Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Resistance and Weight Reduction Strategies
Overview
Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
The total drag on a moving tractor-trailer is the sum of the air resistance acting on it and
the mechanical friction in the wheel bearings and tires. Technologies that aim to reduce
drag will make the tractor-trailer more efficient either aerodynamically or mechanically.
Drag reduction measures are of particular interest with respect to GHG reductions, as
many of these options can be introduced to the fleet relatively quickly through retrofit, as
opposed to most other vehicle and powertrain options, which must be implemented
through redesign of new vehicles.
Aerodynamic drag is particularly sensitive to vehicle speed, with power requirements
increasing with the square of velocity. Rolling resistance also increases directly with
speed and mass (Vyas et al., 2002). Aerodynamic improvements will have the greatest
impact for those vehicles traveling at highway speeds (60+ mph). When heavy-duty
trucks are traveling at these speeds, approximately 65 percent of the engine power
produced is used to overcome aerodynamic drag (Vissier, 2005). In addition, about
70 percent of fuel use by Class 8 HDVs is estimated to occur on trips greater than 100
miles, which occur primarily at highway speeds. By contrast, the majority of fuel use for
smaller delivery trucks (as well as vocational trucks and transit buses) is attributable to
trips of less than 100 miles, typically occurring at lower urban speeds (21 st Century Truck
Partnership, 2009). Accordingly, aerodynamic improvement options are evaluated here
only for Class 8 freight trucks, although some benefits also are possible for smaller, urbanbased
trucks as well.
The aerodynamic drag of tractor-trailer rigs has dropped significantly since the 1980s,
improving fuel efficiency by about 15 percent per truck (Schubert and Kromer, 2008).
Some of the currently available truck/tractor drag reduction options include a cab top
deflector, sloping hood, and cab side flares, as shown in Figure 3.8. These design
concepts already are widely implemented for line-haul trucks. In fact, implementation on
Class 7 and 8 cabs and hoods already have achieved a high level of market penetration
(about 70 percent by 2006). 45
However, there is potential to further increase the
aerodynamic efficiency of current Class 8 HDVs by making additional changes to the
shape of these vehicles, from the front bumper of the truck to the back of the trailer.
45 At 70 percent market share, according to AEO 2009 – Table aeo2009.d120908a.
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