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					SAE TECHNICAL PAPER SERIES

2006-21-0026

Innovative Driveline System Technology
Harald Deiss, H. Krimmel and G. Horsak
ZF Friedrichshafen AG

Convergence 2006 Detroit, Michigan October 16-18, 2006
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-0790 Web: www.sae.org

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. For permission and licensing requests contact: SAE Permissions 400 Commonwealth Drive Warrendale, PA 15096-0001-USA Email: permissions@sae.org Tel: 724-772-4028 Fax: 724-776-3036

For multiple print copies contact: SAE Customer Service Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: CustomerService@sae.org ISSN 0148-7191 Copyright © 2006 Convergence Transportation Electronics Association and SAE International Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published in SAE Transactions. Persons wishing to submit papers to be considered for presentation or publication by SAE should send the manuscript or a 300 word abstract to Secretary, Engineering Meetings Board, SAE. Printed in USA

2006-21-0026

Innovative Driveline System Technology
Harald Deiss, H. Krimmel and G. Horsak
ZF Friedrichshafen AG

Copyright © 2006 Convergence Transportation Electronics Association and SAE International

ABSTRACT
Increasing dynamics and agility as well as simultaneously reducing fuel consumption and emissions – these are today's challenges in the field of driveline development. Now, hybrid technology is in the pipeline. ZF is developing hybrid components, modules, and transmissions, as well as complete hybrid systems. Among the key issues of hybrid technology discussion is also the topic of energy storage. In addition to the evolving trend focusing on hybrid technology, the market perceives a number of innovations with reference to automatic transmissions, dual clutch transmissions, and all-wheel drive systems. These technologies also enable a substantial reduction in fuel consumption as well as CO2 emissions. The networking of these new driveline and chassis systems allows for an optimized power management and enhanced driving dynamics with respect to stability and agility.

All-wheel drives become more and more common. In comparison to traditional differentials, Torque-OnDemand transfer cases provide additional potential to reduce fuel consumption. Moreover, variable drive torque distribution provides new opportunities for controlling drive dynamics. Agility and vehicle stability constitute the essential assets of new axle drives with variable lateral torque distribution. In particular when looking at the CO2 balance, networking of driveline systems with chassis systems becomes indispensable. The electric power consumers must not impact the on-board supply all at the same time. The functional networking of driveline and chassis is discussed in the present elaboration on the basis of an exemplary concept car, the SUC (Sports Utility Convertible).

1

INTRODUCTION

In the course of the last years, the oil price has risen considerably. This is most certainly one of the reasons why, for the time being, hybrid drives have attracted a lot of attention. A significant, critical success factor for hybrid drives is their respective capacity to store energy. This is why we are discussing the interrelation between energy accumulation parameters and mileage. Currently, important innovations in the driveline (figure 1) relate also to automatic transmissions, dual clutch transmissions, and all-wheel drives. Thus, today, automatic transmissions open up the opportunity to reduced fuel consumption as well as CO2 emissions by several percent thanks to a higher number of gears and an optimization of efficiency. Figure 1: Current innovations in the driveline and chassis.

2

HYBRID

Today, customers have very high expectations with regard to hybrid technology. In particular, they are expecting considerable savings in fuel consumption as well as further reduced emissions which go hand in hand

with the well-known driving dynamics and the high vehicle utility. Add-on costs must remain within a bearable scope and the reliability of additional components – in particular of the power electronics and the battery – must be warranted for the vehicle’s lifetime. To put it in a nutshell: They are expecting just a “tiny miracle”! 2.1 POTENTIALS AND MARKET OPPORTUNITIES

2.1.1

Hybrid alliances

In addition to the GM / DC / BMW platform, further cooperations have evolved within Europe: • • • Robert Bosch - Getrag Siemens-VDO - Magna Steyr Conti Automotive Systems - ZF

For the time being, automotive markets are highly inhomogeneous with regard to supply and demand of hybrid vehicles. Particularly in Europe, the upcoming hybrid segment has to face up to a very strong diesel fraction. However, it has become clear in the meantime that the hybrid will not be a short-time fashion fancy. In the long-run, only those concepts will prove successful on the market which feature a well-balanced cost-benefit ratio and which offer noticeable customer satisfaction characteristics i.e. primarily fuel consumption and/or tax advantages. Figure 2 shows the amortization of hybrid costs over the mileage, using the example of an SUV (sports utility vehicle).
350.000 milage amortisation [km] 300.000 250.000 200.000
realistic range for Realistischer Bereich für practical operation gemischten Praxisbetrieb Otto Hybrid vs. Diesel konv.

It is the objective of such alliances to jointly develop cost-efficient hybrid systems by standardizing components across all variants and splitting development costs. The lead by Japanese automotive manufacturers can only be recovered in Europe when the supplier industry is adequately involved. 2.1.2 ZF / Conti Automotive Systems positioning

The marketing of complete passenger car hybrid systems is the declared objective of ZF and Conti Automotive Systems. Components and competence in the fields of driveline, transmissions, E-machines, power electronics, and batteries complement one another with know-how and experience gained on interfaces and functions as well as driveline control and battery management (refer to figure 4).

