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									2005-01-1021
SAE TECHNICAL PAPER SERIES

Dual Clutch Transmissions – Lessons Learned and Future Potential
Bernd Matthes
BorgWarner Transmission Systems GmbH, Germany

Reprinted From: Transmissions and Drivelines Symposium–4WD/AWD (SP-1979)

2005 SAE World Congress Detroit, Michigan April 11-14, 2005
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org

The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. 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: 724-772-4891

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2005-01-1021

Dual Clutch Transmissions – Lessons Learned and Future Potential
Bernd Matthes
BorgWarner Transmission Systems GmbH, Germany

Copyright © 2005 SAE International

ABSTRACT Dual Clutch Transmissions (DCTs) are providing the full shift comfort of traditional step automatics but offer significantly improved full efficiency and performance. With up to 15% better fuel efficiency compared to planetary-ATs, the DCTs are the first automatics to provide better values than manual transmissions. Higher top speed and, more important in everyday driving, better acceleration compared to planetary-ATs and CVTs are additional benefits. DCTs are also the first automatic transmissions well suited for high torque Diesels and high revving gas engines alike. The DCT concept was originally developed in the 1940’s but it took until recently that high volume production solutions were made possible and allowed for the mainstream vehicle market entry. Beside the electronic hard- and software the key enablers for the DCT are the dual wet clutch and the electrohydraulic transmission control module. On the clutch side new friction elements have been designed. Combined with an enhanced transmission fluid more than two million shifts per clutch and a consistent launch quality over the transmission lifetime are achieved. The dual clutch design itself supports very responsive clutch engagements and disengagements as well as good controllability under slipping conditions. On the controls side the electronics, sensors and electrohydraulics have been integrated into a mechatronic module for minimized tolerance bands and improved reliability. Low hysteresis, direct acting solenoids enable the precise and fast clutch control that is the key for the DCT function.

INTRODUCTION Today the powertrain and in particular the transmission are facing development challenges that are more diverse and more difficult to meet than in many years before. First of all the governmental regulations on emissions and fuel consumption are getting tighter all over the world. In Europe the carbon dioxide emissions have been reduced by 10% from 1995 to 2003 1 . The tools to achieve the goal have been evolutionary improvement of the transmissions going from 4 to 5 to 6 speeds and larger ratio spread and additional internal efficiency improvements, e.g. through drag reduction and the reduced number of clutches in the Lepelletier design [2].
The engine side, however, has been the real driver for efficiency improvements so far. Direct injection Diesel engines (TDI) already cover almost half of the European passenger car market and direct injection gas engines (GDI) are starting to replace the traditional gas engines. The targets set for a European fleet average of 140g CO2/km require another 25% emission reduction over the five year period from 2003 to 2008. 140g CO2 / km is equivalent to approximately 6 liter/100km (42 mpg / 16.7 km/l) for gas engines and approximately 5 liter/100 km (50 mpg / 20 km/l) for Diesel engines. These are difficult numbers as a fleet average.

Figure 1: Torque and power density development for direct injection Diesel engines

INDUSTRY TRENDS The industry is working on radical solutions such as the fuel cell technology, hydrogen powered cars and, more evolutionary, on hybrids. Inappropriate energy storage media (batteries), a missing hydrogen infrastructure and costs are remaining major roadblocks for a near future and / or high volume introduction into the market place. For the coming years the combustion engine and the transmission will therefore have to cover the main portion of the vehicle fuel efficiency improvement. On the engine side downsizing and further improved direct injection Diesel and gas engines can be identified as major trends. Higher engine speeds on the gas engine side and the usage of turbo chargers (all TDIs and an increasing number of GDIs) or super chargers (some gas engines) compensate for the displacement reduction. In North America, Europe and Asia, cylinder deactivation for 6 or 8 cylinder engines results in a virtual displacement reduction under most driving conditions.

