles
 

produced mixed results (possibly due to the small number of FI
 

vehicles in the samples).
 

The differences in the distributions between carbureted
 

(Carb) and FI among the 1986 and newer model year trucks in the
 

CRC testing program is illustrated in the following table and in
 

Figure 6-3.
 

Comparing Carbureted Trucks to FI Trucks
 

1986-95 CRC 

LDT Carbs 

Sample 

Size 

7 

Median 

6.15 

Mean 

9.31 

Standard 

Deviation 

8.28 

1986-95 CRC 

LDT FIs 

43 2.85 5.65 6.92 
-13- DRAFT
 

July 13, 1999
 

Figure 6-3
 

Cumulative Distributions of FIs and Carb Trucks
 

RTD Emissions in the CRC Testing Program
 

24-Hour RTD Emissions (grams of HC)

30 

20 

10 

0 

CRC 86-91 Trk Carb 

CRC 86-91 Trk FI 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

The differences in the distributions between carbureted
 

(Carb) and FI among the 1986 and newer model year LDVs in the EPA
 

testing program is illustrated in the following table and in
 

Figure 6-4. Both the distributions shown in Figure 6-4 and the
 

characterizations of those two EPA samples represented in the
 

following table have been weighted (using Table 4-1) to correct
 

for recruitment bias.
 

Comparing Carbureted LDVs to FI LDVs
 

Sample
 

Size Median Mean 

1986-95 EPA 6 10.56 10.34 

LDV Carbs 

1986-95 EPA 64 3.41 9.50 

LDV FIs 
-14- DRAFT
 

July 13, 1999
 

Figure 6-4
 

Weighted Cumulative Distributions of FIs and Carbs LDVs
 

RTD Emissions in the EPA Testing Program


24-Hour RTD Emissions (grams of HC)

30 

20 

10 

0 

EPA 86-95 LDV Carb 

EPA 86-95 LDV FI 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

In each of the two preceding figures, the sample sizes of the
 

carbureted vehicles are relatively small. However, it is
 

noteworthy that every carbureted vehicle in each sample had RTD
 

test results higher than the median of the corresponding fuel
 

injected vehicle sample. (An unlikely situation if the RTD
 

emissions of the fuel injected and carbureted vehicles were to be
 

indistinguishable from each other.)
 

Therefore, EPA proposes to treat the carbureted vehicles and
 

the FI vehicles as distinct strata for the remaining analyses.
 

6.3 Comparing Cars and Trucks
 

Determining the appropriateness of combining the RTD test
 

results of LDVs with those of LDTs presented different problems.
 

Specifically, the CRC sample was exclusively trucks except for the
 

1971-77 stratum, and the EPA sample (using 6.7-6.9 RVP fuel) was
 

almost exclusively cars. The obvious solution was to compare the
 

CRC trucks with the EPA cars. However, because of the difference
 

in recruitment methods, we first had to omit from the CRC sample
 

those vehicles which would not have been recruited in the EPA
 

sample (i.e., those failing both purge and pressure), and we then
 

weighted the remaining results (as we did with the EPA sample).
 

This produced the following two strata with which to investigate
 

the differences in RTD results between cars and trucks:
 
-15­ DRAFT 

1) 

July 13, 1999 

in the combined EPA and CRC testing programs, the weighted 

results of 13 1980-85 carbureted LDVs and 44 corresponding 

carbureted trucks (Figure 6-5), and 

2) in the combined EPA and CRC testing programs, the weighted 

results of 62 1986 and newer FI LDVs and 42 corresponding 

carbureted trucks (Figures 6-6 and 6-7). 

The distributions in Figures 6-5 and 6-6 and the 

characterizations of those strata (in the following table) have
 

been weighted to correct for the actual recruitment bias in the
 

EPA sample and the simulated bias in the CRC sample.
 

1980-85 LDVs
 

Carbureted
 

1980-85 LDTs
 

Carbureted
 

1986+ FI LDVs
 

1986+ FI LDTs
 

Sample 

Size Median Mean 

13 10.22 11.29 

44 10.55 10.58 

62 3.40 9.48 

42 3.11 5.99 

Figure 6-5 

Weighted Cumulative Distribution of Cars and Trucks
 

RTD Emissions in the EPA and CRC Testing Programs
 

(1980-1985 Model Year Carbureted Vehicles)
 

24-Hour RTD Emissions (grams of HC)

40 

30 

20 

10 

0 

EPA 80-85 Carb 

LDVs 

CRC 80-85 Carb 

Trucks 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 
-16- DRAFT
 

July 13, 1999
 

Figure 6-6
 

Weighted Cumulative Distribution of Cars and Trucks
 

RTD Emissions in the EPA and CRC Testing Programs
 

(1986 and Newer Model Year FI Vehicles)


24-Hour RTD Emissions (grams of HC)

40 

30 

20 

10 

0 

EPA 86-95 FI LDVs 

CRC 86-91 FI Trucks 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

In Figure 6-6, the distributions of the FI 1986 and newer
 

cars and trucks are virtually identical up to about the 50
 

percentile point, after which they diverge. However, much of that
 

divergence is the result of a RTD test on a single truck in the
 

CRC sample (vehicle 9143). If that single truck had not been
 

recruited, then the (re-weighted) distribution of the remaining 41
 

FI trucks (given below in Figure 6-7) is quite similar to that of
 

the corresponding 62 FI cars.
 
-17-	 DRAFT
 

July 13, 1999
 

Figure 6-7
 

Weighted Cumulative Distribution of Cars and Trucks
 

RTD Emissions in the EPA and CRC Testing Programs
 

(1986 and Newer Model Year FI Vehicles)
 

(Excluding CRC LDT No. 9143)
 

24-Hour RTD Emissions (grams of HC)

40 

30 

20 

10 

0 

EPA 86-95 FI LDVs 

CRC 86-91 FI Trucks 

(minus 9143) 

0% 20% 40% 60% 80% 100% 

Cumulative Percentage (%) 

Based on the similarity of the cumulative distribution curves, the
 

close fit of the means for the 1980-85 vehicles, and on the close
 

fit of all of the medians, we merged the cars and trucks into a
 

single stratum for the remaining analyses. This conclusion seems
 

reasonable based on the fact that the larger fuel tanks (and hence
 

potentially larger vapor volumes) of trucks are offset by the
 

reportedly larger canister volumes.
 

6.4 Summarizing Stratification Parameters
 

For each combination of the pass/fail results on the
 

(screening) purge test and pressure test (i.e., recruitment
 

groups), we stratified the combined 119 vehicle EPA and 151
 

vehicle CRC data into the following five strata:
 

Model Year Range 

1971-1979 

Number of 

Carbureted 

Vehicles 

57 

Number of 

Fuel Injected 

Vehicles 

* 

1980-1985 65 12 

1986 and Newer 15 121 

* No data were available for this stratum. 	We simply
 

applied the results of the 1971-79 carbureted vehicles
 

to characterize this stratum.
 
-18- DRAFT
 

July 13, 1999
 

These five (tested) strata, in the above table, were then
 

subdivided to include the recruitment criteria and yielded the 20
 

substrata listed in Appendix C. Three of these 20 strata were not
 

tested, and two of the remaining had only limited coverage. These
 

five missing or poorly covered strata are comprised of vehicles
 

that failed both the purge and pressure tests.
 

6.5 Evaluating Untested Strata
 

As noted in the previous section, the strata that are either
 

missing or poorly represented in our sample fall into two
 

categories:
 

1) No pre-1980 model year vehicles equipped with fuel 

injection were recruited because of the small numbers of 

pre-1980 model year vehicles in the in-use fleet. 

2) The vehicles that failed both the purge and the pressure 

tests: 

• were systematically excluded from the EPA sample and 

• were missing or poorly represented in CRC's sample of 

the newer model year vehicles due to their relative 

rarity among the newer vehicles (see Table 4-1). 

For the MOBILE model, EPA proposes that the RTD emissions of
 

the (untested) pre-1980 fuel injected vehicles are identical to
 

the corresponding emissions of the pre-1980 carbureted vehicles.
 

This should be a safe assumption since any actual differences
 

between these strata should be balanced by the relatively small
 

number of these vehicles in the in-use fleet.
 

Eighteen vehicles that failed both the purge and the pressure
 

tests were tested (all by CRC). Four of those were identified as
 

gross liquid leakers and analyzed separately. Thirteen (of the
 

remaining 14) were pre-1980 carbureted vehicles. For those 13
 

vehicles, the mean (24-hour) RTD emissions was 25.11 grams (with a
 

standard deviation of 12.00). The corresponding stratum of pre1980 vehicles that passed the purge test but failed the pressure
 

test contains 20 vehicles (18 CRC and 2 EPA) has a mean (24-hour)
 

RTD emissions of 24.39 grams (with a standard deviation of 7.77).
 

Based on the similarity of those means, we will combine those two
 

strata into a single stratum of vehicles that failed the pressure
 

test (regardless of their results on the purge test). (This
 

approach permits us to avoid having to estimate emissions from the
 

untested strata of newer vehicles that fail both the purge and
 

pressure tests.)
 

7.0 Evaporative Emissions Represented by the RTD
 

The results from the real-time diurnal (RTD) tests can be
 

used to model the following two types of evaporative emissions:
 
-19- DRAFT
 

July 13, 1999
 

1) "Diurnal" emissions are the pressure-driven emissions
 

resulting from the daily increase in temperature.
 

2)	 "Resting loss" emissions are the relatively stable
 

emissions that are always present.
 

7.1	 Resting Loss Emissions
 

Examinations of the RTD data suggest that, for virtually all
 

of the tests (regardless of the temperature cycle, fuel RVP, or
 

vehicle type), the hourly HC evaporative emissions had stabilized
 

and were relatively constant for hours 19 through 24. (See Figure
 

7-1.) This suggests that the average hourly emissions during the
 

final six (6) hours of the 24-hour RTD cycle correspond to what
 

this paper refers to (in the previous section) as hourly “resting
 

loss” emissions.
 

