United States Air and Radiation EPA420-P-99-005
Environmental Protection M6.EVP.004
Agency February 1999
&EPA Update of Hot Soak
Emissions
> Printed on Recycled Paper
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EPA420-P-99-005
February 1999
of Hot
M6.EVP.004
Assessment and Modeling Division
Office of Mobile Sources
U.S. Environmental Protection Agency
Prepared for EPA by
ARCADIS Geraghty & Miller, Inc.
EPA contract No. 68-C6-0068
Work Assignment No. 1-01
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.
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I. INTRODUCTION
The U. S. Environmental Protection Agency's (EPA) highway emission factor model, currently
MOBILESa, calculates in-use emission factors for exhaust and evaporative emissions using national
average values as supplemented by user-supplied input (e.g., temperature, fuel volatility, etc.). EPA
is currently working to develop a new version of the model (MOBILE6) to further improve its accuracy
and include more "real world" data.
Evaporative "hot soak" (trip-end) emissions represent one area where data now exists to better
characterize conditions observed during "real world" driving conditions. A hot soak is defined as the
evaporative losses produced as fuel evaporates from the carburetor and fuel tank in carbureted vehicles,
or from the fuel tank in fuel injected vehicles, as a result of heating of the fuel tank and fuel system
above ambient temperatures. Average temperatures that occur during a hot soak event are shown in
Figure 1. As can be seen from this figure, fuel system temperatures greatly exceed ambient
temperatures during a hot soak event.
Hot soak emissions generally occur during the one-hour period1 after the engine is shut down
and are measured in a sealed housing for evaporative emission determination (SHED). Results from
SHED tests are in grams per one-hour test (g/test). Level of emissions during a hot soak is a function
of fuel volatility (Reid Vapor Pressure [RVP]) and ambient temperature, as well as other variables.
In previous versions of the MOBILE model, hot soak emissions were characterized using data
derived from laboratory testing of light-duty vehicles and trucks. Testing was conducted under EPA-
derived fuel RVP and temperature criteria. These criteria stem from the EPA certification test
procedure, and specifically consisted of a certification test fuel with a fuel volatility level of 9.0 psi, a
fuel tank fill level of 40%, and an ambient temperature of approximately 82°F. Two additional fuels
with RVP levels of 10.4 and 11.7 psi were also used during testing performed in 1984 through 1989.
In 1990, data from testing in Hammond, Indiana was also added to the emission factor database. This
test program involved the procurement of vehicles tested in Indiana Inspection and Maintenance (I/M)
program lanes, where vehicles were driven on an EVI240 transient test cycle, and testing of the
evaporative emissions control systems was performed to see if either failures existed due to improper
pressure and/or improper purging of vapors. This testing is known as pressure/purge tests. Some
vehicles that failed either test were also tested for their diurnal and hot soak emissions, in an attempt
to assess whether failure of pressure/purge testing could be correlated to high diurnal and hot soak
emissions.
The MOBILE model contains correction factors for the effects of RVP and temperature on hot
soak emissions that allow the user to adjust these conditions to correspond to local values. These
correction factors have been developed though statistical analysis of the EPA hot soak emissions data.
Since the development of the latest version of MOBILE, EPA has recognized the need to
incorporate additional hot soak data into its modeling efforts. The data used to generate hot soak curve
1 The majority of hot soak emissions occur within 10 minutes after engine shut-off, but
are measured during a hot soak test for a 1 hour period.
1
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220
200
180
160
Air Under Hood
Main Fuel Jet
Carburetor Bowl
Fuel-Pump Inlet
Atmospheric Temperature
Appro*. 60° F
I4O
120
100
5 10 5 0
Time Before Idle
5 10 15 20
Time After Shutdown to Idle, min.
fits for MOBILESa did not incorporate low RVP fuels, which are now regulated in various parts of the
country. Furthermore, the data did not fully represent "real world" conditions, as "real world" vehicles
Figure 1. Temperatures occurring during a hot soak event
never have certification fuel in their fuel tanks and are operated under a wide range of ambient
temperature conditions.
To this end, several studies have recently been conducted characterizing hot soak emissions at
higher ambient temperatures and over a wider range of fuel RVPs than those contained in previous
EPA testing. The two most significant studies were conducted by the Auto/Oil Air Quality
Improvement Research Program (AQIRP) and by EPA, under contract with the Automotive Testing
Laboratories (ATL). Both studies recruited vehicles from Arizona Inspection and Maintenance (I/M)
testing lanes, and the testing was performed under comparable conditions. Table 1 lists the testing
conditions and average results of emissions testing for both studies. In addition, two other EPA work
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assignments (Contract 68-C3-0006, Work Assignments 0-07 and 0-11) contain hot soak testing on
smaller numbers of vehicles. This report details an analysis of these "real world" databases and
develops correction factors for RVP and ambient temperature based upon this data. These databases
encompass vehicles with "real world" gasolines in their fuel tanks, have a variety of tank fill levels, are
tested as received and at a variety of ambient temperatures.
Table 1. Comparison of testing criteria for "real world" hot soak studies
Testing dates
Number of test vehicles
Type of vehicles
Model years represented
Location of Testing
Daytime temperature range
Hot soak cut-point
As-received average HC
emissions across fleet
Range of emissions
No. of high emitters
Percent high-emitters
Auto/Oil
June 15, 1993 to
September 15, 1993
299
In-use LDVs and LDTs
1983 - 1993
Automotive Testing
Laboratory, Mesa, Arizona
82°Ftoll2°F
2.0 grams/test
1.53 grams/test
0.04 to 49.39 grams/test
46 out of 299
15.3%
EPA
July?, 1995 to
September 29, 1995
181
In-use LDVs and LDTs
1981 - 1994
Automotive Testing
Laboratory, Mesa, Arizona
at least 80 °F
2.0 grams/test
1.76 grams/test
0.06 to 46.95 grams/test
28 out of 181
15.6%
II.
