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. These experiments have highlighted that the wind tunnel results were optimistic and suggest a need for on-road testing to more accurately evaluate the benefit of aerodynamic devices for trucks. The on-road results finally are used to predict the resultant fuel economies for various loads and speed conditions.