Improving the Aerodynamic Efficiency of Heavy-Duty Vehicles – Wind Tunnel Test Results of Trailer-Based Drag Reduction Technologies

Executive Summary

Through its ecoTECHNOLOGY for Vehicles program, Transport Canada commissioned the National Research Council Canada (NRC) to investigate the aerodynamic improvements possible with current and emerging drag reduction technologies for heavy-duty vehicles, with the intent of guiding future implementation and regulation of such technologies for Canada's transportation industry. A wind-tunnel test campaign was undertaken in the NRC 9 m Wind Tunnel to evaluate the aerodynamic performance of various drag reduction concepts, with an emphasis on those for trailers, using a 30% scale model of modern tractor-trailer combinations. Project stakeholders also include Environment Canada's Transportation Division.

A wind-tunnel approach was taken for the project because of its ability to provide a precise measure of the aerodynamic differences between vehicle configurations. Advancements in testing techniques developed for the project have improved the accuracy of the results, such that they reflect better the aerodynamic performance of a tractor-trailer combination under real-world conditions. These advancements consisted of a modular model that can represent various tractor configurations (sleeper-cab and day-cab with various roof-fairings for each) and trailer configurations (40 ft dry-van, 53 ft dry-van, 53 ft half-height dry-van, 53 ft flatbed with various cargo configurations, tandem 28 ft long-combination vehicle arrangement). The model has spinning wheels matched to an appropriate ground-effect simulation consisting of a boundary-layer suction system and a moving ground plane. Cooling drag is simulated with an engine compartment through which the flow-rate is proportional to that of a real vehicle. The 30% scale of the model is small enough to minimize wall-interference effects in the wind tunnel even with a 53 ft-equivalent trailer, yet is big enough to provide the aerodynamic performance of a full-scale vehicle through appropriate Reynolds-number scaling. In addition to the model, a new Road Turbulence System (RTS) has been introduced in the NRC 9m Wind Tunnel that provides road-representative turbulence in the wind. All of these advances create the appropriate relative motions between the vehicle, the ground, and the wind.

The overall test program described herein included distinct sub-studies to address drag reduction techniques for various regions of the vehicle or for different vehicle types. For each vehicle configuration tested, the wind-tunnel drag-coefficient measurements were used to calculate a wind-averaged-drag-coefficient that represents a long-term average of the aerodynamic performance for typical North-American wind conditions, from which fuel savings and greenhouse-gas reductions have been estimated based on typical Canadian driving distances. The table on the next page summarizes the main findings of the study, with fuel savings and greenhouse-gas reduction estimates for some of the configurations tested.

Drag-Reduction Technique  Fuel Saved
 CO2 Reduction
Tractor-Trailer Gap:
reduce tractor-trailer gap by 12"  800 ± 200  2,100 ± 500
add trailer fairing for sleeper-cab w/ 36" gap   600 ± 200 1,600 ± 500
add trailer fairing for day-cab w/ 36" gap  1,600 ± 500 4,200 ± 1,300
Trailer Underbody:
add side-skirts to tandem axle trailer 2,900 ± 800 7,700 ± 2,100
add extended side-skirts to tandem axle trailer 3,300 ± 900 8,700 ± 2,400
add side-skirts to tridem axle trailer  3,800 ± 1,100 10,000 ± 2,900
Trailer Base:
add long or short 4-panel boat-tail to trailer base 1,900 ± 500 5,000 ± 1,300
add tapered-side 3-panel boat-tail to trailer base 1,600 ± 500  4,200 ± 1,300
Trailer Upper-Body:    
profile the trailer roof (top 6")  1,000 ± 300 2,600 ± 800
48" to 36" gap, trailer fairing, side-skirts, boat-tail (sleeper)  6,700 ± 1,900  17,700 ± 5,000
48" to 36" gap, trailer fairing, extended skirts, boat-tail, profile roof (sleeper) 8,300 ± 2,300 21,900 ± 6,100
48" to 36" gap, trailer fairing, side-skirts, boat-tail (day-cab) 7,900 ± 2,200 20,900 ± 5,800
48" to 36" gap, trailer fairing, extended side-skirts, boat-tail (day-cab) 8,600 ± 2,400 22,700 ± 6,300
Flatbed Trailers:
add side-skirts to flatbed with high irregular cargo 2,900 ± 800 7,700 ± 2,100
add side-skirts to flatbed with low irregular cargo 1,600 ± 400 4,200 ± 1,100
Long Combination Vehicles - LCVs:
add trailer fairing to LCV trailer-trailer gap 1,400 ± 400 3,700 ± 1,100
reduce LCV trailer-trailer gap from 5 ft to 3 ft 1,900 ± 500 5,000 ± 1,300
add trailer fairing and reduce gap, and add full aero package to LCV 7,900 ± 2,200 20,900 ± 5,800
Tractor-Trailer Height Matching:
remove full-height fairing from day-cab with low dry-van trailer 5,400 ± 1,500  14,300 ± 4,000
remove full-height fairing from day-cab with full-height dry-van trailer  -5,400 ± 1,500 -14,300 ± 4,000

