Dual-Clutch Transmissions: "Lucky Number Seven"
(1-Sep-2008, Ward's AutoWorld)
Getrag GMBH & CIE KG Becomes the latest high-profile player in the dual-clutch-transmission segment as BMW AG and Ferrari Automobiles SpA rush to incorporate the German supplier's new drivetrain technology in their newest performance cars.
Many praise-worthy headlines are to be expected of these new transmission systems, which deliver more than ever the performance benefits of a sequential-manual gearshift with the smoothness of a conventional torque-converter automatic.
Rushed is the wrong word to describe the similar DCT system Porsche AG developed with ZF Friedrichshafen AG for the '09 911. The German auto maker originally planned to launch the 7-speed PDK (Porsche-Doppelkupplung) gearbox with the release of the current 997-edition model, but “development issues” delayed the program by at least three years.
However, rushed correctly applies to Getrag's 36-month development for its two all-new DCT gearboxes, which debut in the BMW M3 this fall and upcoming '09 Ferrari California.
The new 7-speed transmissions — 7DCI600 for the M3 and 7DCL750 for the California — were developed in parallel, but according to Stephan Rinderknecht, Getrag's head of research and development, “They were independent programs; there are no common parts.”
To broaden the 7DCI600's appeal, Getrag offers the gearbox in two basic forms, both capable of accepting up to 443 lb-ft (600 Nm) of torque.
For the M3, the high-speed version offers a ratio spread (between first and seventh gears) of 4.8, with a direct top gear that permits 9,000 rpm.
The alternative variant (yet to be seen on a production car), in which fifth gear is direct and sixth and seventh are overdrive gears, offers a planned ratio spread of 6.7, although this could stretch to “beyond 7.0.”
Because the overdrive ratios would increase prop-shaft speeds to well over 10,000 rpm, this version is limited to engine speeds of 7,500 rpm.
“The main focus in the development was a superb level of economy,” Rinderknecht says.
In the M3, the high-speed DCT delivers a 5% fuel-economy improvement over a 6-speed automatic. However, Rinderknecht claims the wide-ratio gearbox carries at least a 10% advantage.
“We also wanted to offer flexibility between comfort and sporting (gear) shifting and to allow short shifting in low-speed driving, so that it is capable of performing at virtually the (same) level as an automatic gearbox.”
Still, he admits software upgrades to further improve low-speed smoothness already are in the pipeline.
The M3 is just the first of many high-performance, front-engine/rear-drive cars to use the new 7DCI600 transmission, called M-DCT by BMW.
Getrag's factory, 31 miles (50 km) north of Stuttgart, which produces the new gearbox, is set up to allow a “flexible concept on volumes,” Rinderknecht says, noting the factory architecture allows for a production of 100,000 units per year.
Because the M3 only is expected to absorb about 15,000 units annually, and the addition of a DCT option for certain '09 3-Series models won't take up the remaining capacty, that leaves plenty of room for other programs.
However, the BMW M5 and M6, which currently are saddled with the oft-criticized single-clutch SMG automated manual, will have to wait until their next iterations to sport a DCT, despite the new gearbox's ability to cope with the 5.0L V-10's 384 lb-ft (520 Nm) of torque.
Getrag's dual-clutch transaxle unit for the Ferrari California is more robust than the BMW version and capable of absorbing 553 lb-ft (750 Nm) of torque. This is far beyond the maximum torque of Ferrari's latest V-8 powertrain, which is expected to produce in the California slightly more than the F430's 343 lb-ft (465 Nm).

Image: Getrag 7DCL750 (Ferrari California DCT)
It, too, is capable of extreme revs — 9,000-plus rpm, which is well above the 7,500-rpm maximum permitted by even the best conventional automatic — yet manages a range of ratio spreads from 4.7 to 6.1.
The Ferrari gearbox is built in much lower numbers of between 5,000 and 10,000 annually.
“We expect further customers, because it offers important advantages over an automatic at high revs,” says Rinderknecht.
As such, it's not hard to imagine the gearbox becoming standard on all Ferrari road cars, much like how carbon-ceramic disc brakes have proliferated through the auto maker's lineup.
Getrag's involvement with DCTs dates back to the early 1980s, when the supplier worked with Porsche on PDK gearboxes for its 962 Le Mans prototypes.
Rinderknecht admits inadequate electronics slowed joint-development programs with Ford of Europe in 1996 and 1997.
“It was clear that technology was not ready for the product,” he says “The ECU (electronic control unit) processor simply was not fast enough.“
Without a firm commitment from an auto maker, Getrag could not devote the necessary resources to proceed with the development on its own.
It was the Direct Shift Gearbox (DSG) breakthrough by Volkswagen AG and BorgWarner Inc. in late 2003 that provided the impetus to move beyond the crude, single-clutch robotized manuals that slowly gained popularity in the late 1990s.
