United States Air and Radiation EPA420-P-98-012
Environmental Protection November 1998
Agency
v>EPA Modeling Diurnal and
Resting Loss Emissions
from Vehicles Certified
to the Enhanced
Evaporative Standards
Printed on Recycled Paper
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EPA420-P-98-012
- Draft -
Modeling Diurnal and Resting Loss Emissions from
Vehicles Certified to the Enhanced Evaporative
Standards
Larry C. Landman
Report Number M6.EVP.005
November 23, 1998
U.S. EPA
Assessment and Modeling Division
National Vehicle Fuel and Emissions Laboratory
2000 Traverwood Drive
Ann Arbor, Michigan 48105-2425
734-214-4939 (fax)
mobile@epa.gov
NOTICE
These reports do not necessarily represent final EPA decisions
or positions. They are intended to present technical analysis
of issues using data which are currently available. The
purpose in release of these reports is to facilitate the
exchange of technical information and to inform the public of
technical developments which may form the basis for a final
EPA decision, position or regulatory action.
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ABSTRACT
This document reports on EPA's proposed methods of
estimating the resting loss and diurnal emissions from vehicles
certified to the enhanced evaporative standards (i.e., some 1996
through 1998 and all 1999 and newer vehicles). Since this draft
report is a proposal, its analyses and conclusions may change to
reflect comments, suggestions, and new data.
Please note that EPA is seeking comments that might aid us
in modeling any aspect of resting loss or diurnal evaporative
emissions.
Comments on this report and its proposed use in MOBILE6
should be sent to the attention of Larry Landman. Comments may
be submitted electronically to mobile@epa.gov, or by fax to (734)
214-4939, or by mail to "MOBILE6 Review Comments", US EPA
Assessment and Modeling Division, 2000 Traverwood Drive, Ann
Arbor, MI 48105. Electronic submission of comments is
preferred. In your comments, please note clearly the document
that you are commenting on, including the report title and the
code number listed. Please be sure to include your name,
address, affiliation, and any other pertinent information.
This document is being released and posted. Comments will be
accepted for sixty (60) days from the posting date. EPA will
then review and consider all comments received and will provide a
summary of those comments, and how we are responding to them.
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***DRAFT***
Modeling Diurnal and Resting Loss Emission from Vehicles Certified
to the Enhanced Evaporative Standards
Report Number M6.EVP.005
Larry C. Landman
U.S. EPA Assessment and Modeling Division
1.0 Introduction
Evaporative emissions of hydrocarbons (HC) are a significant
portion of the emissions estimated in the MOBILE model. In
recently released draft reports (M6.EVP.001 and M6.EVP.002), the
US Environmental Protection Agency (EPA) proposed methods of
estimating resting loss and diurnal emissions from 1995 and older
model year vehicles. These estimates were based on the results
of real-time diurnal (RTD) tests of in-use vehicles in which the
ambient temperature cycled over a 24-degree Fahrenheit range to
simulate in real-time the daily heating and cooling that parked
vehicles experience over a 24-hour period.
Beginning with the 1996 model year, manufacturers were
required to certify twenty percent of their vehicles using a new
"enhanced" evaporative testing procedure; that percentage is
scheduled to increase to one hundred percent by the 1999 model
year. The phase-in percentages are given below in Table 1
(copied from 40 CFR 86.096-8) .
Table 1
Phase-In of Vehicles with
Enhanced Evaporative Controls
Model Year Percentage
1995 0%
1996 20%
1997 40%
1998 90%
1999 100%
In order to meet the enhanced evaporative standards,
manufacturers have implemented a number of changes, including
(but not limited to):
• "quick connects" that reduce the possibility of
improper assembly when the vehicle is serviced,
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• advanced materials that are less permeable, less
susceptible to puncture, and more durable (i.e.,
elastomeric materials used in hoses and connectors),
• improvements made to the purge system (to enable the
vehicles to pass both the running loss test and the
multi-day diurnal test),
• tethered gas caps, and
• improved fractional-turn gas caps.
Since these changes are expected to result in improved control of
evaporative emissions, EPA proposes to use a separate set of
estimates of both resting loss and diurnal emissions for these
vehicles.
Unlike the analyses for older vehicles that were based on
results of tests of in-use vehicles, the analyses in this report
generally are not based on testing of these in-use 1996 and newer
vehicles because EPA has very few test results on that segment of
the in-use fleet. In this report, EPA proposes methods of
estimating the resting loss and diurnal emissions from these in-
use 1996 and newer vehicles based on RTD testing of older but
similar vehicles.
2.0 Data Sources
EPA proposes to base its estimates of resting loss and
diurnal emissions on the results of real-time diurnal (RTD)
tests. At the time of this analysis, EPA had two available
sources of RTD test data on vehicles that were certified to the
new evaporative standards:
1) a summary of RTD testing used by the ARE and by the EPA
to certify new (1996-97 model year) vehicles (30 and 35
vehicles, respectively) and
2) results of RTD testing performed by Mercedes-Benz on six
of its 1996 model year vehicles (at two years of age) as
part of the proposed Compliance Assurance Program (CAP
2000) .
However, the test data (from these two sources) on the 1996
and newer vehicles have two serious limitations:
• First, all of the 1996 and newer vehicles from these two
sources had properly functioning evaporative control
systems. Since it is likely that some similar in-use
vehicles during the course of their useful life would
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develop malfunctions in their evaporative control
systems, EPA's analysis is limited by not having test
results on such malfunctioning in-use vehicles.
• Secondly, all of these RTD tests were performed using a
single test fuel with a Reid vapor pressure (RVP) of 9.0
psi and using a single temperature cycle (72 to 96
degrees Fahrenheit). Since, all the RTD testing
performed on these 1996 and newer vehicles had been run
at a single combination of temperature cycle and fuel
volatility, EPA is limited in using those data to predict
evaporative emissions at other combinations of
temperature cycle and fuel volatility.
