Greenhouse Gas Emissions Model
(GEM) User Guide
&EPA
United States
Environmental Protection
Agency
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Greenhouse Gas Emissions Model
(GEM) User Guide
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
&EPA
United States
Environmental Protection
Agency
EPA-420-B-11-019
August 2011
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The Greenhouse gas Emissions Model was created by EPA as a means for determining
compliance with EPA's GHG emissions and NHTSA's fuel consumption vehicle standards, for
Class 7 and 8 combination tractors and Class 2b-8 vocational vehicles. The GEM v2.0 model
itself is a part of the final regulation.l It is a free, desktop computer application.
The GEM is designed to operate on a single computer. The downloadable installation
file contains the application executable file and other supporting files, available at
http://www.epa.gov/otaq/climate/gem.htm, and described in this guide. To request a CD of this
software instead of downloading it, or to request assistance if you have trouble with accessibility
of this software, please request through an email addressed to OTAQPUBLICWEB@epa.gov.
After the GEM version 1.0 was released to the public in October 2010 as a part of the
proposed rules, the agencies received many constructive comments and useful suggestions from
the public. Prior to promulgation of the final rules, the agencies continued validations against
additional test data and benchmarking against other commonly used industrial standard vehicle
models. The most noticeable improvements to the GEM version 2.0 are a new driver model, the
simplified electric model in conjunction, and many other refinements including the graphical
user interface.
Chapter 4 of the RIA details model validations against testing data and comprehensive model
benchmark against commonly used vehicle model in industry. This user guide contains the
model documentation with details on the model's input and output files. It also includes
instructions on how to use the GEM. Some of the information provided here can also be found
in Chapter 4 of the RIA.
1. GEM Documentation
This section describes the GEM vehicle model architecture, the list of pre-defined input
parameters, output calculations, and the installation of executable GEM code into the user's
computer.
1.1. Vehicle Model Architecture
The Class 2b-8 vehicle compliance model architecture is comprised of six systems:
Ambient, Driver, Electric, Engine, Transmission, and Vehicle. With the exception of
"Ambient," "Electric," and "Driver," each system consists of one more component models.
Details on the function of each system and their respective component models can be found in
Chapter 4 of the RIA.
1 See Sections II.B and II.D of the preamble and Chapter 4 of the RIA (Regulatory Impact Analysis), available at
http ://www. epa. gov/otaq/climate/regulations. htm.
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1.2. List of Predefined Input Parameters for Class 7/8 Combination
Tractor Models
Although many technologies can potentially achieve GHG emission and fuel
consumption reductions, EPA and NHTSA believe that for the rule's timeframe, some may be
too complex to model for compliance purposes (e.g., hybrid control) while others require
standardization such as the calculation of GHG and fuel consumption benefits due to
aerodynamic improvements. EPA and NHTSA decided to pre-define the tractor frontal area,
tractor-trailer combined and payload weight, gear box and its efficiency, final drive ratio,
engine/transmission/wheel inertia, accessory load, axle base, tire radius, trailer tire coefficient of
rolling resistance (Crr, trailer tires), and engine fuel map. The agencies use these pre-defined
input parameters in the simulation model for all subcategories of combination tractors. Table 1
lists the specific values of these parameters for Class 8 combination tractors, and Table 2 lists the
similar values for Class 7 combination tractors.