† estimated for 125,000±35,000 km/tractor/year @ 100 km/hr

Reducing the aerodynamic drag associated with dry-van trailers was the primary focus of the current effort, and several regions of a tractor-trailer combination were targeted with different drag reduction technologies. For these efforts, the vehicle model represented a modern aero tractor with a 53 ft dry-van trailer. The drag-reduction techniques tested do not represent specific commercial products, although some were designed to achieve drag reduction in a similar manner to technologies on the market.

The gap between the tractor and trailer is a region in which air can circulate and pass through, and is a dominant source of drag for a tractor-trailer combination. Many modern tractors are outfitted with side-extenders that reduce the effective air-gap between the two bodies, and provide a reduction in fuel use, however operational restrictions may prevent the ability to achieve such savings. To better understand the sensitivity of vehicle drag to the gap width, measurements were performed for several gap widths and it was found that the wind-averaged-drag was reduced by 2.6% for every foot the gap was reduced (8.5% per metre). A one foot reduction in gap width, which may be operationally feasible for many vehicles on the road, translates to a reduction in fuel consumption on the order of 800 litres per tractor per year, with CO2 emissions reductions of 2,100 kg per tractor per year. An active fifth-wheel system can provide such benefits at highway speed without adversely affecting low-speed manoeuvering and operations. Another technique to reduce drag associated with the tractor-trailer gap is to introduce a device that prevents air from flowing through the gap region. Of the concepts tested, a large trailer fairing was found to provide the greatest benefit, with drag reductions on the order of 2% for the sleeper-cab tractor variant tested, and 5% for the day-cab variant, providing associated fuel savings of 600 litres and 1,600 litres per tractor per year, respectively. Reducing the gap width and adding a trailer fairing can provide fuel savings in excess of 2,000 litres per tractor per year and greenhouse-gas reductions in excess of 4,000 kg CO2 per tractor per year.

As would be expected based on their prevalent use on North-American highways, side-skirts provide the greatest drag reductions of the trailer-underbody concepts tested. By redirecting the wind around the trailer, they prevent high-momentum air from being entrained in the underbody region and from impinging on the trailer bogie. Drag reductions of 10% were measured for different side-skirt arrangements with a tandem-axle trailer bogie, and extending the skirts over the trailer wheels provided an added benefit such that fuel savings exceeding 3,000 litres per tractor per year may be realized. An even greater reduction in drag was measured for side-skirts applied to a tridem-axle bogie arrangement, with fuel savings of nearly 4,000 litres per tractor per year and greenhouse-gas reductions of 10,000 kg CO2 per tractor per year.

Several boat-tail concepts were tested to examine the influence of a lower panel, the sensitivity to length, and the relative potential for inflatable boat-tails. All showed similar results, with the greatest benefit realized from the four-panel configurations (6-7% drag reduction), providing an estimated fuel savings of 1,900 litres per tractor per year and greenhouse-gas reductions of 5,000 kg CO2 per tractor per year. The short (2 ft full-scale) and long (4 ft full-scale) boat-tail concepts showed the same level of drag reduction. Removing the lower panel and reducing the surface area of the side panels showed only a small reduction in performance (5-6% drag reduction), providing further evidence to support the hypothesis that the manner in which the top panel guides the air downwards towards the ground is the dominant influence on boat-tail performance. Other studies have shown boat-tails to be as effective as side-skirts, reaching drag reductions of 10%. The vertical offset of the top panel tested here (3 inches full-scale), included to leave room for lights at the top edge of the trailer base, may be a reason why the boat-tail concepts tested here have not provided the same magnitude of drag reductions observed for other similar boat-tail concepts. This presents a clear challenge to developing effective boat-tails for real-world applications.

