Development of Evaporative Emissions 

Calculations for the Motor Vehicle 

Emissions Simulator 

(Draft MOVES2009) 

Draft Report 
 

 

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 

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-420-P-09-006 

August 2009 
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 

i
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. 

Table 1 describes the model year group stratifications used for MOVES analysis.

Model year group Emissions standard or technology level 

1971-1977 Pre-control 

1978-1995 Early control 

1996 80% early control, 20% enhanced evap. 

1997 60% early control, 40% enhanced evap. 

1998 10% early control, 90% enhanced evap. 

1999-2003 100% Enhanced evap. 

2004 and later Tier 2, LEV II 

2 Data Sources

• CRC E-9 – Measurement of Diurnal Emissions from In-Use Vehicles1

1
• CRC E-35 – Measurement of Running Loss Emissions in In-Use Vehicles2 3

• CRC E-41 – Evaporative Emissions from Late-Model In-Use Vehicles4 5

• CRC E-65 – Fuel Permeation from Automotive Systems6

• CRC E-65-3 – Fuel Permeation from Automotive Systems: E0, 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 

• HourDayID (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 

2
calculated as for cold soak (equation 2b), but with the initial temperature (Ti) 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. 

 )( air Tank

Tank TTk

dt

dT −= , (1) 

where Ttank is the fuel tank temperature, Tair 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 – Simulink® block diagram of the relation between ambient temperature and fuel tank 

temperature

difference

1.4

k

time1

To Workspace

Ti

Tank Temp at start of soak

Ttank

Output - Tank temperature

1

s xo

Integrator

72_96.mat

Input From File - Ambient 

Temperature Data

Clock

Tair

Ambient temperature

Ttank

dTtank/dt

Tair - Ttank

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 key off to the end of the day. 

Mathematical steps 

1) At time t0 = 0 or KeyOFF (start of soak), Ttank = Ti. 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. 

3
2) Then, for all t > 0 or KeyOFF, the next tank temperature is calculated in this manner: 

 (2a) or i

n

j

T nTank air jTank ⎥ + TtTTk

⎦

⎤ ⎢

⎣

⎡ = ∑ Δ− =

+

0

1 )( )(

air nTank tTTkTT nTank Tank n )( +1 = + − )( Δ (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 Δt should be, to 

improve the solution. MOVES uses a Δt 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 ΔTtank depends on the temperature at the start of 

operation. It also depends on vehicle type. The convention used in this algorithm is that ΔTtank

applies over a 4300 second period, which is the length of the running loss test performed by 

manufacturers for certification. To find ΔTtank, we must first find ΔTtank95, the average increase in 

tank temperature at a standard 4300 second @ 95F running loss test. 

• If the vehicle is evap-enhanced, then ΔTtank95 = 24F 8

 

• If the vehicle is pre-enhanced, the vehicle type affects ΔTtank95. 

o If LDV, then ΔTtank95 = 35F. 

o If LDT, then ΔTtank95 = 29F. 

We can use these values for ΔTtank95 for 95F to calculate the ΔTtank for other starting fuel tank 

temperatures (other trips) using the following equation: 

 , 95 95(352.0 ) ΔTTank = − TTank KeyON + ΔTTank (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. 

keyON Tank KeyON

Tank

Tank Ttt

T

T , )( 3600/4300+−

Δ = (4) 

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 

HourDayID. 

• 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 72-96-72°F diurnal test (also known as 

resting loss). In these six hours, the emissions rate and the ambient and fuel tank temperatures 

4
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 Age group Base permeation 

rate [g/hr] 

10-14 0.192 

1971-1977 15-19 0.229 

20+ 0.311 

0-3 0.0554 

4-5 0.0554 

6-7 0.0913 

8-9 0.0913 

10-14 0.124 

15-19 0.148 

1978-1995 

20+ 0.201 

0-3 0.046 

4-5 0.046 

6-7 0.075 

8-9 0.075 

10-14 0.101 

15-19 0.120 

1996 

20+ 0.163 

0-3 0.037 

4-5 0.037 

6-7 0.059 

8-9 0.059 

10-14 0.079 

15-19 0.093 

1997 

20+ 0.125 

0-3 0.015 

4-5 0.015 

6-7 0.018 

8-9 0.018 

10-14 0.022 

15-19 0.024 

1998 

20+ 0.029 

0-3 0.0102 

4-5 0.0102 

6-7 0.0102 

8-9 0.0102 

10-14 0.0102 

15-19 0.0102 

1999-2003 

20+ 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: 

 (5) (0385.0 ) Tank TT base

adj base ePP − =

5
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 

E0, 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 E0. The percent difference between the 

ethanol rate and the E0 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 

fuelModelYearID. Table 3 shows the fuel adjustments used for E5 through E10 for the 

fuelModelYearID’s used in MOVES. 