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Table 1: Class 8 Combination Tractor Modeling Parameters
MODEL TYPE
Regulatory
Subcategory
Fuel Map
Gearbox
Gearbox Ratio
Gearbox
Efficiency
Engine Inertia (kg-
m2)
Transmission
Inertia (kg-m2)
All Axle Inertia
(kg-m2)
Loaded Tire
Radius (m)
Tractor Tare
Weight (Ibs)
Trailer Weight
(Ibs)
Payload (Ibs)
Total weight (Ibs)
Total weight (kg)
Frontal Area (m2)
Coefficient of
Aerodynamic Drag
Axle Base
Electrical
Accessory Power
(W)
Mechanical
Accessory Power
(W)
Final Drive Ratio
Tire CRR
(kg/metric ton)
Trailer Tire CRR
(kg/metric ton)
Steer Tire CRR
(kg/metric ton)
Drive Tire CRR
(kg/metric ton)
Vehicle Speed
Limiter (mph)
CLASS 8
Sleeper Cab
High Roof
CLASS 8
Sleeper Cab
Mid Roof
CLASS 8
Sleeper Cab
Low Roof
CLASS 8
Day Cab
High Roof
CLASS 8
Day Cab
Mid Roof
CLASS 8
Day Cab
Low Roof
15L - 455 HP
10-speed
Manual
4.17
5
360
0.489
19,000
13,500
38,000
70500
31978
10.4
OEM Input
5
350
1,000
2.64
10-speed
Manual
10-speed
Manual
10-speed
Manual
10-speed
Manual
148 1095 809 597 446 3 32 245 181 135 1
0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98, 0.98
4.17
5
360
0.489
18,750
10,000
38,000
66750
30277
7.7
OEM Input
5
350
1,000
2.64
4.17
5
360
0.489
18,500
10,500
38,000
67000
30391
6.9
OEM Input
5
350
1,000
2.64
4.17
5
360
0.489
17,500
13,500
38,000
69000
31298
10.4
OEM Input
5
350
1,000
2.64
4.17
5
360
0.489
17,100
10,000
38,000
65100
29529
7.7
OEM Input
5
350
1,000
2.64
10-speed
Manual
4.17
5
360
0.489
17,000
10,500
38,000
65500
29710
6.9
OEM Input
5
350
1,000
2.64
= 0.425 x Trailer CRR + 0.425 x Drive CRR + 0. 15 x Steer CRR
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
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Table 2: Class 7 Combination Tractor Modeling Parameters
MODEL TYPE
Regulatory Subcategory
Fuel Map
Gearbox
Gearbox Ratio
Gearbox Efficiency
Engine Inertia (kg-m2)
Transmission Inertia
(kg-m2)
All Axle Inertia (kg-m2)
Loaded Tire Radius (m)
Tractor Tare Weight (Ibs)
Trailer Weight (Ibs)
Payload (Ibs)
Total weight (Ibs)
Total weight (kg)
Frontal Area (m2)
Coefficient of Aerodynamic
Drag
Axle Base
Electrical Accessory Power
(W)
Mechanical Accessory
Power (W)
Final Drive Ratio
Tire CRR
(kg/metric ton)
Trailer Tire CRR
(kg/metric ton)
Steer Tire CRR
(kg/metric ton)
Drive Tire CRR
(kg/metric ton)
Vehicle Speed Limiter
(mph)
CLASS 7
Day Cab High
Roof
CLASS 7
Day Cab Mid
Roof
CLASS 7
Day Cab
Low Roof
11L-350HP
10-speed
Manual
10-speed
Manual
10-speed
Manual
11.06, 8.19, 6.05, 4.46, 3.34, 2.48, 1.83, 1.36, 1,
0.75
0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98,
0.98
3.36
5
233.4
0.489
11,500
13,500
25,000
50000
22680
10.4
OEM Input
4
350
1000
3.73
3.36
5
233.4
0.489
11,100
10,000
25,000
46100
20910
7.7
OEM Input
4
350
1000
3.73
3.36
5
233.4
0.489
11,000
10,500
25,000
46500
21092
6.9
OEM Input
4
350
1000
3.73
= 0.425 x Trailer CRR + 0.425 x Drive CRR +
0.15 x Steer CRR
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
6
OEM Input
OEM Input
OEM Input
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1.3. List of Predefined Input Parameters for Class 2b-8 Vocational
Vehicle Models
Likewise, EPA and NHTSA standardized a set of parameters for the three Class 2b-8
vocational vehicle types, which the agencies refer to as Vocational Light Heavy-Duty (VLHD),
Vocational Medium Heavy-Duty (VMHD), and Vocational Heavy Heavy-Duty (VHHD). These
predefined parameters include the coefficient of aerodynamic drag, truck frontal area, truck total
and payload weight, the gear box and its efficiency, final drive ratio, engine/transmission/wheel
inertia, accessory load, axle base, tire radius, and the engine fuel map. The specific values of
these parameters are listed in Table 3.