Figure 7-1
 

Identifying Resting Losses
 

(Stable Portion of RTD Hourly Emissions)
 

100° 1.5 

Temperature 

Hourly 

Emissions 

0	 1 2 2 4 

Ambient Temperature (°F) 

RTD Emissions (grams / hr)

90° 1.0 

80° 0.5 

70° 0.0 

Time (hours) 

The “resting loss” emissions component of each RTD test was
 

calculated as the average (i.e., mean) hourly RTD emissions for
 

hours 19 through 24, at the nominal temperature for the twenty-


fourth hour. In this example, the average emissions for that 6­

hour period (0.10 grams per hour) would represent this vehicle's
 

hourly resting losses at a stable 72°F with a fuel having RVP of
 

6.8 psi. The mean hourly resting loss emissions (temperatures of
 

60°, 72° and 82°) for each of the strata in Section 6.4 are given
 

in Appendix C.
 
-20- DRAFT
 

July 13, 1999
 

7.2 Diurnal Emissions
 

Subtracting the hourly resting loss emissions (calculated in
 

Section 7.1) from the hourly RTD emissions, should yield an
 

estimate of the hourly emissions that result from the daily rise
 

in temperature (i.e., “diurnal” emissions). Although the hourly
 

resting loss emissions will vary as the ambient temperature cycles
 

over the full range of the RTD test (see Section 8.0), the
 

variation is small relative to the RTD hourly emissions.
 

Therefore, using a constant resting loss value rather than a
 

“temperature adjusted” value will not affect the analysis. (Using
 

a "temperature adjusted" resting loss value will result in a
 

slightly higher level of resting loss emissions over the day, and
 

a corresponding lower level of diurnal emissions over that day.
 

The total emissions will be unchanged.)
 

In the following figure, the hourly resting loss emissions
 

correspond to the unshaded area. The remaining (i.e., shaded)
 

area then corresponds to the hourly diurnal emissions which are
 

primarily pressure-driven vapor leaks. This approach produces
 

calculated hourly diurnal emissions that approach zero as the SHED
 

Figure 7-2
 

Estimating Diurnal Emissions
 

(Pressure Driven Vapor Leaks)
 

1.5 

1.0 

0.5 

0.0 

Time (hours) 

RTD Hourly Emissions (grams)

Pressure Driven 

Vapor Leaks 

Resting 

Losses 

0 6 12 18 24 30 
-21- DRAFT
 

July 13, 1999
 

(i.e., "ambient") temperature drops to near the starting
 

temperature.
 

The average (mean) 24-hour RTD emissions for each of the
 

strata in Section 6.4 are given in Appendix C.
 

7.3 Separating Out Gross Liquid Leakers
 

The largest quantity of RTD data (combining data from the EPA
 

and CRC programs) was generated using fuel with an RVP ranging
 

between 6.7 and 7.0 psi over the 72°-96° F temperature cycle.
 

These test conditions were used by a total of 96 vehicles in the
 

EPA program and all 151 vehicles in the CRC program. Using the
 

preceding method to estimate hourly resting loss emissions (at
 

72°F) for each of those 247 vehicles, we then plotted the full 24­

hour RTD emissions versus those hourly resting loss emissions
 

(Figure 7-3).
 

Figure 7-3
 

Comparison of RTD versus Resting Loss Emissions
 

(72°-96°F Cycle Using 6.7-7.0 RVP Fuel)
 

800 

600 

400 

200 

0 

Resting Loss Emissions (grams per hour) 

This graph clearly illustrates that the test results of all
 

but five of the vehicles are tightly clustered with RTD results
 

under 100 grams (per 24-hours) and with hourly resting losses
 

under 1.5 grams per hour. The test results from each of the
 

remaining five vehicles are quite distinct from those of the
 

corresponding 242 tightly clustered vehicles. Each of these five
 

extremely high emitting vehicles was also identified, by the
 24-Hour RTD Emissions (grams of HC)

0.0 5.0 10.0 15.0 20.0 
-22- DRAFT
 

July 13, 1999
 

mechanics who examined them, as having significant leaks of liquid
 

gasoline (as opposed to simply vapor leaks).
 

The RTD data in Figure 7-3 suggest that the evaporative
 

emissions from these five vehicles can exceed the emissions of
 

corresponding vehicles by one to two orders of magnitude. For
 

this reason, this report treats these “gross liquid leakers” as a
 

separate category of evaporative emitters. It is important to
 

note that this category (i.e., “gross liquid leakers”) is not a
 

new or previously unaccounted for source of emissions, since the
 

emissions from these vehicles had previously been included with
 

the resting loss and diurnal emissions. Thus, modeling these
 

vehicles separately should have no impact on the total evaporative
 

emissions.
 

To define this category of “gross liquid leakers,” we first
 

assumed that the effects of a significant liquid fuel leak should
 

be evident during the resting loss portion of the RTD test. This
 

report, therefore, defines a “gross liquid leaker” to be any
 

vehicle whose resting loss emissions are at least two grams per
 

hour. These five gross liquid leakers were all part of the CRC
 

study. Using this definition, we classified two vehicles in the
 

EPA study as likely gross liquid leakers. (These two are only
 

"likely" gross liquid leakers because no mechanic's inspections
 

were performed. We inferred their status based solely on their
 

resting loss emissions.) These two additional gross liquid
 

leakers do not appear in Figure 7-3 because they were tested only
 

on 6.3 and 9.0 psi RVP fuels.
 

It is important to note that another type of liquid leaker is
 

possible. Some leaks can occur only if the vehicle is operating
 

(e.g., leaks associated with the fuel pump). Preliminary results
 

from a running loss testing program being run by CRC suggests that
 

vehicles with such leaks may have higher hourly evaporative
 

emissions (in grams per hour) while they are operating than the
 

hourly (RTD) emissions from the gross liquid leakers in this
 

analysis. However, the gross liquid leakers identified in this
 

analysis have high evaporative emissions every hour of the day;
 

while, the other type of liquid leaker would probably have high
 

evaporative emissions only during the hours the vehicle is
 

actually operating. The effects of that other type of liquid
 

leaker will be included in the running loss component of the
 

evaporative emissions in the MOBILE model.
 

8.0 Characterizing Resting Loss Emissions
 

Resting loss evaporative emissions are functions primarily of
 

ambient temperature. There are several distinct mechanisms
 

contributing to resting loss emissions:
 

• permeation of the liquid fuel through the walls of both
 

hoses and (if applicable) plastic fuel tanks,
 
________________________________ 

-23- DRAFT
 

July 13, 1999
 

• seepage of vaporized fuel at connectors and through cracks
 

in hoses, fuel tanks, etc.,
 

• permeation and seepage at the canister, and
 

• undetected (minor) liquid leaks of fuel.
 

Some of these components of resting loss emissions are strongly
 

related to temperature changes while others such as the minor
 

liquid leaks are relatively unaffected by temperature changes.
 

As the first step in characterizing the effects of changes in
 

temperature and volatility on the hourly evaporative emissions, we
 

identified 57 vehicles in the EPA program that were each tested:
 

• using both the 6.8 and the 9.0 RVP fuels and
 

• over all three temperature cycles.
 

Using this sample permitted us to have exactly the same vehicles
 

being tested at each combination of fuel RVP and temperature;
 

thus, avoiding many of the problems associated with vehicle-tovehicle test variability. This sample of 57 vehicles consisted
 

of:
 

• 12 1974-85 model year carbureted vehicles and
 

• 45 1985-94 model year fuel injected vehicles.
 

In the following graph (Figure 8-1), we plotted the mean hourly
 

resting loss emissions for the carbureted vehicles and the fuel
 

injected vehicles.
 

Based on the graphs in Figure 8-1 (on the following page), we
 

can make the following observations:
 

• Hourly resting loss emissions increase with increasing
 

ambient temperature.*
 

• The rate at which the resting loss emissions are increasing
 

appears to be a linear function of the ambient temperature.
 

• For the fuel injected (i.e., the larger sub-sample), the
 

graph appears to contain a slight non-linear component.
 

However, with measurements at only three temperatures,
 

there are insufficient data to confirm that observation.
 

*
 An increase in hourly resting loss emissions corresponding to an increase
 

in fuel RVP was also noted (especially for the fuel-injected vehicles).
 

This apparent relationship is believed to simply be an artifact of the
 

vehicles always being tested in the same (not randomized) order rather
 

than being a true relationship between resting loss emissions and Reid
 

vapor pressure. In the previous version of MOBILE, it was noted that
 

resting loss emissions appear to be insensitive to the fuel volatility
 

level, and EPA proposes to continue to use that same assumption in this
 

version of MOBILE.
 
-24- DRAFT
 

July 13, 1999
 

Figure 8-1
 

Mean Hourly Resting Loss Versus Temperature
 

(averaged at each temperature)
 

(Sub-Sample of 57 Vehicles)
 

0.3 

0.2 

0.1 

0.0 

Ambient Temperature (°F) 

Therefore, for these 57 vehicles, the functions that most
 

reasonably model the hourly resting loss emissions (within the
 

tested range) are linear functions of temperature. That is:
 

Hourly Resting Loss = A + [ B * Temperature (°F) ] 

Where: 

"A" 

-0.032040 

"B" 

0.002973 For Carb Vehicles 

-0.123027 0.002769 For FI Vehicles 

The two slopes (i.e., the "B" values in the above table) are 

obviously close to each other in value. Since the difference 

between the slopes was not statistically significant, the 

regression was rerun, producing a single slope of 0.002812. 

Having a single value for the slope (regardless of the stratum) 

indicates that an increase in ambient temperature of ten degrees 

Fahrenheit will, on average, correspond to an increase of 0.028 

grams in each hour's resting loss emissions. Hourly Resting Loss (grams / hr) 

Carb (12) 

FI (45) 

Regressions 

50° 70° 90° 110°
 
________________________________ 

-25- DRAFT
 

July 13, 1999 

While only the test results from the 57 vehicles that were 

tested over the full range of fuel RVPs and temperature cycles 

were used to determine the coefficient ("B") which determines the 

slope of the lines. The full data set was used only to solve for 

the individual constant terms ("A"). 

