COVER You STORY will find

Dual mentioned Clutch in Gearbox the figures this article in the German issue of ATZ 11/2003 beginning on page 1022.

Das neue Doppelkupplungsgetriebe von Volkswagen

The New Dual Clutch Gearbox from Volkswagen

By Wolfgang Schreiber, Falko Rudolph and Volker Becker

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Unifying the advantages of the manual gearbox and the stepped automatic gearbox was the development target for the new dual clutch gearbox of Volkswagen. Based on the principle of the dual clutch which is familiar, above all, from the field of motor sport, the past six years have seen the development of the so-called Direct Shift Gearbox (DSG) from Volkswagen, which due to its innovative concept with two wet multi-plate clutches and various automatic shift modes, meets the standards of drivers of vehicles equipped with automatic gearboxes and also of drivers of vehicles with manual gearboxes.

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1 Introduction

The current world of gearboxes is dominated by manual gearboxes in Europe and by stepped automatic gearboxes with converters in the USA and Japan. Both types of gearbox have specific advantages and disadvantages: the manual gearbox is particularly characterised by the best efficiency, great robustness, low production costs and – due to the direct link between the engine and the vehicle – also by driving enjoyment and sportiness. In contrast, the stepped automatic gearbox particularly offers the driver a high level of comfort; driving away and changing gears are carried out without jolting and without any interruption in tractive power. Against this background, Volkswagen set itself the goal of unifying the advantages of both types of gearbox in one single product. Based on the principle of the dual clutch which is familiar, above all, from the field of motor sport, the past six years have seen the development of the new Direct Shift Gearbox (DSG) from Volkswagen, Title Figure. Due to its innovative concept with two wet multi-plate clutches and various automatic shift modes, this gearbox meets the high standards of comfort demanded by the drivers of vehicles equipped with automatic gearboxes on the one hand, whilst also offering drivers of vehicles with manual gearboxes pure driving enjoyment due to the possibility of exerting a direct influence and extremely rapid, jolt-free shifting. Its fuel consumption lies at the same low level as that of a manual gearbox. 2 Development Goals

The central goals involved in the development of the DSG included the achievement of top values in the fields of performance (to impart vehicle dynamics and driving enjoyment), overall efficiency (to achieve low fuel consumption values) and shifting times (as short as possible with maximum possible shifting comfort). In addition, consideration also had to be given to the trend towards engines with ever increasing torque and output values. A trend which has long since reached the compact class, thanks in no small measure to the successful TDI engines. Volkswagen therefore designed the DSG for a high torque and output capacity. In order not to merely offer customers a technically interesting product which is superior in many regards, one additional, important goal was to design the DSG to be highly competitive in comparison with modern six-speed gearboxes in terms of

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production costs. The installation space requirements provided for installation in all Volkswagen Group passenger car platforms with transverse engine layouts. In summary, the following development goals were to be achieved: ■ fuel consumption and performance better than or identical to manual gearboxes ■ shortest shifting times ■ shifting comfort better than or equal to that of conventional automatic gearboxes ■ usage for an engine torque of up to 350 Nm and engine outputs of up to 200 kW; optionally as four-wheel drive ■ design as a six-speed gearbox with a ratio greater than 8 between first and top gear with free ratio selection ■ compact design for use in all frontmounted, transverse passenger car platforms ■ use of inexpensive, modular components from existing manual gearboxes ■ wet multi-plate clutches as a drive-away and shift element via a module ■ imparting a sense of vehicle dynamics and driving enjoyment. 3 Description of the Concept

