Commercial Vehicle Aerodynamic Drag Reduction:
Historical Perspective as a Guide
Kevin R. Cooper
National Research Council of Canada
Ottawa, Canada
Introduction
The aerodynamics development of commercial vehicles has evolved over many
years. Sixty-five years ago, the Labatt Brewing Company developed a
streamlined truck for advertising purposes and to provide larger capacity and
higher cruising speeds, Figure 1. The success of this effort is demonstrated by
the fact that while trucks of the day travelled at 35 mi/h, the Labatt truck
could cruise at 50 mi/h with a fifty percent larger load. The focus today is no
longer on speed, but on energy conservation. It is beneficial for a country to
minimise its energy utilisation and equally beneficial for its trucking industry
to make money while doing so.
Fig. 1: 1947 Labatt Streamliner
The opportunity offered by aerodynamic drag reduction was successfully
exploited by North American industry twenty-five years ago and is being
revisited in a second effort by industry and government. The previous
SAE/DOT Voluntary Truck and Bus Fuel Economy Program had an
10
K.R. Cooper
important impact on the implementation of fuel-saving aerodynamics. The
current DOE program might do the same.
The SAE/DOT program had the benefit of being first and thus being
able to utilise the large gains obtained from reshaping the front ends of the
trucks, which were relatively easy to deal with and gave the largest drag
reductions. Much of the work focussed on the tractor, since only one truck
needed to be modified, no matter how many trailers were in the fleet. At the
same time, trailer mounted devices or trailer modifications also found wide
acceptance, and rounded-edged truck bodies have become the standard.
Many other areas were investigated, including: tractor-trailer gap seals,
trailer skirts, trailer boat-tailing and tractor-trailer integration. These have not
been successful in the marketplace, due to operational difficulties, due to their
small return on investment, or due to the complexity of fleet-wide integration.
It has been known for many decades that more integrated tractor-trailer
combinations were beneficial aerodynamically, but the complexities of doing
so have precluded development in this area, except for demonstration vehicles.
Steady increases in fuel prices over the years now make some of the
unused technology economically viable. A major new initiative to improve
truck fuel economy seems appropriate. The important question is how to do
so? We have the option of seeking to further hone the aerodynamics of the
truck. Much of this work has been done and so we face the law of
diminishing returns – a greater and greater effort to provide a smaller and
smaller gain.
A more effective scenario would be to apply what is already known in the
short term and to work toward more integrated configurations in the longer
term. The latter task is a major challenge, even though it can be shown to
offer considerable benefit, because of the importance of not compromising the
investment in current fleet hardware and warehousing. The issue is not how
to lower the drag coefficient by a further 0.002, but rather to work with fleets,
manufacturers, researchers and legislators to apply what we already know.
Without appropriate legislation, the acceptance of the operators and
collaboration between the OEMs, no real improvements will be made. We
need to find solutions by implementing our existing and substantial body of
knowledge. We do not need to study the problem much more.
A new effort, sponsored by the DOE, is being mounted now to further
improve truck aerodynamics, primarily based on CFD calculation and some
experiment. This paper provides a review of previous aerodynamic research
and technology-transfer initiatives as a way of placing the new program in
perspective. It seeks to ensure that the existing, rich aerodynamic history is
not ignored and that lessons learned previously in technology transfer are not
lost.
Commercial Vehicle Aerodynamic Drag Reduction
11
Prior Art
In the 1950’s, a serious effort to improve truck fuel consumption was
undertaken at the University of Maryland [1,2,3] through an examination of
the aerodynamics of tractors and trailers, funded by Trailmobile. This work
provided an early, detailed look at truck aerodynamics and may have triggered
the development of the air deflector in the 1960’s by Seldon Saunders and
Chet Wiley of Airshield – the first successful add-on aerodynamic device.
These studies also presaged the advent of trailer streamlining, by looking at
edge rounding, rounded trailer front faces, skirts and boat-tailing. At about
the same time that Airshield was developing the cab-mounted deflector, Joe
Fitzgerald, working at Thermoking, had realised that their refrigeration units
reduced truck fuel consumption. He decided to take this concept a step
further and developed the Nose Cone trailer streamlining fairing. Thus, the
modern truck aerodynamic age was born.