DATA ANALYSIS
There are several variables which directly affect hot soak emissions. Hot soak emissions in
vehicles (with newer evaporative emissions control systems) are usually due to small leaks in the
evaporative emission control system (joints, lines, valves) and permeation of the fuel hoses and tank.
These fuel vapor leaks are generally driven by the heating of the fuel system above ambient conditions
during a hot soak event As seen in Figure 1, fuel system temperatures greatly exceed ambient
temperatures during a hot soak event. Fuel tank temperature is usually close to ambient but can
increase in fuel injected vehicles due to fuel returning from the hot engine compartment. Typically,
tank temperatures in fuel injected vehicles can exceed ambient temperatures by 5 to 15°F. Thus hot
soak emissions are not a direct function of ambient temperature.
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Data from the "real world" data set are characterized in Table 2. In addition to the categories
of the data used in MOBILESa, two new strata were added. The first is "gross liquid leaker." This
refers to vehicles which produce abnormally high evaporative emissions as a result of a fuel leak and
which have hot soak emissions of over 10 grams per test. The second is the addition of two new model
year groupings (1981-1985, and 1986 and newer) for vehicles that passed both the pressure and purge
tests. This stratification of model year groups was used to capture the significant improvement of
evaporative emissions systems in most automobiles that occurred beginning with the 1986 model year.
Other strata used in MOBILESa were continued, i.e., hot soak data from vehicles that passed both the
pressure and purge tests wer stratified by fuel system type (carbureted [Carb], throttle body fuel
injected [TBI], and port fuel injected [PFI]) and by vehicle type classification (passenger cars [LDV]
and light-duty trucks [LOT]).
Table 2. Data strata
Fuel
System
Carb
FI
TBI
PFI
Carb
TBI
PFI
Carb
TBI
TBI
TBI
TBI
PFI
PFI
PFI
PFI
Carb
Carb
Carb
Carb
Pressure Purge
Test Test
All
All
Fail
Fail
Fail
All
All
All
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Total
All
All
All
All
All
Fail
Fail
Fail
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Pass
Leaker
Category
Liquid
Liquid
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vapor
Vehicle
Type
All
All
All
All
All
All
All
All
LDV
LDV
LOT
LOT
LDV
LDV
LOT
LOT
LDV
LDV
LOT
LOT
Model
Year
All
All
All
All
All
All
All
All
81-85
86+
81-85
86+
81-85
86+
81-85
86+
81-85
86+
81-85
86+
Vehicles/Average Emissions
Sample
Size
2
7
19
40
21
12
23
12
17
56
0
29
15
225
0
39
45
38
14
16
630
Average
Hot Soak
(g/test)
14.60
57.79
5.30
2.50
6.39
1.71
10.69
4.52
0.54
0.61
-
0.48
0.51
0.66
-
1.17
2.27
1.35
3.68
1.29
2.50
Standard
Deviation
(g/test)
0.09
26.71
9.52
2.80
3.93
2.48
9.90
2.95
0.37
1.24
-
0.35
0.43
2.37
-
2.54
3.50
1.67
4.18
1.42
2.99
Hose permeation can also be a large source of hot soak emissions, particularly in fuel injected
vehicles. PFI systems typically run at pressures of 40-50 psi, while TBI systems run around 10 psi.
Permeation of fuel through elastomers in the fuel and evaporative control system can be very
temperature sensitive and can be a large source of hot soak emissions in newer vehicles. Injector leaks
in fuel injected systems can result in very high hot soak emissions (liquid leakers).
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A further factor in real world hot soak emissions is the different molecular weight of the fuel
vapor. Because different fuels have different light ends and there is some weathering (loss of light ends
over time) of fuel components in fuel tanks, hot soak emissions can vary in molecular weight by up to
50%. In the Auto/Oil Air Quality Improvement Research Program (AQIRP), over 50 current vehicles
were tested on a variety of fuels. Hot soak emission molecular weight varied from 65.6 g/g mole to
92.3 g/g mole with an average molecular weight of 84.1 g/g mole. These large fluctuations in
molecular weight can significantly affect the mass of emissions emitted during a hot soak event.
With these factors in mind, it was not surprising to find significant scatter in the real world hot
soak data. Furthermore, most of the data represented RVPs of 5 to 7 psi. Extrapolation of this data
past 9 psi is not recommended due to the narrow range of RVPs in the data set. Therefore curve fits
using real world data were only generated up to 9 psi RVP. For RVPs over 9 psi, previous MOBILESa
curves were used. This presented an additional challenge to the regression analyses, making sure that
the two curves met at 9 psi RVP at all temperatures. This required that the functional form of the
equation be identical to those presently used in MOBILESa and that the temperature coefficient in those
equations be the same. Thus regression analyses were performed only on RVP using the real world
data.
In some cases, the data produced a negative coefficient with regard to RVP (i.e. as RVP
increased, predicted hot soak emissions decreased). As this is intuitively incorrect, additional data
points were generated using the MOBILESa curve fits at 9 psi RVP and added to the data sets that
produced a negative RVP coefficient until the resulting curve produced by regression analysis had a
positive RVP coefficient. Further discussion is found in each of the sections below.
The following sections describe the stratification of the hot soak data sets, the methodology
used to determined curve fits of that data, and a discussion of the results of the curve fits.
III. GROSS LIQUID LEAKERS
Liquid fuel leaks from fuel systems can result in very high hot soak emissions. This is
particularly true for fuel injection systems that operate at high pressure (40-45 psi). If an injector is
leaking due to damage or incorrect position, pressure built up in the fuel system will bleed off through
that injector. Liquid leaks can also exist in carbureted fuel systems as a result of leaking carburetor
gaskets or a defective fuel shut off at the carburetor bowl. The real world data set included 17 liquid
leakers, 9 of which fell into the gross liquid leaker category. Gross liquid leakers were defined as those
vehicles with liquid fuel leaks that were measured at over 10 grams per test of hot soak emissions.
