Development of Evaporative Emissions Calculations for the Motor Vehicle Emissions Simulator MOVES2010


            Development of Evaporative Emissions

            Calculations for the Motor Vehicle

            Emissions Simulator MOVES2010
            Final Report
&EPA
United States
Environmental Protection
Agency

-------
                Development of Evaporative Emissions
                   Calculations for the Motor Vehicle
                   Emissions Simulator MOVES2010

                                   Final Report
                               Assessment and Standards Division
                              Office of Transportation and Air Quality
                              U.S. Environmental Protection Agency
                 NOTICE

                 This technical report does not necessarily represent final EPA decisions or
                 positions. It is intended to present technical analysis of issues using data
                 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.
&EPA
United States
Environmental Protection
Agency
EPA-420-R-12-027
September 2012

-------
Table of Contents

  1 Background	2
  2 List of Data Sources	5
  3 Design and Analysis	6
    3.1 Fuel Tank Temperature Generator	6
    3.2 Permeation	8
    3.3 Tank Vapor Venting	11
    3.4 Inspection/Maintenance (I/M) Program effects	18
    3.5 Liquid Leaks	21
    3.6 Refueling	22
  Appendix A - Notes on Evaporative Emission Data	25
  Appendix B - CumTWCoeffs Table	27
  Appendix C - MOVES  Operating Modes and Emission Processes	29
  Appendix D - Evaporative Pollutants	29
  Appendix E - Evaporative Failure Frequency	30
  References	31

-------
1 Background

Vehicles emit hydrocarbon (HC) emissions in addition to exhaust emissions due to driving. HC
emissions "evaporate" from the fuel system while a vehicle is sitting or driving, and are largely
driven by changes in the ambient temperature. For gasoline fueled vehicles, evaporative
emissions can account for a significant portion of the total gaseous hydrocarbon inventory. The
processes are unique from exhaust and require their own modeling approach. In the MOBILE
series of models, and in certification test procedures, evaporative emissions were quantified in
distinct modes based on the test procedures used to measure them:

• Running Loss - vapor lost during vehicle operation.
• Hot Soak - vapor lost while parked, immediately after turning off a vehicle.
• Diurnal / Cold Soak - vapor lost while parked and with a stabilized temperature.
• Refueling Loss - vapor lost and spillage occurring during refueling.

However, for MOVES, a change from this approach was proposed to better account for the
underlying physical processes involved in evaporation of fuels, and thus to better model
evaporative emissions under different ambient temperatures and fuel types. For example,
Ethanol (EtOH) has a unique effect on permeation, which is distributed among the modes listed
above. Instead MOVES groups evaporative emissions based on the evaporative mechanism,
using the following "processes":

• Permeation - the migration of hydrocarbons through elastomers in the vehicle's fuel
system.
• Tank Vapor Venting (TW) - vapor generated in fuel system that is expelled into the
atmosphere.
• Liquid Leaks - liquid fuel leaking from the fuel system, eventually evaporating into the
atmosphere.
• Refueling Emissions - vapor in the fuel system that is displaced into the atmosphere as a
vehicle refuels and spillage.

These processes can occur for each of the modes (Running Loss, Hot Soak, Cold Soak) used in
the MOBILE models. MOVES calculates permeation, tank vapor venting, and liquid leaks in
each of the operating modes. The benefit is that each emission process can be modeled using the
different factors that affect it, independently of how the vehicle is operating. This makes for
easier, more accurate modeling of scenarios that do not perfectly replicate the test procedures.
The emission processes used by MOVES and the operating modes used for evaporative
processes are shown in Appendix C.

The graphic below shows the evaporative emission processes. Permeation of the fuel through
the tank walls, hoses and seals occurs continuously, but is affected by the tank temperature.
Vapor is generated by a change in tank temperature and is intended to be captured by the
charcoal canister. If the canister is not purged enough or if there are leaks in the system, vapors
can be expelled when temperatures rise. Liquid leaks can occur in connections, hoses and the
tank itself. Refueling displaces the vapor in the tank and can result in spillage.