Table 3 – Increase in emissions due to ethanol levels of 5 to 10% compared to E0 (gasoline) 

Model years 

(via fuelModelYearID in MOVES)

Percent increase due to ethanol 

(E5 through E10) 

1995 and earlier 37.3 

1996 69.4 

1997-2000 175 

2001 and later 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 

6
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 (TVV) 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. 

( ) *RVPB CT CT2 = x − eeAeTVG [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. 

Gasoline E10 

Constant Sea Level Denver alt. Sea Level Denver alt. 

A 0.00817 0.00518 0.00875 0.00665 

B 0.2357 0.2649 0.2056 0.2228 

C 0.0409 0.0461 0.0430 0.0474 

TVG is the amount of vapor generated in the tank. We will establish a relationship between 

Cumulative TVV and TVG for inputs into MOVES. 

7) We constructed quadratic curves (zero intercept) of CumTVV vs. TVG, stratifying by model 

year group, age group, and pressure test result. 

2 CumTVV 1 += 2TVGaTVGa (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
)1( , , , FaFaa aggx = failx + passx − , (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: 

2

,

2 2

,

2

, , , , )1( aggx failx passx ay ay ay = sFs −+ sF (9) 

 As a result, the CV’s for the aggregate coefficients are calculated: 

2 2

,

2 2 2

,

2

,

, , , )1( 1aggx afailx failx apassx passx

aggx

a CVaF CVaF

a

CV = −+ (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 nonI/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 cumTVVCoeffs table. 

a1 a2

model year group age group 

pass fail aggregate pass fail aggregate 

10-14 1.227 11.314 1.941 2.175 0.402 2.049 

1971-1977 15-19 5.406 9.254 5.835 2.331 3.117 2.419 

20+ 5.406 9.254 6.127 2.331 3.117 2.479 

0-3 1.578 3.073 1.589 0.440 1.338 0.446 

4-5 1.578 3.073 1.604 0.440 1.338 0.455 

6-7 1.578 3.073 1.610 0.440 1.338 0.459 

8-9 1.578 3.073 1.623 0.440 1.338 0.466 

10-14 0.849 11.314 1.283 2.095 0.402 2.025 

15-19 3.743 9.254 4.120 2.246 3.117 2.305 

1978-1995 

20+ 3.743 9.254 4.376 2.246 3.117 2.346 

0-3 1.339 3.073 1.354 0.344 1.338 0.352 

4-5 1.339 3.073 1.362 0.344 1.338 0.357 

6-7 1.339 3.073 1.376 0.344 1.338 0.365 

8-9 1.339 3.073 1.392 0.344 1.338 0.374 

10-14 0.756 9.666 1.124 1.668 0.589 1.624 

15-19 3.071 8.017 3.399 1.789 2.762 1.853 

1996 

20+ 3.071 8.017 3.530 1.789 2.762 1.879 

0-3 1.100 3.073 1.120 0.248 1.338 0.259 

4-5 1.100 3.073 1.129 0.248 1.338 0.264 

6-7 1.100 3.073 1.146 0.248 1.338 0.273 

8-9 1.100 3.073 1.163 0.248 1.338 0.283 

10-14 0.663 8.018 0.976 1.241 0.776 1.222 

1997 

15-19 2.399 6.781 2.686 1.332 2.406 1.402 

8
20+ 2.399 6.781 2.791 1.332 2.406 1.428 

0-3 0.502 3.073 0.538 0.009 1.338 0.027 

4-5 0.502 3.073 0.553 0.009 1.338 0.035 

6-7 0.502 3.073 0.575 0.009 1.338 0.046 

8-9 0.502 3.073 0.596 0.009 1.338 0.057 

10-14 0.429 3.897 0.589 0.174 1.244 0.223 

15-19 0.719 3.691 0.907 0.189 1.516 0.273 

1998 

20+ 0.719 3.691 0.959 0.189 1.516 0.296 

0-3 0.383 3.073 0.422 -0.039 1.338 -0.019 

4-5 0.383 3.073 0.438 -0.039 1.338 -0.011 

6-7 0.383 3.073 0.461 -0.039 1.338 0.001 

8-9 0.383 3.073 0.483 -0.039 1.338 0.012 

10-14 0.383 3.073 0.508 -0.039 1.338 0.025 

15-19 0.383 3.073 0.552 -0.039 1.338 0.048 

1999-2003 

20+ 0.383 3.073 0.595 -0.039 1.338 0.070 

0-3 0.124 3.073 0.151 -0.013 1.338 -0.001 

4-5 0.124 3.073 0.161 -0.013 1.338 0.004 

6-7 0.124 3.073 0.175 -0.013 1.338 0.010 

8-9 0.124 3.073 0.187 -0.013 1.338 0.016 

10-14 0.124 3.073 0.203 -0.013 1.338 0.023 

15-19 0.124 3.073 0.229 -0.013 1.338 0.035 

2004 and later 

20+ 0.124 3.073 0.255 -0.013 1.338 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: 

