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7. Aerodynamic Drag Reduction Practices

Aerodynamic Drag Reduction Practices

Simple aerodynamic practices requiring little or no investment could bring considerable 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

Aerodynamic drag significantly increases fuel consumption: it has been estimated that decreasing drag by 2% can improve fuel economy by approximately 1%. FPInnovations tests confirmed the potential of add-on aerodynamic devices such as trailer rear deflectors and trailer skirts, which showed fuel improvements up to 5.6% and 7.2%, respectively.

Besides the add-on and built-in aerodynamic features, there are also simple practices requiring little or no investment, but which could bring considerable fuel savings.

Speed Reduction

The easiest and most cost-effective approach for reducing aerodynamic drag and increasing fuel efficiency is to reduce highway speed. Excessive speed is the largest single factor in reduced fuel efficiency because air resistance at high speeds represents the bulk of power demand. The rule of thumb for the influence of speed on fuel consumption is that for every 10 km/h over 90 km/h, you use roughly 10% more fuel. The impact of speed on fuel consumption was evaluated with two FPInnovations track-tests: compared with the baseline stage at 98 km/h, the test stages showed a 2.0% fuel economy for the constant speed of 95 km/h, and 5.1% for the constant speed of 92 km/h. Therefore, the test results confirm the above-mentioned rule of thumb.

Speed Limiters

Speed limiters electronically restrict the maximum attainable road speed and are standard equipment on the majority of heavy-duty trucks. Their activation through the electronic control module (ECM) is relatively simple, but requires manufacturer’s software and sometimes is password-protected. An operational observation conducted by FPInnovations showed a 6.3% fuel improvement for a vehicle combination equipped with speed limiters, low rolling resistance tires, and trailer side skirts in the movement of Less-than-Truckload (LTL) freight. The speed limiter in the Detroit Diesel ECM was activated to limit the top speed to 100.6 km/h.

Containers vs. Vans

Either because of increased overseas imports or simply because of changes in shipping practices, transport companies have been using more and more transcontainers over the past several years. With their wavy walls that are built for durability and easy maintenance, their aerodynamic performance would be expected to be lower than those of smooth-walled vans. Energotest^TM track-test results confirmed that transcontainers are much less efficient than van semi-trailers. With a 15% increase in fuel consumption, transcontainers do not represent a fuel-efficient alternative for road transportation, even if they present advantages for railway and water transportation. Transporters might consider this information when reviewing their practices. Thus, for long-distance transport, it could be worthwhile to transfer freight from containers to van semi-trailers, with the additional cost being recovered from the saved fuel. Another solution could be different rates for transcontainers and van semi-trailers. Energotest^TM track tests were also conducted comparing van semi-trailers to curtain-sided semi-trailers with comparable payloads, and no significant differences in fuel consumption were measured.

Tarp Systems

FPInnovations conducted track tests to evaluate the influence on the fuel consumption of unloaded dump trucks and tractors pulling dump semi-trailers, when the empty boxes are sealed with the tarp system. Sealing the top of the body improves the aerodynamic shape of the trailer and prevents air being forced against the tailboard at the back of the trailer, thus reducing resistance to movement and aerodynamic drag. The tests showed fuel savings of 8.8% for the truck, 13.8% for a 44-ft. dump semi-trailer, and 14.9% for a 53-ft. dump semi-trailer.

Scope of Application

Speed limiters and speed reduction strategies can be used on any commercial vehicles. Using tarp systems on empty dump trucks and trailers, and transferring freight from containers to van semi-trailers would be more efficient for longer trips and higher average 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 payback for speed limiters and speed reduction strategies is instantaneous because practically no investment is necessary (except the labour time for modifying ECM settings), but it is a worthwhile exercise to determine the savings.

