Experimental Measurement of the Flow Field of Heavy Trucks
Flat flaps that enclose the trailer base on the sides and top are known to reduce truck drag and reduce fuel consumption. Such flapped-truck geometries have been studied in laboratory wind tunnels and in field tests. A recent review of wind tunnel data for a variety of truck geometries and flow Reynolds numbers show roughly similar values of peak drag reduction, but differ in the determination of the optimum flap angle. Optimum angles lie in the range 12 degrees-20 degrees, and may be sensitive to Reynolds number and truck geometry. The present field test is undertaken to provide additional estimates of the magnitude of the savings to be expected on a typical truck for five flap angles 10, 13, 16, 19, and 22 degrees. The flaps are constructed from a fiberglass-epoxy-matrix material and are one-quarter of the base width in length (about 61 cm, or 2 feet). They are attached along the rear door hinge lines on either side of the trailer, so that no gap appears at the joint between the flap and the side of the trailer The flap angle is adjusted by means of two aluminum supports. The present test is performed on the NASA Crows Landing Flight Facility at the northern end of the San Joaquin valley in California. The main runway is approximately 2400 meters in length, and is aligned approximately in a north-south direction The test procedure is to make a series of runs starting at either end of the runway. All runs are initiated under computer control to accelerate the truck to a target speed of 60 mph (96 6 km/hr), to proceed at the target speed for a fixed distance, and to decelerate at the far end of the runway. During a run, the broadcast fuel rate, the engine rpm, forward speed, elapsed time--as well as several other parameters (10 in all)--are digitized at a rate of 100 digitizations per second. Various flapped-conditions are interspersed with the ''no flaps'' control, and are sequenced in a different order on different days. Approximately 310 runs are accumulated over the 5-day test period, May 17-21, 2004. The runway slopes rather uniformly upward from north-to-south. Over the distance of 2424 meters between our two ''start'' markers at either end of the runway, the net change in elevation is a little over ten meters. Test results clearly show the greater fuel consumption required to lift the truck against gravity in the southbound direction For this reason, it is important that the tests be averaged over a round trip circuit--that is, a run in both directions over the identical portion of the roadway. Northbound-southbound averages require an overlap segment of the runway (near the middle of the runway) where the truck--starting from either end--has achieved its target speed. For the target truck speed of 60 mph, this overlap region is approximately 700 meters in length. Typically a run and the return run are accomplished within a time interval of 6 minutes. Analysis of the data show fuel consumption savings at all flap angle settings tested, when compared to the ''no flaps'' condition. The most beneficial flap angle appears to be 13 degrees, for which the fuel consumption is 0.3778 {+-} 0.0025 liters/km compared to the ''no flaps'' control of 0.3941 {+-} 0.0034 liters/km. The error bounds expressed above mark the 99% confidence interval in the mean values given. That is, additional estimates of the mean fuel consumption would be expected to lie within the bounds given, approximately 99% of the time. The fuel consumption saving is--to reasonable accuracy--about 1.63 liters/100 kilometers. These savings represent the increment associated only with the change in drag due to the presence or absence of flaps. The result will hold for any truck of similar size and shape and engine performance regardless of the loading of the truck or the rolling resistance. The economy achieved by use of base flaps can be compared to the economy resulting from driving two trucks in a tandem configuration. In December 2003, such fuel consumption tests were performed at the same Crows Landing testsite. In the tests, two identical trucks are operated at headways in the range 3-10 meters. The trucks are steered by hand, but longitudinal control is provided by a closed-loop control system. Laser ranging measures truck-to-truck distance, and the control system maintains a truck separation to within about {+-} 3 centimeters. From these tests it is concluded that both trucks save fuel by close-following, that the fuel saving increases with decreasing spacing, and that the trail truck saves more fuel. An average value of fuel saving for each of the two trucks at spacings of 6-10 meters can be taken to be 3.0 liters/100 km.
- Research Organization:
- Univ. of Southern California, Los Angeles, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC26-98EE50512
- OSTI ID:
- 912757
- Country of Publication:
- United States
- Language:
- English
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