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					SAE TECHNICAL PAPER SERIES

2006-01-3245

Lubricant Technology for Dual Clutch Transmissions
S. Hurley, C. D. Tipton and S. P. Cook
Lubrizol Limited and The Lubrizol Corporation

Powertrain & Fluid Systems Conference & Exhibition Toronto, Canada October 16-19, 2006
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-0790 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: 724-776-3036

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 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

2006-01-3245

Lubricant Technology for Dual Clutch Transmissions
S. Hurley, C. D. Tipton and S. P. Cook
Lubrizol Limited and The Lubrizol Corporation
Copyright © 2006 SAE International

ABSTRACT
The advent of the dual clutch transmission, which combines the performance advantages and hardware systems of both manual and conventional automatic transmission designs, has led to the development of new lubricant technology. A dual clutch transmission lubricant requires a new, specialized additive technology to meet the unique, often competitive, requirements of the different elements of the hardware system: specific clutch performance requirements related to the wet clutches along with high load-carrying and thermal stability associated with the manual transmission. We report the development of specialized lubricant technology that can be tailored to the specific requirements of the next generation, dual clutch transmissions. Features of the new lubricant technology for dual clutch transmission fluid include: • • • • • • • excellent wet clutch friction performance excellent anti-shudder friction durability high thermal and oxidative stability high load-carrying ability excellent bearing performance strong anti-corrosion performance excellent compatibility

INTRODUCTION
FUEL EFFICIENCY Today, both manufacturer and motorist agree that fuel efficiency is one of the deciding factors in the production and purchase of passenger car vehicles. The incentive for the manufacturer include the agreement between the Association of European Automotive Industry and the European Commission to reduce CO2 emissions from 180g/km to 140g/km for 2008 [1] and the 25% reduction in Corporate Average Fuel Economy limits for 2008: 6 liters/100km for gasoline and 5 liters/100km for diesel [2]. The incentive for the motorist is cost of ownership as evidenced by increased sales of the more fuel efficient diesel passenger cars versus gasoline. In Europe, over 50% of new passenger cars sold are equipped with diesel engines—evidence that a 30% savings on fuel sells vehicles. TRANSMISSION CHOICE Transmission design and choice not only contribute to fuel efficiency but also to driving comfort and pleasure. Traditionally, the transmission of choice in North America is the automatic transmission (AT). In Japan, the trend is toward continuously variable transmissions (CVT), and in Europe, the manual transmission (MT) is favored. The graph in Figure 1 provides an overview of the transmission choices in Europe, Japan, Korea and North America in 2004.

This paper will concentrate on the frictional performance of the wet clutch. This new hardware, which acts as both the vehicle launch and gear shifting device, is the most critical component addressed by the new fluid technology.

Figure 1 Transmission choices 2004

Increasing congestion on European roadways is pushing the traditional MT driver toward more automated transmissions. Initially, automated manual transmissions (AMT) for small/medium passenger cars and 6- and 7-speed automatic transmissions for the larger luxury vehicles were introduced. Examples of the latter were the ZF ‘mytronic6’ introduced in the BMW 7 series and DaimlerChrysler 7-speed automatic transmissions. Although the 6- and 7-speed stepped automatic transmissions are 10-15% more fuel efficient than 4-speed stepped automatic transmissions, an additional cost is still passed on to the motorist and to the manufacturer utilizing MT production technology. Thus selling stepped automatics transmissions in Europe is not easy The solution may be the dual clutch transmission (DCT), which combines the advantages of the manual gearbox and the stepped automatic transmission. For Volkswagen this is the Direkt Shalt Getriebe/Direct Shift Gearbox, or DSG. It is being sold as optional equipment on the Golf, Touran, Mini Peoplecarrier, Audi TT coupe, Roadster, Skoda Octavia and Seat Leon. ADVANTAGES For the motorist, the advantages offered by the DCT are the driving enjoyment and low fuel consumption of manual gearboxes as well as the comfort of stepped automatic transmissions. A further bonus is that the DCT is better matched to modern high-torque diesel engines. Torque-controlled clutch operation enables uninterrupted gear change under load. The manufacturer saves money by using existing MT technology, manufacturing facilities and equipment rather than needing to invest in new facilities and equipment associated with the move from MT to AT. The DCT can be manufactured in either a concentric or a parallel design. The former enables a shorter clutch length and lower production cost. The parallel design is longer and allows independent cooling of the clutches, which may benefit higher energy shifts.