-------
                          Vapor Losses
                                      Charcoal
                                       Canister
                            Liquid Leaks
                                                        Permeation
Evaporative emissions depend on a number of variables. In MOVES, we model the impact of
the following factors:

    •   Ambient Temperature
    •   Fuel Tank Temperature
    •   Model year group (standard)
    •   Vehicle age
    •   Vehicle class
    •   Fuel Properties
          o  Ethanol content
          o  Reid Vapor Pressure (RVP)
    •   Failure Modes
    •   Presence of inspection and maintenance programs

Both ambient temperature and engine operation can  increase  fuel tank temperature.  Any
increase in fuel tank temperature will generate vapor in  the tank and pressure  in the tank.
Charcoal canisters connected to the gas tank are intended to vent these vapors through activated
charcoal and remove the hydrocarbons. When the engine  is operated the canister is "purged"
periodically and the captured hydrocarbons are diverted to the intake manifold of the engine and
burned. The emission standards (model year and vehicle class) determine the amount of capacity
the canister system is designed to hold. If the amount of vapors generated exceeds the capacity
                                        3

-------
of the canister, the vapors are vented to the atmosphere. This can occur if a vehicle is not driven
and undergoes several days of ambient temperature rises.
Fuel properties are important for evaporative emissions. Since evaporative emissions are just
fuel, the fuel properties will have a significant effect on the amount and types of hydrocarbons
emitted.

Fuel systems will sometimes generate leaks that allow liquid fuel or vapors to circumvent and
escape the control systems on the vehicle. Inspection and maintenance (I/M) programs
sometimes explicitly include inspections intended to identify vehicle in need of evaporative
system repairs. Stage 2 programs are designed to capture the vapors released during refueling.

The model year groups for evaporative emissions are shown in Table 1. They depend on
evaporative emission standards and related technological improvement designed to control
evaporative emissions.

Table 1 - Model Year Groups used for MOVES
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.
1 0% early control, 90% enhanced evap.
100% Enhanced evap.
Tier 2, LEV II
All evaporative emissions from vehicles derive directly from the fuels used and are not affected
by the combustion process. This means that hydrocarbons not present in the fuels that are a
product of combustion (such as methane) will not appear in evaporative emissions. Appendix D
contains a list of the non-combustion pollutants calculated by MOVES from evaporative
emissions.

The data used for this evaporative analysis was collected on Light-Duty gasoline vehicles but
will also be used in application for Heavy-Duty gasoline vehicles. For diesel vehicles, there are
no evaporative emission losses except for refueling spillage. All other diesel evaporative losses
are considered negligible.

-------
2 List of Data Sources

The evaporative emissions in MOVES are based on data from a large number of studies.

   •  CRC E-9 - Measurement of Diurnal Emissions from In-Use Vehicles
   •  CRC E-35 - Measurement of Running Loss Emissions in In-Use Vehicles
   •  CRC E-41 - Evaporative Emissions from Late-Model In-Use Vehicles
   •  CRC E-65 - Fuel Permeation from Automotive Systems
   •  CRC E-65-3 - Fuel Permeation from Automotive Systems: EO, E6, E10, and E857
   •  CRC E-77 - Vehicle Evaporative Emission Mechanisms: A Pilot Study
   •  CRC E-77-2 - Enhanced Evaporative Emission Vehicles
   •  CRC E-77-2b - Aging Enhanced Evaporative Emission Vehicles (DRAFT)
   •  CRC E-77-2c - Aging Enhanced Evaporative Emission Vehicles with E20 Fuel
   •  BAR Gas Cap Study
   •  API Gas Cap Study
   •  EPA Compliance Testing

Appendix A has a summary of the test programs mentioned above.

-------
3 Design and Analysis
Fuel tank temperature was empirically found to be the driver of the transient emissions
processes, permeation and vapor venting in the CRC E-77 pilot testing program which included
ten vehicles. Therefore, determining fuel temperature is critical in predicting emissions in each
operating mode. Fuel tank temperature is dependent on the daily ambient temperature profile,
vehicle operation patterns, and vehicle model year. As standards have tightened, fuel system
materials and connections have become more efficient at containing gaseous HC's from escaping
to the atmosphere. Purge systems and canister technologies have also advanced resulting in
lower emissions. The calculated fuel tank temperature can be used in modeling permeation and
vapor venting. Because liquid leaks are liquid rather than vapor, they are not dependent on
temperature. .

