Development of Evaporative Emissions
Calculations for the Motor Vehicle
Emissions Simulator
(Draft MOVES2009)
Draft Report
United States
Environmental Protection
Agency
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Development of Evaporative Emissions
Calculations for the Motor Vehicle
Emissions Simulator
(Draft MOVES2009)
Draft Report
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
v>EPA
NOTICE
This technical report does not necessarily represent final EPA decisions or
positions. It is intended to present technical analysis of issues using data
that 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.
United States EPA-420-P-09-006
Environmental Protection . ^ _
Agency August 2009
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Table of Contents
1 Background 1
2 Data Sources 1
3 Design and Analysis 2
3.1 Fuel Tank Temperature Generator 2
3.1.1 Input parameters 2
3.1.2 General steps 2
3.1.2.1 Calculating soak temperatures (as a function of ambient temperature) 3
3.1.2.2 Calculating fuel tank temperatures during operation 4
3.2 Permeation 4
3.2.1 Base Rates 4
3.2.2 Temperature adjustment 5
3.2.3 Fuel Adjustment 6
3.3 Tank Vapor Venting 6
3.3.1 Cold Soak 7
3.3.2 Hot Soak 9
3.3.3 Inspection/Maintenance (I/M) Program effects 11
3.4 Liquid Leaks 14
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1 Background
Evaporative emissions account for a significant portion of the total gaseous hydrocarbon
inventory. Its processes are unique and require a unique modeling approach. For a long time,
evaporative emissions were thought of being quantifiable in three distinct modes and subsequent
test procedures: running loss (during vehicle operation), diurnal/resting loss (stabilized parked
emissions), and hot soak (parked emissions immediately after vehicle operation). However, it has
become evident that different factors, such as ambient temperature and fuel type for example,
affect evaporative emissions more noticeably in the emissions processes, rather than the
aforementioned three modes. Evaporative emissions can be broken up into three main processes:
Permeation - the migration of hydrocarbons through elastomers in a vehicle's fuel system
Tank Vapor Venting - expulsion into the atmosphere of fuel vapor generated from
evaporation of fuel in the fuel system
Liquid Leaks - fuel, in liquid form, leaking from the fuel tank or fuel system, which then
evaporates into the atmosphere
These three processes occur and can be addressed in each mode. Therefore, we can measure
and/or calculate permeation, tank vapor venting, and liquid leaks in each of the three testing
regimes prevalent in major evaporative emissions test programs. Then, we can relate the
emissions from each of the processes to different factors that occur independently of modes. This
makes for easier, more accurate modeling of scenarios that do not perfectly replicate the test
procedures.
The factors that affect permeation, vapor venting, and leaks that we considered were:
Ambient temperature
Fuel tank temperature
Model year
Age
Vehicle class
Fuel (ethanol %, RVP)
Failure modes
Presence of I/M
The model year groups used for evaporative emissions are shown in Table 1. They depend on
evaporative emission standards and related technological improvement designed to control
evaporative emissions.
Tabh
; 1 describes the model year group stratifications used for MOVES analysis.
Model year group
1971-1977
1978-1995
1996
1997
1998
1999-2003
2004 and later
Emissions standard or technology level
Pre-control
Early control
80% early control, 20% enhanced evap.
60% early control, 40% enhanced evap.
10% early control, 90% enhanced evap.
100% Enhanced evap.
Tier 2, LEV II
2 Data Sources
CRC E-9 - Measurement of Diurnal Emissions from In-Use Vehicles:
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CRC E-35 - Measurement of Running Loss Emissions in In-Use Vehicles23
CRC E-41 - Evaporative Emissions from Late-Model In-Use Vehicles45
CRC E-65 - Fuel Permeation from Automotive Systems6
CRC E-65-3 - Fuel Permeation from Automotive Systems: EO, E6, E10, and E857
BAR Gas Cap Study
API Gas Cap Study
EPA Compliance Testing
Appendix A has a summary of most of the test programs mentioned above.
3 Design and Analysis
We found fuel tank temperature to be the driver of the two transient emissions processes,
permeation and vapor venting. Determining fuel tank temperature was critical in predicting
emissions in each of these Operating Modes. Fuel tank temperature is dependent on the daily
ambient temperature profile, times that the vehicle is operating, and the model year of the vehicle.
Then, we can use the calculated fuel tank temperature profile to calculate permeation and vapor
venting. Other factors were included as needed. Liquid leaks were not dependent on
temperature. This section will first describe the Fuel Tank Temperature Generator, and then
explain how we used the fuel tank temperature to determine emission rates for each of the
emissions processes.
3.1 Fuel Tank Temperature Generator
This section explains how to generate fuel tank temperature through time for a given day's
ambient temperature profile and a vehicle's trip times. Generating fuel tank temperature allows
for the calculation of permeation and vapor venting, two major fuel-related evaporative emissions
processes. As a result, this algorithm is instrumental in modeling evaporative emissions in
MOVES.