A dual-clutch gearbox is principally a parallel circuit involving two autonomous subgearboxes. In this case, each of these subgearboxes is functionally designed in the same manner as a conventional manual gearbox. As Figure 1 shows, the engine's torque is transmitted via two clutches (C1 and C2) to the relevant sub-gearbox, whereby sub-gearbox 1 is assigned with 1st, 3rd, 5th and R gears, and 2nd, 4th and 6th gears are assigned to sub-gearbox 2. Whilst torque is transmitted from the engine to the wheels in one sub-gearbox when the clutch is closed, the gearbox's next shifting process can be prepared by disengaging or engaging gears in the second gearbox – which is torque-free due to the fact that the clutch is open. Once the shifting point has been reached, one sub-gearbox must be disconnected from the flow of power and the other connected by mutually opening or closing the relevant clutch. This overlap is carried out very rapidly and without any interruption in tractive power. Precisely this provides the driver with shifting times never before achieved with a maximum level of comfort. Result: the direct drive train feeling is unique in a vehicle fitted with an automatic gearbox. Particular focus was placed on the development and continuous optimisation of "direct" shifting without any interruption in tractive power within a sub-gearbox. In the past, this "Achilles heel" in the dualclutch gearbox shifting process has fre-

Dual Clutch Gearbox

quently led to negative project decisions for this gearbox concept, as this type of shifting was regarded in particularly critical terms with regard to shifting speed and shifting comfort. Development at Volkswagen has resulted in shifting processes – e.g. as can be seen in rapid downshifting from 6 to 2 (see Chapter 5) – which are not currently achieved by any other automatic gearbox. For the first time in the world, Volkswagen is showing, in the form of the DSG, how this dual-clutch gearbox "system problem" can be convincingly solved. 4 Gearbox Structure, Gears and Shafts

Figure 2 shows the main section of the gearbox. The engine's torque is conducted into the clutch unit via a dual-mass spring damper and splines. An intermediate plate separates the dry clutch chamber from the wet clutch chamber. The radial layout of both clutches upstream of the gearbox leads to a very compact drive-away/shifting element. The outer of the two clutches (referred to as C1 in the following) has been designed as a driveaway/shifting clutch for both first and reverse gear. C1 is also assigned to 3rd and 5th gears as a shifting clutch. The inner of the two clutches (C2) serves as a shifting element for the even gears – 2nd, 4th and 6th. When C1 is closed, the engine's torque is conducted to the selector gears for the 1st, 3rd and 5th gears via the inner gearbox input shaft (IS1), and to the reverse gear's selector gear via the intermediate shaft. Clutch C2 transmits the torque to the selector gears for the 2nd, 4th and 6th gears via the outer gearbox input shaft (IS2). A third shaft – concentrically positioned in IS1 – uses the engine's rotational speed to drive the oil pump which is located opposite the dual clutch. The selector gears for the 1st, 2nd, 3rd and 4th gears are located on the first output shaft (OS1) and transmit the torque to the final drive. This is also connected to the second output shaft (OS2), on which the selector gears for the 5th, 6th and R gears are positioned in turn. The special feature of a dual-clutch gearbox is made clear by the layout of the individual gears. Unlike conventional manual gearboxes, the 1st and 3rd, 2nd and 4th and 6th and R gears are each assigned to one shift mechanism. In the case of the DSG, the parking lock, which is obligatory in an automatic gearbox to prevent the vehicle from rolling away, has been integrated into the final drive. Like all other assemblies, the gears and shafts are mounted in a two-piece aluminium housing. As a unit comprising the

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Dual Clutch Gearbox

electrohydraulic control unit and the electronic control module with sensor module, the mechatronics are located vertically in front of the wheel housing. The interface between the vehicle and the gearbox has been implemented using a plug-type connector. 4.1 Dual Clutch

