ABSTRACT
Four simple, low cost aerodynamic drag reduction devices have been developed for application to
the trailer of a tractor-trailer truck. Two vortex flow devices have undergone extensive
operational fleet testing where they have amassed over 85,000 miles of use. These two vortex
flow technologies have shown a combined fuel savings of 8% at an average speed of 47.5 mph.
This improvement in fuel economy correlates to an equivalent drag reduction of approximately
20% with a corresponding drag coefficient of 0.45. Observations and anecdotal evidence from
the test activity have shown that the addition of these devices to the trailers has not had a
negative impact on either the operational utility of the trailers or the maintenance procedures
and requirements. Two base mounted devices have been developed to reduce the base drag of
trailers. The two base devices have been developed through computational design and wind
tunnel testing. Each of these two base mounted devices have been shown to reduce the drag of
heavy trucks by more than 8%, which equates to a 4% fuel savings at highway speeds. The
estimated combined fuel savings of the vortex flow and base devices is greater than 12%.
INTRODUCTION
To curb the impact of rising fuel costs the nations heavy truck community is exploring a number of
advanced technologies that include aerodynamic drag reduction for both the tractor and the trailer. An
assessment of on-road heavy truck energy consumption, for still air conditions, shows that the primary
energy loses are due to drive-train, rolling friction, and aerodynamic drag, Hucho, 1988. It is well
documented that as vehicle speed is increased the force required to overcome both aerodynamic drag and
rolling friction increase, however, the rate of increase in aerodynamic drag with increasing vehicle speed is
much greater than that for rolling friction such that at speeds greater than 45 miles per hour aerodynamic
drag is the dominant resistance force. The importance of aerodynamics on heavy truck fuel economy is
further highlighted by the work of Tyrrell in 1987 that showed operational and environmental factors act
to further increase the impact of aerodynamics on heavy truck fuel economy resulting in aerodynamic
drag becoming the dominant resistance force for these class of vehicles. These operational and
environmental concerns include factors such as vehicle interference, atmospheric effects, and road
conditions that must be addressed when developing aerodynamic based fuel economy improvement
technologies for heavy vehicles. It is estimated that application of existing aerodynamic innovations to
the tractor and trailer will increase the fuel economy by 20 percent. The heavy truck community has
been slow to adopt the existing innovations due to their concerns related to operations and maintenance
that is driven by device complexity, weight and cost.
DISCUSSION
To understand the technical challenge and economic payoff offered by aerodynamic drag reduction, it is
important to understand the distribution of the drag between the tractor and trailer, see figure 1. The
data used to develop the drag distributions depicted in figure 1 were obtained from a review of the
available historical data, as represented by the data contained in reports by Drollinger, 1987 and Sovran,
1978. The schematic of figure 1 depict the dominant drag regions on a tractor-trailer truck are the tractor
front face, tractor-trailer gap, undercarriage/wheels, and trailer base. The data show that under
representative operational conditions a crosswind is present and the distribution of aerodynamic drag
between the tractor and trailer is 35% tractor and 65% trailer. The drag of the trailer can be further
qualified as having equal parts trailer front face drag, trailer base drag, and trailer undercarriage/wheel
drag. To address the needs of the trucking community it is critical that aerodynamic drag reduction
efforts address vehicle operations, maintenance, safety, weight, cost, stability and handling, braking,
splash and spray, and tire wear, Barnard, 1986.

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