The first years were difficult. Fuel was cheap and truckers did not want
those gadgets on their rigs. However, the 1970’s energy crunch arrived and
the new devices were rapidly accepted. They saved fuel and made profit for
the trucker. They also reduced direct operating cost and strengthened the
competitive position of the trucking industry with the railroads.
Standard tractor-trailer
Standard straight truck
Equipped with deflector
Equipped with Nose Cone
Fig. 2: Smoke Flow Over Standard and Modified Trucks
12
K.R. Cooper
In the late 1970s, the National Research Council of Canada (NRC) took
on the task of comparing the commercial devices of the day [4], with the
intention of convincing truckers of the benefit they provided and helping
them choose the best type of device for their operation. Smoke pictures like
those in Figure 2 made a lot of believers, as did a growing body of road
measurements of fuel savings. When truckers saw the two pairs of
photographs, they had no difficulty in making a choice.
The growing activity attracted the attention of the SAE and the US
DOT, leading to the SAE/DOT Voluntary Bus and Truck Fuel Economy
study of the late 1970s and early 1980s [5]. By this time, OEM’s and aftermarket suppliers in North America and Europe were actively improving fuel
consumption through aerodynamic means, resulting in the reduced-drag fleet
of today.
The SAE/DOT study was a major government/industry cooperative
venture. Its goal was to demonstrate that truck fuel consumption could be
significantly reduced. The study was centred on a set of four pairs of trucks,
two tractor-trailer combinations and two straight trucks. Each pair consisted
of a standard truck for the time and an identical partner fitted with an
aerodynamic package, advanced tyres, a fuel-saver motor, improved lubricants
etc. These trucks were track and road tested, and run in fleet service. The
trucks are shown in Figure 3.
Fig. 3: SAE/DOT Demonstration Trucks
Commercial Vehicle Aerodynamic Drag Reduction
13
The study was multi-faceted, developing test technology for the
laboratory and for the fleet. It was also applied and practical, in that it had a
large component of on-road testing and user involvement. A series of SAE
Recommended Practices were developed that are still in use today, including
those for wind tunnel testing [6], coast-down testing [7} and on-road fuel
measurement [8, 9]. They were verified by wind tunnel and road trials. This
program involved fleets, trucking associations, equipment manufacturers, the
SAE, legislators, government laboratories and university researchers. It was a
hands-on project that had great impact on the acceptance of the new
technologies. The trucking industry believed the findings because they were
part of the process.
Early Aerodynamic Development
Considerable wind tunnel aerodynamic development of commercial vehicles
has occurred over the past 50 years. The University of Maryland study is of
particular historical importance because it was an early piece of work and it
was well done. In fact, it provides most of the answers required to develop the
year 2010 aerodynamic truck. Other authors have also published widely and
again, have shown the way. Notable is the work of Buckley et al [8], Mason &
Beebe [9, 10] and that of various European authors, including Hans Gotz of
Daimler-Benz [11] and Alfons Gilhaus of Ford Cologne [12]. Their survey
papers provide a wealth of material on the aerodynamics of heavy commercial
vehicles. The combination of the three groups of authors provides a broad
overview of significant past developments.
The NRC was active in this program also, taking on the task of
comparing commercially available, drag reducing devices to advise truckers of
the best choices for their equipment and operations. The NRC was deeply
involved in the SAE/DOT Voluntary program. It built 1:10-scale models of
the four vehicle pairs that were road tested in this program. These models cost
$160,000.00 (1980 dollars) to design and build, and were used to support the
road tests and to demonstrate the effectiveness of the SAE Recommended
Practice for the Wind Tunnel Testing of Trucks and Buses, J1252 [13]. In all,
the models were tested in up to 11 wind tunnels world wide [9, 10].The
Aerodynamics Laboratory of the NRC actively worked with several OEM’s
and many aerodynamic-device manufacturers to calibrate and improve their
products [14, 15, 16] and did research on basic concepts, including edge
rounding and base-drag reduction [17], and trailer skirts [18].
The body of work from these sources easily permits very low drag vehicles
to be designed now.
We know most of the answers; we need to apply them.
14
K.R. Cooper
The University of Maryland Study
As a demonstration of this point, I have summarised the most pertinent data
from the second of the University of Maryland Trailmobile studies [2]. The
configuration chosen was the COE tractor with the Model A van trailer. The
model was built at 1:6 scale and was tested at 150 mi/h, giving a Reynolds
number that was 42% of full scale at 60 mi/h road speed. The various
configurations are shown in the photographs of Figure 4.
