DEVELOPMENT Exhaust Outlet Sound

2 ATZ worldwide 2/2004 Volume 106

xDrive is the new, intelligent four-wheel drive concept with an active vehicle dynamicscontrol system for the BMW X3 and BMW X5 model series as of model year 2004. Thisnot only offers the best possible traction in the event of difficult road surface conditions,but also simultaneously provides increased agility, vehicle dynamics and driving stabilitythanks to permanently variable torque distribution between the front and rear axle. Theinnovative xDrive concept thus combines the dynamic advantages familiar from BMW'sstandard drive with the traction advantages of a four-wheel drive system.

1 Introduction

The goals of a new four-wheel drive con-cept were defined at the beginning of theconcept phase for the X3 series and the X5facelift model: ■ increased vehicle dynamics, traction,agility, driving stability and driving safety■ improved performance values (accelera-tion, elasticity and top speed) while simul-taneously increasing economy.

In addition to using the available en-gines and the 6-speed manual and auto-

matic transmissions, this also led to the re-quirement of a new 4x4 drive line concept.

Those measures that were important forachieving these goals will be described ingreater detail in this article, which is princi-pally focused on describing the new BMWxDrive four-wheel drive system.

2 The New X3 and X5 Powertrain

The engine and transmission range of theX3 and X5 is shown in Table 1.

By Gerhard Fischer,

Werner Pfau,

Hans-Stefan Braun

and Christian Billig

xDrive – Der neue Allradantrieb

im BMW X3 und BMW X5

You will find the figures mentioned in this article in the German issue of ATZ 2/2004 beginning on page 92.

xDriveThe New Four-WheelDrive Concept in theBMW X3 and BMW X5

3ATZ worldwide 2/2004 Volume 106

2.1 The Engine RangeThe X3 will be fitted with the 2.5-litre and3.0-litre in-line six-cylinder petrol enginesand the re-designed 3.0-litre in-line six-cylinder diesel engine. The X5 will beequipped with the 3.0-litre in-line 6-cylin-der petrol engine, the re-designed 3.0-litrein-line 6-cylinder diesel engine and the 4.4-litre V8 petrol engine known from the new7 Series with a 4.8-litre variant for the newX5is. In some cases, these engines reveal asignificant increase in torque output, witha simultaneous increase in efficiency, com-pared to the previous engines, leading toconsiderable improvements in perfor-mance, Figure 1, and fuel consumption,Figure 2.

2.2 The Transmission RangeDepending on their engines, the X3 and X5will also be fitted with the new 6-speedmanual and automatic transmissions.When equipped with the six-cylinder petroland the 3.0-litre diesel engines, both willalso be fitted with the new 6-speed manualtransmission. In the case of the X5, 6-speedautomatic transmissions will be fitted inthe 3.0-litre diesel and 8-cylinder petrol en-gine variants.

By simplifying the interface between allmanual and automatic transmissions andthe transfer case, success has been achievedin serving all engine and transmission vari-ants with one transfer case for the X3 and aslightly different one for the X5. The en-gine/transmission unit has been shortenedby approx. 70 mm compared to the previ-ous model. In combination with its lowweight, the compact, short transfer casehas resulted in high inherent drive trainbending and torsional frequency, therebycontributing towards improving acousticcomfort.

2.3 The Drive TrainThe new four-wheel drive system for the X3and the X5 is fundamentally based on thearchitecture of the BMW 4x4 drive line,which has proven its worth over a numberof years. In this case, the front axle differen-tial is flanged to the engine oil pan on theleft in the direction of travel, while theright drive shaft is guided in a tube withinthe engine oil pan below the connectingrod sweep and by a supporting bearing out-side the oil pan. This has enabled theachievement of drive shafts of an identicallength without asymmetrical self-steeringbehaviour.

The propeller and drive shafts as well asthe front and rear axle differentials havebeen primarily integrated from the prede-cessor and the existing BMW module.

The transfer case, which is an entirely

new development, is the “core element” ofthe new four-wheel drive concept. Thetransfer case is positioned “in-line” up-stream of the manual and automatic trans-missions in an extremely compact “singleoffset” design.