However, the availability of more cylinders or turbo chargers are still leading to bottom line upgrades of engine power, torque and vehicle performance. Most dramatic is this trend for TDI engines. Figure 1 3 shows the development of torque and power density over the last years. The significant increase of torque, however, comes at the expense of a smaller usable rpm range. A typical direct injection Diesel engine today reaches its max torque already at 1500 rpm, but beyond 4000 rpm there is an immediate torque drop, resulting in the need for more gears and a larger overall ratio spread for the transmission. On the gas engine side, the area of best engine efficiency has been growing over the last years, a trend that allows the engine to run at best efficiency even in combination with a stepped transmission.

Figure 13: DualTronic TM controllability and responsiveness

20 Relative F uel consumption A T/MT 15 10 4 _ S te p A T 5 0 -5 -1 0 -1 5 0 1 00 0 2 00 0 3 0 00 4 0 00 E n g in e D is p la c e m e n t 5 6 C D S te p A T S te p A T VT CT

Figure 3: Transmission comfort and fuel efficiency comparison

Finally the end customer does appreciate the comfort of an automatic transmission when driving through cities and on congested roads but especially in Europe, he is also looking for the sporty, responsive and connected drive feel of a manual. As a result more and different transmission concepts will be visible in the market than there are today. Figure 2 shows the expected development of the European transmission market for passenger cars from 2004 through 2013. Even though there is a dramatic change from pure manuals towards automated manuals and automatics, the layshaft type base gearbox remains serving an almost unchanged market share of about 80%, with planetary automatics and CVTs covering the rest. What are the drivers for the change? Beyond the move to automation for reduced fuel consumption the trend to TDI engines is the second driver. These engines are already covering almost half of the European passenger car market and it is expected that their share will increase further. Belt and chain type CVTs are limited in torque capacity and do have difficulties with the higher vibration level of Diesels. Traditional step automatics have the efficiency and performance disadvantage of the torque converter and the fixed ratios. TDI engines reach their maximum torque level at very low speeds where torque converters are not very efficient. In addition the high vibration level, especially for four cylinder engines requires a soft torque converter further deteriorating the efficiency. As a result today’s step ATs show a 15% to 25% higher fuel consumption compared to a manual if applied

to the same 4 cylinder TDI engine. Replacing the torque converter through a wet start clutch 3 can reduce the gap. The higher multiple clutch transmission drag and the inflexibility to adjust gear ratios and ratio spreads to the individual engine and vehicle remain as limitations. High engine speeds on the gas engine side can not be appropriately handled by either CVTs or planetary-ATs, limiting their application. CVTs in addition have the disadvantage that the transmission efficiency itself is the worst of all transmission concepts discussed here due to high pump and systems losses caused by the high oil pressures required and the need to avoid a slipping belt or chain under any circumstances as this would destroy the transmission. Automated manual transmissions (AMTs), using an automated single dry clutch for shifting, provide the best fuel efficiency and the lowest cost, but do not meet customer expectations due to the uncomfortable torque interrupt when shifting in particular in the lower gears. The only transmission with no application limitations is the wet clutch DCT. It provides excellent fuel efficiency, responsiveness and the shift and launch comfort of a good planetary-AT. It is the only automatic transmission suitable for high torque Diesel engines and high revving gas engines alike and offers competitive packaging and weight as well as easy and cost effective adjustment of gear ratios and ratio spread to best suit the various types of engines and cars. Figure 3 shows a comfort and fuel efficiency table for the transmissions discussed in more detail in the following.

Figure 4: Fuel efficiency vs. engine displacement comparison for automatic transmissions

The fuel efficiency data are further detailed by engine size in Figure 4 [4]. In Figure 5 the expected market segment coverage for automated and automatic transmissions in Europe based on the specific advantages of the different transmission types and the customer expectations is described. Combining existing investment, duty cycles and customer preferences, it is expected that North America, Japan and Europe will have very different transmission technologies in the next years. In North America traditional Step-ATs with improvements in ratio spread (6 speeds) and efficiency will cover most of the market with hybrids becoming a more important niche player. Japan is expected to be the only market with a significant share of CVTs with the rest of the market covered mainly with step- ATs plus a growing share of hybrids using both transmission technologies.

In Europe 80% of the market will still be transmissions based on the manual gearbox, many of them automated (AMT, torque interrupt type) or automatic (DCT, without torque interrupt) though. However, also in Asia (including China, India, Japan and Korea) and North America the interest in DCTs is rapidly growing. Initial applications will be performance oriented cars.

