which further complicates assigning an operating weight to the truck. Considering the multiplicity of factors influencing fuel consumption and the complexity of larger vehicle systems and operations, the committee concludes that the notion of a single metric being applied identically to all classes of vehicles appears to be problematic. However, the committee is confident that a standard measurement protocol coupled with different standards and metrics will provide a means of assessing fuel consumption on the basis of work task for medium- and heavy-duty vehicles.
Class 2b and 3 vehicles tend to be higher volume general-purpose vehicles with less custom built content. The high production volume of this vehicle class is conducive to a more general metric such as gallons per mile, gallons per mile per person weight, or gallons per ton mile. Buses also have substantial variability. Of the bus categories, the long-distance motor coach not only transports passengers but they also transports freight, and therefore the task-based metric would need to consider both freight (baggage and package cargo) and passenger mass. Passenger mass can be estimated using “typical passenger” mass multiplied by the number of available seats. Freight mass can be estimated by using a “typical” freight density term multiplied by the cubic capacity of available cargo space.
For most truck transportation, the nature of the freight task can be classified as volume limited or mass limited. Mass-limited freight is of sufficiently high density that the GVW will be reached before the volumetric capacity of the vehicle is fully utilized. Volume limited freight is of sufficiently low density that it occupies the available cargo space before the GVW is achieved. It is estimated that the split between volume-limited and mass-limited freight on the U.S. highway network is approximately 50/50. Vehicles are often designed on the basis of mass or volumetric capacity, and the characteristics of these vehicles are somewhat sensitive to the methods used to calculate fuel consumption. The following example illustrates practical considerations that will be necessary when developing a fuel consumption regulatory instrument.
Consider the real-world example of two tractor trailers having identical power units but with trailers of different cargo mass capacity and identical volumetric capacity (Figure 8-3). Vehicle A has a GVW of 80,000 lb and a cargo capacity of 48,000 lb. Vehicle B has a GVW of 97,000 lb and a cargo capacity of 61,000 lb (allowing 4,000 lb for the extra axle, suspension and additional trailer structure). Both trailers have identical cargo volume capacity of 3,650 ft3.
It is clear that Vehicle A is better suited to cargo weighing 48,000 lb or less, and vehicle B is better suited to cargo weighing more than 48,000 lb. There is no difference in the volumetric capacity of these vehicles; therefore, the cargo mass dictates the vehicle choice. On the surface this case appears to be ideally suited to the vehicle mass fuel consumption metric (gal/cargo ton-mile). However, when examined more closely, it is apparent that the identical tractor would have different fuel consumption values for each of the two cases given the difference in the GVW and cargo mass capacity. It is likely that if the mass metric were applied, Vehicle B would always outperform Vehicle A (assuming that a proportionate cargo mass is used). This would be counterproductive for low-density, volume-limited freight applications because the mass metric would encourage heavier capacity vehicles with higher tare weight. In such cases, this problem can be offset by considering an alternate metric such as gallons per cargo ft3-mile. Further discussion of alternative metrics can be found in Annex 8-2 to this chapter.
The choices of possible methods for certification and compliance of fuel consumption standards for medium and heavy duty vehicles involve some of the most challenging regulatory design issues.
One broad choice pertains to whether it would be possible to establish average standards by corporate entity, as is done under the light duty vehicle CAFE program, or whether the breadth and diversity of the medium and heavy-duty vehicle market precludes such an option. In general there are important benefits associated with a corporate average standard in that it allows corporations flexibility to focus improvements on vehicle types within the retooling cycle. The challenge with a corporate average standard is that the medium- and heavy-duty vehicle market is extremely diverse and would require establishing categories of vehicles by type and application. In addition, the light-duty vehicle CAFE program placed full line, largely domestic, manufacturers at