3.1.1 Input parameters
Hourly ambient temperature profile
Key on and key off times
HourDaylD (day and hour) of first KeyON
Vehicle Type (LDT/LDV)
Pre-enhanced or enhanced evaporative emissions control system
MOVES defines these input parameters via the sampleVehicleTrip and zoneMonthHour tables
and the sourceBinID variable.
3.1.2 General steps
1) Input parameters must be defined.
2) Fuel tank temperature is computed up to the start of the first trip, assuming that the
vehicle has been parked for a long time (overnight). This is done through the block
diagram in
3) Figure 1 below, which represents the differential equation in equation 1. All soaks (hot
and cold) are calculated using this portion of the algorithm.
4) Next, for each trip, the fuel tank temperature is computed for the operation period and the
corresponding soak period after the key off for that trip. It computes fuel tank
temperature until the start of the next trip (next key on), at which point this step is
repeated, or until the end of the day. The fuel tank temperature during operation is
calculated using equations (3) and (4). The fuel tank temperature during hot soak is
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calculated as for cold soak (equation 2b), but with the initial temperature (T,) changed to
the temperature at the end of each trip, and the time interval modified to accommodate
the key on/off times.
3.1.2.1 Calculating soak temperatures (as a function of ambient temperature)
The following equation was used to model tank temperature as a function of ambient temperature.
This was used for hot and cold soaks.
-TTank), (1)
at
where Ttank is the fuel tank temperature, Talr is the ambient temperature, and k is a constant
proportionality factor (k = 1.4). The value of k was verified by trial and error against EPA
compliance data. There was no distinction made between hot soak and cold soak calculations.
We assumed that during either soak, the only factor affecting fuel tank temperature was the
ambient temperature profile and the fuel tank temperature at the start of the soak. The block
diagram below simplifies the equation into several mathematical steps, which are explained
below.
Figure 1 - Simulinkฎ block diagram of the relation between ambient temperature and fuel tank
temperature
Ambient temperature
Input From File-Ambient
Temperature [
TankTemp at start of soak
Output - Tanktemperature
^'"^ To Workspace
The time periods for which this part of the algorithm is used depends on the key on and key off
times. Since this equation can be used only for cold soaks and hot soaks (all parked
conditions), it applies for the following time intervals only:
from the start of the day to the first trip,
from all key off to key on times, and
from the last ke off to the end of the da.
Mathematical steps
1) At time t0 = 0 or KeyOFF (start of soak), TtalA = Tt. This value will either be the ambient
temperature (at the very start of the model) or the fuel tank temperature at the end of a
trip.
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2) Then, for all t > 0 or KeyOFF, the next tank temperature is calculated in this manner:
(2a) or
.7=0
= TTankn+k(Tair-TTank}nM (2b)
(Tair - Ttank) is a function of time. Since analytical integration is too complicated (the
input ambient temperature data is in a table), numerical integration should be used to
perform this step. The method of numerical integration varies based on the accuracy
desired. The above method represents the Euler method, one of the simplest methods of
integration. The less accurate the method, the smaller the time step A^ should be, to
improve the solution. MOVES uses a A^ of 15 minutes, which is accurate enough for our
modeling purposes.
3.1.2.2 Calculating fuel tank temperatures during operation
Operation periods (trips) are relatively short compared to the length of the day or modeling
period. Therefore, even though the fuel tank temperature profile during operation is not exactly
linear, assuming a linear increase in temperature makes calculations easier without compromising
accuracy. However, the increase in temperature ATtank depends on the temperature at the start of
operation. It also depends on vehicle type. The convention used in this algorithm is that ATtank
applies over a 4300 second period, which is the length of the running loss test performed by
manufacturers for certification. To find ATtank, we must first find ATtank95, the average increase in
tank temperature at a standard 4300 second @ 95F running loss test.
If the vehicle is evap-enhanced, then ATtank95 = 24F 8
If the vehicle is pre-enhanced, the vehicle type affects hTtank95.
o If LDV, then Mtank95 = 3 5F.
o If LOT, then kltank95 = 29F.
We can use these values for ATtank95 for 95F to calculate the ATtank for other starting fuel tank
temperatures (other trips) using the following equation:
MTank =0.352 (95 - TTank ^KeyON ) + MTank 95 (3)9
Since this gives us the increase in tank temperature, we can create a simple linear function that
models fuel tank temperature for each trip.
T = Is* (t -1 \ + T (4}
1 Tank 43 oo / 3 600 keyฐN Tc"*,KeyฐN ^ '
The 4300/3600 appears, as the running loss test done by manufacturers is 4300 seconds long, and
we convert that to hours maintain consistency in the algorithm.
Assumptions:
The first trip is assumed to start halfway into the hour stated in the first trip's
HourDaylD.
We assumed the effect of the ambient temperature or change in ambient temperature
during a trip was negligible compared to the effect of operation.
The KeyON tank temperatures will be known by way of the calculations of the tank
temperatures from the previous soak.
3.2 Permeation
3.2.1 Base Rates
We first determined base rates for permeation. We define these rates as the non-leak hydrocarbon
gram-per-hour emission rate during the last six hours of a 72-96-72ฐF diurnal test (also known as
resting loss). In these six hours, the emissions rate and the ambient and fuel tank temperatures
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are relatively stable or constant, leading us to believe that the constant permeation process is the
only emissions process occurring. We stratified these rates by model year group and age group.
