Peer Review of the Greenhouse Gas 

Emissions Model (GEM) and EPA’s 

Response to Comments 
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.

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

Sections prepared for EPA by

Research Triangle Institute International

 EPA Contract No. EP-C-08-008

 Work Assignment No. 3-02

Peer Review of the Greenhouse Gas 

Emissions Model (GEM) and EPA’s 

Response to Comments 

EPA-420-R-11-007

August 2011
Table of Contents

I. RTI’s Technical Report of the Peer Review of EPA’s Greenhouse gas Emissions Model 

(GEM) 

1. Background 

2. Description of Peer Review Process

3. Summary of Peer Review Comments

4. Background Materials 

II. Appendices 

1. Appendix A: Charge Questions 

2. Reviewer Resumes

3. Cover Letters

4. Individual Peer Reviews

i. Aris Babajimopoulos 

ii. Daniel L. Flowers

iii. Shawn Midlam-Mohler 

iv. Elliott Ortiz-Soto 

III. EPA’s Response to Peer Reviewer Comments

 

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RTI International is a trade name of Research Triangle Institute

TO: Kent Helmer, Chien Sze, U.S. Environmental Protection Agency, Office of 

Transportation and Air Quality (OTAQ) 

FROM: Tony Lentz, Paramita Sinha, and Karen Schaffner, RTI 

DATE: January 28, 2011

SUBJECT: Peer Review of EPA’s Heavy-Duty Greenhouse Gas Emission Model (GEM)

1. Background 

EPA and the National Highway Traffic Safety Administration are considering a first-ever 

program to reduce GHG emissions and improve fuel efficiency in the heavy-duty highway 

vehicle sector. This broad vehicle sector, ranging from large pickups to sleeper-cab tractors 

(Classes 2b through 8), represents the second largest contributor to transportation GHG 

emissions after light-duty passenger cars and trucks. The agencies are proposing to evaluate both 

fuel consumption and CO2 emissions from heavy-duty highway vehicles through a whole-vehicle 

operation simulation model. 

EPA has created a model called “Greenhouse Gas Emissions Model (GEM),” which is 

specifically tailored to predict truck GHG emissions. As the model is designed for the express 

purpose of vehicle compliance demonstration, it is less configurable than similar commercial 

products and its only outputs are GHG emissions and fuel consumption. This approach gives a 

simple and compact tool for vehicle compliance without the overhead and costs of a more 

sophisticated model. 

To assure that the regulated community gets the highest quality predictive tools that EPA can 

provide and to assure its stakeholders that the proposed model structure (and overall 

development process) will result in a tool that is simple, accurate and well suited for 

certification, EPA sought an independent peer review of its GEM model. 

2. Description of Review Process 

EPA’s Office of Transportation and Air Quality (OTAQ) contacted RTI International in October 

of 2010 to facilitate the peer review of its Heavy-Duty Greenhouse Gas Emission Model (GEM). 

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RTI began the review process on October 19, 2010 and concluded January 21, 2011, a period of 

slightly more than 3 months. 

EPA provided a short list of subject matter experts from academia and the public sector 

(Appendix B of the work assignment 3-02) to RTI, and this served as a “starting point” from 

which we assembled the list of subject matter experts. RTI selected three1 independent (as 

defined in Sections 1.2.6 and 1.2.7 of EPA’s Peer Review Handbook) subject matter experts to 

conduct the requested reviews. Subject matter experts familiar with MATLAB, Simulink, 

Stateflow and Visual Basics software, as well as having expertise in vehicle operations and 

analysis, linkages between mobile source emission modeling and transportation modeling and 

planning, or application of current mobile source emissions models for analysis for regulatory 

purposes were selected. 

To ensure that the review process was conducted in a timely manner, RTI contacted potential 

reviewers within 10 days of submitting the work plan and determined that each reviewer would 

be able to perform work during the period of performance. To make the review process as 

credible as possible, RTI did not consult the Agency in the final determination of reviewers. RTI 

obtained the resumes of the selected reviewers, and these are included in Appendix B. 

EPA provided RTI with the necessary model review material via email on November 12, 20102. 

This was forwarded to the reviewers; and in addition to the review material, RTI forwarded a set 

of charge questions prepared by the EPA (these questions were later revised). 

On November 17, 2010, RTI organized and held a teleconference between EPA, the three 

reviewers, and RTI to provide an opportunity to the panel to discuss any questions or concerns 

they may have regarding the material provided and expected deliverables. The call concluded 

when all participants’ questions and concerns were addressed and a mutually agreed upon 

deliverable date was set. Based on the discussion during the call, EPA sent RTI an updated set of 

charge questions on November 23, 2010 and this was forwarded to the reviewers on November 

29, 2010. These charge questions are included in Appendix A of this memorandum. 

Following the first bi-monthly progress report call between RTI and EPA (November 16, 2010) 

and subsequent email correspondence (November 18, 2010), it was also agreed upon that a 

fourth subject matter expert should be identified and selected as a reviewer. EPA sent RTI an 

expanded short list from which the fourth reviewer was identified and contacted by RTI. The 

 

1 Initially, RTI identified 3 subject matter experts to serve as reviewers. Following the first bi-monthly progress report call 

between RTI and EPA, it was agreed upon that a fourth subject matter expert should be identified and selected as a reviewer. 

2 EPA distributed all the necessary review material to RTI in an email, which contained hyperlinks (“weblinks”) to the review 

material along with weblinks to both, the executable and MATLAB/Simulink, versions of the GEM and an accompanying 

model guide. 

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necessary material and charge questions were forwarded to him upon his acceptance to 

participate in this peer review. 

Completed reviews from the panel were sent to EPA on Wednesday, December 22, 2010. These 

reviews included the response to the charge questions and any additional comments the reviewer 

may have had (e.g., margin notes on review materials). RTI also obtained a cover letter from 

each reviewer stating the reviewer’s name; the name and address of their organization if 

applicable; and a statement of any real or perceived conflict(s) of interest. The cover letters and 

reviews are included in Appendices C and D, respectively. EPA’s comments in response to the 

reviewers’ assessments are included in Appendix E. 

3. Summary of Review Comments 

Aristotelis Babajimopoulos (University of Michigan, College of Engineering), Dan Flowers 

(Lawrence Livermore National Laboratory, Combustion and Alternative Fuels, E Program), 

Shawn Midlam-Mohler (Ohio State University, Department of Mechanical Engineering), and 

Elliott Ortiz-Soto (University of Michigan, College of Engineering) reviewed EPA’s GEM. This 

section provides a summary of the comments received from them. 

3.1 EPA’S OVERALL APPROACH TO THE STATED PURPOSE OF 

THE MODEL (MEET AGENCIES’ COMPLIANCE 

REQUIREMENTS) AND WHETHER THE PARTICULAR 

ATTRIBUTES FOUND IN RESULTING MODEL EMBODIES 

THAT PURPOSE. 

All four reviews addressed the model’s ability to meet the agencies’ compliance requirements. 

One reviewer explicitly detailed whether the model’s particular attributes embody the stated 

purpose of the model. In general, the reviewers reported that the model does an acceptable job 

testing different vehicle configurations from different vehicle manufacturers for compliance 

purposes. 

Dr. Flowers comments, “Overall, the concept of using a generic vehicle model has merit to limit 

the need to test the myriad possible vehicle configurations. The use of a generic powertrain 

(engine and transmission) is problematic because a well-integrated powertrain can significantly 

improve vehicle performance.” (Additional discussion of powertrain issues are provided in 

subsection 3.2.1 below.) 

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Dr. Babajimopoulos remarks, “GEM is a very detailed vehicle simulation that could capture with 

reasonable accuracy the impact of changes in aerodynamic drag coefficient, tire rolling 

resistance and tire weight reduction on overall vehicle fuel economy and CO2 emissions. The 

model itself is almost too detailed for this purpose, but this should not be a problem, provided 

that not all details of the model are discussed in such great length with the users. However... it is 

hard to envision a compliance tool that does not account for fuel economy improvements coming 

from the development of advanced combustion technologies by the engine manufacturers.” 

Dr. Midlam-Mohler addresses the five modeling attributes3 needed for the model to serve as a 

primary compliance tool. Regarding each attribute, he comments: 

1. “The model fidelity of the type proposed should be capable of achieving the desired 

objectives. The model reviewed, however, has a number of issues which cast doubt 

upon the specific implementation of the model. Specifically, a number of issues were 

found in the electrical subsystem as well as the engine subsystem.” 

2. “Providing source code as a Simulink diagram is necessary for this objective but not 

sufficient. Additional documentation on the equations and references behind the 

Simulink code should be developed and released to the public.” 

3. “The compiled version of the code is free and easy to use. The Simulink version 

requires a Matlab license which is not free but fairly common in industry.” 

4. “The current structure satisfied this objective.” 

5. “By releasing an official and unalterable executable version of the model this 

objective is met.” 

While Mr. Ortiz-Soto provides specific comments on multiple aspects of the model (including 

comments on inputs, outputs, model and submodels, see sections below for details), he also notes 

that “In general, the rest of the model looks good.” Mr. Ortiz-Soto reports, “Overall, the model is 

in great shape and should be a strong starting point for a dedicated simulation oriented to 

compliance purposes.” 

 

3 The five attributes listed in EPA’s updated charge questions are: 1) capable of modeling a wide array of medium- and heavyduty vehicles over different drive cycles; 2) contains open source code, providing transparency in the model’s operation; 3) 

freely available and easy to use by any user with minimal or no prior experience; 4) contains both optional and preset 

elements; and 5) managed by the Agencies for compliance purposes. 

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3.2 THE APPROPRIATENESS AND COMPLETENESS OF THE 

CONTENTS OF THE OVERALL MODEL STRUCTURE AND ITS 

INDIVIDUAL SYSTEMS, AND THE COMPONENT MODELS, IF 

APPLICABLE (i.e., USING THE MATLAB/SIMULINK VERSION) 

This section is broken down into 4 subsections with each subsection containing one or more 

comments from the reviews. In general, each reviewer commented on one or more of the 

following subsections. 

3.2.1 The Elements of Each System to Describe Different Vehicle 

Categories 

The GEM model has 6 systems (Driver, Ambient, Electric, Engine, Transmission, and Vehicle) 

used to describe different vehicle categories. There was little, to no, discussion among the 

reviewers concerning 2 systems, the ambient and driver systems. In the remaining four systems 

(electric, engine, transmission, and vehicle), the reviewers found errors and identified issues that 

raised questions about the overall ability of the systems to accurately depict different vehicle 

categories. 

Dr. Babajimopoulos details issues with the component models of the engine, transmission and 

vehicle systems. He comments, “....engine fuel maps and drivetrain parameters are hardwired in 

the model and the user has no option of changing them. However, it seems counterintuitive that a 

tool for determining compliance with emissions standards would ignore efforts on the part of the 

manufacturers to make improvements on the engine itself. Moreover, in order to take full 

advantage of any improvements in combustion and engine-out emissions, the vehicle 

transmission needs to be optimized for a particular vehicle/engine/driving schedule combination, 

so that the engine can operate near its optimum efficiency points at all times.” 

Dr. Flowers noted that in the GEM model, “the engine and transmission is not optimized to the 

vehicle” and “The use of a generic powertrain (engine and transmission) is problematic because 

a well-integrated powertrain can significantly improve vehicle performance.” “In practice, the 

engine and transmission can be appropriately sized to best take advantage of the reduced overall 

vehicle load. By requiring only one engine and transmission be used, drag reduction efforts could 

be penalized.” 

Mr. Soto also commented on the engine fuel maps. He stated that “One of the most important 

input dat[um] for a fuel economy drive-cycle simulation is the engine mechanical load and fuel 

consumption maps. The mechanical load maps are usually simple because only the WOT (or 

Diesel equivalent) values are required, but obtaining full range fuel consumption values is much 

more difficult. Several engine maps appear to be available for each vehicle class, but making 

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these completely standard with a prescribed displacement volume and operating range might be a 

limiting factor for some manufacturers. A more flexible approach would be to have normalized 

load and fuel consumption maps, given in BMEP and BSFC values. The current maps can be 

easily converted into BMEP and BSFC with the data available. The user could then provide the 

engine displacement and possibly another key parameter such as rated torque or power and the 

engine speed, and an algorithm could automatically manipulate the normalized maps to obtain 

more representative absolute values for the engine in question. Even though this compliance tool 

assumes that the engines have already been certified, the fuel economy and CO2 values that the 

simulation predicts are directly related to the maps given, and manufacturers might want to 

ensure the engines in their vehicles are properly accounted for.” 

Dr. Flowers conducted a comparison of the GEM model output values and direct calculated 

values for the same parameters for a particular vehicle configuration and drive cycles. He 

determined that the direct calculated torque is 3 percent lower than the GEM-modeled torque, 

and noted a possible explanation may be due to the speed variation during the constant desired 

speed portion of the drive cycle. Referring to the chassis component model contained in the 

vehicle system, Dr. Flowers reports that the powertrain inertial mass should be zero during a 

certain drive cycle. He states, “The GEM model uses an “effective mass” formulation that 

includes powertrain inertial effects. In the GEM code, the vehicle static mass 

(vehicle.chsmass_static) is added to the representative powertrain inertial mass (tire_mass_out). 

For steady speed vehicle operation the powertrain inertial mass should be zero.” Dr. Flowers 

compared GEM model output values and calculated values for fuel usage, fuel consumption, and 

GHG emissions (using GEM output values for torque and speed), and he noted that errors were 

small (less than 0.3 percent). 

Dr. Midlam-Mohler summarizes his comments of the model systems and their underlying 

components models by stating, “The overall approach of using a relatively simple model 

structure based in Matlab-Simulink is sound provided that models are calibrated and validated to 

a sufficient level.” 

Dr. Midlam-Mohler stated that some issues in the Engine subsystem need to be addressed and he 

stated “The method of handling negative brake torques in the model does not seem to be 

appropriate.” Dr. Midlam-Mohler notes that “A map-based engine model should be sufficient to 

achieve the desired objectives. The engine model implemented in the current version of the 

software does not appear to be as well implemented as it could be. Given the importance of this 

in the overall objectives of the simulator this needs to be addressed. Using fuel maps which have 

torque indices ranging from a negative brake torque to the maximum rated torque would alleviate 

much of the uncertainty in the model. Driver accelerator requests should then be linearly scaled 

from minimum value to the maximum value on this map with the exception of idle conditions in 

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which alternative measure must be taken. This approach also automatically takes into account 

deceleration fuel cut-off as well.” 

Dr. Midlam-Mohler notes some recommendations for the Vehicle subsystem, stating “The most 

serious item is considered to be the fact that the “Vehicle Weight Reduction” parameter is 

specifically cited as being able to model light-weight wheels. The existing model structure would 

not accurately do this as it does not take into account the inertial aspect of the wheels which 

would have a greater impact on the vehicle.” 

Dr. Midlam-Mohler noted that in the Driver subsystem, the PID values are fixed in the GEM 

model but that it may be worth adding this as an advance feature or using a more sophisticated 

control concept, such as augment the current PID control with a feedforward component. He did 

note that large errors in velocity tracking were not observed in the model. 

Mr. Ortiz-Soto notes that “Control for most of the vehicle components seems to be achieved by 

fairly standard PID controllers. Usually the gains for these controllers are tuned to a specific 

plant, but in this case they remain fixed for all the vehicle configurations. Were these gains tuned 

for all the plants individually and then somehow averaged to account for all of them, or were 

they computed for a single vehicle? Although for the test cases do not show any major problems 

with following the prescribed velocity profile, simulation of some vehicles or with a different set 

of parameters could possibly suffer if the controller gains are not appropriate. For the driver, for 

example, more elaborate, robust and reusable driver models exist, and it might [be] useful to 

investigate the possibility of incorporating one of these in order to avoid possible issues with the 

simulations.” 

3.2.2 The Performance of Each Component Model, Including the 

Reviewer’s Assessment of the Underlying Equations and/or 

Physical Principles Coded into That Component 

Four of the GEM model systems (electric, engine, transmission and vehicle) are made up of 

underlying component models. The reviewers assessed the performance of those component 

models, including the equations and physical principles of the component model, and reported 

their findings. Each reviewer noted that one or more of the component models performed 

inadequately and recommended these component model inadequacies be addressed to improve 

the robustness of the compliance tool. Additionally, a couple of the reviewers identified nontrivial errors in the equations of some of the component models. For example, one reviewer 

states, “A number of errors were found in models within GEM. None of these errors are expected 

to contribute to larger errors to the output results but should be corrected nonetheless.” 

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Three of the reviewers commented on flaws in the electric system. 

Dr. Babajimopoulos reports, “The model of the electric subsystem is particularly detailed and 

convoluted. GEM includes submodels for the starter, alternator, battery and electric accessories. 

This complexity seems unnecessary for the stated purposes of GEM. Careful examination of the 

results reveals that the starter has almost zero effect on overall fuel economy and CO2 emissions. 

Moreover, the overall effect of the electrical system on fuel economy and CO2 emissions is 

almost negligible.” 

Dr. Midlam-Mohler comments, “Very significant issues were found in the electric subsystem 

which require attention. In particular, the battery model appears to [have] an error which causes 

battery voltage to decrease with battery state of charge which is exactly opposite of the desired 

behavior. Furthermore, it appears that the sign convention used for the starter, accessories, 

alternator have the wrong sense. The alternator generates negative current which decreases SOC. 

The other two currents, which are current sinks, actually increase the SOC of the battery. Even 

with the above issues aside, the alternator model appears to not consider the mechanical to 

electrical efficiency of the device and the control is naïve of actual alternator capabilities and 

control.” 

Mr. Ortiz-Soto comments, “The electric components and EES seem to be fixed for all the 

vehicles in the simulation, but in reality the electrical system is probably designed for a given 

application to account for the particular load requirements. It is understandable that due to the 

complexity of acquiring parameters such as these, the system model is standardized, but it could 

also result in simulation inaccuracies. It might be more appropriate to provide at least some basic 

scaling capability for the overall electrical system so that with one or two additional inputs, the 

electrical components and EES are scaled to match the actual setup more closely.” “A similar 

observation can be made regarding the starter and alternator models.” While these are not critical 

components, a scaling factor should be applied. 

Dr. Babajimopoulos found that the density of air value in the ambient system “seems to be rather 

low” and could impact model results in a non-trivial fashion depending on the cycle. 

3.2.3 The Input and Output Structures and How They Interface with 

the Model to Obtain the Expected Result; i.e., Fuel Consumption 

and CO2 over the Given Driving Cycles 

Overall, the reviewers commented the input and output structures interfaced well with the model 

to obtain the expected results. Several reviewers offered minor suggestions that could help the 

end user when using the model. These suggestions are found in subsection 3.6.1 of this report. 

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3.2.4 The Default Values Used for the Input Files, as Shown in the GEM 

User Guide 

All of the reviewers commented that the default values for the input files should be allowed to 

change to reflect manufacturer improvements. The reviewers’ comments reflect a concern that 

the model does not allow for sufficient flexibility in certain respects. For example, Dr. Flowers 

expresses his concern about standardization when he remarks, “My main concern with the 

overall approach is the standardization of the vehicle and powertrain combination. This seems to 

have the potential to devalue efforts towards vehicle and powertrain integration and optimization 

towards GHG reduction.” 

Dr. Midlam-Mohler recommends that EPA allow some of the model parameters to change with 

respect to vehicle class. He suggests, “A number of parameters were noted which should change 

with respect to the vehicle class. The reviewer is certain that there are others that were not noted 

in this review. It is recommended that the EPA investigate this and take an appropriate action. In 

many cases, these components will not have a serious impact on the overall performance of the 

vehicle. By way of example, many of the inertias simulated in the model will not have a large 

impact on the results in contrast to the large inertia of the vehicle. If this is the case, then these 

inertias could be discarded from the model with little impact on performance. If the detailed 

inertias remain in the model, then they should accurately reflect the vehicle class.” 

Given the overall importance of fuel consumption and CO2 emissions to the model’s objective, 

three reviewers specifically address the engine maps default values. 

Dr. Babajimopoulos commented, “If the assumption is that engines will be relatively similar for 

the same class vehicles coming from different manufacturers, then it is safe to assume that GEM 

would be an appropriate tool for determining compliance with fuel economy and CO2 emissions 

standards based on vehicle design changes alone. Nevertheless, it would be proper to allow for 

the provision to change the engine fuel map and transmission characteristics used by GEM.” 

Mr. Ortiz-Soto and Dr. Midlam-Mohler provide comments on the default values for the engine 

fuel maps in subsection of 3.2.1 of this report. 

3.3 USING THE STANDARD OF GOOD ENGINEERING 

JUDGMENT, THE PROGRAM EXECUTION IS OPTIMIZED BY 

THE CHOSEN METHODOLOGIES 

One reviewer commented that he interprets this statement to be referring to “the performance of 

the code as an effective tool for this application [regulatory application].” The reviewer states the 

code seems to be developed in such a way that it provides detail on both the vehicle and 

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powertrain dynamics. Because the model is complex and is a “highly interconnected system,” he 

expresses concern about the model documentation and believes more detail should be provided 

about the physical models implemented. He feels that transparency in the details of the model is 

important for regulatory application and the model may suffer without sufficient detailing of the 

underlying physics and engineering assumptions. 

3.4 CLARITY, COMPLETENESS AND ACCURACY OF THE 

OUTPUT/RESULTS (CO2 EMISSIONS OR FUEL EFFICIENCY 

OUTPUT FILE) 

Two reviewers stated the model output was clear and one commented that it was complete. One 

reviewer added, “The four tab format with the first tab being summary data and others being 

cycle data was sufficient.” A second review concluded, “The model reports the individual drivecycle results and weighted average results, which is what is most important to the end user.” The 

reviewer added, “All the inputs needed to reproduce the results are reported.” 

Two reviewers express concern about the clarity of the results with respect to the output file 

naming scheme. 

Mr. Ortiz-Soto comments, “...naming the files based on date and time is not very useful or 

descriptive. When multiple simulations are performed, it becomes difficult to determine what file 

you should be looking into, unless you actually open it. The file names should include at least 

some sort of indication of what the simulation configuration was. The second problem I found 

was the lack of flexibility to specify where these output files are saved. There should be an 

option allowing the user to browse and select the main directory where these files are to be 

saved. As a final comment on this, there is really no reason for each of these files to be saved to a 

different folder if there is just a single output file. This simply adds an unnecessary layer to the 

file structure.” 

Dr. Babajimopoulos raises a similar concern when he remarks, “It would be good if the message 

indicating where the results will be stored also include the drive (C:) in the path (e.g., 

‘C:\GEM_Results\December_14_2010-0135PM instead of \GEM_Results\December_14_2010-

0135PM).’” 

Regarding the accuracy of the output/results, Dr. Flowers indicates, “accuracy of the results is 

difficult to assess, since that requires specific comparison to experimental data to evaluate the 

performance of the model. Based on my testing efforts and experience, the results seem of 

reasonable magnitude for these kinds of vehicles.” Dr. Flowers concludes: 

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 “The model is quite detailed with regard to powertrain and vehicle dynamics. There is 

a danger here that imbedded assumptions can affect results in unexpected and 

undesirable ways. The example of the 3% difference in torque for analytical versus 

GEM simulation calculated torque for steady state operation may be indicative of 

these kinds of issues.” 

 “Detailed description of the physics and assumptions imbedded in the models and 

submodels should be documented and made available to users.” 

 “It may be worth considering if the model could be streamlined to provide greater 

clarity and transparency while still providing a tool for quantitatively estimating fuel 

consumption and GHG emissions.” 

3.5 ANY RECOMMENDATIONS FOR SPECIFIC IMPROVEMENTS 

TO THE FUNCTIONING OR THE QUALITY OF THE OUTPUTS 

OF THE MODEL 

The reviewers made several recommendations for improving the functioning and quality of the 

outputs. Two reviewer recommendations have been detailed in section 3.4 regarding the output 

file naming scheme. Additional reviewer recommendations are detailed below: 

6. One reviewer recommends including additional results in the output. He believes, “It 

would be informative to have the fraction of each drive-cycle used in the average 

reported somewhere in the output.” 

7. Dr. Midlam-Mohler suggests, “End users will likely want to see more detail in the 

output file then just the vehicle target speed and achieved speed. Making a limited 

number of “internal” parameters available to allow end users a glimpse inside the 

model without having to use Matlab-Simulink would be sufficient. These should be 

limited to things relevant to their inputs, such as aerodynamic drag over the cycle, 

rolling losses over the cycle, etc.” 

8. Mr. Ortiz-Soto offers a couple recommendations for improving the quality of the 

outputs. He suggests for compliance purposes, “...it would be good to see the actual 

target value next to the simulation result, and probably some sort of percentage 

difference between these. It would give the manufacturer/user an idea of how their 

product performs with respect to the expected regulation standard.” Mr. Ortiz-Soto 

also believes some additional results will be helpful when he recommends, “...some 

additional results might be helpful for manufacturers to determine if the simulation is 

representative of their vehicle. Because many model parameters and vehicle operating 

strategies have been standardized using internal assumptions and algorithms, the 

overall behavior of the vehicle in question could end up being very different from 

what the vehicle manufacturer actually observes. This can result in a significant overestimation of fuel consumption and CO2 emissions, and possibly non-compliance. For 

this reason, it is fair that the manufacturer be able to assess the validity of the 

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simulation without having to investigate the model in detail. This could be achieved 

by providing a series of additional results, which could be related to the engine 

operation over the drive-cycles, the shifting strategy, the electrical system, etc.” He 

noted that it not practical to have to close each plot in order to see the next one or to 

run another simulation; he suggested that a small table with drive output would be 

useful to see along with the plots. He suggested that plots of the engine map and 

shifting strategy be included, along with various drive-cycle visitation points plotted 

on the engine map. 