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Table 3: Vocational Vehicle Modeling Parameters
Model Type
Regulatory Subcategory
Fuel Map
Gearbox
Gearbox Ratio
Gearbox Efficiency
Engine Inertia (kg-m2)
Transmission Inertia
(kg-m2)
All Axle Inertia (kg-m2)
Loaded Tire Radius (m)
Payload (Ibs)
Total weight (Ibs)
Total weight (kg)
Frontal Area (m2)
Coefficient of
Aerodynamic Drag
Axle Base
Electrical Accessory
Power (W)
Mechanical Accessory
Power (W)
Final Drive Ratio
Tire CRR
(kg/ton)
Trailer Tire CRR
(kg/metric ton)
Steer Tire CRR (kg/metric
ton)
Drive Tire CRR
(kg/metric ton)
Heavy Heavy-Duty
Vocational Truck
(Class 8)
15L - 455 HP
10-speed Manual
14.8, 10.95,8.09,5.97,4.46,
3 32 245 1 81 1 35 1
0.96, 0.96, 0.96, 0.96, 0.98,
0.98,0.98,0.98,0.98,0.98
4.17
5
200
0.489
15,000
42,000
19,051
9.8
0.7
3
350
1,000
2.64
Medium Heavy-Duty
Vocational Truck
(Class 6-7)
7L - 270 HP
6-speed Manual
9.01,5.27,3.22,2.04,
1.36, 1
0.92,0.92,0.93,0.95,
0.95,0.95
2.79
0.5
60
0.389
11,200
25,150
11,408
9.0
0.6
2
300
1,000
3.36
Light Heavy-Duty
Vocational Truck
(Class 2b-5)
7L - 200 HP
6-speed Manual
9.01, 5.27, 3.22, 2.04,
1.36, 1
0.92,0.92,0.93,0.95,
0.95,0.95
2.79
0.5
60
0.378
5,700
16,000
7,257
9.0
0.6
2
300
1,000
2.85
= 0.5 x Drive CRR + 0.5 x Steer CRR
Not applicable Not applicable Not applicable
OEM Input OEM Input OEM Input
OEM Input OEM Input OEM Input
1.4. Output Processes
The outputs produced by the GEM include post processes to calculate the final weighted
results.
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The GEM produces a cycle-weighted grams CO2/ton-mile and gallons/1,000 ton-mile
result which incorporates the drive cycle weightings of the ARB transient cycle, 55 mph steady
state cruise, and 65 mph steady state cruise cycle, as shown in Table 4.
Table 4: Drive Cycle Weighting
CATEGORY
ARB Transient
55 mph Cruise
65 mph Cruise
CLASS 8
SLEEPER CAB
TRACTORS
5%
9%
86%
CLASS 7/8
DAY CAB
TRACTORS
19%
17%
64%
CLASS 2b-8
VOCATIONAL
VEHICLES
42%
21%
37%
The GEM converts the mile per gallon result into ton-mile space by using the payload for
each regulatory class - 19 tons for Class 8 tractors, 12.5 tons for Class 7 tractors, 7.5 tons for
HHD vocational vehicles, 5.6 tons for MHD vocational vehicles, and 2.85 tons for LHD
vocational vehicles.
GEM calculates the gallons/1,000 ton-mile weighted result by converting the weighted
grams CCVton-mile result. The gram CCVton-mile result is multiplied by 1,000 and divided by
10,180 grams CC>2 per gallon of diesel fuel.
2. GEM Installation Instructions
Certain minimum computer requirements must be met before installation. The system
required to run GEM needs a minimum RAM of 1 GB (4 GB is highly recommended), and
approximately 900 MB of disk storage. Window XP or Window 7 operating system is required.
The validation has not been tested in other computer operating systems. The 2007 Microsoft
Excel or later version is required.
The executable form of GEM named "GEM_Setup.exe" can be downloaded from the
EPA website, at http://www.epa.gov/otaq/climate/gem.htm. Before installation, please close all
other programs. Follow the procedure below to perform installation.
• Down load GEM_Setup into a temporary directory.
