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Double-jubilee for Subaru in 2005: For 25 years the manufacter-er of AWD passenger cars has been selling his products on thegerman market. 33 years ago Subaru introduced the All-Wheel-Drive into mass-produced passenger cars. In 1972 the standardpart-time All-Wheel-Drive of the Leone Station AWD started anew area in drivetrain-technology. Today the “SymmetricalAWD” with its horizontally opposed engine and the lateral sym-metrical layout is the Subaru core-technology.

By Koji Matsuno

Entwicklung des Allradantriebs

bei Subaru

You will find the figures mentioned in this article in the German issue of ATZ 6/2005 beginning on page 498

Development of theSubaru All-Wheel-Drive

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1 Introduction

Subaru’s mass-produced 4WD passengervehicle, which is the forerunner to theAWD car, made its debut in 1972. At thattime, AWD vehicles were generally heavy-duty, cross-country types of vehicles. Incor-porating AWD systems in passenger vehi-cles was a revolutionary idea, wherebySubaru aimed at having the best of bothworlds in a single design on-road comfortand superior driving traction. The develop-ment was initiated by the Tohoku ElectricSupply Company, which provided electrici-ty to Tohoku Prefecture in the north of theJapanese main island of Honshu which is aheavy-snow-area. Subaru developed theLeone Station Wagon AWD, Figure 1, and inSeptember 1972 it was delivered to the cus-tomer. This was the first mass-producedAWD-Passenger Car in automobile history.The car created a demand by all the compa-nies who needed AWD-performance in thesnowy and mountainous areas of Japan.But even in export-markets the car was suc-cessful: The Subaru Leone Station WagonAWD became the best-selling AWD-passen-ger car of the world and the nucleus of Sub-aru AWD-technology.

In the ensuing years, Subaru has contin-ued to refine the technologies of its originalAWD system to assure higher levels of on-road safety and stability. Subaru is widelyacclaimed throughout the world for itstrack record in AWD technologies, such asdramatic improvements in safety for high-speed driving, and for enabling both topdriving safety and superb handling.

2 Subaru Symmetrical All-Wheel-Drive

The main feature of Subaru All-Wheel-Dri-ve (AWD) is the layout. The foundations ofthe AWD system are Subaru’s unique Hori-zontally-Opposed Engine and the originalAWD left-right symmetrical layout, withthe longitudinally mounted, Horizontally-Opposed Engine; transmission, transfercase, propeller shaft, drive shaft, and reardifferential arranged linearly, Figure 2. Thislayout, with its low centre of gravity and itsexcellent balance of weight, is ideal for theAWD system, enabling top driving stabilityand handling performance as well as out-standing “Active Safety”. This layout pro-vides the neutral balance that is essentialfor superior weight transfer both laterallyand longitudinally, that gives enhanceddriving pleasure and vehicle handling. No-tice that all the heavy components such asthe transmission, transfer case, etc., arearranged within the wheelbase of the vehi-cle. This cuts out superfluous overhang of

these components front and rear, reducesthe moment of inertia involved in steeringand spectacularly improves the driving per-formance of the vehicle itself. Clearly, thedifference in performance the power trainlayout makes cannot be matched by otherdevices, and this advantage gives Subaruautomobiles superior performance that issecond to none. Furthermore, Subaru’s sim-ple AWD layout results in qualities, such ashigh reliability, reduced noise, and low fuelconsumption that are essential for AWDcars to become widely popular. With itshighly original combination of Horizontal-ly-Opposed Engine and Symmetrical AWDsystem, Subaru offers pleasurable yet safedriving with a confident feeling of stability,through high level performance plus su-perb weight balance.

3 Horizontally-Opposed Engineand the System of Lateral Symmetry

Subaru believes that the ideal engine forthe greatest driving pleasure is the Hori-zontally-Opposed Engine. This engine, witha layout that positions the pistons laterallyto the crankshaft at an angle of 180 degrees,delivers multiple benefits. This Horizontal-ly-Opposed Engine is the ideal power unitbecause the opposed pistons mutuallycounteract vibration and give superior bal-ance. This results in smoother engine per-formance throughout all speed ranges –with no need for a balancer shaft – as wellas giving unparalleled response to the dri-ver. The next advantages are the size andweight. The Horizontally-Opposed pistonlayout means an engine that is both short-er and lower than a conventional in-linedesign. In addition, you have a more com-pact shape, lighter weight and the ability tomount the engine much lower in the chas-sis. The overall result is a low centre ofgravity and balance that cannot bematched with other engine types. So theHorizontally-Opposed Engine is the onefactor that really makes Subaru Symmetri-cal AWD possible.

