THE NEW ALL-WHEEL DRIVE GENERATION FROM BMW
With the EfficientDynamics strategy, the BMW Group aims to achieve
a sustained reduction in fuel consumption and exhaust emissions while
simultaneously improving driving functions. With the BMW 7 Series,
a new generation of all-wheel drives will be produced in series and also
deployed in future as construction kits for other model series. With this
new generation, the fuel consumption difference between all-wheel
drive and standard drive vehicles can be minimised while keeping the
performance the customer values at the same level.
INDUSTRY ALL-WHEEL DRIVE
FOCUS ON FUEL CONSUMPTION
The objectives of the BMW Group also include sustainability. Particularly in the premium segment, it is likely that mobility will be defined to a much more significant degree than before with sustainability as a factor, starting with the development of low-consumption and environmentally friendly drives, going all the way to resource conser-vation in production, distribution and main-tenance along the entire product life cycle.
However, for the most part, vehicle con-cepts that focus exclusively on the reduc-tion of fuel consumption have not yet been crowned with success. Particularly in the premium segment, emotional aspects and driving dynamics that can be experienced play an extensive role in the purchasing decision. Here, the customer expects the best possible resolution of the conflict of objectives between consump-tion and vehicle characteristics. For all-wheel-drive vehicles, this means that the customer will consider the additional con-sumption and additional functions of the all-wheel drive to an increasing extent.
In what follows, BMW will present measures on the all-wheel drive system and vehicle concept that reduce as far possible with todays technology addi-tional consumption due to factors inher-ent in the system. Furthermore, details of other future measures and the limit poten-tials of mechanical all-wheel drives will be provided.
SIMULATION-BASED CONSUMPTION OPTIMISATION ON THE LEVEL OF THE COMPLETE VEHICLE
It also applies to an all-wheel drive that the singular optimisation of an individual system only leads to a limited utilisation
of potential . A global rather than local optimisation can only be achieved by examination of the complete system. The basic requirement for this is a detailed understanding of the active chains on the level of the vehicle.
Control levers are identified and their efficiency is assessed on the basis of phys-ical models embedded in a complete vehi-cle simulation. This enables assessment of new vehicle concepts in various relevant load cases with regard to fuel consump-tion even in the early development phase.
Subsequently, the identified starting points are to be prioritised in accordance with the logic in : the most important and first starting point is the reduction of the energy requirement for functions. What is not required does not have to be converted and provided. The consump-tion optimisation of the all-wheel drive system is in this first and most important stage. On the second stage is the avoid-ance of losses during energy conversion. The third stage is the supply of functions in line with requirements. An efficient supply should only be used when it is actually required to perform the function. Under certain circumstances, supply in line with requirements also compensates for poorer rates of efficiency, depending on the frequency of use or operating time of a function. Moreover, energy that dissi-pates unused by the customer despite all previous efforts should be recuperated (fourth stage).
On examination of the results of the complete vehicle analysis of an all-wheel drive vehicle, it becomes apparent that the additional consumption of the all-wheel drive can be broken down into a direct and an indirect proportion. The direct proportion is approximately 4 to 5 %. The indirect proportion of approxi-
DR.-ING. MIHIAR AYOUBIwas Head of Development for
Manual Transmissions, All-wheel Drive Vehicle and Drive Train until 1 July 2010 at the BMW Group in
DIPL.-ING. MICHAEL HEITZERwas Head of Vehicle Energy System, Driving Dynamics until 1 May 2010
at the BMW Group in Munich (Germany).
DIPL.-ING. HOLGER FELIX MAYERis Head of System Development for
All-wheel Drive at the BMW Group in Munich (Germany).
DIPL.-ING. GERD RIEDMILLERis Head of Brake Control System Integration at the BMW Group in
Prioritisation of consumption-reducing measures on the all-wheel drive in stages 1 to 4
11I2010 Volume 112 45
mately 5 to 6 % includes the influences of the vehicle concept, for example shorter rear-axle ratios to compensate for the additional weight or the increased aerody-namic drag due the increased vehicle ride height. The direct proportion includes the increases caused directly by the all-wheel-drive transmission such as the additional friction in the drive train and the addi-tional weight.
