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Comprehensive Truck Size and Weight Limits Study - Modal Shift Comparative Analysis Technical Report

Chapter 5: Scenario Impacts on Energy and Emissions

5.1 Scope

The purpose of this subtask is to evaluate the effect of alternative truck configurations on the fuel consumption and greenhouse gas emissions of the fleet for each scenario. The baseline vehicles and alternative configurations were evaluated on a range of drive cycles to determine their load-specific fuel consumption and emissions. The results of this analysis will be combined with modal shift data to represent the overall fuel consumption and emissions impacts on the fleet.

5.2 Methodology

In previous truck size and weight studies such as the 2000 CTSW Study, a simple table showing truck fuel economy in miles per gallon as a function of vehicle configuration and combined vehicle weight was used as an input to the energy and emissions analysis. For example in the 2000 CTSW Study, a triple 28-foot trailer combination is listed as having 11 to 17 percent greater fuel economy than that of a 3-axle, 53-foot box van trailer operating at the same vehicle weight. In practice, the 28-foot triple combination should suffer from higher aerodynamic drag than the 53-foot box van trailer, and thus would be expected to have lower fuel efficiency.

The more recent OECD report “Moving Freight with Better Trucks” uses fuel consumption values from road tests conducted by the German trucking magazine Lastauto Omnibus (page 152). These tests are run at maximum GCW over a defined route on German highways.

The approach selected for this study is to use baseline engine and vehicle models that are calibrated against experimental data, and then modify the models to represent the range of alternative truck configurations selected for this 2014 CTSW Study. This approach was first used in a 2009 report by the Northeast States Center for a Clean Air Future (NESCCAF) entitled Reducing Heavy-Duty Long Haul Combination Truck Fuel Consumption and CO2 Emissions. The approach used here is also being used by the Southwest Research Institute (SwRI) in a study of fuel efficiency technologies being conducted for the National Highway Traffic Safety Administration (NHTSA), under contract GS-23F-0006M/DTNH22-12-F-00428. The models used in this project have been previously developed and verified as part of this NHTSA project.

The engine selected for this project is a 2011 model Detroit DD15. This is a widely used long- haul truck engine that has more than 20 percent of the long haul market share. The DD15 meets US EPA 2010 emissions requirements, and a slightly modified version of the engine has since been certified to meet the EPA’s 2014 greenhouse gas requirements. From a proprietary benchmarking program, SwRI has an extensive set of performance, emissions, and fuel consumption data on this engine. Under the NHTSA contract, the experimental data was used to build and calibrate a GT-POWER simulation model of the engine. GT-POWER is a commercially available engine simulation tool. Four different ratings of the engine were developed in GT-POWER for this study: 428 HP, 485 HP (the baseline rating), 534 HP, and 588 HP.

The alternative engine power ratings were developed in order to maintain power-to-weight ratios for some of the alternative vehicle scenarios. For some scenarios, a much higher power would be required to maintain baseline vehicle performance. For example, if gross combined weight (GCW) is increased from 80,000 pounds to 129,000 pounds, the baseline engine rating of 485 HP would need to increase to 782 HP in order to maintain the same vehicle acceleration and grade performance. Since engines with greater than 600 HP are not available in the US truck market, the USDOT study team decided to limit engine power to 588 HP and accept performance penalties for the highest vehicle weights. Detailed results of the engine simulation analysis are provided in Appendix D.

The tractor selected for this 2014 CTSW Study is a Kenworth T-700 high roof sleeper tractor. This truck is not offered with the DD15 engine, but it is offered with the Cummins ISX, another 15 liter engine with similar performance, emissions, and fuel consumption characteristics. Coast-down testing[7] of the tractor with a 53 foot box van trailer was performed by SwRI under an EPA project to obtain aerodynamic drag and rolling resistance characteristics of the tractor. The T-700 is an aerodynamic tractor using standard (not SmartWay[8]) tires, and the baseline trailer has no aerodynamic or low rolling resistance features. This tractor-trailer combination represents approximately the average current fleet vehicle performance from an aerodynamic and rolling resistance perspective.