Hybrid module

100.000 50.000

20% 40% 40 %

Otto Hybrid vs. Otto conv.

Electric motor + power electronics + functions + clutches + vibration dampers + functions + regenerative braking + functions + transmission + functions

0 1,30 1,40 1,50 1,60 1,70 1,80 1,90 2,00 2,10 2,20 2,30 2,40 2,50 fuel cost [€/l]

Figure 2:

Cost-benefit calculation for the SUV.

Currently, various sources say that fuel consumption savings of 15 percent can be obtained for hybrid drives. These are implemented by the classical hybrid functions, such as start/stop, electric launch, recuperation, and shifting of the combustion engine’s operation point to higher loads, also see figure 3.

+ battery + functions

Figure 4:

ZF/Conti – from the component to the system.

consumption emissions

recuperation

boost

dynamic

This portfolio is further enhanced by comprehensive expertise on the integration of components on the one hand, and E/E integration of (networked) systems on the other hand (see figure 5).

engine start-stop

electric launch modulation of operating point ICE

electric driving

functionality
requirement function

comfort

Figure 3:

Typical hybrid functions.

Hybrid system

Hybrid comp.

150.000

reduction of fuel Verbrauchsconsumption 10% reduzierung 10%

Hybrid transmission

OEM Continental/ZF
Hybrid module

system cost w/o battery

Brake control unit

2 x 50 kW
Brake

2 x 65 kW Full Hybrid power-split

E-Motor

higher

Transmission
Transmission control unit

similar Performance Full Hybrid parallel 1 x 35 kW 1 x 61 kW

Engine control unit

Power electronics
DC/DC converter

BM Battery control unit

Battery

Benchmark (w/o Hybrid)
Strategic driving functions Driveline management Operational hybrid functions

worse

better

Interfaces

fuel economy (MVEG-cycle)

Figure 5:

Supplier scenario hybrid from the point of view of the OEM.

Figure 7:

Comparison of concepts: Parallel vs. power-split hybrids, SUV application.

The different hybrid concepts explained in figure 6 can be assessed according to their properties and expenses for specific operating conditions in the respective vehicle classes. The characteristics concerning the performance (mild, full, or power hybrid) or capability of all-wheel application also play a role.

Within an environment of still unknown trends, the parallel hybrid offers the major advantage of modularity. Thus, hybridization can be set up as an "add-on" for existing transmission concepts, and any kind further developments that increase transmission efficiency will primarily benefit the overall driveline, whether a hybrid or not. Thanks to the opportunity of using existing driveline components, development costs and risks are kept at a minimum. ZF has already established hybrid drives for all vehicle categories (refer to figure 8).
Hybrid modules and e-drives
Hybrid module for cars

Transmission systems
Hybrid transmissions for SUVs

CSGHybrid

SerialHybrid

ParallelHybrid

TransAxleHybrid

PowerSplitHybrid
Hybrid driveline for vans

Cars

Figure 6:

Different hybrid concepts.

Hybrid system for local delivery trucks

The conceptual alignment of the ZF/Continental cooperation is focused on the parallel hybrid system which, from a joint point of view, constitutes the most cost-effective solution for the implementation of all important hybrid functions (refer to figure 7). The advantage of continuously variable functions, which is typical for power-split hybrids, has not the same significance on the European market (as it does in Asia or the US) and will not create additional fuel savings or extra performance.

Commercial vehicles

Figure 8:

Examples for (implemented) ZF hybrid projects.

Only in the case of the city bus, the serial hybrid concept seems to be a sensible alternative to parallel hybrids. Here, ZF is already looking back on 10 years of practical experience and more than 5 million km run in line traffic (refer to figure 9).

alternative traction systems
trolly wire, diesel-electric

Hybrid categories weight (example) kg M icro M ild Power el. driving 10km 10 30 30 70 Energy Wh 50 400 200 2000 Spec. energy W h/kg 50 133 67 286 Delta-SOC % 10 10 10 10 Power kW 10 20 80 60 Spec. power kW /kg 1,00 0,67 2,67 0,86

diesel-electric

Ragone Diagram for Electrical Energy Storage System
300

El. driving

Spec. energy [Wh/kg]

250 200 150 100 50 0 1

battery/hybrid

Lithium NiMH UC
10 100

Mild-Hybrid Micro-Hybrid Power-Hybrid

fuel cell

• • • •

low floor vehicle flexibility in arrangement of the components high comfort (no switching) flexibility for changeover to alternative traction systems

1.000

10.000

Spec. power [W/kg]
Basis: 50-70 kg battery for el. driving 30-40 kg battery for power and Mild-Hybrid 10-20 kg battery for Micro-Hybrid

