areas, typically of higher technology risk and cost, 21CTP processes are very important. The ability of the Department of Energy (DOE) to assist in understanding how to achieve the goals through its Vehicle Technologies Program, including specifically the Vehicle Systems Simulation and Testing (VSST) protocols (DOE, 2010b), should be very beneficial and is particularly encouraged.
Aerodynamic drag arises principally from the pressure differentials on fore and aft body surfaces. Surface friction is a much less significant issue if surfaces are substantially smooth, which is a common but not yet a universal design feature; a notable example is container boxes (DOE, 2010a). Full-truck on-road testing following Society of Automotive Engineers (SAE) protocols (e.g., SAE J1263 coast-down testing; SAE J1321, Type II over-the-road testing with control truck) is relatively imprecise for evaluating the aerodynamic effect of design changes. Wind tunnel tests, also following SAE protocols (e.g., SAE J1252), are an improvement in precision and, importantly, allow an evaluation of the effects of off-axis forces (yaw) due to prevailing wind. Computational fluid dynamics (CFD) is often useful for fine-tuning component design to take account of the aerodynamic contribution, but it is in limited use for full-truck behavior.
Historically, the truck manufacturers have not reported Cd values for tractors. This situation is likely due to the competitive nature of those values, especially in light of the imprecision of prevailing standard test procedures.
Four regions of the tractor-trailer combination truck are amenable to aerodynamic design improvements. These regions include (1) the various tractor-related “aero” details, (2) the tractor-trailer gap, (3) the trailer skirt (or underbody), and (4) the trailer “base” fairing, which are all illustrated in Figure 5-2, along with the approximate fuel-consumption reductions related to each region that are estimated to be achievable in the near term.
The DOE funded a $4.2 million study, initiated in 2007, on trailer aerodynamic improvements to reduce fuel consumption and presented a project status report to the committee (DOE, 2010a). The authors provided the committee a pre-publication draft of their results and analysis. The study combined CFD studies with a large battery of full-size tractor-trailer wind tunnel tests at 65 mph in the huge National Full-Scale Aerodynamics Complex (NFAC) at the National Aeronautics and Space Administration (NASA) Ames Research Center. This research was supported by numerous industrial partners. These data are reported as yaw-wind averaged drag, which is acquired only in a wind tunnel, and which most aerodynamicists agree represents the best on-road performance figure. The study clearly is an excellent one, with very thorough evaluation of numerous candidate design improvements for all three of the trailer regions in Figure 5-2. The analysis is quite insightful, providing very helpful commentary to clarify why certain design details yield the observed results. The best performance observed for the case of a 2008 long sleeper tractor and straight frame (LS/SF) tractor-trailer was a Cd reduction of 23 percent. The draft did not proffer a base case Cd for this LS/SF combination. However, if a Cd of 0.63—used in this chapter for a 2006-2008 pre-SmartWay tractor—is assumed, a fuel-consumption reduction of about 12 percent would be expected. This can be compared to the average fuel consumption of 9.3 percent for a full package described in the section below titled “TIAX Summary” and Figure 5-3.
EPA SmartWay Transport Program
In 2004 the EPA developed and implemented SmartWay, an organized effort to specify a collection of current and emerging technologies for creating fuel-efficient tractor-
FIGURE 5-2 Tractor-trailer (T-T) combination truck showing areas of energy-saving opportunities. NOTE: Percentage changes refer to fuel consumption with base Cd = 0.625. SOURCE: Personal communication, Richard M. Wood, Solus, LLC.