• Click GEM_Setup.exe to install the package and GEM Installation window will be
displayed.
• Click Next; a "Choose Install Location" window will appear. The user has the option to
select the folder location or just use default folder on screen. Click Next.
• A "Select Shortcuts" window will be displayed. A default checkbox for Create a
Desktop Icon is already selected. Click Next.
• A Window "Ready to Install" will be displayed. Click Install.
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In the next few steps, it will install 2008 Microsoft Visual C++ Redistributable Setup and
the MATLAB® Runtime 7.15 if you have not installed those previously. If these two were
installed previously, click Cancel once Choose Setup Language window displays. The
installation is finished.
For Microsoft Visual C++ Redistributable Setup, just follow on-screen instruction to
install. If Visual C++ Redistributable Setup has been installed previously, the installation will
not prompt you to install, and you just go to the next step for MATLAB® Runtime 7.15
installation.
For MATLAB® Runtime 7.15 installation, follow the steps below.
• Installation of MATLAB® Runtime 7.15 will be automatically initiated either right after
installation of Visual C++ Redistributable Setup or click OK once Choose Setup
Language window displays.
• An InstallShield Wizard window will be displayed, prompting the user to install the
MATLAB® Runtime 7.15. Click Next.
• Click Install, and then follow the steps below to complete the installation
a. Enter the names of user and organization, and then click Next.
b. A window "Destination Folder" will be displayed. Click Next if a default folder is
selected.
c. Click Install to complete MATLAB® Compiler Runtime. It will take a while to
complete the installation.
d. Click Finish, and entire installation is completed.
The GEM folder contains these visible files:
• GEM.exe shortcut
• GEM_input.xlsx
It should be pointed out that the GEM is compatible with MATLAB® Runtime 7.15.
Different versions of MATLAB® Runtime may result in compatibility issues.
To uninstall, the user may manually delete the GEM by following the steps below:
• Delete the GEM folder entirely
• Delete the short cut on your desktop screen.
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3. GEM Running Instructions
To run GEM, double click the GEM.exe on the desktop icon. A graphic user interface
GUI will pop as shown in Figure 1.
Greenhouse Gas Emissions Model (GEM)
^Identification-
Manufacturer Name:
Vehicle Family:
Vehicle Configuration:
Vehicle Model Year:
Date:
V
r Regulatory Subcategory
, Class 8 Combination - Sleeper Cab - High Roof
_ • Class 8 Combination - Sleeper Cab - Mid Roof
_':• Class 8 Combination - Sleeper Cab - Low Roof
j Class 8 Combination - Day Cab - High Roof
Class 8 Combination - Day Cab - Mid Roof
'. Class 8 Combination - Day Cab - Low Roof
Class 7 Combination - Day Cab - High Roof
j Class 7 Combination - Day Cab - Mid Roof
Class 7 Combination - Day Cab - Low Roof
'. • Heavy Heavy-Duty - Vocational Truck (Class 8)
Medium Heavy-Duty - Vocational Truck (Class 6-7)
'~j Light Heavy-Duty - Vocational Truck (Class 2b-5)
i-Simulation Inputs-
Coefficient of Aerodynamic Drag:
Steer Tire Rolling Resistance [kg/metric ton]:
Drive Tire Rolling Resistance [kg/metric ton]:
Vehicle Speed Limiter [mph]:
Vehicle Weight Reduction [Ibs]:
Extended Idle Reduction:
rSimulation Type
O Single Configuration
D Plot Output
O Multiple Configurations
Figure 1 - GEM Input GUI for Single Configuration Run
With the GEM GUI launched, the user has two options to run the GEM. The first option
is to run single configuration, and the second option is to allow the user to run multiple
configurations or a batch job under the same regulatory subcategory and vehicle model year.
For the single configuration, use the following instruction:
Figure 1 allows the user to enter the required parameters into the model. There are
essentially two types of input parameters. The first set of parameters is required to provide
information to EPA and NHTSA. These include the following:
• Manufacturer Name
• Date
• Vehicle Family
• Vehicle Configuration
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• Vehicle Model Year
The second set of parameters affects how GEM calculates the final results. The list
below describes each input and how it is used within GEM.