Since the set of liquid leaker data was so small, all that could be defined was an average value for two
different fuel systems, namely carbureted (Carb) and port fuel injected (PFI). Carb vehicles had an
average gross liquid leaker hot soak value of 14.60 g/test, while PFI vehicles had an average liquid
leaker hot soak value of 57.79 g/test. It is reasonable that fuel injected systems would have much
higher liquid leak emissions as they are usually under higher fuel pressure. While there is no data on
TBI liquid leakers in the data sets, Bernoulli's equation indicates that the leak rate for TBI systems
would be about one half that for PFI systems (the square root of the ratio of operating pressures).
Therefore, without further data, the author suggests assuming that TBI liquid leakers might emit
approximately half the emissions of PFI systems.
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IV. PRESSURE TEST FAIL VEHICLES
Of the 630 vehicles tested, 80 vehicles that did not fall into the gross liquid leaker category
failed the pressure test. Data within this strata had significant scatter and in several cases there was not
enough data to support further stratification by fuel system type, so all pressure fail data were
aggregated together similar to what was done for MOBILESa. The MOBILESa curve fit for pressure
failed vehicles was in the form of:
Hot Soak = exp(A*(RVP-9.0) +B*(Temp - 82) + C) (IV. 1)
Since the data was at various ambient temperatures, each hot soak test value was adjusted to 95 °F using
the MOBILESa temperature correction as shown below:
Adjusted Hot Soak = Hot Soak * exp(1.774+0.05114*(95-82))/exp(1.774+0.05114*(Temp-82)) (TV2)
In addition, to be consistent with MOBILESa, all fuel injected test data (TBI and PFI) were divided by
0.88, the fuel system adjustment factor in MOBILESa (in MOBILESa, this factor is multiplied by all
fuel injected vehicle results to adjust for the difference between in-use and FTP fuel tank levels). A
regression analysis was run on the adjusted hot soak versus RVP data to determine coefficient B in
equation IV. 1 above. A t-statistic of 2.88 resulted for the coefficient with a P-value of 0.0051. The
coefficient C was determined so that the calculated hot soak results using the real world curve fit
matched the MOBILESa curve fit at 9 psi RVP at all temperatures. This still resulted in a P-value of
0.0195 for coefficient C. The equation for all pressure fail vehicles for less than 9 psi RVP is:
Hot Soak = exp(0.413356*(RVP - 9.0) + 0.05114*(Temp - 82) + 1.774) (IV.3)
This may be compared to the MOBILESa equation for RVP less than 9.0 psi, which is:
Hot Soak = exp (0.4443 *(RVP - 9.0) + 0.05114*(Temp - 82) +1.774) (IV.4)
For fuel injected vehicles, the fuel system adjustment factor of 0.88 should be multiplied by both
equations IV.3 and IV.4 to obtain hot soak emission results.
Predicted hot soak emissions calculated using equation IV.3 are shown in Table 3 for pressure
fail vehicles before application of the fuel system adjustment factor. MOBILESa estimates calculated
using equation IV.4 are also included for reference. Figure 2 shows the real world and MOBILESa
curve fits as well as the real world data for TBI vehicles. Figure 3 shows the real world and
MOBILESa curve fits as well as real world data for PFI vehicles. Figure 4 shows the real world and
MOBILESa curve fits as well as real world data for Carb vehicles. Figures 2 and 3 show both real
world and MOBILESa curve fits with the fuel system adjustment factor applied. While the curve fits
are the same in all three figures (except for application of the fuel system adjustment factor), real world
data was shown divided by fuel system type in Figures 2 through 4 so that the reader could see how the
real world data compared against the new and MOBILESa curve fits.
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Table 3. Pressure fail hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
New
Curve Fit
0.79
1.70
3.66
7.88
1.19
2.57
5.53
11.91
1.80
3.88
8.36
18.00
2.73
5.87
12.64
27.22
4.12
8.87
19.11
41.15
MOBILESa
Curve Fit
0.70
1.50
3.23
6.96
1.09
2.34
5.04
10.85
1.69
3.65
7.86
16.92
2.64
5.69
12.25
26.39
4.12
8.87
19.11
41.15
* unadjusted for fuel system
Figure 2. Estimated TBI Pressure Fail Hot Soak Emissions and Real World Data
15.0
Real World Data
TBI Curve Fit
• MOBILESa
Test Points at Varying T, Curve Fits atT=95 deg F
-- 12.5
10.0
-- 7.5
-- 5.0
&
1
CO
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Rgure 3. Estimated PR Pressure Fail Hot Soak Emissions and Real World Data
15.0
Real World Data
PFI Curve Fit
• MOBILESa
Test Points at Varying T, Curve Fits at T=95 deg F
-- 12.5
10.0
-- 7.5
I/)
4-1
0>
co
O
co
Rgure 4. Estimated Carb Pressure Fail Hot Soak Emissions and Real World Data
15.
Real World Data
Carb Curve Fit
MOBILESa
Test Points at Varying T, Curve Fits at T=95 deg F
0.0
9.0
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As seen in Table 3 and Figures 2, 3 and 4, the new pressure fail curve fits predict slightly higher
emissions in the 5 to 7 psi RVP range than the previous MOBILESa curve fits. This indicates that real
world pressure test fail data shows slightly higher levels of hot soak emissions than previously
estimated from the laboratory data used to generate the curve fits for MOBILESa.