Figure 9: 2.2

Serial hybrid in the city bus. BATTERIES

Figure 10: Ragone chart for battery systems. In order to warrant the lifetime of electrochemical batteries, only comparably minor energy volumes are cyclically charged/discharged back and for (Delta State of Charge, SOC); 5 to 10 percent of the nominal capacity are standard in practice. Thus, also the benefit of such accumulators is considerably reduced (just as the [theoretical] range of electric driving), see figure 11.
Frequency distribution ΔSOC More than 90% of loads cause less than 1% of SOC change Basis: Toyota Prius with a 2 kWh NiMH battery

The battery is an essential element of any given hybrid concept. It defines in particular the customer’s perception in regard to consumption (recuperation), performance (launch, boosting), and experience (electric driving). Costs, weight, and dimensions establish the limits for the design of an ”optimum“ battery. Depending on the concept selected (micro, mild, or full hybrid), technological criteria also determine the design of the battery (refer to figure 10). 2.2.1 Electrochemical batteries

The omnipresent and cost-effective lead battery (as included in starter batteries) – even as improved versions (e.g. AGM battery) - are not particularly suitable for hybrid applications because of their low energy and performance density as well as the low cycle rigidity. Currently available as an alternative and significantly more mature are NiMH batteries with their considerable advantages concerning weight and performance. Soon, NiMH batteries will be replaced by the significantly more powerful Li-Ion (Lithium ion batteries) systems. However, it holds true that even the most powerful battery systems from research laboratories are still miles away from the energy density of today’s fuels. Figure 11: Typical SOC changes. 2.2.2 Electrostatic capacitor

The development of supercapacitors (double layer capacitors - DLCs) has drastically increased storage capacity when compared to electrolytic capacitors. Their energy density is approximately 10 to 20 times lower than that of electrochemical batteries, however, 50 to 80 percent of the energy can be discharged and (re)charged cyclically without impacting the overall lifetime. In parallel, even smaller-size models can be implemented with very high charging/discharging flows. As a consequence, also DLCs with the corresponding hybrid design may constitute an alternative for electrochemical batteries.

2.2.3

Outlook

1.680

Pel [W]
1.500

chassis + steering
1.590

propulsion

comfort

For the time being, there is no battery that lives up to the requirements of a cost-efficient hybrid concept featuring high customer benefit. Moreover, it represents the “Achilles heel“ of the requested and sustainable market success of hybrid vehicles as such. In addition to high costs (approximately 50 percent share in hybrid components), their performance and energy density represent another set of items calling for significant improvements. Doubts relating to lifetime may well lead to strong loss in value for second-hand cars. Hybrid systems open up new avenues and chances on the road to the "Dry Vehicle" in terms of new vehicle electrics structures (higher voltage) and new electric systems (e.g. Electric Power Steering), which allow vehicle integration at a higher level of efficiency. The electric power steering (EPS) demonstrates in an exemplary manner how the use of electric actuators and controllers optimizes fuel consumption, because energy is only consumed during steering, see figure 12. With hydraulic solutions, some basic energy consumption is always associated. The EPS example also shows that electromotive packages can offer many benefits. Torque-on-demand transfer cases, electrically controlled differential locks, and electrically operated active chassis systems are other examples.

high power components

… peak power [split second] … mean power [30 minutes]

1.000

150

680

500

580

400

150

60

120

40

100 ERC (eABC)

EPS VW Golf

EPS BMW

TV-HAG (VTG)

40

windshield heater

seat heater

Figure 13: Electric power requirements of selected consumers.

3.

AUTOMATIC TRANSMISSIONS

With the introduction of the first automatic 6-speed transmission by ZF in 2001, a milestone was established for passenger car driveline technology which featured significant improvements compared to the previous 5speed transmissions. In addition to the reduction in fuel consumption (by approximately 5 percent) and an improvement in acceleration (by at least 4 percent), the transmission’s weight was reduced by approximately 12 percent. This transmission generation established itself very swiftly on the market and has, more or less, completely replaced the previously existing 5-speed transmission range. Within 2 years’ time, a complete transmission model range with 3 basic transmissions was entered into volume production at ZF. Continuously rising requirements relating to automatic transmissions have led to a technical revision of this transmission with the objective of significantly increasing customer-relevant parameters such as power-to-weight ratio, fuel consumption, and shift dynamics. Thanks to targeted optimization measures, the power transmission capacity was significantly improved, also see figure 14. Thereby, the already good power-toweight ratio balance is improved by approximately 15 percent. This aspect constitutes an active contribution to the extension of the scope of applications as well as the preservation of resources in the area of product environmental compatibility. A consistent shift strategy for the torque converter lock-up clutch (also called CC, converter clutch) helps to optimize fuel consumption and focuses on the aim of completely closing the CC as soon as possible. As a result, the development of a completely new, mechanical damper was necessary for the converter.