3.1 Fuel Tank Temperature Generator
This section explains how MOVES generates fuel tank temperature over a diurnal ambient
temperature profile and the vehicle trip schedule. Tank temperature is used in later calculation of
permeation and vapor venting, which makes this an instrumental algorithm in modeling
evaporative emissions for MOVES.

3.1.1 Fuel Tank Temperature Calculator General Steps

Define Input Parameters:
• Hourly ambient temperature profile (zoneMonthHour table)
• Key on and key off times (sampleVehicleTrip table)9
• Day and hour of first KeyON (hourDay table)
• Vehicle Type (Light-duty vehicle, Light-duty truck, Heavy-duty gas truck)
• Pre-enhanced or enhanced evaporative emissions control system

Tank temperature is modeled for the three primary operating modes of a vehicle. Operating, hot
soak, and cold soak. Operating vehicles tanks are warmer than ambient due to recirculation from
the engine. Hot soak is the period after a vehicle's engine ceases to run, and the vehicle begins
to cool to ambient temperature. Cold soak is the period in which a vehicle is not running and is
not hot soaking, therefore the primary driver of tank temperature of a cold-soaking vehicle is the
ambient temperature.

3.1.2 Calculate fuel tank temperature for hot and cold soaks
The following equation is used to model tank temperature as a function of ambient temperature.

-TTank) Equation 1
at
Trank is the fuel tank temperature, Tair is the ambient temperature, and & is a constant
proportionality factor (k = 1.4 hr"1, reciprocal of time constant). The value of & was established
by trial and error using EPA compliance data. Compliance data was available on 77 vehicles that
underwent a 2-day diurnal test and had a 1 -hour hot soak. There was no distinction made

-------
between hot and cold soak calculations. We assume that during any soak, the only factor driving
change in the fuel tank temperature is the difference between the tank temperature and the
ambient temperature.

As stated before this equation only applies during all parked conditions, which include the
following time intervals:
   •   From the start of the day (midnight) until the first trip (keyON)
   •   From a keyOFF time until the next keyON time
   •   From the final keyOFF time until the end of the day

For more information on the activity data used to determine the time of keyOn and keyOff
events, see the MOVE technical report10 and supporting contractor reports11'12.
Mathematical steps
   a)  At time to = 0 or KeyOFF (start of soak), Trank = Tt. This value will either be the ambient
       temperature at the start of the day, or the fuel tank temperature at the end of a trip.
   b)  Then, for all t > 0 and KeyOFF, the next tank temperature is calculated in this manner:
                         \1-
                           Tank
+ Ti           Equation 2a
                         (TTank )» + ! = TTank n + k(Tmr ~ TTank )„ A^            Equation 2b
(Tair - Ttan/f)  is a function of time.   Since analytical integration is too complicated (the input
ambient temperature data is  in a table), numerical integration is 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 smaller the time
step A?, the more accurate the  solution.  MOVES uses a A? of 15 minutes, which is accurate
enough for our modeling purposes without causing tremendous strain on computing resources.

3.1.3 Calculate fuel tank temperature during operation
Vehicle trips are relatively short  compared to the length of the day or modeling  period.
Therefore,  even though the fuel tank temperature profile  is not perfectly linear with time,
assuming  a linear  increase  in  temperature makes  calculations easier without compromising
accuracy.

The convention used in this  algorithm is that ATtank applies over a 4300 second period. This is
the length of the running loss test performed by manufacturers at certification. To find ATtank, we
must first find ATtank95, the average  increase in tank temperature during a standard 4300 second
95°F running loss test. The increase in fuel tank temperature depends on the temperature of the
tank at the KeyON time. It also depends on  vehicle type and  technology, due to different
configuration and design that affect heat loss.  The different ATtank95 temperatures are as follows:

    •  If the vehicle is evap-enhanced, then ATtank95 = 24°F 13
    •  If the vehicle is pre-enhanced, the vehicle type affects A

-------
o If LDV, then Mtank95 = 3 5°F.
o If LOT, then Mtank95 = 29°F.

We use these values to calculate to calculate the ATtank for other starting fuel tank temperatures
using equation 3.