North American and Asian markets, and current forecasts are more geographically widespread. The high power density of the DCT is a definite benefit from a transmission installation point of view. The DCT may be fitted in front, front-longitudinal and rear longitudinal formats, so it can be adapted in small and compact passenger cars. In addition, it can easily be adapted to all-wheel-drive vehicles and trucks, broadening the range of application. Furthering the predicted popularity of the DCT is the arrival of units capable of handling high torque levels. This enables the DCT to be used in a wider range of vehicles, from small cars to luxury, sport utility and light commercial vehicles. ZF recently launched its ZF 7DCT50, capable of handling 500 Nm torque. The Bugatti Veyron with 1001 HP and 1250 Nm is fitted with a 7-speed AWD DCT [2]. There have been many predictions of the DCT share of the transmission market. Getrag, for example, has announced plans to build more than a half million DCT units in 2010. Certainly a consideration is that DCT growth will be at the expense of the MT share, and it may well halt the growth of the 6- and 7- speed stepped ATs in the European market. Taking an average of the various predictions, a forecast for the three key markets in 2010 is in Figure 2 below.

Figure 2 Predicted transmissions used in 2010

The unique twin shaft and clutch design of the DCT provides the comfort of driving a conventional AT, coupled with the responsiveness and sporty feel of an MT. Absence of a torque converter means there is a direct link between engine and driver, so there is no time lag when accelerating. Using a wet clutch to launch the vehicle provides much greater starting acceleration and also allows for very fast up and downshifting in higher gear. The benefits of improved shift response, speed and quality, coupled with increased power-to-weight ratio, lead to the prediction that use of the DCT will increase. THE TRANSMISSION OF THE FUTURE Initially, Europe was forecasted to be the primary DCT market, but the technology is being investigated for the

DUAL CLUTCH TRANSMISSION OPERATION The dual clutch system consists of two separate input shafts, twin multi-plate wet clutches, a pump system, mechatronic module incorporating the transmission control unit and dual mass flywheel [Figure 3].

PERFORMANCE REQUIREMENTS OF THE DUAL CLUTCH TRANSMISSION FLUID (DCTF) The lubricant in a dual clutch transmission (DCTF) differs from that in either a manual or a conventional automatic transmission in that it has to provide: • • •
Figure 3 Schematic of the DCT system (Borg Warner)

lubrication of the clutches, gears, shafts, bearings and synchronizers wear and corrosion protection of the above components hydraulic actuation of the clutches and gear actuation cooling for the entire system

•

Gears 1, 3, 5 and reverse are mounted on the inner input shaft and 2, 4 and 6 on the hollow, outer shaft. These shafts can be selected individually by each of the two wet clutches, operating under hydraulic piston pressure. Gear changes occur without interruption, with clutch and flywheel loaded, by a controlled torque transfer between the two clutches. Synchronizer elements aid the smooth selection of the gears. Using the same friction material for the clutch and synchronizer elements simplifies the transmission design. The DCT employs the wet clutch for start-up and launch as well as for shifting between gears. The clutch friction material lining is specially designed to provide smooth engagement and disengagement in combination with the lubricant. The pump and lubricant system provide cooling hydraulic pressure for clutch and gear actuation. FEATURES OF THE DUAL CLUTCH TRANSMISSION One of the most significant features of the DCT is the high power-to-weight ratio, hence improved fuel economy. The DCT has an average weight increase of 45% compared to a MT [1], but this is offset by the excellent power advantage (particularly with high-torque diesel engines or high-revving gasoline engines). Volkswagen claims that this technology may provide fuel savings of 5-10% over MTs and 10-20% over ATs. Certainly the 6-speed DCT provides fuel economy comparable to a 7-speed, conventional AT. Energy losses are minimized because only two clutches provide all gear changes. Further fuel economy savings are gained by zero drag from the alternately redundant clutch.