For each of the strata identified in Section 6.4, we 

calculated the value of "A" that would minimize the difference 

between the predicted and the actual resting losses (i.e., the 

residuals). If more tests had been conducted at a given 

combination of temperature and fuel RVP (e.g., 72 °F using 6.8 psi 

RVP fuel), then the average resting loss emissions at that 

combination was then more heavily weighted in the process to 

calculate the value "A". 

This process produced a regression equation for each of the
 

18 strata; however, the predicted results based on the vehicle's
 

pass/fail status on the purge test were inconsistent. This
 

inconsistency is not surprising since the types of mechanical
 

problems that would cause a purge failure are not likely to
 

contribute to resting loss emissions.* To address this situation,
 

the population was stratified based simply on whether the vehicles
 

pass or fail just the pressure test. The regression equations for
 

each of the 12 resulting strata are given in Appendix D. The
 

regression equations are unique for each stratum in which testing
 

was performed. The untested strata of pre-1980 fuel-injected
 

vehicles used the regression equations of the pre-1980 carbureted
 

vehicles.
 

Using these 12 equations, we calculated an estimate of the
 

hourly resting loss emissions at each hour of the three
 

temperature cycles. Then, adding the hourly estimates for the
 

first 24 hours of each test produced the daily resting loss
 

emissions (for each of the 24 strata). These equations all
 

predict that the full day's resting loss emissions (in grams)
 

would be 24 times the hourly resting loss (calculated at the day's
 

low temperature) plus 0.766.
 

These equations predict resting loss emissions of the
 

carbureted vehicles to be higher than for the fuel injected
 

vehicles. While these regressions can be used to calculate
 

reasonable estimates of resting loss emissions within the range of
 

temperature and fuel RVPs that were actually tested, we must
 

determine (see Section 11) how to extrapolate beyond the limits of
 

the test data.
 

*
 In the previous version of MOBILE, it was noted that resting loss
 

emissions are independent of the canister state (i.e., whether the
 

canister is saturated or fully purged).
 
________________________________ 

-26­ DRAFT 

July 13, 1999 

9.0 Characterizing 24-Hour Diurnal Emissions 

Diurnal evaporative emissions, like most other evaporative
 

emissions, are functions of both fuel volatility and temperature
 

which are themselves interdependent. The RVP is a measure of
 

vapor pressure (VP)* at a single temperature, 100°F. The
 

Clausius-Clapeyron relationship was used to estimate the vapor
 

pressure at each temperature and for each of the fuels (RVPs of
 

6.8 and 9.0 psi) used in this testing program. (See Appendix B.)
 

To characterize the diurnal emissions, we again (see Section
 

8.0) identified the 57 vehicles in the EPA program that were
 

tested over a wide range of vapor pressures. These test vehicles
 

were distributed among 12 strata (of the 18 potential strata
 

identified in Section 6.5). Within each stratum, we then
 

attempted to regress the diurnal emissions against combinations of
 

fuel volatility and temperature.
 

A similar approach was attempted to characterize resting loss
 

emissions (see previous section) but had not been successful.
 

However, this approach produced more satisfactory results in
 

characterizing the diurnal emissions even in strata that were
 

sparsely tested. Most likely this difference was due to the
 

effect that the test-to-test variability was substantially larger
 

relative to the resting loss emissions than to the diurnal
 

emissions. Therefore, any test-to-test variability was less
 

likely to hide patterns evidenced in the diurnal emissions
 

measurements.
 

For each RTD test, the Clausius-Clapeyron relationship was
 

used to estimate the vapor pressure at both the low and the high
 

temperatures. Using these estimates, we calculated both the
 

average of the low and the high vapor pressures, as well as the
 

difference between the low and the high vapor pressures (both in
 

kPa). Multiplying these two quantities together produced a single
 

product term (VP*?VP) that incorporates the parameters of the RTD
 

test (i.e., both the temperature cycle and the fuel's RVP).
 

The mean diurnal emissions (calculated in the previous
 

section by subtracting a daily resting loss value from the RTD
 

test results) were repeatedly regressed against a polynomial of
 

that product term of vapor pressures within each stratum. The
 

independent variable used in the regressions was either:
 

1)	 the product term (i.e., the average vapor pressures
 

times the difference of the vapor pressures) or
 

2)	 the square of that product term (to allow for possible
 

non-linearity).
 

*
 In Appendix B, we illustrate how the Clausius-Clapeyron relationship can
 

be used to estimate a fuel's vapor pressure at each temperature if the
 

fuel's RVP is known.
 
________________________________ 

-27-	 DRAFT
 

July 13, 1999
 

However, when we graphed the mean diurnal emissions against this
 

"vapor pressure product" term, we noted that the affect of the RVP
 

of the test fuel on diurnal emissions was not being completely
 

accounted for by the "vapor pressure product" term. We,
 

therefore, reran the previous regressions and included RVP as one
 

of the independent variables. Thus, in each of those 12 strata,
 

we generated both a nonlinear (i.e., quadratic) model and a linear
 

model* . A two step process was used to choose among those models:
 

1)	 We performed a visual inspection of the data. (This
 

approach, in and of itself, is not very precise, but we
 

wanted to make certain that the model selected would be
 

both reasonable and accurately represent the test data.)
 

2)	 We compared the statistical parameters associated with
 

each of those regressions. (That is, we identified the
 

model that optimized: the F-ratio, the statistical
 

significance of the independent variable, and the R-


squared value.)
 

The regression analyses performed did not always (i.e., in all 12
 

strata) identify the fuel RVP as a statistically significant
 

variable. However, for consistency, RVP was used as an
 

independent variable in all of the strata regardless of its
 

significance level.
 

Although the equations that we developed in this analysis are
 

empirical (i.e., data driven) models, we did impose the following
 

three restrictions that were based on engineering experience with
 

diurnal emissions:
 

•	 The diurnal emissions should increase with increasing
 

fuel RVP (with all other parameters held constant).
 

•	 The diurnal emissions should increase with increasing
 

temperature cycles (with all other parameters held
 

constant).
 

•	 For each combination of fuel delivery system (i.e., fuel
 

injected versus carbureted) and purge/pressure category,
 

the diurnal emissions should increase with each
 

successively older model year grouping (for each
 

combination of temperature cycle and fuel RVP).
 

*
 Theoretically, in each of those models, a zero change in daily temperature
 

(hence, in ?VP) should result in zero diurnal emissions. This physical
 

necessity would result in the constant term in each regression being zero.
 

However, this requirement was dropped because:
 

(1)	 of the resulting low r-squared values,
 

(2)	 of the lack of test data having diurnal temperature ranges less than
 

24 degrees, and
 

(3)	 we will require, for any diurnal emissions, a difference between the
 

daily high and low temperatures of at least five degrees Fahrenheit.
 
________________________________ 

-28- DRAFT
 

July 13, 1999
 

Seven separate strata required additional effort to meet
 

these three criteria (that were based on engineering experience):
 

• the three strata of 1972-1979 model year carbureted
 

vehicles,
 

• the 1980-1985 model year FI vehicles that passed the
 

pressure test, and
 

• the three strata of 1986 and newer model year carbureted
 

vehicles.
 

Basing the estimates of diurnal emissions for the 1972-1979
 

model year carbureted vehicles resulted in predicted diurnal
 

emissions (for some temperature cycle / RVP combinations) that
 

were lower than for the newer (1980-85 model year) vehicles
 

(possibly due to the small number of 1972-79 vehicles tested over
 

different temperature cycles and with different fuel RVPs). As a
 

result, we used a modification of the equations that resulted from
 

the analysis of the 1980-85 model year carbureted vehicles.
 

Specifically, we used the same coefficients, but we altered the
 

constant terms so that when the modified equations were used to
 

estimate the emissions of the Pre-1980 vehicles, the sum of the
 

residuals (within each purge / pressure stratum) was zero.
 

The strata of 1980-85 FI vehicle that passed both the purge
 

and pressure tests was represented by only a single vehicle that
 

was tested over the full range of temperature cycles and fuel
 

RVPs. Therefore, the results of those tests were combined with
 

the tests on the three 1980-85 FI vehicles the failed the purge
 

test but passed the pressure test into a single stratum of
 

vehicles that passed the pressure test (represented by four
 

vehicles). The regression of these data was used to determine the
 

coefficients for both the stratum of 1980-85 FI vehicle the passed
 

both the purge and pressure tests and the stratum of 1980-85 FI
 

vehicle the failed only the purge test. The coefficient for each
 

stratum was the value that would make each sum of residuals zero.
 

The last three problem strata were the 1986 and newer
 

carbureted vehicles. As is illustrated in Appendix C, only four
 

combinations of temperature cycle and fuel RVP were tested (in
 

each of the three purge/pressure substrata). The two untested
 

combinations were the combinations that would have yielded results
 

at the highest and the lowest VP values. Having test data over
 

such a narrow range (i.e., only the four middle values) of vapor
 

pressures makes selecting the proper regression curve difficult.
 

*
 Theoretically, in each of those models, a zero change in daily temperature
 

(hence, in ?VP) should result in zero diurnal emissions. This physical
 

necessity would result in the constant term in each regression being zero.
 

This requirement was dropped because of the resulting low r-squared values
 

and because for any diurnal emissions we will require a difference between
 

the daily high and low temperatures of at least five degrees Fahrenheit.
 
