The drag coefficient of an articulated vehicle is about 0.6-0.65. This is unlikely to go down much more in future, especially if there is no change in the European regulations. Assuming a well-designed tractor in the first place, a roof fairing/deflector, tractor side panels, side panels and an air dam can be fitted to reduce drag. This is as far we can go with simple modifications; to achieve a further drag coefficient reduction the semi-trailer needs to be included in the aerodynamic analysis.

Volvo drag coefficient development over time

Volvo drag coefficient development over time

Under the European legislation the length of a truck is defined as the distance between the foremost and rearmost points of the vehicle combination, whereas in the United States the maximum length relates to the length of the semi-trailer. As a result, tractor cabs in Europe have become much more compact and the freedom of design of tractor fronts is much more restricted than in the US, where conventional tractors have better aerodynamics owing to the engine hood.

A proposal to introduce a 300mm crumple zone in front of the windscreen for safety reasons is currently under discussion in Europe. A study has shown that this zone can also be used to improve the airflow around the cab. An example of a tractor of this kind.

Exceptions in the legislation could also provide opportunities for reducing the fuel consumption of trucks. A recent amendment in the United States, for instance, allows the rear of a semi-trailer to be extended by 1.5m to fit fuel-saving aids. These must of course meet certain safety requirements. The additional space must not be used to carry more cargo. This has legalised the use of boat tails, enabling a fuel saving of several percent to be achieved.

There is no limit on the height of semi-trailers in Britain, so semi-trailers with teardrop roofs can be used. The curve on the top of the semi-trailer reduces drag, resulting in 16% less fuel consumption. As most bridges on the continent of Europe have a headroom of just over 4.20m, the law limits the maximum height of semi-trailers to 4m which makes a semi-trailer with a teardrop roof unattractive because of the loss of load space.

The aerodynamics could be enhanced still further if the law permitted the use of cameras instead of mirrors: cameras are aerodynamic and much more attractive from the safety point of view.

A major gain can also be achieved by reducing the gap between the tractor and semi-trailer or the truck and trailer at higher speeds. The gap is needed to keep the turning circle of the vehicle combination small at low speeds, but at high speeds it has an adverse effect on fuel consumption. A system that pulls the trailer closer to the truck at higher speeds, thus reducing drag, could therefore be used, but it is unclear whether the Dutch legislation permits this.

Active aerodynamics offers prospects of reducing the drag of trucks still further. These active aerodynamic aids employ moving parts that anticipate such things as wind speed, travelling speed and wind direction, enabling the aids to be adjusted to the best setting in every situation, thus maximising the fuel savings.