It also has to possess sufficiently low viscosity at high and low temperature to provide good fuel efficiency. Due to the severe gear mesh conditions of the DCT, viscosity has to be balanced with wear protection. It is unlikely that a conventional AT fluid (ATF) would provide sufficient wear protection for the helical gear and synchronizer system of the DCT. Similarly, a conventional MT fluid (MTF) would not be optimal for clutch friction performance. It is clear that the new generation of DCT fluids (DCTFs) require a specialized additive technology in order to meet the often contradictory requirements of the DCT. VISCOMETRICS The viscosity requirements of a DCTF are similar to those in modern ATF and MTFs. The kinematic viscosity at 100o C as measured by DIN 51562 is typically between 6 and 7 cSt. Low-temperature Brookfield viscosity at minus 40o C, as measured by DIN 51398, is typically <10,000 cP. It is very important to consider the viscometrics, particularly at low temperature, due to the presence of the pump and hydraulic control unit. If low-temperature viscosity is too high, the performance of these components is reduced, leading to impaired clutch, synchronizer and gear operation, energy losses and eventually decreased fuel economy. Low-temperature fluidity is also important for protection of components during start-up. The lowtemperature performance of the DCTF is largely determined by the properties of the lubricant base fluid; however, pour point depressants can also be used to improve it. Commonly, Group III and IV base fluids are used, which have lower intrinsic viscosity at low temperature. It is also imperative for the DCTF to have good shear stability, particularly to withstand the high shear conditions of the gear mesh. Sufficient viscosity is required to provide adequate film thickness to protect these components. In order to meet these stringent viscometric and shear stability demands, a viscosity modifier (VM) is often used. Care must be taken to ensure that the combination of additive package, base fluid and VM is optimized for the system.

FRICTION This discussion will concentrate mainly on the frictional performance of the wet clutch. It is probably the most critical component addressed by the lubricant additive system and acts as both the start-up and gear shifting device. The friction material and lubricant system must possess correct friction characteristics for controlled clutch engagement and disengagement. In addition, the combination must be optimized for low wear, high heat capacity and good low-temperature behavior. The clutch lining material must be composed of a thermally and mechanically durable material, commonly a carbon composite. An open and porous structure facilitates circulation of the fluid in the friction material, enhancing cooling and clutch durability. In order to preserve the frictional durability, it is also essential that the material and lubricant combination are designed to remain as clean as possible, minimizing clogging of pores. Excellent cooling properties and flow regulation from the pump minimizes thermal shock, providing performance comparable to a torque converter. The wet clutch provides improved shift comfort and is better suited to higher torque applications than a dry clutch system. The DCTF additive system must be compatible with the friction material to impart good frictional properties, resulting in smooth gear shifts. Ideally, the lubricant and friction material must be developed in combination to ensure synergy. In addition to the friction material, the surface finish of the separator plates and groove design impact shift quality. Unoptimized friction characteristics, low material elasticity and surface defects can contribute to shudder. For each shift, both clutches need to be engaged and disengaged simultaneously, leading to an estimated 2 million shifts over the transmission life [2]. The launch mode is particularly severe and accounts for ~80% of the frictional impact on the clutch, making highperformance friction materials and an optimized lubricant cooling system necessary. Therefore, the DCTF must be formulated to protect the clutch and sustain consistent launch quality and anti-shudder durability for the lifetime of the DCT. Many studies have detailed friction characteristics within start-up and shifting clutches [3] [4] [5] [6] [7] [8]. Appropriate friction characteristics will mean controlled, comfortable and fast shift times. In general, a steady increase in friction coefficient with slipping speed is required for correct shifting without noise or shudder. The total friction during a clutch engagement is the sum of hydrodynamic and asperity friction [Figure 4] [9]. At the beginning of the engagement, the hydrodynamic friction is the governing factor; later during the engagement, asperity friction dominates. The DCTF plays a major part in controlling the frictional response. The bulk properties of the lubricant have more effect during the hydrodynamic phase. The friction modifiers and other surface active components in the additive

package are largely responsible for controlling friction at the low-speed end of the engagement. The dynamic friction value strongly influences the resultant clutch torque capacity. The static value determines the holding capacity of a clutch. In general, it is desirable for these values to be as high as possible, without compromising a positive friction vs. speed (mu-v) curve, which gives rise to stick-slip and shudder.

