To capture fully the fuel consumption benefits of technologies in future regulatory phases, more engineering will be needed (and in fact is under way). The committee notes, however, that the selected drive cycles do not include external effects such as road grade or cross-winds (i.e., yaw angle), which are particularly significant for Class 7 and Class 8 vehicles. The simulations will thus not fully reflect the benefits of certain types of technologies for reducing fuel consumption. The NRC Phase One Committee had noted that road grade variations, for example, are absent from practically all widely used test cycles. A further example is adaptive cruise control, a subsystem actuated by radar systems to set a desired speed and offering the option of maintaining a set following interval from a vehicle directly in front. The longitudinal control this technology provides offers co-benefits for fuel consumption. These systems have full systems control. Further, as noted by the agencies, many of the vehicle specifications (transmission and final drive ratio) are left to the routine specification process (EPA and NHTSA, 2011a, p. 57158). The GEM simulation is based on a few user input parameters, including rolling resistance, aerodynamic drag coefficient, and vehicle weight reductions (EPA and NHTSA, 2011b, p. 4-10). Hence the question of expected change in performance is not fully answered and neither is the question of whether the Phase I Rule will have a favorable impact.

A lingering concern in GEM is the inability of manufacturers of Class 8 vehicles to take into account the engine choice, actual engine efficiency, and integrated power train design optimization, items that can provide as much benefit as some aerodynamic features.7 Also, the cooling system is neglected: It may be unfavorably affected by efficient aerodynamics yet not accounted for at the engine. The cooling system can represent 5 percent of the vehicle power demand during some operations.8

Both GEM and current test cycles are time based and may not accurately reflect fuel consumption to accomplish a mission. Improved productivity may not be recognized, yet it saves fuel because commercial vehicles will run until the mission (work, distance, etc.) is completed.

Finding: The current certification procedures rely on computer simulations that have only a few unbound variables that can be user-specified. Further, GEM specifically does not allow for synergy between components, the operation of components in a most efficient way, or the engendering of efficiency through operation of a smaller component at higher relative load. Vehicle designs that are optimized for the conditions of the simulation may not be optimized in the real-world operation.

Recommendation 1.2: NHTSA should conduct a real-world evaluation to validate the simulated fuel consumption outputs in light of the input data used. The evaluation should include a sensitivity analysis on key parameters, such as gross combination vehicle weight, to judge whether the variation in these parameters leads to a source of error in the simulation. NHTSA will need to test a reasonable number and variety of vehicles to further refine and validate the Greenhouse Gas Emissions Model (GEM) simulations.

Pilot Program and Evolution of the Regulatory Regime

The NRC Phase One Report included the following recommendation:

Recommendation 8-6. NHTSA should conduct a pilot program to “test drive” the certification process and validate the regulatory instrument proof of concept. It should have these elements:

  • Gain experience with certification testing, data gathering, compiling, and reporting. There needs to be a concerted effort to determine the accuracy and repeatability of all the test methods and simulation strategies that will be used with any proposed regulatory standards and a willingness to fix issues that are found.
  • Gather data on fuel consumption from several representative fleets of vehicles. This should continue to provide a real-world check on the effectiveness of the regulatory design on the fuel consumption of trucking fleets in various parts of the marketplace and in various regions of the country. (NRC, 2010)

It appears that the administration’s schedule for issuing a rule quickly was a key factor in not conducting a pilot program. It is recognized that the entire NHTSA regulation was on a mandatory fast track, as requested by President Obama (White House, 2010). The committee compliments the Agencies on getting a Phase I Rule in place quickly, to promote fuel savings as soon as feasible.

The recommendation that NHTSA conduct a pilot program had two broad purposes: first, the agency would gain experience with certification testing, data gathering, compiling, and reporting. The trial period was envisaged as a means for developing and refining the regulatory processes before the official start date of the program. Second, the pilot program would include gathering data on fuel consumption from several representative fleets of commercial trucks (e.g., long-haul, delivery vans, specialty vehicles, and large pickups). These data would provide a real world check on the effectiveness of the regulatory design on the fuel consumption of trucking fleets in various parts of the marketplace and in various regions of the country (NRC, 2010, p. 188).


7 David Kayes, Daimler Trucks North America, “Lessons Learned from FE/GHG Phase 1 Regulations, and Ways to Incorporate the Most Likely Future Technologies into FE/GHG Phase 2 and 3 Regulations,” Presentation to the committee, March 20, 2013.

8 Nigel Clark, West Virginia University, Morgantown, “Engine Models and Maps for Truck Certification,” Presentation to the committee, June 20, 2013.

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