Aerodynamic Investigation of Truck-Trailer Combinations

Licentiate thesis, 2011

In today’s society, increasing fuel prices and the constantly evolving emission legislation, forces the vehicle industry to develop new techniques for the propul- sion of vehicles, and optimization of existing technology. Trucks constitute a significant proportion of the transportation system, consequently it is of great importance to ensure as low an impact as possible on the environment. By improving the aerodynamic properties of the trucks there is potential for sub- stantial fuel savings. It is a known fact that the aerodynamic drag becomes the dominant factor at speeds above approximately 80km/h, as a consequence of the aerodynamic drag force increasing with the square of the velocity. For long-haul trucks, operating at speeds about 90km/h, the potential for savings is apparent. Replacing tractor-semi-trailer combinations with longer transport concepts would be one way of improving the transport efficiency. By replacing three tractor- semi-trailer combinations, with a total length of 16.5m, with two vehicle com- binations with a total length of 25.25m, the same amount of freight can be transported with less vehicles. Several studies have shown that both fuel con- sumption and emission levels, as well as the logistics system, would benefit from such transport concepts. However, the research on the aerodynamic properties of longer vehicle concepts is somewhat limited. This thesis deals with the aerodynamics of truck-trailer combinations, in order to investigate the possibilities for drag reductions. In total, 7 different vehicle combinations were investigated in terms of aerodynamic properties, by using numerical simulations, in the form of Computational Fluid Dynamics (CFD). The results from the investigations showed that the longer transport concepts had great potential for fuel savings; the power required per loading length was shown to be less compared to conventional vehicle combinations, according to the simplified analysis used in the thesis. However, the aerodynamic properties of the longer vehicle combinations were significantly different from the shorter combinations. It was concluded that the gap size between the cargo-units was an important factor in determining the aerodynamic behaviour. When analyz- ing the efficiency of two common drag-reducing devices, roof deflector and cab side-extenders, it was seen that they were efficient, but the effects of these de- vices were diminishing moving downstream. It was hence concluded that it is important to work with the entire vehicle combination in order to maintain the positive effects created in the cab region. In an attempt to achieve an optimized transport concept, different drag-reducing strategies for the gap between cargo- units and chassis skirts for trailers were also evaluated. The results showed that there is great potential for such drag-reducing strategies.