UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2565 PLYMOUTH ROAD

ANN ARBOR, MI 48105-2498

 OFFICE OF

 AIR AND RADIATION

June 2015

MEMORANDUM

SUBJECT: Possible Tractor, Trailer, and Vocational Vehicle Standards Derived from Alternative 

Road Grade Profiles

FROM: Lauren Steele, Houshun Zhang, OTAQ/ASD

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

Introduction

The purpose of this memorandum is to illustrate some possible effects on the calculated fuel 

consumption and CO2 emissions of vehicles caused by variations in the test procedure for simulating 

road grade in the Greenhouse gas Emissions Model (GEM). This memorandum presents possible road 

grade profiles other than the one presented in the proposal, as well as an analysis of exploratory work 

using the Phase 2 GEM with alternate road grades.

1

 The road grade profiles described below include 

two that were developed by the Department of Energy’s National Renewable Energy Laboratory

(NREL) and one that was developed by the agencies. For completeness the proposed profile that is in 

Phase 2 GEM is described as well.

It is a goal of the agencies to select the most appropriate road grade profile(s) on the basis of being 

nationally representative as well as reasonably similar to real-world driving conditions. It is not the 

agencies’ intent to cause the standards to become more or less stringent as a result of selection of road 

grade profile. Any effect on stringency is minimized by the fact that both the baseline case and the 

stringency case will be simulated in GEM with the same road grade profile. However, it is expected that 

various engine and driveline technologies will perform differently over different road grade profiles, 

thus changing the resulting numerical results in all cases.

The agencies acknowledge that if a road grade profile is very aggressive, some vehicles may not be able 

to follow the full trace at speed. In the preamble to this rulemaking we request comment on the merits 

of developing variable speed highway cycles. Our goal is to adopt a cycle that is challenging enough to 

ensure that vehicles continue to be built for real world performance, while maintaining the simulated 

vehicle operation within a range for which GEM has been validated. 

 

1

In the preamble this memo is referred to as being dated in April (note 194 Section III) and May (note 240 Section IV and 

note 264 Section V). The exploratory work described below was conducted in April and May 2015.
2

Proposed Road Grade Profile

The agencies are proposing an interim road grade profile for development of the proposed standards, as 

shown below in Figure 1, in both the 55 mph and 65 mph cycles. The grade profile was developed by

Southwest Research Institute (SwRI) on a 12.5 mile stretch of restricted-access highway during on-road 

tests conducted for EPA’s validation of the Phase 2 version of GEM. The minimum grade in the interim 

profile is -2.1 percent and the maximum grade is 2.4 percent. The profile includes 30 percent of the 

distance at grades of +/- 0.5 percent. Overall, the profile includes approximately 50 percent of the time 

in relatively flat terrain with road gradients of less than 1 percent.

Figure 1: Proposed Road Grade Profile for 55 mph and 65 mph Drive Cycles

The following charts illustrate the degree to which the SwRI road grade profile in GEM matches the 

nationally representative profile, as characterized by NREL. In Figure 2, the GEM profile has generally

lower magnitude grades than the national data, with a lack of grades between 2 and 4%. In Figure 3 the 

majority of half hills in the GEM profile are of length 0.5 to 1.5 miles, with a lack of half hills less than 

0.5 mi and longer than 1.5 miles.

-4

-2

0

2

4

0 2 4 6 8 10 12 14

Grade (%)

Distance (mi)

Road Grade on the 55 mph and 65 mph Cycles
3

Figure 2: Comparison of National Cumulative Absolute Road Grade with SwRI Route (Source: NREL)

Figure 3: Comparison of National Cumulative Half Hill Distance with SwRI Route (Source: NREL)
4

NREL Road Grade Profiles 

A complete description of the methodology for generating the NREL road grade profiles, as well as 

several other possible alternate profiles, is found in a May 2015 report titled, “Development of Road 

Grade Profiles Representative of US Controlled Access Highways,” available in the docket. Numerical 

representations of these distance-based profiles are also available in the docket, in a format that is ready

for modeling.

The minimum grade in the example 55 mph NREL profile shown in Figure 4 is -3.1 percent and the 

maximum grade is 3.8 percent. The majority of the profile is within a range of grades of +/- 2 percent. 

Of the five nationally representative 55 mph profiles generated by NREL in the final report, this one was 

selected as the example because the shorter length of the synthetic profile (compared to the relatively 

long local routes) has many advantages. Also, the road grade is steeper than in the other synthetic 

profile.

The minimum grade in the example 65 mph NREL profile shown in Figure 5 is -2.2 percent and the 

maximum grade is 3.3 percent. The majority of the profile is within a range of grades of +/- 2 percent. 