To compensate for those significant limitations, EPA
proposes to supplement those data with the results of RTD testing
of older vehicles that were not certified to the enhanced
evaporative standards. Two sources of those test results were:
3) RTD testing performed on 119 in-use 1971-95 model year
vehicles for EPA by its testing contractor and
4) RTD testing performed on 151 in-use 1971-91 model year
vehicles for the Coordinating Research Council (CRC).
Although none of the 270 in-use vehicles tested in the EPA
or CRC programs (sources 3 and 4) had been certified to the new
evaporative standards, the combined sample does include both:
• in-use vehicles with malfunctions in their evaporative
control systems
as well as
• vehicles for which the RTD test were performed over three
different temperature cycles and using fuels with
different RVPs.
A fifth source of information on these vehicles certified to
the enhanced evaporative standards was:
5) an analysis of evaporative emissions (not explicitly
based on RTD testing) of vehicles certified to the
enhanced evaporative standard presented to the California
Air Resources Board (ARE) (in December 1997) by the
American Automobile Manufacturers Association (AAMA) and
the Association of International Automobile Manufacturers
(AIAM).
In Section 3.0, EPA proposes how to use RTD test results
from some of the older (i.e., 1990-95) vehicles (i.e., from
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sources 3 and 4) to compensate for the limitations of the test
results on the 1996 and later vehicles.
3.0 Simulating Test Data from In-Use 1996 and Newer Vehicles
The MOBILE model must be able to estimate the resting loss
and diurnal evaporative emissions from the 1996 and newer
vehicles over a variety of daily temperature cycles and with a
variety of fuel RVPs. However, as noted in the preceding
section, the only test data on those vehicles available at this
time are with a single combination of fuel volatility (RVP of 9.0
psi) and daily temperature profile (i.e., ambient temperatures
cycling between 72 and 96 degrees Fahrenheit). EPA, therefore,
proposes to use results of RTD tests on older vehicles (i.e.,
model years 1990 through 1995) to estimate the effects on the
actual "base line" emissions (from source 1) of different fuel
volatility and different temperature cycles on the resting loss
and diurnal evaporative emissions of the 1996 and newer vehicles.
For the purpose of characterizing the effects of varying the
fuel RVP and/or the temperature cycle, EPA proposes to continue
(from the previous analyses) the approach of dividing the in-use
fleet into four strata. The first of these strata consists of
vehicles having substantial leaks of liquid gasoline (as opposed
to simply vapor leaks); these vehicles were labeled "gross liquid
leakers." EPA proposed (in M6.EVP.001) using as a definition for
such vehicles the requirement that the hourly resting loss (at 72
degrees Fahrenheit) be at least 1.0 grams per hour of HC. EPA
realizes that such a definition could result in potentially
ignoring a vehicle having a substantial leak that is apparent
only when the engine is operating (e.g., some fuel line leaks).
While neither the purge test nor the pressure test measures
evaporative emissions, a vehicle's failure on either test is
indicative of potential malfunctions of the vehicle's evaporative
control system. Additionally, the recruitment of the vehicles in
the third data source was intentionally skewed to recruit a
larger proportion of vehicles with potentially malfunctioning
evaporative control systems (i.e., a stratified random
recruitment). Therefore, the results of any analysis must be
weighted to correctly represent the entire in-use fleet. Thus,
the analyses will be stratified to match the recruitment process.
EPA proposes to use the results on the purge and pressure tests
to define the remaining three strata. This approach produces the
following three additional strata each containing:
1) vehicles that pass both the purge and pressure tests,
2) vehicles that fail the pressure test (regardless of their
performance on the purge test), and
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3) vehicles that fail only the purge test.
As discussed previously, it is necessary to make use of the
RTD tests performed on older vehicles to predict the effects on
the evaporative emissions of changes to the temperature cycle or
the fuel RVP. In order to make use of those RTD tests on some of
those 270 vehicles, EPA made the following assumptions:
1) The 1996 and newer vehicles are expected to be port fuel
injected (PFI); therefore, EPA chose the 1990 to 1995
model year vehicles that were equipped with PFI as
appropriate surrogates. (RTD tests on a total of 25
vehicles were found and appear in Appendix A.)
2) RTD tests were performed on 65 1996-97 model year
(enhanced evaporative procedure certified) vehicles using
a daily temperature profile in which the ambient
temperatures cycled between 72 and 96 degrees with a fuel
having an RVP of 9.0 psi (data source number 1). EPA
proposes to use those test results to obtain baseline
evaporative emissions for these "properly functioning"
vehicles for that specific temperature cycle and for that
test fuel. To "correct" those baseline values for
temperature and RVP changes, EPA identified, in its RTD
data base (data source 3), ten 1990-95 model year PFI
vehicles that:
• passed both the purge and pressure tests,
• had a RTD test performed using a daily temperature
cycle ranging between 72 and 96 degrees with a fuel
having an RVP of 9.0 psi (i.e., at the baseline
conditions used for the vehicles in data source 1),
and
• had one or more additional RTD tests performed using
a fuel with an RVP of 6.8 psi.
3) EPA believes that the RTD emissions from malfunctioning
enhanced evaporative control vehicles (i.e., vehicles
that developed problems with their evaporative control
systems) will be similar to the RTD emissions from the
1990 to 1995 model year vehicles that also develop
problems with their evaporative control systems. That
is, those 1996 and newer model year vehicles that had
failed either EPA's purge or pressure tests are expected
to have evaporative emissions similar to those 1990 to
1995 model year PFI vehicles that also failed the same
test.
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Thirteen such vehicles were identified in the combined
EPA/CRC sample (eight of the 13 failing only the purge
test and the remaining five failing the pressure test).
(See Appendices B and C, respectively.) EPA proposes to
use these 13 vehicles to estimate the effects on both the
resting loss and diurnal emissions for the malfunctioning
enhanced evaporative control vehicles of changes to the
temperature cycle and/or to the RVP of the fuel.
