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Technical Paper

ABCD - An Improved Coast Down Test and Analysis Method

An improved coast down method that incorporates the input of data measured with an onboard anemometer into the governing equation of motion is used to evaluate models representing the aerodynamic and mechanical drag characteristics of a wide variety of vehicles. Results from the application of the improved technique are presented to demonstrate the repeatability and precision of the analyzed aerodynamic drag coefficients, to address the merits of different wind tunnel blockage correction methods, to illustrate the method's usefulness in the study of tractor-trailer and light truck-travel trailer aerodynamic drag, to evaluate the influence of floor blowing in wind tunnel studies of the effects of various changes to the underbody of an automobile, and to compare track and dynamometer determinations of the variation of mechanical drag with speed on a vehicle equipped with a four-speed automatic transmission.
Technical Paper

An Improved Over-the-Road Test Method for Determining the Fuel Savings Benefit of a Truck Aerodynamic Drag-Reducing Device

The problem of evaluating the fuel savings benefit of a truck aerodynamic drag-reducing device in an over-the-road test is addressed. A demonstration of the limitations of current SAE short-term test methods led to the development of a modified, wind-monitoring-based procedure which facilitates a more fundamental analysis of the test data from which a more meaningful estimate of the long-term fuel savings benefit can be derived. Sample results from the application of the new test and analysis procedure are reported to demonstrate the usefulness of the improved test technique. The procedure also provides findings that are of interest in wind-tunnel-to-road correlation exercises.
Technical Paper

Comparison of On-Road and Wind-Tunnel Tests for Tractor-Trailer Aerodynamic Devices, and Fuel Savings Predictions

Wind tunnels which are large enough for full-scale trucks are rare, and the cost of satisfactorily-detailed models for smaller tunnels is high. The work presented shows the results from the application of a method which provides an over-the-road evaluation of the incremental changes in fuel consumption and drag coefficient produced following the addition of a variety of aerodynamic drag reducing devices to a tractor-trailer truck combination. The devices tested were an aerodynamic sunvisor, a roof-mounted air deflector, cab extenders, cab skirts, a trailer nose fairing, a set of trailer quads (quarter-rounds), and trailer skirts which were mounted on a low-forward-entry tractor and high box-van trailer. The significant differences between the wind tunnel and on-road drag reductions suggest that the effects of on-road wind turbulence can substantially reduce the wind tunnel results even though a 1.5% turbulence intensity level was used in the tunnel experiments.
Technical Paper

The Effect of Tractor-Trailer Flow Interaction on the Drag And Distribution of Drag of Tractor-Trailer Trucks

A wind-tunnel study was carried out to determine the influence of some tractor and trailer geometries on the distribution of aerodynamic drag of tractor-trailer trucks. Also investigated were the effects on the drag distribution of drag-reducing devices and of exhaust stacks. The measurements were made on 1/8th-scale models at a Reynolds number of 106. The shape of the tractor strongly influenced the drag on the trailer. Streamlining the tractor could increase the drag on the trailer to the point that the drag on the tractor-trailer combination increased. A relatively streamlined conventional tractor was responsible for between 45% and 57% of the total aerodynamic drag at 0° yaw, whereas a relatively unstreamlined cab-over-engine tractor was responsible for between 73% and 82% of the zero-yaw drag. A streamlined fairing mounted on the tractor increased the drag on the tractor, but decreased the drag on the trailer substantially more than the drag on the tractor increased.
Technical Paper

An Evaluation of the Aerodynamic Drag Reductions Produced by Various Cab Roof Fairings and a Gap Seal on Tractor-Trailer Trucks

A study has been carried out to determine the reduction in the aerodynamic drag of tractor-trailer trucks brought about by fairings mounted on the cab roof and a vertical seal mounted across the gap between the tractor and the trailer. This study was conducted using one-eighth-scale models in a wind tunnel at a Reynolds number of about 25 percent of that of full scale trucks at 50 mph. Reductions of up to 35 percent of the zero-yaw drag coefficient, and of over 25 percent of the wind-averaged drag coefficient were obtained. The drag reductions obtained with roof fairings, with and without gap seals, significantly exceeded those obtained with a commercially-available roof deflector, with and without a vortex stabilizing device. Comparison with other model tests and a full-scale test indicates that the results obtained in this study should be directly applicable to full-scale trucks.
Technical Paper

Test Procedures for the Evaluation of Aerodynamic Drag on Full-Scale Vehicles in Windy Environments

Procedures for conducting full-scale coast down tests for the purpose of determining the aerodynamic drag forces on tractor-trailer trucks are presented. Tests were conducted on the baseline vehicle and on the baseline vehicle modified with drag reduction devices. Techniques for the evaluation of mechanical drag forces prior to the coast-down tests are given and a description of the instrumentation is included.
Technical Paper

Analysis of Coast-Down Data to Assess Aerodynamic Drag Reduction on Full-Scale Tractor-Trailer Trucks in Windy Environments

A data reduction procedure is presented and sucessfully used to analyze coast-down data obtained in a windy environment to provide a measure of the aerodynamic drag on a full-scale tractor-trailer combination as a function of the yaw angle of the vehicle. Full-scale drag coefficients were evaluated for a vehicle operated in the baseline mode, and following the addition of four different drag reducing combinations, over a yaw angle range of from -10° to 10°. Comparison with wind tunnel measurements suggests that the wind-tunnel provides a reasonable simulation of the effects of winds on vehicle drag. The full-scale drag reductions measured in the presence of winds were generally lower than those found in wind-tunnel tests, except in situations where a vertical gap seal device was present. In this case, there was good agreement between the results of the coast-down tests and the wind-tunnel tests.
Technical Paper

Comparisons of Effectiveness of Commercially Available Devices for the Reduction of Aerodynamic Drag on Tractor-Trailers

Wind tunnel experiments, with emphasis on cross-wind effects, have been used to evaluate the effectiveness of a number of commercially available devices for reducing the aerodynamic drag of a tractor-trailer combination. The evaluations included consideration of the effects of tractor type, trailer height, and the bluffness of the tractors and/or trailers. A wind-averaged drag coefficient was introduced to interpret the basic data for the prediction of average drag in a highway environment. The average drag of the base-line vehicles was found to be a strong function of the bluffness of the tractor and/or trailer, and a weak function of the tractor type. Estimates of the average fuel savings that would result from the application of the various drag reducing devices ranged from -400 to 3300 gallons per 100,000 miles, depending on the combination of tractor, trailer and device that is used.