Under conditions like a high-speed lanechange to avoid danger, where the vehicleis pushed to its limits, the differences in dri-ving stability due to differences in weightbalance can be seen more realistically. WithFF (front engine, front-wheel-drive) and FR(front engine, rear-wheel-drive), where theengine power is distributed to either frontor rear wheels, the different traction char-acteristics can easily result in an unstablesituation. And, even though AWD normallyoffers superior driving stability, the highcentre of gravity of normal AWD vehiclesmakes it impossible to obtain the same de-

gree of stability possible with Subaru AWD.This is because in general, vehicles with alow centre of gravity have superior steeringstability. It means that the position of the“pendulum” where centrifugal force acts islow, and this means less rolling. Less rollingmeans more responsive manoeuvering, be-cause there are fewer uncontrolled changesin attitude, and there’s a more stable rela-tionship between the road surface and thetyres, which contributes to vehicle stability.This is why Subaru AWD uses a Horizontal-ly-Opposed Engine with a low centre ofgravity. Subaru has also successfully elimi-nated surplus overhang. The straight, sym-metrical layout of the power train, differen-tial, transmission, etc., provides idealweight balance, which contributes to thepleasure and safety of driving.

This symmetrical layout not only givesoptimum driving performance, but con-tributes greatly to safety in the event of acollision because it allows for plenty ofspace on both sides of the engine compart-ment. This makes it possible to design theside frames, which play a major role in ab-sorbing the impact of frontal collision, sothat they extend straight from the cabin tothe bumper. At the same time, it also al-lows body components to be in a laterallysymmetrical layout with the sturdy framein a straight arrangement. Ideal frame con-struction made possible by the longitudinalmounting of the Horizontally-Opposed En-gine. So the entire chassis construction caneffectively act as a crumple zone, to protectpassengers in the event of a frontal colli-sion. The low mounting of the Horizontal-ly-Opposed Engine and the longitudinal po-sitioning of the transmission along the cen-tre line of the vehicle also lets them shiftbelow the floor tunnel in frontal collisions.So there’s less deformation of the passen-ger compartment and less intrusion intothe cabin, which adds to greater safety.

4 Basic Advantages of All-Wheel-Drive

There are three basic advantages of AllWheel Drive: traction, handling and stabil-ity. Basically AWD supplies the entiretorque to the transmission and the wheels.Torque transfer is crucial because drive andtorque distribution define the momentwhen the maximum frictional connectionbetween road and tire is exceeded. Thesafety-benefit of AWD is obvious duringcornering when three forces influence thebehaviour of the car: the engine-power, thefrictional connection between tire and sur-face and the cornering force which is coun-teracting to the lateral force. The strongerthe cornering force is the safer is the cor-

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nering behaviour. Cornering force increas-es proportional to decreasing propellingpower: With identical engine power theAWD car generates more cornering forcethan the 2WD car. This is the basic differ-ence which is effective under all conditionsand becomes extremely important slipperyroads with low grip. The more dangerousthe situation the more evident is the ad-vantage of AWD to 2WD. AWD makes themost of tire grip by distributing power to allfour wheels, to provide more stable tractionand driving performance under all roadconditions.