3.6 OTHER COMMENTS 

The following subsections contain additional reviewer comments. 

3.6.1 “Input” Format 

Multiple reviewers suggested improvements to the input boxes to streamline its ease for the user; 

suggestions included: 

 “The coefficient of aerodynamic drag can only be specified with a pull-down list of 

values from 0.50 to 0.85, with step 0.05. As a result, not all intermediate values for 

Cd can be specified, including the recommended values provided by EPA in Table 5 

(e.g. 0.69, 0.76, 0.81 etc.). Considering the significant impact of Cd on fuel economy 

and its importance in achieving compliance, the value of Cd should be allowed to be 

entered in a textbox.” 

 “...it is not clear why there should be a dropdown menu for the “Coefficient of 

Aerodynamic Drag” parameter. Furthermore, the dropdown menu allows the values 

to be overwritten by the user, so the dropdown menu has no real purpose... A better 

approach would be to just provide a sample value in the parameter name to give the 

user an idea of what would be an expected input in the box. Basically, it should look 

something like the “Steer Tire RR” and “Drive Tire RR” input boxes.” 

 One reviewer suggests that input boxes should become inactive (“grayed out”) when 

it is not desirable for those input values to be changed. 

 “The windows executable version has predefined values for C_d in a dropdown menu 

with preset values in increments of 0.02. The C_d value should just be an entry box, 

like the C_rr values.” 

 “The inputs for weight reduction, speed limiter, and idle reduction are not consistent 

between the matlab version and the windows executable. For example in the matlab 

version. In matlab, zero “Weight Reduction” defaults to “N/A,” which causes an error 

in the windows version. The windows version does not accept “N/A” for idle 

reduction.” 

 “The location of the “Vehicle Model Year” dropdown menu is not intuitive. This is 

one of the most important parameters of the simulation and it is part of the inputs that 

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affects the results, but it has been grouped with the identification parameters. These 

should be separated as they currently are, but somehow the “Vehicle Model Year” 

was left in the top section.” 

 “Having radial buttons with all of the vehicle configurations in the “Regulatory 

Class” section is not necessary. It occupies space and reduces the GUI’s flexibility to 

add other parameters in the future. This type of list is probably better addressed 

through the use of a drop down menu. It would reduce the profile of this parameter 

list, and it would show much more clearly what vehicle type is being used. Currently, 

closer attention has to be paid to the GUI to notice which radio button of the ten 

available is selected, whereas with the dropdown menu it is only necessary to read 

what is displayed.” 

3.6.2 Further Validation of the GEM Model 

Two reviewers remarked that further validation is needed to ensure confidence of the model 

results. 

Dr. Midlam-Mohler addresses model validation in remarking, “Based on the issues noted in (2) 

[Parameter values for Different Vehicle Classes] above, it is important to validate the model 

across vehicle classes. Because the model structure is relatively low-fidelity it has a greater 

burden of proof when “extrapolating” results. To have confidence in the model some further 

level of validation should be conducted.” 

Dr. Flowers comments, “It should be confirmed whether the various controllers in the GEM 

model are well tuned and result in a vehicle response consistent with empirical data.” 

3.6.3 Uncertainty/Sensitivity Analysis 

One reviewer suggests that a sensitivity analysis should be conducted to better understand the 

propagation of error in the input parameters. He recommends that, “It would be useful to have a 

better understanding [of] the propagation of error in the input parameters. For the proposed 

configuration for the class 8 high-roof sleeper cab the sensitivity of the CO2 result to errors in Cd 

is approximately 50%. This implies that a 10% error in Cd will result in a 5% error in prediction 

of CO2 emissions. For rolling resistance, the impact of a 10% error in the tire rolling resistance 

causes a 2.3% error in prediction of CO2 emissions. These sensitivities should be compared to 

the reduction in CO2 emissions required as well as the accuracy of the key input parameters in 

the model. This analysis would also be useful in determining which parameters might be 

superfluous with respect to the desired output. As discussed above, there are some models which 

likely have more complexity then necessary.” 

The reviewer concludes, “A rigorous study of the sensitivity of key input parameters should be 

conducted. Our ability to measure and estimate input parameters is not perfect, hence, the output 

Page 14 of 104
Kent Helmer 

January 28, 2011 

Page 14 

of the model is affected by this uncertainty. If our ability to measure the coefficient of drag is +/- 

x.y % then that has an impact on the model output. This uncertainty can then be compared to 

required accuracy to make a judgment on the validity of this method at estimating green house 

gas emissions or fuel economy.” 

3.6.4 Complexity, Detail, Depth of Some Parts Seem Unnecessary 

A couple reviewers note that they believe the model has a level of detail and complexity that 

may be unnecessary for the stated purpose of the model. 

Mr. Ortiz-Soto provides a couple of examples of detail that seem unnecessary. He reports, 

“Some blocks go into deeper levels unnecessarily. Examples can be found in the electrical 

system and in the driver models. Although the approach used in this model of grouping models 

into blocks based on their physical components or functionality is fairly intuitive, adding extra 

layers can also make the model more difficult to follow if done excessively.” Adding to this, he 

comments, “Some models, such as the electrical system, appear to be extremely complex and 

detailed for this type of dedicated simulation. Unless there is a particular reason, such as future 

extensions to GEM for hybrid-electric trucks or different drive-cycles, where such details are 

necessary, then the electrical system model can probably be stripped down substantially without 

sacrificing much fidelity in the simulation.” 

Dr. Midlam-Mohler similarly reports, “...that there is a higher than necessary level of fidelity in 

many of the models.” He suggests, “EPA could reduce the complexity of many of the models 

with little impact on the accuracy of the simulation – this would then lead to a reduced set of 

parameters that v[a]ry with vehicle class and therefore need to be determined.” Following up on 

this he concludes, “Several of the sub-models had complexity that far outweighed their impact 

on the results. The battery was one such sub-model which also contained some serious errors in 

its formulation. Many of these models could be simplified which will also reduce the number of 

parameters required...” 

3.6.5 User Guide 

One reviewer provides comments on the user guide. The reviewer believes that the model 

description, as presented in the user guide, is too detailed and may “generate unnecessary 

confusion to the users of GEM.” He provides examples of “features of the model that are 

irrelevant and outside the scope of GEM, even though these features are present in the model.” 

4. References 

Environmental Protection Agency (EPA). “Greenhouse Gas Emissions Model (GEM) User 

Guide.” EPA-420-B-10-039, October 2010. 

Page 15 of 104
Kent Helmer 

January 28, 2011 

Page 15 

Environmental Protection Agency (EPA). “Draft Regulatory Impact Analysis: Proposed 

Rulemaking to Establish Greenhouse Gas Emissions Standards and Fuel Efficiency 

Standards for Medium- and Heavy-Duty Engines and Vehicles.” EPA-420-D-10-901, 

October 2010. 

Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration 

(NHTSA). “Preamble: Greenhouse Gas Emissions Standards and Fuel Efficiency 

Standards for Medium and Heavy-Duty Engines and Vehicles.” EPA-HQ-OAR-2010-

0162; NHTSA-2010-0079, October 2010. 

Environmental Protection Agency (EPA). “EPA and NHTSA Propose First-Ever Program to 

Reduce Greenhouse Gas Emissions and Improve Fuel Efficiency of Medium- and HeavyDuty Vehicles: Regulatory Announcement.” EPA-420-F-10-901, October 2010. 

Page 16 of 104
APPENDICES

Page 17 of 104
1

 Appendix A. Elements to be addressed in the Review of EPA’s GEM model

(The model and its documentation can be downloaded from the EPA website at: 

http://www.epa.gov/otaq/climate/regulations. Background information, the pre-publication draft 

Preamble, Regulations and Regulatory Impact Analysis, can also be found on the same website.) 

EPA’s vehicle simulation model, GEM, was created to serve as the primary tool to certify Class 

7/8 combination tractors and Classes 2b – 8 vocational vehicles in meeting EPA’s and NHTSA’s 

proposed vehicle GHG emission levels and fuel efficiency requirements. As both agencies’ 

proposed compliance tool, GEM needed the following modeling attributes: 

1) capable of modeling a wide array of medium- and heavy-duty vehicles over different 

drive cycles; 

2) contains open source code, providing transparency in the model’s operation; 

3) freely available and easy to use by any user with minimal or no prior experience; 

4) contains both optional and preset elements; and 

5) managed by the Agencies for compliance purposes. 

The design of GEM parallels the proposed regulations, which focus on the application of 

technologies having the largest impact on reducing vehicle GHG emission reductions or fuel 

consumption in the 2014-2017 timeframe. For the given timeframe, the model would allow 

various inputs to characterize a vehicle’s properties (e.g., weight, aerodynamics, and rolling 

resistance) and predict how the vehicle would behave when it to be operated over a particular 

driving cycle. 

EPA has validated GEM based on the chassis test results from “SmartWay”-certified tractors 

tested at the Southwest Research Institute. Since many aspects of one tractor configuration (such 

as the engine, transmission, axle configuration, tire sizes, and control systems) are similar to 

those used on a manufacturer’s sister models, the validation work conducted on these vehicles is 

representative of the other Class 8 tractors. 

The input values needed for the simulation model (e.g., drag coefficient, tire rolling resistance 

coefficients, tire/wheel weight reduction, vehicle speed limiter and extended idle reduction 

technologies) are obtained as manufacturer testing or model default values. At the present time, 

the agencies are proposing test procedures for generating aerodynamic drag and tire rolling 

resistance coefficient inputs. Likewise, the agencies are proposing a range for vehicle speed 

limiter and default extended idle reduction technology benefit variables. All other aspects of 

vehicle conformation as defined by the agencies are fixed within the model and are not variable 

for the purpose of compliance. 

After parameters are input to the graphical user interface, GEM predicts the individual and cycleweighted fuel consumption and CO2 emissions for three proposed test cycles – a Transient cycle, 

a 55 mph steady-state cruise cycle, and a 65 mph steady-state cruise cycle. The model can also 

be used to determine a level of technology necessary for a vehicle to meet a specified GHG 

standard and allows a manufacturer to estimate the benefits and costs of those changes to a 

Page 18 of 104
2

particular vehicle for that level of GHG reductions. 

In general, EPA is looking for the reviewer’s opinion of the concepts and methodologies upon 

which the model relies and whether or not the model can be expected to execute these algorithms 

correctly. Toward this end, we suggest that reviewers comment on the following items: 

1) EPA’s overall approach to the stated purpose of the model (meet agencies’ compliance 

requirements) and whether the particular attributes found in resulting model embodies 

that purpose. 

2) The appropriateness and completeness of the contents of the overall model structure 

and its individual systems, and their component models, if applicable (i.e., using the 

MATLAB/Simulink version), such as: 

a) The elements of each system to describe different vehicle categories; 

b) The performance of each component model, including the reviewer’s assessment 

of the underlying equations and/or physical principles coded into that component. 

c) The input and output structures and how they interface with the model to obtain 

the expected result, i.e., fuel consumption and CO2 over the given driving cycles; and 

d) The default values used for the input file, as shown in the GEM User Guide. 

3) Using the standard of good engineering judgment, the program execution is optimized 

by the chosen methodologies; 

4) Clarity, completeness and accuracy of the output/results (CO2 emissions or fuel 

efficiency output file); and 

5) Any recommendations for specific improvements to the functioning or the quality of the 

outputs of the model. 

In making comments to the model, reviewers should distinguish between recommendations for 

clearly defined improvements that can be readily made based on data or literature reasonably 

available to EPA and improvements that are more exploratory or dependent on information not 

readily available to EPA. Any comment(s) should be sufficiently clear and detailed to allow a 

thorough understanding by EPA or other parties familiar with the model. EPA would appreciate 

the reviewers not releasing any peer review materials or their comments to the public until the 

Agency makes its GEM model and supporting documentation public. 

If a reviewer has questions as to what is required to complete this review or needs additional 

background materials, please have that person contact the RTI project manager. If a reviewer has 

a question about the EPA peer review process itself, please have that person contact Ms. Ruth 

Schenk in EPA’s Quality Office by phone (734-214-4017) or e-mail schenk.ruth@epa.gov. 

Page 19 of 104
Curriculum Vitae: Aris Babajimopoulos 

Personal 

 Degrees 

- Ph.D. in Mechanical Engineering, February 2005 (conferred April 2005), The 

University of Michigan, Ann Arbor, MI 

- M.Sc. in Mechanical Engineering, August 2002, The University of Michigan, Ann 

Arbor, MI 

- Diploma in Mechanical Engineering, October 1998, Aristotle University of 

Thessaloniki, Thessaloniki, Greece 

 Positions at the University of Michigan 

- Assistant Research Scientist, January 2006 – present 

- Research Fellow, February 2005 – January 2006 

 Honors and Awards 

- 2006 SAE Excellence in Oral Presentation Award 

Teaching 

 Ph.D. Committees 

1. Mr. Michael Mosburger (member, chair: Volker Sick, Mechanical Engineering, 

expected 2011) 

2. Mr. Jerry Fuschetto (member, chair: Dennis Assanis, Mechanical Engineering, 

expected 2011) 

3. Dr. Jason Martz (member, chair: Dennis Assanis, Mechanical Engineering, October 

2010) 

4. Dr. Seung Hwan Keum (member, co-chairs: Dennis Assanis and Hong Im, 

Mechanical Engineering, February 2009) 

5. Dr. Heejun Park (member, co-chairs: Dennis Assanis and Dohoy Jung, Mechanical 

Engineering, February 2009) 

 M.Sc. Committees 

1. Mr. Prasad Shingne ((co-chair with Dennis Assanis, Mechanical Engineering, 

August 2010) 

2. Mr. Janardhan Kodavasal (co-chair with Dennis Assanis, Mechanical Engineering, 

May 2010) 

Page 20 of 104
3. Mr. Elliott Ortiz-Soto (co-chair with Dennis Assanis, Mechanical Engineering, May 

2010) 

4. Mr. Sotiris Mamalis (co-chair with Dennis Assanis, Mechanical Engineering, 

December 2009) 

5. Mr. Anastasios Amoratis (co-chair with Dennis Assanis, Mechanical Engineering, 

June 2009) 

6. Mr. Prasad Challa (co-chair with Dennis Assanis, Mechanical Engineering, 

December 2008) 

 ME590 Students Supervised 

1. Mr. Chang-Ping Lee (Winter term, 2009) 

2. Mr. Sourabh Goel (Winter term, 2009) 

 UROP Students Supervised 

1. Mr. Joshua Busuito (2 credits/term, September 2009 – April 2010) 

2. Ms. Christine Siew (3 credits/term, September 2006 – April 2007) 

 Courses 

1. ME 538: Advanced Internal Combustion Engines, Winter semester 2009 (Three 

lectures. Also prepared the three homework sets for the class and advised student 

teams on their class projects). 

Research 

 Grants and Contracts: 

1. “Advanced Combustion Controls – Enabling Systems and Solutions (ACCESS) for 

High Efficiency Light Duty Vehicles,” funded by the Department of Energy, 

National Energy Technology Laboratory, 9/30/2010 – 9/29/2014, total funding 

~$25,000,000 including 50% cost share, CO-PI (PI: Hakan Yilmaz, Robert Bosch 

LCC).

2. “A University Consortium on Efficient and Clean High Pressure Lean Burn 

(HPLB) Engines,” funded by the Department of Energy, Office for Energy 

Efficiency and Renewable Energy, 9/1/2009 – 12/31/2012, total funding 

$3,750,000, CO-PI (PI: Dennis Assanis).

3. “Development of a High Fidelity Vehicle Energy Assessment Simulation and 

Implementation in the GT-Drive Framework,” funded by the TARDEC FED/CASSI 

Team, 9/1/2009 – 12/31/2010, total funding $525,000, CO-PI with a share of 

approximately $50,000 (PI: Dennis Assanis). 

4. “Thrust Area 4: JP-8 Engine Combustion Modeling,” plus-up project funded by the 

Department of Defense (U.S. Army) through the Automotive Research Center, 

1/1/2009 – 12/31/2009, $100,000, PI. 

5. General Motors-University of Michigan Engine Systems Research Collaborative 

Research Lab (GM/UM ESR CRL), Phase 3, funded by General Motors 

Page 21 of 104
Corporation, 1/1/2009 to 12/31/2013, total funding $3,750,000, CO-PI with a share 

of approximately $450,000 (PI: Dennis Assanis). 

5. “Modeling and Experimental Study of the Boosted HCCI Engine," funded by 

General Motors Corporation, 7/1/2007 – 12/31/2009, total funding $1,365,000, COPI with a share of approximately $350,000 (PI: Dennis Assanis). 

6. “Simulation-Based and Experimental Assessment of Variable Valve Timing 

Strategies for HCCI Engines,” funded by Borg Warner, 1/1/2007 – 12/31/2009, total 

funding $300,000, CO-PI (PI: Dennis Assanis). 

7. “University Consortium on Low Temperature Combustion for High-Efficiency, 

Ultra-Low Emission Engines,” funded by the Department of Energy, 1/1/2006 – 

12/31/08, total funding $4,697,000, CO-PI (PI: Dennis Assanis). 

8. “Numerical Assessment of a Free Piston Linear Alternator for Series HEV (Hybrid 

Electric Vehicle),” funded by General Motors Corporation, 6/1/2005 – 5/31/07, total 

funding $525,000, CO-PI (PI: Dennis Assanis). 

 Publications: 

- Articles in Journals and Transactions

1. Ortiz-Soto, E., Assanis, D.N. and Babajimopoulos, A. (2010) A Comprehensive 

Engine to Drive-Cycle Modeling Framework for the Evaluation of Future Engine 

and Combustion Technologies. Submitted to International Journal of Engine 

Research. 

2. Keum, S., Park, H., Babajimopoulos, A., Assanis, D.N. and Jung, D. (2010) 

Modeling of Heat Transfer in Internal Combustion Engines with Variable Density 

Effect. Submitted to International Journal of Engine Research. 

3. Martz, J.B., Middleton, R.J., Lavoie, G.A., Babajimopoulos, A. and Assanis, D.N. 

(2010) A Computational Study and Correlation of Premixed Isooctane-Air 

Laminar Reaction Front Properties under Spark Ignited and Spark Assisted 

Compression Ignition Engine Conditions. Accepted for publication in 

Combustion and Flame.

4. Mamalis, S., Nair, V., Andruskiewicz, P., Assanis, D. and Babajimopoulos, A. 

(UM); Wermuth, N. and Najt, P. (GM) (2010) Comparison of Different Boosting 

Strategies for Homogeneous Charge Compression Ignition Engines – A Modeling 

Study. SAE International Journal of Engines, vol. 3, no. 1, pp. 296-308 

(Presented as SAE Paper 2010-01-0571 at the SAE World Congress, Apr 13-15, 

2010, Detroit, MI). 

5. Babajimopoulos, A., Challa, P., Lavoie, G.A. and Assanis, D.N. (2010) Modelbased assessment of two variable cam timing strategies for HCCI engines: 

Recompression vs. rebreathing. Accepted for publication in the ASME Journal of 

Engineering for Gas Turbines and Power (also presented at the ASME Internal 

Combustion Engine Division 2009 Spring Technical Conference, May 3-6, 2009, 

Milwaukee, WI, as ASME Paper ICES2009-76103). 

Page 22 of 104
6. Babajimopoulos, A., Lavoie, G.A., Assanis, D.N. (2007) On the role of top dead 

center conditions in the combustion phasing of homogeneous charge compression 

ignition engines. Combustion Science and Technology, vol. 179, no. 9, pp. 2039-

2063.

7. Chang, K., Babajimopoulos, A., Lavoie, G.A., Filipi, Z.S., Assanis, D.N. (2006) 

Analysis of load and speed transitions in an HCCI engine using 1-D cycle 

simulation and thermal networks. 2006 SAE Transactions, Journal of Engines, 

vol. 115, no. 3, pp. 621-633 (Presented as SAE Paper 2006-01-1087 at the SAE 

World Congress, Apr 3-7, 2006, Detroit, MI). 

8. Babajimopoulos, A., Assanis, D.N., Flowers, D.L., Aceves, S.M., Hessel, R.P. 

(2005) A fully coupled computational fluid dynamics and multi-zone model with 

detailed chemical kinetics for the simulation of premixed charge compression 

ignition engines. International Journal of Engine Research, vol. 6, no. 5, pp. 497-

512.

9. Aceves, S.M., Flowers, D.L., Espinosa-Loza, F., Babajimopoulos, A., Assanis, 

D.N. (2005) Analysis of premixed charge compression ignition combustion with a 

sequential fluid mechanics-multizone chemical kinetics model. 2005 SAE 

Transactions, Journal of Engines, vol. 114, no. 3, pp. 252-262 (Presented as SAE 

Paper 2005-01-0115 at the SAE World Congress, Apr 11-14, 2005, Detroit, MI). 

10. Sjöberg, M., Dec, J.E., Babajimopoulos, A., Assanis, D.N. (2004) Comparing 

enhanced natural thermal stratification against retarded combustion phasing for 

smoothing of HCCI heat-release rates. 2004 SAE Transactions, Journal of 

Engines, vol. 113, no. 3, pp. 1557-1575 (Presented as SAE Paper 2004-01-2994 at 

the Powertrain and Fluid Systems Conference and Exhibition, Oct 25-28, 2004, 

Tampa, FL). 

11. Babajimopoulos, A., Assanis, D.N., Fiveland, S.B. (2002) Modeling the effects of 

gas exchange processes on HCCI combustion and an evaluation of potential 

control through variable valve actuation. 2002 SAE Transactions, Journal of 

Fuels and Lubricants, vol. 111, no. 4, pp. 1794-1809 (Presented as SAE Paper 

2002-01-2829 at the Powertrain and Fluid Systems Conference and Exhibition, 

Oct 21-24, 2002, San Diego, CA; also included in Homogeneous Charge 

Compression Ignition (HCCI) Engines – Key Research and Development Issues, 

SAE PT-94, 2003). 

- Articles in Refereed Conference Proceedings

1. Manofsky, L., Vavra, J., Babajimopoulos, A. and Assanis, D. (2010) Bridging the 

Gap between HCCI and SI: Spark-Assisted Compression Ignition. Submitted to 

the 2011 SAE World Congress. 

2. Delorme, A., Rousseau, A., Wallner, T., Ortiz-Soto, E., Babajimopoulos, A. and 

Assanis, D. (2010) Evaluation of Homogeneous Charge Compression Ignition 

(HCCI) Engine Fuel Savings for Various Electric Drive Powertrains. To be 

presented at the 25th World Battery, Hybrid and Fuel Cell Electric Vehicle 

Symposium & Exhibition ̧ Nov 5-9, 2010, Shenzhen, China. 

Page 23 of 104
3. Shingne, P., Assanis, D. and Babajimopoulos, A. (UM); Keller, P., Roth, D. and 

Becker, M. (2010) Turbocharger Matching for a 4-Cylinder Gasoline HCCI 

Engine Using a 1D Engine Simulation. SAE Paper 2010-01-2143, Presented at 

the SAE Powertrains, Fuels and Lubricants Meeting, Oct 25-27, 2010, San Diego, 

CA. 

4. Mamalis, S. and Babajimopoulos, A. (2010) Model-Based Estimation of 

Turbocharger Requirements for Boosting an HCCI Engine. ASME Paper 

ICEF2010-35122. Proceedings of the ASME Internal Combustion Engine 

Division 2010 Fall Technical Conference, Sep 12-15, San Antonio, TX. 

5. Lee, C.-P., Goel, S., and Babajimopoulos, A. (2010) The Effects of Stroke-toBore Ratio on HCCI Combustion. SAE Paper 2010-01-0842, Presented at the 

SAE World Congress, Apr 13-15, 2010, Detroit, MI. 

6. Hessel, R., Foster, D., Aceves, S., Davisson, M., Espinosa-Loza, F., Flowers, D., 

Pitz, W., Dec, J., Sjöberg, M. and Babajimopoulos, A. (2008) Modeling isooctane HCCI using CFD with multi-zone detailed chemistry; Comparison to 

detailed speciation data over a range of lean equivalence ratios. SAE Paper 2008-

01-0047, Presented at the SAE World Congress, Apr 14-17, 2008, Detroit, MI. 

7. Murotani, T., Hattori, K., Sato, E., Chryssakis, C., Babajimopoulos, A. and 

Assanis, D.N. (2007) Simultaneous reduction of NOx and soot in a heavy-duty 

diesel engine by instantaneous mixing of fuel and water. SAE Paper 2007-01-

0125, Presented at the SAE World Congress, Apr 16-19, 2007, Detroit, MI. 

8. Chang, K., Lavoie, G.A., Babajimopoulos, A., Filipi, Z.S. and Assanis, D.N. 

(2007) Control of a multi-cylinder HCCI engine during transient operation by 

modulating residual gas fraction to compensate for wall temperature effects. SAE 

Paper 2007-01-0204, Presented at the SAE World Congress, Apr 16-19, 2007, 

Detroit, MI. 

9. Aceves, S.M., Flowers, D.L., Chen, J.-Y., Babajimopoulos, A. (2006) Fast 

prediction of HCCI combustion with an artificial neural network linked to a fluid 

mechanics code. SAE Paper 2006-01-3298, Presented at the Powertrain and Fluid 

Systems Conference and Exhibition, Oct 16-19, 2006, Toronto, Canada. 

10. Flowers, D., Aceves, S., Babajimopoulos, A. (2006) Effect of charge nonuniformity on heat release and emissions in PCCI engine combustion. SAE Paper 

2006-01-1363, Presented at the SAE World Congress, Apr 3-7, 2006, Detroit, MI. 