The DSG's dual clutch has been jointly developed by Volkswagen and Borg-Warner, the development partner. Figure 3 shows the main assemblies of the dual clutch. Following the flow of torque, the engine torque transmitted by a dual-mass spring damper is conducted into the clutch input hub via splines. From there, it is passed on to the clutch housing and the outer plate carrier of clutch C1 via the drive plate, and continues on to the main hub and the outer plate carrier of clutch C2. The main hub of clutch is mounted on IS2 using two lowfriction needle bearings. The torque flows from the steel plates located on the engine side to the friction plates assigned to the inner plate carriers and on to IS1 and IS2. The actuating pistons of both clutches are centrifugally compensated using oil and work against friction-optimised return springs. The pressure oil used to actuate the clutch is fed to the piston chambers from annular ducts via a rotary passage sleeve with axial ducts. The dual clutch is intensively supplied with an infinitely adjustable flow of cooling oil via ducts which are positioned axially in the main hub. A sensor located in the clutch chamber monitors the temperature of the centrifugal oil emerging from the clutch and serves to control the optimum quantity of cooling oil for the clutch functions. Thanks to the flow of cooling oil, 20 l/min, which is available if required in combination with the dual clutch's high thermal storage capacity, frictional power of up to 70 kW can be temporarily induced. The clutch's rated torque of 350 Nm is transmitted at an actuation pressure of 10 bar. This good control capability provides a drive-away element which can be optimally adapted to the relevant engine, driving situation and ambient conditions; this is not possible using a hydrodynamic converter. Thanks to the use of this dual clutch in combination with fast gear changes, the driver experiences a direct "drive train feeling" which is entirely new in an automatic gearbox; in addition, the designation of the gearbox as Direct Shift Gearbox is also intended to express this fact. 4.2 Gearshift Actuator System

The six forwards gears and the reverse gear are shifted via hydraulically actuated selec-

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tor forks, which are mounted in the housing, Figure 4. The DSG contains four selector forks which have to be independently actuated; these act directly on the selector sleeves for gears 1/3, 2/4, 6/R and 5. On one hand, the defined position of each selector fork is the result of locking sleeves which are fixed in the housing and, on the other hand, is due to the actuating pistons which act on both ends of the rail. The oil pressure which acts on the pistons moves the entire unit in the relevant shifting direction. A pole plate with two magnets supplies an analogue travel signal via a Hall sensor. Mounting is carried out via two ball sleeves, which are supported on steel sleeves in the housing. At the same time, these sleeves form the cylinder chamber for the actuating pistons. The pressure required for actuation can be variably adjusted between 0 and max. 20 bar. This enables a variable shifting speed to be set whilst also enabling extremely precise movement to, and monitoring of, one position with the aid of the sensor system. 4.3 Oil System: Lubrication, Cooling and Filter Concept

All of the DSG's hydraulic functions are carried out using a common oil system. The housing has been designed in such a way that approx. 6.5 litres of automatic gearbox oil in the sump do not lead to gearbox efficiency impediments due to excessive penetration depths on the part of rotating components. Significant gearbox oil functions include: ■ lubricating/cooling gears/shafts/ bearings/synchroniser assemblies ■ lubricating/cooling/actuating the dual clutch ■ operating the hydraulic control unit ■ actuating the gear actuator system. A special, fully-synthetic automatic gearbox oil has been developed in order to accomplish these diverse functions. In doing so, meeting the tribological requirements – particularly in terms of the dual clutch's friction value curve – had to be ensured throughout its entire service life. The demands of minimising clutch drag torque, operating the hydraulic control unit and meeting the requirements of toothing lubrication had to be harmonised. The automatic gearbox oil is intaken from the sump by the gearbox oil pump via the suction filter and a connecting tube, Figure 5. The oil pump is designed as an internal gear pump with trochoidal toothing in a two-piece housing. It is designed for a maximum operating pressure of 20 bar. As the oil pump's power consumption contributes extensively to the efficiency of au-