Figure 4

Frictional response during a clutch engagement [Lam]

Many bench [2] [4] [6] [10] and transmission tests [11] have been developed to assess friction response and durability over thousands of cycles. These types of tests have been used extensively during DCTF additive development. Inappropriate friction response and energy losses will ultimately impact fuel economy, a major driver in development of the DCT [12] [13] [14]. The clutch pack drag test developed at SWRI [10] can be used to measure individual losses from factors such as the friction materials, grooving, oil flow, component speeds and axial clearance. A simple yet versatile bench friction test, the Variable Speed Friction Tester (VSFT) [4], has been used to screen DCTF lubricant performance. This tester consists of a small, rotating steel annulus loaded against a disc of clutch friction material. Lubricant can be pumped into the system chamber. Speed, load and temperature conditions are computer controlled. The apparatus can run short, discrete tests or longer, multiphase cycle programs to stress the fluid and friction material combination in order to assess long-term friction durability. The VSFT has proved extremely useful for ATF, MTF and DCTF development. Not only can tests be designed to simulate real conditions (such as those associated with the wet start-up clutch or synchronizer element), but the device can be used to replicate field problems and correlate with standard full-scale test procedures such as CEC, JASO, and ASTM. The VSFT is particularly

useful for rapid screening of the relative performance of fluids, rather than absolute friction values. The VSFT programs used in DCTF development consist of 2 phases: (1) Durability mode: Friction coefficient is measured at constant speed, over time. This mode is used to stress the fluid and friction material combination, promoting aging and indicating long-term friction durability. (2) Single cycle sweep mode: This discrete test measures friction coefficient as a factor of speed and temperature and represents frictional response over a clutch engagement and disengagement. Tests can be designed to alternate between the durability and single sweep mode. In this way, the quality of the clutch engagement and disengagement can be assessed after long aging cycles. A delta mu-v graph can be used to show the mu-v gradient change of each sweep cycle over time. The gradient of the mu-v slope between 0.06 and 1.4 ms-1 is calculated at a 2hour interval. A standard 16-hour test run at 185o C, 1.5 ms-1 and 1.5 MPa can approximate clutch performance over 100,000-150,000 km. An added benefit is the option to examine both the used lubricant and friction material at the end of the test. Specifically, the size of the friction disk (2.5 cm diameter) means that it can fit into most surface analysis apparatus chambers. Figures 5 and 6 show VSFT data comparing a new generation DCTF with a conventional ATF. A modern wet clutch friction lining material used in DCT applications and standard durability conditions mentioned above were used. Figures 5 and 6 show the frictional response during single-cycle sweep mode. The DCTF shows a positive mu-v friction response over the entire speed, temperature and time range, indicating excellent antishudder friction durability. The ATF begins positive, but after time the response becomes negative, leading to shudder.

Figure 6

VSFT mu-v response for ATF

The frictional performance of the DCTF has also been assessed in a full-scale start clutch test [4] where the engagement and disengagement torque traces were measured over 2000 cycles. The test represents the high energy (3300 rpm, 400 kPa) conditions in the wet clutch. Figure 7 shows the test data for the new DCTF. The fluid performed extremely well; each friction trace shows a positive (descending) torque value, which endures throughout the test, indicating excellent material-fluid synergy and anti-shudder durability.

Figure 7

Start clutch test data with DCTF

After the test, the friction material and steel separator plate surfaces were analyzed to check that the critical additive components had adsorbed and that no wear had taken place. A comparative test was also carried out on an industrial MTF [Figure 8]. This fluid did not perform well; the torque traces begin positive, but the coefficient of friction decreases throughout the test, and the slow speed region of the engagements becomes negative. This effect is known as a rooster tail (highlighted) and results in poor shift performance and eventually shudder. Visual inspection of the plates after this test showed

Figure 5

VSFT mu-v response for DCTF

evidence of clutch plate glazing due to clogging of the material pores with oxidation products. The glazing prevents the desirable additive chemistry to adsorb and control friction. This effect clearly shows the need for a new and dedicated additive system for the DCTF. In addition to providing correct friction control, the lubricant additive technology must promote cleansing of the lining material, prolonging the performance life of the clutch.