11. Babajimopoulos, A., Lavoie, G.A. and Assanis, D.N. (2003) Modeling HCCI 

combustion with high levels of residual gas fraction - A comparison of two VVA 

strategies. SAE Paper 2003-01-3220, Presented at the Powertrain and Fluid 

Systems Conference and Exhibition, Oct 27-30, 2003, Pittsburgh, PA. 

12. Babajimopoulos, A., Lavoie, G.A. and Assanis, D.N. (2003) Numerical modeling 

of the effects of temperature and composition stratification on HCCI combustion 

for high levels of residual gas fraction. Proceedings of the 6th International 

Conference on Engines for Automobile, Sep 14-19, 2003, Capri, Italy. 

Page 24 of 104
- Patents

1. Assanis, D.N., Babajimopoulos, A., Filipi, Z.S., Kuo, T.-W., Lavoie, G.A., Najt, 

P.M., and Rask, R.B. (2009) Hybrid Powertrain System Using Free Piston Linear 

Alternator Engines. Patent application filed to the U.S. Patent Office, US Serial 

No. 12/504502, filed 16-JUL-2009. 

- Book Chapters

1. Aceves, S.M, Flowers, D.L., Dibble, R.W. and Babajimopoulos, A. (2007) 

Overview of modeling techniques and their application to HCCI/CAI engines. In 

Zhao, H. (Ed.) HCCI and CAI engines for the automotive industry, Chap. 18, pp. 

456-474, Woodhead Publishing Limited, Cambridge, England. 

- Articles in Non-Refereed Conference Proceedings

1. Hessel, R., Foster, D., Aceves, S., Flowers, D, Pitz, B., Dec, J., Sjöberg, M. and 

Babajimopoulos, A. (2007) Modeling HCCI using CFD and detailed chemistry 

with experimental validation and a focus on CO emissions. Proceedings of the 

17th International Multidimensional Engine Modeling User’s Group Meeting, Apr 

15, 2007, Detroit, MI. 

2. Babajimopoulos, A., Assanis, D.N., Flowers, D.L., Aceves, S.M., Hessel, R.P. 

(2005) A fully integrated CFD and multi-zone model with detailed chemical 

kinetics for the simulation of PCCI engines. Proceedings of the 15th International 

Multidimensional Engine Modeling User’s Group Meeting, Apr 10, 2005, Detroit, 

MI. 

- Presentations in Conferences without Proceedings

1. Babajimopoulos, A., Manofsky, L., Shingne, P., Spater, J., Lavoie, G., and 

Assanis, D. (2009) UM FFVA Engine: Experimental Recompression Results and 

GT-Power Modeling. HCCI University Working Group Meeting at USCAR, 

October 8, 2009, Southfield, MI. 

2. Assanis, D.N., Lavoie, G., and Babajimopoulos, A. (2009) Advanced Combustion 

for High Efficiency Ultra-Clean Engines. Keynote Lecture, 6th US National 

Combustion Meeting, May 17-20, 2009, Ann Arbor, MI. 

3. Babajimopoulos, A., Nair, V., Lavoie, G. and Assanis, D. (2009) Exploring 

supercharged HCCI using GT-Power based simulation tool. HCCI University 

Working Group Meeting at Sandia National Laboratories, February 12, 2009, 

Livermore, CA. 

4. Babajimopoulos, A., Challa, P., Mamalis, S., Lavoie, G., Filipi, Z. and Assanis, 

D. (2008) System modeling of turbocharging for HCCI. HCCI University 

Working Group Meeting at USCAR, August 21, 2008, Southfield, MI. 

5. Babajimopoulos, A., Lavoie, G., Filipi, Z., Challa, P., Mamalis, S. and Assanis, 

D. (2008) System modeling of valve actuation strategies and turbocharging for 

HCCI. HCCI University Working Group Meeting at Sandia National 

Laboratories, March 20, 2008, Livermore, CA. 

Page 25 of 104
6. Babajimopoulos, A., Lavoie, G., Filipi, Z., Challa, P., and Assanis, D. (2007) 

System modeling of valve actuation strategies for HCCI with a realistic 

combustion model. HCCI University Working Group Meeting at USCAR, October 

4, 2007, Southfield, MI. 

7. Babajimopoulos, A., Lavoie, G., Filipi, Z., Mo, Y., Chang, K. and Assanis, D. 

(2007) A CFD-based HCCI combustion correlation for use in system models. 

HCCI University Working Group Meeting at Sandia National Laboratories, 

February 8, 2007, Livermore, CA. 

8. Babajimopoulos, A., Lavoie, G.A., Assanis, D.N. (2006) On the role of top dead 

center conditions in the combustion phasing of homogeneous charge compression 

ignition engines. Oral only presentation at the Powertrain and Fluid Systems 

Conference and Exhibition, October 16-19, 2006, Toronto, Canada. 

9. Chang, K., Lavoie, G., Babajimopoulos, A., Filipi, Z., and Assanis, D. (2006) 

Simulation of a multi-cylinder HCCI engine during transient operation by 

modulating RGF – Compensating for the wall temperature effects. HCCI 

University Working Group Meeting at USCAR, June 29, 2006, Southfield, MI. 

10. Babajimopoulos, A., Lavoie, G., Filipi, Z., Im, H., Mo, Y., Chang, K., 

Hamosfakidis, V. and Assanis, D. (2006) HCCI modeling: System modeling of 

coolant temperature effect and update on CFD results. HCCI University Working 

Group Meeting at Sandia National Laboratories, February 9, 2006, Livermore, 

CA.

11. Babajimopoulos, A., Lavoie, G. and Assanis, D. (2005) Scaling of HCCI 

combustion phasing: The role of TDC conditions. HCCI University Working 

Group Meeting at USCAR, September 15, 2005, Southfield, MI. 

12. Babajimopoulos, A., Lavoie, G., Filipi, Z., Mo, Y., Chang, K. and Assanis, D. 

(2005) Recent HCCI modeling results and application to system models. HCCI 

University Working Group Meeting at Sandia National Laboratories, February 3, 

2005, Livermore, CA. 

13. Assanis, D., Filipi, Z., Lavoie, G., Babajimopoulos, A., Chang, K. and Mo, Y. 

(2004) Modeling HCCI for control and system simulation. HCCI University 

Working Group Meeting at USCAR, June 24, 2004, Southfield, MI. 

14. Babajimopoulos, A., Lavoie, G., Mo, Y. and Assanis, D. (2004) Developments in 

HCCI modeling at the University of Michigan. HCCI University Working Group 

Meeting at Sandia National Laboratories, January 29, 2004, Livermore, CA. 

15. Assanis, D., Filipi, Z., Lavoie, G., Babajimopoulos, A. and Chang, J. (2003) 

Progress in HCCI thermo-kinetic Modeling and engine experiments. HCCI 

University Working Group Meeting at USCAR, June 26, 2003, Southfield, MI. 

16. Babajimopoulos, A., Assanis, D. and Fiveland, S. (2002) Sequential use of an 

open cycle CFD code and a multi-zone model for assessment of VVA control 

strategies. HCCI University Working Group Meeting at USCAR, June 12, 2002, 

Southfield, MI. 

Page 26 of 104
- Invited Seminars

1. Babajimopoulos, A. (2008) An introduction to Homogeneous Charge 

Compression Ignition (HCCI). Dept. of Naval Engineering, National Technical 

University of Athens, September 22, 2008, Athens, Greece. 

2. Babajimopoulos, A. (2008) An introduction to Homogeneous Charge 

Compression Ignition (HCCI) and the ongoing work at the University of 

Michigan. Graduate Seminar Series, Dept. of Mechanical Engineering, Marquette 

University, April 10, 2008, Milwaukee, WI. 

Service 

 Co-organizer for the Kinetically Controlled CI Combustion (including HCCI) session, 

SAE 2011 World Congress, April 12-14, 2011, Detroit, MI 

 Co-organizer for the Kinetically Controlled CI Combustion (HCCI) session, SAE 2010 

World Congress, April 13-15, 2010, Detroit, MI 

 Review coordinator for the Low Temperature Combustion session, ASME Internal 

Combustion Engine Division 2009 Fall Technical Conference, September 27-30, 2009, 

Lucerne, Switzerland 

 Co-organizer for the Homogeneous Charge Compression Ignition session, SAE 2009 

International Powertrains, Fuels and Lubricants Meeting, June 15-17, 2009, Florence, 

Italy 

 Co-chair for the Multi-dimensional Modeling session, ASME Internal Combustion 

Engine Division 2009 Spring Technical Conference, May 3-6, 2009, Milwaukee, WI 

 Co-organizer for the Homogeneous Charge Compression Ignition session, SAE 2009 

World Congress, April 20-23, 2009, Detroit, MI 

 Judge, UM Engineering Graduate Student Symposium, November, 2006 

 Reviewer for 

1. SAE/JSAE 

2. ASME/IMECE 

3. The Combustion Institute 

4. Transactions of the ASME – Journal of Engineering for Gas Turbines and Power 

5. International Journal of Engine Research 

6. Proceedings of the Institution of Mechanical Engineers, Part D, Journal of 

Automobile Engineering 

7. IEEE/ASME Transactions on Mechatronics 

8. Combustion and Flame 

9. Journal of Energy Resources Technology 

Page 27 of 104
10. Combustion Science and Technology 

11. Energy 

Page 28 of 104
Daniel L. Flowers 

!"!#

Brief Biography

Daniel Flowers is the Associate Program Leader for Combustion and Alternative Fuels at 

Lawrence Livermore National Laboratory, where his work focuses on experimental and 

analytical research in thermal sciences and combustion. He has been working in the 

area of Homogeneous Charge Compression Ignition (HCCI) engine combustion since 

joining LLNL in 1998. Flowers leads several combustion research projects at LLNL in the 

areas of HCCI, hydrogen and Diesel combustion. On leave from LLNL Flowers led 

research and development at Cleeves Engines, an energy research startup company. 

Flowers served as Associate Technical Editor of the ASME Journal of Energy Resource 

Technologies in 2007 and 2008. Flowers holds Ph.D. (2001), M.S. (1997), and B.S. 

(1996) degrees in Mechanical Engineering from the University of California, Davis.

Work History

Lawrence Livermore National Laboratory, September 1998 to present

Title: Principal Investigator/Project Leader

Responsibilities:

• Principal Investigator – DOE OFCVT Combustion and Fuels Programs ($1M/FY07)

o Leading Ongoing LLNL activities in HCCI research, developing multidimensional 

modeling tools

o Leading collaborations with Universities and Other National Labs

o Program highly ranked at annual program review

o Integral part of a world recognized team that has developed the most advanced 

analysis tools for HCCI combustion

o Extending analysis tools for HCCI combustion – Continuing to advance 

multidimensional HCCI combustion modeling tool

o Continuing development of massively parallel tool for simulation of 

multidimensional HCCI and PCCI combustion

o Collaborating with US auto industry partners to guide development of new 

combustion systems

o Investigating HCCI applications for biofuels and non-standard fuels: biodiesel, 

“wet ethanol,” “trash gas”

• Principal Investigator – DOE NETL ($300K/FY07)

o Separate project on HCCI working with International Engine Company funded by 

NETL

o Modeling to support International Engine’s HCCI Engine Development program

• Principal Investigator - DOE/OFCVT

o Modeling of Hydrogen Spark Ignition Combustion ($150K FY06)

o Modeling of Smokeless Rich Diesel Combustion ($150K FY06)

• Principal Investigator/Project Leader – HCCI engines for stationary power generation 

(California Energy Commission, 3 years $2M)

o Leading development of an experimental HCCI engine for stationary power 

generation applications

o Completed 2006

Cleeves Engines Incorporated (San Carlos, CA), February 2008 to June 2009

Title: Senior Combustion Engineer

Responsibilities:

• Leading Research and Development activities on an advanced technology concept.

• Concept development advanced operating strategies for an advanced internal 

combustion engine strategy

• Developing test cell hardware, methods, and protocols for demonstration of engine 

Page 29 of 104
Daniel L. Flowers 

!$!#

concept

• Guiding and conducting numerical analysis activities for prototype engine 

development, including CFD (Fluent), Engine dynamic modeling (GT-Power), and 

FEA (Cosmos, Ansys)

• Cleeves Engines is an Energy Technology startup developing advance internal 

combustion engine concepts 

Education

Ph.D in Mechanical Engineering – University of California, Davis

Dissertation: Combustion in Homogeneous Charge Compression Ignition Engines: 

Experiments and Detailed Kinetic Modeling

M.S. in Mechanical Engineering - University of California, Davis

Thesis: Application of Morphology Dependent Resonance Spectroscopy to Droplet 

Sizing

B.S. in Mechanical Engineering – University of California, Davis (Highest Honors)

Professional Activities

Associate Technical Editor, ASME Journal of Energy Resources Technology

The Combustion Institute, Member (Alternate on Executive Committee)

Society of Automotive Engineers, Member

American Society of Mechanical Engineers, Member

Symposium Co-Chair, Advanced Energy Systems, ASME IMECE, 2004

Session Organizer and Chair, Advanced Energy Systems, ASME IMECE, Multiple years

Session Chair, Society of Automotive Engineers, Multiple years

Mentoring and Education

Mentor to LLNL Graduate Research Fellows and LLNL Graduate Student Employees

Long-standing collaboration with Profs. Robert Dibble and J.Y. Chen at UC Berkeley.

Direction and Research Guidance to UC Berkeley Graduate Students.

Mentor and project leader to several LLNL Undergraduate Engineering Interns.

Invited Mini-course, Universidad de Guanajuato, Mexico: Introduction to Kiva3v.

Patent

Daniel L. Flowers, “Controlling and Operating Homogeneous Charge Compression 

Ignition (HCCI) Engines,” U.S. Patent 6,923,167

Book Chapter

S. M. Aceves, D. L. Flowers, R. W. Dibble and A. Babajimopoulos, “Overview of 

modeling techniques and their application to HCCI/CAI engines,” in HCCI and CAI 

engines for the automotive industry, Hua Zhao, Ed., in press.

Peer Reviewed Publications

Journal Papers

Killingsworth, N.J., Aceves, S.M., Flowers, D.L., Espinosa-Loza, F.J., and Kristic, M., 

"HCCI Engine Combustion Timing Control: Optimizing Gains and Fuel Consumption Via 

Extremum Seeking,” IEEE Transactions on Control Systems Technology, in press 

(2008).

Page 30 of 104
Daniel L. Flowers 

!%!#

D.L. Flowers, S.M. Aceves, R.W. Dibble, “Effect of Laser-induced Excitation of Oxygen 

on Ignition in HCCI Engines Analyzed by Numerical Simulations,” Combustion Theory 

And Modeling, Vol. 11, No. 3, 2007: 455-468.

J.H. Mack, B.A. Buchholz, D.L. Flowers and R.W. Dibble. “Using Biofuel Tracers to 

Study Alternative Combustion Regimes,” Nuclear Instruments & Methods B (2006) in 

press.

Aristotelis Babajimopoulos, Dennis Assanis, Daniel Flowers, Salvador Aceves, Randy 

Hessel“A Fully Integrated CFD and Multi-zone Model with Detailed Chemical Kinetics for 

the Simulation of PCCI Engines” International Journal of Engine Research, Volume 6, 

Number 5, October 2005, pp. 497-512(16).

Parag Mehresh, Daniel Flowers, Robert Dibble, “Experimental and Numerical 

Investigation of Effect of Fuel on Ion Sensor Signal to Determine Combustion Timing in 

HCCI Engines,” International Journal of Engine Research, Volume 6, Number 5, October 

2005, pp. 465-474(10).

Parag Mehresh, Jason Souder, Daniel Flowers, Uwe Riedel, Robert Dibble, 

“Combustion Timing in HCCI Engines Determined by Ion-Sensor: Experimental and 

Kinetic Modeling,” Proceedings of the Combustion Institute, Vol 30, Part 2, 2005: 2693- 

2700.

Hunter Mack, Bruce Bucholtz, Daniel Flowers, Robert Dibble, “Investigation of HCCI 

Combustion of Diethyl Ether and Ethanol Mixtures Using Carbon 14 Tracing and 

Numerical Simulations,” Proceedings of the Combustion Institute, Vol 30, Part 2, 2005: 

2701-2709.

Martinez-Frias J, Aceves SM, Flowers D, Smith JR, Dibble R, “Thermal charge 

conditioning for optimal HCCI engine operation”ASME Vol. 124 No. 1, March 2002: 67-

75.

Flowers, D. L, Aceves, S.M., Martinez-Frias, J., and Dibble, R. W., “Prediction of 

Carbon Monoxide and Hydrocarbon Emissions in Isooctane HCCI Engine Combustion 

Using Multi-Zone Simulations,” Proceedings of the Combustion Institute, Vol 29, Part 1, 

2002: 687-694.

Flowers, D. L., Aceves, S. M., Westbrook, C. K., Smith, J. R., Dibble, R. W., “Detailed 

Chemical Kinetics Simulation of HCCI Combustion Gas Composition Effects and 

Investigation of Control Strategies,” ASME Journal of Engineering for Gas Turbines and 

Power, Vol. 123, No. 2, April 2001.

Santangelo, P. J., Flowers, D. L., and Kennedy, I. M., “Demonstration of droplet size 

and vaporization rate measurements in the near field of a two-phase jet with droplet 

lasing spectroscopy,” Applied Optics, 1998 Aug 20, Vol 37 No. 24: 5573-5578.

SAE Transactions Papers

Hessel, R.P, Aceves, S.M., Flowers, D.L., “A Comparison on the Effect of Combustion

Chamber Surface Area and In-Cylinder Turbulence on the Evolution of Gas Temperature 

Distribution from IVC to SOC: A Numerical and Fundamental Study,” SAE Paper 2006- 

01-0869 SAE Transactions, Journal of Engines, 2006.

Page 31 of 104
Daniel L. Flowers 

!&!#

Aceves, S.M., Flowers, D.L., “A Detailed Chemical Kinetic Analysis of LowTemperature, Non-Sooting Diesel Combustion,” SAE Paper 2005-01-0923, SAE 

Transactions, Journal of Engines, 2005.

Aceves, S.M., Flowers, D. L., Espinosa-Loza, F.J., Babajimopoulos, A., Assanis, D., 

“Analysis of Premixed Charge Compression Ignition Combustion With a Sequential Fluid 

Mechanics-Multizone Chemical Kinetics Model,” SAE Paper 2005-01-0115, SAE 

Transactions, Journal of Engines, 2005.

Salvador Aceves, Daniel Flowers, “Analysis of the Effect of Geometry Generated 

Turbulence on HCCI Combustion by Multi-Zone Modeling” SAE Paper 2005-01-2134, 

SAE Transactions, Journal of Engines, 2005.

Parag Mehresh, Daniel Flowers, Robert Dibble, “EGR effect on Ion Signal in HCCI 

Engines,” SAE Paper 2005-01-2126, SAE Transactions, Journal of Engine, 2005.

Hunter Mack, Bruce Bucholtz, Daniel Flowers, Robert Dibble, “Effect of the Di-Tertiary 

Butyl Peroxide (DTBP) additive on HCCI Combustion of Fuel Blends of Ethanol and 

Diethyl Ether” SAE Paper 2005-01-2135, SAE Transactions, Journal of Fuels and 

Lubricants, 2005.

Salvador M. Aceves, Daniel L. Flowers, Francisco Espinosa-Loza, Joel Martinez-Frias, 

John E. Dec, Magnus Sjöberg, Robert W. Dibble and Randy P. Hessel, “Spatial Analysis 

of Emissions Sources for HCCI Combustion at Low Loads Using a Multi-Zone Model,” 

SAE Paper 2004-01-1910, SAE Transactions, Journal of Fuels and Lubricants, 2004.

Daniel L. Flowers, Salvador M. Aceves, Joel Martinez-Frias, Randy Hessel, and Robert 

W. Dibble, “Effects of Mixing on Hydrocarbon and Carbon Monoxide Emissions, 

Predictions for Isooctane HCCI Engine Combustion Using a Multi-Zone Detailed Kinetics 

Solver,” SAE Paper 2003-01-1821, SAE Transactions, Journal of Fuels and Lubricants, 

2003.

Salvador M. Aceves, Daniel L. Flowers, Francisco Espinosa-Loza, Joel Martinez-Frias, 

Robert W. Dibble, Magnus Christensen, Bengt Johansson, Randy P. Hessel, “PistonLiner Crevice Geometry Effect on HCCI Combustion by Multi-Zone Analysis,” SAE 

Paper 2002-01-2869, SAE Transactions, Journal of Engines, Volume 111, pp. 2691-

2698, 2002.

Aceves, S.M., Martinez-Frias, J., Flowers, D. L., Smith, J. R., Dibble, R. W., “A 

Decoupled Model of Detailed Fluid Mechanics Followed By Detailed Chemical Kinetics 

for Prediction of Iso-Octane Hcci Combustion, “ SAE Paper 2001-01-3612, SAE 

Transactions, Journal of Fuels and Lubricants, 2001.

Aceves, S. M., Flowers, D. L., Martinez-Frias, J., Smith, J. R., Westbrook, C. K., Pitz, 

W. J., Dibble, R. W. “Multi-Zone Analysis of Propane HCCI Combustion,” SAE Paper 

2001-01-1027, SAE Transactions, Journal of Engines, 2001.

Martinez-Frias, J. M., Aceves, S. M., Flowers, D. L., Smith, J. R., Dibble, R. W., “HCCI 

Engine Control by Thermal Management,” SAE Paper 2000-01-2869, SAE Transactions, 

Journal of Engines, 2000.

Page 32 of 104
Daniel L. Flowers 

!'!#

Aceves, S.M, Flowers, D. L., Westbrook, C. K., Smith, J. R., Pitz,W., Dibble, R. W., 

Christensen, M., and Johansson, B., 2000, “A Multi-Zone Model for Prediction of HCCI 

Combustion and Emissions,” SAE paper 2000-01-0327, SAE Transactions, Journal of 

Engines, 2000.

Other SAE Papers

Hessel, R.P., Foster, D., Steeper, R., Aceves, S.M., Flowers, D.L., “Pathline Analysis of 

Full-cycle Four-stroke HCCI Engine Combustion Using CFD and Multi-Zone Modeling,” 

SAE Paper 2008-01-0048.

Hessel, R., Babajimopoulos, A., Foster, D., Aceves, S., Davisson, M, Espinosa-Loza, 

F.J., Flowers, D.L., Pitz, W., Dec, J., Sjoberg, M., “ Modeling Iso-octane HCCI using 

CFD with Multi-Zone Detailed Chemistry; Comparison to Detailed Speciation Data over a 

Range of Lean Equivalence Ratios, 2008-01-0047.

Flowers, D.L., Aceves, S.M, Martinez-Frias, J., “Improving Ethanol Life Cycle Energy 

Efficiency by Direct Utilization of Wet Ethanol in HCCI Engines,” SAE Paper 2007-01- 

1867/JSAE Paper 20077037.

S.M. Aceves, D.L.Flowers, J.Y. Chen, A. Babajimopoulos, “Fast Prediction of HCCI 

Combustion With an Artificial Neural Network Linked to a Fluid Mechanics,” SAE Paper 

2006-01-3298.

R.P. Hessel, N. Abani, S. Aceves, D. Flowers, "Gaseous Fuel Injection Modelling Using 

a Gaseous Sphere Injection Methodology,” SAE Paper 2006-01-3265.

Flowers, D.L., Aceves, S.M., Babajimopoulos, A., “Effect of Charge Non-uniformity on 

Heat Release and Emissions in PCCI Engine Combustion,” SAE Paper 2006-01-1363.

Salvador M. Aceves, Daniel Flowers, Joel Martinez-Frias, Francisco Espinosa-Loza, 

William J. Pitz, Robert Dibble, “Fuel and Additive Characterization for HCCI 

Combustion,” SAE paper 2003-01-1814.

Salvador M. Aceves, Joel Martinez-Frias, Daniel Flowers, J. Ray Smith, Robert Dibble, 

J.Y. Chen, “A Computer Generated Reduced Iso-Octane Chemical Kinetic Mechanism 

Applied to Simulation of HCCI Combustion,” SAE Paper 2002-01-2870.

James W. Girard, Robert W. Dibble, Daniel L. Flowers, Salvador M. Aceves, “An 

Investigation of the Effect of Fuel-Air Mixedness on the Emissions from an HCCI 

Engine,” SAE Paper 2002-01-1758.

Martinez-Frias, J., Aceves, S.M., Flowers, D. L., Smith, J. R., Dibble, R. W., 

“Equivalence Ratio-Egr Control of Hcci Engine Operation and the Potential for Transition 

to Spark-Ignited Operation, “ SAE Paper 2001-01-3613.

Flowers, D. L., Aceves, S. M., Martinez-Frias,J., Smith, J. R., Au, M. Y., Girard, J. W., 

Dibble, R. W., 2001, “Operation of a Four-Cylinder 1.9 L Propane Fueled Homogeneous 

Charge Compression Ignition Engine,” SAE Paper 2001-01-1895.

Page 33 of 104
Daniel L. Flowers 

!(!#

Au, M., Girard, J. W., Dibble, R. W., Seibel, C., Maas, U., Aceves, S. M., Flowers, D. L., 

Martinez-Frias, J., Smith, J. R., 2001, “Four-Cylinder HCCI Engine Operation with

Exhaust Gas Recirculation,” SAE Paper 2001-01-1894.

Flowers, D. L., Aceves, S. M., Smith, J. R., Torres, J., Girard, J., and Dibble, R. W., 

“HCCI in a CFR Engine: Experiments and Detailed Kinetic Modeling,” SAE paper 2000- 

01-0328.

