04CVT-40

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04CVT-40

Development of High Performance CVT Components

A. Englisch, H. Faust, M. Homm, A. Teubert, M. VornehmLuK GmbH & Co. oHG, Bhl, Germany

Copyright 2004 SAE International

ABSTRACT

Based on the mass produced CVT components for350 Nm and ratio coverage of 6, the advances of thefollowing LuK components are described: the pulley sets,

the hydraulic control with integrated pump and the chainwith guide rail. The potential ranges from 400 Nm withthe new fully variable torque sensor and optimized chainto 550 Nm and ratio coverage 7 in power splitgearboxes. The further investigations on hydraulics andon powersplit hybrid design will also be shown.

INTRODUCTION

A comparison of the recently launched 6-speedautomatic and dual clutch gearboxes with the manualtransmission gearbox reveals that not all theexpectations for these gearboxes in terms of perfor-

mance and efficiency are met. The 6-speed automatictransverse gearbox for example, exhibits a significantlyhigher consumption in the MVEG in the order of 12 %,Figure 1. The 6-speed automatic for rear-driven vehiclesexhibits, depending on the engine, higher consumptionsin the order of 9 %, with decreasing tendency with biggerengines.

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multitronic d iesel multitronic gas 6 AT east west gasdual clutcht/m diesel dual clutcht/m gas 6 AT rear wheeldrive diesel

6 AT rear wheeldrivegas

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Fig. 1: Fuel consumption of AT/DCT/CVT compared to MT

Compared with manual transmissions, consumption withthe new dual clutch gearboxes increases by some 7 to

12 %. Only the 3.2 l petrol engine, in comparison to thesportive, short geared, 6-speed manual transmission,can achieve significant fuel savings in the MVEG drivingcycle.

By contrast, multitronic

[1, 2] exhibits at least neutralfuel consumptions on diesel and an approx. 3 %advantage on petrol engines. In terms of combining fuelconsumption and comfort, this gearbox continues torepresent the optimal design.

multitronic diesel multitronic gas 6 AT east west gasdual clutcht/m diesel dual clutcht/m gas 6 AT rear wheeldrive diesel

6 AT rearwheeldrivegas

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0 50 100 150 200 2500 50 100 150 200 250engine power in kW

Fig. 2: Acceleration of AT/DCT/CVT compared to MT

The primary objective when developing the dual-clutchgearbox was to create a sports "automatic" gearbox andthus improve driving dynamics and driving pleasure.Comparing the acceleration time of 0-100 km/h withthose of the manual transmission, we see that despitethe absence of a traction interruption, no significantimprovement has been achieved. The 6-speedtransverse automatic gearbox reduces driving per-formance by between 6 % and 10 %. The drivingperformance achieved with 6-speed automatic gear-boxes for rear-driven vehicles only approximates that ofmanual transmissions when combined with powerfulengines.

By contrast, multitronic offers the benefit of continuouslyvariable transmission even with less powerful engines

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and driving performance approximates that of themanual transmission.

The following features contribute to the impressiveperformance and consumption values of the CVTcomponents supplied by LuK [6]:

Chain with low internal losses and high torquecapacity with wide ratio spread and low minimalrunning radii, without limiting the max. vehicle speed;

pulley sets with LuK double piston principle with theclamping cylinders being directly hydraulicallyconnected and when adjusted, only the small ratioadjustment cylinders are to be operated by thepump;

hydro-mechanical torque sensor with dual stageresponse curve modified according to the ratiocontrolling the clamping pressure at high dynamicswithout external signals;

hydraulic control with small number of valves, narrowgaps and small pump with gap compensation.

In summary, LuK believes that the CVT with the structureselected for multitronic

represents the best combination

in terms of driving comfort, driving performance andconsumption. Thanks to the LuK double piston concept,the shift times are limited practically only through theengine's capacity for self acceleration, which, similar tothe dual clutch principle, also offers to the ambitioussports driver an adequate solution.

The aim of continued development is to further increasetorque capacity and at the same time, further improve

overall efficiency by optimizing the components.

VARIATOR DESIGN

Figure 3 shows the variator with primary-side hydro-mechanical torque sensor.

Fig. 3: Pulley sets with double piston and dual stage torque sensor

The response curve of the dual stage torque sensor isautomatically switched between the two differentcharacteristics through the position of the axiallymoveable sheave by means of opening and closing twobores [6]. The simultaneously employed double pistonprinciple ensures that the outward-lying pressure cylinder

of the driven pulley set and the ratio adjusting cylinder onthe driving side suffer no leakage which would otherwiseoccur in the sliding seats.