The drive train architecture of the X5 isbasically identical to that of the X3, Figure3. Thanks to its compact layout, this con-cept guarantees a low centre of gravity po-sition and thus makes an important contri-bution towards improving dynamic han-dling characteristics [1].

2.4 Transfer Case for xDriveThe improvements desired in terms of ve-hicle dynamics, traction and performancenecessitated a new drive concept with ahigh degree of variability in torque distrib-ution between the front and rear axle. Thishas led to a departure from rigid drivetorque distribution (the familiar solutionfor classic, permanent four-wheel drive),with a planetary gear set in the transfercase in a symmetrical (50/50) or asymmet-rical (e.g. 38/62) configuration.

Analysis of the different concepts forlongitudinal torque distribition (transfercase) finally led to this solution, with a rigiddrive-through to the rear axle and variabletorque transfer to the front axle with a con-trollable clutch. This concept, with its func-tional integration into the existing vehicledynamic control systems, is called xDrive atBMW.

In order to meet the dynamic handlingrequirements, particular attention waspaid to a high level of actuator system dy-namism and accuracy for the controllableclutch in the transfer case.

2.4 Transfer CaseA transfer case family (ATC400/ATC500)with a high percentage of common partswas developed for the X3 and X5 vehiclemodel series. The primary difference be-tween the two transfer cases is the geomet-rical design of the front axle output. Thosecomponents that are relevant with regardto function are identical for both transfercases.

3.1 Description of the Transfer CaseIn a conventional transfer case, Figure 4left, the planetary gear differential is locat-ed between the front axle and rear axle out-put, and input is generally carried out viathe planetary gear carrier.

The xDrive concept, Figure 4 right, en-compasses a rigid main shaft to the rearaxle, on which the controllable, wet multi-plate clutch is seated. There, the torque forthe front axle is conducted, with infinite

variability, to the front output. A G-rotor oilpump, which ensures that the multi-plateclutch and the remaining components aresupplied with cooling and lubricating oil, islocated on the main transfer case shaft.

In both transfer case concepts, torque istransferred to the front axle via a rockerlink chain.

For the xDrive, great emphasis wasplaced on a compact transfer case design,with low weight and short length, Figure 5.As in the case of all BMW applications, thefront axle output is located on the left inthe direction of travel. The actuator module(electric motor with worm drive) is locatedto the rear of the chain case.

The function of the xDrive transfer casewill be described in the following by ex-plaining the flow of torque, Figure 6. In thetransfer case, the torque is guided from thetransmission input shaft (1) in the rigid dri-ve-through to the rear axle (2) and to thefront axle (3) via the controllable clutch inthe ramification path. The actuator moduleoperates the spreader mechanism with anactuator lever and ball ramp system via thecontrol plate. The axial force generated onthe multi-plate clutch supplies the desiredfront axle torque.

3.2 Description of the Clutch Actuator SystemThe multi-plate clutch actuator system iscomprised of electromechanical actuation.The rotary movement of the direct currentmotor is transferred, by a worm gear bymeans of a control plate, via the actuatorlevers onto a ball ramp system and fromthere onto the multi-plate clutch via an ax-ial pressing motion. The necessary clutchtorque precision is guaranteed by charac-teristic curve-controlled direct current mo-tor position regulation.

Thanks to the specific design of theworm gear, success was achieved in imple-menting highly dynamic actuation withmoderate electric motor current loads.Whilst higher current peaks may temporar-ily occur in the event of highly dynamic ac-tuation movements, the current required tomaintain stationary torques is very low.Subject to dynamic handling requirements,any desired state, from rear-wheel drive tofully-locked four-wheel drive, can beachieved rapidly and precisely.

3.3 Transfer Case Control ModuleThe transfer case control module (VGSG) isresponsible for delivering the torque re-quested by the DSC to the front axle pre-cisely and in a highly dynamic manner.

The characteristic clutch curves, whichassign the clutch torque to the actuator's

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actuation movement and enable precisetorque adjustment, are stored in the trans-fer case control module software. Precisionof the xDrive clutch actuation is ensured bymeans of actuator system position control. A calibration routine is started at regularintervals to compensate for effects such asclutch plate wear and settling.