Figure 12: Electric Power Steering (EPS) offering new features. Due to high-power electric actuators, especially in chassis applications (figure 13), hybrid vehicles with higher battery voltages open up new dimensions for the reduction of current (today sometimes above 100 A); consequently, the efforts for cable wiring and power electronics are decreased as well. Concurrently, new performance categories are opened up which cannot be attained with today's 12 volts on-board network.

40

Engine torque (Nm)

increase of planets in the gear set 1, the diameter optimization of the main shafts, and the reinforcement of the ring gear of the Ravigneaux gear set. Also refer to figure 16, the example for the new automatic ZF 6 HP 28 6-speed transmission.
6 HP 34

6 HP 28 Torque (Nm)

6 HP 21

Figure 14: ZF 5- and 6-speed transmission range. 3.1 DEVELOPMENT TARGETS AND INNOVATIONS

For a sustainable CO2 reduction, fuel consumption was decreased according to the EU cycle by 3 percent in combination with combustion engines and by 6 percent in diesel engines. In order to support a vehicle weight reduction at the transmission-end as well, higher engine torques have to be served with smaller transmission models. Efficiency and driving dynamics benefit from a reduction of drag losses and inertia torques. Furthermore, the objective envisaged to establish a new automatic 6-speed transmission with shift dynamics which, until date, sporty drivers could only experience with vehicles featuring a manual transmission. This aspect also encompasses a reduction of the shift and response times as well as more spontaneous downshifts and a direct engine connection in order to improve agility, precision, and response. 3.2 INCREASED POWER-TO-WEIGHT RATIO

Weight (kg)

Figure 15: Weight of ZF automatic 5- and 6-speed transmissions.

The still continuing trend for higher engine torques used with diesel engines but also new injection and accumulator technologies with combustion engines make it necessary to further optimize the transmission's transfer capacities. On the one hand, this leads to a weight reduction for vehicle applications because higher engine torques can be served with smaller transmission models; on the other hand, the application restrictions for transmissions employed in more powerful applications are further expanded. Comprehensive testing with a large number of customers and different engine / vehicle combinations identified the limiting components with respect to rising torque and/or power transmission capacity. Thus, a targeted set of optimization measures could be launched in order to further improve the torque capacity of individual transmissions without significantly increasing the overall transmission weight (also refer to figure 15). Optimization primarily related to the increase of transmission capacities of the torque converter, the

Figure 16: ZF 6 HP 28 3.3 REDUCTION IN FUEL CONSUMPTION

The prime motivation for the revision of the automatic 6speed transmission by ZF resided in another reduction of fuel consumption and CO2 emissions. Thus, a major contribution to the self-obligation of the ACEA was to be made with 140 g CO2/km. Considerable improvements in the consumption cycle can be made by means of even earlier closing of the converter lock-up clutch (CC) and the thereto related reduction in driving shares with controlled e.g. slipping CC. One prerequisite for opening up this potential is to significantly improve the vibration behavior of the driveline so that early closing of the CC has no negative impact on the vehicle’s acoustics. Here, the objective is to establish a mechanical damping system which – at a very low moment of inertia - enables optimum rotary and torsional vibration decoupling. The

simulation of new damper designs in the engine – transmission network has generated the following result: There is no joint system for combustion and diesel engines which lives up to all the requirements. Individual, tailored converter concepts must be used for combustion and diesel engines which have one thing in common: The opportunity of reducing slip in the converter clutch in a targeted manner, as shown in figure 17. Thus, the new converters can make a considerable contribution to attaining the envisaged fuel consumption reduction and moreover, leads to a direct engine connection which helps to uplift agility and response.
KD Accelerator pedal in % 3-2 open =>closed-loop control open =>closed-loop control 2-3

Reviewed damper systems:
Drivetrain Output Drivetrain Output Drivetrain Output

without TD Engine Pump

with OD Turbine

with TTD Transmission

ZDW

Without torsion damper (OD)

Depicted as spring-mass systems:

Torsion damper (TD)

Turbine torsion damper (TTD)

Combination: TD+TTD = ZDW (Twin torsional damper)

Figure 18: Spring – mass systems. Detailed advantages and efficiency are not further examined within the context of the present elaboration. /1/ By means of the measures described, the CC engagement points can be reduced by up to 300 revolutions/min in relation to the throttle plate. All in all, the use of the converter with TTD and Twin TD technologies leads to fuel consumption advantages compared to the 6 HP model range for gasoline applications (approximately 3 percent) and for diesel applications (approximately 6 percent), also refer to figure 19. Premature closing of the CC and compensation of inertia torques by smaller converter models was also considered in order to meet the objective of improved driving dynamics, in addition to the target of reduced fuel consumption. If, from an objective point of view, acceleration data do not reveal a disadvantage, then, subjectively, the connection of the driveline to the engine was improved and the driver experiences a more direct response of the vehicle when changing the accelerator pedal setting.
100% 100%