Aceves, S. M., Smith, J. R., Perkins, L. J., Haney, S. W., Flowers, D. L., “Optimization 

of a CNG Series Hybrid Concept Vehicle,” SAE paper 960234.

Other Conference Papers

Killingsworth NJ, Aceves SM, Flowers DL, Krstic M. “A simple HCCI engine model for 

control.” IEEE Conference on Computer Aided Control System Design, 2006 IEEE 

International Conference on Control Applications, 2006 IEEE International Symposium 

on Intelligent Control. IEEE. 2006, pp. 6.

Walther, D.C., Fernandez-Pello, A.C., Dibble, R., Aceves, S.M., Flowers, D. “The use of 

hydrogen combustion for power generation” 3rd International Energy Conversion 

Engineering Conference, v 3, Collection of Technical Papers - 3rd International Energy 

Conversion Engineering Conference, 2005, p 1919-1938.

Flowers, Daniel L., Martinez-Frias, Joel, Espinosa-Loza, Francisco, Killingsworth, Nick, 

Aceves, Salvador M., Dibble, Robert, Kristic, Miroslav, Bining, Avtar, "Development and 

testing of a 6-cylinder HCCI engine for distributed generation," Proceedings of the 2005 

Fall Technical Conference of the ASME Internal Combustion Engine Division, 2005, p 

643-651.

Martinez-Frias, Joel, Flowers, Daniel, Aceves, Salvador M., Espinosa-Loza, Francisco, 

Dibble, Robert, "Thermal management for 6-cylinder HCCI engine: Low cost, high 

efficiency, ultra-low NOx power generation," Proceedings of the 2004 Fall Technical 

Conference of the ASME Internal Combustion Engine Division, 2004, p 833-839.

Martinez-Frias, Joel, Aceves, Salvador M., Flowers, Daniel, Smith, J. Ray, Dibble, 

Robert, “Exhaust energy recovery for control of a homogeneous charge compression 

ignition engine,” American Society of Mechanical Engineers, Advanced Energy Systems 

Division (Publication) AES, v 40, 2000, p 349-356.

Santangelo, P.J. (Univ of California Davis); Flowers, D.; Kennedy, I.M. “Measurements 

of droplet size in the near field of a droplet laden jet using MDR spectroscopy,” Chemical 

and Physical Processes in Combustion, Fall Technical Meeting, The Eastern States 

Section, 1997, p 265.

Thesis and Dissertation

Flowers, D. L, “Combustion in Homogeneous Charge Compression Ignition Engines: 

Experiments and Detailed Chemical Kinetic Simulations” Ph.D. Dissertation, University 

of California, Davis, 2001

Flowers, D. L., “Application of Morphology Dependent Resonance Spectroscopy to 

Droplet Sizing” Masters Thesis, University of California, Davis, 1997.

Page 34 of 104
CURRICULUM VITAE

SHAWN W. MIDLAM-MOHLER, PH.D. 

3938 Norbrook Dr. 

Columbus, Ohio 43220 

(614) 307-4176 

midlam-mohler.1@osu.edu 

EDUCATION 

Engineering Education 

Ph.D. Mechanical Engineering 6/2005 

The Ohio State University Columbus, OH 

Dissertation Title: "Modeling, Control, and Diagnosis of a Diesel Lean NOx Trap Catalyst" 

M.S. Mechanical Engineering 3/2001 

The Ohio State University Columbus, OH 

Thesis Title: "A Novel Fuel-Operated Heater for Automotive Thermal Management" 

B.S. Mechanical Engineering Summa cum Laude 6/1999 

 Wright State University Dayton, OH 

Senior Design Project: “Aerodynamic Design and Simulation of a Wind-Turbine” 

Academic Fellowships 

Graduate Automotive Technology Education Program – Ph.D. Studies Source: Dept. of Energy 

• Awarded to select graduate students conducting research supporting DOE goals for transportation research

University Fellowship – M.S. Studies Source: Ohio State University 

• Awarded in a university-wide search to attract high-caliber graduate students 

RESEARCH 

EXPERIENCE 

Research Appointments 

Research Scientist 10/2008 to present 

Ohio State University Center for Automotive Research, Columbus, OH 

• Conduct research in the area of clean and efficient transportation, including emissions reduction, Diesel 

engines, alternative combustion, hydrogen generation, heavy fuel atomization, and advanced powertrains 

• Directed and advised graduate students in this area of research 

Senior Research Associate 11/2005 to 9/2008 

Ohio State University Center for Automotive Research, Columbus, OH 

• Conducted research in the area of clean and efficient transportation 

• Directed and advised graduate students in this area of research 

Research Associate II 2/2004 to 10/2005 

Ohio State University Center for Automotive Research, Columbus, 

• Conducted research in the area of clean and efficient transportation 

Page 35 of 104
Research Intern 6/2003 to 9/2003 

Ford Scientific Research Labs, Dearborn, MI 

• Conducted research on emissions reductions for gasoline hybrid-electric vehicles 

• Three-month assignment resulted in three Ford invention disclosures and two U.S. patents 

Research Funding 

As a PI or co-PI, Dr. Midlam-Mohler has averaged over a half million dollars in research per year since 2005. 

These projects are identified in the following sections. 

Projects as PI / Co-PI:

$50,000/1 years Title: Analysis of Secondary Powertrain Systems in HEVs Start: 10/2009 

 Source: CAR Industrial Consortium Role: PI 

 

$40,000/0.5 years Title: Life Cycle Analysis of Landfill Derived Natural Gas Start: 4/2009 

 Source: FirmGreen Role: PI 

$99,000/2 year Title: Fleet Studies of Plug-In Electric Hybrid Vehicles Start: 1/2009 

Source: SMART@CAR Consortium Role: PI 

 

$2,000,000/3 years1

 Title: EcoCAR Challenge Hybrid Electric Vehicle Project Start: 6/2008 

 Source: US Department of Energy and numerous other sponsors Role: Co-PI 

$943,108/4 years Title: Coordinated Diesel Engine and Aftertreatment Control Start: 4/2008 

 Source: Cummins Role: PI 

$724,531/3 years Title: Hierarchical Approach to Engine Modeling Start: 4/2007 

 Source: General Motors Role: Co-PI 

 $234,760/2 years Title: Soot Filter Regeneration though External Heat Addition Start: 11/2005 

 Source: Tenneco Automotive Role: Co-PI 

$673,550/3 years Title: On-Board Fuel Reformation for Diesel Aftertreatment Start: 11/2005 

 Source: Tenneco Automotive Role: Co-PI 

Minor Projects as PI/co-PI:

$45,000 Miscellaneous small projects 2009 

 Source: Hi-Stat, Henkel 

$22,500 Miscellaneous small projects 2008 

 Source: National Energy Technology Lab, Nextech Materials 

Projects with Major Research Role (not co-PI):

$940,863/4 years Title: Next Generation Charge Estimation for IC Engines Start: 7/2004 

 Source: General Motors Role: Researcher 

$1,327,954/5 years Title: Next Generation AFR Control for IC Engines Start: 7/2004 

 Source: General Motors Role: Researcher 

 

1

 This is the estimated cost of the research conducted under this problem if funded from an external sponsor. This 

project is heavily leveraged by the Department of Energy, General Motors, Ohio State University, and a number of 

other sponsors through in-kind contributions as well as direct funding and fellowships. 

Page 36 of 104
TEACHING 

EXPERIENCE 

Instructional Appointments 

Adjunct Assistant Professor 7/2009 to present 

Ohio State University Department of Mechanical Engineering, Columbus, OH 

• Granted in recognition of significant educational service to the Mechanical Engineering Department

• Service includes one-on-one student advising, student project advising, and supervision of undergraduate 

research

Instructor 4/2007 to present 

Ohio State University Department of Mechanical Engineering, Columbus, OH 

• Sole instructor of record for two applied thermal and fluids courses on internal combustion engines

Course Development 

ME 631 - Powertrain Laboratory (3 CR) 1/2009 

Ohio State University Department of Mechanical Engineering, Columbus, OH 

• Developed course material for two quarter hours of classroom lecture which reinforced lab work

• Developed eight new lab experiments based on in-depth knowledge of the automotive industry

• Facilitated donation of a gasoline engine from General Motors and a Diesel engine from Cummins, both 

with a calibration system to provide students access to cutting-edge equipment

ME 730 - Internal Combustion Engine Modeling (3 CR) 4/2007 

Ohio State University Department of Mechanical Engineering, Columbus, OH 

• Developed all new lecture material to bring in personnel research experience

• Developed new homework assignments to better engage students by building a fully functioning engine 

model in stages of greater fidelity and complexity

• Facilitated the donation of industry-standard engine simulation software for use by students 

• Developed capstone project which allowed students to become engaged in a topic of interest

Seminar - Alternative Fuels Short Course 1/2007 

Ohio State University Center for Automotive Research Distance Education Program 

• Developed 10 hours of lecture and lecture notes for industrial distance education program

• Provided case studies of alternative-fueled vehicles to reinforce concepts for the industry audience 

Teaching Experience 

ME 631 – Powertrain Laboratory (3 CR) Sole Instructor of Record 1/2010 

Overall Teaching Rating: 5.0/5.0 Class Size: 15 

ME 730 - Internal Combustion Engine Modeling (3 CR) Sole Instructor of Record 4/2009 

Overall Teaching Rating: 4.4/5.0 Class Size: 7 

 

ME 631 – Powertrain Laboratory (3 CR) Sole Instructor of Record 1/2009 

Overall Teaching Rating: 4.8/5.0 Class Size: 12 

ME 730 - Internal Combustion Engine Modeling (3 CR) Sole Instructor of Record 4/2007 

Overall Teaching Rating: 4.5/5.0 Class Size: 8 

Page 37 of 104
Academic Advising 

Since 2005, Dr. Midlam-Mohler has become increasingly involved in student advising. He has served in an 

advisory or supervisory capacity to the following students at the M.S. and Ph.D. level: 

 

Degree Student Role

Graduation Date or 

Expected Graduation Date

Ph.D. Quiming Gong Research Supervisor 2012 

Vis. Scholar Bernhard Grimm Research Supervisor 2010 

M.S. John Davis Co-Advisor 2011 

Ph.D. Jason Meyer Research Supervisor 2011 

Honors B.S. Katherine Bovee Acting Advisor 2010 

Honors B.S. John Davis Acting Advisor 2010 

Honors B.S. Ryan Everett Acting Advisor 2010 

Ph.D. Kenny Follen Research Supervisor 2010 

M.S. Beth Bezaire Acting Advisor 2010 

M.S. Brad Cooley Acting Advisor 2010 

M.S. Chris Hoops Acting Advisor 2010 

M.S. Ming Fang Acting Advisor 2009 

Honors B.S. Chris Hoops Acting Advisor 2009 

M.S. Dave Ortiz Supervisor 2009 

M.S. Rajaram Maringanti Acting Advisor 2009 

M.S. Joshua Supplee Acting Advisor 2009 

Vis. Scholar Adalbert Wolany Supervisor 2009 

Ph.D. Sai Rajagopalan Committee Member 2009 

Ph.D. Sergio Hernandez Acting co-advisor 2008 

Vis. Scholar Andrea Pezzini Supervisor 2008 

Vis. Scholar Patrick Rebechi Supervisor 2008 

Honors B.S. Rhisee Bhatt Acting co-advisor 2007 

Vis. Scholar Simone Bernasconi Supervisor 2007 

M.S. Josh Cowgill Acting co-advisor 2007 

M.S. Kenny Follen Acting co-advisor 2007 

M.S. Courtney Coburn Acting Advisor 2006 

M.S. Adam Vosz Acting Advisor 2006 

M.S. Eric Snyder Acting co-advisor 2005 

Undergraduate Student Research Assistants: 

 

Dr. Midlam-Mohler has supervised the following students on research outside of a formal degree program: 

Degree Student Role Year

B.S. John Macauley Supervisor 2009-10 

B.S. Alixandra Keil Supervisor 2009-10 

B.S. Jennifer Loy Supervisor 2009-10 

B.S. Sean Ewing Supervisor 2009 

B.S. David Griffin Supervisor 2009 

Page 38 of 104
B.S. Ross Wang Supervisor 2009 

B.S. Orlando Inoa Supervisor 2008-09 

B.S. Al Godfrey Supervisor 2008-09 

B.S. John Lutz Supervisor 2008 

B.S. Konrad Svzed Supervisor 2008 

B.S. Joshua Supplee Supervisor 2007 

Mentor for Local High School Students 

Dr. Midlam-Mohler has mentored six local high school students for ~30 hours of activity per student since 2007. 

Student Organization Advising 

EcoCAR Challenge Hybrid Electric Vehicle Team 6/2008 - present

Ohio State University 

• Co-advise 40 member (~80% undergraduate) student design project team competing in U.S. Department 

of Energy sponsored vehicle competition

• Oversee day-to-day operation of team as they design, build, and test a hybrid electric SUV

• Team won 1st place in first year, 4th place in second year

• Nominated by team for “NSF Advisor of the Year Award”

Challenge-X Hybrid Electric Vehicle Team 8/2006 – 6/2008

Ohio State University 

• Co-advised primarily undergraduate team competing in Department of Energy Sponsored advanced 

technology vehicle completion

• Over the course of the four year competition from 2004 – 2008, OSU placed 3rd, 4th, 4th, and 3rd

respectively in the premier advanced technology vehicle competition

Professional Development and Service - Education 

Lecturer for Groups Touring the Ohio State Center for Automotive Research 1/2007 - present

Ohio State University Center for Automotive Research 

• Provide 30 – 60 minute presentation and discussion on topic of energy use in transportation to groups

• Reached over 500 individuals including elementary school students, college students, and community 

groups

OSU Continuing Education Program, Columbus, OH 9/2010

Presenter 

• Provided one hour seminar to practicing engineers on green vehicle design

Lilly Conference on College Teaching 11/2009 

Miami University Teaching Conference 

• Attended three day conference on college teaching 

Teaching at Ohio State Orientation 9/2009 

Ohio State University Faculty Development Workshop 

• Attended the following seminars over the course of two and a half days: Introduction to Teaching and 

Learning; Fair and Efficient Grading; Designing Assignments Quizzes, and Tests; and Seven Habits of 

Effective Teachers - Universal Design for Learning; and Developing Effective Presentation Skills 

OSU Continuing Education Program, Columbus, OH 9/2009

Presenter 

• Provided one hour seminar to practicing engineers on green vehicle design 

Page 39 of 104
Ohio State Mechanical Engineering Curriculum Development Retreat, Columbus, OH 7/2009 

Participant in Design Focus Group 

• Requested by Dept. Chair to serve in Design Focus Group 

• Participated with faculty colleagues and alumni to evaluate engineering curriculum as OSU 

Summer Institute on Course Design 6/2009 

Ohio State University Faculty Development Workshop 

• Attended 15 hour, hands-on seminar on effective course design 

• Learned structured techniques for developing courses 

• Defined course goals, learning objectives, course content, and methods of assessment for a course

PROFESSIONAL 

SERVICE 

Professional Service 

Clean Fuels Ohio, Columbus, OH 9/2009 to present 

Member of the Board of Directors 

• Elected to Board of Directors of Clean Fuels Ohio, a non-profit committed to cleaner transportation fuels 

State of Indiana 4/2009 

Proposal Reviewer 

• Reviewed multi-million dollar proposal for Indiana grant program in area of internal combustion engines 

Natural Gas Fleet Stakeholders Meeting, Grove City, OH 11/2008 

Panel Member 

• Served as panel technical expert on alternative vehicular fuels 

• Meeting attended by designees’ from the Governor’s office and from both of Ohio’s U.S. Senators’ staff 

McMaster Fuel Ltd., Perrysburg, OH 9/2006 to 1/2007 

Independent Consultant 

• Provided analysis of a hydrogen production technique against other methods of hydrogen production 

• Provided analysis of these techniques for emissions reduction 

• Assisted McMaster Fuel Ltd. in making strategic decisions regarding their technology 

Publication Reviewer Continuous

• Review numerous publications for conferences and journal submission of ASME, SAE, IEEE, etc.

 

PUBLICATIONS 

Scholarly Publications 

Journal Articles:

1. J. Meyer, S. Yurkovich, S. Midlam-Mohler, “An Approach for Cylinder Specific AFR Prediction,” in 

preparation for submission to ASME Journal of Dynamic Systems, Measurements, and Controls. 

2. S. Midlam-Mohler, R. Maringanti, M. Fang, “Inverse-Distance Interpolation Methods for Diesel Engine 

Combustion Control,” in preparation for submission to ASME Journal of Dynamic Systems, Measurements, 

and Controls. 

Page 40 of 104
3. M. Canova, S. Midlam-Mohler, P. Pisu, A. Soliman, “Model-Based Fault Detection and Isolation for a Diesel 

Lean NOx Trap Aftertreatment System,” Control Engineering Practice, November 2009. 

4. M. Canova, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, "Mean Value Modeling and Analysis of HCCI 

Diesel Engines with External Mixture Formation,” ASME Journal of Dynamic Systems, Measurement and 

Control, Vol. 131, No. 11, 2009. 

5. M. Canova, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, “Theoretical and Experimental Investigation on 

Diesel HCCI Combustion with External Mixture Preparation,” International Journal of Vehicle Dynamics, 

Volume 44, Nos 1-2, 2007. 

6. N. Szabo, C. Lee, J. Trimboli1, O. Figueroa, R. Ramamoorthy, S. Midlam-Mohler, A. Soliman, H. Verweij, P. 

Dutta and S. Akbar, “Ceramic-Based Chemical Sensors, Probes and Field-Tests in Automobile Engines,” 

Journal of Materials Science, November, 2003. 

Conference Papers:

1. Qi. Gong, S. Midlam-Mohler, V. Marano, G. Rizzoni, Y. Guezennec, “Statistical analysis based PHEV fleet 

data study”, 2010 IEEE Vehicle Power and Propulsion Conference, September, 2010. 

2. Kerem Bayar, Beth Bezaire, Brad Cooley, John Kruckenberg, Eric Schact, Shawn Midlam-Mohler, Giorgio 

Rizzoni, “Design of an Extended-Range Electric Vehicle for the EcoCAR Challenge”, ASME 2010 

International Design Engineering Technical Conference, August, 2010. 

3. J. Meyer, S. Yurkovich, S. Midlam-Mohler, “An AFR Control Architecture Comparison: Phase Lock Loop 

Versus Duty Cycle Control,” 2010 American Controls Conference, June, 2010. 

4. R. Maringanti, S. Midlam-Mohler, M. Fang, F. Chiara, M. Canova, “Set-Point Generation using Kernel-Based 

Methods for Closed-Loop Combustion Control of a CIDI Engine,” ASME DSCC2009, September, 2009. 

5. J. Meyer, S. Rajagopalan, S. Midlam-Mohler, Y. Guezennec, S. Yurkovich, “Application of an Exhaust 

Geometry Based Delay Prediction Modal to an Internal Combustion Engine,” ASME DSCC2009, September, 

2009. 

6. M. Fang, S. Midlam-Mohler, R. Maringanti, F. Chiara, M. Canova, “Optimal Performance of Cylinder-byCylinder and Fuel Bank Controllers for a CIDI Engine,” ASME DSCC2009, September, 2009. 

7. S. Midlam-Mohler, E. Marano, S. Ewing, D. Ortiz, G. Rizzoni, “PHEV Fleet Data Collection and Analysis,” 

IEEE VPPC09, September 2009. 

8. L. Headings, G. Washington, S. Midlam-Mohler, J. Heremans, “Thermoelectric Power Generation for HybridElectric Vehicle Auxiliary Power,” Proc. SPIE Int. Conference on Smart Structures and Materials, 2009, Vol. 

7290, No. 13. 

9. M. Canova, S. Midlam-Mohler, G. Rizzoni, F. Steimle, D. Boland, M. Bargende, “A Simulation Study of an 

E85 Engine APU for a Series Hybrid Electric Vehicle,” 9th Stuttgart International Symposium on Automotive 

and Engine Technology, Stuttgart, Germany, 2009. 

10. S. Rajagopalan, S. Midlam-Mohler, S. Yurkovich, Y. Guezennec, K. Dudek, “Control Oriented Modeling of a 

Three Way Catalyst Coupled with Oxygen Sensors,” ASME Dynamic System and Controls Conference, Ann 

Arbor, MI, 2008. 

11. L. Headings, S. Midlam-Mohler, G. Washington, and J. P. Heremans, “High Temperature Thermoelectric 

Auxiliary Power Unit for Automotive Applications,” ASME Conference on Smart Materials, Adaptive 

Structures and Intelligent Systems, 2008, Paper #610. 

12. K. Sevel, M. Arnett, K. Koprubasi, C. Coburn, M. Shakiba-Heref, K. Bayar, G. Rizzoni, Y. Guezennec, S. 

Midlam-Mohler, “Cleaner Diesel Using Model-Based Design and Advanced Aftertreatment,” SAE 2008-01-

0868, 2008 International Congress, Detroit, MI, April 2008. 

13. K. Dudek, B. Montello, J. Meyer, S. Midlam-Mohler, Y. Guezennec, and S. Yurkovich, “Rapid Engine 

Calibration for Volumetric Efficiency and Residuals by Virtual Engine Mapping,” International Congress on 

Virtual Power Train Creation 2007, Munich, Germany, October 24-25, 2007. 

14. M. Canova, S. Midlam-Mohler, Y. Guezennec, A. Soliman, and G. Rizzoni, “Control-Oriented Modeling of 

NOx Aftertreatment Systems,” SAE ICE’07 Conference, Capri, Italy, September 2007. 

15. M. Canova, F. Chiara, J. Cowgill, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, “Experimental 

Characterization of Mixed-Mode HCCI/DI Combustion on a Common Rail Diesel Engine,” 8th International 

Conference on Engines for Automobile (ICE2007), Capri, Italy. 

16. M. Canova, F. Chiara, M. Flory, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, “Experimental Characterization 

of Mixed Mode HCCI/DI Combustion on a Common Rail Diesel Engine,” submitted to SAE ICE’07 

Conference, Capri, Italy, September 2007. 

Page 41 of 104
17. M. Canova, M. Flory, Y. Guezennec, S. Midlam-Mohler, G. Rizzoni, and F. Chiara, “Dynamics and Control of 

DI and HCCI Combustion in a multi-cylinder Diesel engine,” Paper 44, submitted to 5th IFAC Symposium on 

Advances in Automotive Control, Pajaro Dunes/Seascape, CA, August 2007. 

18. A. Vosz, S. Midlam-Mohler, and Y. Guezennec, “Experimental Investigation of Switching Oxygen Sensor 

Behavior Due to Exhaust Gas Effects,” Proc. of IMECE ’06, Paper IMECE 2006-14915, Chicago, IL, 

November 2006. 

19. S. Midlam-Mohler and Y. Guezennec, “A Temperature-Based Technique for Temporally and Spatially 

Resolved Lean NOx Trap Catalyst NOx Measurements,” Proc. of IMECE ’06, Paper IMECE 2006-14887, 

Chicago, IL, November 2006. 

20. M. Canova, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, L. Garzarella, M. Ghisolfi, and F. Chiara, 

“Experimental Validation for Control-Oriented Modeling of Multi-Cylinder HCCI Diesel Engines,” Proc. of 

IMECE ’06, Paper IMECE 2006-14110, Chicago, IL, November 2006. 

21. A. Soliman, S. Midlam-Mohler, Z. Zou, Y. Guezennec, and G. Rizzoni, “Modeling and Diagnostics of NOx 

Aftertreatment Systems,” Proc. FISITA ’06, Yokohama, Japan, October 2006. 

22. Z. Zou, S. Midlam-Mohler, R. Annamalai, Y. Guezennec, V. Subramaniam, "Literature Survey of On-Board 

Hydrogen Generation Methods for Diesel Powertrains,” Global Powertrain Conference, Novi, MI, Not Peer 

Reviewed, September 2006. 

23. K. Follen, S. Midlam-Mohler, Y. Guezennec, “Diesel Particulate Filter Regeneration with an External Burner,” 

Global Powertrain Conference, Novi, MI, Not Peer Reviewed, September 2006. 

24. S. Midlam-Mohler and Y. Guezennec, “Regeneration Control for a Bypass-Regeneration Lean NOx Trap 

System,” American Control Conference ’06, Minneapolis, MN, Invited paper, June 2006. 

25. A. Soliman, I. Choi, S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, “Modeling and Diagnostics Of NOx AfterTreatment Systems,” SAE Paper 2006-05-0208, 2006 International Congress, Detroit, MI, April 2006. 

26. S. Midlam-Mohler and Y. Guezennec, “Design, Modeling and Validation of a Flame Reformer for LNT 

External By-Pass Regeneration,” SAE Paper 2006-01-1367, 2006 SAE International Congress, Detroit, MI, 

April 2006. 

27. S. Midlam-Mohler, and Y. Guezennec, “Modeling of a Partial Flow Diesel, Lean NOx Trap System,” Proc. of 

IMECE ’05, Paper IMECE 2005-80834, Orlando, FL, November 2005. 

28. M. Canova, L. Garzarella, M. Ghisolfi, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “A Control-Oriented 

Mean-Value Model of HCCI Diesel Engines with External Mixture Formation,” Proc. of IMECE ’05, Paper 

IMECE 2005-79571, Orlando, FL, November 2005. 

29. A. Soliman, P. Jackson, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “Diagnosis of a NOx 

Aftertreatment System,” ICE 2005 7th International Conference on Engines for Automobiles, Capri, Italy, 

September 2005. 

30. M. Canova, L. Garzarella, M. Ghisolfi, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “A Mean-Value 

Model of a Turbo-Charged HCCI Diesel Engine with External Mixture Formation,” ICE 2005 7th International 

Conference on Engines for Automobiles, Capri, Italy, September 2005. 