-29- DRAFT
 

July 13, 1999
 

We first, therefore, attempted to enlarge the scope of the data by
 

estimating the diurnal emissions at the two missing extreme
 

values. We did this by observing that the diurnal emissions of
 

the 1986-95 carbureted vehicles (at the four tested combinations
 

of fuel RVP and temperature cycle) were between the corresponding
 

diurnal emissions of the 1986-95 FI vehicles and the 1980-85
 

carbureted vehicles for each tested combination of fuel RVP,
 

temperature cycle, and purge/pressure result. If this pattern were
 

to hold true for the two untested combinations, then the diurnal
 

emissions of the 1986-95 carbureted vehicles would be:
 

• for tests using 6.8 RVP fuel over the 60-86 °F cycle:
 

•• between 4.815 and 9.519 for vehicles failing the
 

pressure test,
 

•• between 4.372 and 5.100 for vehicles failing only the
 

purge test, and
 

•• between 0.187 and 2.976 for vehicles passing both the
 

pressure and the purge tests.
 

• for tests using 9.0 RVP fuel over the 82-106 °F cycle:
 

•• between 28.26 and 45.456 for vehicles failing the
 

pressure test,
 

•• between 21.046 and 50.67 for vehicles failing only
 

the purge test, and
 

•• between 9.932 and 36.565 for vehicles passing both
 

the pressure and the purge tests.
 

We then experimented, using the tested values for the 1986-95
 

carbureted vehicles with the coefficients determined for the 1980­

85 carbureted vehicles and for the 1986-95 FI vehicles to
 

determine which set would most closely predict the preceding
 

estimates of the untested configurations. While neither set was
 

perfect, the coefficients developed for the 1986-95 FI vehicles
 

came closer and were selected.
 

Once the coefficient values of the equation were determined
 

for each of the 15 strata, we then transformed the constant term
 

(for each stratum) to minimize the sum of the differences between
 

the predicted and calculated diurnal emissions. The resulting
 

equations are given in Appendix E. The statistics associated with
 

the eight regressions are given in Appendix F.
 

In the five strata in which the vehicles passed both the
 

purge test and the pressure test, the data strongly suggest a nonlinear relationship (i.e., quadratic) between the diurnal
 

emissions and that "vapor pressure product" term. In the various
 

strata containing vehicles that failed either the purge or
 

pressure (or both) tests, the relationship between diurnal
 

emissions and the vapor pressure product term was sometimes linear
 

and sometimes non-linear.
 
-30- DRAFT
 

July 13, 1999
 

10.0 Gross Liquid Leakers
 

Three issues related to vehicles with gross liquid leaks need
 

to be addressed:
 

1) the frequency of the occurrence of gross liquid leakers 

(possibly as a function of vehicle age), 

2) the magnitude of the emissions from gross liquid 

leakers, and 

3) the effects of changes in vapor pressure on the diurnal 

and resting loss emissions of these gross liquid 

leakers. 

Analyses of these issues were hampered by a lack of a substantial
 

number of identified gross liquid leakers. We anticipate revising
 

the following initial estimates for MOBILE7 based on additional
 

data.
 

10.1 Frequency of Gross Liquid Leakers
 

In a concurrent report (Document Number M6.EVP.006, entitled
 

"Estimating Weighting Factors for Evaporative Emissions in
 

MOBILE6"), EPA first uses data from EPA testing programs, CRC
 

testing programs, and an American Petroleum Institute (API)
 

testing program to estimate the occurrence of the gross liquid
 

leakers at three different vehicle ages:
 

Frequency of
 

Vehicle "Gross Liquid


 Age Leakers" 


5.62 0.20%


 12.50 2.00%


 21.29 7.84%
 

EPA then found a logistic growth curve that closely approximates
 

these three values:
 

0.09063 Gross Liquid Leaker Rate = 1 + 337.2*exp[-0.3625 * AGE] 

In this analysis, vehicle age was estimated by subtracting
 

the model year from the test year. Since the test dates averaged
 

(both mean and median) early July, the preceding equation actually
 

estimates the occurrence of gross liquid leakers as of July of
 

each given calendar year. However, the MOBILE models base their
 

estimates as of January 1 of each calendar year. Therefore, EPA
 

proposes to modify the preceding equation so that its predictions
 

are based on January first:
 

0.09063 Gross Liquid Leaker Rate = 1 + 337.2*exp[-0.3625 * (AGE - 0.5)] 
-31- DRAFT
 

July 13, 1999
 

Plotting both the unmodified curve (i.e., based on ages as of
 

July) and the preceding set of three failure rates produces Figure
 

10-1 below:
 

Figure 10-1
 

Frequency of Gross Liquid Leakers
 

10% 

5% 

0% 

Frequency (%) 

0	 1 0 2 0 3 0
 

Vehicle Age (years) 

The dotted line in Figure 10-1 is the logistic growth
 

function. The rapidly increasing proportion of gross liquid
 

leakers in the in-use fleet tends to be offset by the decreasing
 

number of older vehicles in the in-use fleet. This graph (or the
 

preceding equation) predicts:


 •	 Fewer than one-half a percent of vehicles (at each age) up 

to eight years of age will be "gross liquid leakers."

 •	 "Gross liquid leakers" do not reach one percent of the 

fleet until the vehicles exceed 10 years of age.

 •	 "Gross liquid leakers" reach two percent of the fleet for 

vehicles exceeding 13 years of age.

 •	 The portion of the fleet that is "gross liquid leakers" 

then rises (almost linearly) to about eight percent for 

vehicles that are 22 years old.

 •	 The increase in the frequency of "gross liquid leakers" 

then levels off and the frequency approaches just over nine 

percent (about age 30). 
-32-	 DRAFT
 

July 13, 1999
 

It is important to note that this model of the frequency of
 

gross liquid leakers is based on the assumption that modern
 

technology vehicles will show the same tendency toward gross
 

liquid leaks as do the older technology vehicles at the same age.
 

However, if the modern technology vehicles were to exhibit a lower
 

tendency to leak (due to the more stringent demands imposed by the
 

new evaporative emissions certification procedure as well as
 

heightened attention to safety, e.g., fuel tank protection and
 

elimination of fuel line leaks), the effect would be to replace
 

that single logistic growth function with a family of two or three
 

curves.
 

Since EPA has no data to indicate that the multiple curve
 

scenario is the correct approach, EPA proposes to use the single
 

curve approach to estimate the occurrence in the in-use fleet of
 

these vehicles that have substantial leaks of liquid gasoline
 

(i.e., "gross liquid leakers").
 

10.2 Magnitude of Emissions from Gross Liquid Leakers
 

In Section 10.1, we concluded that the frequency of gross
 

liquid leakers is a function of vehicle age. The question as to
 

whether the magnitude of the emissions are also a function of age
 

cannot be answered with the available data.
 

Seven vehicles (five in the CRC study and two in the EPA
 

study) have been identified as gross liquid leakers. However, two
 

of the five CRC vehicles exhibited questionable results.
 

Specifically:
 

1)	 For vehicle number 9111, the RTD test was aborted after
 

only 16 hours due to the high evaporative emissions.
 

CRC used the emissions measured during the first 16
 

hours to estimate the emissions during the final eight
 

hours. (The cumulative HC through 16 hours was 616.71
 

grams which was extrapolated to 777.14 for the full 24
 

hours.) Therefore, the calculated resting loss
 

emissions (i.e., the mean of the untested hours 19
 

through 24) might be in error.
 

2)	 Vehicle number 9129 exhibited relatively normal
 

emissions for the about the first nine hours of the RTD
 

test, after which the hourly emissions quickly rose then
 

stabilized at about 11 grams per hour. This suggests
 

that the leak actually developed during the RTD test
 

(around the tenth hour). Therefore, while this
 

vehicle's resting losses (i.e., the mean of hours 19
 

through 24) were representative of other gross leakers,
 

the calculated diurnal emissions are likely not
 

representative of other gross leakers. (The calculated
 

resting loss emissions at 72°F from this vehicle were
 

10.77 grams per hour. Had that level of emissions
 

simply continued for the full 24 hours, the total
 

resting loss emissions would have been 258.48 grams
 

compared to the 181.79 grams actually measured for the
 
________________________________ 

-33- DRAFT
 

July 13, 1999
 

entire 24-hour RTD test. Computationally, this would
 

result in a substantial negative estimate of diurnal
 

emissions.)
 

An additional difficulty is caused by the two vehicles in the
 

EPA sample not being tested with the same fuel as the five CRC
 

test vehicles. However, since the major mechanism driving the
 

emissions of these vehicles is the leaks of liquid gasoline, the
 

effects of changes in temperature or fuel RVP should be relatively
 

small (see Section 10.3). If we, therefore, simply average the
 

emissions of these two vehicles, we obtain the following table:
 

Veh No 

5002 

RVP Temp Cycle 

9.0 72.to.96 

RTD 

91.09 

Hourly RstL 

1.88 

9.0 82.to.106 158.80 3.81 

Means: 124.95 2.85 

5082 6.3 72.to.96 54.80 1.45 

6.3 82.to.106 99.35 2.88 

9.0 72.to.96 87.26 2.07 

Means: 80.47 2.13 

If we then average the preceding two means with the results
 

from the five vehicles in the CRC sample (omitting the non-resting
 

loss data from vehicle 9129), we obtain:
 

Veh No 

9049 

RTD 

181.35 

Hourly RstL 

4.87 

9054 316.59 10.58 

9087 478.16 14.12 

9111 777.14 16.51 

9129 Ignore 10.77 

5002 122.01 2.96 

5082 77.58 2.09 

Means: 325.47 8.84 

Std Dev: 264.96 5.62 

A third complication becomes apparent when the hourly
 

emissions for these tests are examined.* Several of the tests
 

exhibit high and decreasing hourly emissions for the first three
 

hours. (We expected the tests to exhibit increasing emissions for
 

the first few hours.) EPA believes that the unexpectedly high
 

emissions for the first two hours resulted from the evaporation of
 

*
 A more thorough analysis of the hourly emissions is contained in report
 

M6.EVP.002.
 
-34-	 DRAFT
 

July 13, 1999
 

gasoline that had leaked prior to the start of the test.
 

Compensating for that (hypothesized) problem results in reducing
 

the above mean of the RTD emissions from 325.47 grams per day down
 

to 312.45 (a decrease of 13.01 grams).
 