FULLY VARIABLE TORQUE SENSOR FOR POWERENHANCEMENT

To further increase the torque capacity to 400 Nm ofvariator torque, in addition to optimization measures onthe chain and on the tribological system of chain andpulley surface, a new type of torque sensor with a fullyvariable characteristic map curve even over the ratio isbeing developed [7], Figure 4. Within certain ratioranges, this torque sensor relieves the chain, the

tribological contacts, the disks, the shafts, the bearings,the housing structure and finally also the pressure levelfor pump, hydraulic control and pressure gaskets.Besides the higher torque capacity, the efficiency of thevariator is improved and the power to drive the pump isreduced. The result is further enhanced drivingperformance and better fuel consumption, as well as areduced cooling requirement.

Fig. 4: Primary pulley set with fully variable torque sensor (VTS)

The key to the functional principle is that the two activeramps of the VTS exhibit variable tangential angles overthe radius, Figure 5. These continuously altering anglesact over the radial position of the balls, which transfer thetorque from the driving to the driven side ramp. The ballsare positioned radially in accordance with the actualvariator ratio via guiding surfaces, which are arranged onthe axially moveable sheave. The desired map curve iscreated through an appropriate ramp contour andguiding surface configuration.

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Fig. 5: Fully variable torque sensor as drawing and exploded view

By integrating the entire ball/ramp mechanism, includingball guides, into the pressure cylinders, the pulleybecomes very compact as the length of the moveablesheave is used for the ball mechanism. The initialdurability runs at 400 Nm variator torque have confirmedthe endurance capability of the VTS and the associatedpotential for increasing the torque applicability of theentire CVT system. The achievements of the dual stagetorque sensor, such as high accuracy, dynamics androbustness, e.g. with respect to chip tuning ofturbocharged engines, remain fully sustained. As anexample, Figure 6 displays the measurement with torque

jumps on the test rig demonstrating the reliable function,also in respect of the powertrain vibrations occurring withthis maneuver as they would in the vehicle with about100 Nm of dynamic overtorque.

Fig. 6: Measured behavior of the fully variable torque sensor when

exposed to fast torque steps

HYDRAULIC COMPONENTS

PUMP CONCEPT

The LuK dual flow vane pump can make a furthercontribution towards fuel economy. The use of a two-

stroke ring contour can achieve double take-in /expulsion per revolution. The pump is therefore verysmall and suitable for an arrangement as a compactpump in the gearbox. In terms of hydraulics, each half ofthe pump is a pump in itself that can operate at differentpressure levels, Figure 7.

n

Q A+B(constant-pump)

Q B

Q demand

Advantage:

+ good adjustment to demandflow+ compactstyle

Disadvantage:

- minor increased hydraulic effort

QB

Q

QA

Q

n

Q A+B(constant-pump)

Q B

Q demand

Advantage:

+ good adjustment to demandflow+ compactstyle

Disadvantage:

- minor increased hydraulic effort

QBQB

Q

QAQA

QQ

Fig. 7: Dual flow vane pump

In the simplest case, one pump half can be switched tolow pressure operation as a function of rotation speed.Altering the strokes on the contour ring also allows thepump to be split asymmetrically. Thus, a dual flow vanepump enables three different delivery volumes to beprovided: QA, QB and QA+QB. The selection function canbe integrated into the hydraulic control with small extraeffort. When a pump flow is brought onto or eliminatedfrom the system, an abrupt change occurs in thevolumetric flow. In order to avoid disruptions in thecontrol pressure, the relevant valves must equalize thechange in volumetric flow at high speed by assuming anew operating position.

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Fig. 8: Measured switching behavior of the dual flow vane pump

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In the measurement shown in Figure 8, one pump flow ofapprox. 53 bar system pressure is switched over toapprox. 3 bar back pressure. This barely affects thesystem pressure. To prevent toggling when driving withinthe shift range, a shift hysteresis is also employed.

Fig. 9: Torque of different pump concepts at p=20 bar, T=90 C,Q(1000 rpm) = 10.5 L/min

Figure 9 shows a comparison of various pump principleswith regard to torque pick-up over pump speed. Allpumps have been standardized so that at a speed of1000 rpm and a pressure of 20 bar, the delivery rateamounts to 10.5 L/min. In this example, it is assumedthat the volumetric flow of 15 L/min is sufficient to supplyall functions of the gearbox control with an adequateamount of oil. The most suitable pump must be selectedbased on gearbox concept, vehicle and engine. Theeffect of the pump's power draw on maximum enginespeed is significant, especially with petrol enginescapable of high rpms, and justifies the slight investmentin a dual flow vane pump. With a theoretical pumpdelivery volume of 10.5 cm3 and a pressure of 20 bar, anapprox. 1200 Watt saving can be achieved in enginepower and at the very end also in cooling capacity.

