UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MI 48105-2498

 OFFICE OF

 AIR AND RADIATION

July 8, 2015

MEMORANDUM

SUBJECT: Numerical Standards for Tractors, Trailers, and Vocational Vehicles Based on the GEM 

Executable Code; and Derivation of the Numerical Engine Standards 

FROM: Angela Cullen, Engineer, Assessment and Standards Division

Jessica Brakora, Engineer, Assessment and Standards Division

Lauren Steele, Engineer, Assessment and Standards Division

TO: Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and 

Heavy-Duty Engines and Vehicles - Phase 2 - Docket EPA-HQ-OAR-2014-0827

Numerical Standards for Tractors, Trailers, and Vocational Vehicles

The agencies established the proposed standards by evaluating specific vehicle technology

packages using a prepublication version of the Phase 2 GEM. This prepublication version was an 

intermediate version of the GEM source code, rather than the executable file version of GEM, which is 

being docketed for this proposal and is available on EPA’s GEM Webpage. Both the GEM source code 

and the GEM executable file are generally functionally equivalent.

Formal external peer review and expert external user review was then conducted on the version 

of the GEM source code that was used to calculate the numerical values of the proposed standards. It 

was discovered via these external review processes that the GEM source code contained some minor 

software “bugs.” These bugs were then corrected by EPA and the Phase 2 proposed GEM executable 

file was derived from this corrected version of the GEM source code. Moreover, we expect to also 

receive technical comments during the comment period that could potentially identify additional GEM 

software bugs, which would lead EPA to make additional changes to GEM before the Final Rule.

Thus, even without the agencies making any changes in our projections of technology

effectiveness or market adoption rates, it is likely that further revisions to GEM could result in us 

finalizing different numerical values for the standards. It is important to note that the agencies would 

not necessarily consider such GEM-based numerical changes by themselves to be changes in the 

stringency of the standards. Rather, we believe that stringency is more appropriately evaluated in 

technological terms; namely, by evaluating technology effectiveness and the market adoption rates of 

technologies.

The tables below compare results from the prepublication GEM source code with results from

the proposed GEM executable for combination tractors, vocational vehicles, and trailers. The 

combination tractor results are directly from each version of GEM. The vocational vehicle GEM results 
2

underwent post-processing to recognize idle reduction technologies as well as to make some adjustments 

as described below. The trailer results are based on equations developed from both the prepublication 

and proposed GEM versions. Note that NHTSA fuel consumption values are not shown in the tables 

below. These values have been calculated using a conversion factor of 10,180 grams CO2 per gallon of 

diesel fuel, or 8,887 grams CO2 per gallon of gasoline, and are presented in the supporting 

documentation for GEM. It should also be noted that the version of the GEM executable code that was 

used for developing the stringencies uses unrounded inputs, which is different than the compliance

version of the GEM executable, which will round the inputs.

