using the maximum gross combination vehicle weight rating (GCVWR) for the vehicle weight class. This captures the situation that over half of trucks on the road are volume limited,5 meaning the trailer is filled up with containers without reaching the weight limit. In such a case the combined tractor trailer is not at full GCVW of 80,000 lb, the maximum allowed weight for un-permitted interstate transit.6 Appropriately, the Agencies indicated the need and intent to gather additional data on the weight/payload in actual service. They addressed the work-factor metric in Class 2b by accounting for the payload capability of these vehicles in the rule instead of setting a payload for evaluation, which overall addresses the Phase One Committee recommendations. The Agencies chose not to consider a metric for volume-limited freight, which might otherwise have been useful in the assessment of longer combination vehicles (LCVs). The NRC Phase One Report found that such vehicles “offer potential fuel savings for the trucking sector that rival the savings available from technology adoption for certain vehicle classes and/or types” (NRC, 2010, p. 176). Payload and its relationship to LSFC remain important considerations.

Recommendation 1.1: NHTSA should evaluate the load-specific fuel consumption (LSFC) at more than one payload to ensure there is not an undesirable acute sensitivity to payload by a particular truck power train and to reflect the fact that some states allow vehicles to operate with gross combination vehicle weight ratings well in excess of the values adopted for the simulation.

Classes of Vehicles to Regulate

The Phase One Report included the following finding:

Finding 8-1. While it may seem expedient to focus initially on those classes of vehicles with the largest fuel consumption (i.e., Class 8, Class 6, and Class 2b, which together account for approximately 90 percent of fuel consumption of MHDVs), the committee believes that selectively regulating only certain vehicle classes would lead to very serious unintended consequences and would compromise the intent of the regulation. Within vehicle classes, there may be certain subclasses of vehicles (e.g., fire trucks) that could be exempt from the regulation without creating market distortions. (NRC, 2010)

The Agencies agreed with the NRC that regulating all MHDV classes at the outset of the regulation was important. As noted in the Phase One Report, if NHTSA were to regulate only Classes 2b, 6, and 8, this would encompass 90 percent of the fuel used by all medium- and heavy-duty vehicles. The Phase One Committee was quick to note, however, that uneven policy application may cause disruptions in the marketplace and create the potential for reclassifying various classes of vehicles, as has been done in light-duty vehicles (LDVs). Other unintended consequences might result, such as changes in market behavior to avoid higher prices due to regulation (e.g., if Class 2b is regulated but not Class 3, then buyers might buy more of the larger Class 3 trucks because they would become less expensive relative to 2b trucks). In view of these considerations, the committee believes regulating all MHDVs should remain the “agencies’ objective.” As the regulatory framework becomes more defined and comprehensive, additional effort needs to be applied to avoid unintended consequences, as addressed in the Phase One Report.

Certification Procedures

The Agencies adopted the general recommendation of using simulation to handle the wide range of vehicle configurations and equipment and drive cycles, while building on existing protocols of engine testing for criteria emissions. The use of simulation for the vehicle, with a separate engine test, generally followed the NRC Phase One Report recommendation to certify entire vehicles. The Phase One Report included the following recommendation:

Recommendation 8-4. Simulation modeling should be used with component test data and additional tested inputs from power train tests, which could lower the cost and administrative burden yet achieve the needed accuracy of results. This is similar to the approach taken in Japan, but with the important clarification that the program would represent all of the parameters of the vehicle (power train, aerodynamics, and tires) and relate fuel consumption to the vehicle task. (NRC, 2010)

The Agencies developed a relatively measured regulation in 2011 (EPA and NHTSA, 2011a) in that the fuel efficiency targets are modestly challenging for some vehicle classes, and the certification process builds largely on current methods. The exception where extensive engineering was required was the development of the GEM for Classes 2b-8 vehicle compliance (see Chapter 3 for more discussion of GEM). GEM, a MATLAB/Simulink-based model, uses the same physical principles as many other existing vehicle simulation models to derive governing equations that describe driveline components, engine, and vehicle. These equations are then integrated in time to calculate transient speed and torque (EPA and NHTSA, 2011b, p. 4-2). The development and benchmarking of GEM are found in EPA reports (EPA, 2011; EPA and NTHSA, 2011b). The Agencies reduced the engineering challenge by simplifying the model, excluding hybrid powertrains and several widely used component technologies (e.g., automatic transmissions).


5 Federal Register 57158 states that “These payload values represent a heavily loaded trailer, but not maximum GVWR, since as described above the majority of tractors ‘cube-out’ rather than ‘weigh-out.’”

6 GCVW of more than twice this weight is possible with special permits on certain roadways.

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