A TTmk = 0 .352 (95 - TTmk ^KeyON ) + A TTank 95 Equation 3

This equation comes from GM regression analyses of light-duty vehicles driving the running loss
drive cycle with several different starting temperatures. 14 The average slope of fuel temperature
increase to starting fuel temperature is -0.352 with a standard error of the mean of 9.5%. This is
to say that the lower the starting fuel tank temperature, the bigger the increase over a drive cycle.
This gives us the increase in tank temperature so we can create a linear function that models fuel
tank temperature for each trip.
Tank A^f\f\ I ^^f\f\ ^ hsyON ) ~ ± Tank ,KeyON
4300 /3600 Equation 4

The 4300/3600 is in the denominator as a conversion from seconds to hours, maintaining
consistency in the algorithm.

Assumptions:
• The first trip is assumed to start halfway into the hour stated in the first trip's
HourDaylD.
• The effect of a change in ambient temperature during a trip is a negligible compared to
the temperature change caused by operation.
• The KeyON tank temperature is known from calculation of tank temperature from the
previous soak.

3.2 Permeation
Permeation emissions are fuel vapor emissions that escape through micro-pores in pipes, fittings,
fuel tanks, and other vehicle components. They differ from leaks in that they occur on the
molecular level and do not represent a mechanical/material failure in a specific location.

3.2.1 Base Rates
We first determined base rates for permeation. These were determined using the per-hour
emission rate during the last six hours of a 72-96-72°F diurnal test (also known as cold
soak/resting loss) The diurnal tests analyzed are federal cycle (72F-96F) tests from the CRC E-9
and E-41 programs. These two programs together represent a total of 151 vehicles with model
years ranging from 1971 to 1997.1'4'5 In the final six hours of the diurnal; the emissions rate,
ambient temperature, and fuel temperature are relatively stable or constant. This leads us to
believe that permeation is the only process occurring. The rates are broken out by model year
group and age group. The base permeation rates are in Table 2. Model years 1996-1998 are
represented individually to reflect 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 and Newer
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
In the E-65 permeation study, it was found that permeation rates, on average, double for every
18°F (10°C).6 This study included permeation testing performed on 10 vehicle fuel rigs at 85°F
and 105°F. The vehicles ranged in model year from 1978-2001. In MOVES the base permeation
rates are calculated at 72°F.

The following equation is derived from that study and used to adjust the base permeation rate:

p = p e °-0385 V™ -T^e ) Equation 5
adj base

Phase is the base permeation rate, Ttank is the tank temperature, and Tbase is the base temperature
for a given temperature cycle (e.g. 72°F for a 72-86-72°F diurnal test).

3.2.3 Fuel Adjustment
E10 affects evaporative emissions from gasoline vehicles due to the increased permeation of fuel
vapors through tanks and hoses. The model separates permeation emissions from vapor venting
emissions to allow better accounting of vapor losses 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; and CRC's E-77-2 and E-77-2b programs, which measured evaporative
emissions from sixteen vehicles. For this analysis, we separated the evaporative enhanced and
Tier 2 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 using a mixed model. The vehicle's evaporative certification, fuel ethanol
content, and fuel RVP were modeled as fixed effects and the particular vehicle modeled as a
random effect. The natural log of the emission rates over the 65-105-65°F diurnal cycle provided
a normally distributed dataset to the model. Due to a relatively small amount of data, in order to
find a significant effect of ethanol within different evaporative certifications (enhanced AND
Tier 2 vs. pre-enhanced), we had to combine E5.7 and E10 results into one category of "Ethanol"
fuel. Ethanol fuel could then be seen to have a significant influence on permeation from a
baseline EO fuel. 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 E85 for the fuelModelYearlD's used in MOVES.
10

-------
Table 3 — Increase in Emissions
Due to ethanol levels of 5% to 85% compared to EO (gasoline)
Model years
(via fuelModelYearlD in
MOVES)
1995 and earlier
1996
1997-2000
2001 and later
Percent increase due to ethanol
(E5 through ESS)
65.9
75.5
107.3
113.8
There is more information regarding permeation emissions in the final releases of the CRC E-77-
2b and E-77-2c studies which can be used to update the permeation estimates in future versions
of MOVES.