5 Six-speed-gearbox with Centre Differential, Helical LSDand DCCD

The Impreza WRX STi (1994), basis of theImpreza WRC, modifies the basic designcentre differential with viscous coupling –to sporty conditions. The torque distribu-tion ratio fore and aft was designed as 35:65by use of a double pinion type planetarygear set. It was a response to a demand ofthe market for the manual transmissionversion of VTD. The electro magnetic deviceand ball-ramp mechanism are adopted forcentre differential L.S.D. because the manu-al transmission does not have a hydraulicpressure source. The L.S.D. torque can bechanged from open to lock of the centre dif-ferential by electric current according to aposition of the manual dial. Additionally,when driver pulls hand brake lever, centredifferential is opened for spin-turn. In 2002DCCD with auto-mode was released. Themechanical constructions are the same asprevious DCCD, but the computer controlwas added. The target of this AWD controlwas the best cornering on dry asphalt or fora circuit driving. Additionally, the torquedistribution ratio of centre differential wasset back from 45:55 to 35:65 to realize moreagile performance. Even though the torquedistribution ratio was set back to 35:65, theelectronic control can keep the vehicle sta-bility on slippery road by use of lateral ac-celeration sensor e.g. The controller detectsdriving situation and road condition by lat-eral acceleration signal. The L.S.D. torque isoptimized for simultaneous pursuit of agili-ty, stability and traction. In 2004 DCCDwith auto-mode was upgraded. The yawrate sensor was added to detect a spin mo-tion of the car. When the controller detectsan excessive yaw motion, L.S.D. torque is in-creased to keep stability of the car. Thisfunction seems similar to VDC function,but this control is smoother than VDC anddoes not conflict with driver’s demand foraggressive driving. This function is very ef-

fective for high-speed driving or criticalconditions.

6 Multi-plate Clutch Designand Function (Automatic Transmission with VTD)

This centre differential gear is in the formof composite a planetary gear system with-out a starter ring. The output torque fromtransmission to the planetary gearbox istransferred to the primary sun gear whichdrives the planetary gears. The planetarygears are joined by an interference fit withthe planet carrier which is connected to thedrive gear of the front axle. The outputshaft to the rear axle is driven by the rearplanetary gears via the secondary sun-gear.The distribution to the rear axle is effectedby a hydraulically actuated multi-plate-clutch which locks the planetary gearboxbetween front- and rear axle when rev-dif-ferences occur. Due to the multi-plate-clutch the torque distribution of normally36:64 % is continuously variable. The lock-ing-degree depends on the rev-differencesand may reach 100 % when the difference isextremely high. With the planetary geardead-locked the output torque of transmis-sion is distributed 60 % front and 40 % rearaxle.

7 Multi-plate Clutch Designand Function (Automatic Transmission ACT-4))

The multi-plate-clutch with its alternatinginner and outer plates is running in an oilbath. The outer plates (steel plates) arejoined by an interference fit with the planetcarrier. The inner plates (frictional plates)are situated on the multi-plate carrierwhich is joined by an interference fit to theoutput shaft of the rear axle. The piston inthe transfer-case is actuated hydraulically.The interior piston oil-pressure is influ-enced by a mapped and load controlledmagnetic valve receiving operating datafrom the gearbox control device. The crucialinput signals for the automatic gearbox de-vice to control the locking are the rev-sig-nals of the speed sensors 1 and 2 and the in-put data of the vehicle dynamics control.On this data-base the electronic device rec-ognizes the vehicle load and eventual rev-differences between the axles. Based onthese data the control device calculates thebest possible oil-pressure at the piston ofthe multi-plate clutch (AWD-clutch): Thehigher the pressure acting on the entiremulti-plate package the higher is the pres-sure force of the piston and the higher isthe torque distribution to the rear axle.

8 AWD with VTD and VehicleDynamics Control

VTD is a driving torque distribution systemusing a centre differential, which splitshigher torque to the rear wheels than thefront wheels, with a computer controlledhydraulic L.S.D. clutch. This centre differen-tial gear is in the form of composite a plan-etary gear system without a starter ring.The output torque of transmission is dis-tributed 45 % to front wheels and 55 % torear wheels. It improves the handling per-formance on dry asphalt when the transferclutch is open. The integration of VTD andVehicle Dynamics Control, Figure 3, systemoptimizes the driving stability by imple-menting the electronic control evolved byuse of some sensor signals from VDC con-troller through the CAN communication.First topic was a μ (road friction coefficient)estimation technology. μ estimation is akey technology for enhancement of AWDvehicle dynamics because μ is very impor-tant but unknown variable for drivingtorque distribution control. If μ can be de-tected by AWD controller, a conflict be-tween stability on slippery road and agilityon dry asphalt will be solved in very highlevel. The μ estimator is realized based onthe yaw response comparison between ac-tual vehicle and a model in the controller.By the optimization of L.S.D. torque for VTDaccording to the result of μ estimation, thevehicle dynamics of VTD system was en-hanced significantly. Additionally, whenVehicle Dynamics Control is activated,L.S.D. torque is optimized to improve con-trollability of each wheel speed by VDC-controller. VTD provides the best solutionto combine a traction performance and thecontrollability for the VDC-controller. Yawrate feed back control is built in this systemalso to improve a stability of vehicle yawmotion which improves stability when μchanges suddenly.