31. M. Canova, R. Garcin, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “A Control-Oriented Model of 

Combustion Process in HCCI Diesel Engines,” American Control Conference ’05, Portland, OR, June 2005. 

32. C. Musardo, B. Staccia, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “Supervisory Control for NOX 

Reduction of an HEV with a Mixed-Mode HCCI/CIDI Engine,” American Control Conference ’05, Portland, 

OR, June 2005. 

33. M. Canova, A. Vosz, D. Dumbauld, R. Garcin, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “Model and 

Experiments of Diesel Fuel HCCI Combustion with External Mixture Formation,” 6th Stuttgart International 

Symposium on Motor Vehicles and Combustion Engines, Stuttgart, Germany, Not peer reviewed, February 

2005. 

34. S. Midlam-Mohler, S. Haas, Y. Guezennec, M. Bargende, G. Rizzoni, S. Haas, and H. Berner, “Mixed-Mode 

Diesel HCCI/DI with External Mixture Preparation,” Paper F2004V258, Proc. FISITA ’04 World Congress, 

Barcelona, Spain, May 2004. 

35. Y. Guezennec, C. Musardo, B. Staccia, S. Midlam-Mohler, E. Calo, P. Pisu, and G. Rizzoni, “Supervisory 

Control for NOx Reduction of an HEV with a Mixed-Mode HCCI/DI Engine,” Paper F2004F233, Proc. FISITA 

’04 World Congress, Barcelona, Spain, May 2004. 

36. M. Gilstrap, G. Anceau, C. Hubert, M. Keener, S. Midlam-Mohler, K. Stockmeier, J-M Vespasien, Y. 

Guezennec, F. Ohlemacher, and G. Rizzoni, “The 2002 Ohio State University FutureTruck – the 

BuckHybrid002,” 2003 SAE International Congress and Exposition, Detroit, MI, March 2003. 

Page 42 of 104
S. Midlam-Mohler C.V. Page | 9 

37. Y. Guezennec, S. Midlam-Mohler, M. Tateno, and M, Hopka, “A 2-Stage Approach to Diesel Emission 

Management in Diesel Hybrid Electric Vehicles,” Proc. 2002 IFAC Meeting, Barcelona, Spain, July 2002. 

38. M. Hopka, A. Brahma, Q. Ma, S. Midlam-Mohler, G. Paganelli, Y. Guezennec, and G. Rizzoni, “Design, 

Development and Performance of Buckeyebrid: The Ohio State Hybrid Electric FutureTruck 2001,” SAE SP1701, Not peer reviewed, March 2002. 

Scholarly Presentations Independent of Paper Publications:

1. S. Midlam-Mohler and Y. Guezennec, “Lean NOx Trap Modeling Based on Novel Measurement Techniques,” 

CLEERS Conference Workshop 3, Not peer reviewed, May 4, 2006. 

2. S. Midlam-Mohler, and Y. Guezennec, “Design, Modeling and Validation of a Flame Reformer for LNT 

External By-Pass Regeneration,” 2005 DEER Conference, Chicago, IL, Not peer reviewed, August 2005. 

3. M. Canova, S. Midlam-Mohler, Y. Guezennec, and G. Rizzoni, “Control-Oriented Modeling of HCCI 

Combustion,” 2005 DEER Conference, Chicago, IL, Not peer reviewed, August 2005. 

4. S. Midlam-Mohler and Y. Guezennec, 2004 DEER Conference, San Diego, CA, Not peer reviewed, August 

2004. 

5. S. Midlam-Mohler, Y. Guezennec, G. Rizzoni, M. Bargende, and S. Haas, “Mixed-Mode Diesel HCCI with 

External Mixture Preparation,” 2003 DEER Conference, Newport, R. I., Not peer reviewed, August 2003. 

6. S. Midlam-Mohler, Y. Guezennec, “An Active, Thermo-Chemically Managed Diesel NOx After-Treatment 

System,” CLEERS Conference Workshop 2, Not peer reviewed, October 11, 2001. 

Intellectual Property Activity 

Issued Patents:

1. S. Midlam-Mohler, B. Masterson, "System System for Controlling NOx Emissions During Restarts of Hybrid 

and Conventional Vehicles,” U.S. Patent 7,257,493, awarded 3/21/07. 

2. S. Midlam-Mohler, "System and Method for Reducing NOx Emissions after Fuel Cut-Off Events,” U.S. Patent 

7,051,514, awarded 5/30/06. 

Patent Applications:

1. S. Liu, K. Dudek, S. Rajagopalan, S. Yurkovich, Y. Hu, Y. Guezennec, S. Midlam-Mohler, “Off-Line 

Calibration of Universal Tracking Air Fuel Ratio Regulators,” U.S. Patent Application 20090271093, 

10/29/2009. 

2. S. Rajagopalan, K. Dudek, S. Liu, S. Yurkovich, S. Midlam-Mohler, Y. Guezennec, Y. Hu, “Universal 

Tracking Air-Fuel Regulator for Internal Combustion Engines, U.S. Patent Application 20090266052, 

10/29/2009. 

3. K. Dudek, S. Rajagopalan, S. Yurkovich, Y. Guezennec, S. Midlam-Mohler, L. Avallone, I. Anilovich, “Air 

Fuel Ratio Control System for Internal Combustion Engines,” U.S. Patent Application 20090048766, 

2/19/2009. 

4. Y. Guezennec and S. Midlam-Mohler, Shawn, “Fuel Preparation System for Combustion Engines, Fuel 

Reformers and Engine Aftertreatment,” U. S. Patent Application 20040124259, 7/1/04 

5. S. Midlam-Mohler and B. Masterson, "System and Methods for the Reduction of NOx Emissions after Fuel 

Cut-Off Events,” U.S. Patent application 20060021326, filed 2/2/03. 

6. S. Midlam-Mohler and B. Masterson, "Strategy for Controlling NOx Emissions During Hot Restarts for Hybrid 

and Conventional Vehicles,” U.S. Patent Application 20060021330, filed 2/2/03. 

Patent Applications in Preparation:

1. J. Meyer, S. Midlam-Mohler, K. Dudek, S. Yurkovich, Y. Guezennec, Topic: Engine emissions control, Status: 

submitted to patent office 9/09. 

2. J. Meyer, S. Midlam-Mohler, K. Dudek, S. Yurkovich, Y. Guezennec, Topic: Engine emissions control, Status: 

submitted to patent office 9/09. 

3. S. Midlam-Mohler, S. Rajagopalan, K. Dudek, S. Yurkovich, Y. Guezennec, Topic: Catalyst modeling for 

improved emissions control, Status: Patent application being prepared by outside counsel. 

Page 43 of 104
ADDRESS: 2373 LESLIE CIRCLE, ANN ARBOR, MI 48105 • PHONE: 787-475-0241 • EMAIL: EORTIZSO@UMICH.EDU

1 

Curriculum Vitae 

ELLIOTT ORTIZ-SOTO 

EDUCATION

UNIVERSITY OF MICHIGAN – ANN ARBOR (U – M) In Progress Ann Arbor, MI 

PhD Pre-Candidate in Mechanical Engineering (4

th

 Year) 

Relevant Graduate Coursework: Turbulent Combustion, Turbulent Flow, Combustion Processes, Advanced Internal 

Combustion Engines, Hybrid Electric Vehicles, Gas Turbine Propulsion, Advanced Heat Transfer, Advanced Fluid 

Mechanics, Advanced Thermodynamics, Computational Fluid Dynamics, Internal Combustion Engines, Heat 

Transfer Physics, Partial Differential Equations, Probability & Statistics

UNIVERSITY OF MICHIGAN – ANN ARBOR May 2010 Ann Arbor, MI 

Master of Science in Mechanical Engineering 

Thesis: Dual-Mode SI-HCCI Operation for Improved Drive-Cycle Fuel Economy: Modeling Framework 

Development and Implementation in Comparative Fuel-Economy Study 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY (MIT) June 2006 Cambridge, MA 

Bachelor of Science in Mechanical Engineering GPA: 4.2/5.0

Language Concentration in German 

Thesis: Design of Oil Consumption Measuring System to Determine the Effects of Evolving Oil Sump 

Composition over Time on Diesel Engine Performance and Emissions 

RESEARCH 

WALTER E. LAY AUTOMOTIVE LAB (U – M) Fall 2007 – Present Ann Arbor, MI 

 Researching the physics behind novel combustion approaches, involving high pressures, ultra high dilution, 

spark-assisted compression ignition (SACI) and alternative fuels, and began combustion modeling and coding 

work for the implementation in GT-Power as user-developed subroutines. 

 Developed complete heat release analysis program in Matlab for improved experimental heat release analysis of 

multi-mode combustion engines and future combustion model development. 

Increased computational speed and functionality through full Matlab implementation 

Superior accuracy in temperature, heat transfer and heat release calculations through: 

o Better properties estimation using in-house properties and equilibrium functions (based on 

JANAF tables). 

o Updated residual estimation techniques for unconventional valve actuation strategies. 

o Single-zone and two-zone heat release analysis options to account for various combustion modes. 

Fully functional Matlab GUI for enhanced utility and ease of use. 

 Developed complete modeling and simulation framework for fuel-economy evaluation and mode transition 

studies of Dual-Mode SI-HCCI engines involving: 

Detailed system-level engine models of spark-ignition (SI) and HCCI engines using GT-Power 

Experimental validation of engine, combustion, heat transfer, knock, and emissions submodels based on 

Fully-Flexible Valve Actuation Engine at the U-M Auto Lab. 

Full range SI and HCCI engine operating map generation using Design of Experiments optimization 

Flexible architecture vehicle model using a coupled GT-Suite/Simulink approach for intuitive physical 

modeling and improved controls development 

Drive-cycle simulations to assess real fuel-economy benefits of Dual-Mode SI-HCCI operation over 

conventional SI engines 

 Performed simulation study exploring the potential synergy between the HCCI engine system and three hybrid 

electric vehicle (HEV) configurations, proposed the supervisory control strategy to maximize the benefits 

combining the two technologies. 

Page 44 of 104
2 

Developed Matlab/Simulink conventional, split-hybrid and parallel-hybrid vehicle models 

Implemented fuel-consumption maps for SI and HCCI engines and created bsfc-optimized shifting 

strategies for each engine operating mode. 

Developed rule-based control strategy to maximize HCCI engine operation and minimize mode 

transitions 

 Developed new HCCI engine cycle simulation using a zero-dimensional thermodynamic combustion approach 

with detailed chemical kinetics within the Cantera-Matlab environment, and investigated the effects of engine 

speed, fueling and variable valve actuation on ignition timing

 Proposed practical design to achieve constant-volume combustion using advanced split-cycle engine concept and 

performed a modeling study to compare efficiency benefits over conventional and other split-cycle engines. 

OAK RIDGE NATIONAL LABORATORY (ORNL) Summer 2010 Oak Ridge, TN 

Fuels, Engines and Emissions Research Center (FEERC)

 Started work on improved experimental engine heat release analysis program for in-depth evaluation of multimode combustion, model development and validation. 

 Researched current state-of-the-art flame propagation and chemical kinetics models for SI and HCCI 

combustion simulation, and evaluated their possible implementation as simplified models for system-level 

simulations. 

OAK RIDGE NATIONAL LABORATORY (ORNL) Summer 2009 Oak Ridge, TN 

Fuels, Engines and Emissions Research Center (FEERC)

 Began work on comprehensive, physics-based Spark-Assisted HCCI model for use in system-level simulations. 

 Presented in detail components and implementation of the U-M HCCI Combustion correlation. 

 Developed improved GT-Power engine model of experimental single-cylinder engine with fully-flexible valve 

actuation capable of multi-mode SI and HCCI operation. 

 Performed validation study of engine and combustion models with available experimental data. 

SLOAN AUTOMOTIVE LABORATORY (MIT) Fall 2005 – Spring 2006 Cambridge, MA 

 Set up experimental single-cylinder diesel engine for emissions and oil consumption studies 

 Studied formation and evolution of inorganic emissions from different diesel fuel compositions and evaluated its 

effect on diesel particulate filter performance 

WORK EXPERIENCE

M RACING Fall 2009 – Present Ann Arbor, MI 

Formula SAE Powertrain Division

 Serving as experienced modeling consultant for development of improved engine model in GT-Power. 

 Current engine model capable of reproducing similar experimental engine behavior; expected improvements with 

further model enhancements in near future. 

FORD MOTOR COMPANY Summer 2008 Dearborn, MI 

Intern – Transmission/Driveline Research & Advanced Engineering 

 Performed hydraulic, transmission and vehicle level simulations (Matlab/Simulink & Ford Software), validated 

models with experimental data for Stop-Start w/ Assisted Direct Start (Micro-Hybrid) technology development. 

 Studied formation and evolution of inorganic emissions from different diesel fuel compositions and evaluated its 

effect on diesel particulate filter performance. 

FORD MOTOR COMPANY Summer 2007 Livonia, MI 

Intern – Automatic Transmission New Product Center (Electro-Hydraulic Components) 

 Assessed theoretical performance of competitive 6-speed automatic transmission pumps. 

 Established target comparison metrics and presented preliminary data suggesting design improvements for 

increased efficiency. 

ZF FRIEDRICHSHAFEN AG July 2006 – December 2006 Friedrichshafen, Germany 

Intern – Automatic Transmission New Product Center (Electro-Hydraulic Components)

Page 45 of 104
3 

 Worked on new simulation approaches with Dymola (Modelica) and prepared training material for new users. 

 Researched new control techniques for disturbance reduction in future hybrid transmission systems. 

 Optimized powertrain/vehicle level models for real-time simulations (DSpace) used in pre-development and 

serial production projects. 

FORD MOTOR COMPANY Summer 2005 Dearborn, MI 

Intern – Automatic Transmission New Product Center (Electro-Hydraulic Components)

 Tested and analyzed competitive air induction system performance in environmental wind tunnels. 

 Presented data to recommend and support possible air induction system redesign/placement. 

MIT MOTORSPORTS Fall 2005 – Spring 2006 Cambridge, MA 

Formula SAE Powertrain Division

 Redesigned complete formula race car air induction system. 

PUBLICATIONS

 Ortiz-Soto, E., Babajimopoulos, A., Lavoie, and G., Assanis, D., “A Comprehensive Engine to Drive-Cycle 

Modeling Framework for the Evaluation of Future Engine and Combustion Tehcnologies,” International Journal 

of Engine Research (IJER). (Submitted) 

 Lawler, B., Ortiz-Soto, E., Gupta, R., Peng, H., and Filipi, Z.S, “Hybrid Electric Vehicle Powertrain and Control 

Strategy Optimizatino to Maximize the Synergy with a Gasoline HCCI Engine,” SAE Paper 11PFL-0963. 

(Submitted) 

 Delorme, A., Rousseau, A., Wallner, T., Ortiz-Soto, E., Babajimopoulos, A., and Assanis, D., “Evaluation of 

Homogeneous Charge Compression Ignition (HCCI) Engine Fuel Savings for Various Electric Drive 

Powertrains,” The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition, 

Shenzhen, China, November 5-9, 2010. 

PRESENTATIONS

 Ortiz-Soto, E., Babajimopoulos, A., Lavoie, G., and Assanis, D., “Dual-Mode SI-HCCI Operation for Improved 

Drive-Cycle Fuel Economy: Engine Modeling and Map Generation Framework,” USCAR, May 12, 2010 

 Ortiz-Soto, E., Babajimopoulos, A., Lavoie, G., and Assanis, D., “Dual-Mode SI-HCCI Operation for Improved 

Drive-Cycle Fuel Economy: Modeling Framework Development,” Low Temperature Combustion (LTC) 

University Consortium Meeting, USCAR, October 7-8, 2010 

PROFESSIONAL DEVELOPMENT

 GT-Power Advanced Training Seminar, November 2010 

 Direction in Engine-Efficiency and Emissions Research (DEER) Conference, September 2010 

 High-Pressure Lean Burn (HPLB) Consortium Meeting, USCAR, October 2010 

 Princeton-CEFRC Summer Program on Combustion: 2010 Session, June 27 – July 3, 2010 

 Low Temperature Combustion (LTC) Consortium Meeting, USCAR, October 2009 

 WINDPOWER 2009 Conference, May 2009 

 Society of Automotive Engineering (SAE) World Congress, April 2008 

 High Performance Engine Design and Development Seminar, April 2008 

 U-M Graduate School Recruiter @ SHPE Conference, October 2007 

 Certified Engineering in Training (E.I.T) 

AWARDS & ACHIEVEMENTS

 2010 and 2009 ScholarPOWER Award for Master’s Student Achievement, University of Michigan – Ann Arbor 

 Awarded GEM Fellowship for PhD (2009) and Master’s (2007) studies in Mechanical Engineering 

 2006 and 2005 Lufthansa Award for Excellence in German Studies, Massachusetts Institute of Technology 

PROFESSIONAL ORGANIZATIONS

 Society of Automotive Engineers (SAE) 

Page 46 of 104
4 

 American Society of Mechanical Engineers (ASME) 

 Society of Hispanic Professional Engineers (SHPE) 

 Alliance for Graduate Education and the Professoriate (AGEP) 

 Latino Engineering Graduate Organization (LEGO) 

SKILLS

 Languages: Fully bilingual and bicultural (English and Spanish). Fluent in German. 

 Computer: Matlab/Simulink, GT-Power/GT-Suite, Fortran, Mathematica, Fluent, SolidWorks and MS Office 

Page 47 of 104
Aris Babajimopoulos 

2325 Leslie Circle 

Ann Arbor, MI 48105 

12/22/2010 

Tony Lentz 

RTI International 

3040 Cornwallis Road 

RTP, NC 27709 

Dear Mr. Lentz, 

Enclosed is my review of the EPA GEM model. In reviewing the material, I did not 

encounter any real or perceived conflicts of interest. Please note that this review was 

conducted outside of my normal job duties as an Asst. Research Scientist at the W.E. Lay 

Automotive Laboratory of the University of Michigan. 

I appreciate the opportunity to review the EPA GEM model and hope that my comments 

are helpful. I would be happy to address any questions or concerns that may arise. 

Sincerely, 

Aris Babajimopoulos, PhD 

enclosure 

Page 48 of 104
Daniel'L.'Flowers,'Ph.D.

San'Leandro,'CA'94577

dlfenergyconsulting@gmail.com

12/27/2010

Tony'Lentz

RTI'International

3040'Cornwallis'Road

RTP,'NC'27709

Mr.'Lentz,

Enclosed'is'my'review'of'the'EPA'GEM'model.''In'reviewing'the'material,'I'did'not'encounter'any'real'or'

perceived'conflicts'of'interest.''This'review'was'conducted'as'a'private'consultant'outside'of'my'normal'

job'duties'as'a'member'of'the'technical'staff'at'Lawrence'Livermore'National'Laboratory.'

I'appreciate'the'opportunity'to'review'the'model'and'hope'that'my'comments'are'helpful'to'the'review'

process.

Sincerely,

Daniel'L.'Flowers

enclosure

Page 49 of 104
3938 Norbrook Drive

Columbus, Ohio 43220

12/11/2010

Tony Lentz

RTI International

3040 Cornwallis Road

RTP, NC 27709

Mr. Lentz,

Enclosed is a review of the EPA GEM model.  In reviewing the material, I did not encounter any real or

perceived conflicts of interest.    This review was conducted outside of my normal job duties as a

Research Scientist at the Ohio State University Center for Automotive Research; however, my

experience from this position was invaluable for conducting the review.

I appreciate the opportunity to review the model and hope that my comments are helpful to the review

process.

Sincerely,

Shawn Midlam‐Mohler

enclosure

Page 50 of 104
2373 Leslie Circle

Ann Arbor, MI 48105

01/11/2011

Tony Lentz

RTI International

3040 Cornwallis Road

RTP, NC 27709

Mr. Lentz,

Enclosed is a review of the EPA’s Greenhouse‐Gas Emissions Model (GEM).  During the review process, I

did not encounter any real or perceived conflicts of interest.  This peer review was conducted outside of

my normal job duties as a Research Assistant at the University of Michigan Walter E. Lay Automotive

Laboratory; however, my work at the lab has provided me with the knowledge and experience that was

indispensable for conducting the review.

I appreciate the opportunity to become part of the reviewer team and hope this review provides some

useful feedback in the development and improvement of the GEM compliance simulation tool.

Best regards,

Elliott Ortiz‐Soto

enclosure

Page 51 of 104
Review of Greenhouse gas Emissions Model (GEM) 

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Page 52 of 104
Introduction 


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Main model components and assumptions 



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Page 53 of 104
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electric.acc.power engine.acc.power 

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General comments on the Matlab code 

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Page 54 of 104
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load('drive_cycles\ARB_Transient')ǡ

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General comments on the GEM user guide 


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GEM download, installation and execution 

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Comments on GEM executable 

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Page 55 of 104
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GEM as a tool for determining vehicle compliance 




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Page 57 of 104
Table 1. Effects of the variation of various model parameters on the simulation 

results for a baseline vehicle (Class 8 D Sleeper cab D high roof, MY 2010) 

Baseline No electrical 

accessories

Electrical 

acc. power 

added to 

mechanical 

acc. power

Final drive 

equal to 2.77 

instead of 

2.64

Air density 

equal to 

1.205 instead 

of 1.1071

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 [lbs]

extendedIdleReductionLabel

Fuel Consumption for Entire Cycle [mpg] 3.51 3.51 3.49 3.38 3.49

CO2 Emissions [g/ton-mile] 152.86 152.52 153.64 158.68 153.41

Fuel Consumption during Steady State [mpg] 7.40 7.43 7.41 7.28 7.15

CO2 Emissions [g/ton-mile] 72.38 72.14 72.32 73.59 74.98

Fuel Consumption during Steady State [mpg] 6.19 6.21 6.20 6.01 5.91

CO2 Emissions [g/ton-mile] 86.52 86.22 86.40 89.22 90.67

Weighted Fuel Consumption [mpg] 6.17 6.19 6.17 5.99 5.90

--> in gal/1000 ton-mile 8.70 8.67 8.69 8.97 9.08

Weighted CO2 Emission [g/ton-mile] 88.57 88.27 88.49 91.29 92.40

N/A

N/A

Model Inputs

Transient Cycle Simulation

55 mph Steady-State Cycle Simulation

65 mph Steady-State Cycle Simulation

Cycle-Weighted Results

0.69

7.8

8.2

N/A

Page 58 of 104
Table 2. Impact of Cd and vehicle speed limiter on the simulation results for a 

Heavy Heavy-Duty D Vocational Truck (Class 8) 

Case 1 Case 2 Case 3

Coefficient of Aerodynamic Drag 0.8 0.6 0.8

Steer Tire Rolling Resistance [kg/metric ton] 9 9 9

Drive Tire Rolling Resistance [kg/metric ton] 9 9 9

Vehicle Speed Limiter [mph] 65 65 55

Vehicle Weight Reduction [lbs] 0 0 0

extendedIdleReductionLabel 0 0 0

Percent Time Missed by 2mph [%] 1.51 1.5 1.51

Fuel Consumption for Entire Cycle [mpg] 3.51 3.55 3.51

CO2 Emissions [g/ton-mile] 152.74 150.89 152.74

Percent Time Missed by 2mph [%] 0.23 0 0.23

Fuel Consumption during Steady State [mpg] 6.47 7.24 6.47

CO2 Emissions [g/ton-mile] 82.75 74.04 82.75

Percent Time Missed by 2mph [%] 0 0 0

Fuel Consumption during Steady State [mpg] 5.34 6.18 6.48

CO2 Emissions [g/ton-mile] 100.41 86.69 82.75

Weighted Fuel Consumption [mpg] 4.81 5.3 5.23

--> in gal/1000 ton-mile 11.66 10.9 11.02

Weighted CO2 Emission [g/ton-mile] 118.68 111 112.15

Cycle-Weighted Results

Model Inputs

Transient Cycle Simulation

55 mph Steady-State Cycle Simulation

65 mph Steady-State Cycle Simulation

Page 59 of 104
Review&of&EPA&GEM&model&for&as&a&tool&for&evaluation&of&medium&and&heavy9

duty&vehicle&GHG&emissions&

Daniel!L.!Flowers,!Ph.D.

danflowers@gmail.com!

19!Dec!2010

This!report!reviews!the!methodology!developed!by!EPA!for!evaluating!greenhouse!

gas!(GHG)!emissions!reductions!from!medium!and!heavyIduty!road!vehicles![1].!!

This!model!focuses!on!GHG!emissions!improvements!based!on!vehicle!drag!

reduction!and!rolling!resistance!reduction,!and!would!be!used!by!EPA!as!a!

regulatory!tool!to!evaluate!compliance!by!vehicle!manufacturers.!!!

In!general,!the!goal!of!this!program!is!to!provide!a!framework!for!fairly!evaluating!

GHG!emissions!from!medium!and!heavyIduty!vehicles![2].!Thus,!a!key!mission!of!

this!review!is!evaluating!how!well!the!modeling!approach!developed!serves!as!a!

regulatory!and!compliance!tool.!!

We!reviewers!have!been!asked!to!comment!on!the!model!with!regard!to!5!specific!

items:!

1) EPA’s!overall!approach!to!the!stated!purpose!of!the!model!(meet!agencies’!

compliance!requirements)!and!whether!the!particular!attributes!found!in!

resulting!model!embodies!that!purpose.!

