Flaps Out, Costs Down

Adaptive flaps at the truck rear end reduce fuel consumption by 2.4 percent under real-world conditions

Viewed in purely aerodynamic terms, trucks are a complete and utter disaster: a great big block that air has to flow around and with a wake at the rear end that pulls it in the opposite direction to which it is traveling. This results in noticeably higher fuel consumption and unnecessarily high diesel costs. Flaps at the tail end improve aerodynamics – especially if they adapt to the particular driving situation. IAV has developed adaptive flaps with a payback period of twelve months.

Amending its directive 96/53/EC, the EU has cleared the way: since the beginning of 2016, flaps in a maximum length of 0.5 meters have been permissible on trucks to reduce the impact of the wake at the rear end. They slightly shift the recirculation zone towards the back, increasing pressure at the rear of the trailer which ultimately reduces driving resistance. ”The longer the flaps are, the greater the effect is – until a point of saturation is reached”, explains Dr. Lars Henning, head of the Commercial Vehicle Powertrain Electronics – Algorithm Development team at IAV. “Flaps of a meter in length would doubtlessly be better, but even with today’s permissible 50 centimeters, we can make a significant fuel saving.” The effect is noticeable from approx. 50 kilometers an hour which means that the flaps should only be deployed from that speed and over. It then increases more or less in proportion to the square of vehicle speed.

Truck model in the university wind tunnel

To examine the potential saving in detail, a 1:10 scale model of a truck was built as part of an in-house development project, fitted with adaptive rear flaps and tested in the wind tunnel at Berlin University of Technology. In the wind tunnel tests, 16 pressure sensors at the rear of the model measured the extent to which pressure changes in relation to flap angle. Aerodynamic drag was determined on the basis of measurements with a 3-D wind tunnel balance. The result: the flaps are particularly effective if they adapt to the actual driving situation. In a head-on wind, they should be pitched at an angle of 14 degrees to the direction of wind. This increases pressure at the rear end by about 31 percent and aerodynamic drag can be reduced by 8 percent.

Side wind, however, changes the situation. If wind meets the truck at an angle of 8 degrees, for example, the flap exposed to the greatest force of wind should adjust to 20 degrees. This adaptation reduces aerodynamic drag further by one percent. The optimum angle can be calculated from a formula and included in the control concept. “This adaptation of the flap to airstream gives IAV a genuine USP”, Dr. Henning reports. “Solutions with static flaps already exist on the market, but in terms of aerodynamics, fuel savings and, above, all operating convenience, our system is superior to them.”

Applying the widely used Lastauto Omnibus test cycle, the team has calculated the savings that can be expected on the road – it involves phases at high and low vehicle speed, various hill climbs as well as wind from different directions. Without flaps a 40-tonne truck with
a 380-kilowatt engine and a twelve-gear transmission would use 34.8 liters over 100 kilometers. With static flaps, this level falls to 34 liters and adaptive flaps reduce fuel consumption by a further 0.3 percent. Converted into the costs for diesel at a price of € 1.20 per liter and annual mileage of 150,000 kilometers, static flaps save € 1,296 whereas their adaptive counterparts even reduce fuel expenses by € 1,494.

Payback possible after twelve months

The interdisciplinary project also involved IAV’s cost optimization experts. According to their calculations, the adaptive solution pays off after 14 months based on an annual volume of 30,000 units – after then, the truck owner saves money from every kilometer driven. “But there is even further optimization potential”, Dr. Henning says. “This means we can achieve a payback time of twelve months, as demanded by the market from our talks with potential customers.” The solution comes with no drawbacks. Being designed with spindle actuators, the flaps take up no valuable cargo space. The entire system is fitted on the outside of the truck’s tail end, also making it suitable as a retrofit solution.

Besides flaps, an optimized driver’s cab can also significantly reduce a truck’s aerodynamic drag. IAV is currently developing its own design which is also being tested in the wind tunnel and takes account of all statutory requirements on trucks, such as keeping to the defined turning circle or “circular area” as stipulated by German Road Traffic Regulations. On maneuvering through a roundabout, the semi-trailer truck must not be allowed to move in too closely on the inside nor must it project too far on the outside. Combining adaptive flaps with an aerodynamically optimized truck front design can reduce a truck’s fuel consumption even further.