ZDW TTD

closed-loop control =>closed closed-loop control =>closed

WK-KL 6HP (TTD) 3.G --- WK-KL 6HP (ZDW) 3.G

500

1000

2000

n_output in rpm

Figure 17: Consumption potential due to early closing of the converter lock-up clutch. Now, as a standard, and in combination with combustion engines, turbine torsion dampers (TTD) are used (also see figure 18). The turbine torsion damper interacts with a ring-spring package. Thanks to the coupling of the converter’s turbine mass upstream of the spring and a serial connection of the spring in combination with a comparably soft transmission input shaft, the principle of a single-mass oscillator is created where the natural frequency is below the drive range thanks to a special design. In combination with diesel engines, twin torsional dampers (Twin TDs) are used as starting elements, just as shown in figure 18. In the case of the Twin TD, the spring elements are located in a radial serial manner for both, the inner and outer side. Principle-wise, the turbine torsion damper is supplemented by another spring from the turbine. Thus, the individual rigidity of the single spring elements is added up reciprocally which, in turn, leads to a very low overall rigidity and to an improved decoupling from the engine’s rotational irregularity in the driveline.

3-Gang 3-speed

4-speed 4- Gang

5-speed 5-Gang

6-speed 6-Gang

new 6-speed 6-Gang TÜ

95% 95%

- 6%
90% 90%

- 2%

Fuel Verbrauch consumption
85 % % 85 80 % % 80

- 5%
diesel
Diesel

petrol Benzin

- 3%

- 6%

Premises
Level of amendment expenditure commensurate with best possible cost-benefit ratio
No worsening in objective performance figures

Figure 19: Contribution to the CO2 and/or fuel consumption reduction thanks to ZF transmissions. 3.4 IMPROVEMENT IN SHIFT DYNAMICS.

Just as before, automatic transmissions are in demand thanks to their excellent shift comfort. Many vehicle manufacturers attribute major importance to the sporty characteristics of their respective vehicles. In addition to the engine and the chassis, a sporty driving impression

is primarily determined by the transmission. Shift dynamics: A characteristic which – in addition to the number of gears and the gradation of the transmission – is considered highly important. With the further development of the automatic ZF 6-speed transmission range, shift dynamics have been considerably upgraded. Moreover, engineers have successfully reduced response and shift times by up to 50 percent. Shift dynamics is mainly determined by the values derived from response time and slip time. The response time is defined as the time elapsing from the output of the shift command by the driver (e.g. by tipping the shift paddle or triggering the command via the accelerator pedal) to the noticeable response of the engine speed. The slip time constitutes the amount of time elapsing from quitting the previous gear and attaining the point of synchronization of the new gear. Figure 20 depicts a typical shift sequence with the corresponding response and slip time. Both parameters are relevant for the subjective recognition of a shift process. Prolonged response times are recognized by the driver as unpleasant delays and a steep speed gradient supports the “more sporty” association linked to a shift process. Short response and slip times lead to the fact that in the case of a downshift, the higher tractive force of the lower gear is made available at a respectively earlier point in time. Both, subjective recognition as well as objective measurement values, are positively influenced.
Shift Command

contribution to the improvement of the shift dynamics is generated through the optimization of the interface between the engine and the transmission. Upon request, the transmission controls the engine torque i.e. by reducing it to a value that is close to zero or raising it up to maximum. The extremely high speed resulting from these torque modifications constitutes one of the key parameters for the optimization of shift dynamics. Improved response times of the further developed automatic ZF 6-speed transmission range is illustrated in the figure 21; here, various shift sequences are compared with the previous version and typical values derived from volume production dual clutch transmissions.
Comparison of response times Vergleich Reaktionszeiten
1200 6HP19 1000
Reaktionszeit in ms Response time in ms

6HP21 DKG

800 600 400 200 0
RS 32 SR S 43 SR S 54 SR S 65 SR S ZH S ZR S ZH S ZR S ZR S ZH S ZH S 21 23 34 45 56 32 54 21 S 53 ZR S 42 43 12 65 64 ZR S ZH S ZR S ZR S ZR S

Figure 21: Response times of the further developed automatic ZF 6-speed transmission. 3.5 EXPERIENCE AND OUTLOOK

Turbine engine speed

Sports shift

Comfort shift

Response Duration of time change in ratio

Time (ms)

Example of traction downshift

Time (ms)

Figure 20: Response and slip time of automatic transmissions. In order to attain a high level of shift dynamics, measures implemented in the field of hydraulics, transmission mechanics, and interfaces, as well as in the vehicle became necessary. In the case of the transmission’s hydraulics for example, an independent pressure control unit is used for each clutch to be actuated. Thus, highly flexible shifting can be performed at any given point in time. Furthermore, the clutch fill volume was reduced in a targeted manner. Intelligent fill models (i.e. clutch filling adapted to the type of shift, comfort or sports) and double downshifts have led to further functional optimizations. An important