3.4 Dynamics of the Clutch Actuator System The characteristic of the controllable clutchwithin the xDrive system is described bythe actuator accuracy and dynamics. In or-der to achieve the requirements of tractionand dynamic stability, a very fast and high-ly precise adjustment of torque is neces-sary. The response time by increasing thetarget torque (Mk_soll) from 200 Nm to 600Nm is shown in Figure 7. The new targettorque is achieved within 90 ms.

4 Active Vehicle Dynamics andTraction Control with the NewBMW xDrive Four-Wheel DriveConcept

4.1 xDrive Control ConceptIn the xDrive concept, the Dynamic Stabili-ty Control (DSC) system, with its two brakemanagement (BM) and engine manage-ment (MM) control loops, has been extend-ed by an additional control loop – longitu-dinal torque management (LM).

Longitudinal torque management hasthe task of optimally adjusting the drivetorques between the front and rear axlesaccording to the driving situation via thecontrollable clutch in the transfer case.Thanks to the xDrive concept, both dynam-ic stability and traction characteristics cannow be positively influenced via a total ofthree control loops.

The control algorithms for longitudinaltorque management are implemented in amodular form in the DSC system within thexDrive system network, Figure 8. The nomi-nal clutch torque for the front axle is contin-uously calculated and transmitted to thetransfer case control module via CAN. Thetransfer case control module adjusts the re-quired nominal clutch torque in the transfercase by means of the clutch actuator system.

4.2 xDrive Strategy for ActiveVehicle Dynamic ControlKamm's theory describes the relationshipbetween the maximum forces that can betransmitted in a longitudinal and trans-verse direction at the wheel.

If longitudinal and lateral forces simul-taneously occur at the wheel, the results ofthis cannot exceed a certain limit value.Thanks to variable drive torque distribu-

tion, the longitudinal force potential at thefront and rear axle wheels can be influ-enced.

If the lateral forces at the wheels change,the torque equilibrium around the vehicle'svertical axis also changes. By making use ofdifferent lateral force potentials at the frontaxle and rear axle, an unstable vehicle (e.g.oversteering) can be stabilised again bymeans of yawing moment compensationvia longitudinal torque management (LM).

4.3 Active Vehicle DynamicControl with xDriveThe manner in which variable torque dis-tribution acts to actively control vehicle dy-namics in the driving situations of over-steering, Figure 9, and understeering, Fig-ure 10, during cornering will be qualitative-ly explained in greater detail in five phases.

4.3.1 OversteeringStable entry into the bend (1): The vehicledrives into a left-hand bend in a stablemanner with an ideally assumed torquedistribution of 40/60 (front axle/rear axle).Tendential oversteering (2): Even at thisstage, the vehicle dynamics control systemrecognises the beginnings of a slight ten-dency to oversteer on the part of the vehi-cle, and causes the longitudinal torquemanagement system to re-distribute thetorque, for example 50/50. The drive torqueat the rear axle is reduced, thereby increas-ing the lateral forces that can be transmit-ted. At the front axle, the drive torque is in-creased and the lateral forces are reduced.Corresponding counter-torque to compen-sate for the vehicle's yawing moment canbe generated by specifically influencing thelongitudinal and lateral forces at the frontand rear axles.

Increased oversteering (3): The vehiclenow tends to oversteer even more. This isrecognised in the vehicle dynamics controlsystem, and the xDrive clutch is set to an"overlocked” state. A torque distribution of,for example, 65/35 cannot be actively ad-justed in this range, but is set solely on thebasis of the slip and therefore frictionalconditions at the tyres. In this oversteeringdriving situation, the torque ratios shift infavour of the front axle (absolute slip at therear axle is greater than that at the frontaxle; as a result of this, the increase in lon-gitudinal force potential at the front axle isgreater than that at the rear axle).

The greater longitudinal forces at thefront axle and the increased lateral forcepotential at the rear axle in the overlockedstate generate a counter-torque. This is suf-ficient to compensate for the vehicle's yaw-ing moment. The vehicle begins to stabiliseand approaches the desired path curve.

Stabilisation (4): The vehicle dynamicscontrol system recognises that the vehicleis becoming stable again and calculates anew torque distribution of, for example,50/50, in order to exit the bend in a neutralmanner and with the best possible tractivepower.