4.0 Analysis
As noted in two previous reports (M6.EVP.001 and
M6.EVP.002), EPA proposes to use the results of the RTD test to
model two distinct mechanisms of evaporative emissions:
1) "Resting loss" emissions are always present, regardless
of vehicle activity, and are relatively weakly related to
the ambient temperature as opposed to diurnal emissions
which are related to the rise in temperature.
The earlier reports calculated the hourly resting loss
emissions to be the mean of the RTD emissions from hours
19 through 24 at the nominal temperature for hour 24.
This method permitted EPA to estimate the hourly resting
loss emissions at three distinct temperatures (60, 72,
and 82 degrees Fahrenheit). In those analyses, resting
loss emissions were determined to be independent of the
RVP of the test fuel.
2) "Diurnal" emissions are the pressure-driven emissions
resulting from the daily increase in temperature.
The diurnal emissions were calculated by first estimating
the resting loss value for the ambient temperature at
each hour of the 24-hour cycle, and then subtracting that
temperature-adjusted resting loss estimate from the RTD
hourly test results.
This approach permitted EPA to analyze separately the relatively
constant resting loss emissions and the (pressure driven) diurnal
emissions.
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4.1 Resting Loss Emissions
4.1.1 Resting Loss Emissions of Properly Functioning Vehicles
In Section 3.0, EPA proposed using data from 10 of the older
(1990-95) model year vehicles to characterize the effects on the
baseline evaporative emissions of altering either the temperature
cycle or the fuel RVP for the 1996 and newer model year vehicles
with properly functioning evaporative control systems. Since the
resting loss emissions were calculated as the average hourly
emissions during hours 19 through 24 of the RTD test, to
calculate the baseline resting loss emissions (from the RTD tests
of the 65 vehicles in data source 1) we need the emissions to be
measured and reported hourly (or at least in six hour intervals).
Unfortunately, the test results in data source 1 were reported
for each 24-hour period; therefore, EPA does not currently have
the necessary baseline resting loss data for that stratum. Thus,
EPA must determine not only the correction factors but also the
baseline resting loss values.
EPA believes that, while that sample of 10 vehicles
(proposed in Section 3.0) is appropriate for estimating the
effects of varying either the temperature cycle or fuel RVP, the
actual emission values are not necessarily representative of
actual 1996 and newer vehicles. Hence, those vehicles might not
be appropriate for estimating the baseline resting loss
emissions. To find an appropriate subset of the 10-vehicle
sample composed of vehicles whose emissions are representative of
the 1996 and newer vehicles, we examined the data from the 65
certification vehicles (from data source 1). Those vehicles had
a mean RTD emission for the first 24-hours of a 72-hour RTD test
of 0.745 grams (for a temperature cycle from 72 to 96 degrees and
with a 9.0 RVP fuel). The corresponding median RTD for those 65
tests was 0.635 grams. In the 10-vehicle sample, the five
vehicles that had RTD emissions of at most 1.0 grams (over the
same temperature cycle and using the same test fuel) had mean and
median RTD emissions of 0.726 and 0.653 grams, respectively.
This sample of five vehicles consisted of four light-duty
vehicles and one light-duty truck. Based on the similarity of
RTD emissions as indicated by both the means and medians of the
65-vehicle sample and this five-vehicle sample, EPA proposes to
use the smaller sample to estimate both the baseline and the
temperature corrections of the hourly resting loss emissions of
the properly functioning 1996 and newer vehicles.
We then estimated the hourly resting loss associated with
60, 72 and 82 degrees Fahrenheit for each of those five vehicles
(see Appendices A and D) and regressed the resting loss emissions
against the ambient temperatures to obtain Table 2. A linear
(rather than exponential or polynomial) regression was used for
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consistency with the modeling of resting loss emissions of the
pre-1996 vehicles (see reports M6.EVP.001 or M6.EVP.002).
Table 2
Linear Regression of Hourly Resting Loss Emissions
(Vehicles Passing Both Purge and Pressure Tests)
Dependent variable is
No Selector
R squared = 40.2%
s= 0.0061 with 15-
Source
Regression
Residual
Variable
Constant
Temperature
R squared (adjusted)
= 35.6%
Resting Loss
2 = 13 degrees of freedom
Sum of Squares
0.000320
0.000477
Coefficient
-0.027859
0.000514
df
1
13
s.e. of Coeff
0.0125
0.0002
Mean Square
0.000320
0.000037
t-ratio
-2.23
2.95
F-ratio
8.72
prob
0.0441
0.0112
Converting that linear regression analysis into an equation
produces equation (1) below:
Hourly Resting Loss (grams/hr) = -0.027859 + [0.000514 * Temperature (°F)] (1)
EPA proposes to use equation (1) to estimate the hourly resting
loss emissions (in grams per hour) of that portion of the fleet
of 1996 and newer model year vehicles with properly functioning
evaporative control systems.
Equation (1) predicts that the mean hourly resting loss
emissions (for the fleet of 1996 and newer model year vehicles
with properly functioning evaporative control systems) would be
negative for all ambient temperatures below or equal to 54.2
degrees Fahrenheit. EPA will assume that the resting loss
emissions at those temperatures will be zero grams per hour.
Equation (1) also predicts that the mean hourly resting loss
emissions (for the fleet of 1996 and newer model year vehicles
with properly functioning evaporative control systems) at 72
degrees Fahrenheit would be 0.009149 grams per hour. Repeating
that calculation for each hour of the full 24 hours of the RTD
test, and then adding the 24 "temperature corrected" hourly
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resting loss emissions produces the full day's total resting loss
for the fleet of 0.3596 grams per vehicle. Subtracting that
value from the mean RTD test results (0.745 grams per day) yields
total diurnal emissions of 0.385 grams per vehicle per day (which
will be used as the baseline diurnal emission in Section 4.2.1).