On page 25, we noted that the daily resting loss emissions
 

(assuming a daily temperature profile similar to those in Appendix
 

A) would be 24 times the hourly resting loss (at the day's low
 

temperature) plus 0.766. Since including the 0.766 term will
 

increase the day's total resting loss less than 0.4 percent, , we
 

will assume the resting loss emissions are completely independent
 

of temperature (see Section 11.1). Therefore, based on the means
 

in the preceding table, we propose to use, in MOBILE6, for the
 

category of gross liquid leakers:
 

•	 DAILY RESTING LOSS = ( 24 * HOURLY RESTING LOSS )
 

= ( 24 * 8.84)
 

= 212.16 (GRAMS / DAY )
 

and
 

• Full Day's DIURNAL = MEAN RTD - DAILY RESTING LOSS
 

= 312.45 - 212.16
 

= 100.29 (GRAMS / DAY )
 

Thus, while the occurrence of these gross liquid leakers is
 

relatively rare among newer vehicles (Section 10.1), their
 

presence has a substantial effect on the total resting loss and
 

diurnal emissions of the in-use fleet.
 

10.3	 Effects of Vapor Pressure Changes on Gross Liquid
 

Leakers
 

As previously discussed, the true vapor pressure is a
 

function of both the ambient temperature and the Reid vapor
 

pressure of the fuel. Since only two of the seven vehicles that
 

have been identified as gross liquid leakers were tested over a
 

range of fuel RVPs, there are not enough data to relate changes in
 

diurnal and resting loss emissions to changes in fuel RVP.
 

However, as noted in the preceding section, changes in fuel RVP
 

are expected to have only minimal (proportional) effects on the
 

total diurnal and resting loss emissions of vehicles whose primary
 

mechanism of evaporative emissions is leaking liquid gasoline.
 

Thus, until additional data are available, EPA proposes to treat
 

the diurnal and resting loss emissions of the gross liquid leakers
 

as independent of fuel RVP.
 

In the previous section, EPA proposed to treat the hourly
 

resting emissions of these gross liquid leakers as if they are
 

independent of ambient temperature as well. In a concurrent
 

report (document number M6.EVP.002, entitled "Modeling Hourly
 

Diurnal Emissions and Interrupted Diurnal Emissions Based on
 

Real-Time Diurnal Data"), EPA was able to use the hourly diurnal
 

emissions to estimate the effects of temperature changes on the
 

diurnal emissions of these gross liquid leakers. That report
 

concludes that the full-day's diurnal emissions of gross liquid
 

leakers is dependent only upon the daily temperature range (i.e.,
 
-35-	 DRAFT
 

July 13, 1999
 

the difference between the daily high and low temperatures).
 

Thus, for any of the three temperature cycles in Appendix A, the
 

mean of the full-day's diurnal emissions of gross liquid leakers
 

is the constant 100.29 grams (calculated in the previous section).
 

Therefore, EPA is proposing that both the hourly resting loss
 

emissions and full-day's diurnal emissions of gross liquid leakers
 

are independent of vapor pressure for any of the three temperature
 

cycles in Appendix A.
 

11.0 Other Topics
 

Several topics were not discussed in the preceding analysis
 

because either:
 

1) They will be discussed in forthcoming reports. 

or 

2) No changes are planned in how they were handled in 

MOBILE5. 

11.1 Temperature Ranges
 

All of the tests used in this analysis were performed using
 

one of the three temperature cycles in Appendix A. This results
 

in all of the resting loss data being at only three temperatures
 

(i.e., 60, 72, and 82 °F). In Section 8, we developed regression
 

equations to estimate hourly resting loss emissions at
 

theoretically any temperature. We will limit that potentially
 

infinite temperature range as we did in the previous version of
 

MOBILE, specifically:
 

1)	 We will assume, for light-duty vehicles other than gross
 

liquid leakers, there are no resting loss emissions when the
 

temperatures are below or equal to 40°F. (This assumption
 

was used consistently for all evaporative emissions in
 

MOBILE5.)
 

2)	 We will assume, for light-duty vehicles other than gross
 

liquid leakers, that when the ambient temperatures are above
 

105°F that the resting loss emissions are the same as those
 

calculated at 105°F.
 

Since vehicles classified as gross liquid leakers were not handled
 

separately in MOBILE5, we will now make a new assumption
 

concerning those vehicles' emission performance as relates to
 

temperatures. Specifically:
 

3)	 For the vehicles classified as gross liquid leakers, we will
 

assume the resting loss emissions are completely independent
 

of temperature, averaging 8.84 grams per hour.
 

The equations developed in this report to estimate hourly
 

diurnal emissions theoretically could also be applied to any
 
-36- DRAFT
 

July 13, 1999
 

temperature cycle. We will limit those functions by assuming that
 

the 24-hour diurnal emissions will be zero for any temperature
 

cycle in which the difference between the daily high and low
 

temperatures (i.e., the "diurnal temperature range") is less than
 

five degrees Fahrenheit.
 

11.2 Heavy-Duty Vehicles (HDGVs)
 

The analyses in this report were based only on RTD tests of
 

light-duty gasoline-powered vehicles (LDGVs) and light-duty
 

gasoline-powered trucks (LDGTs). Since the data did not indicate
 

a significant difference between the RTD emissions from LDGVs and
 

LDGTs, they were combined in a single group of analyses.
 

Since no RTD testing was performed on any HDGVs, we will use
 

the same approach that was used in the earlier version of MOBILE.
 

That is, the ratio of diurnal emissions of the HDGVs to those of
 

the LDGTs is proportional to both the corresponding ratios of the
 

evaporative emission standards and the corresponding market shares
 

(under each of the emission standards). Translating that sentence
 

into an equation yields:
 

DIHDGV = DILDGT * [ ( 1.5 * 0.875 ) + ( 2.0 * 0.125 ) ] 

= 1.5625 * DILDGT 

Where, DIHDGV is the full day's diurnal 

emissions from the HDGVs. 

DILDGT is the full day's diurnal 

emissions from the corresponding 

LDGTs. 

We will use the same formula for resting losses (obviously 

changing DI to "hourly resting losses"). 

11.3 High Altitude Evaporative Emissions
 

We will continue to use the multiplicative adjustment factor 

of 1.30 (from previous version of MOBILE) to adjust both the 

resting loss and diurnal emissions for high altitude. 

11.4 Motorcycles (MC)
 

RTD evaporative emission tests were not performed on
 

motorcycles (MC). In MOBILE5, the resting loss and diurnal
 

emissions from motorcycles were modeled using carbureted vehicles
 

equipped with open-bottom canisters. That approach will continue
 

with MOBILE6.
 

We first identified 109 RTD tests of carbureted vehicles
 

equipped with open-bottom canisters (all 1988 or earlier model
 

years), and calculated both the hourly resting loss (associated
 

with the test's low temperature) and the full-day's diurnal for
 

each of those 109 tests. The diurnal emissions were then regressed
 
 

-37- DRAFT
 

July 13, 1999
 

against both the vapor pressure product term (developed in Section
 

9) and the age of each test vehicle. As illustrated in Table 11­

1, each of those variables is statistically significant. MOBILE6
 

will use the linear regression equation generated by that analysis
 

to calculate the full day's diurnal emissions.
 

Table 11-1
 

Regression of Diurnal Emissions
 

(Simulated Motorcycle Fleet)
 

Dependent variable is: Diurnal 

No Selector 

R squared = 59.0% R squared (adjusted) = 58.3% 

s = 10.20 with 109 - 3 = 106 degrees of freedom 

Source Sum of Squares df Mean Square F-ratio 

Regression 15892.9 2 7946.46 76.4 

Residual 11024.5 106 104.005 

Variable Coefficient s.e. of Coeff t-ratio prob 

Constant -36.7971 4.5620 -8.07 ≤ 0.0001 

age 0.855491 0.1894 4.52 ≤ 0.0001 

VP_Product 0.058251 0.0051 11.5 ≤ 0.0001 

Translating that regression analysis into an equation yields:


 24-Hour Diurnal Emissions (grams) of Motorcycles

 = -36.7971 + ( 0.855491 * Vehicle_ Age )

 + ( 0.058251 * VP_Product _Term ) 

EPA proposes to use this equation to estimate the 24-hour diurnal
 

emissions from motorcycles.
 

Similarly, the hourly resting loss emissions were regressed
 

against both the temperature at which those values were calculated
 

(i.e., the daily low temperature) and the age of each test
 

vehicle. As illustrated in Table 11-2, only the vehicle age is
 

statistically significant. It is possible that temperature was
 

not found to be statistically significant simply due to the fact
 

that most of the resting loss emissions were calculated at the
 

same temperature (72 °F). Since temperature should be an
 

important factor in determining resting loss emissions, EPA
 

proposes to use for MOBILE6 the linear regression equation
 

generated by the analysis (in Table 11-2) that uses both
 

variables.
 
 

-38- DRAFT
 

July 13, 1999
 

Table 11-2
 

Regression of Hourly Resting Loss Emissions
 

(Simulated Motorcycle Fleet)
 

Dependent variable is: Hourly Resting Loss 

No Selector 

R squared = 5.6% R squared (adjusted) = 3.8% 

s = 0.1346 with 109 - 3 = 106 degrees of freedom 

Source Sum of Squares df Mean Square F-ratio 

Regression 0.114078 2 0.057039 3.15 

Residual 1.92123 106 0.018125 

Variable Coefficient s.e. of Coeff t-ratio prob 

Constant 0.044345 0.1572 0.282 0.7784 

age 0.006134 0.0025 2.45 0.0159 

Daily_Low 0.000859 0.0022 0.399 0.6909 

Temp 

Translating that regression analysis into an equation yields:


 Hourly Resting Loss Emissions (grams) of Motorcycles

 = 0.044345 + ( 0.006134 * Vehicle_ Age )

 + ( 0.000859 * Daily_Low _Temperature ) 

EPA proposes to use this equation to estimate the hourly resting
 

loss emissions from motorcycles.
 