2) The!appropriateness!and!completeness!of!the!contents!of!the!overall!model!

structure!and!its!individual!systems,!and!their!component!models,!if!

applicable!(i.e.,!using!the!MATLAB/Simulink!version),!such!as:

a) The!elements!of!each!system!to!describe!different!vehicle!categories;!

b) The!performance!of!each!component!model,!including!the!reviewer’s!

assessment!of!the!underlying!equations!and/or!physical!principles!

coded!into!that!component.!

c) The!input!and!output!structures!and!how!they!interface!with!the!

model!to!obtain!the!expected!result,!i.e.,!fuel!consumption!and!CO2!

over!the!given!driving!cycles;!and

d) The!default!values!used!for!the!input!file,!as!shown!in!the!GEM!User!

Guide.

3) Using!the!standard!of!good!engineering!judgment,!the!program!execution!is!

optimized!by!the!chosen!methodologies;!

4) Clarity,!completeness!and!accuracy!of!the!output/results!(CO2!emissions!or!

fuel!efficiency!output!file);!and

Page 60 of 104
5) Any!recommendations!for!specific!improvements!to!the!functioning!or!the!

quality!of!the!outputs!of!the!model.!

Detailed!discussion!of!each!of!these!items!will!be!described!in!the!following!sections.!

Item&1)&Overall&approach

Based!on!the!description!of!the!proposed!use!in!rulemaking!(EPAI420IDI10I901)!

[1],!the!overall!approach!is!to!provide!a!neutral!framework!upon!which!different!

vehicles!from!different!manufacturers!can!be!compared.!!The!idea!of!this!approach!

is!to!eliminate!manufacturer!differences!by!looking!only!at!the!external!vehicle!loss!

characteristics:!drag!coefficient!and!coefficients!of!rolling!resistance.!!For!vehicles!

from!different!manufacturers!in!each!regulatory!subcategory,!there!are!several!

assumptions!made!about!the!vehicle!characteristics:!

1. The!frontal!area!is!the!same!

2. Accessory!power!required!is!the!same!

3. Vehicle!mass!is!the!same!

4. Distribution!of!weight!on!drive,!steering,!and!trailer!tires!is!the!same!

5. The!engine!is!the!same!

6. The!transmission!and!driveline!losses!are!the!same!

For!a!regulatory!subcategory!of!vehicles!(e.g.!Class!8!Sleeper!Cab!High!Roof),

assumptions!1,!2,!and!3!are!very!reasonable.!!Frontal!area!is!likely!very!similar!for!

subcategory!vehicles,!and!vehicle!mass!is!likely!to!be!similar!based!on!gross!vehicle!

regulated!weight.!!Accessory!loads!vary!from!truck!to!truck!and!applicationItoI

application,!so!constant!accessory!load!for!all!is!a!reasonable!approximation.!

Assumptions!4,!5,!and!6!are!not!necessarily!fully!justified.!!With!regard!to!

assumption!4,!for!nonIvocational!trucks,!the!overall!rolling!resistance!is!specified!as!

42.5%!trailer,!42.5%!drive!wheels,!and!15%!steering!wheels.!!This!has!a!potential!to!

penalize!a!vehicle!that!has!reduced!cab!mass!and!biased!the!load!towards!the!trailer.!!

However,!it!is!likely!to!be!a!small!effect!and!does!not!seem!likely!to!be!frequently!

significant.

Assumptions!5!and!6!are!more!problematic.!!The!engine!and!transmission!can!be!

suitably!sized!to!the!load!characteristics.!!In!this!case,!the!engine!and!transmission!is!

not!optimized!to!the!vehicle.!!This!issue!will!be!discussed!quantitatively!and!in!

greater!detail!in!the!next!section!of!this!review.!!Consider!a!Class!8!tractor!with!a!

drag!coefficient!of!0.69!that!has!the!engine!optimally!sized!for!the!engine!and!

transmission!on!the!drive!cycle.!!Reducing!the!drag!coefficient!by!13%!to!0.60!will!

reduce!the!load!requirements,!shifting!the!operation!to!lower!load!on!the!engine.!!

Diesel!engine!achieve!highest!efficiency!at!highest!load!and!efficiency!decreases!with!

decreasing!load.!!Thus!the!lower!drag!vehicle!may!operate!on!a!lower!efficiency!part!

of!the!engine!map.!

Page 61 of 104
In!practice,!the!engine!and!transmission!can!be!appropriately!sized!to!best!take!

advantage!of!the!reduced!overall!vehicle!load.!!!By!requiring!only!one!engine!and!

transmission!be!used,!drag!reduction!efforts!could!be!penalized.!

The!danger!exists!that!the!manufacturers!would!be!encouraged!to!optimize!vehicles!

to!meet!the!characteristics!that!will!give!the!best!performance!with!the!simulation!

tool,!instead!of!optimizing!the!vehicle!to!achieve!the!true!goals!of!reducing!fuel!

consumption!and!GHG!emissions.!

Overall,!the!concept!of!using!a!generic!vehicle!model!has!merit!to!limit!the!need!to!

test!the!myriad!possible!vehicle!configurations.!!The!use!of!a!generic!powertrain!

(engine!and!transmission)!is!problematic!because!a!wellIintegrated!powertrain!can!

significantly!improve!vehicle!performance.!

Item&2)&Functional&aspects&of&the&overall&model&and&model&components&

This!section!focuses!on!verification!that!the!model!works!as!expected,!as!well!as!

how!the!model!parameters!and!components!affect!the!prediction!of!fuel!

consumption!and!GHG!emissions!in!context!of!regulatory!use.!!The!first!step!is!a!

sanity!check!on!the!results!of!the!model!compared!with!direct!calculation.!

Determining!fuel!consumption!analytically!requires!working!backwards!from!the!

forces!and!accelerations!on!the!vehicle!to!the!engine!fuel!consumption!map.!!

Equation!1!shows!gross!engine!power!in!terms!of!vehicle!parameters!based!on!

working!backwards!from!the!forces!on!the!vehicle.!!!The!full!derivation!with!

description!of!the!parameters!is!in!the!appendix.

€

Pengine,gross = 1

ηtr

crrmgcos(β)V +

1

2

ρcd AfV 3 + mgsin(β)V + maV + Ik

 ̇  ̇ θ k ̇ ( ) θk

k=1

N

∑ '

(

)

*

+

, + Pacc !!(1)

At!constant!speed!and!zero!grade,!the!net!acceleration!and!gravity!terms!become!

zero.

€

Pengine,gross = 1

ηtr

crrmgV +

1

2

ρcd AfV 3 $

%

& '

(

) + Pacc (2)

Table!1!below!shows!a!comparison!between!the!output!of!the!GEM!simulation!

model!and!torque!based!on!calculating!equation!(2)!for!the!same!parameters.!!The!

vehicle!configuration!used!for!Table!1!is!from!the!GEM!manual!for!the!“Class!8!

Combination!I!Sleeper!Cab!I!High!Roof![ref].”!!Torque!is!compared!for!the!constant!

speed!portions!of!the!55!mph!drive!cycle!and!the!65!mph!drive!cycle.!!

The!GEM!simulation!code!calculates!engine!torque!and!speed,!not!power!directly.!!In!

Table!1!the!engine!speed!from!the!GEM!simulation!is!used!with!the!analytically!

determined!power!to!determine!analytical!engine!torque.

Page 62 of 104
Table&1&–&Comparison&of&GEM&simulation&predictions&to&the&calculations&based&on&equations&

derived&in&Appendix&1.&&Comparisons&are&for&“veh_type(i_sim)&=&1”&“Class&8&Combination&9&

Sleeper&Cab&9&High&Roof.”&&The&engine&is&the&first&map&“veh_year=1”&in&engine_map_455.m

Property Units 55&mph 65&mph Source

Air!density kg/m^3 1.1071 1.1071 GEM!model:!ambient_param.m

Gravitational!acceleration m/s^2 9.8066 9.8066 GEM!model:!ambient_param.m

Vehicle!frontal!area m^2 9.8 9.8 GEM!model:!run_preproc.m

Vehicle!static!mass kg 31978 31978 GEM!model:!run_preproc.m

Drag!coefficient

No!

units 0.69 0.69 Input

Drive!wheels!coefficient!of!

rolling!resistance

No!

units 0.0082 0.0082 Input

Steer!wheels!coefficient!of!

rolling!resistance

No!

units 0.0078 0.0078 Input

Trailer!wheels!coefficient!of!

rolling!resistance

No!

units 0.006 0.006 GEM!model:!run_preproc.m

Rolling!resistance!fraction!

from!drive!wheels

No!

units 0.425 0.425 GEM!model:!run_preproc.m

Rolling!resistance!fraction!

from!steer!wheels

No!

units 0.15 0.15 GEM!model:!run_preproc.m

Rolling!resistance!fraction!

from!trailer!wheels

No!

units 0.425 0.425 GEM!model:!run_preproc.m

Net!coefficient!of!rolling!

resistance

No!

units 0.007205 0.007205 Calculated

Mechanical!Accessory!Power W 1000 1000 GEM!model:!run_preproc.m

Electrical!Accessory!Power W 360 360 GEM!model:!run_preproc.m

Vehicle!Speed Mph 55 65

GEM!model:!specified!by!drive!cycle!

(Mild_55_mph.mat,!Mild_65_mph.mat)

Vehicle!Speed m/s 24.6 29.1 Calculated

Vehicle!acceleration m/s^2 0 0

GEM!model:!constant!speed!section!of!drive!cycle!

used!for!analysis

Vehicle!driving!grade Degrees 0 0

GEM!model:!specified!by!drive!cycle!

(Mild_55_mph.mat,!Mild_65_mph.mat)

Aerodynamic!force!on!vehicle N 2262.8 3160.5 Calculated

rolling!resistance!force!on!

vehicle N 2259.5 2259.5 Calculated

Total!resistive!force!on!vehicle N 4522.3 5419.9 Calculated

Vehicle!power!requirement kW 111.2 157.5 Calculated

Engine!speed Rpm 1266.5 1495.6

Output!from!GEM!Model:!Simulink!model!

"GEM_manual_v1/!engine/!engine/!engine_fuel_flow"

Transmission!efficiency

No

units 0.98 0.98

Output!from!GEM!Model:!Simulink!model!

"GEM_manual_v1/!transmission/!gear/!

gear_engaged"!

Engine!Power!required kW 114.8 162.!1 Calculated

Engine!Torque NIm 865.7 1034.8 Calculated!(using!engine!speed!from!GEM!simulation)

Engine!Torque! NIm 892.5 1066.2

Output!from!GEM!Model:!Simulink!model!

"GEM_manual_v1/!engine/!engine/!engine_fuel_flow"!

Difference!in!analytical!versus!

GEM!simulated!torque % 3.0 3.0 Calculated

Page 63 of 104
For!this!case,!the!analytical!torque!is!3%!lower!than!the!torque!determined!by!the!

GEM!simulation!model.!!A!possible!explanation!of!this!discrepancy!may!come!from!

the!formulation!of!the!GEM!model.!!In!the!GEM!model!the!desired!vehicle!speed!is!

specified!and!the!vehicle!dynamic!system!responds!to!try!to!meet!that!by!providing!

needed!engine!torque.!!The!vehicle!speed!is!calculated!in!the!subroutine!of!the!

Simulink!Model!“GEM_manual_v1/vehicle/chassis/vehicle_speed”!as!shown!in!

Figure!2.

Figure&1&9&Section&of&the&GEM&Simulink&model&where&vehicle&speed&is&calculated.

The!vehicle!speed!comes!from!integrating!the!force!balance.!

€

F = meff a = meff

dV

dt ∑ (3)

€

V(t) = 1

meff

( ) ∑F dt

0

t

∫ (4)

The!GEM!model!uses!an!“effective!mass”!formulation!that!includes!powertrain!

inertial!effects.!!In!the!GEM!code,!the!vehicle!static!mass!(vehicle.chsmass_static)!is!

added!to!the!representative!powertrain!inertial!mass!(tire_mass_out).!For!steady!

speed!vehicle!operation!the!powertrain!inertial!mass!should!be!zero.!!Figure!2!

shows!the!vehicle!inertial!mass!(tire_mass_out)!for!the!constant!desired!vehicle!

speed!period!of!the!55!mph!drive!cycle.!!The!inertial!mass!of!1693!kg!during!the!

steady!speed!demand!region!represents!5%!of!the!static!vehicle!mass.!!Figure!2!

shows!that!the!inertial!mass!term!is!not!zero!during!the!constantIdesiredIspeed!

portion!of!the!drive!cycle.!!Figure!3!shows!the!vehicle!chassis!speed!varies!during!

the!constant!speed!period!of!the!65!mph!drive!cycle.!

The!3%!discrepancy!between!analytical!and!GEM!simulated!torque!may!be!due!to!

the!speed!variation!during!this!portion!of!the!drive!cycle.!!The!consistency!of!the!

model!vehicle!dynamics!with!actual!vehicle!dynamics!is!a!possible!way!to!assess!

Page 64 of 104
whether!the!model!is!representative!of!actual!vehicle!dynamics.!!Figure!4!shows!

engine!torque!during!the!acceleration!ramp!leading!up!to!the!55!mph!steady!speed!

demand!region!of!operation.!!Comparing!actual!engine!torque!response!to!this!

dynamic!torque!response!would!be!a!way!of!assessing!whether!the!dynamics!are!

reasonable!or!not.!!The!quality!of!these!response!dynamics!will!be!even!more!critical!

for!transient!drive!cycle!analysis.

Figure&2&9&Inertial&mass&during&the&constant&speed&demand&portion&of&the&55&mph&drive&cycle.

Figure&3&9Actual&vehicle&chassis&speed&during&the&constant&speed&demand&portion&of&the&55&

mph&drive&cycle.

Page 65 of 104
Figure&4&9&Engine&torque&during&acceleration&ramp&in&55&mph&drive&cycle.

The!next!sanity!check!is!whether!the!fuel!consumption!and!GHG!emissions!are!

correctly!calculated!based!on!the!engine!torque!and!speed.!!!Figure!5!shows!the!

torque!versus!engine!speed!contour!map.!!Table!2!shows!a!comparison!of!offIline!

calculations!of!the!output!parameters!from!the!55!mph!and!65!mph!cases!in!table!1!

to!the!output!from!the!GEM!simulation!code.!!The!torque!and!speed!used!for!these!

calculations!are!the!torque!and!speed!calculated!by!the!GEM!simulation!code,!not!

analytically!calculated!torque!and!speed!from!table!1.!!Very!small!error!(less!than!

0.3%)!between!offIline!and!GEM!simulation!calculations!is!seen.!!These!differences

could!be!attributed!to!round!off!or!the!averaging!used!for!offIline!calculations.

Figure&5&9&contours&of&fuel&flow&rate&versus&engine&speed&and&torque&for&15L&engine&from&

"engine_map_455.m".

Page 66 of 104
Table&2&9&Comparison&of&GEM&simulation&and&direct&interpolation&of&fuel&flow&for&55&mph&and&

65&mph&cases&in&Table&1.&Comparisons&are&for&“veh_type(i_sim)&=&1”&“Class&8&Combination&9&

Sleeper&Cab&9&High&Roof.”&&The&engine&is&the&first&map&“veh_year=1.”

Property Units 55&mph 65&mph Source

Engine!speed Rpm 1266.5 1495.6 GEM!simulation

Engine!torque NIm 892.5 1066.2 GEM!simulation

Fuel!flow!rate kg/s 0.00660 0.00932 GEM!simulation

Fuel!flow!rate kg/s 0.00661 0.00934 Interpolated!from!map!in!

engine_map_455.m

Difference!in!calculated!

versus!GEM!simulated!fuel!

flow

% 0.2 0.2 Calculated

Fuel!density kg/L 0.847 0.847 From!“engine.cyl.fuel_desity”!in!

engine_map_455.m

Volumetric!fuel!flow!rate L/s 0.00780 0.0110 Calculated

Volumetric!fuel!flow!rate gal/hr 7.417 10.5 Calculated

Vehicle!speed miles/hr 55 65 Desired!steady!state!speed!from!drive!

cycle

Fuel!consumption miles/gal 7.42 6.21 Calculated

Fuel!consumption miles/gal 7.40 6.19 GEM!simulation!results

Difference!in!calculated!

versus!GEM!simulated!fuel!

consumption

% 0.2 0.3 Calculated

Payload Ton 19 19 From!run_preproc.m

CO2!to!tonImile!conversion g!CO2/!

(mpg*!

payload)

10180 10180 From!run_preproc.m

CO2!emissions g/(tonImile) 72.21 86.56 Calculated

CO2!emissions g/(tonImile) 72.38 86.52 GEM!simulation!results

Difference!in!calculated!

versus!GEM!simulated!CO2!

emission

0.2 0.05 Calculated

Following!up!on!the!earlier!discussion!of!engine!and!vehicle!integration,!Table!3!

shows!an!example!of!the!effect!of!engine!sizing!on!overall!vehicle!performance!when!

drag!reductions!are!implemented.!!The!comparison!is!again!for!the!“Class!8!–!

Sleeper!Cab!–!High!Roof”!vehicle!used!for!the!calculations!in!Tables!1!and!2.!!Three!

cases!are!shown:!1)!base!case!with!drag!coefficient!of!0.69!and!engine_map_455.m!

veh_year=1!engine,!2)!base!case!with!drag!coefficient!reduced!to!0.60,!and!3)!base!

case!with!drag!coefficient!reduced!to!0.60,!and!engine!downsized!to!90%!of!original!

engine.!!As!an!approximation!of!downsizing,!the!engine!map,!torque,!and!maximum!

torque!are!scaled!by!a!factor!of!0.9.!!This!scaling!is!representative!of!the!

performance!changes!that!could!be!achieved!by,!for!example,!reducing!the!

displacement!of!the!engine,!or!changing!the!turbocharger!parameters.

The!results!in!Table!3!show!that!a!generic!engine!has!limitations!demonstrating!

benefits!of!drag!reduction!strategies.!The!vehicle!with!reduced!drag!and!reduced!

engine!size!has!lower!fuel!consumption!and!lower!CO2!emissions!than!the!vehicle!

with!just!reduced!drag!coefficient.!With!a!generic!engine,!this!model!would!give!a!

manufacturer!that!reduces!vehicle!drag!without!consideration!of!vehicle,!engine!and!

powertrain!integration!the!same!performance!as!a!manufacturer!that!does!further!

optimization!of!the!vehicle.!!This!example!is!a!very!simplistic!reduction.!!With!

further!effort!greater!performance!benefits!are!likely!to!be!realized.!

Page 67 of 104
Table&3&9&Comparison&of&reduction&of&fuel&consumption&and&CO2&emissions&due&to&drag&

coefficient&reductions&and&engine9vehicle&integration.&Comparisons&are&for&

“veh_type(i_sim)=1”&“Class&8&Combination&9&Sleeper&Cab&9&High&Roof.”&&The&engine&is&the&first&

map&“veh_year=1”&in&engine_map_455.m.&&Calculated&values&come&from&the&GEM&simulation&

code.

Property Units Base&case Reduced&drag Reduced&drag,&

reduced

engine&size

Drag!coefficient no!units 0.69 0.6 0.6

Engine!scaling no!units 1 1 0.9

Steer!wheels!coefficient!of!rolling!resistance no!units 0.0078 0.0078 0.0078

Drive!wheels!coefficient!of!rolling!resistance no!units 0.0082 0.0082 0.0082

Fuel!consumption,!transient Mpg 3.51 3.53 3.64

Fuel!consumption,!55!mpg!steady Mpg 7.40 7.80 7.96

Fuel!consumption,65!mpg!steady Mpg 6.19 6.66 6.70

Fuel!consumption,!cycle!weighted Mpg 6.17 6.60 6.66

Improvement!in!cycle!weighted!fuel!consumption!relative!

to!base!case

% 0.00 6.97 7.94

CO2!emissions,!transient g/tonI

mile

152.47 151.67 147.15

CO2!emissions,!55!mpg!steady g/tonI

mile

72.38 68.65 67.27

CO2!emissions, 65!mpg!steady g/tonI

mile

86.52 80.48 69.92

CO2!emissions,!cycle!weighted g/tonI

mile

88.55 82.98 82.15

Improvement!in!cycle!weighted!CO2!relative!to!base!case % 0.00 6.29 7.23

Item&3)&Program&execution&

The!objective!of!this!section!is!to!evaluate!if!by!“Using!the!standard!of!good!

engineering!judgment,!the!program!execution!is!optimized!by!the!chosen!

methodologies.”!!I!interpret!this!to!be!asking!about!the!performance!of!the!code!as!

an!effective!and!efficient!tool!for!this!application.!!

The!code!overall!seems!to!be!developed!in!a!way!that!provides!detail!on!the!vehicle!

and!powertrain!dynamics.!!The!model,!like!the!vehicle!it!simulates,!is!a!complex!and!

highly!interconnected!system.!!There!are!many!submodels!in!this!code,!and!there!

are!many!imbedded!assumptions!that!are!not!directly!apparent!without!a!great!deal!

of!reverse!engineering.!It!is!often!difficult!to!test!and!verify!submodels!in!isolation!

because!they!are!highly!interconnected!with!the!main!model!and!significant!effort!

would!be!required!to!recreate!inputs!suitable!for!the!submodel!to!run!on!its!own.!A!

general!rule!in!modeling!is!that!the!level!of!complexity!of!the!model!should!be!the!

minimum!level!needed!to!answer!the!question!posed.!!

The!documentation!available!on!the!model!does!not!provide!a!detailed!description!

of!the!physical!models!implemented.!!This!kind!of!detailed!documentation!is!needed!

to!fully!understand!the!model!and!modeling!assumptions!involved.!

Page 68 of 104
Transparency!in!the!details!of!the!model!is!important!for!a!regulatory!application.!

Transparency!of!this!model!may!suffer!without!detailed!supporting!documentation!

on!the!physics!and!engineering!assumptions!underlying!each!model!and!submodel.!

Item&4)&Clarity,&completeness&and&accuracy&of&output

The!model!output!is!overall!clear!and!complete.!!The!model!reports!the!individual!

driveIcycle!results!and!weighted!average!results,!which!is!what!is!most!important!to!

the!end!user.!!All!the!inputs!needed!to!reproduce!the!results!are!reported.!!I!would!

suggest!that!the!a!code!version!also!be!included,!so!if!the!code!is!changed!in!the!

future!it!will!be!clear!from!which!version!an!output!file!evolved.!

Accuracy!of!the!results!is!difficult!to!assess,!since!that!requires!specific!comparison!

to!experimental!data!to!evaluate!the!performance!of!the!model.!!Based!on!my!testing!

efforts!and!experience,!the!results!seem!of!reasonable!magnitude!for!these!kinds!of!

vehicles.

Item&5)&Recommendations&for&improvements

Following!are!small!issues!I!noticed!during!my!review!of!the!code.!

1) The!syntax!in!the!mIfiles!is!not!compatible!with!unix,!specifically!the!

directory!backslash!“\”!vs!forward!slash!“/”.

2) The!windows!executable!version!has!predefined!values!for!C_d!in!a!

dropdown!menu!with!preset!values!in!increments!of!0.02.!!The!C_d!value!

should!just!be!an!entry!box,!like!the!C_rr!values.

3) The!inputs!for!weight!reduction,!speed!limiter,!and!idle!reduction!are!not!

consistent!between!the!matlab!version!and!the!windows!executable.!For!

example!in!the!matlab!version.!In!matlab,!zero!“Weight!Reduction”!defaults!

to!“N/A,”!which!causes!an!error!in!the!windows!version.!!The!windows!

version!does!accept!“N/A”!for!idle!reduction.!

4) It!would!be!informative!to!have!the!fraction!of!each!driveIcycle!used!in!the!

average!reported!somewhere!in!the!output.!

5) The!fuel!density!variable!is!“engine.cyl.fuel_desity.”!For!clarity!and!

consistency!I!would!recommend!changing!this!to!“engine.cyl.fuel_density.”

Conclusions&

1) My!main!concern!with!the!overall!approach!is!the!standardization!of!the!

vehicle!and!powertrain!combination.!!This!seems!to!have!potential!to!devalue!

efforts!towards vehicle!and!powertrain!integration!and!optimization!towards!

GHG!reduction.

2) The!model!is!quite!detailed!with!regard!to!powertrain!and!vehicle!dynamics.!!

There!is!a!danger!here!that!imbedded!assumptions!can!effect!results!in!

unexpected!and!undesirable!ways.!!The!example!of!the!3%!difference!in!

torque!for!analytical!versus!GEM!simulation!calculated!torque!for!steady!

state!operation!may!be!indicative!of!these!kinds!of!issues.!

3) It!should!be!confirmed!whether!the!various!controllers!in!the!GEM!model!are!

well!tuned!and!result!in!a!vehicle!response!consistent!with!empirical!data.!

Page 69 of 104
4) Detailed!description!of!the!physics!and!assumptions!imbedded!in!the!models!

and!submodels!should!be!documented!and!made!available!to!users.!

5) It!may!be!worth!considering!if!the!model!could!be!streamlined!to!provide!

greater!clarity!and!transparency!while!still!providing!a!tool!for!quantitatively!

estimating!fuel!consumption!and!GHG!emissions.!

References:&

1) “Greenhouse!Gas!Emissions!Model!(GEM)!User!Guide,”!EPAI420IBI10I039,

October!2010.!

2) Jeongwoo!Lee,!“Vehicle!Inertia!Impact!on!Fuel!Consumption!of!Conventional!

and!Hybrid!Electric!Vehicles!Using!Acceleration!and!Coast!Driving!Strategy,”!

Ph.D.!Dissertation,!Virginia!Polytechnic!Institute,!2009,

http://scholar.lib.vt.edu/theses/available/etdI09172009I

234744/unrestricted/ETD_PhD_Dissertation_Jeongwoo_Lee.pdf

3) http://www.epa.gov/otaq/climate/regulations.htm!