In particular with reference to the requirements in the fuel consumption and emissions sector, it is essential to continuously further improve components in the driveline and to consistently implement measures for the efficiency optimization. Simultaneously however, it is about making the transmission more attractive for the user and to generate an extra plus with regard to dynamics and consequently driving & riding pleasure. On the basis of a technical revision of the present model range, ZF succeeded in improving the automatic 6speed transmission with regard to customer-relevant aspects and to take the lead vis-à-vis competitors and their respective, comparable transmission systems. A new generation with a set of further innovations and additional improvement of customer-relevant parameters such as e.g. fuel consumption and driving & riding pleasure will be developed until 2010.

Longitudinal acceleration

4.

LONGITUDINAL AND LATERAL DRIVE TORQUE DISTRIBUTION

Current innovations in the all-wheel driveline relate primarily to variable, driving situation-dependent longitudinal and lateral torque distribution. Here, major attention is given to the Torque-On-Demand transfer case (see Item 4.1) and the Torque Vectoring axle drive

(see Item 4.2). These innovations feature the great potential to equally augment driving pleasure, driving safety, and riding comfort. Active driveline and chassis systems help to adjust wheel force more flexibly than before, even in an individual wheel approach (see figure 22).

4.1

TORQUE-ON-DEMAND TRANSFER CASES

drive

torque

individ

ualiza

tion

The A 350 DAC (DAC: Dynamic All-Wheel Control) transfer case by ZF shown in figure 24 can actively split the input power between the vehicle’s front and rear axles as required depending on driving mode. This capability improves the vehicle’s handling, agility, and safety. Power splitting between the front and rear axles is provided by an electromechanical actuation device and a wet-operation multidisk clutch via the front axle (hang-on principle).
AWD Driveline: A 350DAC:
Multi-disc clutch Gear drive Ball screw

steering angle individualization

u individ l load Whee

o alizati

n
Input Output RA

Sketch: RA

Figure 22: Intelligent wheel dynamics (iWD) based on individualization of longitudinal, lateral, and vertical wheel forces. For example, the advantage of such systems resides in the fact that the drive torque, and, consequently the required friction value, is optimally distributed among the wheels, and thus ensures that the required friction value is the minimum value. As shown in figure 23, the combination of variable drive torque distribution with an active steering system helps to reduce the required friction value for a defined cornering sequence from 0.74 to 0.60. As a result, the vehicle operates at higher safety reserves.
Conventional Steering with drive torque Optimized wheel forces

Output FA

FA

Electric motor

Figure 24: ZF’s torque-on-demand transfer case A 350 DAC. The clutch’s multidisks are pressed on according to the required level of power splitting by means of a brushless and therefore zero-wear electric motor integrated in the idler gear and an actuator gear, consisting of spur gears and a spherical spindle. Featuring great integration credentials, this transmission is characterized by its excellent power-weight ratio and its compact design. High levels of actuation dynamics with short response times and a high level of control quality with precise adjustment of clutch torque control are provided in this transmission thanks to appropriate activation and control algorithms adapted to the hardware. There are two variants of the all-wheel transfer case, see figure 25. In the first version, the transfer case has its own separate housing and an actuation unit powered by electric motor. It is flange-mounted on the automatic transmission. The transfer case in the second version is integrated in the automatic transmission’s housing. It is hydraulically actuated and features a shared oil circuit which in turns means that even less installation space is needed and its weight is reduced yet further. Oil cooling is greatly enhanced by the shared oil circuit making permanent front axle torques possible, also at high speeds. The oil cooling is also independent on road speed and is also possible during reverse travel.

ratio:

μerf

|F| = N vl 0.34 0.31 vr hl 0.65 0.78 hr vl 0.57 0.38 vr hl 0.60 0.57 hr

vl 0.74 0.48 vr hl 0.63 0.35 hr

Figure 23: Longitudinal and lateral drive torque distribution enables optimum use of the road surface friction value.

Add-On
861 782

24kg

The transmission system shown in figure 27 consists of two electromechanically activated multidisk clutches with carbon friction disks. By means of the prevailing ratio and a spherical ramp system, an asynchronous motor presses the multidisks on to the relevant clutch depending on the nominal level of torque to be transferred.
Bremsmoment rechts Braking torque

Integration

AT + 19kg
right

Figure 25: Combination of automatic transmission and transfer case. Add-on (top) and integrated (bottom) version of transfer case. 4.2 LATERAL DRIVE TORQUE SPLITTING
Torque Distribution in the differential Momentenfluss durch Differenzial Torque Distribution including TV-Einheit Momentenfluss bei Bremsbetätigungthe torque vectoring unit