11.5 Pre-Control Vehicles
 

Non-California vehicles prior to the 1972 model year were not
 

required to meet an evaporative emission standard. These
 

uncontrolled vehicles would simply vent vapors to the atmosphere
 

as pressure built up. Since that situation is similar to that of
 

a controlled vehicle with a vapor leak, we hypothesized that the
 

resting loss and diurnal evaporative emissions of the pre-1972
 

vehicles would be comparable to the emissions of the pre-1980
 

vehicles that had failed the pressure test.
 

To characterize the hourly resting loss emissions from these
 

pre-control vehicles, we proceeded in a similar fashion to the
 

approach in Section 8. We first identified the two pre-1980
 

vehicles in our study that both had failed the pressure test and
 

were tested over the full range of fuels and temperature cycles.
 

Possibly due to that small sample size, a regression of those data
 

produced a slope of resting loss versus temperature that was not
 

statistically different from zero. We, therefore, decided to use
 

the same slope (0.002812) that was developed in Section 8. Since
 
 

-39- DRAFT
 

July 13, 1999
 

most of the RTD tests (i.e., 37 of 47) that were performed on the
 

34 candidate vehicles were run over the same temperature cycle
 

(i.e., 72 to 96 degrees), the variable "temperature" would not
 

make a useful independent variable to analyze those 47 resting
 

loss results. However, the variable "age" was found to be
 

statistically significant. Combining the results of regressing
 

the data against age with the previously calculated temperature
 

slope yields the following equation:
 

Hourly Resting Loss (grams) = -0.768438 

+ ( 0.002812 * Temperature in °F ) 

+ ( 0.040528 * Vehicle Age in Years ) 

EPA proposes to use this equation to estimate the hourly resting
 

loss emissions from pre-control vehicles with the restriction that
 

the calculated value must be at least the estimated hourly resting
 

loss of the (newer) 1972-79 model year vehicles (as calculated in
 

Appendix D).
 

To characterize the full day's diurnal emissions from these
 

pre-control vehicles, we proceeded in a similar fashion to the
 

approach in Section 9. In the preceding paragraph we noted that
 

only two of the candidate vehicles (i.e., pre-1980 vehicles that
 

failed the pressure test) were tested over the full range of fuels
 

and temperature cycles. Attempting to analyze the resting loss
 

emissions of those two vehicles as a function of temperature
 

produced only mediocre results. However, the corresponding
 

analysis for diurnal emissions as a function of the vapor pressure
 

product term produced satisfactory results, as shown in Table 11-3:
 

Table 11-3
 

Regression of Diurnal Emissions
 

(Simulated Pre-Control Fleet)
 

(Based on Two Vehicles)
 

Dependent variable is: Diurnal 

No Selector 

R squared = 92.3% R squared (adjusted) = 90.4% 

s = 5.503 with 6 - 2 = 4 degrees of freedom 

Source Sum of Squares df Mean Square F-ratio 

Regression 1456.41 1 1456.41 48.1 

Residual 121.136 4 30.284 

Variable Coefficient s.e. of Coeff t-ratio prob 

Constant -6.52265 6.175 -1.06 0.3504
 

VP_Product 0.05115 0.0074 6.93 0.0023
 
-40- DRAFT
 

July 13, 1999
 

Similar to the statements in the preceding material on the
 

resting loss emissions from these test vehicles, the diurnal
 

emissions from these tests are almost exclusively from tests
 

performed over the 72 to 96 degree cycle using a single fuel RVP.
 

Thus, using a variable for vapor pressure for the full set of 47
 

tests would not be productive. However, as with the resting loss
 

emissions, we used the preceding coefficient (0.05115) to estimate
 

diurnal emissions (based on the vapor pressures) and then regress
 

the calculated residuals against vehicle age. That regression
 

analysis yields the following equation:
 

24-Hour Diurnal (grams) = -40.67512 

+ ( 0.05115 * VP Product Term ) 

+ ( 1.41114 * Vehicle Age in Years ) 

EPA proposes to use this equation to estimate the 24-hour diurnal
 

emissions from pre-control vehicles with the restriction that the
 

calculated value must be at least the estimated full-day's diurnal
 

of the (newer) 1972-79 model year vehicles (as calculated in
 

Appendix E).
 

11.6 Duration of Diurnal Soak Period
 

The analyses in this report were based on diurnals of exactly
 

24 hours in length. In the real-world, the soak period could run
 

for longer or shorter periods of time.
 

Estimating diurnal emissions when the soak period is less
 

than 24 hours are analyzed in report number M6.EVP.002 (entitled
 

"Modeling Hourly Diurnal Emissions and Interrupted Diurnal
 

Emissions Based on Real-Time Diurnal Data").
 

Estimating diurnal emissions when the soak period is more
 

than 24 hours are analyzed in report number M6.EVP.003 (entitled
 

"Evaluating Multiday Diurnal Evaporative Emissions Using RTD
 

Tests").
 

11.7 1996 and Newer Model Year Vehicles
 

Starting with the 1996 model year, EPA began certifying some
 

of the new LDGVs and LDGTs using the RTD test. Estimating the
 

resting loss and diurnal emissions from these vehicles will be
 

analyzed in report number M6.EVP.005.
 
 

-41- DRAFT
 

July 13, 1999
 

Appendix A
 

Temperature Cycles (°F)


 ---Temperatures Cycling Between --­ Change in 

Hour 60°-84°F 72°-96°F * 82°-106°F Temperature 

0 60.0 72.0 82.0 - - -

1 60.5 72.5 82.5 0.5 

2 63.5 75.5 85.5 3.0 

3 68.3 80.3 90.3 4.8 

4 73.2 85.2 95.2 4.9 

5 77.4 89.4 99.4 4.2 

6 81.1 93.1 103.1 3.7 

7 83.1 95.1 105.1 2.0 

8 83.8 95.8 105.8 0.7 

9 84.0 96.0 106.0 0.2 

10 83.5 95.5 105.5 -0.5 

11 82.1 94.1 104.1 -1.4 

12 79.7 91.7 101.7 -2.4 

13 76.6 88.6 98.6 -3.1 

14 73.5 85.5 95.5 -3.1 

15 70.8 82.8 92.8 -2.7 

16 68.9 80.9 90.9 -1.9 

17 67.0 79.0 89.0 -1.9 

18 65.2 77.2 87.2 -1.8 

19 63.8 75.8 85.8 -1.4 

20 62.7 74.7 84.7 -1.1 

21 61.9 73.9 83.9 -0.8 

22 61.3 73.3 83.3 -0.6 

23 60.6 72.6 82.6 -0.7 

24 60.0 72.0 82.0 -0.6 

*	 The temperature versus time values for the 72-to-96 cycle are 

reproduced from Table 1 of Appendix II of 40 CFR 86. 

These three temperature cycles are parallel (i.e., identical
 

hourly increases/decreases). The temperatures peak at hour nine.
 

The most rapid increase in temperatures occurs during the third
 

and fourth hours.
 

For cycles in excess of 24 hours, the pattern is repeated.
 
 

________________________________ 

-42­

Appendix B 

DRAFT 

July 13, 1999 

Vapor Pressure 

Using the Clausius-Clapeyron Relationship 

The Clausius-Clapeyron relationship is a reasonable estimate
 

of vapor pressure over the moderate temperature range (i.e., 60°
 

to 106°F)* being considered for adjusting the diurnal emissions.
 

This relationship assumes that the logarithm of the vapor pressure
 

is a linear function of the reciprocal (absolute) temperature.
 

In a previous EPA work assignment, similar RVP fuels were
 

tested, and their vapor pressures (in kilo Pascals) at three
 

temperatures were measured. The results of those tests are given
 

in the following table:
 

Nominal Measured Vapor Pressure (kPa)

RVP RVP 75° F 100° F* * 130° F 

7.0 7.1 30.7 49.3 80.3 

9.0 8.7 38.2 60.1 96.5 

** The VPs at 100° F are the fuels' RVPs (in kilo Pascals).
 

Plotting these six vapor pressures (using a logarithm scale for
 

the vapor pressure) yields the graph (Figure B-1) on the following
 

page.
 

For each of those two RVP fuels, the Clausius-Clapeyron
 

relationship estimates that, for temperature in degrees Kelvin,
 

the vapor pressure (VP) in kPa will be:
 

Ln(VP) = A + (B / Absolute Temperature), where:
 

RVP A B 

8.7 13.5791 -2950.47 

7.1 13.7338 -3060.95 

* C. Lindhjem and D. Korotney, "Running Loss Emissions from Gasoline-Fueled
 

Motor Vehicles", SAE Paper 931991, 1993.
 
 

Vapor Pressure (kPa) 

RVP 8.7 

RVP 7.1 

0.0030 0.0031 0.0032 0.0033 0.0034
 

-43- DRAFT
 

July 13, 1999
 

Figure B-1
 

Comparison of Vapor Pressure to Temperature
 

100 

10 

Reciprocal of Temp (1/°K) 

We will assume that the specific fuels used in the vehicles that
 

were tested in this analysis had vapor pressure versus temperature
 

curves similar to the curves for these to two test fuels.
 

Extrapolating the trends in either the "A" or "B" values to fuels
 

with nominal RVPs of 6.3, 7.0, and 9.0 psi; and then requiring the
 

lines (in log-space) to pass through the appropriate pressures at
 

100°F, yields the linear equations with coefficients:
 

RVP A B 

6.3 13.810 -3121.05 

6.8 13.773 -3085.79 

9.0 13.554 -2930.67 

We will use the above to estimate vapor pressures for the 6.3,
 

6.8, and 9.0 psi RVP fuels.
 