The base permeation rates are in Table 2. Separate inputs were created from model years 1996-
1998 to account for the 20/40/90% phase-in of enhanced evaporative emissions standards.
Table 2 - Base permeation rates at 72 F
Model year group
1971-1977
1978-1995
1996
1997
1998
1999-2003
Age group
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
Base permeation
rate [g/hr]
0.192
0.229
0.311
0.0554
0.0554
0.0913
0.0913
0.124
0.148
0.201
0.046
0.046
0.075
0.075
0.101
0.120
0.163
0.037
0.037
0.059
0.059
0.079
0.093
0.125
0.015
0.015
0.018
0.018
0.022
0.024
0.029
0.0102
0.0102
0.0102
0.0102
0.0102
0.0102
0.0102
3.2.2 Temperature adjustment
Use following equation for temperature-adjusted permeation rate for each hour not in the last six
hours of a diurnal:
P = P
adj bast
(5)
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where Pbase is the base permeation rate calculated by averaging the last six hours of emissions,
Ttank is the tank temperature, and Tbase is the base temperature for a given temperature cycle (e.g.
72 for a 72-86-72 diurnal test). This is derived from the E-65 permeation study which found that
permeation rate doubles for every 18 degrees F. In MOVES the base permeation rates are
calculated at 72 F.
3.2.3 Fuel Adjustment
E10 affects evaporative emissions from gasoline vehicles due to the increased volatility of E10
blends, the increased permeation of fuel vapors through tanks and hoses, and the increased vapor
emissions due to the lower molecular weight of E10. Each of these effects were modeled using
the draft MOVES model, which separates permeation emissions from vapor venting emissions to
allow better accounting for these different processes.
Fuel effects on permeation were developed from CRC's E-65 and E-65-3 programs, which
measured evaporative emissions from ten fuel systems that were removed from the vehicles on
EO, E5.7, and E10 fuels; fuel systems were removed to ensure that all evaporative emissions
measured were from permeation of the fuel through the different components of the fuel system.
For this analysis, we separated the evaporative enhanced vehicles from the pre-enhanced vehicles.
Enhanced evaporative vehicles began being phased in from 1996 through 1999 and needed to
meet a 2.0 g standard over a 24-hour diurnal test. Pre-enhanced vehicles needed to meet 2.0 g
over a 1-hour simulated diurnal. We estimated the ethanol effect by calculating the percent
increase in average emissions over the 65-105-65 deg F diurnal cycle from each of the two groups
of vehicles. To determine the effect of ethanol blend, we first averaged the E5.7 and E10 results
(where both fuels were tested) for each vehicle to obtain its mean ethanol permeation rate. We
then averaged each vehicle's mean permeation rate on EO. The percent difference between the
ethanol rate and the EO rate was input into MOVES as the fuel adjustment. Due to the phase in
from 1996 to 1999 (20/40/90/100 %), the two fuel adjustments must be properly weighted for
those model years. The fuel adjustment in MOVES is based on a variable called
fuelModelYearlD. Table 3 shows the fuel adjustments used for E5 through E10 for the
fuelModelYearlD's used in MOVES.
Table 3 - Increase in emissions due to ethanol levels of 5 to 10% compared to EO (gasoline)
Model years
(via fuelModelYearlD in MOVES)
1995 and earlier
1996
1997-2000
200 land later
Percent increase due to ethanol
(E5 through E10)
37.3
69.4
175
198
We plan to revisit our permeation emissions estimates with the release CRC E-77 and E-77-2b
studies.
3.3 Tank Vapor Venting
The following explains how vapor venting rates were calculated for each of the operating modes.
For cold soak, MOVES first finds the amount of vapor generated in the tank as a function of fuel
tank temperature and RVP. Then, it determines how much of this vapor is released into the
atmosphere based on several criteria, such as model year and fill pipe pressure test result. The
temperatures will have been generated by the fuel tank temperature generator, and the RVP will
have been generated by the MOVES tank fuel generator. This cannot apply for when vapor is not
generated (when fuel tank temperature is not increasing), such as during a hot soak, but is
released. For these situations, we have aggregated TVV rates after subtracting out permeation
and leaks from the test results. Also, due to the availability of test data for running loss and the
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short length of trips, we determined TVV rates during operation the same way we did for hot
soak.
3.3.1 Cold Soak
1) For each diurnal test, we calculated fuel tank temperature at each hour using the fuel tank
temperature algorithm.
2) After calculating base permeation rate for each vehicle (average of last six hours of HC
evaporative emissions), we used the fuel tank temperature adjustment with the temperatures
calculated in step 1 to calculate the permeation for each hour. The fuel tank temperature is
determined through the fuel tank temperature algorithm for MOVES diurnals.
3) We filtered/reduced data set such that each test met the following requirements:
a. Non-leakers
b. "As received" vehicles (no retests)
c. Hours where tank temperature increased from previous hour
d. Pressure test result must be pass, fail, or blank only (no dashes, slashes, "I", etc.)
