UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MI 48105-2498

 OFFICE OF

 AIR AND RADIATION

May 27, 2015

MEMORANDUM

SUBJECT: Greenhouse Gas Emission Model (GEM) Simulation of the Best of 2027 Tractor-Trailers

FROM: Houshun Zhang, Jessica Brakora, and Angela Cullen

Mechanical Engineers, Office of Transportation and Air Quality

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

Background:

EPA analyzed the potential impact of the proposed Phase 2 Heavy-Duty Greenhouse Gas 

Emission and Fuel Efficiency Standards in terms of what we project could be the best of the tractortrailers that would be built in 2027 and compare it to a typical tractor-trailer built in 2010.

Key Findings:

Based on the GEM simulation results detailed in the following sections, EPA projects the best 

tractor-trailer sold in 2027 will emit approximately 49 to 55 grams of CO2 for every ton of payload 

carried per mile on the highway, which represents a 48 to 51 percent reduction in CO2 emissions when 

compared to a typical tractor-trailer sold in 2010. EPA also projects the best tractor-trailer sold in 2027 

will achieve 10 mpg while traveling at highway speeds. This vehicle would achieve double the fuel 

economy of a typical tractor-trailer sold in 2010.

Engine and Vehicle Inputs to the Simulation Model:

EPA developed the GEM inputs for two types of tractor-trailers – a typical tractor trailer built in 

2010 and a tractor-trailer that represents the best of what could be built in 2027. Details of the inputs 

are included in the following subsections. In addition, since the Phase 1 and Phase 2 GEM models 

differ, details of how EPA compared vehicles across the two models are provided.
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2010 Baseline Engine

EPA developed a baseline 2010 engine fuel map for the HD Phase 1 program. The engine 

represents a typical 15 L engine with a 455 horsepower rating built in 2010. The engine contains a 

selective catalytic reduction (SCR) system and a diesel particulate filter to meet the 2010 EPA standard 

for oxides of nitrogen (NOx) and particulate matter (PM). The 2010 engine fuel map used in simulation 

is shown below in Figure 1.

Figure 1: 2010 Engine Fuel Map

2027 Engine with Waste Heat Recovery

EPA developed a 2027 engine fuel map that included all of the technologies considered in the 

HD Phase 2 program, including waste heat recovery, downsizing, and improvements in parasitic loads, 

friction, aftertreatment backpressure, intake and exhaust flow, and combustion. The corresponding 

engine is a 15 L engine with 455 horsepower rating, the same displacement and output used in the 

2010 baseline engine. The engine was also developed to meet the 2010 EPA standard for NOx and PM.

The impact of each of these technologies relative to a 2017 MY engine over the 13 modes of the 

Supplementary Engine Test used in heavy-duty diesel engine certification is included in 

Table 1. The corresponding fuel map is shown in Figure 2.

The resulting engine has a peak brake thermal efficiency (BTE) of approximately 49.8% at 1150 

rpm and 66% load. The composite BTE and CO2 over the SET is 46.5% and 430.3 g/hp-hr with the 

current 13 mode weightings. With the proposed Phase 2 mode weightings, the composite BTE is 47.5% 

and CO2 is 421.0 g/hp-hr. The optimal engine speed occurs between 1100 and 1250 rpm, consistent 

with trends in the heavy-duty engine industry.