4.1.2 Resting Loss Emissions of Malfunctioning Vehicles
In Section 3.0, EPA proposed using five 1990-95 model year
vehicles to represent the 1996 and newer model year vehicles that
failed the pressure test (four of which were tested over all
three temperature cycles) and using eight vehicles to represent
the 1996 and newer model year vehicles that failed the purge test
(see Appendix D). Repeating the approach used with the vehicles
passing both the purge and pressure tests, we estimated the
hourly resting loss associated with ambient temperatures of 60,
72 and 82 degrees Fahrenheit for each of those 12 vehicles that
were tested over all three temperature cycles. We then regressed
the resting loss emissions against the ambient temperatures
within each of these two strata:
• the stratum represented by the four vehicles failing
the pressure test (see Appendices C and D) and
• the stratum represented by the eight vehicles failing
the purge test (see Appendices B and D).
Additionally, we combined the test results on all 12 vehicles to
create the single stratum of vehicles failing either the purge or
the pressure test, and we again regressed the resting loss
emissions against the ambient temperatures within that third
stratum.
In each of those three strata, the differences in resting
loss emissions from vehicle-to-vehicle exceeded the differences
in emissions resulting from changes in ambient temperature. This
resulted in each regression analysis producing both low R-squared
values and indicated that temperature was not a statistically
significant variable (an unlikely situation). In order to obtain
a model for hourly resting loss emissions based on temperature,
we attempted to reduce the effect of the vehicle-to-vehicle
variability by averaging the resting loss emissions for all 12
vehicles at each of the three temperatures (i.e., producing only
three temperature / resting loss pairs) and then regressing those
three data points. This regression produced an R-squared of
100.0 percent. Since MOBILE6 is designed to predict only the
mean emissions, EPA proposes to use this regression to estimate
the means of the hourly resting loss emissions (in grams per
hour) of the 1996 and newer model year vehicles with
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malfunctioning evaporative control systems. Converting that
linear regression analysis into an equation yields:
Hourly Resting Loss (grams/hr) = -0.076535 + [0.001603 * Temperature (°F)] (2)
Equation (2) predicts that the mean hourly resting loss
emissions (for the fleet of 1996 and newer model year vehicles
with malfunctioning evaporative control systems) will be negative
for all ambient temperatures below 47.8 degrees Fahrenheit. EPA
will assume that the resting loss emissions at those temperatures
will be zero grams per hour.
4.1.3 Resting Loss Emissions of "Gross Liquid Leakers"
In a previous report (M6.EVP.001), EPA proposed that, for
the pre-1996 vehicles classified as gross liquid leakers, the
resting loss emissions are virtually independent of temperature,
averaging a constant 8.84 grams per hour. EPA proposes to
continue that assumption for the 1996 and newer vehicles that
were certified to the enhanced evaporative standard.
4.2 Diurnal Emissions
In Section 4.1, equation (1) indicates that for each one
degree (Fahrenheit) increase in ambient temperature, there is a
corresponding increase of 0.000514 grams per hour in the hourly
resting loss emissions. Applying that temperature correction to
each hour of the full 24 hours of the RTD test, and then adding
the 24 "temperature corrected" hourly resting loss emissions
produces the full day's total resting loss (in grams). This
approach predicts 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 0.140 grams. Subtracting that
quantity from each of the RTD test scores of the ten properly
functioning vehicles yields the estimated diurnal emissions on
each of the 48 RTD tests on those 10 vehicles.
Repeating this procedure for the 13 malfunctioning vehicles
but using the coefficient (0.001603) from equation (2), predicts
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 0.437 grams. Subtracting that quantity from
each of the RTD result score of the 13 malfunctioning vehicles
yields the estimated diurnal emissions.
Two factors that significantly affect a vehicle's diurnal
emissions (see M6.EVP.001 and M6.EVP.002) are:
• the Reid vapor pressure (RVP) of the test fuel and
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• the temperature cycle, as represented by the
combination of the cycle's midpoint temperature and
temperature range.
In previous reports (M6.EVP.001 and M6.EVP.002), we created a
single parameter that incorporated all of those factors. That
new parameter is based on the fuel's vapor pressure (VP). In
those reports, we used both the RVP of the fuel and the ambient
temperature to estimate the vapor pressure curve. (The RVP is
the VP measured at 100 degrees Fahrenheit.) The VP was then used
to create that new parameter which was used as the variable on
which diurnal emissions were calculated. That new parameter is
defined by the following formula, equation (3).
VP_Product_Term = (VPmoH - VPi_ow) * (VPmoH + VPi_ow) / 2 (3)
Where
VPHIGH is the VP (in kiloPascals) associated with the
day's high temperature.
VPLOW is the VP (in kiloPascals) associated with the
day's low temperature.
The analyses in those earlier reports modeled the diurnal
emissions as functions of both that VP product term and the RVP
of the test fuel.
4.2.1 Diurnal Emissions of Properly Functioning Vehicles
In Section 3.0, EPA proposed using the 25 1990-95 model year
vehicles listed in Appendix A to model the diurnal emissions from
the 1996 and newer model year vehicles with properly functioning
evaporative control systems. However, only five of those 25
vehicles were tested at each of the six possible temperature and
RVP combinations. (While a sample of five vehicles is relatively
small, the disadvantages of that small size are offset by having
the identical sample of vehicles at each of those six testing
combinations which eliminates some of the problems caused by
having test results on a different group of vehicles at each of
those combinations.)
The approach used in the earlier reports was to first
regress the mean diurnal emission emissions against the RVP of
the test fuel combined with either the VP product term (in
kiloPascals squared) or the square of that product term. Those
six mean values (for the five vehicles) are given in the
following table.
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DRAFT
Temperature Fuel
Cycle RVP
60 to 84 6.8
72 to 96 6.8
82 to 106 6.8
60 to 84 9.0
72 to 96 9.0
82 to 106 9.0
VP_Product
Term
375
567
789
655
969
1,324
Mean
Diurnal
0.616
0.768
1.781
0.535
1.084
4.008
These means do confirm two reasonable patterns:
• For each temperature cycle, as the fuel volatility
increases, so do the diurnal emissions.
• For a given fuel, as the temperature cycle increases,
so do the diurnal emissions.