Stable exit from the bend (5): The vehicleis stable again and is able to accelerate outof the bend with the ideally assumedtorque distribution of 40/60.

4.3.2 Understeering Stable entry into the bend (1): The vehicledrives into a left-hand bend in a stablemanner with an ideally assumed torquedistribution of 40/60 (front axle/rear axle).Tendential understeering (2): Even at thisstage, the vehicle dynamics control systemrecognises the beginnings of a slight ten-dency to understeer on the part of the vehi-cle, and causes the longitudinal torquemanagement system to re-distribute thetorque, for example 20/80. The drive torqueis increased at the rear axle and reduced atthe front axle, thus resulting in a reductionin lateral force potential at the rear axle andan increase at the front axle.

Increased understeering (3): The vehiclenow tends to understeer even more. A newtorque distribution of, for example, 0/100 istherefore calculated and set. As a result ofthis, the drive torque at the rear axle is fur-ther increased, and the lateral force poten-tial reduced. The lateral force potential in-creases again at the front axle (longitudinalforce-free in this case), and the vehicle ap-proaches the stable path curve.

Stabilisation (4): The vehicle dynamicscontrol system recognises that the vehicleis becoming stable again and calculates anew torque distribution of, for example,10/90, in order to exit the bend in a neutralmanner.

Stable exit from the bend (5): The vehicleis stable again and is able to accelerate outof the bend with the ideally assumedtorque distribution of 40/60.

Cornering (oversteering and understeer-ing) has been described in five phases. Inactual fact, the optimum torque distribu-tion is continuously calculated as the vehi-cle drives through the bend. Due to variablelongitudinal torque distribution betweenthe front and rear axles, the vehicle's self-steering behaviour and therefore its vehicledynamics can be actively influenced. Neu-tral and safe handling is achieved up intothe handling limits. The DSC system, withits engine management and brake man-agement control loops, is active in the back-ground, but only intervenes once the possi-bilities of stabilisation offered by longitudi-nal torque management have been ex-

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hausted. xDrive will reduce the incidence ofthese interventions and perceptibly im-prove ride comfort.

4.4 Active Traction Controlwith xDriveIn addition to variable torque distributionfor the purpose of active vehicle dynamicscontrol, the controllable clutch in the trans-fer case also functions as a centre-lockingdifferential in order to improve tractioncharacteristics. Both axles can be coupled ifrequired. The following example describesa starting manoeuvre by two vehicles withdifferent drive concepts, Figure 11. The rearaxle of both vehicles is set to a low frictionvalue, while the front axle is set to a highfriction value.

Vehicle A (permanent four-wheel drivewith fixed torque distribution of 38/62) isequipped with three open differentials, aDSC system and emulation of the trans-verse and longitudinal locking devices bymeans of active brake intervention at indi-vidual wheels.

In contrast, vehicle B (xDrive) isequipped with two open differentials, aDSC system and emulation of the trans-verse locking devices by means of activebrake intervention at individual wheels.

On acceleration, vehicle A's wheels spinon the rear axle, drive slip occurs and thebrake hydraulic system begins to supplythe pressure to emulate the missing longi-tudinal locking device. This process takesapprox. 0.5 sec. The brake hydraulic systemadjusts the pressure working point request-ed by the traction control system in the DSCat the rear axle wheels, drive torque is builtup at the front axle and the vehicle is set inmotion. The entire process takes approx. 1.0sec. Depending on the friction value, up to62 % of the induced drive torque has to besupported at the rear axle brake in orderprovide a maximum of 38 % of the availabledrive torque at the front axle.

In vehicle B, the clutch in the transfercase is closed, Figure 12, (MK_ist; t2), thanksto the intelligent xDrive system’s pre-con-trol, supported by the accelerator pedalspecification (PW; t1), even before the drivetorque (M_GW_VA, M_GW_HA; t3) hasbeen built up. No additional brake inter-vention is required at the rear axle in thissituation, with the result that no drivetorque is lost in brake support. The vehicleis set in motion spontaneously and withoutdelay (t4).

xDrive therefore makes a significantcontribution towards improving traction,driving safety and agility. In addition,comfort is considerably enhanced thanksto the lack of brake intervention at the rearaxle.