Payback calculation example for speed reduction:

• The annual mileage of the tractor is 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.
• The certified track-test fuel savings is 5% obtained at constant high speed when reducing the speed by 6 km/h, for example 105 km/h to 99 km/h.
• We assume an average pre-implementation fuel consumption of 34 L/100 km.
• The annual fuel savings would be 2 720 L:
   
• At a diesel fuel price of $1.10/L, the annual savings would be:
  2 720 L x $1.10/L = $2 992.

Determining the economic feasibility of transferring loads from containers to van semi-trailers is more complex, because the cost of transloading the freight from the container to a van semi-trailer must be considered.

A tarp system is mandatory to meet safety requirements for gravel or open-topped bulk-hauling vehicles. Therefore, the additional cost of each of these two mechanized tarp systems compared to an ordinary tarp system must be considered, for both the truck and semi-trailer tarp systems.

Payback calculation example for tarp systems:

• The certified track-test fuel savings are 8.8% for a dump truck and 14.9% for a 53-ft. dump semi-trailer, obtained at constant high speed.
• We consider an annual mileage of 100 000 km for both the dump truck and the tractor-dump semi-trailer, and that one tractor uses one semi-trailer.
• We consider only the empty trips on roads with a posted maximum speed of 80 km/h or more: half of 80% of the annual mileage, which gives 40 000 km.
• We assume an empty average fuel consumption of 42 L/100 km for the dump truck and 41 L/100 km for the tractor-dump semi-trailer with regular tarp systems (according to the track test measurements).
• The annual fuel savings would therefore be: 
   

Where      S% = the certified track-test fuel savings
L = the fuel consumption rate

  At a diesel fuel price of $1.10/L, the payback period would be:

Tarp system	Baseline fuel consumption (L/100 km)	Additional cost ($)	Annual savings			Payback period
%			L	$	Years	Months
For dump truck	42	2800	8.8	1478	1626	1.72	21
For 53-ft. dump semi-trailer	41	3500	14.9	2444	2688	1.30	16

Specification Considerations

They are no special issues regarding the specification of electronic speed limiters. The impact on cycle times is generally minimal. As we mentioned, their activation through the ECM is relatively simple: the maximum speed of a vehicle can be pre-set by the manufacturer, dealer, mechanic, or owner using a portable electronic device that is connected to a dedicated port. This operation only takes a few minutes to complete.

Tarp system on a dump truck during track testing
 

The tarp systems should be mechanized to quickly and effectively seal the top of the box. An aerodynamically tight and smooth surface is difficult, if not impossible, to obtain with manual tarp securement. The systems we tested were actuated by electric motors, and the above return on investment calculations are based on the actual costs of these systems. 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.

Regulatory Issues

Starting January 1, 2009, commercial vehicles assembled in or after 1995 with a gross vehicle weight rating of 11 794 kg (26 000 lb.) and more, driven in Ontario and Québec—regardless of where they are licensed—are required to use electronic speed limiters that cap their speed at 105 km/h. Regardless, there are still significant savings to be achieved by operating at lower than mandated speeds.

Tarp system on a 53-ft. dump semi-trailer track-testing
 

A tarp system is mandatory to meet safety requirements when transporting loads with dump trailers and trucks.

Maintenance Considerations

You should activate the speed limiter when you order the new vehicle from the manufacturer.

For the tarp systems, 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

Browand, F. 2005. Reducing aerodynamic drag and fuel consumption. Advanced Transportation Workshop, October 10-11, 2005, Stanford University, CA.

Kenworth Truck Company. 2008. White paper on fuel economy.

Surcel, M.-D. 2009. Fuel consumption track tests of fuel-saving technologies for tractor-trailers: Energotest 2009, Fall Edition. FPInnovations, Pointe-Claire, QC. Internal Report IR-2009-11-24. 77 p.

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

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.

Transport Canada. 2008. Assessment of a heavy truck speed limiter requirement in Canada: Summary Report. Ottawa, ON. TP 14808.

Date modified:

2012-03-09

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