The USDOT study team performed the vehicle simulation using SwRI’s Vehicle Simulation Tool (VST). This software package is based on the National Renewable Energy Lab (NREL) Advisor vehicle simulation program, which has hundreds of users worldwide. SwRI’s VST tool incorporates improvements to the original NREL component models and provides enhanced functionalities in ways that allow the user to define each component of the vehicle. Each component’s set of parameters is defined in a Matrix Laboratory (MATLAB) scripting format that is used in conjunction with a Simulink model. Detailed results of the vehicle simulation analysis are provided in the Appendix D.

Another key factor in any analysis of vehicle fuel consumption and emissions is the drive cycle. For this study, four operational modes were evaluated:

  1. Urban Interstate / Freeway Operation
  2. Rural Interstate / Freeway Operation
  3. Urban Non-Interstate / Non-Freeway Operation
  4. Rural Non-Interstate / Non-Freeway Operation

FHWA used five drive cycles, which are combined to reflect each of the 4 operational modes. These drive cycles are summarized in Table 23:

Table 23: Drive Cycles Used for Simulated Vehicle Operations
Cycle # Cycle Name Comments
1 World Harmonized Vehicle Cycle (WHVC) Same as in NHTSA project
2 Low Speed NESCCAF Same time scale, speed multiplied by 60/68
3 NESCCAF Same as in NHTSA project
4 Urban / Suburban WHVC First 1200 seconds of WHVC
5 GEM Urban (CARB) Same as in NHTSA project

The World Harmonized Vehicle Cycle (WHVC) was developed by the United Nations as a chassis dynamometer emissions and fuel economy test procedure for trucks. The cycle includes three components: a low speed, stop-and-go urban cycle, a medium speed “rural” cycle with one stop, and a higher speed (55 MPH maximum) freeway component. The “urban/suburban” WHVC was created by truncating the cycle at the 1200 second mark (out of 1800 seconds total for the cycle). The NESCCAF cycle was developed for the Reducing Heavy-Duty Long Haul Combination Truck Fuel Consumption and CO2 Emissions Report (NESCCAF, 2009). This cycle had input from vehicle manufacturers, users, and regulators, and represents an attempt to simulate a US long-haul duty cycle. There is some urban driving at the beginning and end of the cycle, with extended periods of high speed (65 to 68 MPH) cruise, and some interruptions in speed designed to mimic a limited amount of traffic congestion. The cruise sections include periods of +/- 1% and +/- 3% grade. The low speed NESCCAF cycle is the exact same cycle scaled down to limit the maximum speed to 60 MPH. Finally, the GEM Urban cycle is the low-speed urban cycle used by the EPA in their Greenhouse Gas Emissions Model, a simulation tool used to certify vehicles for compliance with the EPA’s 2014 greenhouse gas emissions standards. This cycle was developed by the California Air Resources Board (CARB). Details of the vehicle drive cycles are provided in Appendix D.

The drive cycles were combined to handle the four operational modes as shown in Table 24.

Table 24: Mix of Drive Cycles for Four Operational Modes
Urban Rural Road Network
50% WHVC, 50% Low Speed NESCCAF NESCCAF Interstate / Freeway
50% Urban/Suburban WHVC, 50% Gem Urban Low Speed NESCCAF Non-Interstate / Non-Freeway

A key difference between the 2014 CTSW Study and the 2000 CTSW Study is that results are stated in terms of fuel consumption rather than fuel economy. In other words, the results are given in terms of how many gallons of fuel it takes to move the vehicle a mile or to deliver a ton of freight 1000 miles, or how many grams of emissions are emitted per vehicle mile or to move a ton of freight 1 mile. Differences in vehicle efficiency due to variations in tare weight and in aerodynamic drag are accounted for in this project’s methodology. The results provided in this section will be combined with projected vehicle modal shift to provide predictions for the total fleet fuel consumption and emissions levels.