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455 HP / 15 L : 2010 Baseline BSFC (g/kW-hr)

Engine Speed (RPM)

Torque (Nm)

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Table 1: Impact of Engine Technologies on the 13 Modes of the Supplementary Engine Test (SET)

2017 Engine as Baseline

TECHNOLOGY SET WEIGHTED 

REDUCTION (%) 

WHR (RANKINE CYCLE) 3.6%

PARASITIC/FRICTION (CYL KITS, PUMPS, FIE), 

LUBRICATION

1.4%

AFTERTREATMENT (LOWER DP) 0.6%

EGR/INTAKE & EXHAUST MANIFOLDS/TURBO 

/VVT/PORTS

1.1%

COMBUSTION/FI/CONTROL 1.1%

DOWNSIZING 0.3%

Figure 2: 2027 Engine with Waste Heat Recovery Fuel Map

2010 and 2027 Tractor-Trailer Attributes

The 2010 vehicle attributes of tire rolling resistance and coefficient of drag area were 

developed based on EPA’s assessment of the baseline 2010 tractor-trailer in the HD Phase 1 final rule 

(76 FR at 57206). The tractor-trailer coefficient of drag area (CdA) value was calculated based on a 

10.4 m2frontal area and the HD Phase 1 Cd value of 0.68, which represents a HD Phase 1 Bin II high 

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455 HP / 15 L : 2027 Standard BSFC (g/kW-hr)

Engine Speed (RPM)

Torque (Nm)

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roof tractor with a generally aerodynamic shape pulling a box trailer with no aerodynamic devices. The 

GEM inputs used to derive the baseline 2010 tractor-trailer performance are listed below in Table 2.

In general, the 2027 Best Tractor-Trailer represents a combination of the best of each of the 

technologies that the agencies used to demonstrate feasibility of the proposed HD Phase 2 2027 MY 

tractor and trailer standards. The 2027 tractor steer and drive tire rolling resistance values represent

the HD Phase 2 NPRM Level 3 tires. This level of rolling resistance represents the long-term rolling 

resistance value that Michelin predicts could be achieved in the 2025 timeframe. The agencies are 

proposing 2027 MY high roof sleeper cab standards premised on a 25% adoption rate of tires with 

rolling resistance at this level. The 2027 trailer tire coefficient of rolling resistance of 4.7 kg/metric ton 

represents a Level 2 tire in the HD Phase 2 NPRM. This level of trailer tire rolling resistance is available 

today and the agencies developed standards for long box trailers based on nearly full adoption of this 

level of rolling resistance by 2027. The 2027 Best Tractor-Trailer CdA value of 4.0 m2represents a wind 

averaged value and is derived from the proposed HD Phase 2 tractor Bin VI CdA value of 4.7 m2, which 

is proposed at a 20% adoption level in 2027. The 4.7 m

2

tractor-trailer CdA value represents an 

aerodynamic tractor in combination with a box trailer with side skirts. The HD Phase 2 NPRM Trailer 

Aero Bin III represents a trailer with a side skirt and has a CdA delta of 0.3 m

2

. The HD Phase 2 NPRM 

Trailer Bin VI represents a box trailer with a combination of advanced aerodynamic devices (including 

SmartWay Elite) and has a CdA delta equal to 1.0 m

2

. The CdA delta between a trailer with only a skirt 

and a trailer with Bin VI advanced aerodynamics is 0.7 m

2

(equal to 1.0 – 0.3 m2). We subtracted the

trailer CdA delta of 0.7 m

2

from the tractor CdA level of 4.7 m

2

to derive the overall 2027 Best TractorTrailer CdA level of 4.0 m

2

. In addition, we assume a slightly lower final drive ratio of 2.26 than the 

2.34 (3.2 axle ratio multiplied by the 0.73 transmission top gear ratio) value used to derive the 

proposed HD Phase 2 2027 MY tractor standards. A 10-speed automated manual transmission (AMT)

is used in the 2027 vehicle and represents a 50% adoption rate in deriving the proposed HD Phase 2 

tractor standards. The GEM inputs used to derive the Best 2027 Tractor-Trailer performance are listed 

below in Table 2.