tomatic gearboxes, which is poorer than that of manual gearboxes, intensive work was carried out to optimise the pump in terms of volumetric and mechanical losses. This work led to power consumption of a mere 2.0 kW at the operating point at maximum vehicle speed. The pressure oil is fed to the hydraulic control unit, Figure 6, via bores which pass through the gearbox housing. From there, the fluid passes via connecting ducts and bores to the gear actuators, the clutch and the oil-water heat exchanger with its inline pressure filter and oil spray tube. The goal of achieving optimum efficiency with minimal intermesh and churning losses led to the development of an injection lubrication system. The upper shift sleeves are intensively supplied with oil via the oil spray tube with two flat nozzles depicted in Figure 6; six separate nozzles are used to supply oil to the teeth of all gears. The pressure oil which is available at the cooler outlet is therefore used in a very efficient manner for lubrication purposes. In comparison with other automatic gearboxes, the oil cooler's output can be dimensioned relatively low due to the gearbox's good efficiency; expensive, vehicleside heat exchangers are not necessary. The installation space and adaptation to the gearbox housing were selected to enable the use of different coolers, co-ordinated to the relevant maximum power loss, in various engines. 4.4 Mechatronics Module

The mechatronics module comprise an electronic control module with a sensor module and the electrohydraulic control unit (valve chest); this is the gearbox's central control unit, Figure 7. All information is collated here, all actions are initiated and monitored from here. The unit contains a total of ten individual sensors. Eight gear actuator cylinders, two clutch pressures, and the flow of cooling oil are hydraulically regulated and controlled via six pressure modulation valves and five switching valves. The electronic gearbox control module with sensor module was developed together with Conti Temic microelectronic GmbH. Positioned directly in the gearbox, the electronic system forms a compact unit in interaction with the hydraulic control system. The advantages of integrating this control system into the gearbox are clear: ■ the vast majority of the sensor system is integrated into the control module ■ electric actuators are connected directly to the control module ■ separate housings are not required for individual sensors

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Titanium

■ necessary vehicle-side electrical interfaces are implemented by means of just one central connector. This results in minimising the number of plug-type connections and leads which, in turn, leads to considerable weight reductions and even greater electrical and electronic reliability. Integration of the electronic control module into the gearbox makes extremely high demands as regards thermal and mechanical resistance. Possible temperatures of between -40°C and +150°C, and mechanical vibration acceleration of up to 33 g, must not impede the functional capability of the electronics. The electronic circuits are of a state-of-the-art, hybrid design and are additionally protected against environmental influences via a gel. The control module, around which the gearbox oil flows, is also encompassed by a resilient plastic housing. Part of the hydraulic control module, which serves to mount the hybrid circuit and to dissipate heat, etc., is firmly joined to the control electronics' plastic housing. The hydraulic control module's actuators implement the specifications determined by the control electronics and the driving strategy. In this case, the dynamics and control quality of the hydraulic valves for the dual clutch are of particularly high quality, in order to meet the diverse requirements of driving away and shifting under all operating conditions. 5 Control Processes and Functions

The special design of the gearbox demands ingenious approaches to process technology; this particularly applies to engaging the gears and actuating both clutches. With the exception of a spontaneous and rapid change in the speed of the engine and a harmonious change in wheel torque, the driver should not be aware of any of this. An on-board measurement which describes a shifting process is intended to provide an insight into the high level of complexity. Figure 8 shows multiple shifting from 6th to 2nd gear. The total shifting duration is a mere 0.8 s. This sequence can be sub-divided into four phases: ■ In phase I – operation in 6th gear in "D" mode via sub-gearbox 2 – the driver requests maximum engine output and maximum wheel torque via the accelerator pedal. The result is multiple downshifting to 2nd gear. The change in the accelerator pedal and therefore the increase in engine load lead to an increase in engine speed – whilst retaining minimum tractive power at the wheel.

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■ Phase II: After reaching the synchronisation speed for 5th gear, sub-gearbox 1 takes over the engine torque via precisely this gear by applying and closing C1. Clutch 2 is opened simultaneously. As a result of this, tractive power remains available at the wheels. The engine speed gradient curve reveals no change whatsoever during this clutch change. Shifting is carried out entirely without jolting and is not perceptible to the driver. ■ In phase III, a gear change takes place in sub-gearbox 2: 6th gear is disengaged, 2nd gear is synchronised and engaged. Subgearbox 2 is therefore prepared for the final shifting action. ■ In phase IV, tractive power is in turn routed to sub-gearbox 2 by rapidly closing clutch 2 on reaching the engine's target speed. Shifting is therefore completed after a total of less than 0.8 s. This is an absolutely top value in comparison with conventional automatic gearboxes, particularly since the process also offers an optimum, uninterrupted torque curve at the wheels. 6 Control Elements and Gearbox Programmes