DUAL CLUTCH TRANSMISSION (DCT) History and Future Applications The Darmstadt Technical University Professor Rudolph Franke patented the first dual clutch transmission in 1940. It was applied and tested in a truck but never went into production. It took until the 1980’s to become visible for a larger audience. Porsche developed its own dual clutch transmission (Porsche PDK) for racing applications.

Figure 5: Preferred vehicle market segment coverage by transmission type

Figure 6: Dual Clutch Transmission and DualTronic TM transmission and AWD modules

The performance improvements through the elimination of the torque interrupt during the shift and extremely short shift times were impressively demonstrated through superior performance in the Porsche 962 C race car and later also in the Audi Quattro rallye car. The intent to transfer the DCT technology into production cars was found not to be viable at that time due to technological and cost hurdles. Since then many companies have looked into the DCT concept, prototypes were built and patents filed, but none of them made it to production. In the late 1990’s Volkswagen partnered with BorgWarner being the first to start the co-development of a dual clutch transmission suitable for high volume production and mainstream car applications. In 2003 the transverse Volkswagen DQ 250 dual clutch transmission went to market in the VW Golf R32 and the Audi TT 3.2 labelled as DSG (direct shifting gearbox). Meanwhile it is also available in the new VW Golf, the VW Touran Minivan, the new Audi A3 and Audi Sportsback, the Skoda Octavia and the Seat Altea. With its 350 Nm engine torque capability the transmission covers the larger gas engines and the entire range of Diesel engines for these applications already. The transmission is combined with FWD and AWD drivelines. Figure 6 [5] shows a cross section of the VW / Audi DSG with the BorgWarner DualTronic™ clutch and mechatronic transmission control modules and additional BorgWarner transmission and AWD modules that will be applied in future DCT applications. In 2005 the full performance potential of the DCT concept will be visible when a high end

seven-speed dual clutch transmission enters into the market. It is applied in a north-south powertrain with AWD to the strongest engine (1111 HP) and fastest (406 kph / 252 mph) production car available, the Bugatti Veyron. Like a manual transmission the DCT can be applied in every car and truck, whether it is a FWD, RWD or AWD driveline or whether it is in transverse or longitudinal powertrains. The high volume DCT applications in the near future are transverse powertrains for compact and midsize cars starting in Europe and followed by Asia, in particular in combination with Diesel engines where no other viable automatic transmission alternative exists. In a next step developments are under way to replace today’s AMTs and step-ATs through DCTs in sports cars and performance oriented high-end passenger cars worldwide.

WET CLUTCH DCT FUNCTION AND PERFORMANCE The dual clutch transmission is based on a manual gearbox. Unlike a manual the two clutches in a DCT are linked to two input shafts and the shift and clutch actuation is controlled through a mechatronic module integrating the electronic and hydraulic elements. There is no clutch pedal for the driver. Like in a tiptronic type step-AT, the driver can initiate the gear change manually or leave the shift lever in a fully automatic D (comfort oriented, shifting at lower engine speeds) or S (performance oriented, shifting at higher engine speeds) mode. The shift itself is always automatically handled by the shift and clutch actuators.

Figure 7: Dual Clutch Transmission schematic

Figure 7 shows a simplified DCT schematic. The two clutches are linked to separate input shafts. In this example clutch 1 is linked through the inner solid shaft with the gears 1, 3 and 5. Clutch 2 is linked through the outer hollow shaft with the gears 2, 4, 6 and reverse. The input shaft from the engine is connected through the damper with the outer plates of both clutches. When starting the engine the first gear is engaged. As clutch 1 is open there is no torque transfer to the wheels. When clutch 1 is being closed the outer plates of clutch 1 are getting into slipping contact with the inner plates smoothly starting to transfer the engine torque through the solid shaft, gear set and synchronizers of the first gear to the differential and finally the wheels. In parallel the second gear is already pre-selected, which can be done, as clutch 2 does not transmit any torque at this moment. When shifting from first into second gear, full forward thrust is ensured as clutch 1 disengages at the same speed and progression as clutch 2 engages.