Figure 8 Start clutch test data with a conventional MTF Figure 9 GK wet start clutch data after 12,500 cycles for commercial reference DCTF vs. new generation DCTF

The additive technology plays a critical role in clutch friction performance, and good performance cannot be expected from an additive package designed for an existing ATF or MTF application. The GK friction test rig has also been used to evaluate the new DCTF technology for wet clutch friction response. The GK is a standard industry apparatus that can accommodate full-size wet start clutches. The procedure used below is a severe test of the antishudder durability of the DCTF and friction material combination over 25,000 clutch engagement cycles. Inlet temperature is 100o C and pressure is 1 N/mm2. Torque (shown in red) is measured against time in seconds. To increase the severity of the test, the test fluids also undergo some pre-stressing. Figure 9 shows response at 12,500 cycles for a current commercial DCTF (top) compared to the new DCTF (bottom). The reference shows a torque oscillation, centered on 175 Nm, indicating poor anti-shudder performance. Figure 10 shows the response for both fluids at 20,000 cycles. At this point, the reference fluid gives a tremendous torque oscillation; however, the new DCTF still provides excellent anti-shudder performance.

Figure 10 GK wet start clutch data after 20,000 cycles for commercial reference DCTF vs. new generation DCTF

The new DCTF technology has also been tested in a very severe, high-energy launch test. This test simulates 10,000 trailer launches on a 13% gradient with clutch inlet temperature 110° C and outlet 170-190° C. The extremely severe conditions of the test mean that the DCTF, and specifically the additive technology, is pushed to the limit of endurance. For this reason, it is a definitive assessment of friction performance durability. A mu-v slope determination is made in both the 10-250 and 200-500 rpm ranges. For good anti-shudder performance, both curves must remain positive throughout the test; any negativity will result in impaired vehicle launch. The recently developed DCTF gives excellent performance in this test. Figures 11 and 12 show the low- and high-speed mu-v response respectively. In addition to the positive mu-v requirements, other specific frictional characteristics can be assessed in this test. A very sharp radius at low speed can be an issue, leading to control problems; however, the DCTF fluid tested does not show this tendency. Micro-slip also occurs during sudden acceleration, when the clutch is briefly allowed to slip at very low speed. It is important that no shudder occurs in this region. Again, the new DCTF does not exhibit shudder under these conditions, even after extended test duration. After test completion, visual inspection of the clutch test parts reveals no surface damage.

In order to assess the frictional performance of the DCTF on the critical synchronizer components, a fullscale, high-energy synchronizer test was also carried out [4]. The test measured dynamic friction of the elements over 5,000 synchronized cycles. In order for rapid yet controlled synchronization, a specific friction coefficient must be maintained over the entire test, with no evidence of wear. If friction deteriorates or wear occurs, this can lead to synchronizer clash or baulk. The DCTF candidate gave excellent performance. THERMAL STABILITY AND OXIDATION PERFORMANCE Similar to ATFs and MTFs, DCTFs must have excellent thermal and oxidative stability. The fluid must remain clean with minimal viscosity increase during transmission operation. Acidic products of oxidation and thermal breakdown (carbonyl compounds, carboxylic acids, esters and lactones) plus polymerized molecules can lead to undesirable sludges and lacquers that can impair hydraulic and actuator function. Surface temperatures of several hundred degrees Centigrade can exist in the clutch system. It is very important that the friction material pores do not become clogged with oxidized species, leading to glazing. The new DCTF technology provides excellent antioxidancy. The industry standard DKA oxidation test was run (CEC-L-A00/170o C/192hr). The kinematic viscosity change at 100o C was less than 10%, and minimal total acid number increase was observed. A recycling hot tube test was developed [4] to more accurately assess the ability of the DCTF to prevent deposit blockages in the connecting ducts and bores within the DCT unit. In brief, this test consists of a glass tube apparatus. The test fluid is heated to 280o C and circulated under constant air flow for 20 hours. At the end of the test, the tube is rated for cleanliness. The DCTF technology clearly outperformed a conventional MTF, showing the enhanced antioxidant and thermal stability qualities of the additive package. WEAR PROTECTION As previously discussed, gear conditions within the DCT are more severe than a corresponding AT. The epicyclic gear set in a conventional AT consists of a central sun element surrounded by orbital gears that serve to share load, reducing the stress on individual teeth. By contrast, the DCT uses the same helical gear system as the MT and has a corresponding high load-carrying lubricant requirement. The new DCT fluid technology has been developed to fulfill this criterion. FZG test procedures were used to evaluate gear wear performance of the DCTF. FZG A/8.3/90 is the standard FZG scuffing test. A result of fail load stage > 12 is a very strong result even though viscosity is < 7 cSt at 100o C. A more severe version of this test (FZG A10/16.6R/90) was also run and a fail load stage of 10 achieved, indicating very strong wear protection. The improved wear protection of the new generation DCTF distinguishes it from other first-generation fluids,

