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Wind Tunnel and Track Tests of Class 8 Tractors Pulling Single and Tandem Trailers Fitted with Side Skirts and Boat-tails
ISSN: 1946-391X, e-ISSN: 1946-3928
Published April 16, 2012 by SAE International in United States
Citation: Cooper, K., "Wind Tunnel and Track Tests of Class 8 Tractors Pulling Single and Tandem Trailers Fitted with Side Skirts and Boat-tails," SAE Int. J. Commer. Veh. 5(1):1-17, 2012, https://doi.org/10.4271/2012-01-0104.
A 1:10-scale wind tunnel development program was undertaken by the National Research Council of Canada and Airshield Inc. in 1994 to develop trailer side skirts that would reduce the aerodynamic drag of single and tandem trailers. Additionally, a second wind tunnel program was performed by the NRC to evaluate the fuel-saving performance of boat-tail panels when used in conjunction with the skirt-equipped single and tandem trailers.
Side skirts on tandem, 8.2-m-long trailers (all model dimensions converted to full scale) were found to reduce the wind-averaged drag coefficient at 105 km/h (65 mi/h) by 0.0758. The front pair of skirts alone produced 75% of the total drag reduction from both sets of skirts and the rear pair alone produced 40% of that from both pairs. The sum of the drag reductions from front and rear skirts separately was 115% of that when both sets were fitted, suggesting an interaction between both. The reductions in wind-averaged drag coefficient for single trailers with skirts only were 0.0524 for the 14.6-m trailer and 0.0406 for the 16.2-m single trailer. The data are quoted for skirt ground clearances of 305 mm (12 in.).
The 0.50-m-long boat-tail panels combined with the tandem-trailer skirts to increase the wind-averaged drag coefficient reduction to a total of 0.1177. Boat-tails combined with skirts on the single trailers also increased the wind-averaged drag coefficient reductions to totals of 0.1051 and 0.0979 for the 14.6 m and 16.2 m trailers, respectively. Each boat-tail assembly consisted of angled panels attached to the top and the side rear edges of a trailer. The boat-tail panels on the single trailers and the rear trailer of the tandem pair were set at the optimum angle of 15°. The boat-tail panels on the front trailer of the tandem pair were set at 0°.
Skirts were predicted to provide fuel savings for the tandem trailers at 105 km/h of 3.79 liters per hundred kilometers (L/(100 km)) (1.61 US gallons per 100 miles - US gal/(100 mi)) and 2.18 L/(100 km) (0.93 US gal/(100 mi)) and of 1.95 L/(100 km) (0.83 US gal/(100 mi)) for the 14.6-m and 16.2-m single trailers, respectively. It is not known why the longer trailer had the smaller fuel saving.
The addition of boat-tail panels increased the predicted fuel savings for the skirt-equipped tandem trailers to 5.88 L/(100 km) (2.50 US gal/(100 mi)) and increased the fuel savings of the 14.6 m and 16.2 m single, skirt-equipped trailers to 5.25 L/(100 km) (2.23 US gal/(100 mi)) and to 4.89 L/(100 km) (2.08 US gal/(100 mi)), respectively.
Track tests were undertaken using 8.2-m-long, skirt-equipped tandem trailers to verify the fuel-saving predictions based on the wind tunnel drag data. The boat-tails were not track tested. The track tests were performed by the staff at the Goodyear Proving Grounds in San Angelo, Texas. They demonstrated that skirts provided fuel savings of from 0.86 L/(100 km) to 3.54 L/(100 km), with the largest savings in the strongest side winds. The average track-test fuel saving measured over all runs was 2.29 L/(100 km) at an average road speed of 88 km/h. The average wind-tunnel-based prediction for these track runs was 2.25 L/(100 km), using a wind-averaged drag coefficient calculated for the wind conditions of each run. This was within one percent of the average of the track measurements. The wind speeds during the track tests were generally lower than the national average 11.3 km/h wind at truck mid height, averaging only 8 km/h. This indicated that the average fuel saving on the track was less than would be expected annually in North America.
The track-test results confirmed the fuel-saving potential of skirts and demonstrated the reliability of fuel-saving predictions made from high-quality wind tunnel data. This correlation was only possible because the track-side winds were measured during each run.