The build-up of the low-drag model is given in Table 1. The first two
data columns present the measured drag data at the yaw angles indicated. The
barred drag-coefficients in the next two columns are wind-averaged values [13]
at a road speed of 65 mi/h, as indicated. The drag coefficient curves are
plotted in Figure 4. While the initial configuration was not up to today’s
styling and performance standards, the aerodynamic characteristics of the
modified configurations certainly are. The results show the capability of an
integrated tractor/skirted-trailer combination. The antique tractor, with a
fairing merging the tractor and skirted trailer, could compete with the best of
today’s combinations.
Case 2 is taken as the baseline since it is closer to today’s trailer
geometries. Each increment in wind-averaged drag coefficient, DC D (65) , is
the difference between the line item and the preceding configuration. Thus, it
is the result of the underlined, italic description that defines the change from
the preceding case. While the drag levels of the COE tractor are higher than
would be measured today, due to the cab design, the differences due to the
modifications are close to those measured more recently. For example: the
skirts give a drag increment close to that measured at the NRC [18], the roof
fairing gave a result between that for the original curved-plate deflector and the
current cab fairing, closing the gap has a similar gain to that found today and
the drag reduction due to the boat tail on the fully skirted and streamlined
configuration represents close to the total base drag. Rounded trailer rear side
posts were also tried and showed a small gain that was consistent with the
small radius employed. Some of these configurations are unusable on the
road, but they do define the range possible.
Interestingly, although this data set has been available for decades, not all
of the practical techniques it exposes are utilized today. In particular, skirts
and gap closure are not in widespread use and boat tailing is seen only on
some buses and as an add-on device on some trailers. This data set would
permit the design of a low-drag truck without further research.
More interesting information can be gleaned from the drag curves.
Closing the gap is beneficial, especially at large yaw angles. The addition of a
well-streamlined tractor with no gap shows little gain over the faired COE
tractor at small yaw angles, but has much better performance at yaw angle
magnitudes greater than 5 degree. This trend continues as the truck becomes
more closed and integrated, until it is seen to have decreasing drag with yaw
Commercial Vehicle Aerodynamic Drag Reduction
15
angle with the most streamlined configurations - the truck is beginning to sail.
With the exception of the full-height skirts and streamlined tail, all the
modifications could be implemented. Even the seemingly impractical changes
can be utilized in a less extreme fashion. Partial-height skirts and simple boat
tailing can recover a significant fraction of the potential shown above.
Case 1 - COE tractor, square-cornered trailer
Case 3 - COE tractor, trailer with deluxe front
Case 4 - COE tractor, skirts (without bumper)
Case 5 – COE tractor with roof fairing
Case 6 – COE +3/4 skirts, gap fairing
(without bumber)
Case 7 – Streamlined tractor, 3/4 straight skirts
Case 8 –-Streamlined tractor, straight skirts,
boat tail
Case 10 – Fully-streamlined and skirted
tractor-trailer, boat tail
Fig. 4: Trailmobile models
16
K.R. Cooper
Table 1: Summary of the Trailmobile Study
Ca
se
Configuration
CD(0º)
#
CD(1
0º)
C D (65mi / DC D (65mi
1
COE tractor, van trailer, square
front posts
1.017
1.503
1.169
-
2
COE tractor, 12” radius front side
posts
0.900
1.167
1.056
0.113
3
COE tractor, deluxe front on trailer
0.828
1.118
0.994
0.062
4
COE, deluxe front, _ height trailer
skirts
0.803
1.052
0.944
0.050
5
COE, _ skirts, roof fairing
0.641
1.007
0.842
0.102
6
COE, _ skirts, faired gap from
tractor to trailer
0.558
0.825
0.689
0.153
7
Streamlined tractor closing gap, _
skirts
0.555
0.653
0.624
0.065
8
Streamlined tractor, _ skirts, boat
tail
0.460
0.520
0.503
0.121
9
Fully-skirted streamlined tractor
and trailer
0.317
0.329
0.351
0.152
10
Fully-skirted streamlined tractor
and trailer, boat tail
0.184
0.160
0.189
0.169
# Drag coefficients based on reference area equal to trailer roof height times trailer width.
Commercial Vehicle Aerodynamic Drag Reduction
17
1.75
1.50
base COE, square
trailer posts
12" radius side posts
delux trailer front
+ 3/4 trailer skirts
+ roof fairing
Drag Coefficient
1.25
1.00
0.75
faired COE tractor-trailer
gap
streamlined tractor
0.50
streamlined + boat tail
fully streamlined
0.25
fully streamlined + boat tail
0.00
-20
-15
-10
-5
0
Yaw Angle, deg.