4) We subtracted permeation from HC for each hour to get tank vapor venting (TW) rate
5) We summed TVV from beginning of diurnal to each hour to get Cumulative TVV.
6) We then determined Tank Vapor Generated (TVG) from hour 1 to hour x for each hour that
the fuel tank temperature is rising.
TVG =AeB*RVP(eCT*-eCT^ [grams/gal] (6)10
where A, B, and C are constants listed below in Table 4, and Tx is the temperature at hour x.
Table 4 - TVG constants for equation 7.
Constant
A
B
C
Gasoline
Sea Level
0.00817
0.2357
0.0409
Denver alt.
0.00518
0.2649
0.0461
E10
Sea Level
0.00875
0.2056
0.0430
Denver alt.
0.00665
0.2228
0.0474
TVG is the amount of vapor generated in the tank. We will establish a relationship between
Cumulative TWand TVG for inputs into MOVES.
7) We constructed quadratic curves (zero intercept) of CumTWvs. TVG, stratifying by model
year group, age group, and pressure test result.
CumTW = a,TVG + a2TVG2 (7)
Having the zero-intercept ensures that the (0, 0) is a point on the quadratic curve. In other
words, it implies that at 0 TVG, there is no tank vapor venting, which is an accurate physical
assumption.
Curves were generated for model year groups 1971-1977 (ages 15+), 1978-1995 (ages 0-19),
and 1996-2003 (ages 0-9). We also stratified by pressure test result. In failing vehicles, more
of the vapor that is generated in the fuel system will be vented than in passing vehicles, where
the evaporative emission controls should be functioning properly. The remainder of the
coefficients was found by extrapolation or previously determined relationships. The
coefficients of variation (CVs) were calculated by dividing the standard error of the sample
(calculated by SPSS) by the mean for each coefficient in the quadratic equation.
8) After failure frequencies (F) were generated from pressure, gas cap, and OBD test results
from the Phoenix I/M program (see section 3.3.3 Inspection/Maintenance (I/M) Program
effects), aggregate coefficients were calculated:
7
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(8)
where x = 1 or 2, corresponding to quadratic equation 8.
9)
Since the aggregate coefficients were determined using the failure rates, which are
essentially weighting factors, the standard errors of the aggregate coefficients are calculated:
(9)
As a result, the CV's for the aggregate coefficients are calculated:
a
(10)
Table 5 shows the I/M coefficients resulting from the analysis. Ratios between age groups were
used to extrapolate for the 10-14 age group in the 1971-1977 model year groups, and older age
groups where data did not exist were forced to have the same coefficients as their preceding age
groups. The passing coefficients for the 2004 and later model year group were reduced by 32%
from the 1999-2003 model year group, which reflects a reduction in the evaporative emissions
standard from enhanced-evap to Tier 2/LEV II. Separate model year groups are created for 1996
through 1998 due to the phasing of enhanced evaporative standards. These three groups are only
different weightings of the 1978-1995 and 1999-2003 model year groups based on the 20/40/90%
phase-in for 1996/1997/1998. Similarly, though not shown, is a table that was developed for non-
I/M vehicles using non-I/M failure frequencies calculated from the Phoenix I/M data set.
Table 5 - I/M coefficients for equation 7. The aggregate columns are the inputs in the MOVES
model for I/M coefficients in the cumTWCoeffs table.
model year group
1971-1977
1978-1995
1996
1997
age group
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
81
pass
1.227
5.406
5.406
1.578
1.578
1.578
1.578
0.849
3.743
3.743
1.339
1.339
1.339
1.339
0.756
3.071
3.071
1.100
1.100
1.100
1.100
0.663
2.399
fail
11.314
9.254
9.254
3.073
3.073
3.073
3.073
11.314
9.254
9.254
3.073
3.073
3.073
3.073
9.666
8.017
8.017
3.073
3.073
3.073
3.073
8.018
6.781
aggregate
1.941
5.835
6.127
1.589
1.604
1.610
1.623
1.283
4.120
4.376
1.354
1.362
1.376
1.392
1.124
3.399
3.530
1.120
1.129
1.146
1.163
0.976
2.686
a2
pass
2.175
2.331
2.331
0.440
0.440
0.440
0.440
2.095
2.246
2.246
0.344
0.344
0.344
0.344
1.668
1.789
1.789
0.248
0.248
0.248
0.248
1.241
1.332
fail
0.402
3.117
3.117
1.338
1.338
1.338
1.338
0.402
3.117
3.117
1.338
1.338
1.338
1.338
0.589
2.762
2.762
1.338
1.338
1.338
1.338
0.776
2.406
aggregate
2.049
2.419
2.479
0.446
0.455
0.459
0.466
2.025
2.305
2.346
0.352
0.357
0.365
0.374
1.624
1.853
1.879
0.259
0.264
0.273
0.283
1.222
1.402
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1998
1999-2003
2004 and later
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
2.399
0.502
0.502
0.502
0.502
0.429
0.719
0.719
0.383
0.383
0.383
0.383
0.383
0.383
0.383
0.124
0.124
0.124
0.124
0.124
0.124
0.124
6.781
3.073
3.073
3.073
3.073
3.897
3.691
3.691
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
3.073
2.791
0.538
0.553
0.575
0.596
0.589
0.907
0.959
0.422
0.438
0.461
0.483
0.508
0.552
0.595
0.151
0.161
0.175
0.187
0.203
0.229
0.255
1.332
0.009
0.009
0.009
0.009
0.174
0.189
0.189
-0.039
-0.039
-0.039
-0.039
-0.039
-0.039
-0.039
-0.013
-0.013
-0.013
-0.013
-0.013
-0.013
-0.013
2.406
1.338
1.338
1.338
1.338
1.244
1.516
1.516
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.338
1.428
0.027
0.035
0.046
0.057
0.223
0.273
0.296
-0.019
-0.011
0.001
0.012
0.025
0.048
0.070
-0.001
0.004
0.010
0.016
0.023
0.035
0.047
3.3.2 Hot Soak
1) First we found the temperature at the start of the soak for each hot soak test. This is done by
adding on the temperature increase experienced during an LA-4 running loss test cycle (1372
seconds), since the vehicle is put through this test before entering the soak chamber. These
temperature rises depend on the fuel tank temperature at the start of the LA-4 test (ambient).
