# Greenhouse Gas Emissions Model (GEM) User Guide

# Greenhouse Gas Emissions Model (GEM) User Guide

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

EPA-420-B-11-019 August 2011The Greenhouse gas Emissions Model was created by EPA as a means for determining
compliance with EPA’s GHG emissions and NHTSA’s fuel consumption v ehicle 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 r egulation

. 1 It is a free, desktop computer application.

The GEM is designed to operate on a single computer. The downloadabl e 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 Octo ber 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 benchmark ing 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

.

Chap ter 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

.

1

### 1.2. List of Predefined Input Parameters for Class 7/8 Combination

Tractor Models

Alt hough 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 decide d 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 spec ific values of these parameters fo r Class 8 combination tractors, and Table 2 lists the

similar values for Class 7 combination tractors.

2Table 1 : Class 8 Combination Tr actor Modeling Parameters

MODEL TYPE CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8 CLASS 8

Regulatory Subcategory Sleeper Cab High Roof Sleeper Cab

Mid Roof Sleeper Cab

Low Roof Day Cab High Roof Day Cab

Mid Roof Day Cab

Low Roof

Fuel Map 15L

- 455 HP

Gearbox 10

- speed

Manual 10

- speed

Manual 10

- speed

Manual 10

- speed

Manual 10

- speed

Manual 10

- speed

Manual

Gearbox Ratio 14.8, 10.95, 8.09, 5.97, 4.46, 3.32, 2.45, 1.81, 1.35, 1

Gearbox Efficiency 0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98, 0.98

Engine Inertia (kg

m 2 ) 4.17 4.17 4.17 4.17 4.17 4.17

Transmission Inertia (kg

- m 2 ) 5 5 5 5 5 5

All Axle Inertia (kg

- m 2 ) 360 360 360 360 360 360

Loaded Tire Radius (m) 0.489 0.489 0.489 0.489 0.489 0.489

Tractor Tare Weight (lbs) 19,000 18,750 18,500 17,500 17,100 17,000 Trailer Weight (lbs) 13,500 10,000 10,500 13,500 10,000 10,500

Payload (lbs) 38,000 38,000 38,000 38,000 38,000 38,000

Total weight (lbs) 70500 66750 67000 69000 65100 65500

Total weight (kg) 31978 30277 30391 31298 29529 29710

Frontal Area (m 2 ) 10.4 7.7 6.9 10.4 7.7 6.9

Coefficient of Aerodynamic Drag OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input

Axle Base 5 5 5 5 5 5

Electrical Accessory Power (W) 350 350 350 350 350 350

Mechanical Accessory Power (W) 1, 000 1, 000 1, 000 1, 000 1, 000 1, 000

Final Drive Ratio 2.64 2.64 2.64 2.64 2.64 2.64

Tire C RR (kg/metric ton) = 0.425 × Trailer C RR + 0.425 × Drive C RR + 0.15 × Steer C RR

Trailer Tire C RR (kg/metric ton) 6 6 6 6 6 6

Steer Tire C RR (kg/metric ton) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input

Drive Tire C RR (kg/metric ton) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input

Vehicle Speed Limiter (mph) OEM Input OEM Input OEM Input OEM Input OEM Input OEM Input