Of the four nationally representative 65 mph profiles generated by NREL in the final report, this one 

was selected as the example because the shorter length of the synthetic profile (compared to the 

relatively long local routes) has many advantages. Also, the length of distance spent in a steep downslope is less than in the other synthetic profile.

Figure 4: Example NREL road grade profile for 55 mph cycle
5

Figure 5: Example NREL road grade profile for 65 mph cycle

EPA Road Grade Profile 

The profile depicted in Figure 6 is 11.48 miles (18.44 km) long, its longest half hill spans 2.87 miles (4.47 

km) and its highest grade equals 2.5%. It was developed to align with the national road grade data using a 

methodology differing from NREL’s and may offer some advantages. Of several profiles developed by EPA, 

this one was selected for analysis in part due to its aggressive uphill.

A complete description of the methodology for generating the EPA road grade profile is found in a May

2015 memorandum titled, “Development of an Alternative, Nationally Representative, Activity 

Weighted Road Grade Profile for Use in EPA GHG Certification of Medium- and Heavy-Duty 

Vehicles,” available in the docket.
6

Figure 6: Example EPA road grade profile for both cruise cycles

Exploratory Work Analyzing the Effect in GEM of Alternative Road Grade Profiles

The agency has conducted exploratory work to evaluate the effects on vehicle performance as simulated 

in GEM of applying different road grade profiles. The objective of this analysis is to understand how the 

default transmission auto-shift responds to demands to move heavy vehicles over different road grades. 

This auto-shift is being used for the stringency development as well as the future compliance tool under 

the Phase 2 rulemaking.2,3 The results are inconclusive, but do offer insights as to how GEM could be 

further improved to represent more realistic real-world driving conditions. 

For this analysis, three profiles were examined: (1) the EPA profile described above and presented in 

Figure 6 (Profile 1); an amplified version of Profile 1 referred to as Profile 2, and (3) the proposed 

profile described above and presented in Figure 1 (“Baseline”). Figures 7 and 8 show the road grades

and elevations, respectively, of these three profiles explored in this study. 

 

2 Newman, K., Dekraker, P., Zhang, H. Sanchez, J., et al, “Development of Greenhouse Gas Emissions Model (GEM) for 

Heavy- and Medium-Duty Vehicle Compliance,” SAE Technical Paper, October 6-9, 2015.

3 Newman, K., Kargul, J., and Barba, D., “Development and Testing of an Automatic Transmission Shift Schedule Algorithm 

for Vehicle Simulation,” SAE Int. J. Engines 8(3):2015, doi:10.4271/2015-01-1142
7

Figure 7 Alternate Road Grade Profiles Explored

Figure 8 Alternate Elevation Profiles Explored

It can be seen that Profiles 1 and 2 are similar in term of shape, except that Profile 2 has a higher road 

grade up to 4%. In fact, Profile 2 was derived from Profile 1 by multiplying a constant to Profile 1 in 

order to achieve a maximum road grade of 4%. This hypothetical road grade is mainly used to check 

how the auto-shifting algorithm responds with a road grade up to 4%, and was not designed to align with 

the national data. The Baseline and Profile 1 have similar road grade in terms of maximum road grade, 

although elevation is quite different. Compared to the Baseline, both Profiles 1 and 2 have a long hill, 

which can test how the default transmission auto-shift responds to this condition. 

In order to evaluate the impact of the road grade on the vehicle performance as well as check the fidelity

of the default transmission auto-shift, a vehicle with the key parameters defined in Table 1 is selected for 

-4

-3

-2

-1

0

1

2

3

4

5

0 5 10 15 20 25

R

oad

Grad

e (%)

Distance (km)

Profile 1

Profile 2

Baseline

-60

-40

-20

0

20

40

60

80

E

0 5 10 15 20 25 le

vatio

n (m)

Distance (km)

Profile 1

Profile 2

Baseline
8

this study, which represents a class 8 tractor with specifications aligning with some technologies that 

could be adopted in the 2027 timeframe.

Table 1 Class 8 Combination Tractor Sleeper Cab Modeling Parameters

Regulatory Class Class 8 Combination

Sleeper Cab - High Roof

Engine 455hp rating map for 2027

Transmission gear ratio [0 14.8 10.95 8.09 5.97 4.46 3.32 2.45 1.81 1.35 1]

Gearbox Efficiency 98% for the gear with a 1:1 gear ratio, and 96% for all other 

gears

CdA 4

Steer CRR (kg/t) 4.3

Drive CRR (kg/t) 4.5

Trailer CRR (kg/t) 4.7

Axle Mechanical Efficiency 96%

Total weight (kg) 31978

Axle ratio 2.26

Number of Axles 5

Default Axle Configuration 6x4

Electrical Accessory Power (W) 300

Mechanical Accessory Power (W) 1000

Environmental Air Temperature (°C) 25

Payload (tons) 19

Figures 9 and 10 below present the simulated results of the vehicle described in Table 1 over Profile 1

and a 65 mph drive cycle.

