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.

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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 today‘s 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 [1]. 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 ➊: 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

Munich (Germany).

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

Munich (Germany).

AUTHORS

➊ Prioritisation of consumption-reducing measures on the all-wheel drive in stages 1 to 4

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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 [2] 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, ➋.

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 ➌. 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

➋ Construction kit with the ATCx00 transfer box (left) as predecessor and the new ATCx50 (right)

➍ Lubrication and cooling concept in the ATC450 transfer box

➌ Efficiency measures for consumption optimi-sation of the all-wheel drive in the vehicle

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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.

➍ 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), ➎.

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 for example dry-sump lubrication or cascading lubrication of the “consumer units”.

The expected limit potential of one of today‘s mechanical all-wheel drives lies at 4 to 5 % increased consumption com-pared to the standard drive, ➏. Any con-sumption optimisation that goes beyond this requires active intervention in the drive architecture.

A prospect for the future involves tech-nologies in the all-wheel drive train that enable the temporary immobilisation of the hang-on drive train as a function and, depending on an efficient control strategy, offer more potentials for consumption reduction in the conflict of objectives involving all-wheel functionality against CO

2 savings [4]. Decisive for the assertion

of these technologies in the market will be the ratio of CO2 reduction potential to additional costs.

FUNCTIONAL ASPECTS OF ALL-WHEEL DRIVES

In accordance with the BMW Efficient-Dynamics strategy, improvements in effi-ciency are to go hand in hand with retain-ing the function performance of the xDrive the customer values in the fields of “traction” and ‚“lateral dynamics/driving safety”. In order to be able to maintain the top positioning of dynamic handling char-acteristics in BMW vehicles, even as the demands for efficiency are rising, intelli-gent functional extensions that do not lead to additional increases in consump-tion are the focus of attention.

An example of this is the extension of xDrive to include the “Performance Con-trol” lateral torque distribution function,

which is based on the intelligent network-ing with the series standard brake control system DSC in the „Integrated Chassis Management“ and which gets by without additional hardware components (intro-duction since first deployment in March 2007 in the BMW 5 Series, successively in every all-wheel drive vehicle).

During dynamic cornering, the lateral torque distribution function intervenes on the rear axle with a one-sided build-up of brake pressure to further enhance driving agility or to stabilise the vehicle – the simultaneous automatic driving torque increase means that the driver does not notice the brake interventions. The basis of this is the operating principle of the torque vectoring system “Dynamic Per-formance Control” from the BMW X6 [5].

The integral operating principle of xDrive and Performance Control with understeering as an example is shown in ➐ in five phases:

➎ Consumption advantage of the new ATCx50 all-wheel drive trans-fer box compared to the predecessor ATCx00 in the NEDC

➏ Next optimi-sation steps and limit potential of mechanical all-wheel drive

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: Phase 1: on entering a curve, the xDrive system distributes the driving power at a ratio of approximately 40:60 between the front axle and rear axle, and the Performance Control at a ratio of 50:50 between the left-hand and right-hand rear wheel (no lateral torque distribu-tion is active).

: Phases 2 and 3: long before the limit range is reached, xDrive begins to with-draw longitudinal forces from the front axle and Performance Control shifts up to 1000 Nm of torque to the outer rear wheel. The yaw moment created that turns in towards the curve counteracts the tendency to understeer.

: Phases 4 and 5: as soon as neutral driv-ing characteristics have returned, the torque distribution of the two systems can be returned to the basic setting as the curve progresses.

The electronic torque distribution func-tions of xDrive described above provide the possibility to simulate different driv-ing characteristics flexibly by means of software application alone with standard hardware that is used across the board. This enables different torque distribution strategies without component modifica-tions – adapted to each vehicle segment (saloon, SUV/SAV (X model), sport ver-sion (M model)).

The integrated technology approach is also used over and above automatic adap-tations to define a torque distribution strategy that can be enabled by the driver depending on needs in order for the driver to be able, for example, to choose between function attributes that are oriented to consumption, traction or driving dynam-ics. This means that the maximisation of dynamic handling characteristics and effi-ciency-improving functional advantages

goes hand in hand with the consistent use of economies of scale while simul-taneously minimising the overhead for verification.

SUMMARY

Building on a simulation-based com-plete analysis on vehicle level, control levers to optimise energy flows were identified and prioritised by BMW. On the all-wheel drive system level, this acquired knowledge was consistently implemented jointly with the develop-ment partner Magna Powertrain in the development of the new ATCx50 all-wheel drive construction kit. The result is a significant increase and improve-ment in efficiency.

The new ATCx50 all-wheel drive con-struction kit in the BMW xDrive concept represents a common all-wheel drive plat-form across all product lines with signifi-cant commercial scale effect. At the same time, the construction kit features the greatest possible flexibility in the applica-tion of individual all-wheel drive charac-teristics for a vehicle model. This flexibil-ity is based on the electronic adaptation via software and networking of the all-wheel drive system within the driving dynamics framework.

The responsibility of the BMW com-pany towards society is anchored in its EfficientDynamics strategy for sustainabil-ity. The customer wish to have emotional driving dynamics with simultaneous CO

2 reduction is not to be viewed as a contra-diction here. The new generation of BMW xDrive models makes a contribution by linking the highest efficiency with the maximum driving dynamics the customer can experience.

REFERENCES[1] Dirndorfer, A.; Ayoubi, M.; Billig, C.; Heitzer, M.; Klaiss, T.; Liebl, J.; Mayer, H. F.: BMW All-wheel Drives. Full Performance and Less CO2 Emission. Congress paper, 9. European All-wheel Drive Congress, Graz, Austria, 2009[2] Billig, C.; Nistler, G.; Pfau, W.; Rastel, H.; Straub, M.: BMW xDrive in der 3er- und 5er Reihe. Der BMW-Allradantrieb für Limousine und Touring. In: ATZ 107 (2005), No. 10, pages 862 – 871[3] Gratzer, F.; Lippitsch, K.: CO2-optimiertes Allradfahrzeug. In: ATZ 110 (2008), No. 7 – 8, pages 654 – 661[4] Gassmann, T.; Schwekutsch, M.: Verringerung des allradbedingten Mehrverbrauchs durch dynamische Allradabschaltung. In: ATZ 111 (2009), No. 9, pages 672 – 679[5] Billig, C.; Boedrich, H.; Brack, J.; Holle, M.; Höll, B.; Kimmich, F.: Die Dynamic Performance Control von BMW. In: ATZ 110 (2008), No. 11, pages 984 – 994

➐ Combined longitudinal and lateral torque distribution in five phases enables higher cornering speeds and enhances agility

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