Table 1: Comparison of GEM Results for Combination Tractors

Class 7 Class 8

Heavy

Haul

Day Cab Day Cab Sleeper Cab

Low

Roof

Mid

Roof

High

Roof

Low

Roof

Mid

Roof

High

Roof

Low

Roof

Mid

Roof

High

Roof

Baseline

Pre-Publication Source Code 107 118 121 86 93 95 79 87 88 57

Proposed GEM Executable 106 117 120 85 92 94 79 87 88 57

2021

Pre-Publication Source Code 97 107 109 78 84 86 70 78 77 54

Proposed GEM Executable 96 107 109 77 84 86 70 78 77 54

2024

Pre-Publication Source Code 90 100 101 72 78 79 64 71 70 52

Proposed GEM Executable 89 99 100 72 78 78 64 71 70 51

2027

Pre-Publication Source Code 87 96 96 70 76 76 62 69 67 51

Proposed GEM Executable 86 96 96 70 76 76 62 69 67 51

Table 2: Comparison of Coefficients for GEM-Based Trailer Compliance Equations

Subcategory GEM

Version

GEM-Based Equation Constants

Intercept Aero CRR WR

Long Dry Van

Pre-Publication Source Code 77.4 -6.1 1.7 -0.001

Proposed GEM Executable 77.1 -6.1 1.7 -0.0011

Long Refrigerated Van Pre-Publication Source Code 78.3 -6.0 1.8 -0.001

Proposed GEM Executable 78.0 -6.0 1.8 -0.0011

Short Dry Van Pre-Publication Source Code 134.0 -10.5 2.2 -0.003

Proposed GEM Executable 133.4 -10.2 2.2 -0.0031

Short Refrigerated Van Pre-Publication Source Code 136.3 -10.3 2.4 -0.003

Proposed GEM Executable 136 -10.1 2.4 -0.0031

Table 3: Comparison of GEM-Based Equation Results for Trailers

Dry Van Refrigerated Van

Long Short Long Short

Baseline

Pre-Publication Source Code 85 147 86 151

Proposed GEM Executable 84 147 86 150

2018

Pre-Publication Source Code 83 144 84 147

Proposed GEM Executable 82 143 84 147
3

Dry Van Refrigerated Van

Long Short Long Short

2021

Pre-Publication Source Code 81 142 82 146

Proposed GEM Executable 81 142 82 145

2024

Pre-Publication Source Code 79 141 81 144

Proposed GEM Executable 79 140 80 144

2027

Pre-Publication Source Code 77 140 80 144

Proposed GEM Executable 77 139 79 143

Table 4: Comparison of Post-Processed GEM Results for Vocational Vehicles

LHD MHD HHD

Pre-Publication 

Source Code

Proposed GEM 

Executable

Pre-Publication 

Source Code

Proposed GEM 

Executable

Pre-Publication 

Source Code

Proposed GEM 

Executable

CI Baseline

U

a

316 315 201 200 212 224

M 325 324 203 203 214 228

R 339 338 199 199 203 221

E 318 317 200 200 223 236

SI Baseline

U 334 333 213 212 224 237

M 344 343 215 215 226 241

R 358 358 211 210 214 234

CI 2021

U 296 295 188 187 198 209

M 305 304 190 189 200 213

R 318 317 186 186 189 207

E 312 311 195 196 215 231

SI 2021

U 320 319 203 202 214 226

M 329 329 205 205 216 230

R 343 342 201 201 204 223

CI 2024

U 284 282 179 179 190 199

M 292 291 181 181 192 203

R 304 303 178 177 182 197

SI 2024

U 312 310 197 197 208 219

M 321 320 199 199 210 223

R 334 333 196 195 199 217
4

LHD MHD HHD

Pre-Publication 

Source Code

Proposed GEM 

Executable

Pre-Publication 

Source Code

Proposed GEM 

Executable

Pre-Publication 

Source Code

Proposed GEM 

Executable

CI 2027

U 272 265 172 168 182 188

M 280 273 174 170 183 192

R 292 285 170 166 174 186

SI 2027

U 299 292 189 185 196 207

M 308 301 191 187 198 211

R 321 314 187 183 188 205

Note:

a U = Urban, M = Multipurpose, R = Regional, E = Emergency

Derivation of Vocational Vehicle Standards

The agencies developed a method to normalize the standards so that vehicles certified over the 

Regional composite cycle would have similar stringency as vehicles certified over the Multi-Purpose or 

Urban cycles. The reason for this is that the agencies expect each vehicle to be tested with an 

appropriate duty cycle for its intended application and want to minimize unintended incentives to place a 

vehicle in a less appropriate subcategory to receive a benefit of a less stringent standard. A summary of 

the normalizing process can be found in the draft RIA Chapter 2.9.2.1.

Characteristics of the baseline vehicles that were used for each subcategory can be found in the 

draft RIA: Predefined modeling parameters are presented in Table 4-8, 4-9 and 4-10 of the draft RIA 

Chapter 4; and User-enterable modeling parameters are presented in Table 2-43, 2-44, and 2-45 of draft 

RIA Chapter 2.9.2. 

As described in draft RIA Chapter 2.9.2.1, the agencies identified a “best baseline vehicle” with 

which to normalize the performance of the defined baseline vehicles of the three duty cycles within each 

weight class. Table 5 lists some characteristics of the vehicles that the agencies chose as the “best 

baseline vehicles” for each weight class. For the HHD weight class, these are the vehicles intended to 

be certified over the Regional cycle, while for the MHD and LHD weight classes, these are the vehicles 

intended to be certified over the Multi-Purpose cycles. In the draft RIA at 2.9.2.1, it is stated that the 

best vehicle for LHD and MHD was the Urban vehicle, however that statement was an error. In the 

LHD weight class, the Multi-Purpose vehicle is identical to the Urban vehicle, and in the MHD weight 

class, the Multi-Purpose vehicle is nearly identical to the Urban vehicle, with a slightly different tire 

radius.
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Table 5: Best Vocational Baseline Vehicles

Weight 

Category Engine Transmission Tire Radius (m)

Drive Axle 

Ratio

Steer CRR 

(kg/t)

Drive CRR 

(kg/t)

HHD 455 hp 10 spd MT 0.483 3.76 7.7 7.7

MHD 270 hp 5 spd MH AT 0.462 4.88 7.7 7.7

LHD 200 hp 5 spd MH AT 0.378 4.56 7.7 7.7

GEM was used to simulate each of the vehicles over its intended duty cycle as well as over the 

other two composite cycles. Following the process described in the draft RIA 2.9.2.1, the normalization 

process using the proposed GEM executable yields values presented in Table 6, which can be compared 

with the results obtained using the pre-publication source code, as presented in Table 2-46 of the draft 

RIA.