V. PURGE TEST FAIL VEHICLES
Of the 630 vehicles tested, 47 vehicles that did not fall into the gross liquid leaker category
failed the purge test. Data within this strata had significant scatter and in several cases there was not
enough data to support further stratification by fuel system type, so all purge fail data were aggregated
together similar to what was done for MOBILESa. The MOBILESa curve fit for purge failed vehicles
was in the form of:
Hot Soak = exp(A*(RVP-9.0) +B*(Temp - 82) + C) (V. 1)
Since the data was at various ambient temperatures, each hot soak test value was adjusted to 95 °F using
the MOBILESa temperature correction as shown below:
Adjusted Hot Soak = Hot Soak * exp(1.76223+0.05114*(95-82))/exp(1.76223+0.05114*(Temp-82)) (V.2)
In addition, to be consistent with MOBILESa, all fuel injected test data were divided by 0.88, the fuel
system adjustment factor in MOBILESa. A regression analysis was run on the adjusted hot soak versus
RVP data to determine coefficient B in equation V.I above. A t-statistic of 2.37 resulted for the
coefficient with a P-level of 0.0222. The coefficient C was determined so that the calculated hot soak
results using the real world curve fit matched the MOBILESa curve fit at 9 psi RVP at all
temperatures. This coefficient resulted in a P-value of 0.0261. The equation for all purge fail vehicles
for less than 9 psi RVP is:
Hot Soak = exp(0.552175*(RVP - 9.0) + 0.05114*(Temp - 82) + 1.76223) (V.3)
This may be compared to the MOBILESa equation for RVP less than 9.0 psi, which is:
Hot Soak = exp (0.4443 *(RVP - 9.0) + 0.05114*(Temp - 82) +1.76223) (V.4)
For fuel injected vehicles, the fuel system adjustment factor of 0.88 should be multiplied by both
equations V.3 and V.4 to obtain hot soak emission results.
Predicted hot soak emissions calculated using equation V.3 are shown in Table 4 for purge fail
vehicles before application of the fuel system adjustment factor. MOBILESa estimates calculated
using equation V.4 are also included for reference. Figure 5 shows the real world and MOBILESa
curve fits as well as the real world data for TBI vehicles. Figure 6 shows the real world and
MOBILESa curve fits as well as real world data for PFI vehicles. Figure 7 shows the real world and
MOBILESa curve fits as well as real world data for Carb vehicles. Figures 5 and 6 show both real
world and MOBILESa curve fits with the fuel system adjustment factor applied. Again, real world data
have been stratified by fuel system type in Figures 5 through 7 for comparison purposes only.
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Table 4. Purge fail hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
New
Curve Fit
0.45
0.96
2.07
4.47
0.78
1.67
3.60
7.76
1.35
2.91
6.26
13.48
2.34
5.05
10.87
23.41
4.07
8.77
18.89
40.67
MOBILESa
Curve Fit
0.69
1.48
3.19
6.88
1.07
2.31
4.98
10.73
1.67
3.61
1.11
16.73
2.61
5.62
12.11
26.08
4.07
8.77
18.89
40.67
* unadjusted for fuel system
Figure 5. Estimated TBI Purge Fail Hot Soak Emissions and Real World Data
8.5
15.0
-- 12.5
• Real World Data
— TBI Curve Fit
MOBILESa
Test Points at Varying T, Curve Fits atT=95 deg F
10.0 _
&
J£
8
CO
9.0
10
-------
Figure 6. Estimated PR Purge Fail Hot Soak Emissions and Real World Data
Real World Data
PFI Curve Fit
• MOBILESa
Test Points at Varying T, Curve Fits at T=95 deg F
15.0
-- 12.5
10.0
-- 7.5
-- 5.0
-- 2.5
D)
~
8
CO
0.0
5.0
5.5
6.0
6.5
7.0
RVP(psi)
7.5
8.0
8.5
9.0
Figure 7. Estimated Carb Purge Fail Hot Soak Emissions and Real World Data
• Real World Data
• — Carb Curve Fit
MOBILESa
Test Points at Varying T, Curve Fits atT=95 deg F
8.5
15.0
0.0
9.0
11
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As seen by Table 4 and Figures 5, 6 and 7, the new purge fail curve fits predict slightly lower
emissions in the 5 to 7 psi RVP range than the previous MOBILESa curve fits. This indicates that real
world purge test fail data shows slightly lower levels of hot soak emissions than previously estimated
from the laboratory data used to generate the curve fits for MOBILESa.
VI. PASS/PASS VEHICLES
Of the 630 vehicles tested, 494 vehicles which did not fall into the gross liquid leaker category
passed both the pressure and purge tests. The hot soak data set of vehicles that passed both the
pressure and purge tests allow some disaggregations, although some of the disaggregations did not
produce statistically significant curves due to large data scatter. In some cases, the data even produced
negative coefficients for RVP and additional data calculated from MOBILESa curve fits at 9 psi RVP
had to be added to produce reasonable trends over the RVP range (5 to 9 psi). The functional form of
current MOBILESa pass/pass vehicle equations and strata were used to perform regression analyses
of the real world data. An additional stratification was added to each set, however. Data was divided
into two model year groupings for each vehicle type. Since manufacturers became more aware of the
need to 'fine tune' evaporative emission systems during the 1981 through 1985 model years, the data
was stratified into two model year groupings, namely 1981-1985 and 1986+. Discussion of the
methodology used and the results of the regression analysis are contained within each subsection
below.
A. TBI
Of the 494 vehicles that passed both the pressure and purge tests, 102 vehicles had TBI fuel
systems. A curve fit similar to that used in MOBILESa for TBI vehicles was used:
Hot Soak = (A + B*RVP)*(C + D*Temp2)/E (VI. 1)
Since the data was at various ambient temperatures, each hot soak test value was adjusted to 95 °F using
the temperature correction factor defined in MOBILESa curve fits for TBI pass/pass vehicles:
Adjusted Hot Soak = Hot Soak * (-2.4636+0.00056161*952))/(-2.4636+0.00056161*Temp2) (VI.2)
In addition, to be consistent with MOBILESa, all fuel injected test data were divided by 0.88,
the fuel system adjustment factor used in MOBILESa. A regression analysis was run on the adjusted
hot soak versus RVP data to determine coefficients A and B in equation VT. 1 above for each strata.