This confirms that the diurnal emissions, rather than being a
function of the VP product term alone, might be a function of
that term in combination with the fuel RVP or temperature cycle.
A number of different combinations were tested, and the one that
proved most successful in predicting the diurnal emissions was a
regression of the logarithm diurnal emissions against:
• the RVP of the test fuel (in psi) and
• the vapor pressure product term (in kiloPascals
squared).
The results of that regression analysis are given in Table 3.
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DRAFT
Table 3
Linear Regression of Mean Diurnal Emissions
(Vehicles Passing Both Purge and Pressure Tests)
Dependent variable
No Selector
R squared = 96.6%
s= 0.1810 with 6-
Source
Regression
Residual
Variable
Constant
VP_Product
RVP
is:
R squared (adjusted)
= 94.3%
Ln_Diurnal
3 = 3 degrees of freedom
Sum of Squares
2.78802
0.098293
Coefficient
0.860614
0.002905
-0.381496
df Mean Square
2 1.39401
3 0.32764
s.e. of Coeff t-ratio
0.5792 1.49
0.0003 8.94
0.0900 -4.24
F-ratio
42.5
prob
0.2340
0.0030
0.0241
In the analyses performed in the previous reports, after we
completed the regression analysis, we then modified the constant
term (produced by the regression) so that the resulting curve
would pass through the mean of all the relevant data (i.e., in
this case, through the mean of the 65 certification RTD tests at
the single temperature / RVP combination). In Section 4.1.1, we
estimated that 0.385 grams per vehicle per day was the baseline
value of the diurnal emissions of the properly functioning 1996
and newer vehicles. Modifying the constant term in the preceding
regression equation so that curve passes through that baseline
value (over the 72 to 96 degree cycle using 9.0 RVP fuel)
produces the equation:
24-Hour Diurnal (grams) = exp [ -0.33599 + (0.002905 * VP_Product_Term )
-(0.381496* RVP)]
(4)
EPA proposes to use equation (4) to estimate the 24-hour
diurnal emissions of all 1996 and newer model year vehicles with
properly functioning evaporative control systems with the
following two modifications:
Regardless of the increase in ambient temperatures, there
are no diurnal emissions until the ambient temperature
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exceeds 40°F. (This assumption was used consistently for
all evaporative emissions in MOBILES.)
For a temperature cycle in which the daily low temperature
is below 40°F, EPA proposes to calculate the diurnal
emissions for that day as an interrupted diurnal (see
M6.EVP.002) that begins once the ambient temperature reaches
40 °F.
2) The 24-hour diurnal emissions will be zero grams for any
temperature cycle in which the diurnal temperature range is
zero degrees Fahrenheit (i.e., a constant temperature
throughout the entire day).
For temperature cycles in which the diurnal temperature
range is between zero and ten degrees Fahrenheit, the 24-
hour diurnal emissions will be a linear interpolation
between the predicted value for the ten-degree cycle (with
the appropriate RVP) and zero grams.
4.2.2 Diurnal Emissions of Vehicles Failing the Pressure Test
In Section 3.0, EPA proposed using five 1990-95 model year
vehicles to model the diurnal emissions from the 1996 and newer
model year vehicles that failed the pressure test (Appendix C).
Only four of those five vehicles were each tested using the same
two fuels (i.e., the 6.8 and 9.0 psi RVP fuels) over each of the
three temperature cycles (i.e., the 60 to 84, the 72 to 96, and
the 82 to 106 degree cycles). Thus, at the six different
combinations of temperature cycle and fuel RVP there were a total
24 measurements of diurnal emissions. In Section 4.2.1, we noted
that the diurnal emissions for the vehicles with properly
functioning evaporative control systems were not a strictly
increasing function of the VP product term. However, for the
vehicles that failed the pressure test, the diurnal emissions
increased as the VP product term increased. We, therefore,
repeated the approach used in the earlier analyses of regressing
the diurnal emission emissions against the cube of the VP product
term (in kiloPascals squared), producing the Table 4. (As a
check, we ran other regressions that included RVP as one of the
variables along with the vapor pressure product term. In none of
them was RVP identified as being statistically significant.)
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Table 4
Linear Regression of Diurnal Emissions
(Vehicles Failing the Pressure Test)
Dependent variable is
No Selector
R squared = 41.1%
s= 12.56 with 24 -2
Source
Regression
Residual
Variable
Constant
Cube of VP Product
1,000,000
R squared (adjusted = 38
= 22 degrees of freedom
Sum of Squares
2417.79
3470.15
4%
df
1
22
Coefficient s.e. of Coeff
7.94455
/ 0.013004
3.475
0.0033
Mean Square
2417.79
157.734
t-ratio
2.29
3.92
Diurnal
F-ratio
15.3
prob
0.0322
0.0007
We then modified the constant term (produced by the
regression) so that the resulting curve would pass through the
mean of all the relevant data (i.e., all 28 tests on the five
vehicles at nine temperature / RVP combinations). This
produced the following equation:
24-Hour Diurnal (grams) = 7.804751 + [0.012985 * Cube of VP_Product] / M (5)
Where M = 1,000,000
EPA proposes to use equation (5) to estimate the mean 24-hour
diurnal emissions of all 1996 and newer model year vehicles that
failed the pressure test with the following two modifications
(repeated from Section 4.2.1):
1) Regardless of the increase in ambient temperatures, there
are no diurnal emissions until the temperature exceeds 40°F.
(This assumption was used consistently for all evaporative
emissions in MOBILES.)
For a temperature cycle in which the daily low temperature
is below 40°F, EPA proposes to calculate the diurnal
emissions for that day as an interrupted diurnal (see
M6.EVP.002) that begins once the ambient temperature reaches
40 °F.
-------
-16-
DRAFT
2) The 24-hour diurnal emissions will be zero grams for any
temperature cycle in which the diurnal temperature range is
zero degrees Fahrenheit (i.e., a constant temperature
throughout the entire day).
For temperature cycles in which the diurnal temperature
range is between zero and ten degrees Fahrenheit, the 24-
hour diurnal emissions will be a linear interpolation
between the predicted value for the ten-degree cycle (with
the appropriate RVP) and zero grams.