To calculate hot soak start temperature Tstart, see section 3.1.2.2 Calculating fuel tank
temperatures during operation.
2) We then found the average temperature in that hour:
T - (T -T )(\-e~k) + T (11)
1 avg ~ , V1 start i air A1 K > ^ i air ^ '
This is derived from the average value of a function over an interval (in this case, between 0
and 1 hour after the start of the hot soak). As stated in the Fuel Temperature algorithm, k =
1.4.
3) We used this average temperature to determine the average permeation rate during the hot
soak via the permeation temperature adjustment using the 72F base permeation rates
determine by model year group and age group.
4) We filtered/reduced the data set such that each test met the following requirements:
a. Non-leakers (emissions less than 10.0 grams11; taken from M6.EVP.009_2.4;
Since hot soak emissions are measured after one hour, the total emissions is
"equal" to its g/hr rate)
b. "As received" vehicles (no retests)
c. Pressure test result must be pass, fail, or blank only (no dashes, slashes, "I", etc.)
5) We subtracted permeation from HC for each hour to get tank vapor venting (TW) rate
6) We averaged TVV rates by model year group, age group, and pressure test result, shown in
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Table 6 - Average hot soak tank vapor venting rates in g/hr by model year group, age group, and
pressure test result.
Model year group
1971-1977
1978-1995
1996-2003
Age group
20+
20+
0-5
0-5
0-9
6-9
6-9
10-14
10-14
10-14
15-19
15-19
all
Pressure test result
F
P
Both
P
F
Both
P
Both
F
P
F
P
Both
TVV rate
6.17
2
1.25
0.56
2.37
1.75
1.38
5.13
3.41
1.76
4.51
2.99
0.1073
7) Like with cold soak, aggregate rates were found using failure rates involving pressure, gas
cap, and OBD tests for non-I/M and I/M. Table 7 below does not reflect the most updated
I/M analysis explained in section 3.3.3 Inspection/Maintenance (I/M) Program effects (unlike
the cold soak coefficients in Table 5) or the enhanced evaporative phase-in, so these numbers
will be updated for the final version of MOVES.
Table 7 - Example of non-I/M hot soak tank vapor venting rates
Model year group
1971-1977
1 978-1 995
1 996-2003
2004 and later
Age group
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
0-3
4-5
6-7
8-9
10-14
15-19
20+
TVV rate [g/hr]
3.099
5.149
5.455
0.627
0.627
1.451
1.471
2.082
3.492
3.817
0.124
0.124
0.150
0.168
0.250
0.383
0.611
0.060
0.060
0.086
0.105
0.187
0.323
0.553
10
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3.3.3 Inspection/Maintenance (I/M) Program effects
Our assumption in MOVES is that tank vapor venting is the only evaporative process where the
effects of I/M are realized. The types of evaporative tests performed in I/M programs (gas cap
test, fill pipe pressure test, OBD scans) do not affect permeation or liquid leaks.
In order to develop I/M and non-I/M tank vapor venting rates, we used available data from I/M
programs to determine the failure frequencies of evaporative control systems. These frequencies
were then used to combine the cumulative tank vapor venting coefficients for failing vehicles and
those for passing vehicles (determined from the TVV analysis). Details of each of the four
programs are in Table 8 below.
Table 8- Description of evaporative characteristics of I/M programs available for analysis 12
Colorado
N. Carolina
Phoenix
Tucson
Gas cap
test
Y
N
Y
Y
OBD
Advisory only
Y
Y
Y
Pressure test
TV
TV
Y
N
Frequency
Biennial
Annual
Biennial
Annual
Network
Hybrid
Decentralized
Centralized
Centralized
Calendar years
2003-2006
2002-2006
2002-2006
2002-2006
Since the Phoenix program contained the most extensive amount of data, we used it to develop
reference I/M evaporative failure frequency. The Tucson, Colorado, and North Carolina data
were used to adjust the Phoenix numbers for differences in I/M programs.
