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Tractor Aerodynamics

Tractor add-on features, such as cab fairings, cab deflectors, axle fenders, and side fairings, can significantly reduce the aerodynamic drag and provide fuel savings.

Important obstacles stand against the adoption of innovative technologies, of which one of the most important is the lack of practical information coming from independent sources. The objective of these fact sheets is to inform those in the trucking industry about new and emerging advanced technologies that have been tested through the Energotest program (run by FPInnovations), which can help increase fuel efficiency and, at the same time, reduce the environmental impact of freight transportation in Canada.

Description

After gross vehicle weight, air resistance (or drag) is the largest factor responsible for vehicle energy consumption. The magnitude of this resistance is affected by the vehicle's shape, frontal area, and travel speed. Drag significantly increases fuel consumption. The greater the frontal area of a vehicle, the greater the aerodynamic drag will be. It is important to specify a well-styled aerodynamic vehicle when purchasing a new vehicle. However, there is a range of add-on features available that can significantly improve the aerodynamics of many existing vehicles, such as cab fairings, cab deflectors, axle fenders, and side fairings.

Fuel Tank Fairings

FPInnovations conducted a track test to evaluate the fuel consumption of a vehicle when the fuel tanks are covered by fairings, compared to when the tanks are exposed. Fairings offer a smoother, more uniform surface, thus reducing air drag, and the test resulted in 1.0% fuel savings.

Axle Fenders

The rear-axle fender aims to optimize the airflow around the axle wheels, thus reducing the turbulence and the aerodynamic drag. The test with a streamlined tractor drive-axle fender showed 1.4% fuel savings.

Cab Fairings

Another FPInnovations track test conducted during the Energotest^TM 2010 campaign assessed the influence of an advanced cab fairing package for day-cab tractors. The tested device was an OEM Aerodynamic Package, composed of a roof air deflector, side air deflectors, and side extenders. The test resulted in 15.6% fuel savings for the vehicle equipped with the aerodynamic package, compared to the bare baseline configuration. The result is explained by the effect of all of the fairings but especially of the cab deflector, which prevented air being forced against an important area of the frontal surface of the semi-trailer (roughly 2.7 m²), reducing resistance to movement and aerodynamic drag.

Heavy-duty Bumpers

There are many other accessories that can increase the aerodynamic drag and fuel consumption, such as over-the-hood air filter housings, lights and horns installed on cab roofs, and heavy-duty bumpers. Many transport companies are choosing heavy-duty bumpers because of the combined effect of very high insurance premiums and the fragility of the lightweight materials used for front-end components. Having a heavy-duty bumper makes it possible to avoid claiming minor collisions but comes with a fuel consumption penalty. The impact on fuel consumption of this approach was also evaluated by FPInnovations during Energotest™ campaigns. Testing resulted in a 2% increase in fuel consumption for the vehicle equipped with the heavy-duty bumper, which can be explained by increased weight and larger surface area compared with the original bumper, as well as possible increased turbulence.

Scope of Application

Drag reduction devices are used on Class 8 tractors and, as with other aerodynamic technologies and approaches, they are best suited for line and regional hauling, which are characterized by long trip lengths and higher average speeds. Because they make tractors more aerodynamically efficient, maximum benefits are only realized at higher cruising speeds.

Return on Investment

The economic impact of the various fuel-saving measures is evaluated based on the payback period, which is calculated by dividing the total additional cost of a modification by the annual net savings it provides. The table below presents payback calculation examples for the various technologies based on certified track-test results, on the pricing information forwarded by the suppliers, and on following assumptions:

• An annual mileage for the tractor of 200 000 km and we assume that the track-test results would apply only under similar conditions, such as trips done on roads with a posted maximum speed of 80 km/h or more: 80% of the annual mileage, resulting in 160 000 applicable kilometres.
• A diesel fuel price of 1.10 $/L.
• A fuel consumption rate of 34 L/100km.
• The annual fuel savings would be:  
  

  The payback period for the various technologies would be:

Device	Additional cost ($)	Annual savings			Payback period
		%	L	$	Years	Months
Fuel tank fairings	2000	1.0	544	598	3.34	40
Rear axle fenders	3900	1.4	762	838	4.65	56
Aerodynamic package for day-cab tractor	1160	15.6	8486	9335	0.12	1.5

Specification Considerations

To increase the lifetime of the device, inquire about protective coatings, material composition, and means of attachment: metallic devices may rust and non-metallic devices may degrade through exposure to the environment.

For a cab roof fairing, rounded edges on the sides are recommended. Better performance is expected if the back is left open, which is feasible when the interior volume is not used for other purposes. If the fairing is not adjustable, ensure that it matches the height of the semi-trailer. However, if it is not possible to adjust the height of the roof fairing to the correct height, it is preferable for the fairing to be adjusted too high rather than too low. The side fairings should cover, as much as possible, the gap between the cab and the trailer.

Regulatory Issues

There are no specific regulatory requirements, but these devices could increase tare weight and marginally reduce payload allowance. When after-market cab roof fairings are installed, ensure that the vehicle will not exceed the legal height of 4.16 m.

California requires that tractors operating in the state must be U.S. Environmental Protection Agency SmartWaySM certified tractors, or retrofitted with SmartWay verified technologies. More information can be found at http://www.arb.ca.gov/cc/hdghg/hdghg.htm.

To be eligible for SmartWay certification, among others, a tractor must be a Class 8 sleeper-cab tractor equipped with an integrated roof fairing, cab side extender fairings, side tank fairings, an aerodynamic bumper, and aerodynamic mirrors.

Maintenance Considerations

You should purchase the cab roof fairing when you order the tractor from the manufacturer. When purchasing a used tractor, look for mounting brackets that are already installed.

Install the aerodynamic device according to the manufacturer’s specifications: correct installation is critical for optimum efficiency. Some manufacturers and dealers provide installation, or training on the installation.

Regularly check that the devices have not been damaged and are secure.

During the winter, check that snow or ice has not accumulated on the device, as its aerodynamic efficiency will be impaired and the weight of the ice itself could damage it.

References

Council of Energy Ministers. 2009. On the road to a fuel-efficient truck: A guide for purchasing aerodynamics for heavy-duty tractors and trailers. Natural Resources Canada, Ottawa, ON. http://fleetsmart.nrcan.gc.ca/index.cfm?fuseaction=rfet.index (accessed November 2010).

Department for Transport. 2007. Freight best practice: Aerodynamics for efficient road freight operations. Queens Printer and Controller of HMSO 2007.

EPA (United States Environmental Protection Agency). 2010. Interim requirements to determine eligibility of SmartWay tractors. EPA-420-F-09-045. Washington, DC.

Surcel, M.-D. 2010. Energotest 2010: Fuel consumption track tests of fuel-saving technologies. FPInnovations, Pointe-Claire, QC. Internal Report 2010-10-28. 93 p.

Surcel, M.-D.; Michaelsen, J.; Provencher, Y.  2008. Track-test evaluation of aerodynamic drag reducing measures for Class 8 tractor-trailers. SAE 2008-01-2600. SAE 2008 Commercial Vehicle Engineering Congress and Exhibition, October 7-9, 2008, Rosemont – Chicago, IL.

Surcel, M.-D.; Provencher, Y.; Michaelsen, J. 2009. Fuel consumption track tests for tractor-trailer fuel saving technologies. Paper no. 2009-01-2891. SAE 2009 Commercial Vehicle Engineering Congress & Exhibition, October 6-8, 2009, Rosemont – Chicago, IL.

Date modified:

2012-03-09

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