. "5 Vehicle Technologies for Reducing Load-Specific Fuel Consumption." Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. Washington, DC: The National Academies Press, 2010.
The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
TABLE 5-5 Class 8 Tractor Aerodynamics Technologies, Considering the 2012 Time Frame
Fuel Consumption Reduction (%)
Cd Improvement (%)
Industry Adoption Rate (%)
Day cab roof deflector
Sleeper roof fairing
SOURCE: TIAX (2009), p. 4-35.
Table 5-5 shows the market shares of various aerodynamic features on aerodynamic-style tractors. Roof shields have a very high market share, while the relatively damage-prone chassis skirts have a lower share. The side-of-cab extender works with the roof shield to minimize the gap between tractors and trailers. This gap has a major role in determining the overall vehicle Cd. The recommended maximum gap3 is typically 30 in. from the rearmost feature on the tractor to the trailer face. Smaller gaps do have drawbacks, however, in that they limit the ability of the vehicle to operate in tight spaces.
The next-generation aerodynamics package shown in Table 5-5 represents the forecast of tractor manufacturers for sleeper cab tractor aerodynamics improvement in the 2012 time frame. These features will be designed and optimized for long-haul applications. It is expected that many of these features may not be compatible with short-haul operations, and thus their application on day cab tractors will be limited. Characteristics of short haul operation include curb encounters, severe road-crossing humps, backing maneuvers, and tight street-side clearances. All of these combine to damage many of the aerodynamic surfaces that could be successful in long-haul duty.
There are two consequences of the fragility of tractor aerodynamic features that must be considered. One is that trucks specified with many aerodynamic features will not be attractive or cost-effective in short-haul operations, because of the fragility of aerodynamic features and because of restricted maneuverability. The other issue is that tractors specified for short-haul operations will be less efficient if they are pressed into long-haul service for any reason. Excessive specialization of tractors can lead to logistics problems for operators, as well as to lower used tractor values in cases where the original operating intent does not match the second buyer’s application. An example of this is sleeper cab tractors with full-height air deflectors pulling flat bed trailers. In this case the aerodynamic feature actually costs fuel rather than saves fuel, because the high roof sleeper increases the frontal area of the truck beyond what the trailer requires. In many cases the application of a high roof sleeper with a flat-bed trailer is just temporary, but in some cases this fuel-wasting combination may be long term.
Another marketplace factor in tractor design that delays full implementation of aerodynamic features is the preference for traditional styling. This preference for a traditional look is prevalent among owner-operators, and many small fleets use traditional styling as a driver retention feature. Notice the differences in traditional styling compared to aerodynamic styling in Figure 5-10. Traditional features known to have high drag-inducing effects include the large, flat bumper, along with features protruding into the airstream such as head lamps, air cleaners, and dual exhaust stacks, as well as “west coast” side mirrors. While manufacturers have made useful aerodynamic improvements to traditional models of years gone by, these traditional features are believed to invoke a fuel consumption increase of at least 5 percent compared to the aerodynamic model. (TIAX, 2009, Table 4-24). Some operators are well aware of the fuel consumption penalty, while others are likely to underestimate it.
Day cab tractors constitute roughly one-third of Class 8 tractors. So far it has not been possible to match the aerodynamic performance of the best sleeper models with day cabs. Fortunately, it appears that many of the day cab and other short-haul tractors accumulate fewer miles and thus consume less fuel than over-the-highway tractors. More specific data gathering is needed to quantify the fuel consumed by various applications of tractor trailers.
A 2-year collaborative study of a variety of design improvements that would reduce aerodynamic drag on tractor trailers was completed in 2007 by four members of the Truck Manufacturers Association (TMA) and DOE. Their research evaluated the effect of post-SmartWay designs on combination tractor-trailer aerodynamics. A number of potential tractor features were evaluated, including alternative rearview mirror designs, treatments of the tractor-trailer gap such as gap fillers and trailer gap flow control devices, and features to manage airflow under the vehicle and between the tractor and trailer. In addition to the tractor features, a number of trailer features also were evaluated.
The 2007 TMA/DOE study started with a computational fluid dynamics (CFD) modeling evaluation of potential aerodynamic feature concepts. The CFD models allowed the researchers to explore the effect of many design parameters on Cd (see in Chapter 2 the section “Computational Fluid Dynamics”). The most promising concepts from the analyti-
“Gap” refers to the distance from the rearmost vertical cab feature to the front of the trailer face; where a cab extender is employed, it is this rearmost feature.