5
10
Fig. 5: University of Maryland Aerodynamic Development
The DOE Study
The current DOE program [19] has as a general goal the reduction of
commercial vehicle fuel consumption. While the major effort is focussed on
diesel motor development, a parallel effort has been aimed at aerodynamic
drag reduction as part of achieving the near-term goal of a 10-mpg Class 8
truck. The stated near-term, aerodynamic target is a 15% reduction in
aerodynamic drag, here assumed to refer to the wind-averaged drag coefficient
at 65-mi/h road speed, C D (65) . The reduction would be DC D (65) = 0.09 ,
referenced to a baseline of C D (65) ª 0.60 . Longer term, more advanced
geometries would be developed to lower drag further.
The DOE program has followed a different route than its DOT
predecessor. It has a major focus on CFD and CFD development, with
relatively small experimental effort. I have some questions concerning the
route that has been chosen for this program and would like to raise them.
The DOE multi-year program [20] to achieve these goals plans to:
“Improve and apply modern computational fluid dynamics codes to tractortrailer systems and identify new configurations to reduce this element of
aerodynamic drag. Follow analysis with design and experimental verification.”
A research program was proposed [21] to satisfy this aerodynamic
objective through the use of advanced CFD methods that were to be
developed as part of the program, with limited experimental benchmarking,
using simple geometries.
18
K.R. Cooper
The chosen technical approach appears to be founded on the following
commentary, quoted directly from [21].
“At present the aerodynamic design of heavy trucks is based largely upon wind
tunnel estimation of forces and moments, and upon qualitative streamline
visualization of flow fields. No better methods have been available
traditionally, and the designer/aerodynamicists are to be commended for
achieving significant design improvements over the past several decades on the
basis of limited quantitative information.
The trucking industry has not yet tapped into advanced design approaches using
state-of-the-art computational simulations to predict optimum aerodynamic
vehicles. Computational analysis tools can reduce the number of prototype tests,
cut manufacturing costs, and reduce overall time to market.”
These two paragraphs are worth careful analysis. A direct reading of the
first paragraph would intimate that experimental aerodynamicists were lucky
to have had any useful results. The opposite, of course, is true.
The wind tunnel permits the measurement, not the estimation, of
aerodynamic forces and the aerodynamicist has had exceptional success at
optimizing the commercial vehicle. Thousands of hours of development have
lead to effective add-on aerodynamic devices and the aerodynamic tractors that
we have today. A major part of the success has come because the physics of
the fluid flow in the wind tunnel is correct. Detailed flow measurement has
not been widely used in the wind tunnel because it does not provide an answer
to the question: “What is the drag?”.
Certainly, wake flow measurements can be and have been used to
measure vehicle drag, but a force balance is much faster and more accurate.
The wind tunnel has shown a remarkable correlation with the road and is a
fast, cost-effective and reliable tool. I am not sure how CFD cuts
manufacturing costs, although if used wisely with experiment it will accelerate
the development cycle.
The second paragraph suggests that CFD can do the optimization better.
It may one day, but cannot now. Firstly, the flow physics are approximated,
resulting in uncertainty in the result. Secondly, the large number of cases that
have to be computed would take much longer than a typical experimental
optimization.
As an example, consider the optimization of a cab-mounted deflector that
was performed in the NRC 2m x 3m wind tunnel. The task was to develop a
map of optimum deflector angle as functions of tractor-trailer gap and height
difference. The test program made 180 measurements over an array of six
gaps and six height differences between the cab roof and the trailer roof. The
deflector angle was adjusted to five values at each combination of separation
and height differential while seeking the best angle. Figure 6 shows the
resulting design table, giving the pin setting that provides the optimum
deflector angle for a selected of tractor-trailer separation and height
Commercial Vehicle Aerodynamic Drag Reduction
19
Tractor-trailer Roof Height
Differential, in.
differential. The test period was 30 hours, the time required for a few
computations. The end result was effective, with most users finding about a
30-50 percent improvement in fuel savings compared to the original factory
chart, which was based on guesswork.