• Vehicle Model Year: The pull-down allows the user to select five model years: "2013",
"2014", "2015", "2016", and "2017 and later". The Vehicle Model Year selects the
appropriate engine fuel map in the model. The 2013 to 2016 model years all use one set of
heavy-duty vehicle's engine fuel maps which meets the 2014 through 2016 model year
engine standards, while a selection of 2017 and later uses another set of the engine fuel maps
which meet the 2017 MY engine standards.
• Regulatory Subcategory: The user must select one of the twelve designated regulatory
subcategories in the radio button menu. The selection directs the model to use the
appropriate predefined inputs and post processing parameters, as outlined in Sections 1.2 and
1.3 of this Guidance.
• Coefficient of Drag: The Cd value is input by the user based on the aerodynamic bins, as
discussed in the preamble, RIA, and regulations. This input is only required for combination
tractors. No input is required for vocational vehicles.
• Steer Tire Rolling Resistance: The coefficient of rolling resistance for the steer tires should
be input by the user in the units of kg/metric ton where the typical value is greater than 5.5
kg/metric ton. The units are not in kg/kg or Ib/lb (the other industry norm) where the values
are typically greater than 0.0055 kg/kg or Ib/lb.
• Drive Tire Rolling Resistance: The coefficient of rolling resistance for the drive tires (i.e. all
tires used in non-steer positions) should be input by the user in the unit of kg/metric ton
where the typical value is greater than 5.5 kg/metric ton. The units are not in kg/kg or Ib/lb
(the other industry norm) where the values are typically greater than 0.0055 kg/kg or Ib/lb.
• Vehicle Speed Limiter: If the vehicle contains a vehicle speed limiter, then input the speed
limit to the nearest single decimal place. Otherwise, use 65 mile/hour. The GEM will limit
the maximum speed of the vehicle to the value selected. If the user is opting to allow an
expiration of the VSL before the full useful life of the vehicle or include soft top features,
then calculate the Effective Speed Limit as described in 40 CFR 1037.640(d). This input is
only available for combination tractors. No input is allowed for vocational vehicles as
indicated by a "grayed-out" feature.
• Vehicle Weight Reduction: If a combination tractor contains lighter weight wheels or tires, or
other lighter components described in the preamble, RIA, and the regulations, then the user
would input the sum of the weight reductions prescribed by the weight bins. This input is
only available for combination tractors. No input is allowed for vocational vehicles as
indicated by a "grayed-out" feature.
• Extended Idle Reduction: If a sleeper cab combination tractor contains an extended idle
reduction technology and a 5 minute automatic engine shut-off, then the user would select
the 5 g/ton-mile reduction. If not, then "0" should be selected. If the user is opting to allow
an expiration of the automatic engine shut-off before the full life of the vehicle, then
calculate the Extended Idle Reduction Credit as described in 40 CFR 1037.660(c) This input
10
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is only available for sleeper cab combination tractors. No input is allowed for day cab
combination tractors and vocational vehicles as indicated by a "grayed-out" feature.
• A check button for "Plot Output" can be selected if the user chooses to output all three cycle
plots for vehicle speed traces as a function of time.
• Click RUN (Figure 1), the GEM will run.
After the simulation is completed, a window will open, prompting the user where the file
is to be saved. The output file will open once clicking OK to save the file. The following output
file will be created with the following naming structure:
GEM_Results_Date_Time.xlsx
The data are in the format of date-month-year, and Time is hour-minute-second-am/pm.
For example, the full name is
GEM_Results_01-Jun-2011_02-25-39-pm.xlsx
The output file has the same structure as the input file, which includes all data used in the
input file plus additional columns for output results. Those additional data are CO2 in g/ton-mile
and fuel consumption in gallon/1,000-ton-mile, which will be used for certification.
If the user wants to close the GEM, click fiH on the GUI window.
For single configuration run, input error checking features have been built in. If the user
forgets entering a value to a required field, a message window will open and the field that
requires entering the values will be in red, asking the user to enter a valid value.
For multiple configurations, use the following instruction
The user must set up the input file in Excel format before initiating "Multiple
Configuration" run. A template input file is provided during installation.