In general, given the fuel RVP, we can approximate A and B with 

these equations: 

B = -3565.2707 + ( 70.5114 * RVP ) 

and
 

A = Ln( 6.89286 * RVP ) - ( B / 310.9 ) 
-44- DRAFT
 

July 13, 1999
 

Appendix C
 

Mean Evaporative Emissions by Strata
 

By Vapor Pressure Products
 

Strata 

Pre-1980 Carburete

 Fail Purge/

 Fail Pressu

Fuel 

RVP 

d 6.8 

re 

Temp. 

Cycle 

72.TO.96 

VP 

times 

?VP 

567.02 

Count 

13 

Mean 

Diurnal 

25.111 

Mean 

Hourly 

Resting 

Loss 

0.452

Pre-1980 Carburete

 Fail Purge/ 

Pass Pressure 

d 6.8 60.TO.84 374.77 1 16.229 0.250

6.8 72.TO.96 567.02 7 21.055 0.307

9.0 60.TO.84 655.07 1 17.511 0.218 

6.8 82.TO.106 789.30 1 36.321 0.204 

9.0 72.TO.96 968.66 1 44.222 0.250 

9.0 82.TO.106 1323.87 1 76.801 0.259 

Pre-1980 Carburete

 Pass Purge/ 

Fail Pressu

d 6.8 60.TO.84 374.77 2 21.284 0.238

6.3 72.TO.96 489.32 1 17.426 0.140

re 6.8 72.TO.96 567.02 20 24.385 0.227 

9.0 60.TO.84 655.07 3 21.572 0.103 

6.3 82.TO.106 683.98 1 24.328 0.175 

6.8 82.TO.106 789.30 2 42.799 0.174 

9.0 72.TO.96 968.66 3 35.331 0.107 

9.0 82.TO.106 1323.87 2 72.263 0.274 

Pre-1980 Carburete

 Pass Purge/ 

Pass Pressur

d 6.8 60.TO.84 374.77 1 7.861 0.167

6.8 72.TO.96 567.02 11 13.240 0.263

e 9.0 60.TO.84 655.07 1 17.423 0.239 

6.8 82.TO.106 789.30 1 32.292 0.293 

9.0 72.TO.96 968.66 1 38.297 0.204 

9.0 82.TO.106 1323.87 1 100.094 0.062 

1980-85 Carbureted 

Fail Purge/

 Fail Pressu

6.8 

re 

72.TO.96 567.02 1 27.401 0.265

1980-85 Carbureted 

Fail Purge/ 

Pass Pressur

6.8 60.TO.84 374.77 3 8.834 0.124

6.3 72.TO.96 489.32 1 16.541 0.185

e 6.8 72.TO.96 567.02 11 17.756 0.163 

9.0 60.TO.84 655.07 4 16.823 0.172 

6.3 82.TO.106 683.98 1 14.962 0.146 

6.8 82.TO.106 789.30 3 19.669 0.169 

9.0 72.TO.96 968.66 4 25.415 0.163 

9.0 82.TO.106 1323.87 3 55.324 0.162 

1980-85 Carbureted 

Pass Purge/ 

Fail Pressu

6.8 60.TO.84 374.77 2 13.383 0.121

6.3 72.TO.96 489.32 1 20.741 0.253

re 6.8 72.TO.96 567.02 8 16.508 0.139 

9.0 60.TO.84 655.07 3 27.768 0.127 

6.3 82.TO.106 683.98 1 43.384 0.444 

6.8 82.TO.106 789.30 2 31.965 0.216 

9.0 72.TO.96 968.66 3 45.319 0.276 

9.0 82.TO.106 1323.87 2 53.615 0.308 
-45- DRAFT
 

July 13, 1999
 

Mean Evaporative Emissions by Strata
 

By Vapor Pressure Products (continued)
 

Strata 

1980-85 Carbureted 

Pass Purge/ 

Pass Pressur

Fuel 

RVP 

6.8 

Temp. 

Cycle 

60.TO.84 

VP 

times 

?VP 

374.77 

Count 

3 

Mean 

Diurnal 

5.302 

Mean 

Hourly 

Resting 

Loss 

0.065

6.3 72.TO.96 489.32 3 16.308 0.195

e 6.8 72.TO.96 567.02 38 9.081 0.107 

9.0 60.TO.84 655.07 7 11.352 0.147 

6.3 82.TO.106 683.98 3 22.047 0.170 

6.8 82.TO.106 789.30 4 14.999 0.169 

9.0 72.TO.96 968.66 7 21.089 0.194 

9.0 82.TO.106 1323.87 3 43.900 0.274 

1986+ Carbureted 

Fail Purge/

 Fail Pressu

N/A 

re 

N/A N/A 0 N/A N/A

1986+ Carbureted 

Fail Purge/ 

Pass Pressur

6.8 72.TO.96 567.02 1 10.230 0.100

9.0 60.TO.84 655.07 1 12.840 0.097

e 6.8 82.TO.106 789.30 1 25.720 0.155 

9.0 72.TO.96 968.66 1 17.670 0.148 

1986+ Carbureted 

Pass Purge/ 

Fail Pressu

6.8 72.TO.96 567.02 2 15.865 0.233

9.0 60.TO.84 655.07 2 21.765 0.342

re 6.8 82.TO.106 789.30 2 21.480 0.124 

9.0 72.TO.96 968.66 2 26.265 0.308 

1986+ Carbureted 

Pass Purge/ 

Pass Pressur

6.8 72.TO.96 567.02 10 9.481 0.138

9.0 60.TO.84 655.07 1 6.440 0.092

e 6.8 82.TO.106 789.30 1 8.630 0.102 

9.0 72.TO.96 968.66 1 8.140 0.075 

1980-85 Fuel Inje

 Fail Purge/

 Fail Pressu

N/A 

re 

cted N/A N/A 0 N/A N/A

1980-85 Fuel Inje

 Fail Purge/ 

Pass Pressur

cted6.8 60.TO.84 374.77 3 4.329 0.010

6.8 72.TO.96 567.02 3 7.910 0.011

e 9.0 60.TO.84 655.07 4 6.556 0.045 

6.8 82.TO.106 789.30 3 10.744 0.041 

9.0 72.TO.96 968.66 4 11.506 0.086 

9.0 82.TO.106 1323.87 4 26.730 0.123 

1980-85 Fuel Inje

 Pass Purge/ 

Fail Pressu

cted6.8 60.TO.84 374.77 2 19.624 0.198

6.8 72.TO.96 567.02 3 19.482 0.206

re 9.0 60.TO.84 655.07 2 25.861 0.184 

6.8 82.TO.106 789.30 2 39.424 0.300 

9.0 72.TO.96 968.66 2 39.065 0.231 

9.0 82.TO.106 1323.87 2 50.255 0.252 

1980-85 Fuel Inje

 Pass Purge/ 

Pass Pressur

cted6.8 60.TO.84 374.77 1 12.943 0.296

6.8 72.TO.96 567.02 4 8.541 0.080

e 9.0 60.TO.84 655.07 2 7.845 0.157 

6.8 82.TO.106 789.30 2 11.861 0.218 

9.0 72.TO.96 968.66 2 13.330 0.227 

9.0 82.TO.106 1323.87 1 25.503 0.348 
-46- DRAFT
 

July 13, 1999
 

Mean Evaporative Emissions by Strata
 

By Vapor Pressure Products (continued)
 

Strata 

1986+ Fuel Inject

 Fail Purge/

 Fail Pressu

Fuel 

RVP 

ed N/A 

re 

Temp. 

Cycle 

N/A 

VP 

times 

?VP 

N/A 

Count 

0 

Mean 

Diurnal 

N/A 

Mean 

Hourly 

Resting 

Loss 

N/A

1986+ Fuel Inject

 Fail Purge/ 

Pass Pressur

e 60.TO.84 321.73 3 3.002 -0.009

6.8 

d 6.3 

60.TO.84 374.77 12 5.413 0.011

e 6.3 72.TO.96 489.32 5 6.027 0.024 

6.8 72.TO.96 567.02 18 9.083 0.060 

9.0 60.TO.84 655.07 17 7.802 0.034 

6.3 82.TO.106 683.98 5 11.068 0.064 

6.8 82.TO.106 789.30 15 14.498 0.073 

9.0 72.TO.96 968.66 17 11.734 0.056 

9.0 82.TO.106 1323.87 12 23.895 0.087 

1986+ Fuel Inject

 Pass Purge/ 

Fail Pressu

e 60.TO.84 321.73 1 5.206 0.037

6.8 

d 6.3 

60.TO.84 374.77 12 6.600 0.042

re 6.3 72.TO.96 489.32 4 10.259 0.038 

6.8 72.TO.96 567.02 19 9.202 0.094 

9.0 60.TO.84 655.07 19 8.611 0.053 

6.3 82.TO.106 683.98 4 14.842 0.088 

6.8 82.TO.106 789.30 16 15.824 0.110 

9.0 72.TO.96 968.66 19 16.193 0.114 

9.0 82.TO.106 1323.87 12 32.116 0.129 

1986+ Fuel Inject

 Pass Purge/ 

Pass Pressur

e 60.TO.84 321.73 2 0.602 -0.001

6.8 

d 6.3 

60.TO.84 374.77 16 1.611 0.027

e 6.3 72.TO.96 489.32 6 2.345 0.032 

6.8 72.TO.96 567.02 69 7.166 0.062 

9.0 60.TO.84 655.07 31 2.398 0.034 

6.3 82.TO.106 683.98 6 3.576 0.049 

6.8 82.TO.106 789.30 24 5.487 0.073 

9.0 72.TO.96 968.66 31 4.426 0.064 

9.0 82.TO.106 1323.87 21 13.640 0.123 
 

 

-47­ DRAFT 

July 13, 1999 

Appendix D 

Modeling Hourly Resting Loss Emissions
 

As Functions of Temperature (°F)
 

In each of the following 12 strata, resting loss emissions (i
 

per hour) are modeled using a pair of numbers (A and B), where


 Hourly Resting Loss (grams) = A + ( B * Temperature in °F ) 

Where 

B = 0.002812 (for ALL strata) and 

"A" is given in the following table: 

Fuel Delivery 

Carbureted 

Model Year

 Range 

Pre-1980 

Pass Pressure

 Test 

0.05530 

Fail Pressure

 Test 

0.07454 

1980-1985 -0.05957 -0.02163 

1986-1995 -0.07551 0.05044 

Fuel Injected Pre-1980* 0.05530 0.07454 

1980-1985 -0.09867 0.02565 

1986-1995 -0.14067 -0.10924

 *	 The untested stratum (Pre-1980 FI vehicles) was represented 

using the Pre-1980 model year carbureted vehicles. 