4) Uwe!Kiencke,!Lars!Nielsen,!“Automotive!Control!Systems:!For!Engine,!

Driveline,!and!Vehicle,”!Springer;!2nd!edition,!2005.!!

Page 70 of 104
Appendix:&Derivation&of&vehicle&and&engine&power&formulas

Figure!a6!shows!a!freeIbody!diagram!of!the!forces!and!accelerations!on!a!vehicle.!

This!vehicle!has!mass!m,!acting!about!the!center!of!gravity.!!Further!reading!on!

these!derivations!is!available!in!the!literature![3,!4].!!Gravitational!acceleration!is!

treated!separately!here!from!the!vehicle!acceleration.!

Figure&a6&9&Free&body&diagram&showing&forces&and&accelerations&on&a&vehicle

The!net!forces!on!the!vehicle!in!the!direction!of!movement!are:

€

Ftractive − Frr − Fdrag − mgsin(θ) = ma!!! ! (a1)

Ftractive!=!required!propulsive!force!on!the!vehicle

Frr!=!resistive!force!due!to!rolling!resistance

Fdrag!=!resistive!force!due!to!aerodynamic!drag!

a!=!net!vehicle!acceleration!in!the!direction!of!travel

m!=!vehicle!mass!(static!vehicle!mass)!

g!=!gravitational!acceleration

β!=!angle!of!vehicle!travel!relative!to!gravity!normal!direction.!

The!engine!transmits!torque!through!the!powertrain!to!the!wheels.!!At!the!wheels,!

the!torque!transferred!becomes!the!propulsive!(or!tractive)!force.!Figure!a7!shows!a!

schematic!of!the!transfer!of!torque!from!engine,!through!the!rotating!components!of!

the!powertrain,!to!the!force!acting!on!the!ground!to!propel!the!vehicle.

Page 71 of 104
Figure&a7&9&Torque&transfer&from&the&engine,&through&the&powertrain,&to&the&tires.

There!are!many!rotating!components!in!the!powertrain,!and!the!engine!torque!

(Tengine)!is!not!fully!transmitted!to!the!powertrain!through!frictional!loss!and!due!to!

rotational!accelerations.!!Figure!a2!represents!the!powertrain!with!N!rotational!

components,!each!k!component!having!a!mass!moment!of!inertia!(Ik)!and!angular!

acceleration!(

€

 ̇  ̇ θk ).!!Each!component!in!the!powertrain!may!have!frictional!losses!

(Tloss,k).!!

€

Tk − Tk−1 − Tloss,k = Ik

 ̇  ̇ θ k !! (a2)

The!tractive!force!can!be!calculated!from!the!sum!of!the!moment!balance!between!

the!engine!and!all!powertrain!components,!including!the!wheels.!!Reff!is!the!effective!

radius!of!the!wheel!over!which!wheel!torque!transfers!to!tractive!force.

€

Ftractive ⋅ Reff = Tengine − Ik

 ̇  ̇ ( ) θ k − Tloss,k

k=1

N

∑ !! (a3)

The!tractive!power!(Ptractive)!can!be!determined!by!calculating!the!product!of!torque!

and!angular!velocity!for!every!component!in!the!powertrain.!

€

Ptractive = Tengine

 ̇ θengine − Ik ̇  ̇ θ k ̇ θ k − Tloss,k ̇ ( θ k )

k=1

N

∑ = Pengine − Ik

 ̇  ̇ θ k ̇ θ k − Tloss,k ̇ ( θk )

k=1

N

∑ !! (a4)

For!convenience,!the!powertrain!power!losses!are!often!defined!as!a!transmission!

efficiency,!ηth.$

€

Tloss,k

 ̇ ( ) θ k

k=1

N

∑ ≡ ( ) ηtr −1 Pengine (a5)

The!transmission!efficiency!could!be!estimated!or!determined!experimentally.!!

Combining!equations!(a4)!and!(a5)!results!in!the!following!equation.!

Page 72 of 104
€

Ptractive = ηtrPengine − Ik

 ̇  ̇ θk ̇ ( ) θ k

k=1

N

∑ ! ! (a5)

Equation!(a1)!can!also!be!used!to!determine!power!by!multiplying!the!forces!by!

vehicle!speed,!V.

€

Ptractive = FrrV + FdragV + mgsin(θ)V + maV (a6)

The!rolling!resistance!is!defined!in!terms!of!a!rolling!resistance!coefficient!(crr)!and!

the!normal!force!of!the!vehicle!(N=mg!cos(β)).

€

Frr = crrmgcos(β)! ! ! ! ! (a7)

Aerodynamic!drag!is!defined!in!terms!of!air!density!(ρ),!drag!coefficient!(cd),!vehicle!

frontal!area!(Af),!and!vehicle!speed.

€

Fdrag = 1

2

ρcd AfV 2

! ! ! ! (a8)

Combining!(a5Ia8),!vehicle!tractive!power!can!be!used!relate!engine!power!to!

€

Ptractive = crrmgcos(β)V +

1

2

ρcd AfV 3 + mgsin(β)V + maV = ηtrPengine − Ik

 ̇  ̇ θ k ̇ ( ) θ k

k=1

N

∑ !(a9)

€

Pengine = 1

ηtr

crrmgcos(β)V +

1

2

ρcd AfV 3 + mgsin(β)V + maV + Ik

 ̇  ̇ θ k ̇ ( ) θk

k=1

N

∑ '

(

)

*

+

, (a10)

Equation!(a10)!completely!describes!the!power!demand!upon!an!engine!due!to!

external!forces!and!powertrain!dynamics.!

The!engine!may!support!vehicle!accessory!loads!(e.g.!air!conditioning,!lights),!and!

these!accessory!loads!will!be!removed!from!the!engine!before!the!transmission.!

Since!accessory!power!(Pacc)!is!removed!before!the!transmission,!accessory!power!

can!be!directly!added!to!the!engine!power!demand.!!Fuel!consumption!maps!are!

based!on!gross!engine!power!(Pengine,gross)!or!torque!and!engine!speed.

€

Pengine,gross = 1

ηtr

crrmgcos(β)V +

1

2

ρcd AfV 3 + mgsin(β)V + maV + Ik

 ̇  ̇ θ k ̇ ( ) θk

k=1

N

∑ '

(

) *

+

, + Pacc !(a11)

A!common!practice!is!to!simplify!the!powertrain!inertia!characteristics!from!the!

final!term!in!equation!(a10)!to!a!proportionality!scaling!of!the!vehicle!acceleration!

(maV)![ref].!The!effective!mass!(meff)!can!be!calculated!dynamically!or!

approximated.!!!

€

maV + Ik

 ̇  ̇ θ k ̇ ( ) θ k

k=1

N

∑ = meff aV ! ! (a12)

Page 73 of 104
Using!this!effective!mass!definition!gives!engine!power!in!terms!of!five!power!

demand!terms:!rolling!resistance,!aerodynamic!drag,!gravity,!acceleration,!and!

accessories.!

€

Pengine,gross = 1

ηtr

crrmgcos(β)V +

1

2

ρcd AfV 3 + mgsin(β)V + meff aV %

&

' (

)

* + Pacc (a13)

Page 74 of 104
PEER REVIEW: 

GEM Vehicle Model

Review Conducted for: 

U.S. EPA 

Review Conducted By: 

Shawn Midlam-Mohler

Review Period: 

11/19/2010 – 12/12/2010 

Page 75 of 104
Shawn Midlam-Mohler GEM Review Page 2 

Contents 

Summary ................................................................................................................................... 4

Introduction ............................................................................................................................... 5

Objective 1: Capable of modeling a wide array of vehicles over different drive cycles ..... 5

Objective 2: Contains open source code, providing transparency in the model ................... 5

Objective 3: Freely available and easy to use by any user ................................................... 6

Objective 4: Contains both optional and preset elements .................................................... 6

Objective 5: Managed by the Agencies for compliance purposes ....................................... 6

Model Structure Evaluation ...................................................................................................... 7

Ambient Subsystem .............................................................................................................. 7

Driver Subsystem .................................................................................................................. 7

GEM Manual Misleading ................................................................................................. 7

Driver PID Values not Configurable ................................................................................ 8

Gear Shifting Control ........................................................................................................ 8

Electric Subsystem ................................................................................................................ 8

Electrical System Parameters not Adjusted with Vehicle Class ....................................... 8

Alternator Model – Current Regulation and Control ........................................................ 8

Alternator Model – Accessory Torque ............................................................................. 9

Starter Model Complexity............................................................................................... 10

Pb Battery Model Accuracy ............................................................................................ 10

Pb Battery Model Complexity ........................................................................................ 11

Electrical Accessories not Adjusted by Vehicle Class ....................................................... 12

Engine Subsystem: .............................................................................................................. 12

GEM Manual is Unclear ................................................................................................. 12

Closed Throttle Engine Torque ....................................................................................... 12

Page 76 of 104
Shawn Midlam-Mohler GEM Review Page 3 

Engine Decel Fuel Cut-Off ............................................................................................. 12

Closed Throttle Engine Fuel Consumption..................................................................... 13

Overall Structure of Engine Model ................................................................................. 13

Mechanical Accessories not Adjusted by Vehicle Class .................................................... 14

Transmission Subsystem ..................................................................................................... 14

Transmission Model Parameters should Change by Class: ............................................ 14

Vehicle Subsystem .............................................................................................................. 14

Vehicle Model Parameters should Change by Class ...................................................... 14

Vehicle Frontal Area ....................................................................................................... 14

Vehicle Weight Reduction for Rotating Components: ................................................... 15

Vehicle Weight Reduction not Implemented for Certain Classes of Vehicle ................ 15

Vehicle Loss Parameters ................................................................................................. 15

GEM Input and Output Files................................................................................................... 17

Format of Output File (xml) .............................................................................................. 17

Clarity of Output File .......................................................................................................... 17

Content of Output File ........................................................................................................ 17

Standard Input Values Specified in GEM Manual ............................................................. 17

Miscellaneous Comments ....................................................................................................... 18

Adjustment of Model Parameters for Different Vehicle Classes ........................................ 18

Model Fidelity ..................................................................................................................... 18

Sensitivity of Parameters .................................................................................................... 18

Conclusions ............................................................................................................................. 20

Page 77 of 104
Shawn Midlam-Mohler GEM Review Page 4 

Summary 

The model fidelity of the type proposed should be capable of achieving the desired 

objectives. The model reviewed, however, has a number of issues which cast doubt upon the 

specific implementation of the model. Specifically, a number of issues were found in the 

electrical subsystem as well as the engine subsystem. In many cases, it is felt that the level of 

modeling used in subsystems, the electrical subsystem being one excellent example, are more 

complicated the necessary given the relatively low impact on the desired outcome. 

From the supporting material, it is clear that the model did an acceptable job at modeling 

a Class 8 SmartWay truck. Further validation across the range of vehicles being modeled would 

be appropriate to provide confidence to the end users and ensure the model is doing an 

acceptable job at modeling green house gas emissions. 

It is also recommended that a better understanding of the propagation of uncertainty in 

the key model input parameters be evaluated. For instance, key parameters like the drag 

coefficient and coefficient of rolling resistance can be measured with a certain degree of 

uncertainty. It is possible to determine how these errors propagate through the model and impact 

the end result of fuel consumption or greenhouse gas emissions. These results should be one part 

of the overall evaluation of the model. This level of uncertainty should then be compared to the 

end use of the model and the expected resolution required to distinguish between different 

technologies. 

Page 78 of 104
Shawn Midlam-Mohler GEM Review Page 5 

Introduction 

The peer review directives suggested addressing a number of different issues. The first 

topic was an overall assessment of the model to meet the stated objectives. In the following 

subjections, there are some high-level comments on the ability of the proposed model to achieve 

the five attributes listed in the peer review statement. 

Objective 1: Capable of modeling a wide array of vehicles over different drive cycles 

The model fidelity of the type proposed should be capable of achieving the desired 

objectives. The model reviewed, however, has a number of issues which cast doubt upon the 

specific implementation of the model. Specifically, a number of issues were found in the 

electrical subsystem as well as the engine subsystem. In many cases, it is felt that the level of 

modeling used in subsystems, the electrical subsystem being one excellent example, are more 

complicated the necessary given the relatively low impact on the desired outcome. 

From the supporting material, it is clear that the model did an acceptable job at modeling 

a Class 8 SmartWay truck. Further validation across the range of vehicles being modeled would 

be appropriate to provide confidence to the end users and ensure the model is doing an 

acceptable job at modeling green house gas emissions. 

Objective 2: Contains open source code, providing transparency in the model 

Providing source code as a Simulink diagram is necessary for this objective but not 

sufficient. Additional documentation on the equations and references behind the Simulink code 

should be developed and released to the public. Even an experienced Simulink user finds it 

difficult to follow somebody else’s code. The code provided is actually laid out quite well but 

more documentation is necessary to avoid confusion. Inexperienced Simulink users would not 

be able to follow the code directly and thus would rely much more heavily on the supporting 

documentation. In later sections there is come critique regarding the current GEM manual in 

how it describes certain aspects of the model. These issues should be addressed as 

documentation is refined. 

Page 79 of 104
Shawn Midlam-Mohler GEM Review Page 6 

Objective 3: Freely available and easy to use by any user 

The compiled version of the code is free and easy to use. The Simulink version requires 

a Matlab license which is not free but fairly common in industry. 

Objective 4: Contains both optional and preset elements 

The current structure satisfied this objective. 

Objective 5: Managed by the Agencies for compliance purposes 

By releasing an official and unalterable executable version of the model this objective is 

met. Providing only a “source-code” version (i.e. Simulink code) would be problematic from 

many perspectives. 

Page 80 of 104
Shawn Midlam-Mohler GEM Review Page 7 

Model Structure Evaluation 

The overall approach of using a relatively simple model structure based in Matlab-Simulink 

is sound provided that models are calibrated and validated to a sufficient level. In the following 

subsections, there are comments on various issues found in the various sub-models in the model. 

The following is a summary of what follows: 

1. Ambient Subsystem: No issues were found in this very simple subsystem. 

2. Driver Subsystem: No major issues were found in this subsystem. 

3. Electrical Subsystem: Several serious problems were found in this subsystem. Most 

notably, there are serious flaws in the battery model, the alternator model, and 

alternator control. 

4. Engine Subsystem: There were problems found in this subsystem which need 

addressed. The main concerns in this subsystem are from the method use to model 

the engine at negative brake torque values. 

5. Transmission Subsystem: No major issues were found in this very simple subsystem. 

6. Vehicle Subsystem: Some issues were found in this subsystem. 

Ambient Subsystem 

The ambient subsystem contains only parameters to describe the ambient conditions. 

There were no relevant comments on this subsystem. 

Driver Subsystem 

The driver subsystem is typical of those found in other models of similar fidelity. There 

were no major issues found within the Driver Subsystem. The following subsections contain 

some comments on models or controls within this subsystem. 

GEM Manual Misleading 

The manual describes that the driver block in a misleading fashion. Once sentence in 

particular: “The search for the proper vehicle speed occurs at every simulation time step.” This 

seems to imply it is something other than a simple PID control. 

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Driver PID Values not Configurable 

From experience, there are times when the PID gains for a driver may need to be adjusted in 

order to drive a particular velocity profile. The PID values are fixed in the current model. If an 

end-user has a vehicle in which the driver does a poor job there is no recourse to correct this. It 

may be worth adding this as an “advanced feature” or using a more sophisticated control 

concept. For example, the driving trace is known as are the overall vehicle characteristics for 

each class, it would not be terribly difficult to augment the current PID control with a 

feedforward component. This being said, large errors in velocity tracking were never observed 

in exercising the model. 

Gear Shifting Control 

The gear shifting strategy was only evaluated by observation. It appears to follow the 

prescribed shift schedule as desired. 

Electric Subsystem 

Very significant issues were found in the electric subsystem which require attention. In 

particular, the battery model appears to an error which causes battery voltage to decrease with 

battery state of charge which is exactly opposite of the desired behavior. Furthermore, it appears 

that the sign convention used for the starter, accessories, alternator have the wrong sense. The 

alternator generates negative current which decreases SOC. The other two currents, which are 

current sinks, actually increase the SOC of the battery. Even with the above issues aside, the 

alternator model appears to not consider the mechanical to electrical efficiency of the device and 

the control is naïve of actual alternator capabilities and control. These issues and others of more 

minor consequence are described below. 

Electrical System Parameters not Adjusted with Vehicle Class 

Many of the model parameters used in the electrical system are not changed based on 

class of vehicle. Many of these would change based on the class of vehicle. 

Alternator Model – Current Regulation and Control 

The alternator model is particularly difficult to follow from the Simulink code. It appears 

that alternator current is directly a function of speed, which is not correct for modern alternators 

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which can regulate voltage quite effectively. Figure 1 shows simulation results for a 45 mph 

simulation case. This shows a few strange behaviors: 1) the voltage drops to 10 volts by 500 

seconds (the vehicle starts moving at 375 sec.); 2) the behavior of the voltage is erratic and not 

typical of what happen in practice. The second point is a direct result of the control that is 

applied to the alternator model in that it turns the alternator on at full rated capacity until it 

reaches a setpoint and then turns it off until voltage drops below a setpoint. This will result in 

much higher internal I2R losses than a more appropriate and more realistic model/control that 

allows the alternator to actually modulate current. 

Figure 1: Irregular Voltage of Battery 

Alternator Model – Accessory Torque 

It appears that the alternator torque is only a function of alternator electrical power demand 

without accounting for the alternator efficiency. This part of the model is shown in Figure 2. If 

this is the case then the model is underestimating the accessory torque required to operate the 

alternator. In looking through the m-file associated with the alternator there was no obvious 

efficiency parameter for the alternator which further raises doubt. 

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Figure 2: Possible Error in Alternator Mechanical Torque Calculation 

Starter Model Complexity 

Given the relative unimportance of the starter in the overall performance of the model, 

the starter model is quite complex. With this level of model, it is clear that parameters should 

change with the engine class – this is currently not implemented in the model. 

Pb Battery Model Accuracy 

Investigating the battery model independently led to the discovery of extremely 

disturbing behavior. With the battery removed and the SOC initialized at zero, a 1-C charge at 

352 amps at 20 deg. C was simulated. The battery SOC moved from 0 to 100 in roughly 3600 

seconds, which was expected. What was not expected was that the value of “ees_volts” behaves 

exactly counter to what it would in an actual battery – with increasing SOC the voltage drops 

very quickly to a minimum value and stays there. The open circuit voltage, which is map based, 

behaves as expected. Figures showing the results of this test are shown in Figure 3. This 

behavior was observed in the vehicle simulation as well although it is difficult to observe 

because of the other dynamics involved. 

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Figure 3: Junction of Three Currents 

Further investigation of the electrical system model yielded further inaccuracies. In the 

model, there is a junction of three different currents: starter, alternator, and accessory current 

(Figure 4). If one disconnects the alternator current and leaves the starter and accessory current 

connected (i.e. disable the ability to charge the battery) one finds that the battery SOC increases. 

If one disconnects the loads and applies an alternator current manually (required because of the 

alternator control and initial SOC) you find the SOC decreases. In both of these cases the 

“ees_volt” value goes the opposite direction of the SOC.

Figure 4: Junction of Three Currents 

Pb Battery Model Complexity 

The battery appears to be unnecessarily complicated with respect to the objective of the 

model. In particular, modeling the thermal dynamics of the battery seems excessive. Over the 

transient cycle for a Class 8 truck, the battery changes temperature by less than one degree. 

Generally, more complicated models than necessary require more calibration parameters and 

could be more prone to inaccurate results. This level of complexity seems unnecessary. 

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Electrical Accessories not Adjusted by Vehicle Class 

The electrical accessory load is not adjusted by vehicle class. The electrical accessory 

loads are not constant between classes. 

Engine Subsystem: 

 The issues found in the engine subsystem are not as serious as those in the electrical 

subsystem, yet they still need to be addressed. The method of handling negative brake torques in 

the model does not seem to be appropriate. Because the engine model is one of the most 

important in the simulator it must be as accurate as possible. Although not a technical flaw, 

many of the variable names in the model are confusing or irrational, such as “closing throttle 

torque” and “closed throttle torque” – use of such language leads one to question the model 

structure and calibration. 

GEM Manual is Unclear 

The manual’s description of the engine model is misleading. In particular, the sentence 

“This map is adjusted automatically by taking into account three different driving types: 

acceleration, braking, and coasting.” This text is not very descriptive of what is actually in the 

model. 

Closed Throttle Engine Torque 

Closed “throttle” is an inappropriate way to describe this parameter for a Diesel engine. 

Diesel engines can have throttles but they are used for purposes other than load control. The 

values seem to be the identical for each of the engine classes as well as being contrived numbers 

since it is precisely equal to -5. This would impact the rate of deceleration and potentially have 

an impact on overall fuel economy predictions. 

Engine Decel Fuel Cut-Off

There is no implementation of fuel cut-off during decelerations. This is a feature that is 

implemented on at least some heavy-duty Diesel engines. This can be observed by plotting the 

fuel flow rate during the transient cycle. 

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Closed Throttle Engine Fuel Consumption 

The method used to calculate “closed throttle” fuel consumption is not clear. The part of 

the code which does this is shown in Figure 5. The use of variable such as “closed throttle 

torque” and “closing throttle torque” do not inspire confidence in the model as they are nonstandard terms – particularly for a Diesel engine. It is difficult to understand exactly why this 

calculation should result in a valid fuel flow. 

It is possible in a lab setting to measure fuel consumption from max rated torque down to 

zero brake torque. With a motoring dyno, it is then possible to measure fuel consumption at 

negative brake torques until the engine reaches a condition where it injects a minimum amount 

of fuel, or in many cases, absolutely no fuel. It is understood that not all engines will have these 

“negative brake torque” fuel maps available, however, there are approximate ways of modeling 

this, such as techniques based on the popular Willans Line method. 

Figure 5: Closed-Throttle Fuel Flow Calculation 

Overall Structure of Engine Model 

A map-based engine model should be sufficient to achieve the desired objectives. The 

engine model implemented in the current version of the software does not appear to be as well 

implemented as it could be. Given the importance of this in the overall objectives of the 

simulator this needs to be addressed. Using fuel maps which have torque indices ranging from a 

negative brake torque to the maximum rated torque would alleviate much of the uncertainty in 

the model. Driver accelerator requests should then be linearly scaled from minimum value to the 

maximum value on this map with the exception of idle conditions in which alternative measure 

must be taken. This approach also automatically takes into account deceleration fuel cut-off as 

well. 

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Mechanical Accessories not Adjusted by Vehicle Class 

The mechanical accessory load is not adjusted by vehicle class. The mechanical 

accessory loads are not constant between classes. 

Transmission Subsystem 

 There were no serious model issues found in this subsystem. As with many other 

subsystems, there are a number of parameters which should change with vehicle class. 

Transmission Model Parameters should Change by Class: 

There are many transmission parameters which currently do not depend on vehicle class, 

such as clutch and gear inertias. In vehicles which across this range of classes the inertias are 

likely much different. These parameters can be found in “transmission_manual_param.m”. 

Vehicle Subsystem 

 There were no modeling errors noted in the vehicle subsystem, however, there are a 

number of things which could be taken as recommendation. The most serious item is considered 

to be the fact that the “Vehicle Weight Reduction” parameter is specifically cited as being able to 

model light-weight wheels. The existing model structure would not accurately do this as it does 

not take into account the inertial aspect of the wheels which would have a greater impact on the 

vehicle. 

Vehicle Model Parameters should Change by Class 

There are many vehicle model parameters which currently do not change with vehicle class 

and should. There are a number of driveline component inertias which do not appear to change 

with vehicle class. These parameters can be found in “vehicle_param.m”. 

Vehicle Frontal Area 

The impact of tractor cab design is one of the key technologies that this simulation is 

intended to evaluate. The equations used to model drag is the typical 0.5 * Cd * A * velocity ^ 

2. The proposed approach constrains the fontal area (A) to fixed values that depend on vehicle 

class. This could dis-incentivize novel cab designs which result in smaller frontal areas for a 

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given class of vehicle. It is recommended that allowing the frontal area be an input parameter to 

the model. In certain disciplines it is common to parameterize a model using a lumped Cd*A 

term because of their interrelation. 

Vehicle Weight Reduction for Rotating Components: 

The “Vehicle Weight Reduction” parameter is described as a way to accommodate, 

among other things, lighter weight wheels. Simply subtracting wheel weight from payload will 

underestimate the impact that light-weight wheels will have on the vehicle because it neglects the 

rotating inertia of the wheels. This could be accommodated given information on the rotating 

inertia of the wheels and subtracting it from the appropriate tire inertias in the mode – this would 

be in addition to the weight reduction already implemented. 

Vehicle Weight Reduction not Implemented for Certain Classes of Vehicle 

The code used to adjust vehicle mass for weight reductions does not do so for many of the 

vehicle classes. This is shown in Figure 6 below. 

Figure 6: Vehicle Weight Reduction Code from run_preproc.m 

Vehicle Loss Parameters 

The vehicle loss parameters used, mainly rolling resistance and drag coefficient, use very 

basic models. Essentially, the rolling losses are characterized entirely by a single constant per 

tire and a single drag coefficient is used to model the aerodynamic losses. Relying on a single 

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parameter may not be sufficient to model these losses accurately. An alternative would be to 

allow alternative forms of entering this data. It is understood that these standards are being 

under development – but it is certainly possible that these parameters are not well modeled by a 

constant. 

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GEM Input and Output Files 

The directions provided for the peer review requested some specific information regarding the 

input and output of the mode. The following subsections address these issues. 