Figure 11 High energy launch test data with DCTF. determination at low speed

Mu-v

Figure 12 High energy launch test data with DCTF. Mu-v determination at medium speed

particularly as friction performance and component compatibility are not compromised. FATIGUE PROTECTION In addition to gear testing, the new additive technology has been evaluated in bearing fatigue tests (test conditions listed below). Test bearings: Shaft speed (rpm): Test load (kN) INA 81212 with PA cage run-in: 400 run-in: 60 test: 750 test: 60 test: 100 test: 210

•

fuel consumption 14.81 liters/100 km (also same below)

Duty cycle These vehicles were driven periodically on the track for 18 months and covered over 24,000 km. The fueling rate indicates the drive severity. Manufacturer recommendations indicate fuel consumption under normal conditions is approximately 7 litres/100 km, which indicates that these vehicles are working twice as hard. Transmission cleanliness rating (CRC Manual 12 DC Method)* • • Factor-fill vehicle New DCTF vehicle 9.7 9.7

Test temperature (° C) run-in: 100 Test duration (hours) run-in: 24

At the end of the test, weight loss of the bearing components is measured. Low values indicate minimum pitting and hence low fatigue. Example results from new DCTF technology are shown below. Weight loss due to pitting is extremely low, indicating high resistance to fatigue. Weight loss (mg): Bearing 1: Bearing 2: Mean: FIELD TESTING inner track 0.0 1.0 0.5 outer track 2.0

(* rating of 10 is completely clean) Gear wear rating (CRC Manual 21) • • Factory-fill vehicle New DCTF vehicle 1.7 1.4

Synchronizer wear 0.0 1.0 Synchronizer wear was most evident on fifth gear for both vehicles. • A small field trial was carried out in manual shift vehicles to confirm the transmission protection properties of the DCTF technology against the current factory fill MTF. In order to maximize stress on the fluids for a limited period of time and mileage, some vehicles were run under severe race track conditions. The transmissions were operated under very heavy load, harsh-duty conditions for over 24,000 km. This field testing, carried out in modern manual transmissions, is a good measure of load-carrying, antiwear protection and anti-corrosion performance of the new DCTF compared with a commercial factory fill MTF. At the end of the test, each transmission was inspected and components rated. Vehicle specifications: • • • • Audi A3 Sport 2.0l FSI - 110 kW 6-speed manual transmission transmission code GB02S300044M test duration 24,000 km • Factory-fill vehicle New DCTF vehicle 0.65 mm gap 0.05 mm gap

Bearing wear • • Factory-fill vehicle New DCTF vehicle 1.5 (Normal) 1.5 (Normal)

In conclusion, the oils perform comparably. Cleanliness is excellent, and seal condition is normal. Both fluids provide a high level of wear protection of gears and bearings. Figure 13 shows two photographs, highlighting the excellent condition of the output shaft roller bearing taken from the new DCTF vehicle.