3Table 2: Class 7 Combination Tr actor Modeling Parameters

MODEL TYPE CLASS 7 CLASS 7 CLASS 7

Regulatory Subcategory Day Cab High

Roof Day Cab Mid

Roof Day Cab

Low Roof

Fuel Map 11L

- 350 HP

Gearbox 10

- speed

Manual 10

- speed

Manual 10

- speed

Manual

Gearbox Ratio 11.06, 8.19, 6.05, 4.46, 3.34, 2.48, 1.83, 1.36, 1,

0.75

Gearbox Efficiency 0.96, 0.96, 0.96, 0.96, 0.98, 0.98, 0.98, 0.98, 0.98,

0.98

Engine Inertia (kg

- m 2 ) 3.36 3.36 3.36

Transmission Inertia (kg

- m 2 ) 5 5 5

All Axle Inertia (kg

- m 2 ) 233.4 233.4 233.4

Loaded Tire Radius (m) 0.489 0.489 0.489

Tractor Tare Weight (lbs) 11,500 11,100 11,000

Trailer Weight (lbs) 13,500 10,000 10,500

Payload (lbs) 25,000 25,000 25,000

Total weight (lbs) 50000 46100 46500

Total weight (kg) 22680 20910 21092

Frontal Area (m 2 ) 10.4 7.7 6.9

Coefficient of Aerodynamic Drag OEM Input OEM Input OEM Input

Axle Base 4 4 4

Electrical Accessory Power (W) 350 350 350

Mechanical Accessory Power (W) 1000 1000 1000

Final Drive Ratio 3.73 3.73 3.73

Tire C RR (kg/metric ton) = 0.425 × Trailer CRR + 0.425 × Drive CRR +

0.15 × Steer CRR

Trailer Tire C RR (kg/metric ton) 6 6 6

Steer Tire C RR (kg/metric ton) OEM Input OEM Input OEM Input

Drive Tire C RR (kg/metric ton) OEM Input OEM Input OEM Input

Vehicle Speed Limiter (mph) OEM Input OEM Input OEM Input

4

### 1.3. List of Predefined Input Para meters for C lass 2b

- 8 Vocational

Vehicle Models

Likewise, EPA and NHTSA standardize d a set of parameters for the three Class 2b -8

vocational vehicle types, which the agencies refer to as Vocational Light Heavy

- Duty (VLH D ),

Vocational Medium Heavy

- Duty (VMH D), and Vocational Heavy Heavy

- Duty (VHH D ). 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.

5Table 3 : Vocational Vehicle Modeling Parameters

Model Type Heavy Heavy

- Duty Medium Heavy

- Duty Light Heavy

- Duty

Regulatory Subcategory Vocation al Truck (Class 8) Vocation al Truck (Class 6

- 7) Vocation al Truck (Class 2b

- 5)

Fuel Map 15L

- 455 HP 7L

- 270 HP 7L

- 200 HP

Gearbox 10

- speed Manual 6- speed Manual 6- speed Manual

Gearbox Ratio 14.8, 10.95, 8.09, 5.97, 4.46, 3.32, 2.45, 1.81, 1.35, 1 9.01, 5.27, 3.22, 2.04,
1.36, 1 9.01, 5.27, 3.22, 2.04,
1.36, 1

Gearbox Efficiency 0.96

, 0.96

, 0.96

, 0.96

, 0.98

, 0.98

, 0.98

, 0.98

, 0.98

, 0.98 0.92

, 0.92

, 0.93

, 0.95

, 0.95

, 0.95 0.92

, 0.92

, 0.93

, 0.95

, 0.95

, 0.95

Engine Inertia (kg -m 2 ) 4.17 2.79 2.79

Transmission Inertia (kg

- m 2 ) 5 0. 5 0. 5

All Axle Inertia (kg -m 2 ) 200 60 60

Loaded Tire Radius (m) 0.489 0.389 0.378

Payload (lbs) 15

, 000 11

, 200 5, 700

Total weight (lbs) 42

, 000 25

, 150 16

, 000

Total weight (kg) 19

, 051 11

, 408 7, 257

Frontal Area (m 2 ) 9.8 9.0 9.0

Coefficient of Aerodynamic Drag 0.7 0.6 0.6

Axle Base 3 2 2

Electrical Accessory Power (W) 350 300 300

Mechanical Accessory Power (W) 1, 000 1, 000 1, 000

Final Drive Ratio 2.64 3.36 2.85

Tire C RR (kg/ton) = 0.5 × Drive CRR + 0.5 × Steer CRR

Trailer Tire C RR (kg/metric ton) Not applicable Not applicable Not applicable

Steer Tire C RR (kg/metric ton) OEM Input OEM Input OEM Input

Drive Tire C RR (kg/metric ton) 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.