Of the 102 TBI tests, 17 corresponded to LDVs with model years between 1981 and 1985, 56
were LDVs model years 1986+, and 29 were LDTs model years 1986+. There were no test data for
LDTs model years 1981-1985.
For the TBI LDVs with model years (MY) between 1981 and 1985, t-statistics of-0.44 and
0.87 and P-values of 0.666 and 0.396 resulted for coefficients A and B, respectively, indicating that
neither produced statistically significant curves due to significant data scatter. It did, however, produce
a reasonable trend with regard to RVP. For TBI LDVs with MY 1986+, t-statistics of -0.44 and 0.65
and P-values of 0.661 and 0.516 resulted for coefficients A and B, respectively, indicating that neither
12
-------
produced statistically significant curves due to significant data scatter. It did, however, also produce
a reasonable trend with regard to RVP. For TBI LDTs with MY 1986+, t-statistics of-3.96 and 6.76
and P-values of 4.9E-04 and 3.0E-07 resulted for coefficients A and B, respectively, indicating that
these coefficients were statistically significant. Coefficients C and D were retained from the
MOBILESa curve fits for TBI pass/pass vehicles and coefficient E was determined so that the
calculated hot soak results using the real world curve fits matched the MOBILESa curve fits at 9 psi
RVP at all temperatures.
The equations for the two model year groupings of LDVs are as follows:
1981-1985 MY LDVs
Hot Soak = (-0.52111 + 0.159322*RVP)*(-2.4636 + 0.00056161 *Temp2)/l.898 (VI.3)
1986+MY LDVs
Hot Soak = (-1.27508 + 0.28853*RVP)*(-2.4636 + 0.00056161*Temp2)/2.748 (VI.4)
For comparison, the MOBILESa equation for MY 1981+ LDV TBI vehicles that pass both the
pressure and purge tests is:
Hot Soak = (0.258327 + 0.041297*RVP)*(-2.4636 + 0.00056161 *Temp2)/l.31 (VI.5)
As explained in Section IV, equations VI.3, VI.4, and VT.5 should be multiplied by the fuel system
adjustment factor of 0.88 to obtain hot soak emission results.
Predicted hot soak emissions for TBI LDVs calculated using equations VI.3 and VI.4 are
shown in Table 5 (with the fuel system adjustment factor applied) along with MOBILESa TBI LDV
estimates (calculated using equation VI.5). Plots of hot soak emissions at 95 °F are shown in Figure
8. Curves shown in Figure 8 also have the fuel system adjustment factor applied.
As seen in Table 5 and Figure 8, the new TBI LDV curve fits predict slightly lower emissions
in the 5 to 7 psi RVP range than the previous MOBILESa curve fits. While no conclusions can be
drawn from these curve fits (as they are not statistically significant), one might assume that the real
world vehicle set used to define these curve fits had lower hot soak emissions in the 5 to 7 psi RVP
range than that estimated from MOBILESa (which was produced from an extrapolation of higher
laboratory data). Furthermore, curve fits for MY 86+ vehicles showed lower hot soak emissions than
the MY 81-85 group, which is reasonable assuming an improvement in evaporative control system
design.
The equation for LDTs derived from the real world data is:
1986+MY LDTs
Hot Soak = (-0.71055 + 0.17803*RVP)*(-2.4636 + 0.00056161*Temp2)/2.596 (VI.6)
13
-------
Table 5. TBILDV pass/pass hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 81-85
LDV
0.09
0.27
0.48
0.72
0.14
0.42
0.75
1.13
0.19
0.57
1.03
1.55
0.24
0.73
1.30
1.96
0.29
0.88
1.58
2.38
MY 86+
LDV
0.04
0.11
0.20
0.30
0.10
0.30
0.54
0.82
0.17
0.50
0.89
1.34
0.23
0.69
1.23
1.86
0.29
0.88
1.58
2.38
MY 81+ LDV
MOBILESa
0.22
0.65
1.16
1.76
0.24
0.71
1.27
1.91
0.26
0.77
1.37
2.07
0.28
0.82
1.47
2.22
0.29
0.88
1.58
2.38
* adjusted for fuel system
Rgure 8. Estimated TBI LDV Pass/Pass Hot Soak Emissions and Real World Data
,...-,....,. _ _ ,_.. . _ „ , ,_
Test Points at Varying T, Curve Fits atT=95 deg F
} a a
• Real World 81-85
D Real World 86+
-- LDV 81-85 Curve Fit
... - LDV 86+ Curve Fit
- MOBILESa
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
in
H
D)
S
Ho
14
-------
The MOBILESa equation for MY 1981+ LDT TBI vehicles that pass both the pressure and
purge tests is:
Hot Soak = (0.078327 + 0.041297*RVP)*(-2.4636 + 0.00056161 *Temp2)/l.31 (VI.7)
Equations VT.6 and VT.7 should be multiplied by the fuel system adjustment factor of 0.88 to obtain
hot soak emission results.
Predicted hot soak emissions for TBI LDTs calculated using equation VI. 6 are shown in Table
6 along with MOBILESa TBI LDT estimates (calculated using equation VT.7) with the fuel system
correction factor applied in both cases. Plots of hot soak emissions for TBI LDT pass/pass vehicles at
95 °F are shown in Figure 9 with the fuel system correction factor applied to the curve fits.
Predicted hot soak emissions using the real world curve fits are lower in the 5 to 7 psi RVP
range than predicted using previous MOBILESa curve fits. The real world data shows lower levels
of hot soak emissions in this region than previously extrapolated from the laboratory data at higher
RVPs used to generate the curve fits for MOBILESa.