When clutch 2 is fully engaged, third gear can be pre-selected, as now clutch 1 does not have to transmit torque and so forth. The driver feels only the clutch-to-clutch shift. For fast shift times the next gear that is linked to the open clutch is generally pre-selected. Choosing the right gear is secured through complex algorithms in the transmission controls software adjusting shift pattern and shift speed to the individual drivers behaviour. As by design maximum differential speeds in a dual clutch transmission are smaller than they are in comparable traditional step-ATs these shifts are easier to handle and can be done quicker or with higher or at least equal comfort than in traditional step-ATs. Through simple control software changes the drive feel can be changed from sportive to high comfort thus allowing a cost effective adaptation to different markets, customer preferences and vehicles. The performance improvements of a VW Golf R32 equipped with a wet clutch DCT transmission over the same car with a 6-speed manual is shown in Figure 8 [5].

Figure 8. Dual Clutch Transmission vs. manual transmission performance

The DCT equipped car reaches the same top speed as the manual transmission equipped car. In addition it beats the manual in acceleration as there is no torque interrupt during the shift and it provides a 10% fuel efficiency improvement, homologated in the European fuel efficiency drive cycle (MVEG), over the manual due to an optimized shift pattern to run the engine continuously in the best efficiency area. There is no other transmission in the market with comparable performance. Fuel efficiency measurements in a variety of vehicles with gas and Diesel engines showed an average fuel efficiency improvement of 5 to 10% over the manual in the MVEG drive cycle. Dry clutch DCTs can further improve the fuel efficiency up to another 1.5% at the expense of significant functional limitations. A key driver for the improved vehicle performance, responsiveness and fuel efficiency is the elimination of the torque converter as a launch device. The DCT uses the wet clutches as shift clutches and launch device at the same time instead. Eliminating the torque converter reduces the mass and inertia significantly. Even more important though is the possibility to independently choose and apply control strategies. The torque converter has a defined characteristic (torque multiplication, creep, vibration dampening etc.). Once the design is selected, it is fixed. An electronically controlled wet clutch used as launch device allows for a variety of launch strategies. The creep function can be chosen freely. Creep can be adjusted to the customer preference, to hill hold, changed vehicle weight or trailer towing conditions. A wide range from smooth starts on slippery ground to performance oriented launches can be freely calibrated in the transmission controls. Depending on throttle position change speeds and wheel slip sensor signals the drivers request and road condition allowance can be identified and the target engine speed will be adjusted also depending on the drive mode (D or S). Especially when combined with Diesel engines or larger gas engines it is a big advantage to be able to disconnect the transmission with its inertia from the engine. The engine can then rev up freely and fast until the clutch is smoothly engaged and kept slipping until clutch input and output speeds match. The same strategy can also be applied when quick and high performance double downshifts are

required. The clutches are opened, the engine can speed up quickly until it is caught by the closing clutch again, thus allowing for fast downshifting and additional acceleration boost through the then higher revving engine and its inertia. In city driving an additional launch clutch fuel efficiency advantage can be achieved through a neutral idle function. Whenever the driver hits the brakes e.g. at the traffic light the creep torque and therefore the losses can be reduced to a minimum, the engine can even be fully disconnected from the transmission. The positively received torque multiplication provided by a torque converter in a conventional step-AT can be compensated through an adjusted first gear ratio and / or modified axle in DCT equipped cars, thus minimizing the acceleration disadvantage when starting from standstill. Under all other driving conditions, e.g. accelerating when already driving, the wet clutch allows for significant improvement over the torque converter through the better clutch and transmission efficiency and the lower mass and inertia. Beside eliminating the torque converter, the fuel efficiency and performance advantages of the wet clutch DCT over CVTs and planetary step-ATs are based on a good mechanical efficiency of the layshaft transmission, the reduced number of clutches and the related drag, controlled and minimized clutch apply pressures and oil feeds and optimised gear ratios and ratio spreads. Unlike CVTs and in particular step-ATs, gear ratios and ratio spreads can be easily and cost effectively changed. Similar to manual transmissions multiple ratio sets per base transmission can be used with ratio spreads from 5 to over 7 to best serve each specific engine and vehicle combination of the application at minimum oncost. Compromising vehicle performance and efficiency by using the same gear set and ratio spread for high torque Diesels and high revving gas engines, for minivans or sports cars like in step-ATs is no longer an issue. BorgWarner has, however, developed wet start clutch modules to boost performance of CVTs and step-ATs in those markets (North America and Japan) where for existing investment reasons DCTs will remain lower volume application in the near and mid term future (Figure 9, [3]). The wet start clutch improves fuel efficiency and responsiveness when