4.2.3 Diurnal Emissions of Vehicles Failing Only the Purge Test
In Section 3.0, EPA proposed using eight 1990-95 model year
vehicles to model the diurnal emissions from the 1996 and newer
model year vehicles that failed only the purge test. Only five
of those eight vehicles were each tested over the six different
combinations of temperature cycle and fuel RVP. Thus, at the six
different combinations of temperature cycle and fuel RVP there
were a total 30 measurements of diurnal emissions. Repeating the
approach used in Sections 4.2.1 and 4.2.2, we regressed the
diurnal emission emissions against the cube of the VP product
term (in kiloPascals), producing the resulting table:
Table 5
Linear Regression of Diurnal Emissions
(Vehicles Failing Only the Purge Test)
Dependent variable is
No Selector
R squared = 45.0%
s = 6.140 with 30- 2 =
Source
Regression
Residual
Variable
Constant
Cube of VP Product
1,000,000
R squared (adjusted) = 43
= 28 degrees of freedom
Sum of Squares
862.12
1055.57
.0%
df
1
28
Coefficient s.e. of Coeff
2.00177
/ 0.006945
1.519
0.0015
Mean Square
862.12
37.6989
t-ratio
1.32
4.78
Diurnal
F-ratio
22.9
prob
0.1983
# 0.0001
-------
-17- DRAFT
We then modified the constant term (produced by the
regression) so that the resulting curve would pass through the
mean of all the relevant data (i.e., all 48 tests on the eight
vehicles at nine temperature / RVP combinations). This produced
the following equation:
24-Hour Diurnal (grams) = 1.134838 + [0.006945 * Cube of VP_Product] / M (6)
Where M = 1,000,000
EPA proposes to use equation (6) to estimate the mean 24-hour
diurnal emissions of all 1996 and newer model year vehicles that
failed only the purge test with the following two
modifications (repeated from Sections 4.2.1 and 4.2.2):
1) Regardless of the increase in ambient temperatures, there
are no diurnal emissions until the temperature exceeds 40°F.
(This assumption was used consistently for all evaporative
emissions in MOBILES.)
For a temperature cycle in which the daily low temperature
is below 40°F, EPA proposes to calculate the diurnal
emissions for that day as an interrupted diurnal (see
M6.EVP.002) that begins once the ambient temperature reaches
40 °F.
2) The 24-hour diurnal emissions will be zero grams for any
temperature cycle in which the diurnal temperature range is
zero degrees Fahrenheit (i.e., a constant temperature
throughout the entire day).
For temperature cycles in which the diurnal temperature
range is between zero and ten degrees Fahrenheit, the 24-
hour diurnal emissions will be a linear interpolation of the
predicted value for the ten-degree cycle (with the
appropriate RVP) and zero grams.
4.2.4 Diurnal Emissions of "Gross Liquid Leakers"
In a previous report (Section 5 of report number
M6.EVP.002), EPA proposed estimating the mean of the diurnal
emissions for each temperature cycle of the vehicles classified
as "gross liquid leakers" using equation (7), below. That
equation predicts diurnal emissions as a function of a single
variable, the diurnal temperature range (i.e., the daily high
temperature minus the daily low temperature):
24-Hour Diurnal (grams) = 20.058 + [ 3.343 * Diurnal_Temperature_Range ] (7)
-------
-18- DRAFT
EPA proposes to continue using equation (7) to estimate the
mean 24-hour diurnal emissions of all gross liquid leakers
regardless of model year whenever the diurnal temperature range
is at least 10 degrees Fahrenheit. The 24-hour diurnal emissions
will be zero grams for any temperature cycle in which the diurnal
temperature range is zero degrees Fahrenheit (i.e., a constant
temperature throughout the entire day). For temperature cycles
in which the diurnal temperature range is between zero and ten
degrees Fahrenheit, the 24-hour diurnal emissions will be a
linear interpolation of the predicted value for the ten-degree
cycle (i.e., 53.49 grams) and zero grams.
5.0 Summary
For the 1996 and newer model year vehicles that were
certified to the enhanced evaporative testing procedure, EPA
proposes to model the resting loss and diurnal emissions
separately for those vehicles having substantial leaks of liquid
gasoline. For the non-leakers, EPA proposes to model the resting
loss and diurnal emissions separately based on the functional
status of the vehicle's evaporative control system.
EPA proposes to model the hourly resting loss emissions (in
grams per hour) using the following three formulae:
• For vehicles with substantial leaks of liquid gasoline
("Gross Liquid Leakers"):
Hourly Resting Loss = 8.84
• For vehicles with properly functioning evaporative control
systems:
Hourly Resting Loss = -0.02786 + [0.000514 * Temperature (°F)] (1)
• For vehicles with malfunctioning evaporative control systems:
Hourly Resting Loss = -0.07654 + [0.001603 * Temperature (°F)] (2)
Additionally, any predicted resting loss emission that is
negative will be replaced with zero.
EPA proposes to model the full (24-hour) day's diurnal
emissions (in grams per day) using the following four formulae:
• For vehicles with substantial leaks of liquid gasoline
("Gross Liquid Leakers"):
Diurnal = 20.058 + [3.343 * Diurnal_Temperature_Range (°F)] (7)
-------
-19- DRAFT
• For vehicles with properly functioning evaporative control
systems:
Diurnal = exp [ -0.33599 + (0.002905 * VP_Product_Term )
-(0.381496*RVP)] (4)
• For vehicles failing the pressure test:
Diurnal = 7.80475+ [ 0.012985 * Cube of VP_Product_Term]/M (5)
Where M = 1,000,000
• For vehicles failing the purge test:
Diurnal = 1.13484+ [ 0.006945 * Cube of VP_Product_Term]/M (6)
Where M = 1,000,000
Additionally, the following two modifications apply to the
diurnal emission that are predicted by any of the four preceding
formulae:
1) Regardless of the increase in ambient temperatures, there
are no diurnal emissions until the temperature exceeds 40°F.