3.3 Tank Vapor Venting
It is important to understand TW emissions and their various sources. As tank temperature rises
and vapor is generated within the tank, it is consequently driven from the fuel system due to
increased pressure. Most modern vehicles are equipped with some kind of control strategy, such
as a carbon canister, to try and capture these vapors as they are generated. Later, the vapors are
consumed as they purge to the engine when the vehicle is operated next. However, the canister is
vented to the atmosphere to prevent unwanted pressure build-up within the fuel system allowing
emissions to "bleed" through the carbon, or even freely pass through a completely saturated
carbon bed. Problems with the evaporative control systems such as tampering, mal-maintenance,
and inadequate durability result in evaporative system failures and can also cause the unintended
release of vapors. The inclusion of these problems in the emission estimate and the use of
inspection and maintenance (I/M) programs to make repairs are handled through the cold soak
calculation.

Integral to the understanding of Tank Vapor Venting (TW) is how to calculate Tank Vapor
Generated (TVG). This is a function of increase in fuel tank temperature, ethanol content,
altitude and RVP. MOVES uses the Wade-Reddy equation for vapor generation.
TVG = AeL
Equation 6
15
In Equation 6, TI is the starting temperature and Tx is the temperature at hour x. Coefficients
A,B,C are required and represent the different altitude and ethanol effects. These coefficients are
shown below in Table 4.
Table 4 - TVG constants for Equation 6

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
11

-------
The following explains how vapor venting rates are calculated for each operating mode. 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 generator16. 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 TW rates after subtracting out permeation
and leaks from the test results. Also, due to the availability of test data for running loss and the
short length of trips, we determined TW rates during operation the same way we did for hot
soak.

3.2.4 Cold Soak
Cold soak vapor emissions are occurring while a vehicle is not operating and therefore only
responding to changes in ambient temperature. These are also commonly referred to as "diurnal"
emissions. To calculate emissions during cold soak, we again use diurnal data from CRC
programs E-9 and E-41. First we need to filter the dataset so that each test meets the following
requirements:

• Vehicle has no vapor leaks
• Vehicle is "As received" (not a retest)
• We only include hours of increasing temperature (no TVG while temperature decreases)
• Pressure test result is pass, fail, or blank (no dashes, slashes, "I", etc.)

1) For each diurnal test, fuel tank temperature is calculated at each hour using the fuel tank
temperature algorithm described in section 3.1.
2) We calculate the base permeation rate using the method described in 3.2.1. (Average of last
six hours of HC evaporative emissions). Then, using the temperature adjustment described in
section 3.2.2 we calculate the temperature adjusted permeation rate for each hour of diurnal.
3) Subtract the temperature adjusted permeation rate from the total HC for every hour. Because
the SHED can only measure total HC, it cannot distinguish permeation from TW, so we
must make this correction.
4) Sum TW from beginning of diurnal to each hour to get Cumulative TW.
5) Using Equation 6, calculate TVG in grams per gallon of vapor space, from hour 1 to hour x.
where hour x is the last hour where temperature is increasing.

TVG is the amount of vapor generated in the tank. We establish a relationship between
Cumulative TW and TVG for inputs into MOVES. This is done by constructing a quadratic
curve of CumTW vs. TVG for each model year group, age group, and pressure test result.

CumTW = a^TVG + a2TVG2 Equation 7

The curve is set to have a zero-intercept (0,0) to ensure that at 0 TVG, there is no tank vapor
venting, an accurate physical assumption. Figure 1 below illustrates the phase-in of evaporative
enhanced vehicles over the model years 1996-1998. For instance, a given amount of vapor
generated (x-axis) a smaller amount of vapor is vented as the technology phases in. The curves

-------
used in Figure 1 use the combined passing and failing coefficients for the case of vehicles in the
6-7 age group. A complete set of coefficients for Equation 7 can be found in Appendix B. The
aggregate columns are the inputs used in the MOVES model for the I/M and non-I/M coefficient
cases in the cumTWCoeffs table.