compared to the torque converter, it cannot eliminate the step-AT disadvantages of fixed ratios and higher mechanical losses though.

In order to cover most vehicles and engines a range of DualTronic clutches has been designed. The clutches address vehicles with FWD, RWD and AWD powertrains, subcompact, compact and midsize cars as well as high performance vehicles. Diesel and gas engine applications (1.4 to 8.0 liter displacement) with engine torques from 150 to 1250 Nm and engine powers from 55 to 850 kW. The DualTronic clutch for the most powerful passenger car engine today (1250 Nm / 850 kW) still remains quite compact with a diameter of 280 mm and a length of only 130 mm. The basic clutch design can differ depending on the application. Common for the wet clutch design are normally open clutches for safety, pressure compensation to avoid apply piston engagement up to max engine speed and low mass and inertia contributing to fast shift times and vehicle responsiveness. Concentric and parallel clutch designs have been realized, verified and are released for production. The concentric design is used to achieve the shortest clutch length possible. As the clutch lube path is the same for both clutches, the control strategy and the hydraulic controls module are simpler. Also supported by the clutch design itself the systems costs for the concentric clutch are generally lower then for parallel designs. When designed to equal torque capacity the parallel clutch is smaller in diameter but longer than the concentric design. The design also allows for independent cooling oil flow to the two clutches and is therefore better suited for high-energy shift conditions. On the downside the cooling oil flow controls are more complex. Even though both clutch designs are very compact, their performance is unique as functional and durability requirements in a DCT are unlike any other automatic transmission. Both clutches are engaged and disengaged in any shift. Over the lifetime of the transmission each clutch needs to be capable to perform 2 million shifts without the need to be replaced. Shift energies can be handled safely as maximum differential speeds during the clutch engagements are typically lower in DCTs then they are in planetary stepATs. The wet clutch concept can handle engine vibration dampening through controlled clutch slip without impact on durability. There is no functional need for an additional torsional

Figure 9: BorgWarner wet start clutch with integrated torsional vibration damper

In order to provide the superior overall performance, two DCT key elements had to be specifically developed for the application, the dual clutch and the mechatronic transmission control module. Both will be described in more detail in the following chapters.

DUALTRONIC

CLUTCH MODULE

Figure 10 shows the cross section of the BorgWarner DualTronic clutch module that is applied in the Volkswagen DSG transmission. In this case the outer clutch is attached to the odd gears 1, 3, 5 and reverse. The inner clutch is attached to the even gears 2, 4 and 6. With a length of only 80 mm and a diameter of 200 mm the clutch is a very compact unit providing a torque capacity of 350 Nm.

Figure 10: DualTronic TM clutch module

vibration damper or even a dual mass flywheel. However, in order to dampen the entire vibration of a 4 cylinder direct injection Diesel engine slip powers of 3.5 kW, with a safety margin, of 4.5 kW and therefore under long slipping time conditions high energy levels would have to be dissipated. The clutches are able to handle this requirement. But from a transmission efficiency point of view it is better do add a simple and cost effective integrated vibration damper to the clutch (Figure 11) thus reducing the clutch slip requirements.