The reality is that CFD may not be the best tool for the job, at least in
the near term. Current numerical simulation physics is challenged by highly
unsteady bluff-body flows. The presence of the natural wind ensures that the
yaw angle is almost always not zero, so that a plane-of-symmetry simulation is
not representative. Because the yaw performance of a truck is important in its
average energy utilisation, it is necessary to compute a sufficient number of
yawed cases to define this behaviour. CFD can provide a great detail of
information about a flow, aiding in understanding, but its use is time
consuming and expensive when a large database is required, particularly if the
computations are unsteady.
70
65
Pin 10
60
9
55
8
7
6
50
5
45
4
40
3
35
20
30
40
50
60
70
80
90
Tractor-trailer Separation, in.
Fig. 6: Wind-tunnel-derived Optimization Chart for a Cab-mounted Air Deflector
As has been suggested by the Trailmobile study, many of the needed
answers are available already. They should be applied. Unfortunately, the
modifications mostly fall on the trailer, or the tractor-trailer interface, which
are hard areas to treat for operational and economic reasons. On the
operational side, any new configurations must interface with the current fleet
and warehousing. They must be mechanically reliable, weather resistant and
not add significantly to yard work or they will not be accepted. Also,
recognising that the savings from base-drag reduction, skirts or gap seals are
small, they are a hard sell, especially when there are at least two trailers for
20
K.R. Cooper
every tractor. The marginal economic advantage is then divided by a factor of
two or more.
The challenge is not to squeeze a fraction more out of a bottle shrunk by
the law of diminishing returns, not to invent a slightly better gadget, but to
transfer current knowledge to industry in a profitable manner. The issue is to
design light, reliable components and encourage industry to use them.
Government can certainly have a role here through encouraging product
development, through education and by providing tax incentives.
It is possible to study problems without solving them. The current
approach seems to do too much studying and too little solving. What has
been achieved to date? It is my opinion that enough is known now to provide
a useful gain in aerodynamic efficiency immediately and that the basis for the
advanced truck exists. Let’s get on with it.
A Case Study – the Future Truck
Three weeks before this conference, I decided that a demonstration project
would serve to emphasise my arguments. As I have stated, a two-pronged
attack – near term and long term - seems like a good idea. I chose to tackle
what might be done in the near term to improve fuel consumption, by
performing a quick test in the NRC 2m x 3m wind tunnel. The project
started with an existing White Road Boss II tractor and 40-foot Dorsey trailer
– a 1:10-scale model of the combination shown in the upper-left photograph
of Figure 3. The plan was to bring this old truck to a higher state of
aerodynamic development using technology that could be applied now. The
results are proffered as a challenge for the DOE CFD program to equal.
The truck was fitted with a contemporary aerodynamics package
consisting of a cab-roof fairing and side extenders. To this baseline were
added:
1. tractor skirts and front trailer skirts back to the trailer wheels
2. beveled base panels (simple boat tail)
3. additional rear skirts behind the trailer wheels
4. a gap seal between tractor and trailer
5. a filler block to completely close and fair the gap
Figure 7 shows the configurations reported and Figure 8 presents a
selection from the drag measurements made. Table 2 summarises the drag
behaviour. It is apparent that the skirts and the rear-end treatment satisfy the
15 percent drag target, and that the gap seal improves the drag further. Both
the skirts and the bevelled rear panels have been tested at full scale in the NRC
9m x 9m wind tunnel, Figure 9. The results obtained were virtually identical
to the model results.
Commercial Vehicle Aerodynamic Drag Reduction
Fully modified truck with full gap fairing, skirts and bevelled rear panels
A view of the 15º bevelled extension panels
The gap seal
Fig. 7: Model Configurations Tested
21
22
K.R. Cooper
1.20
1.10
baseline
Drag Coefficient
1.00
0.90
standard aero package
0.80
+ front trailer skirts
+ rear
bevels
+ gap seal
0.70
0.60
+ rear trailer skirts
0.50
- gap seal + gap filled
0.40
-25
-20
-15
-10
-5
0
5
10
15
20
25
Yaw Angle, deg.
Fig. 8: Low-drag Development of the NRC Tractor-trailer
Fig. 9: Full-scale Test of Trailer Skirts
Fig. 10: Prototype Gap Seal
The gap seal is a device that was patented by Airshield and was fieldtested successfully. However, it never made the transition to market. I do not
know why, although mechanical reliability may have been a major issue. A
prototype Airshield gap seal is shown in Figure 10. It is worth revisiting.