Like the gearbox programmes, the control elements in the Golf R32 with DSG are designed for both comfort and for sporty driving enjoyment and vehicle dynamics. The "D" and "Tiptronic" selector lever positions are enhanced by the sport setting "S" and two "shift paddles" mounted directly on the steering wheel, Figure 9. In selector lever position "D", the characteristic shifting curves stored in the electronic control module ensure vehicle operation at low engine speeds in the case of accelerator pedal values of less than 50 %. This leads to very comfortable cruising with a low level of consumption. If the driver accelerates more intensively, the R32 is increasingly transformed into a very sporty vehicle in which focus is then placed on vehicle dynamics and driving enjoyment. The driver is provided with pure driving enjoyment in selector lever position "S". Here, the DSG exploits its superiority over other automatic gearboxes in terms of spontaneity and shifting speed to maximum effect. Not only its extremely fast gear changes and short reaction times are impressive; its active, double-declutching downshifts, which constantly keep the engine in the optimum speed and output range, are also eminently addictive. The shift mode is designed in such a way that only two gears are used in each vehicle speed range. Multiple upshifting or downshifting, which is rather annoying as regards extreme vehicle dynamics, is avoid-

MATERIALS

ed. This driving programme turns both curvaceous mountain roads, fast motorway driving and even genuine race tracks into a very special experience, something which other contemporary automatic gearboxes are unable to achieve. If the driver wishes to determine gear changes himself, he can do this rapidly and simply in any selector lever position by actuating the shift paddles without removing his hands from the steering wheel. In "D" and "S", the gearbox remains in touch mode until either the vehicle speed falls below 12 km/h or the driver pulls the paddle ("off") for longer than one second, Figure. This guarantees that the driver is able to decide when he would like to return to automatic mode. In the selector lever's "Tiptronic" position, the engaged gear is always held and the gearbox behaves in the same manner as a manual gearbox. In this case, compulsory upshifting is not carried out even on reaching the engine's cut-off speed; on deceleration, downshifting is merely employed to ensure that the engine does not stall. The maximum shifting speeds and shortest reaction times apply to all gear changes carried out in touch mode. This results in an incomparably direct, dynamic and nevertheless comfortable driving sensation. One very special feature is what is called "Launch Control". In the "S" and "Tiptronic" selector lever positions, an engine speed of approx. 3000 rpm is set when the vehicle is stationary and the brake and full throttle are simultaneously actuated. As soon as the brake is released, the R32 shoots off. This manner of driving away, which conjures up images of Formula 1, will leave drivers of manual vehicles shaking their heads in amazement when attempting to "keep up". 7 Performance and Consumption

Due to their design, manual gearboxes have the highest efficiency values of any type of gearbox. Accordingly, the use of a gear set concept comparable with that of a manual gearbox, the minimum of two clutches for achieving shifting without any interruption in tractive power, and optimisation of the oil pump, the electrohydraulic control system and all other consumers forms the basis of an automatic gearbox with an optimised level of efficiency. In combination with the high number of gears and wide spacing, absolutely top vehicle fuel consumption values are virtually guaranteed. Launching this gearbox has enabled us to offer customers a concept