4.5 Driving Safety, Agility andVehicle Handling with xDriveIn terms of safety, agility and handling, theadvantages of the xDrive concept can bevery easily explained in phase diagrams(steering angle over side slip angle) in com-parison between vehicles A and B, eachwith the DSC system switched off (enginemanagement and brake management de-activated), Figure 13.

In vehicle B, only longitudinal torquemanagement is active in order to controlthe variable torque distribution. Thesemeasurements were carried out on a han-dling course prepared with snow in Scandi-navia.

Comparison of the phase diagrams re-veals that the driver is able to keep vehicleA on course only with a high level of steer-ing effort and a relatively large side slip an-gle. In contrast, the steering effort in vehi-cle B, which is fitted with xDrive, is consid-erably lower. Longitudinal torque manage-ment significantly reduces the vehicle'stendency to oversteer on acceleration outof bends. The side slip angle that occurs re-mains comparatively low. Counter-steeringby the driver is only required to a minor de-gree.

Thanks to its active vehicle dynamic andtraction control, the xDrive concept makesa significant contribution towards increas-ing driving safety and agility and towardsimproving handling of the vehicle as awhole.

5 xDrive Controller Structurefor Longitudinal TorqueManagement

The xDrive functional software is sub-di-vided in a modular form between the twoDSC and transfer case control modules(VGSG), Figure 14. The main functions forcalculating the necessary front axle torque(Mk_soll) are implemented in the DSC con-trol module and are derived from the trac-tion and vehicle dynamic criteria internallycalculated by the DSC system. Longitudinaltorque management for actuating theclutch is primarily comprised of the PI con-troller, pre-control (Vorsteuerung) andDelta-n SW blocks.

The PI controller is comprised of the slipcontrol and vehicle dynamic control sec-tions. The slip control system adjusts thefront axle torque in such a way that the dif-ferences in speed between the front axleand the rear axle are minimised. The vehi-cle dynamic control system in the DSC pro-vides data on the current driving condition.Depending on the driving situation, thewheels' longitudinal and lateral forces areadjusted in such a way via variable torque

distribution that stable handling isachieved.

Thanks to pro-active clutch actuation,the pre-control system (Vorsteuerung) pre-vents critical driving situations from occur-ring. The accelerator pedal position (PW),engine torque (Mmot), engine speed(nmot), gear ratio and vehicle speed (vFzg)variables are used to calculate the currentforward drive situation. The front axletorque is determined from this by means ofcharacteristic curve functions. Cornering isrecognised with the steering angle (LW)and vehicle yaw rate (vgi) input variables,and the pre-control values are adapted ac-cordingly.

In the event of different tyre circumfer-ences, the Delta-n module prevents possi-ble distortion torques by means of model-aided monitoring. The four wheel speeds,their correction values (kRad) and the en-gine torque (Mmot) are used as input vari-ables.

In the subsequent torque co-ordinator(Momentenkoordinator), a decision is tak-en, depending on the driving situation, onthe manner in which pre-control is to beadapted via the Delta-n calculation.

Pre-control is additionally influenced bythe power loss (Verlustleistung) and wear(Verschleiß) modules in the transfer casecontrol module. Actual strain on the clutchand transfer case actuator system andclutch wear are taken into consideration inthis case, and pre-control is influenced ifnecessary.

6 Summary

BMW’s intelligent xDrive four-wheel drivesystem heralds the creation of an innova-tive drive concept for the X3 and X5 rear-wheel drive-based four-wheel drive vehi-cles. By implementing the new four-wheeldrive train together with the DSC system,which has been enhanced with longitudi-nal torque management, success has beenachieved in significantly improving drivingstability, driving safety, handling, agility,traction and comfort. Thanks to the combi-nation of xDrive with the re-engineeredrange of engines and transmissions, the X3and X5 vehicles achieve performance val-ues that are outstanding for these vehicleclasses.

Reference

[1] Braun, H.-S.; Lipp, C.; Müller, A.; Schopp, J.: Der Hochleistungsantrieb im X5 4,6is. In: MTZ 63 (2002) Nr.6

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