The tractor used in this study does not have the improved shapes of
contemporary equipment. It is expected that the drag would be reduced
further, by approximately 0.05 £ DC D (65) £ 0.08 with a current tractor and
aero package. The end result would be a drag level of C D (65) £ 0.50 with the
skirts, rear-end treatment and gap seal.
Commercial Vehicle Aerodynamic Drag Reduction
23
The base drag reduction by the beveled plates is not the only possibility.
The use of inset boat-tail plates serves a similar function through the use of a
trapped vortex. This is not a new concept, but was first suggested, to my
knowledge, by J. J. Cornish III, chief engineer, Lockheed-Georgia Company,
in 1968 [22].
This test was completed in 8 hours of tunnel operation and required 6
person days to make the new model parts. The project would have taken
much longer if the models had to be built, but they were available, as are many
other models at 1:8 scale, 1:10 scale and larger scales at various laboratories
and companies. These models provide an inexpensive resource for future
work.
Table 2: Summary of the Low-Drag Development of the NRC Tractor-trailer
Case
Configuration
CD(0º)#
CD(10º)
C D (65 * )
DC D (65)
DC D (65)
re Aero
package
1
White RB II, 9-ft.wide van trailer,
10” front posts
0.765
0.979
0.871
-
-
2
Aero package
(roof fairing + cab
side extenders +
cab skirts)
0.569
0.833
0.724
0.147
-
3
Aero package,
front trailer skirts
0.550
0.710
0.644
0.080
0.080
4
Added rear
bevelled extension
panels
0.511
0.660
0.600
0.044
0.124
5
Added gap seal
0.509
0.615
0.571
0.029
0.153
6
Added rear trailer
skirts + bevel
0.482
0.583
0.540
0.031
0.184
7
Added gap filler
block
0.440
0.513
0.485
0.055
0.239
# Reference area of 97.5 ft2 at full scale
* at 65 mi/h
A fully integrated tractor-trailer combination poses a greater design
challenge than do these add-on components. However, it has been done
successfully with a bus. The example shown in Figure 11 is the Prevost H5-60
articulated highway bus. It is 8.5 feet wide, 13.5 feet high and 60 feet long –
the dimensions of a tractor-trailer combination. It has a sealed articulation
24
K.R. Cooper
and quite low drag. Wind tunnel measurements from a 1:10-scale model of
are presented in Figure 12 and Table 3, courtesy of Prevost Car Inc. Data for
two other configurations of the articulated bus are shown also, as is the data
from Case 6 of Table 2 for comparison. In one bus configuration, the mirrors
have been removed and in another, a more streamlined, but practical, nose and
bevelled rear have been fitted. The single bus is the front unit from the
articulated bus.
Of note is the fact that the articulated bus is 50 percent longer than the
identically shaped single bus but has only 9 percent higher drag. This point
will be revisited in the next section. The advanced articulated bus has very low
drag that is nearly constant with yaw angle and may be near a practical limit
for passive aerodynamics for a geometry having a blunt base. It is apparent
that the articulated bus is superior to the developed tractor-trailer. However,
the difference would diminish with a more rounded cab, and would diminish
further with full cab-trailer integration and skirting. At the limit, the two
vehicles should be identical.
Drag Coefficient
0.8
production Prevost H5-60
articulated bus
White RB II
skirts, bevels, gap seal
0.7
0.6
0.5
Prevost H5-60
no mirrors
0.4
0.3
articulated bus, streamlined front + boat tail
0.2
-20
Fig. 11: Low-drag Articulated Bus,
the Prevost H5-60
-15
-10
-5
0
5
Yaw Angle, deg.
10
15
20
Fig. 12: Drag Characteristics of the Prevost H5-60
Compared to a Tractor-Trailer
Table 3: Bus Drag measurements
Configuration
CD(0º)
C D (65mi / h )
1
Aero RB II, all skirts + rear bevelled panels + gap
seal
0.482
0.540
2
Single Prevost Bus
0.351
0.384
3
Articulated Prevost H5-60 bus
0.378
0.418
4
Articulated Prevost H5-60 bus, no mirrors
0.315
0.344
5
Advanced articulated bus
0.293
0.311
Case
Commercial Vehicle Aerodynamic Drag Reduction
25
Another Concept
As a final thought, the concept of vehicle platoons [23] applies very well to
trucks. The simplest way to decrease the aerodynamic drag of a tractor-trailer
is to add one or two more trailers. This follows the result for the single and
articulated buses just discussed. Truck trains made up of two or three trailers
have been run on selected freeways in some states and provinces. However,
the practice is not widespread. The question is, “Should it be?”. There are
many safety and infrastructure issues to deal with but the returns could be
large, both from energy and road capacity points of view.