SLIP-CONTROLLED CLAMPING PRESSURE

Electronic engine control units still fail to provide preciseinformation on current torque and therefore, electro-

hydraulic claming systems without torque sensor, requirefor safety reasons a high excess clamping pressure.Using a detection of slip between chain and pulley set,the actually required clamping pressure can bedetermined and excess clamping can thus be reduced toa minimum.

The algorithm for slip detection pre-supposes that theclamping pressure is modulated [8, 9]. The idealfrequency range for the modulation within which nocomfort problems are to be expected and no resonancefrequencies occur, is between 20 and 70 Hz.

electroniccontrol unit

solenoidvalve

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Fig. 10: Hydraulics for slip-controlled clamping systems with pressure

modulation

Figure 10 shows the part of the hydraulic control unit thatprovides such a clamping pressure system. Lowhysteresis, high dynamics, high stability in respect ofhydraulic vibrations and high reproducibility of thepressure increase and decrease are the salient require-ments. With this hydraulic concept, the modulatedclamping pressure is realized using a pressure limitingcontrol valve. The pressure valve is permanentlysupplied with a volumetric flow, which is never drained tothe sump but used for cooling and lubricating thegearbox. The advantage of this arrangement is that thepressure valve, both for the pressure increase andpressure decrease, is always at the same control edgeand thus enables high dynamics.

A further aspect is the available volumetric flow. For thepressure increase, the pressure valve closes the controledge and routes the oil supplied by the pump to thepulley sets. As their pressure chambers exhibit a certainelasticity, volumetric flows of up to 10 L/min are brieflyrequired in order to achieve a fast pressure build-up inthe pulley sets. As the pressure drops approx. twice thevolumetric flow is pumped over the pressure valve to thecooling and lubrication circuits and is therefore retained.The presented hydraulic concept is able to satisfy the

dynamic requirements for pressure modulation withoutthe drawback of leakage.

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inbar

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Fig. 11: Test-rig measurement of a slip-controlled clamping system

The left area of the test rig measurement, Figure 11,shows the reaction of the filtered modulated pressure toa change of target slip measure. In the right area, atorque change causes the system to increase theclamping pressure by closed loop control in order tomatch the constant slip target.

LUK CVT CHAIN

Optimization of the chain performance starts withcomponent design and continues with optimization of the

chain assembly. Thanks to highly-developedcomponents and established design procedures, therequirements of a wide range of applications can besatisfied within short periods of time.

CHAIN COMPONENTS

The LuK chain consists essentially of individual linkplates and rocker pins. The aim of developing theseparts is to create components optimized in terms of bothpackaging and strength that can still be efficientlyintegrated into the production process. This complexoptimization process requires certain design tools. Theseinclude commercial FEM programs and also program

extensions for structure or form optimization. However,calculation tools that can cope with the special featuresof the CVT drive are not always available on the marketand specialised software must therefore be developed,e.g. as used by LuK to describe dynamic phenomena.The design tools are completed by the necessary testtechnology.

Through the consistent application of all available tools, ithas been possible to develop an optimized link plategeometry from the current standard production status,which exhibits significant benefits in terms of packagingand strength, Figure 12. Despite an approx. 10 % lower

link weight and 10 % less height, it has been possible tosignificantly increase strength. Chains made from theselink plates displayed an increase in strength of approx.

10 % in the rig test. This new component generation,referred to as the LK 08, is soon coming to massproduction.

Fig. 12: Link plate of the chain: comparison between current series

and new light design

CHAIN ASSEMBLY

The chain characteristics depend not only on theindividual components, but also on how thesecomponents are assembled to form a complete chain.

The key parameters for the chain's strength are thenumber of link plates in one link and the optimizedarrangement of these link plates. Software can be usedto find link arrangements that achieve a balanced loadon the individual components. This includes a favorablecombination between link and rocker pin load.

The measures described above help to create an optimalchain design for the specified application. Themodification of existing chain and/or components to suitalternative applications, e. g. [3], results in the chainbeing composed of many identical components. Byaltering the composition, the characteristics and

installation conditions for the chain can be fundamentallychanged with high confidence due to the use ofextensively tried and tested individual components.