Format of Output File (xml) 

The .xml format used in the output file will be problematic for some users. Most 

operating systems opt to open .xml files with programs (MS Word, internet browsers) which do 

not meaningfully displace the results. The manual states clearly that MS Excel should be used to 

open the file, however, certain users may not head this warning. It may be beneficial to remind 

the user from the software after they click the “RUN” button on the compiled code. 

Clarity of Output File 

The formatting of the output file was clear. The four tab format with the first tab being 

summary data and others being cycle data was sufficient. 

Content of Output File 

End users will likely want to see more detail in the output file then just the vehicle target 

speed and achieved speed. Making a limited number of “internal” parameters available to allow 

end users a glimpse inside the model without having to use Matlab-Simulink would be sufficient. 

These should be limited to things relevant to their inputs, such as aerodynamic drag over the 

cycle, rolling losses over the cycle, etc.

Standard Input Values Specified in GEM Manual 

It was requested that reviewers comment the proposed standard parameters for the 

different vehicle classes shown in the GEM manual. Unfortunately, the reviewer does not have 

the required expertise to make an assessment of the proposed values. 

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Miscellaneous Comments 

The following subsections contain observations which did not fit into the previous sections. 

Adjustment of Model Parameters for Different Vehicle Classes 

A number of parameters were noted which should change with respect to the vehicle 

class. The reviewer is certain that there are others that were not noted in this review. It is 

recommended that the EPA investigate this and take an appropriate action. In many cases, these 

components will not have a serious impact on the overall performance of the vehicle. By way of 

example, many of the inertias simulated in the model will not have a large impact on the results 

in contrast to the large inertia of the vehicle. If this is the case, then these inertias could be 

discarded from the model with little impact on performance. If the detailed inertias remain in the 

model, then they should accurately reflect the vehicle class. 

Model Fidelity 

One overall comment is that there is a higher than necessary level of fidelity in many of 

the models. By way of example, the battery model is particularly complicated and contributes 

very little to the outcome of the simulation. There are also a great number of relatively small 

inertias that are modeled, such as the starter motor inertia. These inertias contribute very little to 

the type of results that are sought after in this simulation. 

The added level of detail also comes with an additional practical consideration in that the 

models require a great deal more parameters to describe the vehicles in each class. By way of 

example, the starter inertia of a Class 2b truck is much different than a Class 8 truck. If the 

starter is modeled as a zero inertia element, then it does not need a defined inertia. If there is an 

inertia parameter, then it should be a representative number even if it does not have a major 

impact on the simulation. EPA could reduce the complexity of many of the models with little 

impact on the accuracy of the simulation – this would then lead to a reduced set of parameters 

that very with vehicle class and therefore need to be determined. 

Sensitivity of Parameters 

It would be useful to have a better understanding the propagation of error in the input 

parameters. For the proposed configuration for the class 8 high-roof sleeper cab the sensitivity 

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of the CO2 result to errors in Cd is approximately 50%. This implies that a 10% error in Cd will 

result in a 5% error in prediction of CO2 emissions. For rolling resistance, the impact of a 10% 

error in the tire rolling resistance causes a 2.3% error in prediction of CO2 emissions. These 

sensitivities should be compared to the reduction in CO2 emissions required as well as the 

accuracy of the key input parameters in the model. This analysis would also be useful in 

determining which parameters might be superfluous with respect to the desired output. As 

discussed above, there are some models which likely have more complexity then necessary. 

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Conclusions 

The overall modeling fidelity and structure of the GEM model should be able meet the objectives 

of the EPA. There are a number of issues with the current version of the GEM model which 

would need to be addressed to best meet the objectives. These issues were described in greater 

detail above, but in summary fit mainly into the following major points: 

1. Accuracy of Sub-Models Structure: A number of errors were found in models within 

GEM. None of these errors are expected to contribute to larger errors to the output 

results but should be corrected nonetheless. 

2. Parameter values for Different Vehicle Classes: Throughout many of the subsystems 

there are a number of minor parameters which should be changed with vehicle class. 

Philosophically, if the model has a parameter that should change with vehicle class then it 

should change with class. If it is determined that changing the parameter with vehicle 

class has not significant impact on the model results then that parameter should be 

considered for elimination. 

3. Model Complexity: Several of the sub-models had complexity that far outweighed their 

impact on the results. The battery was one such sub-model which also contained some 

serious errors in its formulation. Many of these models could be simplified which will 

also reduce the number of parameters required which impacts the comment in (2) above. 

4. Sensitivity Analysis: A rigorous study of the sensitivity of key input parameters should 

be conducted. Our ability to measure and estimate input parameters is not perfect, hence, 

the output of the model is affected by this uncertainty. If our ability to measure the 

coefficient of drag is +/- x.y % then that has an impact on the model output. This 

uncertainty can then be compared to required accuracy to make a judgment on the 

validity of this method at estimating green house gas emissions or fuel economy. 

5. Model Validation at other Classes: Based on the issues noted in (2) above, it is important 

to validate the model across vehicle classes. Because the model structure is relatively 

low-fidelity it has a greater burden of proof when “extrapolating” results. To have 

confidence in the model some further level of validation should be conducted. 

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EPA GEM Peer Review 

Elliott Ortiz-Soto 

PhD Pre-Candidate 

Mechanical Engineering 

University of Michigan – Ann Arbor 

The following document reviews the Greenhouse Gas Emissions Model (GEM) from a user’s 

point of view, as well as providing a more detailed evaluation of the modeling approach and 

assumptions. The review first addresses the executable version of GEM as a black-box 

simulation from an end-user standpoint, commenting and suggesting improvements on the 

overall GUI layout, usability and output. The second part looks into the Matlab/Simulink 

version of GEM. In this case, I attempt to give a more thorough and detailed evaluation of the 

code, assumptions and underlying physical models. The review is organized as a bulleted list, 

and it does not follow a particular order, although I did try to arrange the comments by similar 

subjects. I hope this review provides some useful feedback in the development and improvement 

of the GEM compliance simulation tool. 

GEM Executable and Output

This section provides general feedback on the GEM executable and its output. 

 The location of the “Vehicle Model Year” dropdown menu is not intuitive. This is one of the 

most important parameters of the simulation and it is part of the inputs that affects the results, 

but it has been grouped with the identification parameters. These should be separated as they 

currently are, but somehow the “Vehicle Model Year” was left in the top section. 

 Having radial buttons with all of the vehicle configurations in the “Regulatory Class” section 

is not necessary. It occupies space and reduces the GUI’s flexibility to add other parameters 

in the future. This type of list is probably better addressed through the use of a drop down 

menu. It would reduce the profile of this parameter list, and it would show much more 

clearly what vehicle type is being used. Currently, closer attention has to be paid to the GUI 

to notice which radio button of the ten available is selected, whereas with the dropdown 

menu it is only necessary to read what is displayed. 

 On the other hand, it is not clear why there should be a dropdown menu for the “Coefficient 

of Aerodynamic Drag” parameter. Furthermore, the dropdown menu allows the values to be 

overwritten by the user, so the dropdown menu has no real purpose. Typically dropdown 

menus are used to provide the user with a set of fixed options, which are usually not 

numerical values. A better approach would be to just provide a sample value in the 

parameter name to give the user an idea of what would be an expected input in the box. 

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Basically, it should look something like the “Steer Tire RR” and “Drive Tire RR” input 

boxes. 

 The same issue is seen in the “Speed Limiter” input box. There is no real reason why there 

has to be another dropdown menu. If there are minimum and maximum values for the speed 

limiter, this should simply be stated either in the GUI or in the documentation, and just allow 

the user to input whatever integer value they require within these limits. 

 A similar observation can be made regarding the “Extended Idle Reduction” parameter. 

According to the documentation, this parameter is an on/off option. Providing another 

dropdown menu, which can also be overwritten, is simply confusing. This gives the 

impression that any number of values, maybe between 0 and 5, can be used as inputs, which I 

am not sure is the case here. A checkbox object should be used for this type of on/off 

parameter. 

 It appears that some options are only available when a certain “Regulatory Class” is selected, 

such as the “Vehicle Speed Limiter”, “Vehicle Weight Reduction” and “Extended Idle 

Reduction”. But from the GUI, it is not clear which ones can be selected with the various 

vehicle types. It is generally useful to gray-out the options that are not available in relation to 

another parameter. For example, if one of the vocational vehicles is selected as the 

Regulatory Class, the three options mentioned above should be grayed-out, letting the user 

know unambiguously that these are not available with this vehicle class. Currently, it is not 

clear whether the code is robust enough so that these options are not applied when a certain 

vehicle is selected, or if you would just obtain incorrect results if these were to be selected 

unknowingly. 

 One significant drawback I found relates to the output file naming scheme. First of all, 

naming the files based on date and time is not very useful or descriptive. When multiple 

simulations are performed, it becomes difficult to determine what file you should be looking 

into, unless you actually open it. The file names should include at least some sort of 

indication of what the simulation configuration was. The second problem I found was the 

lack of flexibility to specify where these output files are saved. There should be an option 

allowing the user to browse and select the main directory where these files are to be saved. 

As a final comment on this, there is really no reason for each of these files to be saved to a 

different folder if there is just a single output file. This simply adds an unnecessary layer to 

the file structure. If multiple outputs were generated, then it would make some sense, but 

currently, there is a single xml file within a folder with the exact same name. 

 When the simulations are run, a series of plots with the drive-cycle profiles are generated. It 

is not very practical to have to close each of these in order for the next one to show. These 

should either be generated in different windows or, preferably, in a single tabbed window 

with all three plots. It should also not be necessary to close the plots for another simulation 

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to begin. This way, various simulations could be performed and the users could clearly see 

how the parameters varied affect the vehicle behavior without having to create the plots 

themselves. A small table with some drive-cycle output, such as the one given in the xml 

files, would also be useful to see together with these plots. 

 Within the output file, there are three sheets with the drive-cycle traces. Plots should be 

automatically generated because the explicit profile is not of much use unless it is plotted. 

 Miles per gallon (MPG) is generally assumed to be a measure of fuel economy, not fuel 

consumption. Although this is a small detail, it might be worth revising to be consistent with 

the industry standard. Adding the fuel consumption equivalent in gallons/ hp-hr (liters/hp-hr) 

or liters/100 km might also be useful. 

 For compliance purposes, it would be good to see the actual target value next to the 

simulation result, and probably some sort of percentage difference between these. It would 

give the manufacturer/user an idea of how their product performs with respect to the 

expected regulation standard. 

 Although the idea of the current program is to reduce complexity and provide only the 

necessary information for compliance purposes, some additional results might be helpful for 

manufacturers to determine if the simulation is representative of their vehicle. Because many 

model parameters and vehicle operating strategies have been standardized using internal 

assumptions and algorithms, the overall behavior of the vehicle in question could end up 

being very different from what the vehicle manufacturer actually observes. This can result in 

a significant over-estimation of fuel consumption and CO2 emissions, and possibly noncompliance. For this reason, it is fair that the manufacturer be able to assess the validity of 

the simulation without having to investigate the model in detail. This could be achieved by 

providing a series of additional results, which could be related to the engine operation over 

the drive-cycles, the shifting strategy, the electrical system, etc. Exactly what parameters 

these should be might not be so simple to determine, but it could provide some confidence in 

the simulation results. 

Matlab/Simulink GEM Model

This section provides an evaluation of the GEM Matlab/Simulink model and simulation. 

 General comments on the GUI and Matlab simulation: 

o The internal names (tags) for the objects (buttons, dropdown menus, etc.) in the 

Matlab GUI script should be more explicitly named for clarity and understanding of 

the GUI functionality. 

o Default parameters should be assigned in the GUI opening function so the user has a 

better idea of what to select or provide as input. 

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o The date is not automatically imported as in the executable, and the simulation 

crashes if the user forgets to write it in. 

o If the model is run with any of the Stateflow blocks open, it increases the simulation 

time substantially. 

 In general, the Simulink model is well organized and intuitive. The use of the following 

modeling techniques and Simulink components make the model particularly elegant and easy 

to understand: 

o Multiple “Bus” elements and collecting them into a “System Bus” to keep signals 

clearly labeled and organized. 

o “GoTo” tags to avoid excessive model clutter with connections between blocks. 

o Stateflow instead of explicit Simulink logic blocks, which greatly simplifies 

development and implementation of the various logic controllers. 

o Signal-activated blocks to avoid additional logic blocks for signal generation and 

routing. 

 Even though the use of various “mux” and “demux” components, as well as a series of 

component Buses and an overall System Bus is a very elegant modeling approach, it seems 

that many of the signals are also being output separately, which somehow defeats the purpose 

of having a Bus. I am sure there is some reasoning behind this, but I would have expected 

this Bus component to be used more widely, routing the signals directly to/from the Bus 

everywhere they are required. 

 Some blocks go into deeper levels unnecessarily. Examples can be found in the electrical 

system and in the driver models. Although the approach used in this model of grouping 

models into blocks based on their physical components or functionality is fairly intuitive, 

adding extra layers can also make the model more difficult to follow if done excessively. 

 Is it necessary to have an “Ambient Bus”? The Bus component is used to collect signals 

calculated in the model, whereas the ambient parameters are all prescribed and fixed. They 

could just as easily be called as a variable from the workspace wherever they are needed (this 

seems to be what is typically done in the model anyway). 

 In the pre-processing file, where the parameters for the individual configurations are selected, 

there appears to be a lot of repeated code and “if/else statements”. Most of these parameters 

can simply be collected in arrays, which can then be indexed using the “veh_type” variable. 

This way they can be included in the original parameter files, as they are more explicit and 

easier to read by a user than having to review a long pre-processing script with many 

conditional statements. It would also take advantage of Matlab’s array operations, which are 

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usually more efficient than “for loops” or “if statements”, as well as removing a lot of the 

repeated code. This will most likely result in improved code performance. 

 Control for most of the vehicle components seems to be achieved by fairly standard PID 

controllers. Usually the gains for these controllers are tuned to a specific plant, but in this 

case they remain fixed for all the vehicle configurations. Were these gains tuned for all the 

plants individually and then somehow averaged to account for all of them, or were they 

computed for a single vehicle? Although for the test cases do not show any major problems 

with following the prescribed velocity profile, simulation of some vehicles or with a different 

set of parameters could possibly suffer if the controller gains are not appropriate. For the 

driver, for example, more elaborate, robust and reusable driver models exist, and it might 

useful to investigate the possibility of incorporating one of these in order to avoid possible 

issues with the simulations. 

 In a related comment, Simulink offers pre-developed PID blocks in which only the gains 

must be prescribed. Is there any particular reason why the PID controllers have been 

explicitly created? It might help reduce the profile of the individual models if these were to 

be employed. 

 The engine speed appears to be calculated within the Accessories block, which is not very 

intuitive when reviewing the model. I would expect this to be within the main Engine block 

and then passed to the other engine-related blocks from there. 

 In both the Gear and Clutch blocks of the Transmission model, it is assumed that the gears 

and clutch are either fully engaged, where they pass the total torque being input, or fully 

disengaged, where they pass zero torque. Although this might be a fair simplification for the 

given modeling purposes, there are simple models that can calculate the transmitted torque 

based on the gear/clutch slip or speed differential. This will add a little more complexity to 

the model, but it should result in more realistic vehicle behavior. 

 The electric components and EES seem to be fixed for all the vehicles in the simulation, but 

in reality the electrical system is probably designed for a given application to account for the 

particular load requirements. It is understandable that due to the complexity of acquiring 

parameters such as these, the system model is standardized, but it could also result in 

simulation inaccuracies. It might be more appropriate to provide at least some basic scaling 

capability for the overall electrical system so that with one or two additional inputs, the 

electrical components and EES are scaled to match the actual setup more closely. 

 A similar observation can be made regarding the starter and alternator models. Both appear 

to be parameterized based on HD Class 8 components. Does this mean that these 

components are oversized when used in smaller vehicle classes? If so, would they not impart 

a larger load on the engine, or require a larger amount of electrical power to operate when 

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compared to a right-sized component? These are most likely not critical components of the 

model due to the drive-cycles being used, but again, a scaling factor, even if it is an internal 

value, should be applied to ensure these are representative of the actual system in the vehicle. 

 There seems to be an internal option for an acceleration test. Will this be made available to 

manufacturer users? Acceleration tests are in general much simpler to perform than a full 

transient drive-cycle, so providing this optional capability might give the manufacturers 

another way of validating the model. If acceleration numbers are completely different, then 

it would be hard to expect that a transient drive-cycle simulation would be at all 

representative of the real vehicle. 

 One of the most important input data for a fuel economy drive-cycle simulation is the engine 

mechanical load and fuel consumption maps. The mechanical load maps are usually simple 

because only the WOT (or Diesel equivalent) values are required, but obtaining full range 

fuel consumption values is much more difficult. Several engine maps appear to be available 

for each vehicle class, but making these completely standard with a prescribed displacement 

volume and operating range might be a limiting factor for some manufacturers. A more 

flexible approach would be to have normalized load and fuel consumption maps, given in 

BMEP and BSFC values. The current maps can be easily converted into BMEP and BSFC 

with the data available. The user could then provide the engine displacement and possibly 

another key parameter such as rated torque or power and the engine speed, and an algorithm 

could automatically manipulate the normalized maps to obtain more representative absolute 

values for the engine in question. Even though this compliance tool assumes that the engines 

have already been certified, the fuel economy and CO2 values that the simulation predicts are 

directly related to the maps given, and manufacturers might want to ensure the engines in 

their vehicles are properly accounted for. 

 The closed throttle or motoring torque in all of the engine maps is -5 N-m, except at the idle 

speed. This might be a reasonable simplifying assumption, but in general the motoring 

torque increases with engine speed due to the rise in friction. It might be worth adding some 

sort of speed dependence to ensure correct engine decelerating behavior during non-fueling 

conditions. 

 The shifting strategy can also be considered a significant factor affecting vehicle behavior in 

a drive-cycle simulation such as this. Moreover, they tend to be very specific to the 

combined engine/vehicle configuration, making them hard to obtain from manufacturers or 

extrapolate from ones currently available. The shifting strategy shown in the transmission 

parameter file is only a function of vehicle speed, whereas shifting, in general, is load 

dependent as well. When load is included as a dependency factor, the shifting strategy has to 

be related directly to specific engine map. Most likely these shifting speeds are for WOT, 

but at lower loads the strategy tends to be slightly different to maximize fuel economy. In 

my experience, it is possible to develop reasonable shifting maps optimized for fuel economy 

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based on a given engine map. This is usually achieved by finding the most efficient engine 

speed at various engine load intervals, thus creating an optimum engine operating line, which 

can be related to the vehicle speed through the various gear ratios. The goal of the shifting 

strategy is then to maintain the engine operating as close to this line as possible. This 

approach works well for low/mid load operation. For high loads, where acceleration and 

gradeability are more important, the shifting maps should be corrected, which can be done in 

this case using the available WOT shifting speed numbers. Internally generated shifting 

maps would also allow for engine map scaling as mentioned above, without requiring new 

shifting strategy data. 

 As part of the simulation output and the suggestion for some additional data provided to the 

user, it would be interesting if plots of the engine map and shifting strategy are included. A 

simple assessment of these could give the user a good idea about the appropriateness of the 

given modeling assumptions for their vehicle setup being evaluated. As an extension of this, 

the various drive-cycle visitation points could be plotted on the engine map as well. 

 Some models, such as the electrical system, appear to be extremely complex and detailed for 

this type of dedicated simulation. Unless there is a particular reason, such as future 

extensions to GEM for hybrid-electric trucks or different drive-cycles, where such details are 

necessary, then the electrical system model can probably be stripped down substantially 

without sacrificing much fidelity in the simulation. 

 Similarly, the Stateflow engine logic controller contains some states, such as the ones with 

the engine off, which are probably not seen in any of the simulations, except at the first time 

step. There is no stop-start functionality or cold-start behavior, so it might not be necessary 

to have a full starter model and the engine logic could be somewhat simplified. Another 

related model simplification could be removing the idle controller. A saturation block in 

Simulink could be used to limit the engine operation to a minimum idling speed without 

having an additional controller that can end up slowing down the simulation and increasing 

the complexity of the model. 

 There is a block in the engine model called “trans gear shift” whose output does not appear to 

be actually used anywhere. This block also has a PID controller. It is not clear to me why 

this block is needed, but if it is not, then it should definitely be removed to prevent the 

controller from slowing down the simulation unnecessarily when the block is activated. 

 In general, the rest of the model looks good. I have looked into the various submodels, in 

particular for the engine, transmission and vehicle, and they seem to follow the correct 

approaches. Overall, the model is in great shape and should be a strong starting point for a 

dedicated simulation oriented to compliance purposes. 

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EPA’s Response to Peer

Reviewer Comments

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EPA Response to Peer Review

Overall, the reviewers’ comments toward the Greenhouse gas Emissions Model (GEM) are 

positive and constructive, which can be summarized as follows: 

• Accuracy of systems (i.e., driver, electric, ambient, engine, vehicle and transmission) 

• Parameter sensitivity study related to coefficient of aerodynamic drag (Cd) and

coefficient of tire rolling resistance (Crr) to fuel economy or CO2 emissions

• More model validation

• Model documentation 

• Equations, references, etc. 

At a component level, it can be further summarized as follows

• Driver model with better feed forward components and more configurable PID

(proportional–integral–derivative) gains 

• More consistent electric and accessory models 

• Environment model with more realistic air density and ambient temperature

• Engine model improvements

Many changes have been made since GEM was first released to the public. One of the 

key changes is the driver system model. The enhanced system uses a target vehicle driving 

speed to estimate vehicle torque demand at any given time. Then, the power required to drive 

the vehicle is derived to estimate the required accelerator and braking pedal positions. If the 

driver misses the vehicle speed target, a PID controller applies speed correction logic that adjusts 

accelerator and braking pedal positions for matching targeted vehicle speed at every simulation 

time step. The driver system, with its feed-forward driver controls, more realistically models 

driving behavior. 

Electric system model is modified to use constant electrical power to simulate vehicle 

electronics power consumption. The values for electronic and accessory power consumption are 

modeled as constant over all classes of vehicles. The reason behind modeling power 

consumption in such a manner is that the certification with use of GEM is done on a relative 

basis by comparing the new vehicle model result with the pre-selected engine and vehicle result, 

where all vehicle models use the same electrical and accessory power. The difference in 

selecting electrical or accessory power consumption is not critical and has no impact on the final 

certification results. Since GEM is not used to model absolute vehicle emissions, assigning 

default parameters in the model achieves this objective, even if the absolute emissions may differ 

from those predicted. In other system-level development, the value for ambient density of air 

has been changed to represent more realistic conditions, in accordance with standard SAE 

practices. 

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All bugs noted by the peer reviewers have been identified and fixed with the exception of 

the implementation of an algorithm for “deceleration fuel cut-off” during zero throttle 

deceleration. The agencies recognize that different manufacturers have different fuel cut-off 

control logics and it would be challenging to implement all control logics without manufacturers 

providing the data for final model validation. Consequently, we are delaying implementation of 

a fuel cut-off strategy until a future rulemaking.

In this phase of rulemaking, the agencies have decided to regulate engines and vehicles 

separately, except for heavy-duty pickups and vans. We believe this separation is the most 

appropriate way to achieve the near-term reductions without introducing substantial new testing 

burden on heavy-duty vehicle manufacturers. In the future, though, it may be desirable to certify 

vehicles and their engines to a complete vehicle standard using a complete chassis test procedure 

or a more fully-integrated vehicle model to determine emissions levels for combination tractors 

and some vocational vehicles. At that point, it would be necessary to use fuel maps specific to 

the engines installed in the vehicles being certified. In this first phase of GHG emission 

regulation, though, the GEM model uses fixed engine maps to prevent double counting emission 

reductions from engine improvements (which are subject to compliance with engine standards) 

and then again in the truck model (to comply with vehicle standards). Further, in direct response 

to reviewer comments on the GEM engine system, the engine brake torque value at the closed 

throttle position is no longer negative in the engine fuel map. 

As described in the Regulatory Impact Analysis (RIA) Chapter 4, GEM has been 

validated and benchmarked against test data as well as other well known vehicle simulation tools

since GEM was first released to the public. We extended the model validation to both Class 7 

and Class 8 vehicles using test data. We also benchmarked GEM’s model prediction against the 

GT-Drive model which is commonly used in industry. 

A sensitive analysis of coefficient of aerodynamic drag (Cd) and coefficient of rolling 

resistance (Crr) was conducted following the reviewers’ comments. The study shows that the 

vehicle behavior follows an almost linear relationship between these input parameters and CO2

emissions. Charts in RIA Chapter 2 show the linear trend of the GEM inputs relative to the CO2

emissions results. Nonlinearity is fairly weak in the range of variation of those input parameters. 

Therefore, it is acceptable for the GEM to take as inputs a linear average of fleet Cds and a linear 

average of tire rolling resistances, or Crr. 

The agencies fully recognize the importance of the transmission to overall vehicle 

performance. However, as noted in the peer review, GEM is not designed to model different 

transmissions. Likewise, transmission improvements are not part of the technology package on 

which the GHG emission standard for these vehicles is predicated. GEM’s purpose is to quantify 

the relative effectiveness of a limited suite of technologies and not to discern the absolute GHG 

emissions or fuel consumption of whole trucks. As such, the agencies decided to model only 

those parameters most easily associated with vehicle greenhouse gas emissions reductions. For 

example, in a sleeper cab combination tractor, parameters identified to be the most significant 

include Cd, Crr, weight reduction, governed vehicle speed, and extended idle reduction. 

Transmission improvements could potentially be evaluated as an innovative credit and thus be 

utilized for demonstrating compliance on that basis. 

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