6The GEM produces a cycle- weighted gram s CO 2 /ton

- mile and gallon s /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

C ATEGORY CLASS 8
SLEEPER CAB

TRACTORS CLASS 7/ 8
DAY CAB
TRACTORS CLASS 2 b- 8
VOCATIONAL

VEHICLES

ARB Transient 5% 19% 42% 55 mph Cruise 9% 17% 21% 65 mph Cruise 86% 64% 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

vocation al vehicles.

GEM calculates the gallons/1 ,000 ton-mile weighted result by conve rting the weighted grams CO 2 /ton

- mile result. The gram CO 2 /ton

- mile result is multiplied by 1 ,000 and divided by 10,180 grams CO 2 per gallon of diesel fuel.



### 2. GEM Installation Instructions

C ertain minimum computer requirements must be met b efore installation

. The system require d to run GEM needs a minim um RAM of 1 GB (4 GB is highly recommended), and approximately 9 00 MB of disk storage. Window XP or Window 7 operating system is required. The validation has not been tested in other computer ope rating 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 display ed. A default checkbox for Create a Desktop Icon is already selected. Click Next

.

• A Window “Ready to Install” will be display ed . Click Install

.

7In 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, fol low 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.

• A n InstallShield Wizard window will be display ed , 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 display ed . Click Next if a default folder is

selected.

c. Click Install to complete MATLAB® Comp i ler Runtime. It will take a while to complete the installation.

d. Click Finish, and entire installation is completed.

The GEM folder contains these visible f iles :

• GEM.exe shortcut

• GEM_input.xlsx

It should be pointed out that the GEM is compatible with MATLAB® Runtime 7.15.

Different version s of MATLAB® Runtime may result in compatibility issue s.

To uninstall, the user may manually delete the GEM by follow ing the steps below:

• Delete the GEM folder entirely

• Delete the short cut on your desktop screen.

8

### 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.

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 configuration s 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

9• Vehicle Model Year

The second set of parameters affects how GEM calculates the final result s . 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”,

“2 014”, “2015”, “2016”, and “2017 and later” . The Vehicle Model Year selects the appropriate engine fuel map in the mod el. 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 en gine standards.

• Regulatory Subcategory: The user must select one of the twelve designated regulatory subcategories in the radio button menu . The selection direct s 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 lb/lb (the other industry norm) where the values are typically greater than 0.0055 kg/kg or lb/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 lb/lb (the other industry norm) where the values are typically greater than 0.0055 kg/kg or lb/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 a n 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 combin ation 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

10is only available for sleeper cab combination tractors. No input is allowed for day cab

combination tractors and vocational vehicles as indicated by a “gra y ed

- 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, promptin g 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 t he input file, which includes all data used in the input fi le plus additional columns for output results. Those additional data are CO 2 in g/ton

- mile and fuel consumption in gallon/1,000-ton- mile, which will be used for certification.

If the user wants t o close the GEM, click on the GUI window.

For single configuration run, input error checking features have been built in. If the user forgets enterin g a value to a required field, a message window will open and the field that requires entering the va lues 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 veh icle 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• For all d ay 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 va lues in this figure and other similar figures in the following section are for illustrative purposes only. T he template provided by the agency is called “GEM_Input.xlsx ”. Following the steps below, one can create the input file.

Figure 2 Example of Input file for Multiple Configuration Run

• Enter Manufacturer and Vehicle Family N ames

.

• S elect the Model Year in row 2 and co lumn B through a pull-down menu.

• Select one of twelve regulatory subcategories in row 4 and co lumn B through a pulldown menu.

• En ter an input file name.

• Fill out th e 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. The se values are not us ed by the GEM during simulation

, and they are only used for reference during certificatio n.

• Save the file

.

• Go back to GEM GUI (Figure 1)

.

12• C lick “Multiple Configuration s” in Figure 1 . A window that says “Select the xlsx input file for Multiple Configurations ” will display as follows (Figure 3 ).

Figure 3 GEM Input File S election

• Select the folder where t he input file is located. Click the file, and then click open.

• The following window will appear

. Click OK . The input file setup is completed

.

• 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 display ed

.

• 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 secon ds, a window indicating completion of simulation will appear as

follows. Click Ok

. In the mean time, the Excel output file will open.

13The 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 on the GUI window.

14