CHAIN TYPES

However, if the requirements deviate too far from theprevious ones, it is advisable to even modify theindividual components. In order to expand the applicationspectrum to smaller torques, LuK is currently developing,besides the aforementioned new link geometry LK 08, asmaller scale link type LK 07. This is based on thedescribed experiences and optimization procedures andpermits an even better modification to variators fortorques below 200 Nm.

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The structured procedure described above enables awide range of applications to be served within shortperiods of time in line with requirements. Figure 13shows how this modular system can be used for varioustorque categories as a first indication of the chaindimension to be expected. The external conditions,

including the collective load, are considered and used tofine tune the chain to suit precise requirements. Inaddition to the chain widths stated, other intermediatevalues can easily be prepared on request.

LK37 08

LK33 08

LK24 07

LK28 08

LK26 08

link plate type LK 07

link platetype LK 08

100 200 300 400

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chainwidthinmm

engine torque in Nm

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LK33 08

LK24 07

LK28 08

LK26 08

link plate type LK 07

link platetype LK 08

100 200 300 400

24

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33

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chainwidthinmm

engine torque in Nm

Fig. 13: Recommended chain types for different torque applications

(pulley center distance 170 mm, ratio coverage 5.5 6)

SYSTEM-OPTIMIZATION ON POWER-SPLIT CVTAND HYBRID CVT

Power-split gearboxes [7] are a feasible option for torquevalues of above 400 Nm. The degree to which thesegearboxes can be realized, however, also depends onthe variator load and specifically the chain tensionsand/or stress determined by their structure. Figure 14 tothe left shows one beneficial structure of this gearboxdesign.

Fig. 14: Gearbox structure and resulting variator power load of a

power-split CVT.

The power is split at an input planetary gear set into two

paths, where depending on ratio the variator has totransfer only 35 70 % of the drive power. This isshown in the right-hand part of the diagram. The driven

side is coupled via one of the two couplings K1 or K2 toone of the two variator shafts, where the outputs then re-merge. A reverse gear can be implemented in thedownstream gearbox. Unlike similar nonsplit CVT, forsuch structures a significant torque capacity increase to550 Nm with simultaneous ratio spread expansion to 7 is

expected.

COMBINATION OF POWER SPLITTING,CONTINUOUSLY VARIABLE TRANSMISSION ANDHYBRID FUNCTION

For high performance engines, a potential innovativestep points towards the hybrid with a small electric motorcompared to the capacity of the combustion engine.Figure 15 shows such a structure, on which the electricmotor EM has an arrangement that is different to thoseof conventional starter - generator concepts.

Fig. 15: Gearbox-structure and resulting variator load of a hybrid

power-split CVT.

With KB clutch closed, several established hybridfunctions including generation, recuperation and boostcan be performed with the electric motor. An engine startis also possible if a mechanical minimum clamping of thevariator is present and/or the pump is coupled with theelectric motor. The electric motor can also be usedduring transitions between the two infinitely variableoperating ranges. In this case, comfort can be improvedby feeding the power for the required change ofacceleration of internal gearbox rotating masses directlyinto the gearbox by the electric motor. This allows the

tractive power acting on the wheel to harmonise.

In the event of a slipping or open clutch KB, a furtheradditional function is provided by the electric motor whenthe vehicle is at standstill: If power from the combustionengine is transferred to the electric motor via theplanetary gear within the sense of a generatorfunctionality, the planetary gear generates a positivesupport torque at the gearbox output. This torque can beused with the clutch K1 closed as creep torque or foracceleration in case of consumption-relevant part-throttlemove aways. This torque is therefore not generated byfriction but is gained by battery-charging without

additional energy expenditure. The gearbox thusreceives, completely without impacting the variator, anexpansion of the move away gear ratio, which could be

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referred to as generative partial load geared neutral. Athigher loads, the drive torque is increased by usingclutch KB, whereby the demand-driven and non-hysteresis controllability of the electric motor permitsoptimized initial acceleration, driving comfort andsportiness.

With such gearboxes, the requirements for "conversionof mechanical power" and "electrical energystorage/supply" are met by distinct technologies. Theconversion is primarily mechanical at low cost per kW,low weight per kW and high efficiency of a chain CVT,while the electric motor provides energy storage / supplyas its main function and assists the conversion only to asmall extent. The consumption advantage achievable inthe MVEG alone through elimination of clutch slip underfriction torque at standstill and start-up can be in excessof 0.2l/100km.

SUMMARY

Continuously variable transmissions, especially in thestructure selected for multitronic

with double piston

principle and torque sensor, continue to represent thebest combination in terms of driving performance andconsumption compared with double clutch gearboxesand conventional stepped automatic transmissions.