Figure 9 Vehicle speed traces, gear number, engine speed and torque over 65 mph cycle for Profile 1

1200 1300 1400 1500 1600 1700 1800 1900

50

60

70

80

90

100

110

120

130

Time (S)

V

eh S

p

d/Eng sp

d*10/G

e

ar#*10

Veh Targeted speed (mph)

Veh Speed (mph)

Gear #

Engine speed (rpm)
9

Figure 10 Engine operating points over 65 mph cycle for Profile 1

From Figure 9, it can be seen that the engine is under-powered during the long hill operation because the 

vehicle is unable to follow the targeted vehicle speed trace. The transmission stays in the top gear. The 

engine operating points as shown in Figure 10 seem to make sense, suggesting that the auto-shifting is 

able to handle this road grade profile up to 2.5%. 

The situation changes when higher road grade with a maximum of 4% is evaluated. Figure 11 and 12

below present the simulated results of the vehicle described in Table 1 over Profile 2 and a 65 mph drive 

cycle.

Figure 11 Vehicle speed traces, gear number, engine speed and torque over 65 mph cycle for Profile 2

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

-500

0

500

1000

1500

2000

2500

RPM

T

orque (N

m)

Full Torque

Engine Operating Points

1200 1300 1400 1500 1600 1700 1800 1900

40

60

80

100

120

140

160

180

Time (S)

V

eh S

p

d/Eng sp

d*10/G

e

ar#*10

Veh Targeted speed (mph)

Veh Speed (mph)

Gear #

Engine speed (rpm)
10

Figure 12 Engine operating points over 65 mph cycle for Profile 2

Figure 11 shows that the vehicle cannot meet the targeted speed trace due to the under-powered engine. 

When the vehicle climbs the hill, there is a long delay for 50 seconds before shifting to lower gear. This 

results in the engine operating points moving to the lower speed end along the full torque curve as 

indicated in Figure 12. In reality, the truck running on the road should also have a minimum engine 

speed limit. In contrast, the default auto-shifting algorithm currently allows the engine speed to drop 

below 900 rpm without shifting. These behaviors indicate an opportunity to improve GEM for the final 

rulemaking. Another key observation and area for improvement is how the engine behaves during 

climbing once the transmission shifts to a lower gear. As can be seen in Figure 11, the engine first drops 

the speed a little bit after 1520 seconds, and then keeps rising for more than 80 seconds without shifting. 

This causes the engine to run along the peak torque curve while the speed rises, which is not a very

efficient zone. 

Table 2 shows the tractor performance in grams of CO2 per ton-mile over the individual GEM drive 

cycles as well as the composite cycle with weightings for a sleeper cab tractor, using the three different 

highway cruise road grade profiles explored in this study. No road grade was applied to the ARB 

Transient cycle. As expected, the vehicle emits more CO2 when driving the more aggressive road grade 

using Profile 2.

Table 2 GEM performance (g CO2/ton-mi) of a hypothetical 2027 class 8 sleeper cab tractor over 3 road grade profiles

Baseline Profile 1 Profile 2

55mph 62.24 58.63 68.5

65mph 71.73 67.13 74.28

ARB 128.17 128.17 128.17

Weighted 73.69 69.42 76.45

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

-500

0

500

1000

1500

2000

2500

RPM

T

orque (N

m)

Full Torque

Engine Operating Points
11

This exploratory work illustrates areas for improvement in the calibration of the agency’s transmission 

auto-shift as indicated by Figures 11 and 12, when high road grade is used for highway cruise drive

cycles. To attain the agencies’ goal of representing real world driving, we believe we would need to 

make corrections on the GEM auto-shifting.

In the preamble, the agencies have asked stakeholders for comments, hopefully supported by testing 

data, to help us to calibrate our auto-shift model. More importantly, EPA has developed a 

comprehensive testing plan to obtain first-hand testing data. This plan includes the powertrain tests for 

both medium heavy-duty and heavy heavy-duty powertrains conducted at SwRI, and heavy heavy-duty

powertrains with three transmission setups at Oak Ridge National Laboratory. Up to eight different 

road grade profiles will be tested, and a large data set will be collected to see how the transmissions are 

shifted, thus recalibrating some model constants used in the auto-shifting table. It is expected that a

well-calibrated GEM would be able to address a wider range of operations including high road grade 

routes.