When a second trailer is added to increase capacity by a factor of two, the
weight does not double and aerodynamic drag increases by about 40 percent.
Thus the aerodynamic drag per ton-mile is decreased by 30 percent. It is
unlikely that any other aerodynamic technique with a single trailer will be as
effective.
As an example to illustrate this point, consider the data from a tractor
model that was tested in the NRC 2m x 3m wind tunnel with three trailer
combinations – a single 27-foot trailer, a single 45-foot trailer and a pair of
tandem 27-foot trailers. A photograph of the tandem 27-foot trailer
configuration is seen in Figure 13 and the measured drag results are presented
in Figure 14 and in Table 3. The baseline tractor was equipped with a full
Airshield roof fairing and cab extenders.
1.1
1x27 ft trailer
1x45 ft trailer
2x27 ft trailer
Drag Coefficient
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.0
2.5
5.0
7.5
10.0
12.5
Yaw Angle, deg.
Fig. 13: Tandem 27-foot Trailers
Fig. 14: Drag Behavior of a Tractor Pulling Single
or Double Trailers
26
K.R. Cooper
Table 3: Aerodynamic Performance of Truck Trains
Configuration
CD(0º)
CD(8º)
C D (65)
C D (65) /unit
cargo
1
Conventional tractor,
single 27-ft. trailer
0.515
0.659
0.591
0.591
2
Conventional tractor,
single 45-ft. trailer
0.576
0.752
0.660
0.396
3
Conventional tractor,
two 27-ft. trailers
0.685
0.939
0.805
0.403
Case
The drag-coefficient/unit-cargo for the larger capacity trailer
combinations were found by dividing their measured wind-averaged drag
coefficients by the ratios of the modified trailer lengths to the 27-foot length.
A second 45-foot trailer would provide an even greater reduction than
obtained from the two 27-foot trailers.
Closing remarks
This paper turned out differently than the one that was first planned, which
was a discussion of past technology. As the paper progressed, it became more
and more apparent that most of the required aerodynamic knowledge was in
hand. It also seemed that most of this work was being ignored and that the
effort to advance CFD was retarding the application of known aerodynamic
technology to trucking.
The goal of reducing the aerodynamic drag of commercial vehicles is a
worthy one. It is economically and socially valuable. The development of
advanced CFD is also technically useful and will be of benefit in vehicular
development. However, delaying the introduction of new hardware and
concepts into the fleet while waiting for the evolution of these new CFD tools
is counterproductive, especially since the major issues are not aerodynamic,
but are those of operational effectiveness and mechanical design.
Many of the major tractor and trailer manufacturers have built
demonstration vehicles that incorporate advanced aerodynamic technology
including aerodynamic cabs, completely integrated tractors and trailers, skirts
and rear-end treatment. They all had low fuel consumption. And none of
them are on the market. Why? It must be because they were not
economically viable and because they offered too many impediments to
efficient operation. These are the issues of importance. Economics will take
Commercial Vehicle Aerodynamic Drag Reduction
27
care of itself through a steady rise in fuel prices. It would be aided by the
design of effective, operationally effective and inexpensive components,
perhaps encouraged by tax incentive. The operational issues can best be
resolved by industry-wide collaboration.
A two-pronged approach to the problem of introducing improved
aerodynamics might be beneficial. CFD can be developed for long-term
application while a parallel development of existing technology for near-term
implementation, based on present knowledge and some experiment, is
pursued.
In the latter case, the effort required is that of mechanical design done in
close cooperation with industry. The target would be to select the most likely
candidate technologies for development and, using clever design and modern
materials, produce reliable, cost-effective hardware that will benefit truckers
now, and that will be acceptable to the end users. The designs would have to
capture the necessary aerodynamic benefits without causing operational
difficulties. The answers are out there; neither CFD nor the wind tunnel will
tell us how to apply them.