If we use any temperature profile in which the hourly change in
 

temperature is proportional to the cycles in Appendix A, we find:
 

24-Hour Resting Loss (grams) = ( 24 * Hourly_Resting_Loss_at_Low_Temp) 

+ ( 0.03193 * Diurnal_Temperature _Range ) 

Where B is given above, and where the Diurnal_Temperature_Range is 

the difference of the daily high temperature minus the daily low 

temperature. 
 

 

 

-48-	 DRAFT
 

July 13, 1999
 

Appendix E
 

Modeling 24-Hour Diurnal Emissions
 

As Functions of Vapor Pressure (kPa)and RVP (psi)
 

In each of the following 18 strata, 24-hour diurnal emissions
 

modeled using four constants:
 

A ,
 

B,
 

C, and
 

D. Where,


 24-Hour Diurnal (grams) =
 

= A
 

+ B * RVP (in psi) 

+ C * [(Mean VP) * (Change in VP)] 

+ D * [(Mean VP) * (Change in VP)]2 / 1,000 

For each of the 18 strata, the four constants used to model 


emissions are given below in the following table:
 

Fuel Delivery 

Carbureted 

Model Year

 Range 

1972-79* 

Fail 

Pressure

 Test 

-0.29374 

-0.62160 

0.039905 

0 

Fail Only 

Purge Test 

21.94883 

-2.23907 

0 

0.02990 

Pass Both 

Purge and 

Pressure 

21.13354 

-2.42617 

0 

0.024053 

1980-1985 -1.22213 16.69934 15.50536 

-0.62160 -2.23907 -2.42617 

0.039905 0 0 

0 0.02990 0.024053 

1986­ 18.97709 13.90647 8.37118 

1995** -1.81237 -2.14898 -0.767027 

0 0.021368 0 

0.017098 0 0.005934

 *	 The B, C, and D values are based on 1980-85 carbureted
 

vehicles.


 ** The B, C, and D values are based on 1986-95 FI vehicles. 
 

 

 

-49-	 DRAFT
 

July 13, 1999
 

Appendix E (continued)
 

Modeling 24-Hour Diurnal Emissions
 

As Functions of Vapor Pressure (kPa)
 

In each of the following 18 strata, 24-hour diurnal emissions
 

modeled using four constants:
 

A ,B, C, D. Where,
 

24-Hour Diurnal (grams) = 

= A 

+ B * RVP (in psi) 

+ C * [(Mean VP) * (Change in VP)] 

+ D * [(Mean VP) * (Change in VP)]2 / 1,000 

Fuel Delivery 

Fuel Injected 

Model Year

 Range 

1972-79* 

Fail 

Pressure

 Test 

-0.29374 

-0.62160 

0.039905 

0 

Fail Only 

Purge Test 

21.94883 

-2.23907 

0 

0.02990 

Pass Both 

Purge and 

Pressure 

21.13354 

-2.42617 

0 

0.024053 

1980-1985 7.11253 7.48130 5.62111 

-1.25128 -0.701002 -0.701002 

0.036373 0 0 

0 0.010466 0.010466 

1986-1995 14.19286 9.93656 5.85926 

-1.81237 -2.14898 -0.767027 

0 0.021368 0 

0.017098 0 0.005934

 *	 The three untested strata of Pre-1980 FI vehicles were 

represented using the Pre-1980 model year carbureted 

vehicles (which were themselves based on the 1980-85 model 

year carbureted vehicles). 
 

 

-50­ DRAFT 

July 13, 1999 

Appendix F 

Regression Analyses of 24-Hour Diurnal versus Fuel RVP
 

and Vapor Pressure Product Term
 

Regression of Mean Diurnal Emissions
 

Based on Three 1980-85 Carb Vehicles
 

Passing Both Purge and Pressure Tests
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 97.1% R squared (adjusted) = 95.2% 

s = 2.754 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

765.294 

22.76 

df 

2 

3 

Mean Square 

382.647 

7.58666 

F-ratio 

50.4 

Variable 

Constant 

VP_Product 

Sqrd / 1,000 

Fuel RVP 

Coefficient 

14.3895 

0.024053 

-2.42617 

s.e. of Coeff 

9.439 

0.0027 

1.326 

t-ratio 

1.52 

8.78 

-1.83 

prob 

0.2248 

0.0031 

0.1648 

Regression of Mean Diurnal Emissions
 

Based on Two 1980-85 Carb Vehicles
 

Failing the Pressure Test
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 99.4% R squared (adjusted) = 99.0% 

s = 1.307 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

822.877 

5.12862 

df 

2 

3 

Mean Square 

411.438 

1.70954 

F-ratio 

2 4 1 

Variable 

Constant 

VP_Product 

Term 

Fuel RVP 

Coefficient 

-1.00903 

0.039905 

-0.621600 

s.e. of Coeff 

4.18 

0.0023 

0.650 

t-ratio 

-0.241 

17.0 

-0.956 

prob 

0.8250 

0.0004 

0.4096 
 

 

-51- DRAFT
 

July 13, 1999
 

Appendix F (continued)
 

Regression of Mean Diurnal Emissions
 

Based on Three 1980-85 Carb Vehicles
 

Failing ONLY the Purge Test
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 94.7% R squared (adjusted) = 91.1% 

s = 4.853 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

1256.90 

70.6578 

df 

2 

3 

Mean Square 

628.449 

23.5526 

F-ratio 

26.7 

Variable 

Constant 

VP_Product 

Sqrd / 1,000 

Fuel RVP 

Coefficient 

15.3041 

0.029900 

-2.23907 

s.e. of Coeff 

16.6300 

0.0048 

2.3370 

t-ratio 

0.920 

6.19 

-0.958 

prob 

0.4253 

0.0085 

0.4087 

Regression of Mean Diurnal Emissions
 

Based on Four 1980-85 FI Vehicles
 

Passing the Pressure Test
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 99.6% R squared (adjusted) = 99.3% 

s = 0.4728 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

156.976 

0.670742 

df 

2 

3 

Mean Square 

78.4882 

0.223581 

F-ratio 

3 5 1 

Variable 

Constant 

VP_Product 

Sqrd / 1,000 

Fuel RVP 

Coefficient 

7.29846 

0.010466 

-0.701002 

s.e. of Coeff 

1.620 

0.0005 

0.2277 

t-ratio 

4.50 

22.2 

-3.08 

prob 

0.0204 

0.0002 

0.0542 
 

 

-52- DRAFT
 

July 13, 1999
 

Appendix F (continued)
 

Regression of Mean Diurnal Emissions
 

Based on Two 1980-85 FI Vehicles
 

Failing the Pressure Test
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 94.4% R squared (adjusted) = 90.7% 

s = 3.511 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

626.019 

36.9725 

df 

2 

3 

Mean Square 

313.009 

12.3242 

F-ratio 

25.4 

Variable 

Constant 

VP_Product 

Term 

Fuel RVP 

Coefficient 

7.82649 

0.036373 

-1.25128 

s.e. of Coeff 

11.23 

0.0063 

1.746 

t-ratio 

0.697 

5.77 

-0.717 

prob 

0.5361 

0.0104 

0.5253 

Regression of Mean Diurnal Emissions
 

Based on 16 1986-95 FI Vehicles
 

Passing Both Purge and Pressure Tests
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 97.1% R squared (adjusted) = 95.2% 

s = 0.6560 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

43.7687 

1.29117 

df 

2 

3 

Mean Square 

21.8844 

0.43039 

F-ratio 

50.8 

Variable 

Constant 

VP_Product 

Sqrd / 1,000 

Fuel RVP 

Coefficient 

4.70657 

0.005934 

-0.767027 

s.e. of Coeff 

2.248 

0.0007 

0.3159 

t-ratio 

2.09 

9.09 

-2.43 

prob 

0.1273 

0.0028 

0.0935 
 

 

-53- DRAFT
 

July 13, 1999
 

Appendix F (continued)
 

Regression of Mean Diurnal Emissions
 

Based on 11 1986-95 FI Vehicles
 

Failing the Pressure Test
 

Dependent variable is: 

No Selector 

Diurnal 

R squared = 98.9% R squared (adjusted) = 98.1% 

s = 1.206 with 6 - 3 = 3 degrees of freedom 

Source 

Regression 

Residual 

Sum of Squares 

382.227 

4.36316 

df 

2 

3 

Mean Square 

191.113 

1.45439 

F-ratio 

1 3 1 

Variable 

Constant 

VP_Product 

Sqrd / 1,000 

Fuel RVP 

Coefficient 

14.5718 

0.017098 

-1.81237 

s.e. of Coeff 

4.1330 

0.0012 

0.5807 

t-ratio 

3.53 

14.2 

-3.12 

prob 

0.0388 

0.0007 

0.0524 

Regression of Mean Diurnal Emissions
 

Based on 12 1986-95 FI Vehicles
 

Failing ONLY the Purge Test
 

Dependent variable is: Diurnal 

No Selector 

R squared = 95.7% R squared (adjusted) = 92.8% 

s = 1.578 with 6 - 3 = 3 degrees of freedom 

Source Sum of Squares df Mean Square F-ratio 

Regression 164.793 2 82.3963 33.1 

Residual 7.47312 3 2.49104 

Variable Coefficient s.e. of Coeff t-ratio prob 

Constant 11.0427 5.050 2.19 0.1166 

VP_Product 0.021368 0.0028 7.54 0.0048 

Fuel RVP -2.14898 0.7849 -2.74 0.0715