B. PFI
Of the 494 vehicles that passed both the pressure and purge tests, 279 vehicles had PFI fuel
systems. A curve fit similar to that used in MOBILESa for PFI vehicles was used:
Hot Soak = (A + B*RVP)*(C*Temp)/D (VI.8)
Since the data was at various ambient temperatures, each hot soak test value was adjusted to 95 °F using
the temperature correction factor defined in MOBILESa curve fits for PFI pass/pass vehicles:
Adjusted Hot Soak = Hot Soak * 95/Temp (VI.9)
In addition, as explained in Section IV, to be consistent with MOBILESa, all fuel injected test
data were divided by 0.88, the fuel system adjustment factor in MOBILESa. A regression analysis was
run on the adjusted hot soak versus RVP data to determine coefficients A and B in equation VI.8 above
for each strata.
Of the 279 PFI tests, 15 corresponded to LDVs with model years between 1981 and 1985, 225
were LDVs model years 1986+, and 39 were LDTs model years 1986+. There were no test data for
LDTs model years 1981-1985.
For the PFI LDVs with model years (MY) between 1981 and 1985, the first regression analysis
resulted in a negative B coefficient, implying a decrease in emissions with increasing fuel RVP, which
is intuitively incorrect. To correct this situation, 15 additional data points calculated using the
MOBILESa curve fit at 9 psi RVP and 95 °F were added to the 15 real world data points. This
produced a positive B coefficient with a t-statistic of 2.02 with a P-value of 0.0528. Real world data
for PFI LDVs with MY 1986+ also produced a negative B coefficient. By adding 25 MOBILESa
calculated data points at 9 psi RVP and 95 °F to the 223 real world data points, a positive B coefficient
15
-------
Table 6. TBILDT pass/pass hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 86+
LDTs
0.04
0.13
0.23
0.34
0.08
0.25
0.45
0.68
0.13
0.38
0.68
1.02
0.17
0.50
0.90
1.36
0.21
0.63
1.13
1.70
MY 81+ LDTs
MOBILESa
0.13
0.40
0.71
1.08
0.15
0.46
0.82
1.23
0.17
0.51
0.92
1.39
0.19
0.57
1.02
1.54
0.21
0.63
1.13
1.70
* adjusted for fuel system
Figure 9. Estimated TBI LDT Pass/Pass Hot Soak Emissions and Real World Data
D Real World 86+
.... LDT 86+ Curve Fit
MOBILESa
Test Points at Varying T, Curve Fits at T=95 deg F
B
n
^^^__^^^^^^^^^^^^^^^^^^^^^^ m * ~
n CL- • " " *
„ _--•"" T-"T|I n n
III 1 1 1 1
1 1 1 1 1 1 1
0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9
- 2.25
- 2.00
- 1.75
- 1.50
- 1.25
- 1.00
- 0.75
- 0.50
- 0.25
- 0.00
0
RVP (psi)
C"
«
3
co
O
CO
4-1
O
X
16
-------
resulted with a t-statistic of 1.74 and a P-value of 0.084. For PFI LDTs with MY 1986+, a positive B
coefficient was achieved without addition of MOBILESa points, but the t-statistic and P-value were
only 0.29 and 0.77, respectively, indicating that it was not statistically significant. Coefficient C was
retained from the MOBILESa curve fits for PFI pass/pass vehicles and coefficient D was determined
so that the calculated hot soak results using the real world curve fits matched the MOBILESa curve
fits at 9 psi RVP at all temperatures.
The equations for the two model year groupings of LDVs are as follows:
1981-1985 MY LDVs
Hot Soak = (-0.058967 + 0.100658*RVP)*(0.0055541*Temp)/0.749 (VI. 10)
1986+MY LDVs
Hot Soak = (-0.0097563 + 0.082809*RVP)*(0.0055541 *Temp)/0.651 (VI. 11)
For comparison purposes, the MOBILESa equation for MY 1981+ LDV PFI vehicles that pass
both the pressure and purge tests is:
Hot Soak = (-0.40673 + 0.10297*RVP)*(0.0055541*Temp)/0.46 (VI. 12)
Equations VI. 10, VI. 11, and VI. 12 should be multiplied by the fuel system adjustment factor of 0.88
to obtain hot soak emission results.
Predicted hot soak emissions for PFI LDVs calculated using equations VI. 10 and VI. 11 are
shown in Table 7 (with the fuel system adjustment factor applied) along with MOBILESa PFI LDV
estimates (calculated using equation VI. 12). Plots of hot soak emissions at 95 °F are shown in Figure
10. Curves shown in Figure 10 also have the fuel system adjustment factor applied.
Predicted hot soak emissions using the real world curve fits are generally higher in the 5 to 7
psi RVP range than predicted using previous MOBILESa curve fits. While this could indicate that real
world data shows higher levels of hot soak emissions in this region than previously estimated from the
laboratory data used to generate the curve fits for MOBILESa, it could also be a artifact of the
significant data scatter.
The equation for LOT hot soak emissions derived from the real world data is:
1986+MY LDTs
Hot Soak = (0.3456+0.04906*RVP)*(0.0055541 *Temp)/0.805 (VI. 13)
For comparison purposes, the MOBILESa equation for MY 1981+ LDT PFI vehicles that pass
both the pressure and purge tests is:
Hot Soak = (0.078327 + 0.041297*RVP)*(0.0055541*Temp)/0.46 (VI. 14)
Equations VI.13 and VI.14 should be multiplied by the fuel system adjustment factor of 0.88 to obtain
hot soak emission results.