References
1. DOT/SAE Truck and Bus Fuel Economy Measurement Study Report
P59A. Report No. DOT/TSC – 1007, October, 1976.
2. A. Wiley Sherwood - Wind Tunnel test of Trailmobile Trailers.
University of Maryland Wind Tunnel Report No. 85. College Park, MD,
April 1974.
3. A. Wiley Sherwood - Wind Tunnel test of Trailmobile Trailers, 2nd Series.
University of Maryland Wind Tunnel Report No. 85. College Park, MD,
April 1974.
4. A. Wiley Sherwood - Wind Tunnel test of Trailmobile Trailers, 3rd Series.
University of Maryland Wind Tunnel Report No. 85. College Park, MD,
April 1974.
5. K. R. Cooper - A Wind Tunnel Investigation into the Fuel Savings
Available from the Aerodynamic Drag Reduction of Trucks. Article from
DME/NAE Quarterly Bulletin No. 1976(3), NRC, Ottawa, Canada,
1976.
6. SAE Wind Tunnel Test Procedure for Trucks and Buses. Recommended
Practice, SAE J1252, August 1979.
7. Road Load Measurement and Dynamometer Simulation Using
Coastdown Techniques. SAE Recommended Practice J1263, approved
June 1979.
8. Joint Rccc/SAE Fuel Consumption Test Procedure (Short Term-in-service
Vehicle) - Type I – SAE J1264. SAE Recommended Practice, approved
April 1979.
9. Joint Rccc/SAE Fuel Consumption Test Procedure - Type II – SAE
J1321. SAE Recommended Practice, approved April 1979.
28
K.R. Cooper
10. F. T. Buckley, Jr, C. H. Marks, W. H. Walston – A Study of
Aerodynamic Methods for Improving Truck Fuel Economy. University
of Maryland, College Park, MD, December, 1978.
11. W. T. Mason, P. S. Beebe – The Drag Related Flow field Characteristics
of Trucks and Buses. Aerodynamic Drag Mechanisms of Bluff Bodies and
Road Vehicles. Symposium Held at the General Motors Research
Laboratories. Plenum Press, 1978.
12. H. Götz – Present and Future Trends in Automotive Aerodynamics. VKI
Fluid Dynamics Vehicle Aerodynamics Short Course 1984-01. RhodeSt.-Genese, Belgium, 1984.
13. A. Gilhaus – Aerodynamics of Heavy Commercial Vehicles. VKI Fluid
Dynamics Vehicle Aerodynamics Short Course 1984-01. Rhode-St.Genese, Belgium, 1984.
14. K. R. Cooper, W. T. Mason Jr., W. H. Bettes - Correlation Experience
with the SAE Wind Tunnel Test Procedure for Trucks and Buses. SAE
820375, Int’l Congress & Exposition, Detroit, Michigan, Feb. 22-26,
1982.
15. K. R. Cooper - Wind Tunnel Measurements on the Nose Cone - Tests 1
and 2. LTR-LA-249, NRC, Ottawa, Canada, October 1981.
16. Cooper, K.R. - Wind Tunnel Investigation of the Royal-Air Trailer
Fairing. LTR-LA-256, November 1981.
17. K. R. Cooper - The Wind Tunnel Testing of Heavy Trucks to Reduce
Fuel Consumption. SAE 821285, Indianapolis, November 1982.
18. K. R. Cooper - The Effect of Front-Edge Rounding and Rear-Edge
Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground
Proximity. SAE 850288, Detroit, USA, February 1985.
19. K. R. Cooper - Truck Fuel Savings Through the Use of Trailer Skirts and
Trailer Rear-Corner Rounding. LTR-LA-224, May 1978.
20. Multiyear Program Plan for 1998-2002. Office of Heavy Vehicle
Technologies and Heavy Vehicle Industry Partners. DOE/ORO-2071,
August 1998.
21. A Multi-Year Program Plan for the Aerodynamic Design of Heavy
Vehicles. http://en-env.llnl.gov/aerodrag/
22. J. J. Cornish III - Trapped Vortex Flow Control for Automobiles.
Proceedings of the Second AIAA Symposium on the Aerodynamics of
Sports & Competition Automobiles. Los Angeles, CA, May 1974.
23. M. Hammache, F. Browand – Aerodynamic Forces on Truck Models,
Including Two Trucks in Tandem. SAE 2002-01-0530, SAE 2002
World Congress, Detroit, MI, March 2002.