Table 2: 2010 and 2027 Class 8 High Roof Sleeper Cab Tractor with 53’ Box Trailer GEM Inputs

Vehicle Attribute 2010 Baseline 2027 Best Tractor-Trailer

Fuel Map 2010 15L – 455HP 2027 15L - 455 HP - w/ WHR

Total weight (lb) 70500 70500

CdA (m2) 7.2 4.0

Trailer Tire RR (kg/metric ton) 6.0 4.7

Steer Tire RR (kg/metric ton) 7.8 4.3

Drive Tire RR (kg/metric ton) 8.2 4.5

Transmission model 10 speed AMT with DD 10 speed AMT with DD

Transmission Top Gear ratio 1 .0 1 .0

Rear Axle Ratio 2.64 2.26

Axle efficiency 93.5 % 96%
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The Best 2027 Tractor-Trailer also utilizes additional technologies proposed to be allowed for 

compliance in the HD Phase 2 NPRM, as shown in Table 3.

Table 3: 2010 and 2027 Technology Impacts

Technology CO2 reduction (%) in 2010 CO2 reduction (%) in 2027

6x2 Axle 0.0% 1.5%

Low Friction Axle Lube 0.0% 0.5%

Automated manual transmission (AMT) 0.0% 2.0%

Predictive Cruise control 0.0% 1.0%

Air conditioning load reduction 0.0% 0.5%

Electric Accessories 0.0% 0.5%

Automated Tire Inflation Systems 0.0% 1.0%

The 2010 and 2027 vehicles were simulated using the 55 MPH, 65 MPH, and ARB Transient 

drive cycles. Two sets of runs were conducted, one without road grade in any of the cycles and the 

other with the proposed Phase 2 road grade in the 55 MPH and 65 MPH cycles. The weighted cycle 

results reflect the Phase 1 and proposed Phase 2 cycle weightings of 86% of the 65 MPH cycle, 9% of 

the 55 MPH cycle, and 5% of the ARB transient cycle.

Simulation Results:

The results of the GEM simulation without road grade in the cycles are shown in Table 4. The 

GEM simulation results of the cycles with road grade are shown in Table 5. Based on these results, EPA 

projects the best tractor-trailer sold in 2027 will emit approximately 50 grams of CO2 for every ton of 

payload carried per mile on the highway, which represents a 50 percent reduction in CO2 emissions

when compared to a typical tractor-trailer sold in 2010. EPA also projects the best tractor-trailer sold 

in 2027 will achieve over 10 mpg while traveling at highway speeds. This vehicle would achieve double 

the fuel economy of a typical tractor-trailer sold in 2010.

Table 4: Simulation Results without Road Grade in the 55 mph and 65 mph Cycles

2010 Baseline

CO2 (g/ton-mile)

2027

CO2 (g/ton-mile)

2010 Baseline

Fuel Economy 

(mpg)

2027

Fuel Economy 

(mpg)

55 MPH 81.8 40.3 6.6 13.3

65 MPH 100.2 46.9 5.4 11.4 

ARB Transient 159.9 110.1 3.4 4.9 

Weighted Results 101.5 49.4 5.3 10.8
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Table 5: Simulation Results with Road Grade in the 55 mph and 65 mph Cycles

2010 Baseline

CO2 (g/ton-mile)

2027

CO2 (g/ton-mile)

2010 Baseline

Fuel Economy 

(mpg)

2027

Fuel Economy 

(mpg)

55 MPH 85.9 46.3 6.2 11.6

65 MPH 102.7 52.1 5.2 10.3

ARB Transient 159.9 110.1 3.4 4.9

Weighted Results 104.1 54.5 5.2 9.8

The results of the GEM simulation also show that the proposed 2027 model year CO2 emission 

standard for high-roof sleeper cabs is 35 percent lower than the 2010 baseline high-roof sleeper cab. 

The results are show below in Table 6.

Table 6: Simulation Results with Road Grade in the 55 and 65 mph Cycles Compared to the Proposed Standard

Simulated

2010 Baseline

CO2 Emissions 

(g/ton-mile)

Proposed 

2027 MY

CO2 Standards 

(g/ton-mile)

Weighted Results 104 67