17
-------
Table 7. PFILDV pass/pass hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 81-85
LDVs
0.22
0.26
0.30
0.35
0.27
0.32
0.37
0.43
0.32
0.38
0.44
0.51
0.37
0.44
0.51
0.58
0.41
0.50
0.58
0.66
MY 86+
LDVs
0.23
0.27
0.32
0.36
0.27
0.33
0.38
0.44
0.32
0.39
0.45
0.51
0.37
0.44
0.51
0.59
0.41
0.50
0.58
0.66
MY 81+ LDVs
MOBILESa
0.09
0.10
0.12
0.14
0.17
0.20
0.24
0.27
0.25
0.30
0.35
0.40
0.33
0.40
0.47
0.53
0.41
0.50
0.58
0.66
* adjusted for fuel system
Rgure 10. Estimated PFI LDV Pass/Pass Hot Soak Emissions and Real World Data
ET
• Real World 81-85
D Real World 86+
• — LDV 81-85 Curve Fit
- - - LDV 86+ Curve Fit
MOBILESa
1.50
-- 1.25
" 1-00 _
Test Points at Varying T,
Curve Fits atT=95 deg F
-- 0.75
'- 0.50
- 0.25
9.0
18
-------
Predicted hot soak emissions for PFI LDTs calculated using equation VI. 13 are shown in
Table 8 along with MOBILESa PFI LDT estimates (calculated using equation VI. 14) with the fuel
system correction factor applied in both cases. Plots of hot soak emissions for PFI LDT pass/pass
vehicles at 95 °F are shown in Figure 11 with the fuel system correction factor applied to the curve fits.
Predicted hot soak emissions using the real world curve fit are only slightly higher than
previous MOBILESa estimates in the 5 to 7 psi RVP range.
C. Carb
Of the 494 vehicles that passed both the pressure and purge tests, 113 vehicles had Carb fuel
systems. A curve fit similar to that used in MOBILESa for Carb vehicles was used:
Hot Soak = (A + B*RVP)*(C + D*Temp2)/E (VI. 15)
Since the data was at various ambient temperatures, each hot soak test value was adjusted to 95 °F using
the temperature correction factor defined in MOBILESa curve fits for Carb pass/pass vehicles:
Adjusted Hot Soak = Hot Soak * (-2.4636+0.00056161*952))/(-2.4636+0.00056161*Temp2) (VI. 16)
A regression analysis was run on the adjusted hot soak versus RVP data to determine coefficients A
and B in equation VI. 15 above for each disaggregation.
Of the 113 Carb tests, 45 corresponded to LDVs with model years between 1981 and 1985, 38
were LDVs model years 1986+, 14 were LDTs with model years between 1981 and 1985 and 16 were
LDTs model years 1986+.
For Carb LDVs with MYs 1981-1985, the first regression analysis resulted in a negative B
coefficient. To correct this situation, 4 additional data points were calculated using the MOBILESa
curve fit at 9 psi RVP and 95 °F and added to the 43 real world data points. This produced a positive
B coefficient with a t-statistic of 2.29 and a P-value of 0.0266. Real world data for LDVs with MYs
1986+ also produced a negative B coefficient. An additional 4 MOBILESa calculated points at 9 psi
RVP and 95 °F were added to the 38 real world points. This gave a positive B coefficient with a t-
statistic of 1.56 and a P-value of 0.127, indicating that it was not statistically significant at a 95%
confidence level but produced a reasonable trend. For Carb LDTs, a similar trend was found. Twelve
MOBILESa data points (at 9 psi and 95 °F) had to be added to the 11 real world data points for MYs
1981-1985 and 15 MOBILESa calculated data points had to be added to the 16 real world data points
for the MYs 1986+ to obtain a positive B coefficient. The t-statistic for the MY 1981-1985 B
coefficient was 0.56 indicating that is was not statistically significant (P-value of 0.57), but the MY
1986+B coefficient t-statistic was 5.16 indicating it was statistically significant (P-value of 1.6E-05).
Coefficients C and D were retained from the MOBILESa curve fits for Carb pass/pass vehicles and
coefficient E was determined so that the calculated hot soak results from using the real world curve fits
matched the MOBILESa curve fits at 9 psi RVP and all temperatures.
19
-------
Table 8. PFILDT pass/pass hot soak emission estimates*
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 86+
LDTs
0.27
0.32
0.38
0.43
0.29
0.35
0.41
0.47
0.31
0.38
0.44
0.50
0.34
0.40
0.47
0.51
0.36
0.43
0.50
0.57
MY 81+ LDTs
MOBILESa
0.23
0.27
0.32
0.36
0.26
0.31
0.36
0.42
0.29
0.35
0.41
0.47
0.33
0.39
0.46
0.52
0.36
0.43
0.50
0.57
* adjusted for fuel system
Figure 11. Estimated PR LDT Pass/Pass Hot Soak Emissions and Real World Data
n
D Real World 86+
- - - LDT 86+ Curve Fit
MOBILESa
Test Points at Varying T,
Curve Fits at T=95 deg F
5.0 5.5 6.0 6.5 7.0
RVP (psi)
7.5
8.0
8.5
1.50
1.25
-- 1.00
-- 0.75
0.50
0.25
ro
O
(0
0.00
9.0
20
-------
The equations for the two model year groupings of LDVs are as follows:
1981-1985 MY LDVs
Hot Soak = (-1.13591 + 0.39098*RVP)*(-2.4636 + 0.00056161*Temp2)/2.081 (VI. 17)
1986+MY LDVs
Hot Soak = (-1.7318 + 0.45214*RVP)*(-2.4636 + 0.00056161 *Temp2)/2.041 (VI. 18)
For comparison purposes, the MOBILESa equation for MY 1981+ LDV Carb vehicles that pass
both the pressure and purge tests is:
Hot Soak = (0.25593 + 0.13823*RVP)*(-2.4636+0.00056161*Temp2)/1.31
(VI. 19)
Predicted hot soak emissions for Carb LDVs calculated using equations VI. 17 and VT.18 are
shown in Table 9 along with MOBILESa Carb LDV estimates (calculated using equation VI. 19). Plots
of hot soak emissions at 95 °F are shown in Figure 12.