Table 6: Vocational Baseline Normalizing

CLASS 2b-5 CLASS 6-7 CLASS 8

Urban MultiPurposeRegional Urban MultiPurposeRegional Urban MultiPurposeRegional

Straight GEM Baseline Performance

317 327 331 200 204 197 236 233 200

Normalization Factor

0.97 1.00 1.04 1.00 1.01 0.99 0.99 1.01 0.98

Population Weighted Best Vehicle Result

328.8 202.4 193.9

Population Weighted Baseline Result

325.5 201.1 225.3

Normalized Baseline

315 324 338 200 203 199 224 228 221

The numerical stringencies of the proposed standards were determined by applying technology

effectiveness and adoption rates as described in the proposal (see draft RIA 2.9.5), then averaging the 

subcategory-specific percent improvements within each weight class, then calculating the numerical 

value of the standard by applying this averaged percent improvement to the normalized baseline. For 

example, for MY 2027 in the HHD weight class, the technology package as specified in the proposal for 

the Regional subcategory would achieve 11.6% improvement, the Multi-purpose technology package 

would achieve 17.4% improvement, and the Urban technology package would achieve 18.9% 

improvement. These average to 16%, so the HHD 2027 standards were calculated by reducing each of 

the HHD normalized baseline values by 16%.

Derivation of the Proposed Engine Standards

As described in Section II.D(a) of the HD Phase 2 Notice of Proposed Rulemaking, the agencies 

derived the proposed engine standards based on our projections of the effectiveness of various 

technologies over the proposed engine test cycles (i.e., percent reduction) and the extent to which such 

technologies could be adopted by industry by 2021, 2024, and 2027. The values used for tractor engines 

are shown in Table 7 below.
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Table 7: Projected Tractor Engine Technologies and Reduction

SET-WEIGHTED 

REDUCTION (%) 

2020-2027

MARKET 

PENETRATION 

(2021)

MARKET 

PENETRATION 

(2024)

MARKET 

PENETRATION 

(2027)

Turbo compound with clutch 1.8% 5% 10% 10%

WHR (Rankine cycle) 3.6% 1% 5% 15%

Parasitic/Friction (Cyl Kits, 

pumps, FIE), lubrication

1.4% 45% 95% 100%

Aftertreatment (lower dP) 0.6% 45% 95% 100%

EGR/Intake & exhaust 

manifolds/Turbo /VVT/Ports

1.1% 45% 95% 100%

Combustion/FI/Control 1.1% 45% 95% 100%

Downsizing 0.3% 10% 20% 30%

Dis-Synergy Factor 75% 85% 85%

Weighted Reduction for 

Engines w/o TC or WHR 1.45% 3.44% 3.64

Weighted Reduction for 

Engines with TC 2.79% 4.92% 5.12%

Weighted Reduction for 

Engines with WHR 4.08% 6.36% 6.56%

Total Weighted 

Reduction (%)

1.5% 3.7% 4.2%

The proposed standards were derived from the weighted reductions which combined all these 

technologies by multiplying together an effectiveness factor for each technology (equal to 100% minus 

the SET weighted percent reduction times the market penetration) and applying a dis-synergy factor that 

reflects the degree to which combining multiple technologies reduces slightly the effectiveness of each

individual technology. The assumed dis-synergy factors are also shown in Table 7. Because turbo 

compound and Rankine cycle waste heat recovery are mutually exclusive technologies, we calculated 

intermediate values for engines without either turbo compound or Rankine cycle waste heat recovery, 

and for engines with either turbo compound or Rankine cycle waste heat recovery. These intermediate 

values are also shown in Table 7. For example, the average percent reduction in 2021 for engines 

without either turbo compound or Rankine cycle waste heat recovery would be:

[100% - (100%-(45% ×1.4%))×(100%-(45% ×0.6%))×(100%-(45% ×1.1%))×(100%-(10% ×0.3%))]×75% = 1.45%

For each change in the standards (in 2021, 2024, and 2027) these intermediate values were combined 

based on the projected penetrations for turbo compound or Rankine cycle waste heat recovery. For 

example, the total weighted reduction for 2021 was calculated as:

1.45% × (100% - 5% - 1%) + 2.79% × 5% + 4.08% × 1% = 1.543%

This same approach was used to calculate the proposed vocational engine standards, although it 

was not necessary to calculate intermediate values. Table 8 below summarizes the derivation of the 

vocational engine standards.
7

Table 8: Projected Vocational Engine Technologies and Reduction

FTP WEIGHTED 

REDUCTION (%)

2021-2027

MARKET 

PENETRATION

2021

MARKET 

PENETRATION

2024

MARKET 

PENETRATION

2027

Model based control 2.0% 25% 30% 40%

Parasitic /Friction 1.5% 60% 90% 100%

EGR/Air/VVT /Turbo 1.0% 50% 90% 100%

Improved AT 0.5% 50% 90% 100%

Combustion Optimization 1.0% 50% 90% 100%

Dis-Synergy Factor 75% 85% 85%

Total Weighted 

reduction (%)

2.0% 3.5% 4.0%

The numerical tractor and vocational engine standards for Alternatives 2, 4, and 5 were also 

calculated in the same manner as the proposed engine standards.