The Phoenix evaporative I/M program used gas cap tests on all vehicles, OBD scans on OBD-
equipped vehicles, and fill pipe pressure tests on pre-OBD vehicles. The OBD codes used to
determine evaporative failures were P0440, P0442, P0445, P0446, and P0447 for all vehicle
makes and additionally P1456 and P1457 for Honda and Acura vehicles. Vehicles that had one
or more of these faults were flagged as failing vehicles, analogous to pre-OBD vehicles that failed
the pressure test. Very few vehicles failed both the gas cap test and the pressure/OBD test.
Therefore, our total number of failures was the sum of gas cap and pressure/OBD failures.
To determine failure frequencies for I/M areas, from the Phoenix data, we looked at the initial and
final results for each vehicle in a given I/M cycle. For passing vehicles, the initial test and the
final tests were one and the same. We averaged the initial and final failure frequencies (weighted
equally) to calculate an overall I/M failure frequency by model year group and age group. Using
the initial failure frequencies alone would neglect the effect of repair that most failing vehicles
would be required to undergo, and using the final failure frequencies alone would neglect the
existence of the failing vehicles driving around in the fleet in the first place. To determine non-
I/M failure frequencies, we restricted our sample in the Phoenix data to those vehicles with
license plates from states that do not have an I/M program anywhere. Figure 2 gives an example
of how failure frequencies increase with age. Shown are frequencies for model years 1978-1995,
where data was extrapolated for the youngest age groups.
11
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Figure 2 - Evaporative failure frequencies for I/M and non-I/M vehicles in the Phoenix area. This
figure shows model years 1978 to 1995.
20.00% -
15.00% -
5.00% -
0.00%
MOVES age group
The Tucson data was used to determine the effect of program frequency (annual vs. biennial).
For OBD vehicles, Tucson performs gas cap and OBD tests annually, while Phoenix performs
them biennially. Therefore, we were able to develop failure frequencies for annual programs by
analyzing the Tucson data. We applied the ratio between Tucson and Phoenix to determine the
failure frequencies for where we did not have data (e.g. pre-OBD vehicles).
The North Carolina data was used to determine non-I/M failure frequencies for OBD tests. In
North Carolina, expansion of I/M program has led to counties where many vehicles were tested
under the I/M program for the first time. Vehicles were flagged as non-I/M tests if they were
tested:
before the official start of the I/M program,
in a new I/M county and were registered in that same county, or
in a new I/M county and were registered in a non-I/M county or a county that did not
start I/M within the last year.
We compared those failure frequencies to those for vehicles tested in older I/M areas, where
vehicles were previously tested. From the North Carolina data, the average ratio of non-I/M to
I/M OBD failure frequencies is 1.6. This ratio was then applied to the Phoenix OBD and pressure
test failure frequencies to determine non-I/M failure frequencies.
The Colorado data was used to determine non-I/M failure frequencies for gas cap tests. In
Colorado, the I/M data comes mostly from the Denver and Boulder metropolitan areas. Many
residents are new to this area, with many having moved in from non-I/M areas in Colorado or
non-I/M states. Vehicles were flagged as non-I/M tests if their registration state was a 100% non-
I/M state, or if the registration county was a non-I/M county in Colorado. We compared the
failure rates of the flagged vehicles to those of the full tested fleet. The ratio of these two
frequencies was then applied to the Phoenix gas cap failure frequencies to determine non-I/M
failure frequencies. Colorado OBD data was not used, since OBD in Colorado is only advisory,
and does not pass or fail a vehicle.
12
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From the Colorado data, the average ratio of non-I/M to I/M gas cap failure frequencies is 2.2.
This ratio was then applied to the gas cap failure frequencies to determine non-I/M failure
frequencies.
IMfactor
The IM factor lets MOVES interpolate and extrapolate the non-I/M emission rates and the I/M
emission rates depending on the characteristics of the I/M program in each county. Our reference
program, Phoenix, was given an IM factor of 1. The non-I/M rates were given an IM factor of 0.
For each model year group and age group stratification, we used the failure frequencies
determined from the analysis described above to calculate IM factors for the diverse types of
evaporative I/M programs. Figure 3 illustrates how the I/M factor is influenced by the types of
evaporative tests conducted in I/M programs. We modeled the estimated failure frequency
linearly with the I/M factor, with Phoenix, our reference program, always receiving a value of 1,
and our non-I/M failure frequency always receiving a value of 0. Different programs move along
the line, as determined by the analysis from above, based on which evaporative tests they choose
to use. The figure is an example using model year group 1999-2003 and age group 4-5. For
these vehicles, Tucson's OBD and gas cap tests are annual, compared to Phoenix's biennial
requirement, which gives Tucson a lower failure frequency and a higher I/M factor. Colorado's
frequency is biennial, like Pheonix's, but its OBD test is non-enforcing. As a result, their data
shows a higher failure frequency, which results in a lower I/M factor.
Figure 3 - Example of how we calculated the I/M adjustment factor. This figure applies for model
years 1999-2003 ages 4-5.