PROPERTIES OF THE NEW ALL-WHEEL DRIVE CONSTRUCTION KIT BMW XDRIVE
The new BMW xDrive all-wheel drive construction kit with the name ATCx50 replaces the ATCx00 transfer box family  used to date. Starting with deployment in the BMW 7 Series (model year 2010), the new construction kit will be applied to all vehicle models (5 Series, 5 Series GT, X1, X3, X5/X6 LCI).
The all-wheel drive construction kit is a mechatronic all-wheel drive system with
electronic control. Its major feature is the possibility for electronic adaptation to dif-ferent vehicle types without hardware changes. This means each vehicle can be given its own individual all-wheel drive character by means of software applica-tion. The construction kit also enables networking in the chassis and suspension control system framework. This integra-tion in the framework of the driving dynamics functions permits further differ-entiation of the segment-specific attributes.
The second property lies in the com-mon hardware platform as a consistent construction kit across all vehicle models. Alongside the common mechatronics, common function and diagnosis software is used. This consistent path led to signifi-cant cost advantages with the greatest possible functional flexibility.
Even in the early phase of development of the new ATCx50 construction kit, a great deal of overhead was invested by both BMW and the development partner
Magna Powertrain in an optimised pack-age that conforms across all product lines. This consistent approach made it possible to reduce the number of variants result-ing from the base transmission, actuator system and control unit components within the construction kit by 70 %. As a result, a uniform ATC350 transfer box with spur gear power transmission was created for all saloons and an ATC450 transfer box with chain power transmis-sion was created for the X models, .
The components within the ATCx50 construction kit include very many identi-cal components such as the input shaft, clutch, actuator system and add-on con-trol unit with integrated actuator motor. The add-on control unit with highly inte-grated electric-motorised actuator, a brushless synchronous motor, is deployed with a fitting variant for both the CAN and FlexRay onboard networks. In com-parison with the preceding generation, a simplified actuator system with a rolling-element ramp mechanism has been implemented. The add-on control unit also enables simplification of the wiring harness, as plug connections can be eliminated.
MEASURES AND RESULTS OF CONSUMPTION OPTIMISATION
The summary of the efficiency measures for consumption optimisation of the all-wheel drive in the vehicle is shown in . In the ATCx50 all-wheel drive construc-tion kit, optimisations have achieved a weight saving of 2 kg. The implementa-tion of a new efficient lubrication and
Construction kit with the ATCx00 transfer box (left) as predecessor and the new ATCx50 (right)
Lubrication and cooling concept in the ATC450 transfer box
Efficiency measures for consumption optimi-sation of the all-wheel drive in the vehicle
INDUSTRY ALL-WHEEL DRIVE
cooling concept by means of power trans-mission and oil deflectors enables discon-tinuation of the oil pump that used to be separate.
The oil pump used to date was designed as a generated rotor pump, a special form of the gear pump. The gener-ated rotor pump was connected to the main shaft and rotated at propeller shaft speed over the entire load range. On the ATC350 transfer box, the pump is replaced by a gear pump that also makes intelli-gent use of the power-transferring toothed gears (on the ATC450, the chain) as oil-delivering components.
shows the lubrication and cooling concept in the ATC450, in which the chain delivers the oil via a guide rail into the raised reservoir. The oil flows through a line to the stationary actuator ring; from there the lubricating and cooling agent distributes itself to the multidisc clutch as well as to the bearings.
After running through the transmission, the oil is collected in a damping chamber, where the chain can pick it up again. These measures have enabled an improve-ment in efficiency of the components of approximately 30 %, which corresponds to a saving of 0.5 % in the New European Driving Cycle (NEDC), .
Starting from the results achieved, other limit optimisations could be developed in future by reducing mechanical and hydraulic friction losses. These include measures such as the deployment of new bearing concepts, the use of new sealing ring concepts and further lowering of the oil level to reduce churning losses. The above concepts include