“It was always a problem for us, so we were happy that VW took the system and proved it could work. It opened up the complete market,” Rinderknecht says.
VW worked on the technology for more than 15 years before introducing the first 6-speed DSG in 2003.
Among the auto maker's brands, VW, Audi, SEAT and Skoda all offer the feature, as well as the Bugatti Veyron and its unique Ricardo plc-developed 7-speed version.
VW built 400,000 DSG gearboxes in 2007 and recently increased daily output to 1,750 units to keep up with demand.
Earlier this year, the auto maker released a 7-speed version (DQ200), which uses a pair of dry clutches vs. the 6-speed's wet clutches, providing an improvement in efficiency, simplicity and performance in low-power applications.
A high-torque, 7-speed DCT with wet clutches will debut later this year on several high-performance Audi AG models under the S-tronic label.
In addition, Getrag currently has high-volume deals for dual-clutch gearboxes with Ford Motor Co., Volvo Cars, Chrysler LLC and Mitsubishi Motors Corp.
Combined with new transmissions continuing the charge in two of the most-storied nameplates in sports-car history, DCTs surely are destined to become de rigueur in more performance vehicles over the next decade.
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ZF Group: Press Information
(16-Jun-2008)
7-Speed Dual Clutch Transmission by Porsche and ZF Enters Volume Production
- Faster gearshifts without traction interruption.
- Lower consumption with improved driving performance.
- Presentation at the 'Transmissions in Vehicles' Event of the VDI (Association of German Engineers) in Friedrichshafen.
- Volume production launch in the Porsche 911 Carrera.
The new dual clutch transmission is presented for the first time to approximately 900 engineers and press representatives at the VDI 'Transmissions in Vehicles' Event in Friedrichshafen. The 7DT sports transmission is particularly suited for applications with high engine speed requirements. Gear changes without traction interruption allow for very good acceleration and vehicle handling with low fuel consumption. The transmission was developed in Kressbronn, Brandenburg, and Schweinfurt; it is produced at the Brandenburg site of ZF's Car Driveline Technology division.
Drivers of sporty cars expect their vehicle's engine power to be transformed as directly as possible into dynamic propulsion. In addition to this acceleration "linked directly to the accelerator", however, they also attach great importance to automatic shifting comfort. ZF's 7-speed sports transmission accommodates both requirements for vehicles featuring highly powerful engines: the sporty agility of a manual transmission and the shifting comfort of an automatic transmission.
In the dual clutch transmission this becomes possible by connecting two separate transmissions to the engine via two parallel powershift clutches. The even gears are located on one transmission, the uneven ones on the other. This basic principle of the transmission has the advantage that one gear in one of these transmissions ensures propulsion, whereas in the other transmission the next gear is already preselected by the electrohydraulic control unit. When shifting, one clutch is closed while the other opens. During the shifting process, traction is not interrupted. This means that, during acceleration, engine torque is continuously transformed and propels the vehicle – making an essential difference from the manual transmissions which were usually installed in powerful sports cars. In contrast to torque converters, the dual-clutch module by ZF Sachs, the Powertrain and Suspension Components division of ZF, is also suited for higher speeds of up to 8,000 revolutions.
All gear changes – also downshifts – are processed just as evenly and quickly with the 7DT 50 (that is the ZF-internal product designation); so the ZF dual clutch transmission sets a new benchmark for sportiness. A new speed governing concept, which is used for the first time in volume production with the 7DT, also contributes to these extremely short shifting times.
The drivers can choose from fully automatic shifting or manual gear selection via shift paddles or a shift lever. Also in the fully automatic mode, ZF offers several shifting programs: Apart from the comfort mode, there is also a sports and a supersports mode, the latter with considerably increased shifting dynamics (fastest shifting times and race-start function).
The 7DT sports transmission by Porsche and ZF excels thanks to its high power-to-weight ratio: The gears and shafts of the dual clutch transmission, which weighs approximately 120 kg, are produced from case hardened steel according to special, ZF-specific delivery instructions. ZF-engineers have introduced several measures - for example, controlling the cooling fluid flow as required to keep drag losses low. The 7-speed dual clutch transmission not only allows for particularly sporty driving performance but, thanks to its very good efficiency, also achieves consumption values which are below those of a manual transmission.
Especially with sports cars, there is a demand for transaxle transmissions – that is, transmissions with an integrated differential and axle drive – the 7DT sports transmission can also be supplied in this variant, which is suited for mid-engined vehicles with rear axle or all-wheel drive. This transaxle version of ZF's 7-speed dual clutch transmission is starting volume production in the torque range up to 450 Newton meters.
Caption:
New reference with dual clutch transmissions: The 7DT sports transmission by Porsche and ZF is available in two versions, for torques up to 450 and up to 700 Newton meters.

Image: ZF 7DT 45