• • • • •

high thermal and oxidative stability high load-carrying ability excellent bearing performance strong anti-corrosion performance excellent compatibility

The DCTF additive technology is neither an ATF nor an MTF but a new technology tailored to the specific and sometimes contradictory requirements of the DCT. Recent advancements in the technology allow for: •
Figure 13 Photographs of output shaft roller bearing

shudder-free service in excessive conditions longer drain longer transmission life

• •

COMPATIBILITY A very important consideration when formulating any transmission lubricant is the compatibility of the fluid with hardware components such as the metallic mechatronic parts, sealing materials and adhesives. The new generation DCTF additive technology has been specifically tailored to satisfy corrosion and compatibility requirements. The standard industry tests for copper corrosion (ASTM D130) and rust (ASTM D665) were run, with the DCTF providing clear passes in both. The technology has also been extensively tested with a variety of elastomer and seal materials including acrylate, nitrile and silicone. Many parameters, including tensile strength, rupture elongation, weight and volume changes were assessed. The lubricant additive chemistry plays a major part in determining compatibility with these components, although the contribution from the base fluid must be considered. The much improved compatibility performance of the latest additive technology is a defining feature compared with the older DCTF technology.

ACKNOWLEDGMENTS
The authors gratefully acknowledge BorgWarner and Fuchs Europe Schmierstoffe GmbH for permission to use test data. Thanks to Wayne Moore, Michael Gahagan and Jim Sumiejski, Lubrizol Limited and The Lubrizol Corporation.

REFERENCES
1. “DCT market potentials for passenger cars.” R. Voss, R. Herbst. JSAE Annual Congress Proceedings, 67, pp 9-12 2004. 2. “DualtronicTM lessons learned applications.” Bernd Matthes. and future

3. “Review of wet friction component models for automatic transmission shift analysis.” Y. Fuji, W. E. Tobler, G. Pietron, M. Cao and K. W. Wang. SAE International, May 2003. 4. “Lubricating dual clutch transmissions. The development of generation II dual clutch fluid technology.” M. P. Gahagan, S. Hurley, International Tribology (ITC) KOBE 2005. 5. “ATF friction properties and shift quality.” T. M. Cameron, T. McCombs, S. Tersigni and T-C. Jao. SAE International 2004. 6. “Formulating fluids with improved friction durability for wet start clutches.” R. F. Watts, R. C. Castle, K. R. Gorda, R. K. Nibert.

CONCLUSION
The DCT combines the performance advantages of MT and conventional AT designs and has led to the development of new lubricant systems. New and specialized fluid technology has been developed to fulfill the unique requirements of the DCT. The new DCTF fluid technology provides: • • excellent wet clutch friction performance excellent anti-shudder friction durability

7. “Effect of friction material on the relative contribution of thin-film friction to overall friction in clutches.” M. T. Devlin, S. H. Tersigni, J. Senn, T. L. Turner, T-C Jao and K. Yatsunami. SAE International pp 193203, 2004. 8. “Designing paper type wet friction material for high strength and durability.” K. Ito, K. Barber, M. Kubota, S. Yoshida. SAE 982034, 1998. 9. “Prediction of torque response during the engagement of wet friction clutch.” Y. Yang, R. C. Lam, T. Fuji. SAE 980197, Feb. 1998. 10. “The drive for better fuel economy.” L. Grant, Technology Today, SWRI Autumn 2002. 11. “Drivetrain test bench for dual clutch systems.” D. Apold, O. Moseler, P. Prystupa and M. Schiffer. Press release ATZ Worldwide Volume 106 pp 17-19 and 23, June 2004. 12. “The new dual clutch gearbox.” W. Schreiber, F. Rudolph and V. Becker. Press release ATZ Worldwide Vol. 105 pp 2-6, November 2003. 13. “Technologies for modern manual transmission performance.” M. P. Gahagan, T. Yoshimura, J. N. Vinci. JSAE Japan, 2003. 14. “The features of automotive transmissions.” F. Kucukay. JSAE Annual Congress Proceedings, 68, pp1-4, 2004.


				
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