It should be pointed out that there are a few restrictions in using the EPA provided input
template Excel file (GEM_Input.xlsx ) during editing before running the multiple configuration.
• Only one regulatory subcategory and one vehicle model year can be selected for the batch
process.
• Do not change any format used in the template provided.
• It is the user's responsibility to enter the appropriate, valid numerical value for each
mandatory input, which should not be left blank.
• For all vocational vehicles selected in the "GEM_Input.xlsx", those input parameters for
"Cd (Bin)", "Idle Reduction", and "Weight Reduction" will take pre-defined default
values irrespective of the values entered by the user. Those default values can be seen
through "Single Configuration" run when the vocational trucks are selected in their
respective grayed-out areas.
11
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• For all day cab tractors including Class 7 and 8, the input parameter for "Idle Reduction"
will use the pre-defined values irrespective of the values entered by the user. The pre-
defined value can be seen through "Single Configuration" run when the vocational
vehicles are selected in their respective grayed-out area.
One example of the input file can be seen in Figure 2. Note that the values in this figure
and other similar figures in the following section are for illustrative purposes only. The template
provided by the agency is called "GEM_Input.xlsx". Following the steps below, one can create
the input file.
a
GEM_Input_XLS.xls [i
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C.E5
F
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EPA Defined
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GEM Input
Steer Tires
CRR
f.
6
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rive Tires
CRR
E
6
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Idle Reduction
5
5
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Weight Reduction V5L
-:: :
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Figure 2 Example of Input file for Multiple Configuration Run
Enter Manufacturer and Vehicle Family Names.
Select the Model Year in row 2 and column B through a pull-down menu.
Select one of twelve regulatory subcategories in row 4 and column B through a pull-
down menu.
Enter an input file name.
Fill out the input parameters required by the GEM, such as Cd(Bin), CRR, Idle
Reduction, Weight Reduction, and VSL. The instructions on how to enter these values is
the same as those illustrated in the Single Configuration set up in the previous section.
It is the user's responsibility to input "configuration", "Brand/Model Name", "Bin", and
"CdA (Test)" for each case. These values are not used by the GEM during simulation,
and they are only used for reference during certification.
Save the file.
Go back to GEM GUI (Figure 1).
12
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• Click "Multiple Configurations" in Figure 1. A window that says "Select the xlsx input
file for Multiple Configurations" will display as follows (Figure 3).
Select the XLSX input file for Multiple Configurations:
Look in: |QGEM
My Recent
Documents
Desktop
My Documents
0
My Computer
]GEM_Input.xlsx
My Network File name: ||
Places
Files of type: | [*.xlsn]
Figure 3 GEM Input File Selection
• Select the folder where the input file is located. Click the file, and then click open.
• The following window will appear. Click OK. The input file setup is completed.
Greenhouse Gas Emissions Model (GEM)
VEHICLE INPUT PARAMETERS
Vehicle parameters for Multiple Configurations were successfully loaded.
Model Year:
2017 and later
Regulatory Subcategory:
Class 8 Combination - Day Cab - Low Roof
Number of configurations:
6
Click RUN BATCH to start the simulations.
The RUN button in Figure 1 will be changed to RUN BATCH. Click RUN BATCH.
Multiple Configurations will then run. A progress bar window will be displayed.
Once simulation is completed, a window will open, prompting the user where the file is
to be saved. Click OK to save the output file in your desired folder.
After a few seconds, a window indicating completion of simulation will appear as
follows. Click Ok. In the mean time, the Excel output file will open.
13
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) Greenhouse Gas Emissions Mod... [- | |X|
SIMULATIONS COMPLETED
Model Year:
2017 and later
Regulatory Subcategory:
Class 8 Combination - Day Cab - Low Roof
A results file is now available for review at:
C:\GEM
Results file name:
G E M_R esultsj 3 J ul-2011 _01 -21 -13-pm. xlsx
To run more simulations click RESET.
The same output file structure as the single configuration run is used, but with more
results depending on the total number of cases running.
If the user wants to close the GEM program, click IS on the GUI window.
14
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