The fuel consumption methodology used in this project matches the methodology being used for the NHTSA contract. This methodology has been reviewed with NHTSA, EPA, and with the National Research Council committee on Reducing the Fuel Consumption and Greenhouse Gas Emissions of Medium- and Heavy-Duty Vehicles, Phase 2.

For carbon dioxide (CO2) emissions, the study assumes that standard petroleum-based diesel fuel is used. There is a fixed relationship between a gallon of fuel and the amount of CO2 generated by burning it: 10.15 kilograms of CO2 are generated for every gallon of diesel fuel consumed.

For the purpose of this study, the assumption was made that all involved vehicles comply with 2010 EPA nitrous oxide (NOx) requirements of 0.2 grams per brake horsepower-hour, with a
10 percent engineering margin. Based on benchmarking tests performed by SwRI, this is a conservative assumption for the types of vehicle operation simulated for this project. Also, the assumption is made that brake-specific NOx emissions are independent of engine speed and load. Again, SwRI’s internally developed benchmarking data shows this to be a reasonable assumption over a fairly wide range of speed and load.

One additional assumption is required to allow a calculation of NOx emissions. For this 2014 CTSW Study, the team assumed that the average brake specific fuel consumption of the engine over the drive cycles is 200 g/kW-hr. In actual practice, a range of 190 to 220 g/kW-hr can be expected. Using these assumptions, 3.8 grams of NOx can be expected for every gallon of fuel consumed.

5.3 Results

A total of eight vehicle scenarios were run. Two of these alternative truck configurations represent the baselines: a 5-axle 53-foot trailer limited to 80,000 pounds and a 5-axle 28-foot double combination, also limited to 80,000 pounds. The configurations evaluated are listed below in Table 25.

Table 25: Tractor-Trailer Vehicle Scenarios Evaluated
Scenario Configuration # Trailers # Axles Tare Wt. (Pounds) Allowed GCW (lb.)
Empty Cell 5-axle vehicle (3-S2) [control vehicle] 1 5 34,622 80,000
1 5-axle vehicle (3-S2) 1 5 34,622 88,000
2 6-axle vehicle (3-S3) 1 6 36,255 91,000
3 6-axle vehicle (3-S3) 1 6 36,255 97,000
Empty Cell Tractor plus two 28-ft trailers (2-S1-2) [control vehicle] 2 5 31,376 80,000
4 Tractor plus two 33-foot trailers (2-S1-2) 2 5 33,738 80,000
5 Tractor plus three 28-foot trailers (2-S1-2-2) 3 7 41,454 105,500
6 Tractor plus three 28-foot trailers (3-S2-2-2) 3 9 47,852 129,000

Each vehicle was simulated over a range of payloads, up to the maximum GCW. Vehicles that had maximum GCWs above 80,000 pounds were evaluated with both the baseline engine and a higher rating intended to maintain performance or, in the case of Scenarios 5 and 6, at least limit the performance penalty for the higher GCW. Table 26 below depicts the payloads along with the number of drive cycles evaluated and the total number of simulation runs required:

Table 26: Scenario Payloads, Engine Ratings, and Drive Cycles Evaluated
Scenario Payloads To Be Simulated by Drive Cycle (Pounds) Engine Ratings Drive Cycles # Of Runs
1 2 3 4 5
Control Vehicle 15,378 30,378 45,378 N/A N/A 485 HP All 5 15
1 15,378 30,378 45,378 53,378 N/A 485 HP, 534 HP All 5 40
2 15,378 30,378 45,378 54,745 N/A 485 HP, 534 HP All 5 40
3 15,378 30,378 45,378 60,745 N/A 485 HP, 588 HP All 5 40
Control Vehicle 15,378 30,378 45,378 48,624 N/A 485 HP, 428 HP All 5 40
4 15,378 30,378 46,262 N/A N/A 485 HP All 5 15
5 15,378 30,378 45,378 64,046 N/A 485 HP, 588 HP 1, 2, 3 24
6 15,378 30,378 45,378 64,046 81,148 485 HP, 588 HP 1, 2, 3 30

In addition to the payloads shown above, each vehicle was also simulated under two additional conditions: a zero payload (empty vehicle) simulation and a GCW of 200,000 pound payload simulation. These two additional simulations for each vehicle along with the payload scenarios shown above allowed estimation of all possible emissions and energy consumption rates at all possible payloads for each vehicle analyzed.