desired friction characteristic for a selfdampening function and for controlled clutch slip during launch and shift is a steady increase in friction coefficient with increasing slip speed. The friction characteristic is influenced through the separator plate surface finish, the friction material and groove design and the transmission fluid. Dual clutch transmission fluids are based on automatic transmission fluids. The additive package has been modified to meet the complex requirements of high shear stress and pressure in the gears, high thermal stability as well as positive friction characteristics in the clutches and synchronizers. Using the same type of friction material for the synchronizers and the clutches simplifies the transmission fluid development. In order to meet the friction requirements, mechanically and thermally extremely stable carbon composite friction materials are used. Together with its inherent positive friction characteristic, the open and porous surface structure allows a high number of friction additive molecules to accumulate on the surface and further contribute to a positive friction characteristic. The open structure also prevents fast clogging of the pores through oil and additive degradation products that would deteriorate the friction characteristic, thermal capability and lifetime. As result of the effort, the BorgWarner DualTronic clutch with 200 mm diameter can dissipate 70 kW slip energy during launch or stall tests without being damaged meeting all OEM requirements on gradebility, multiple steep uphill launches, a clutch slip based hillhold and creep function and trailer towing capabilities. Even with more than 10 GJ slip and shift energy input over the lifetime there is no wear, just initial setting of the composite friction material and no need for clutch replacement. DUALTRONIC MODULE MECHATRONIC

Figure 11: DualTronic TM clutch module with integrated torsional vibration damper

On top of the shift and vibration dampening function the clutches are also used as launch device. During the launch very high specific friction energies combined with high slip speeds occur. The total friction energy impact on the clutch over the lifetime is caused by approx. 80 % through the launch procedures, 15% through the shifts and another 5% through other clutch slip conditions. In order to safely absorb and dissipate the energies, the cooling oil flow strategy and the oil routing have to be optimised and in addition new application specific friction materials, friction element designs and DCT specific transmission fluids had to be developed. Excellent clutch controllability during engagement, disengagement and launch is key for high comfort but also for fast shifts. Important are a short oil route between valve body and clutch, the oil routing in the clutch itself, pressure compensated apply pistons with appropriate return spring design and very fast yet well controllable clutch actuation solenoids (see next chapter). Finally a self-dampening function of shudder vibration in the clutch is desirable. Contributing to shudder are inappropriate friction characteristics, low elasticity of the friction material and geometrical surface defects such as waviness or dishing. The

The mechatronic module combines the electronic transmission control unit (TCU), the sensors and the hydraulic solenoid module and the valve body into one system. Figure 12 shows the BorgWarner DualTronic mechatronic module used in the Volkswagen DSG transmission. The mechatronic transmission module consists of 4 major elements:

Electronic Transmission Control Unit (TCU) Sensor Cluster Solenoid Assembly with Lead Frame Valve Body

The entire mechatronic module is installed inside the transmission and has only one connector to the outside for electric power supply and for communication with the vehicle and engine CAN system. The TCU is specifically developed by Temic for the use inside the gearbox and withstands high temperatures up to 150°C. It is based on a specific insulating and high temperature capable ceramic substrate technology and utilizes the oil flow in valve body and worm plate for cooling. The TCU and the sensor cluster, integrating sensors for temperature, speeds and shift actuator positions are moulded into one piece, again increasing reliability by eliminating electrical connectors. As highlighted in the clutch section, excellent clutch slip control is life critical for the performance of a dual clutch transmission. Beside appropriate software and calibration it is the design and interaction of clutch actuation solenoids and clutch that decides on the performance. Therefore BorgWarner has developed special low hysteresis direct acting variable force solenoids (VFS) for its DualTronic™ applications. Direct acting VFS solenoids are required for the speed, responsiveness and precision in pressure level controls needed for the clutch slip adjustment, especially at low clutch apply pressures and low slip speeds. Figure 13 shows that clutch apply pressure variations down to 0.05 bar can be well controlled by the direct acting VFS solenoids. The clutch itself is so responsive and controllable that these small changes in apply pressure can be used for adjusting the transmitted torque to +/- 1 Nm under slipping conditions on a 30 Nm base torque level.

Figure 12: DualTronic TM mechatronic transmission controls

The key advantages of a mechatronic module are very tight tolerances and reduced cost at high volume production. The calibration of the entire systems is done at the end of line functional test when the mechatronic module is fully assembled. Instead of calibrating single components such as solenoids, sensors or the TCU and adding up the single component tolerances to the tolerance band of the assembly, the module itself is calibrated to the desired set of output functions (e.g. oil pressure or flow vs. electric current) with a very tight tolerance band. Beside the elimination of multiple component calibration steps, the system cost was further reduced through the elimination of wiring harness and connectors. The lead frame is directly connecting the solenoids with the TCU and sensor cluster. The elimination of connectors and unnecessary electrical contacts is also an important aspect for increased reliability.