air

k

avg start air TeTT

k

T +−−= − ( )1)( 1 (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 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 (TVV) rate 

6) We averaged TVV rates by model year group, age group, and pressure test result, shown in 

9
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 Age group Pressure test result TVV rate 

20+ F 6.17 

1971-1977 

20+ P 2 

0-5 Both 1.25 

0-5 P 0.56 

0-9 F 2.37 

6-9 Both 1.75 

6-9 P 1.38 

10-14 Both 5.13 

10-14 F 3.41 

10-14 P 1.76 

15-19 F 4.51 

1978-1995 

15-19 P 2.99 

1996-2003 all Both 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 Age group TVV rate [g/hr] 

10-14 3.099 

15-19 5.149 1971-1977 

20+ 5.455 

0-3 0.627 

4-5 0.627 

6-7 1.451 

8-9 1.471 

10-14 2.082 

15-19 3.492 

1978-1995 

20+ 3.817 

0-3 0.124 

4-5 0.124 

6-7 0.150 

8-9 0.168 

10-14 0.250 

15-19 0.383 

1996-2003 

20+ 0.611 

0-3 0.060 

4-5 0.060 

6-7 0.086 

8-9 0.105 

10-14 0.187 

15-19 0.323 

2004 and later 

20+ 0.553 

10
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

Gas cap 

test OBD Pressure test Frequency Network Calendar years

Colorado Y Advisory only N Biennial Hybrid 2003-2006

N. Carolina N Y N Annual Decentralized 2002-2006

Phoenix Y Y Y Biennial Centralized 2002-2006

Tucson Y Y N Annual Centralized 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 OBDequipped 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 nonI/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
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. 

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

0-3 4-5 6-7 8-9 10-14 15-19 20+

MOVES age group

Failure frequency

I/M vehicles

Non-I/M vehicles

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% nonI/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
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. 

IM factor 

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. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.50% 2.00% 2.50% 3.00% 3.50% 4.00%

Failure frequency [%]

I/M factor

Phoenix (reference I/M) 

Colorado 

Tucson 

Non-I/M 

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
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, age. 

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-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.) 

6) Subtract permeation from HC for each hour to get tank vapor venting (TVV) rate 

7) 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 TVV mean [g/hr] 

1971-1977 12.59 

1978-1995 11.6 

1996-2003 0.72 

2004 and later 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 Liquid leak rate

Cold Soak 9.85 g/hr 

Hot Soak 19.0 g/hr 

Operating 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 Percentage of leakers in fleet 

0-9 0.09 % 

10-14 0.25 % 

15-19 0.77 % 

20+ 2.38 % 

14
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 Cold soak Hot soak Operating 

0-9 0.009 0.017 0.158 

10-14 0.025 0.048 0.450 

15-19 0.075 0.145 1.36 

20+ 0.235 0.452 4.23 

 

15
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 

16
Fuel System: Port Fuel Injection 

Unleaded Cert Fuel 

S u lfu r R V P D a te

w t %

0 .0 0 4 8 9 .0 4 Ju l-9 8

0.0045 9.2 D ec-98

0.0063 9.04 A ug-99

0.0048 8.99 M ay-00

0.0042 9.05 S ep-01

0.003 9.12 D ec-01

0.003 9.12 D ec-02

0.0031 8.8 M ay-03

0.0035 8.91 A pr-04

0.0027 8.95 Jun-04

CARB Phase II Fuel 

Sulfur RVP Date

wt %

0.0023 6.92 Aug-99

0.0023 6.92 May-00

0.0038 6.92 Jan-01

0.0033 6.92 Oct-02

0.0036 6.77 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_1A: 1 hour Breathing Loss Evap. Test – Gas Cap left “On” 

 BL_1B: 1 hour Breathing Loss Evap. Test – Canister as recd. 

 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
 

1 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. Project E-41-2. Evaporative Emissions from Late-Model In-Use Vehicles. October 1999. 

http://www.crcao.com/reports/emission/e41.htm. 6

 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: E0, 

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

8

 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. 

18