The result of these simulations were a set of rates describing the amount of fuel consumed and CO2 and NOx emitted from each vehicle at different payloads for each of the four operational modes. These rates were then applied to the weight specific VMT distributions developed by the modal shift analysis. Only those vehicles that were analyzed by the modal shift analysis were considered in the energy and emissions analysis. The results of this analysis are discussed below.

Fuel Consumption Results

Each scenario demonstrates reductions to fuel consumption relative to the base case. This is consistent with the reductions in travel made possible by the increases in payload tested in each scenario. While most scenarios show comparable improvements, Scenario 3 shows the greatest overall reduction in fuel consumption. Table 27 shows the changes to fuel consumption between each of the scenarios.

Table 27: Truck Fleet Annual Fuel Consumption (millions of gallons)
Scenario Fuel Consumed Change from Base Case
Base Case 21,797.9 0.0
Scenario 1 21,690.9 -107.0 (-.5%)
Scenario 2 21,688.8 -109.1 (-.5%)
Scenario 3 21,488.7 -309.2 (-1.4%)
Scenario 4 21,553.3 -244.7 (-1.1%)
Scenario 5 21,564.8 -233.2 (-1.1%)
Scenario 6 21,567.1 -230.9 (-1.1%)

GHG Emissions Results

Each scenario demonstrates reductions to greenhouse gas emissions relative to the base case scenario. This is consistent with the reduction of travel made possible by the increases in payload tested in each scenario. While most scenarios show comparable improvements, Scenario 3 shows the greatest overall reduction in greenhouse gas emissions. Table 28 shows the changes to CO2 emissions between each of the scenarios.

Table 28: Truck Fleet Annual CO2 Emissions (millions of kilograms)
Scenario CO2 Emitted Change from Base Case
Base Case 221,249.2 0.0
Scenario 1 220,162.9 -1,086.2 (-.5%)
Scenario 2 220,141.8 -1,107.4 (-.5%)
Scenario 3 218,110.5 -3,138.7 (-1.4%)
Scenario 4 218,765.7 -2,483.5 (-1.1%)
Scenario 5 218,882.7 -2,366.5 (-1.1%)
Scenario 6 218,906.0 -2,343.2 (-1.1%)

NOx Emissions Results

Each scenario demonstrates reductions to NOx emissions relative to the Base Case scenario. This is consistent with the reduction of travel made possible by the increases in payload tested in each scenario. While most scenarios show comparable improvements, Scenario 3 shows the greatest overall reduction in NOx emissions. Table 29 shows the changes to NOx emissions between each of the scenarios.

Table 29: Truck Fleet Annual NOx Emissions (millions of grams)
Scenario NOx Emitted Change from Base Case
Base Case 82,832.2 0.0
Scenario 1 82,425.5 -406.7 (-.5%)
Scenario 2 82,417.6 -414.6 (-.5%)
Scenario 3 81,657.1 -1,175.1 (-1.4%)
Scenario 4 81,902.4 -929.8 (-1.1%)
Scenario 5 81,946.2 -886.0 (-1.1%)
Scenario 6 81,955.0 -877.3 (-1.1%)


[7] Coast-down testing is a technique for establishing the dynamometer load which simulates the vehicle road load during EPA dynamometer fuel economy and emission testing. Return to Footnote 7
[8] SmartWay tires are certain low-resistance tire models that the EPA has determined can reduce NOx emissions and fuel use by 3 percent or more, relative to the best selling new tires for line haul class 8 tractor trailers. See Source: for more information. Return to Footnote 8

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