Figure 13: DualTronic TM controllability and responsiveness

All safety functions and a limp home mode are included in the mechatronic transmission control module. In case of a total electric system breakdown both clutches will open avoiding safety concerns. These safety concerns are present with normally closed clutches, typically applied in dual dry clutch transmissions to maximize transmission efficiency. In case of total electrical systems failure both normally closed clutches in a dual clutch transmission would lock up, blocking the transmission and the wheels immediately.

Further reduced drag, mechanical and pump losses and inertias through design improvements including lower weight solutions are in the evaluation phase and will result in even better fuel efficiency and responsiveness increasing the gap to today’s automatics. An important reference for the future market beyond Europe will be 6 and 7 speed dual clutch transmissions installed in north-south powertrains currently in the production development phase and entering the market in the coming years beyond the initial north-south DCT super car applications. The BorgWarner DualTronic dual clutch and mechatronic control modules are the key enablers for dual clutch transmissions. The advantages of the DualTronic dual clutch modules include: Very compact packaging with a lower mass and significantly lower inertia leading to better performance and responsiveness. Excellent controllability under all shift, launch and slip conditions. Transmission lifetime durability without wear. Very high slip energy capabilities allowing for hill hold and heavy trailer towing functions, thus meeting the specifications for torque converter type step-automatic transmissions. Inherent safety through normally open clutches. The DualTronic mechatronic transmission control modules integrating the electronics (TCU), sensors and the hydraulic controls (solenoid module and valve body) offer tighter tolerances, lower cost and higher reliability than the traditional separated hydraulic control block and TCU approach. Highly responsive direct acting VFS solenoids are key enablers for a precise clutch shift and slip control. REFERENCES [1] C. Bock, Die ACEA-Vereinbarung zur Flottenverbrauchsreduzierung und ihre möglichen Konsequenzen auf zukünftige Getriebekonzepte, Tagung CVT-Getriebe, Haus der Technik, Essen, Germany, 2000

SUMMARY AND FUTURE OUTLOOK Dual clutch transmissions provide the convenience and comfort of a step automatic or CVT transmission and are the first automatics with up to 10% better fuel economy and at the same time better performance than a manual. The DCT meets customer demands for a sporty, responsive driving experience but also for improved fuel economy and the convenience of a non-torque interrupt-shifting automatic while navigating congested cities. Through the dual clutch transmission controls and clutch designs the launch and shifting characteristics can be easily adapted to meet any customer and market preferences and is the first transmission that allows for branding just through software change. The dual clutch transmission is also the only automatic transmission well suited for standard engines, high torque Diesels and high revving gas engines alike. It is therefore expected that the DCT will rapidly gain market share, first with performance models and with Diesel engines, then as a mainstream transmission. High volume applications started in Europe where the assets for manual transmission manufacture and therefore for the DCT base transmission were already available. In order to further improve the competitive position of the dual clutch transmission special focus is now on cost and even better efficiency. The transmission is already cost competitive to the latest 6 speed step-ATs which is remarkable as those are currently produced in much higher volumes and can look back on a 50 year plus history of continuous improvement. The cost reduction activities started will result in an increased cost advantage over step-ATs and an even larger difference to CVTs.

[2]

Pierre Lepelletier, Le concept Lepelletier de transmission automatique planetaire a 6 vitesses, Conference Proceedings, Innovations pour Transmissions Automobiles, Paris, 2004 K.H. Bauer, Nasslaufende Anfahrkupplung in modernen Automatikgetrieben, 1. Internationales IIR-Symposium „Innovative

Fahrzeuggetriebe“, Offenbach, Germany, 2003 [4] K.H. Bauer, Dual Clutch Transmissions, Conference Proceedings, Innovations pour Transmissions Automobiles, Paris, 2004 Volkswagen AG, DSG Press Release, Wolfsburg, Germany, November 2002

[3]

[5]


								
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