9 Dynamics Enhancement ofAWD Vehicles

Vehicle Dynamics Enhancement of AWDVehicle is what Subaru is working reallyhard on. The crucial point is the further in-tegration of electronics into the AWD-sys-tem: The refinement of centre differentialdesign, the development of electronicallycontrolled centre differential LSD and theoptimization of Front & Rear LSD designnearly automatically lead to the dynamicsenhancement of the AWD vehicle.

10 μ-Estimator

Development target is to optimize stability

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on slippery road (centre differential lock),handling performance on dry road (opti-mum torque distribution) and μ-estima-tion. This is supposed to be done by cooper-ative control with Vehicle Dynamics Con-trol and yaw velocity feedback control, Fig-ure 4.

The concept of μ Estimator does not re-quire additional or exclusive sensors andshows robustness and sensitivity. We fo-cused on the parameter identification lawof adaptive control theory. We thoughtthat, by the use of the adaptive control the-ory, we could estimate the tire characteris-tics which are under the influence of μ. Inaddition, we add the modification which isbased on the lateral acceleration of vehicleto improve a response of μ estimation. Thismodification is effective when input sig-nals for parameter identification is insuffi-cient because, in adaptive control theory,identification signals must have the persis-tently exciting (fluctuant) characteristics,Figure 5.

The tire characteristics are tuned by pa-rameter identification logic to reduce theerror between the model and the actual ve-hicle. The estimated μ is calculated fromthe identified value of tire characteristics,and basic transfer torque is set according tothis estimated μ. On the other hand the es-

timated μ under aggressive driving on dryasphalt was increased by the parameteridentification law, and modified by the lat-eral acceleration, then the high μ road wasdetected. There was momentary decreaseof estimated μ caused by unmodelled dy-namics of the adaptive vehicle model. But,by the modification according to lateral ac-celeration, estimated μ was quickly recov-ered.

The evaluation of our control system onsnow covered roads under typical operat-ing condition is different, Figure 6. The mo-mentary stability factor is the index ofsteering behaviour. K is the momentarystability factor which is calculated from thesteering angle, yaw velocity and vehiclespeed. If the steering behaviour is over-steer, the momentary stability factor isnegative. Otherwise, when the steering be-haviour is excessive understeer, the mo-mentary stability factor is a large positive.When the centre differential was open, thefrequency of neutral or oversteering behav-iour was relatively high. Conversely, whenthe centre differential was locked, the fre-quency of excessive understeering behav-iour was relatively high. So we introduced adistortion of momentary stability factor,Figure 7. It is an index of the third power ofdeviation between the momentary stabili-

ty factor and its average. As the red lineshows, when the frequency of oversteeringbehaviour is high, distortion is negative orrelatively small. In the case of our new VTDcontrol, the controllability of wheel speedwas improved by the optimizing of thetransfer torque control on VDC activation.

The μ estimator has been put to practicaluse in 1998. We have optimized transfercontrol according to μ, and yaw velocityfeedback has brought excellent handlingperformance and stability. Cooperativecontrol with Vehicle Dynamics Controlachieved sufficient stability and controlla-bility, Figure 8.

11 Outlook

Subaru has consequently continued the de-velopment which started in September1972 with the Leone Station Wagon AWD.The equation Subaru = All-Wheel-Drivedefinitely applies to the future. There is stillpotential in the Symmetrical AWD whichwill remain the core technology of theAWD-pioneer who 33 years ago started anew era the industrial production of pas-senger cars. And still Subaru continues towork on improving and optimizing its coretechnology. ■