For a temperature cycle in which the daily low temperature
is below 40°F, EPA proposes to calculate the diurnal
emissions for that day as an interrupted diurnal (see
M6.EVP.002) that begins once the ambient temperature reaches
40 °F.
2) The 24-hour diurnal emissions will be zero grams for any
temperature cycle in which the diurnal temperature range is
zero degrees Fahrenheit (i.e., a constant temperature
throughout the entire day).
For temperature cycles in which the diurnal temperature
range is between zero and ten degrees Fahrenheit, the 24-
hour diurnal emissions will be a linear interpolation of the
predicted value for the ten-degree cycle (with the
appropriate RVP) and zero grams.
-------
-20-
DRAFT
Appendix A
Twenty-Five 1990-1995 Model Year Vehicles
Passing Both the Purge and Pressure Tests
Vehicle
No.
4912
4923
4928
4932
5032
5038
Fuel
RVP
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
9.0
9.0
9.0
Temp Cycle
72 - 96
82 - 106
60 - 84
72 - 96
72 - 96
82 - 106
60 - 84
72 - 96
72 - 96
82 - 106
60 - 84
72 - 96
72 - 96
82 - 106
60 - 84
72 - 96
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
VP Product
Term
567
789
655
969
567
789
655
969
567
789
655
969
567
789
655
969
375
567
789
655
969
1,324
567
789
655
969
1,324
RTD
(grams)
0.980
5.120
1.930
3.350
0.670
4.480
1.710
2.550
4.830
8.230
4 . 170
4.370
1.700
2.850
1.490
2.080
0.374
0.772
1.231
0.473
0.741
2.433
0.615
1.011
0 .441
1.302
4.366
Hrly Rst
Loss*
0.012
0.102
-0.005
0.045
0.000
0.048
0.018
0.032
0.065
0.142
0.045
0.058
0.017
0.037
0.023
0.017
0.004
0.006
0.012
0.005
0.008
0.018
0.005
0.007
0.002
0.004
0.006
Daily Rst
Loss
0.428
2.588
0.020
1.220
0.140
1.292
0.572
0.908
1.700
3 .548
1.220
1.532
0.548
1.028
0.692
0.548
0.236
0.284
0.428
0.260
0.332
0.572
0.260
0.308
0.188
0.236
0.284
Diurnal
(grams)
0.552
2.532
1.910
2.130
0.530
3 .188
1.138
1.642
3 .130
4.682
2.950
2.838
1.152
1.822
0.798
1.532
0.138
0.488
0.803
0.213
0.409
1.861
0.355
0.703
0.253
1.066
4.082
"Hourly Resting Loss" emissions are calculated at the lowest temperature
of each cycle.
-------
-21-
DRAFT
Appendix A (Continued)
Vehicle
No.
5046
5047
5052
5066
5068
Fuel RVP
6.8
6.8
6.8
9.0
9.0
9.0
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp Cycle
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
60 - 84
72 - 96
82 - 106
VP Product
Term
375
567
789
655
969
1,324
655
969
1,324
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
RTD
(a rams)
0.439
0.565
1.498
0.360
0.971
9.716
0.366
0.653
0.906
3.502
4 .273
8.937
2.966
5.853
11.820
0.390
0.351
0.605
0.295
0.397
0.581
0.281
0.626
1.936
0.814
0.580
1.150
0.368
0.839
1.391
0.638
1.385
2.132
Hrly Rst
Loss*
0.011
0.007
0.020
0.004
0.013
0.041
0.005
0.012
0.015
0.032
0.071
0.114
0.039
0.106
0.205
-0.007
0.001
0.006
0.000
0.003
0.004
-0.001
0.007
0.011
0.006
0.006
0.009
0.003
0.009
0.018
0.009
0.010
0.029
Daily Rst
Loss
0.404
0.308
0.620
0.236
0.452
1 . 124
0.260
0.428
0.500
0.908
1 .844
2.876
1.076
2.684
5.060
-0.028
0.164
0.284
0.140
0.212
0.236
0.116
0.308
0 .404
0.284
0.284
0.356
0.212
0.356
0.572
0.356
0.380
0.836
Diurnal
(a rams)
0.035
0.257
0.878
0.124
0.519
8.592
0.106
0.225
0.406
2.594
2.429
6.061
1.890
3 .169
6.760
0.418
0.187
0.321
0.155
0.185
0.345
0.165
0.318
1.532
0.530
0.296
0.794
0.156
0.483
0.819
0.282
1.005
1.296
-------
-22-
DRAFT
Appendix A (Continued)
Vehicle
No.
5081
9009
9026
9028
9033
9038
9040
9048
9056
9059
9088
9135
9141
9143
Fuel RVP
6.3
6.3
9.0
9.0
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
Temp Cycle
72 - 96
82 - 106
60 - 84
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
72 - 96
VP Product
Term
489
684
655
969
567
567
567
567
567
567
567
567
567
567
567
567
567
RTD
(grams)
0.647
1.187
0.326
0.639
35.565
1.755
16.984
0.879
5.818
0.810
9.443
3 .095
1.009
2.750
1.591
10.328
7.904
Hrly Rst
Loss*
0.001
0.009
0.005
0.007
0.095
0.031
0.024
0.003
0.106
0.006
0.228
0.046
0.013
0.023
0.012
0.209
0.070
Daily Rst
Loss
0.164
0.356
0.260
0.308
2.420
0.884
0.716
0.212
2.684
0.284
5.612
1.244
0.452
0.692
0.428
5.156
1.820
Diurnal
(grams)
0.483
0.831
0.066
0.331
33.145
0.871
16.268
0.667
3.134
0.526
3.831
1.851
0.557
2.058
1.163
5.172
6.084
-------
-23-
DRAFT
Appendix B
Eight 1990-1995 Model Year Vehicles
Failing (Only) the Purge Test
Vehicle
No.