The intent of the current study was to evaluate ways of reducing the drag associated with dry-van trailers without changing cargo capacity. In an effort to modify the shape of the roof while minimizing any influence to the cargo volume, the top 6 inches of the trailer were modified in three ways: rounding the front edge, rounding the side edges, and tapering the aft edge. The aft taper provided the greatest benefit of the three, however the combined profiled roof provided a drag reduction of 3.5%, which translates to 1,000 litres per tractor per year in fuel savings and a reduction in greenhouse gas emissions of 2,600 kg CO2.

Of the various technologies tested, some did not provide any measurable drag reductions and some showed increased drag. A partial plate seal applied to the front face of the trailer and paired to the sleeper-cab with a 36 inch tractor-trailer gap showed no significant reduction in wind-averaged drag. Removing the landing gear, smoothing the trailer underbody, and adding an underbody diffuser fairing all showed a small increase in wind-averaged drag. For these attempts, it was found that by reducing the resistance to flow in the underbody region, a greater flow-rate is introduced in this region which increases the drag of the trailer bogie. Roof mounted vortex generators also showed increased wind-averaged drag. These various poorly-performing concepts do not represent specific commercial products or concepts and the designs used have not been optimized. These test results should not be taken to mean such concepts will not work, only that they show much lower potential for fuel savings than the well-performing technologies and that they must be carefully optimized.

The best performing techniques for each region of the dry-van trailer were combined to examine the additive properties of the various technologies, and similar combinations were paired with both the day-cab and sleeper-cab variants. Significant drag reductions of up to 29% have been observed for some combinations. Fuel savings in excess of 8,000 litres per tractor per year are predicted for some combinations (greater than $10,000 per year at current diesel rates). Greater reductions were observed for the day-cab than the sleeper-cab tractor, and have been attributed to the sleeper-cab guiding the wind over the gap region in a smoother manner as a result of its length, thus receiving less gains from the gap devices. Of particular note, it was found that side-skirts and boat-tails have a mutually beneficial interaction that provides a reduction in drag from their combined use that is greater than the sum of their individual drag reductions. An additional 3% drag reduction was observed in the current study when the extended side-skirts and boat-tail were paired. This interaction has been identified as a possible source of discrepancy for performance claims reported in literature of side-skirts and boat-tails when tested in a combined manner as opposed to when tested individually.

In addition to the full-height 53 ft single dry-van trailer, the current project examined other trailer types including a 53 ft flatbed trailer with different cargo configurations, a tandem 28 ft dry-van trailer, and a 53 ft half-height dry-van trailer. This was done in an attempt to identify fuel savings measures for a greater proportion of tractor-trailer combinations found on the road. Different tractor roof configurations were also tested for some trailer configurations to examine the sensitivity to proper matching of the tractor with the trailer

Side-skirts were beneficial for all the flatbed configurations tested, but the magnitude of the drag reductions varied (5% to 8%). A mid-height tractor roof was shown to benefit all of the flatbed cargo configurations, even for a set of large boxes with a maximum height the same as a full-height dry-van trailer.

For the tandem 28 ft trailer, which was used to represent a long combination vehicle (LCV), reducing the trailer-trailer gap from 5 ft to 3 ft was most beneficial, but adding a trailer fairing or full-plate seal in the trailer-trailer gap provided measurable drag reductions. The same magnitudes of drag reductions were not realized when the rest of the trailer regions were treated with side-skirts, a boat-tail at the base of the aft trailer, and a fairing on the front of the forward trailer. A 25% drag reduction was measured for the full aerodynamic treatment of the LCV configuration.

Aerodynamic matching of the tractor and trailer was examined by testing different tractor-roof configurations with various trailers. The most interesting finding was that the drag increase when adding a full-height roof fairing to a low-tractor/low-trailer configuration is as great as the drag increase when removing the fairing from a high-tractor/high-trailer. The improperly paired configurations can result in an increased fuel use in excess of 5,000 litres per tractor per year and increased greenhouse-gas emissions in excess of 14,000 kg CO2 per tractor per year.

The results presented in this study are intended to provide guidance to Canadian regulators and Canada's transportation industry on effective ways to reduce the fuel consumption and emissions, through aerodynamic means, from the transportation of goods on Canadian roadways. Descriptions of the way in which the technologies affect the flow-field around a heavy-duty vehicle should also be helpful in providing guidance to technology developers, and in particular to trailer manufacturers that have the opportunity to design high-efficiency trailers for the next generation of heavy-duty vehicles.

The full report can be found at:〈=en

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