Several options are available for individual wheel splitting of drive torque on the rear wheels. One system, comparably simple in terms of its mechanics, for the lateral splitting of input torque is the rear axle drive with individual wheel torque distribution, see figure 26. This drive includes a differential plus two additional clutches which split the torque available between the wheels of the rear axle as necessary depending on operating mode.
Torque Vectoring axle gear
in

Figure 27: Variable drive torque distribution at the rear axle. Torque flow via the differential and the superimposed unit. 4.3 NETWORKING LONGITUDINAL AND LATERAL DRIVE TORQUE DISTRIBUTION (TORQUE VECTORING)

out

out

output input electric motor

Torque-vectoring unit right

Torque-vectoring unit left

The Torque-On-Demand transfer case and the Torque Vectoring axle drive both impact traction and driving dynamics. Thus, functional networking becomes indispensable. Particularly in the driveline and chassis, a major share of advantages can only be generated when networking the units in a system: • First: Multiplexing makes it possible to incorporate additional levels of functionality which standalone systems were unable to offer. • Second: Comprehensive coordination helps to prevent undesired interaction effects within the system. There are several aspects to networking an all-wheel transfer case and torque vectoring axle drive: • Firstly, the systems are networked to gain optimum traction. In other words, if one of the two systems detects slipping, this is taken into account in activating the other component. For example, if the torque-on-demand transfer case puts more drive torque to the rear axle, the torque vectoring axle gear uses this information in the feed forward control of its clutches. A connection with brake and engine interventions rounds off the traction optimization feature. This realizes a traction control going beyond what is known today.

Differential

Figure 26: Dual clutch rear axle transmission without differential gear. By using what it is essentially wheel load-dependent splitting, traction can be improved not only for straightahead travel but also when cornering. Agility is increased at the same time. By increasingly splitting the torques on the rear axle asymmetrically to the wheel on the outside of the bend, torque turning into the bend can be exerted on the vehicle. If both clutches are locked, a blocked rear axle differential can be simulated. The clutch to the wheel on the outside of the bend is normally closed and the splitting of torque of the rear axle is controlled by the slipping clutch on the inside of the bend. In situations where for example oversteering needs to be avoided, the torque on the inner clutch is increased and the torque on the clutch on the outside of the bend reduced, thus producing a stabilizing torque turning out of the bend.

• Secondly, the systems are networked in order to set the input torque to be as individual as possible for each wheel. In the general spirit of torque vectoring this realizes maximum stability and agility as shown in figure 23 and figure 28. This allows for braking interventions to be limited to very critical driving maneuvers. • Thirdly, these systems are networked with the brakebased drive dynamic stabilization intervention. In other words, when stabilizing brake interventions occur, the transfer case and the torque vectoring axle drive support the control objectives by using the torque split (in both traction and coasting modes) to achieve supportive self-steering characteristics. • Fourthly, if we network the torque vectoring systems with the active steering or a steer-by-wire-system, then yaw torques, which arise during traction optimization, can be balanced as part of yaw torque control. For example, when applying maximum braking power on a µ-split surface, automatic intervention in the steering system can prevent the vehicle from being pulled sideways. On the other hand, the brake intervention can be harmonized more strongly which, in turn, will lead to a reduction in braking distance.
Steering Angle [°] 80 60 40 20 0 -20 -40 -60 -80
ESP brake intervention Torque Vectoring car

such as management of the main power supply, main supply faults, bus communication, vehicle status management, diagnosis, and flashing behavior. Such services are similarly complex to drive dynamics functions. Moreover, they require just as thorough and comprehensive test series. Figure 29 provides for an overview on the functions and aspects to be tested.
component test bench component test bench network test bench network test bench

diagnosis error detection error handler safety functions code variants network management power supply flash communication (CAN, ...)

distributes functions network reaction in case of error feedback control behavior car configuration variants diagnosis

Figure 29: Hardware-in-the-loop tests at the components and networked systems test bench. The ZF hardware-in-the-loop test bench established for networked control units comprises all major driveline and chassis systems. This includes the following range of units: The automatic 6HP transmission (automatic 6speed multi-ratio transmission), the transfer case (VTG/all-wheel drive transfer case), the Torque Vectoring rear axle drive (TV-HAG), the superimposed steering system (AFS/Active Front Steering), the active stabilizer (ARS/Active Roll Stabilization), the variable damper (CDC/Continuous Damping Control), the leveling system based on airspring technology (NIV/Leveling System) and finally, the brake control unit (ESP/Electronic Stability Program), also refer to figure 30.

no correction necessary! Safety

smaller steering angle optimized handling

1

1.5

2

2.5

3

3.5

4

4.5

5

Figure 28: Hardware-in-the-loop test of networked driveline and chassis systems. 4.4 HARDWARE-IN-THE-LOOP-TEST OF NETWORKED SYSTEMS

Major efforts are required within the vehicle in order to “merge” the different components. Often, the first time they interact at all is in a test vehicle. Here, the complexity of the individual control units must be mastered but also the one of the control unit network. As a consequence and as soon as possible, the networked system must be tested on a hardware-in-the-loop test bench for networked control units. The failsafe provided for the control unit network not only includes the drive dynamics functions but also aspects

Figure 30: ZF’s hardware-in-the-loop test bench for networked driveline and chassis systems.