Table 9. Carb LDV pass/pass hot soak emission estimates
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 81-85
LDVs
0.27
0.82
1.47
2.21
0.40
1.21
2.17
3.27
0.54
1.60
2.87
4.33
0.67
2.00
3.57
5.38
0.80
2.39
4.27
6.44
MY 86+
LDVs
0.18
0.54
0.97
1.46
0.33
1.00
1.79
2.70
0.49
1.46
2.62
3.95
0.64
1.93
3.44
5.19
0.80
2.39
4.27
6.44
MY 81+ LDVs
MOBILESa
0.50
1.51
2.70
4.07
0.58
1.73
3.09
4.66
0.65
1.95
3.48
5.25
0.72
2.17
3.88
5.85
0.80
2.39
4.27
6.44
21
-------
Figure 12. Estimated Garb LDV Pass/Pass Hot Soak and Real World Data
5.0
4.5
4.0
3.5
3.0
2.5
Test Points at Varying T, Curve Fits atT=95
• Real World 81-85
D Real World 86+
. — LDV 81-85 Curve Fit
- - - LDV 86+Curve Fit
MOBILESa
7.0
RVP(psi)
9.0
Predicted hot soak emissions using the real world curve fits are lower than the previous
MOBILESa estimates in the 5 to 7 psi RVP range. While no conclusions can be drawn from these
curve fits (as additional data needed to be added to make the curves show a positive trend with RVP),
one might assume that the real world vehicle set used to define these curve fits had lower hot soak
emissions in the 5 to 7 psi RVP range than that estimated from MOBILESa (which was produced from
an extrapolation of higher laboratory data). Furthermore, curve fits for MY 86+ vehicles showed lower
hot soak emissions than the MY 81-85 group, which is reasonable assuming an improvement in
evaporative control system design.
The equations for LDTs derived from the real world data are:
1981-1985 MY LDTs
Hot Soak = (1.29368+0.08904*RVP)*(-2.4636 + 0.00056161 *Temp2)/2.541 (VI.20)
1986+MY LDTs
Hot Soak = (-1.8687+0.43908*RVP)*(-2.4636 + 0.00056161 *Temp2)/2.527 (VI.21)
For comparison purposes, the MOBILESa equation for MY 1981+ LDT Carb vehicles that pass
both the pressure and purge tests is:
Hot Soak = (-0.164070 + 0.13823*RVP)*(-2.4636+0.00056161*Temp2)/1.31
(VI.22)
Predicted hot soak emissions for Carb LDTs calculated using equations VT.20, and VI.21 are
shown in Table 10 along with MOBILESa Carb LDT estimates (calculated using equation VI.22).
Plots of hot soak emissions for Carb LDT pass/pass vehicles at 95°F are shown in Figure 13.
22
-------
Table 10. Carb LDT pass/pass hot soak emission estimates
(g/test)
RVP
psi
5.0
6.0
7.0
8.0
9.0
Temp
op
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
75
90
105
120
MY 81-85
LDTs
0.48
1.43
2.55
3.85
0.50
1.50
2.68
4.05
0.52
1.57
2.81
4.24
0.55
1.65
2.94
4.44
0.57
1.72
3.07
4.64
MY 86+
LDTs
0.09
0.27
0.48
0.73
0.21
0.63
1.13
1.70
0.33
0.99
1.78
2.68
0.45
1.36
2.43
3.66
0.57
1.72
3.07
4.64
MY 81+ LDTs
MOBILESa
0.28
0.84
1.50
2.26
0.35
1.06
1.89
2.86
0.43
1.28
2.29
3.45
0.50
1.50
2.68
4.04
0.57
1.72
3.07
4.64
Figure 13. Estimated Carb LDT Pass/Pass Hot Soak and Real World Data
• Real World 81-85
D Real World 86+
. _ LDT 81-85 Curve Fit
- - - LDT 86+ Curve Fit
MOBILESa
Test Points at Varying T, Curve Fits at T=95 deg F
23
-------
As can be seen from Figure 13 and Table 10, real world data for Carb LDTs with MYs 81-85
predict higher hot soak emissions than MOBILESa and MYs 86+ predict lower hot soak emissions
than MOBILESa. It would be expected that newer model Carb LDTs would have significantly lower
emissions than older model LDTs due to improvements in the evaporative emission control system.
VII. DISCUSSION OF RESULTS
The new "real world" curve fits provided reasonable trends in hot soak emissions relative to
RVP and temperature. For RVPs between 5.0 and 9.0, the "real world" curve fits provided a more
accurate picture of "real world" hot soak emissions for MY 1986+ vehicles. However, the data sets
analyzed contained no data over 9.0 RVP and thus extrapolations beyond 9 psi RVP could not be
developed. This created some dilemma as to meeting the MOBILESa curves at 9 psi RVP. The
methodology used in this report provides a better real world curve fit for lower RVPs and still allows
using MOBILESa curve fits above 9 psi RVP without a discontinuity.
In most cases the curve fits provided reasonable agreement with previous data. The addition
of a liquid leaker category adds better definition of the real world conditions. In addition, the additional
stratification of model year groups provides a better picture of hot soak emissions as technology
improves.
To improve the curve fits developed in this report, additional data are needed, particularly in
the 9 psi RVP range and higher. Previous data in this region were generated using laboratory tests and
may not be indicative of real world conditions. Furthermore, new vehicles now entering the market
have significantly improved evaporative emission control systems. These vehicles should also be
tested to give a more accurate picture of in-use emissions from the current and future U.S. vehicle fleet.
24
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