Tucson
Phoenix (reference I/M)
Non-I/M
2.50% 3.00%
Failure frequency [%]
3.3.4 Running Loss
1) For each vehicle, we calculated fuel tank temperature at the end of the running loss test using
the fuel tank temperature algorithm (see section 3.1.2.2 Calculating fuel tank temperatures
during operation). The running loss test performed was the typical 4375-second LA-4 -
NYCC - NYCC - LA-4 sequence, with two minute idle periods following the first LA-4, the
second NYCC, and the final LA-4 (CRC E-41).
2) We found the average temperature during the test by assuming a linear increase in
temperature during the test. Thus, the average was calculated by averaging the start
temperature of the test and the final temperature of the test found in step 1.
13
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3)
4)
5)
6)
7)
We used this average temperature to determine the average permeation rate during the hot
soak via the permeation temperature adjustment using the 72F base permeation rates
determine by model year, age.
We calculated gram/hour rates by dividing total emissions by the duration of the running loss
test (4300 seconds)
We filtered/reduced the data set such that each test met the following requirements:
a. Non-leakers (emissions less than 137.2 g/hour; taken from M6.EVP.009_2.4)
b. "As received" vehicles (no retests)
c. Pressure test result must be pass, fail, or blank only (no dashes, slashes, "I", etc.)
Subtract permeation from HC for each hour to get tank vapor venting (TW) rate
After analysis of TVV data, we found that the best way to stratify running loss TVV was by
model year only. Table 9 shows the results of the analysis.
Table 9 - Final running loss tank vapor venting emission rates.
Model year group
1971-1977
1978-1995
1996-2003
2004 and later
TVV mean [g/hr]
12.59
11.6
0.72
0.23
8) Since model year group is the only stratification, the running loss TVV rates are not affected
by the failure rates. Therefore, the I/M and non-I/M rates are the same and equal to the table
above.
3.4 Liquid Leaks
Liquid leaks are the final evaporative emissions process discussed in this document. To calculate
the average leaking rate, we used the leaking vehicles excluded from the previous analysis for
tank vapor venting. We estimated permeation and tank vapor venting on these vehicles using the
methods described above. We assumed the remainder of emissions to be caused by liquid leaks.
We averaged these emissions by the three different modes, shown in Table 10.
Table 10 - Emission rates for liquid leakers by mode.
Mode
Cold Soak
Hot Soak
Operating
Liquid leak rate
9.85 g/hr
19.0 g/hr
178 g/hr
The rates in Table 10 must be multiplied by the percentage of leakers in the fleet to get an
average liquid leaking emission rate. For this we relied on the studies by BAR and API. Our
estimates of the percentage of liquid leakers are shown in
Table 11. On average, we assume that most leaks do not occur until vehicles are 15 years or
older.
Table 11 - Percentage of liquid leakers by age.
Age group
0-9
10-14
15-19
20+
Percentage of leakers
in fleet
0.09 %
0.25 %
0.77 %
2.38 %
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Combining Table 10 and Table 11, we get Table 12, which shows the liquid leaking rate of the
entire fleet. We assume that this rate does not change with model year nor is it affected by I/M.
Table 12 - Final liquid leak rates in g/hr by age group and mode.
Age group
0-9
10-14
15-19
20+
Cold soak
0.009
0.025
0.075
0.235
Hot soak
0.017
0.048
0.145
0.452
Operating
0.158
0.450
1.36
4.23
15
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Appendix A - Notes on Evaporative Emission Data
Parameters: Vehicle Numbers, Test #, Ambient Temperature, RVP, Model Year, Fuel
System, Purge, Pressure, Canister, Gram HC, Retest
E-41 CRC Late Model In-Use Evap. Emission Hot Soak Study (1998):
50 vehicles (30 passenger cars and 20 light duty trucks)
Age: 1992 to 1997 model year
Average RVP: 6.5 psi
Diurnal Temperature: 72 to 96F
Fuel System: Port Fuel Injection, Throttle Body Injection
Vehicle fuel tank drained and refilled to 40% of capacity with Federal
Evaporative Emission Test Fuel
Driving schedule will be a full LA-4-NYCC-NYCC-LA4 sequence, with two
minute idle periods following the first LA-4, the second NYCC, and the final
LA-4.
Hydrocarbon readings will be taken continuously throughout the running loss test.
Cumulative mass emissions will be reported at one minute intervals.