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which achieves fuel consumption values below those of comparable manual gearboxes. Figure 10 shows the values achieved by the Golf R32 fitted with the DSG in comparison with a six-speed manual gearbox. The very good acceleration values achieved by the manual gearbox are lowered by a further 0.2 s. This is attributable to the fact that DSG shifts from "1 to 2" and "2 to 3" without any interruption in tractive power, Figure 10. The top vehicle speed reached with the manual gearbox is also reached in combination with DSG. The DSG's excellent overall efficiency, particularly in terms of this criterion, is also revealed at full throttle. Above all, however, the DSG consumption value measured in the MVEG cycle – a mere 10.3 l/100 km in comparison with 11.5 l/100 km with the manual gearbox – is not currently achieved by any other conventional automatic gearbox. The consumption values are compared again in Figure 11. Volkswagen is convinced that DSG technology can be used to reduce vehicle consumption values by 5 to 10 % in comparison with those of manual gearboxes. 10 to 20 % lower consumption is to be assumed as against conventional automatic gearboxes. In the DSG, Volkswagen is offering an automatic gearbox which also achieves the consumption values of vehicles fitted with manual gearboxes in combination with TDI technology for the first time. These low consumption values are only made possible by the gearbox's good efficiency, which certain of the automatic gearboxes currently available on the market spectacularly fail to achieve.

Due to principle-related gear changes which interrupt tractive power, however, the AMG reveals dramatic weaknesses as regards ride comfort. Primarily as a result of this, it is not a full-quality automatic gearbox and is not therefore a serious competitor. Certain customers regard the CVT as the ride comfort benchmark due to the fact that it is able to continuously vary its ratio. However, the DSG does not lag significantly behind in this regard, but performs on a par with good, conventional automatic converter gearboxes (AG). In terms of vehicle dynamics and driving enjoyment, however, the DSG is vastly superior to all other concepts. The driver's sensation of being linked directly to the vehicle via the accelerator pedal is unique and gives rise to pure driving enjoyment. Volkswagen is convinced that the combination of features offered by the DSG will permanently change the world of gearboxes. The DSG can look forward to a bright future, particularly in Europe, where manual gearboxes dominate the small and medium vehicle segments. In combination with increasingly popular diesel engines or the new FSI engines, features which were previously difficult to combine, such as consumption and vehicle dynamics or driving enjoyment and ride comfort, can be combined in an almost ideal way and optimised in terms of both the market and customers thanks to the DSG. Whether – as with the DSG from Volkswagen – with wet clutches or, in the future, with dry clutches in the case of other dual-clutch gearboxes, this new type of transmission will prevail.

8 Summary and Outlook

References

Volkswagen's DSG not only presents all of the gearbox concepts currently available on the market with serious competition, it is also – as the comparison of characteristics of significance to customers shows, Figure 12 – a superior product thanks to the totality of its features. In terms of performance, in which top speed and acceleration from 0 to 100 km/h were evaluated, the DSG even prevails against the previous market leader – the manual gearbox – thanks to its very good efficiency and its ability to change gears without interrupting tractive power. As regards fuel consumption, it is only outperformed by the automated manual gearbox (AMG), which is not equipped with a constantly operating oil pump, and which therefore displays even better overall efficiency. All other concepts are inferior to the DSG in terms of fuel consumption.

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Förster, H. J.: Das kraftschlüssige Schalten von Übersetzungsstufen in Fahrzeuggetrieben. In: VDI-Zeitung 99 (1957) Nr. 27, S. 1319-1331 Schröder, F.: Schalten ohne zu kuppeln – Neue Porsche-Halbautomatik. In: Auto Zeitung 7/86 Flegl, H.; Wüst, R.; Stelter, N.; Szodfridt, I.: Das Porsche-Doppelkupplungs-(PDK-)Getriebe. In: ATZ 89 (1987) Nr. 9, S. 439-452 Franke, R.: Doppelkupplungsgetriebe für Nutzfahrzeuge. In: ATZ 91 (1989) Nr. 3 , S. 159162 Franke, R.: Kraftschlüssiges Zahnradstufenschaltgetriebe mit Vorwählung. DBP Nr. 923402 ab 1940 Köpf, P.: Systeme Automatischer Getriebe im Vergleich. In: Antriebstechnik April 1990, S. 36-44 Wagner, G.: Doppelschaltungen bei Doppelkupplungsgetrieben. In: VDI-Berichte Nr. 1170, 1994

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