The newly developed, fully variable torque sensorexpands applicability to up to 400 Nm in gearboxstructures without power split. The application of the VTSoptimizes the clamping characteristic map, which leadsto lower loading of the overall system and increases

variator efficiency. As an alternative, the illustratedhydraulics enables high-quality pressure modulation for aslip-controlled clamping, which at stationary operatingpoints reduces overclamping to a minimum. The dualflow vane pump reduces the power spent on pump drive,in order to improve the overall gearbox efficiency,especially at high engine speeds.

Thanks to optimized "light" links, chain LK 08 can beused for new applications in higher torque ranges of upto 400 Nm or as chain LK 07 assembled of finer links inthe segment below 200 Nm.

The combination of power split CVT and electric motorcreates new solutions for cutting fuel consumption inhigh-performance vehicles with continuously drivingcomfort.

REFERENCES

1. Nowatschin, K.; Fleischmann, H.-P.; Gleich, Th.;Franzen, P.; Hommes, G.; Faust, H.; Friedmann, O.;Wild, H.: multitronic

Das neue Automatikgetriebe

von Audi. ATZ 102 (2000), H. 7/8, S. 548 - 553, H. 9,S. 746 753;

[multitronic

The New Automatic Transmission

from Audi. ATZ worldwide, no. 7-8/2000, p. 25 27,no. 9/2000, p. 29 31].

2. Nowatschin, K.; Fleischmann, H.-P.; Franzen, P.;

Hommes, G.; Faust, H.; Wild, H.: multitronic

Dieneue Getriebegeneration von AUDI. [AUDImultitronic

The New Generation of Automatic

Transmission]. VDI Berichte Nr. 1565 (2000), S. 195 213.

3. Wagner, G.; Remmlinger, U.; Fischer, M.: CFT30 A Chain Driven CVT for FWD 6 Cylinder Application.

2004 SAE World Congress, Detroit, Michigan, March8-11, 2004. SAE Technical Paper Series 2004-01-0648.

4. Fleischmann, H.-P.; Gutz, H.; Kumpf, G.; Martin, F.;Schffmann, M.: Die neuen Getriebe im Audi A6.

ATZ 106 (2004) Sonderheft Audi A6, S. 128 138.5. Audi [Ed.]: Audi A4 Details, September 2003.

6. Faust, H.; Linnenbrgger, A.: CVT Development atLuK. 6

thLuK Symposium (1998), p. 157 - 179.

7. Englisch, A., Faust, H., Homm, M., Teubert, A.,Reuschel, M., Lauinger, C.: Entwicklungspotenziale

fr stufenlose Getriebe. ATZ 105 (2003), H. 7/8,S. 676 - 685.[Potentials for the Development of Continuously

Variable Transmissions. ATZ worldwide, no. 7-8/2003, p. 11 14.]

8. Faust, H.; Homm, M.; Bitzer, F.: Efficiency-OptimisedCVT Clamping System - Reduction of FuelConsumption through Increased Slip? 7

thLuK

Symposium (2002), p. 75 - 87.9. Faust, H.; Homm, M.; Reuschel, M.: Efficiency-

Optimised CVT Hydraulic and Clamping System.CVT 2002 Congress, Mnchen, 7./8. Oktober 2002,

VDI-Berichte Nr. 1709 (2002), p. 43 58.10. Wagner, U., Teubert, A., Endler, T.: Entwicklung von

CVT-Ketten fr Pkw-Anwendungen bis 400 Nm.

Getriebe in Fahrzeugen 2001, Tagung, VDI-BerichteNr. 1610 (2001).

11. Indlekofer, N.; Wagner, U.; Fidlin, A.; Teubert, A.:Latest Results in the CVT Development. 7

thLuK

Symposium (2002), p. 63 - 72.12. Englisch, A.; Lauinger, C.; Vornehm, M.: 500 Nm

CVT LuK Components in Power Split. 7th

LuK

Symposium (2002), p. 91 - 103.13. Englisch, A., Lauinger, C., Vornehm, M., Wagner, U.:

500 Nm CVT LuK-Components in Power Split.

CVT 2002 Congress, Mnchen, 7./8. Oktober 2002,VDI-Berichte Nr. 1709 (2002), p. 147 163.

CONTACT

Dipl.-Ing. Dipl. Wirtsch.-Ing. Andreas EnglischManager Product Line CVTLuK GmbH & Co. oHGBussmatten 2D-77815 BhlPhone: +49 7223 941 [email protected]

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04CVT-40