4925
4933
5004
5035
5040
Fuel
RVP
6.8
6.8
9.0
9.0
6.8
6.8
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
Temp Cycle
72-96
82-106
60-84
72-96
72-96
82-106
60-84
72-96
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
567
789
655
969
567
789
655
969
375
567
789
655
969
1,324
375
567
789
655
969
1,324
375
567
789
655
969
1,324
RTD
(grams)
4.170
4.450
2.170
3.830
10.750
18.670
7.170
12.120
0.989
1.673
2.924
1.025
5.440
20.391
5.593
5.869
22.973
14.493
24.068
24.872
0.667
1.143
6.961
1.065
2.930
20.658
Hrly Rst
Loss*
0.063
0.080
0.035
0.058
0.145
0.352
0.137
0.228
0.003
0.023
0.031
0.015
0.018
0.047
-0.016
0.016
-0.033
0.015
0.032
0.040
0.003
0.010
-0.013
-0.003
0.012
-0.008
Daily Rst
Loss
1.949
2.357
1.277
1.829
3.917
8.885
3.725
5.909
0.509
0.989
1.181
0.797
0.869
1.565
0.053
0.821
-0.355
0.797
1.205
1.397
0.509
0.677
0.125
0.365
0.725
0.245
Diurnal
(grams)
2.221
2.093
0.893
2.001
6.833
9.785
3.445
6.211
0.480
0.684
1.743
0.228
4.571
18.826
5.540
5.048
23.328
13.696
22.863
23.475
0.158
0.466
6.836
0.700
2.205
20.413
-------
-24-
DRAFT
Appendix B (Continued)
Vehicle
No.
5069
5070
5087
Fuel
RVP
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
9.0
9.0
Temp Cycle
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
72-96
82-106
60-84
72-96
VP Product
Term
322
489
684
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
489
684
655
969
RTD
(grams)
1.774
3.593
6.810
1.322
1.953
9.565
7.082
12.372
20.430
0.351
0.690
1.209
0.375
0.745
1.176
0.416
1.381
9.141
1.830
2.435
1.478
2.533
Hrly Rst
Loss*
0.001
0.012
0.003
0.004
0.011
0.039
-0.017
0.007
0.080
0.002
0.001
0.016
0.002
-0.004
0.007
0.003
0.019
0.057
0.042
0.048
0.029
0.043
Daily Rst
Loss
0.461
0.725
0.509
0.533
0.701
1.373
0.029
0.605
2.357
0.485
0.461
0.821
0.485
0.341
0.605
0.509
0.893
1.805
1.445
1.589
1.133
1.469
Diurnal
(grams)
1.313
2.868
6.301
0.789
1.252
8.192
7.053
11.767
18.073
0.000
0.229
0.388
0.000
0.404
0.571
0.000
0.488
7.336
0.385
0.846
0.345
1.064
-------
-25-
DRAFT
Appendix C
Five 1990-1995 Model Year Vehicles
Failing the Pressure Test
Vehicle
No.
4937
5008
5021
5044
5067
Fuel
RVP
6.8
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.8
6.8
6.8
9.0
9.0
9.0
6.3
6.3
6.3
6.8
6.8
6.8
9.0
9.0
9.0
Temp Cycle
72-96
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
60-84
72-96
82-106
VP Product
Term
567
375
567
789
655
969
1,324
375
567
789
655
969
1,324
375
567
789
655
969
1,324
322
489
684
375
567
789
655
969
1,324
RTD
(grams)
3.330
12.853
17.632
29.663
19.811
35.202
57.174
7.789
15.477
23.810
17.246
24.840
41.963
0.286
0.523
0.706
0.467
0.494
1.914
5.206
13.206
21.981
12.128
8.644
18.697
7.106
29.697
50.741
Hrly Rst
Loss*
0.028
-0.018
0.013
0.054
-0.002
0.038
0.014
0.004
0.029
0.065
-0.003
0.038
-0.034
0.004
0.011
0.014
0.011
0.007
0.005
0.037
0.056
0.113
0.025
0.070
0.062
0.036
0.107
0.040
Daily Rst
Loss
1.109
0.005
0.749
1.733
0.389
1.349
0.773
0.533
1.133
1.997
0.365
1.349
-0.379
0.533
0.701
0.773
0.701
0.605
0.557
1.325
1.781
3.149
1.037
2.117
1.925
1.301
3.005
1.397
Diurnal
(grams)
2.221
12.848
16.883
27.930
19.422
33.853
56.401
7.256
14.344
21.813
16.881
23.491
42.342
0.000
0.000
0.000
0.000
0.000
1.357
3.881
1 1 .425
18.832
11.091
6.527
16.772
5.805
26.692
49.344
-------
-26-
DRAFT
Appendix D
Resting Loss Data Used in Regression Analyses
1990-1995 Model Year Vehicles
(By Ambient Temperature)
Category
Pass Both
Pass Both
Pass Both
Pass Both
Pass Both
Veh. No.
5032
5046
5047
5066
5081
Means:
60»F
0.0043
0.0075
0.0047
-0.0024
0.0052
0.0039
72»F
0.0069
0.0097
0.0117
0.0036
0.0038
0.0071
82»F
0.0150
0.0306
0.0150
0.0071
0.0090
0.0153
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Only Purge
Fail Pressure
Fail Pressure
Fail Pressure
Fail Pressure
4925
4933
5004
5035
5040
5069
5070
5087
Means:
5008
5021
5044
5067
Means:
0.0350
0.1367
0.0088
-0.0007
0.0001
-0.0042
0.0024
0.0290
0.0259
-0.0102
0.0006
0.0073
0.0324
0.0075
0.0608
0.1867
0.0203
0.0236
0.0108
0.0101
0.0052
0.0000
0.0397
0.0253
0.0332
0.0088
0.0778
0.0363
0.0800
0.3517
0.0392
0.0031
-0.0106
0.0407
0.0266
0.0000
0.0663
0.0341
0.0155
0.0096
0.0714
0.0327
Fail Either:
0.0198
0.0386
0.0551
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