Ambient Temperature in running loss enclosure: 95F
E-9 CRC Real Time Diurnal Study (1996):
151 vehicles (51 vehicles from MY 1971 through 1977, 50 vehicles from
MY1980 through 1985, 50 vehicles from MY 1986 through 1991)
Odometers range from 39,000 to 439,000 miles
Fuel tank volume was 15% of the rated capacity
RVP: 6.62 psi (average sum of 47 vehicles)
Diurnal temperature: 72 to 96F
Fuel System: Port Fuel Injection, Carburetor, Throttle Body Injection
CRC E-35 Running Loss Study (1997)
150 vehicles
Ambient Temperature in running loss enclosure: 95F
RVP: 6.8 psi
Fuel System: Port Fuel Injection, Carburetor, Throttle Body Injection
EPA Compliance Data:
2-Day Test
Length of the hot soak: 1 hour
77 vehicles
RVP: average 8.81 psi
Ambient Temperature:
Federal Standard (72 to 96 F) Diurnal
Cal. (65 to 105 F) Diurnal
Hot Soak: 81.67F
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Fuel System: Port Fuel Injection
Unleaded Cert Fuel
S u Ifu r
wt %
0.0048
0.0045
0.0063
0.0048
0.0042
0.003
0.003
0.0031
0.0035
0.0027
RVP
9.04
9.2
9.04
8.99
9.05
9.12
9.12
8.8
8.91
8.95
D ate
Jul-98
D ec-98
A ug-99
M ay-00
S e p-0 1
D ec-0 1
D ec-02
M ay-03
A pr-04
J u n-04
CARS Phase II Fuel
Sulfur
wt%
0.0023
0.0023
0.0038
0.0033
0.0035
RVP
6.92
6.92
6.92
6.92
6.77
Date
Ajg-99
May-00
Jan-01
Oct-02
Mar-04
MSOD (Mobile Source Observation Database):
Hot Soak: 1 hour hot soak evaporative test
FTP: Federal test procedure (19.53 mph), also referred to as the UDDP schedule
NYCC: New York City Cycle Test (7.04 mph)
BL_1 A: 1 hour Breathing Loss Evap. Test - Gas Cap left "On"
BL_1B: 1 hour Breathing Loss Evap. Test - Canister as reed.
ST01: Engine Start cycle test
4HD: 4 hour Diurnal test
24RTD: 24 Hour Real Time Diurnal
33RTD: 33 Hour Real Time Diurnal
72RTD: 72 Hour Real Time Diurnal
3Rest: 3 Hour Resting Loss Evap. Emission Test (follows 1 HR Hot Soak)
CY6084: Real time diurnal temperature pattern: range 60 to 84 F
CY7296: Real time diurnal temperature pattern: range 72 to 96 F
CY8210: Real time diurnal temperature pattern: range 82 to 102 F
DIURBL: Standard temperature rise for 1 hour diurnal or breathing loss
evaporative emission test
F505: Bag 1 of federal test procedure (25.55 mph)
ASM: Acceleration Simulation Mode Test Procedure
ATD: Ambient Temperature diurnal evaporative Test, shed temp constant, vehicle
begins 24 degree cooler
17
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CRC Report No. 610, Project E-9. Measurement of Diurnal Emissions from In-Use Vehicles. September 1998.
http: //www.crcao. com/reports/emission/e9 .htm.
2 CRC Report No. 611. Project E-35. Measurement of Running Loss Emissions from In-Use Vehicles. Automotive
Testing Laboratories. Februrary 1998. http://www.crcao.com/reports/emission/e35.htm.
3 CRC Report No. 612. Project E-35. Running Loss Emissions from In-Use Vehicles. Harold Haskew and Associates,
Inc. Februrary 1999. http://www.crcao.com/reports/emission/e35.htm.
4 CRC Report No. 622. Project E-41-1. Real World Evaporative Testing of Late-Model In-Use Vehicles. October
1999. http://www.crcao.com/reports/emission/e41 .htm.
5 CRC Report No. 622. ProjectE-41-2. Evaporative Emissions from Late-Model In-Use Vehicles. October 1999.
http://www.crcao.com/reports/emission/e41.htm.
Haskew, Harold M., Thomas F. Liberty and Dennis McClement, "Fuel Permeation from Automotive Systems," Final
Report, for the Coordinating Research Council and the California Air Resources Board, CRC Project E-65, September
2004. http://www.crcao.com/reports/recentstudies2004/E65%20Final%20Report%209%202%2004.pdf.
Available in Docket EPA-HQ-OAR-2005-0161.
7 Haskew, Harold M., Thomas F. Liberty and Dennis McClement, "Fuel Permeation from Automotive Systems: EO,
E6, E10, E20, and E85" Final Report, for the Coordinating Research Council and the California Air Resources Board,
CRC Project E-65-3, December 2006. http://www.crcao.com/reports/recentstudies2006/E-65-3/CRC%20E-65-
3%20Final%20Report.pdf
Certification fuel tank temperature profiles of top selling passenger cars and light-duty trucks provided by
manufacturers.
9 T. Cam, K. Cullen, and S. L. Baldus, Running Loss Temperature Profile, SAE. 930078, Society of Automotive
Engineers, Warrendale, Pa., 1993;
10 R.S. Reddy, Prediction of fuel vapour generation from a vehicle fuel tank as a function of fuel RVP and temperature,
SAE 892089, 1989.
11 Landman, Larry C. U.S. EPA MOBILE6 Technical Document M6.EVP.009. Evaporative Emissions of Gross
Liquid Leakers in MOBILE6 (EPA420-R-01-024). April 2001. http://www.epa.gov/otaq/models/mobile6/r01024.pdf.
12 Sierra Research Report No. SR2005-12-03. United States Motor Vehicle Inspection and Maintenance (I/M)
Programs. December 2005.
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