5.

CONCLUSION

Moreover, the hybrid drive opens up new opportunities for fuel consumption reduction and is consequently at the center of attention for the time being. But also conventional driveline and particularly all-wheel drive configurations provide for manifold opportunities to actively reduce CO2 emissions, also refer to figure 31.
Engine: Direct injection (high pressure), variable valve control, variable charging, cylinder cutout, alternative fuels, ... Higher torques and efficiency Transmission: Wider ratio spread, optimized shift strategies closed-loop starting control, more gears, … Better efficiency rating and engine adaptation

The Torque-On-Demand transfer case and the TorqueVectoring axle drive open up new ways for driving dynamics stabilization and agility increases by means of variable drive torque distribution. Networking of the driveline and chassis systems has become a necessity because of the increasing functionality and complexity of the systems. The power consumers in these systems are coordinated through power management based on actual driving situations. Optimum longitudinal and lateral vehicle dynamics is achieved on the basis on the car’s actual electric power reserve. ZF has already set up various test units with innovative, networked all-wheel and driving dynamics functions in order to highlight their inherent advantages and to provide for highly innovative system solutions.

REFERENCES:
Solutions call for a holistic, integrated approach Hybrid: Recuperation of braking energy, start/stop, driving and setting off using electrical power, … Higher utilization of fuel and better dynamics

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Figure 31: Solution-finding strategies for future requirements vis-à-vis performance and fuel consumption. The future of hybrid vehicles will be primarily determined by accumulator technologies. Thus, it represents the major ”unknown factor“ when it comes to the interdependence of the various components. Here, research institutes and suppliers are called upon to provide input. Major investments made in this sector will, in parallel, open up major market opportunities. As a result, the automatic transmission will generate further fuel consumption reduction percentages until 2010, also see figure 32. For the all-wheel drive, the Torque-On-Demand transfer case will also provide potentials for reducing fuel consumption as such.
3-speed
100%

ZF Getriebe GmbH, H. Hörmann: Die technisch verbesserten ZF 6-GangAutomatgetriebe - Verbrauchsoptimiertes Fahren mit sportlicher Schaltdynamik. (EN: Technically enhanced automatic ZF 6-speed transmissions – fuel consumption-optimized driving with sporty shift dynamics.) VDI reports no. 1943, 2006, pages 697-718. BMW AG, Dr. C. Breitfeld: Die überarbeiteten SechsgangAutomatikgetriebe von BMW (EN: The further developed automatic 6-speed transmission by BMW) VDI reports no. 1943, 2006, pages 719-735. Krimmel, Horst; Deiss, Harald; Runge, Wolfgang; Schürr, Heinrich, “Elektronische Vernetzung von Antriebsstrang und Fahrwerk” (EN: Electronic networking of driveline and chassis), Automobiltechnische Zeitschrift [automotive magazine] (ATZ), page 368, May 2006. Rosemeier, Thomas; Granzow, Claus; Peter, Robert, “Besseres Fahren durch aktive Längskraftbeeinflussung“ (EN: Improved driving thanks to active, longitudional force management), congress on transmissions in vehicles 2006, Friedrichshafen, June 27-28, 2006. Schürr, Heinrich; Kutsche, Thomas, “Elektronisch gesteuerte Federbeinsysteme“ (EN: Electronically controlled spring strut systems), 4th All-Wheel Drive Congress in Graz, February 13-14, 2003.

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4-speed

5-speed

6-speed

6-speed opt.

Model year 2010

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95%

- 6%
90%

- 2%

85 %

- 5% - 3%

*)

80 %

- 5%
*) For diesel applications, cost reduction of 6 %

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Figure 32: Further development of ZF automatic transmissions.

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Deiss, Harald; Krimmel, Horst; Maschmann, Oliver; “Hardware and Software-in-the-LoopTests“, Congress Virtual Vehicle Creation 2006, Stuttgart, March 14-15, 2006. Vahlensieck, Bernd; Sattler, Martin; Blome, Frank; „Hybridantriebe: Konzepte und Lösungen für PKW und NKW“ (EN: Hybrid drives: Concepts and solutions for passsenger cars and commercial vehicles), VDI transmission conference 2006, Friedrichshafen, June 27-28, 2006. Paul, Michael; Vahlensieck, Bernd; „Bewertung und Auswahl von Hybridantriebssystemen für unterschiedliche Fahrzeugkonzepte“ (EN: Evaluation and selection of hybrid drive systems for various different vehicle concepts), 10th annual meeting of the German Handelsblatt magazine: Driveline and vehicle concepts, Stuttgart, June 1, 2006.

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