Figure 1 - MOVES Composite Vapor Generation Curves
Model Years 1996-1998
60
50
40

-------
7) Since the aggregate coefficients are determined using the failure rates, which are essentially
weighting factors, the standard errors of the aggregate coefficients are calculated:
sy>a*,°BS V Sy'a*.f 6
o
Q.
3
2
—Non-IM

—IM
2 4 6 8 10

Tank Vapor Generated (g/gal head space)
12
14
-------
3.2.5 Hot Soak
Hot soak vapor emissions refer to the vapor vented immediately after a car ceases operation and
begins to cool; until it reaches the ambient temperature. In MOVES, the TW apportioned to Hot
Soak is a simpler process than the Cold Soak calculations. Base rates in grams per hour are used
for each model year and age group. Pass and fail results are weighted together to form the
aggregate rate, similarly to Cold Soak. The process for developing the rates is described below:

Data is used from CRC E-41 Late Model In-Use Evap. Emission Hot Soak Study. This study
included 50 vehicles (30 passenger cars and 20 light duty trucks) ranging from model year 1992
to 1997. The driving schedule is a full LA-4, NYCC, NYCC, and LA-4 with a two minute idle
period following the first LA-4, the second NYCC and the final LA-4.

The data must first be filtered by the following criteria:
a. Vehicles are Non-leakers (emissions less than 10.0 grams17; 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. Vehicles are "As received" (no retests)
c. Vehicle pressure test result must be pass, fail, or blank only (no dashes, slashes, "I",
etc.)

I) First, we find the temperature at the start of the soak for each hot soak test. This is done by
adding the calculated 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. The temperature rise depends on the fuel tank temperature at the start of the LA-4
test. To calculate the temperature Tstart, see section 3.1.3 Calculating fuel tank temperatures
during operation. Then we find the average tank temperature in that hour:

Tavg=^(Tstart-Tmr)(l-e-k) + Tair Equation 11

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 section 3.1.2, k = 1.4 hrs"1.

TW rates are averaged by model year group, age group, and pressure test results, shown in
the table below.
15
-------
Table 5 - Average hot soak tank vapor venting rates
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
Unknown
P
F
Unknown
P
Unknown
F
P
F
P
Unknown
TW
rate
(g/hr)
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
Number
of Cases
5
4
5589
901
170
202
255
2
31
64
5
14
83
3) As with cold soak, aggregate rates are found using failure rates involving pressure, gas cap,
and OBD tests for non-I/M and I/M. Table 6 reflects the most updated I/M analysis explained
in section 3.4 Inspection/Maintenance (I/M) Program effects (unlike the cold soak
coefficients in Table 5) or the enhanced evaporative phase-in.
16
-------
Table 6 - Hot soak tank vapor venting rates
Model year
group
Pre-1971
1971-1977
1978-1995
1996-2003
2004 and later
Age
group
20+
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+
Non-I/M
TW rate
[g/hr]
5.455
3.099
5.149
5.455
0.627
0.627
1.451
1.471
2.082
3.492
3.817
0.124
0.124
0.15
0.168
0.25
0.383
0.611
0.06
0.06
0.086
0.105
0.187
0.323
0.553
I/M
TW rate
[g/hr]
5.116
2.957
4.881
5.116
0.612
0.612
1.43
1.455
1.963
3.225
3.49
0.099
0.1
0.1
0.103
0.113
0.137
0.198
0.035
0.036
0.036
0.039
0.05
0.074
0.137
3.3.3 Running Loss
Running Loss is the process of vapor venting that occurs during vehicle operation. Data for
developing running loss emission rates came from CRC E-352'3 and CRC E-414'5. Between these
two programs 200 vehicles were testing with model years ranging from 1971-1997.

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). The running loss test performed in E-
41 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.
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.
3) We used this average temperature to determine the average permeation rate during the hot
soak via the permeation temperature adjustment using the 72°F base permeation rates
determine by model year, age.
17
-------
4) We calculated gram/hour rates by dividing total emissions by the duration of the running loss
test (4300 seconds)
5) We filtered/reduced the data set such that each test met the following requirements:
a. Non-liquid-leakers (emissions less than 137.2 g/hour17; taken from M6.EVP.009,
Section 2.4, Table 2-1)
b. "As received" vehicles (no retests)
c. Pressure test result must be pass, fail, or blank only (no dashes, slashes, "I", etc.)
6) Subtract permeation from HC for each hour to get tank vapor venting (TW) rate
7) After analysis of TW data, we found that the best way to stratify running loss TW was by
model year only. The table below shows the results of the analysis.