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Samuel Kiefer :
Flap closed, CO2 down


Samuel Kiefer :
BMW introduces also at the new X6 actively controllable air flaps. Partner for development and production at the 1 and 3 series as well as the X5 and X6 is Röchling Automotive. The adjustable jalousies behind the kidneys and the air inlet below the buffer bar reduce the drag coefficient by 10 percent and thus fuel consumption by 1 percent on the European driving cycle.

Astonishingly enough the flaps stay closed on the whole European driving cycle. This is also true for 70 percent of the practically relevant driving operation. They have to be opened only in the remaining driving situations. For example in a traffic jam, at maximum speed, on an uphill drive or when the air conditioning is blowing at full steam. Under these conditions it is important that the inlet area is covered only at a minimum share by the open flaps.
Energy consumption of the flaps actuation is a minor factor at 4.2 W standby power. The system weight of for example 1.1 kg for the 1 and 3 series is overcompensated by the benchmark numbers of added customer benefit. “At a constant speed of 120 km/h the new 330i with direct injection and lean operation consumes 0.1 litres less fuel, at 180 km/h it is 0.3 litres less,” says Dr. Vanja Zivkov, project manager for aerodynamics at BMW.
Air flow through the engine compartment accounts for about 15 percent of the total vehicle drag. These air flow losses can be reduced 70 percent by closed air flaps. “Active aerodynamics,” is the slogan currently promoted by BMW anchorman Johannes Liebl at several events on CO2 reduction.
Locking pressure required for high speeds is no challenge for the electromechanic flap actuation. Contrary to a magnetic clamp such an actuator allows for an active adjustment, especially so for the closing against strong airflow. Energy consumption of electric items accounts for a growing share of fuel consumption. Comprehensive aerodynamic optimizations with testing sequences on the complete car in climatic wind tunnels resulted in a 30 percent reduction of the operation current for the closing of the flap. Thus, there is another correcting element for the thermal management at the disposal of the engine control unit. With the mechatronic actuation the full potential may be leveraged by the regarding algorithms.
Because of future exhaust emission standards focussing much more on the cold start phase, another advantage of the flaps will gain in importance: Thanks to the shielding the engine heats up much quicker and stays warm longer. The problematic cold start exhaust emissions are decreasing.
Against the backdrop of all these impacts the BMW CO2 committee ranks the active air flaps among the most efficient single measurements. “We currently receive requests from all parties. Obviously we owe this also to the PR of BMW. There, the active air flap is mentioned in the same breath together with the start-stop-system or the brake energy recovery, when the benchmark numbers for fuel consumption are explained,” says Ludwig Huber, R&D chief of Röchling Automotive.
Therefore fuel economy is actually in line with sheer driving pleasure. Part of this emotional experience is also a refined engine sound. The accumulated level of exterior noise at a diesel cold start is reduced by 1 dB with the flaps shut. Thus, a BMW pleases also its surrounding. “Regarding acoustics as well as aerodynamics the active air flap is part of an integrated system together with the full sized underbody we also provide for the 1 and 3 series,” explains Dr. Klaus Pfaffelhuber, head of pre-development at Röchling Automotive.
For the bold concept of the air shutter to come true, first of all cost and weight had to be cut. Thus, the horizontal jalousies of the 1 and 3 series are only 2 mm thick – at a spread of over 22 cm. At 200 km/h this subtle structure must withstand a wind load of 235 N. Stone chipping impact is tested with a shooting machine. 50 kg of hard cast pellets crash into the surface at a rate of 500g per 10 seconds, propelled by 2 bar acceleration pressure.
A similar shielding function is provided by the closed shutter against splattering snow and splashing water. The corrosion damage of the cooler is contained, “come snow and high water”. Though a passenger car is no amphibious vehicle, slow and fast water crossings are part of the specification. Head water pressure at 30 km/h builds up to 4,290 N - a crash test, also for the joints on each side.
Here the shutters have a pivot mount. They consist of PA GF 30 and the friction with the PP GF 30 of the outer frame produces absolutely no noise. Both components are equipped with elastic seal lips from Styrene-Ethylene-Butylene-Styrene (SEBS) along their longitudinal edge. The 2K-process applied reduces cycle time and material handling. The additional cavity is opened in the same tool by sliders being pulled back after the injection moulding of the stiff part. This robust process helped that failures could be avoided and cost targets met.
Endurance may be regarded as proven due to the experience gained with the air flap launched in 2003 at the 5 series. Contrary to the current generation those were passive, i. e. without electric actuation. The mechanical design principle has been maintained yet widely. Thus the physical strain is comparable. Already at the 5 series Röchling Automotive was development partner and series supplier.
There are even some advantages for the looks of the car. “Behind the grill there is now only a mat black background recognizable,” explains Christoph Kemper, product manager for air flaps at Röchling Automotive, “Thus chrome plated grill structures are highlighted better. And the cooler core behind the shutter stays out of sight. Those would otherwise shimmer through in a unfavorable way. This loss of contrast is obviously disturbing designers and drivers in such a way, that cooler cores have to be painted black normally, what in turn compromises heat exchange. This additional cost and weight of the painting can be spared by the air flap.”

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