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Getrag Adds Second-Generation DCT With Dry Clutch

Growing awareness of CO₂ emissions, the impending launch of fuel economy color coding in Europe, and the threat of gas guzzler penalties, also in Europe, have focused industry attention on efficiency like never before. Transmissions, with the capacity to make the most of efficiency gains being made in the engine area, are at the front line of this campaign. “We’re fighting for every gram of CO₂,” observed Getrag’s group vice president for R&D, Ralf Najork.

No longer are the easy gains made by first-generation dual clutch transmissions sufficient, especially in the small-car sector where fuel economy and CO₂ emissions have become perhaps the most important differentiator between the many dozens of competing models. Efficiency is everything here, prompting automakers and transmission suppliers to look to dry clutches in DCTs so as to eliminate the energy loss of the oil pump in wet-clutch systems.

Even the clutch and gear actuating systems have been under sustained review, again to reduce or eliminate the energy losses associated with hydraulic pumps and actuators. Additionally, replacing hydraulic systems with electro-hydraulic or electro-mechanical operation opens the door to compatibility with two other key fuel-saving technologies: micro-hybrid stop-start systems and full hybrids, where electric motors are integrated into the power train to actually propel or assist the vehicle.

It is not surprising, therefore, that the new second-generation product from leading transmissions supplier Getrag reflects precisely these concerns. Aimed at small and medium cars (B- and light C-segment) with torque outputs of up to 280 Nm, the 6DCT250 is a compact dual dry-clutch unit with electromechanical shift and clutch actuation, stop-start, hybrid and AWD capable, and requiring no additional cooling system.

The 6DCT250 complements Getrag’s existing wet-clutch transmissions, the 6DCT450 and 6DCT470. These are familiar on the market, with customers such as Ford and Volvo. In development – Getrag does not say how far away – are the smaller five-speed 5DCT150, capable of up to 180 Nm, and the seven-speed 7DCT480, targeted at transverse front and AWD applications of 350 to 530 Nm torque. The clutch solutions for these units have not been disclosed.

Impressive Specification

The 6DCT250 shares the three-shaft layout of its higher-torque counterparts, but is notably more compact and lighter. Weighing as little as 72 kg (depending on the flywheel configuration) complete with control unit and lubricant, it is almost 20 kg slimmer. It is shorter end-to-end, too, at as little as 350 mm, aiding installation in compact B-segment cars – just as with the new Fiat Powertrain DDCT 635. Fiat does not quote an axial length for its unit but, like the Getrag transmission, it emphasizes compactness.

The headline figures make impressive reading: six speeds, a maximum gear ratio spread of 7.2, simple synchromesh and actuator systems, and a claimed efficiency 20 percent better than that of a state–of–the–art, six-speed planetary automatic transmission. Indeed, says Getrag, the average electrical power consumption of the actuators over a typical drive cycle is just 15 watts – very significantly less than would have been absorbed by hydraulic or even electro-hydraulic actuation systems.

Proof comes from Renault, the chosen launch customer for this second-generation transmission. In the Mégane C-segment range, the French company says the transmission saves 17 percent in fuel consumption, equivalent to a reduction of 30 grams per kilometer in CO₂ emissions. This helps achieve what many automakers have long dreamed of: a model line where buyers who wish for automatic transmission are not penalized with higher fuel consumption or extra taxation-related CO₂ emission penalties.

With production already underway in Italy and soon to start in both Mexico and, later, China, Getrag’s ambitions for the new transmission are substantial. “We are starting with [a volume of] 500,000 units a year,” says Dr. Carsten Bünder, senior manager of automatic transmission engineering at the company’s Untergruppenbach facility in Germany. “Once all the applications are in place, a million units will be realized.”

Ford, which has a joint venture with Getrag for transmission production, has announced that its new Focus platform will be available with a new dual dry-clutch transmission. With Ford promising ten different model lines on the platform and up to 2 million units of annual production by 2012, this will give a further major boost for the second-generation DCT transmission family.

Detailed Design

“This is a high efficiency, next-generation Powershift transmission for the world market,” says Bünder. “So design flexibility is important, as is manufacture. All products in this family are identical in terms of assembly, and the electromechanical actuator module can be rotated to adapt to the packaging conditions in each application.”

The three-shaft layout locates the odd-numbered gears on one shaft and reverse and the even gears on the other; synchronizers are single-cone apart from first and second gears. Four different versions, with center distances between 183 and 205 mm are available, but the majority of the components – especially the internal parts – are common between all versions.

Crucially, says Bünder, there is no drag torque in the neutral position; again, this is important in improving overall efficiency, especially when no stop-start system is fitted.

Two different types of double-clutch assembly are available. The cardanic type double clutch with an integrated slip-control disc damper mechanism is specified for lower torque applications with low engine excitations, particularly non-turbocharged gasoline engines. The total thermal mass of the three pressure plates is 6.2 kg, says the company. For diesel and turbocharged gasoline applications, there is a dual mass flywheel. In this case, the total thermal mass of the clutch assembly rises to 6.9 kg.

The clutches are both normally open – in contrast to the Fiat DCT, where the launch and odd-gear clutch is normally closed – and are each actuated by a novel, low-energy electric motor unit. Each motor drives a threaded spindle, which in turn moves a roller pressing onto a ramp on the lever, which actuates the clutch itself. The twin motor units are mounted on either side of the casing for the mechatronic actuator module (MAM), which also contains the brushless DC gearshift motors and their interconnectors, the control board, and the power board.

The MAM is completely self-contained and is fitted on the non-hot side of the transmission, away from the engine’s exhaust and turbocharger. This leads to lower ambient temperature conditions than mechatronic modules integrated within the envelope of the transmission casing, says Bünder. A single electrical connector interfaces the MAM with the vehicle wiring harness. Again, says Getrag, this system is common to all applications and allows complete "plug and play" fitment.

The shift motors actuate the twin shift drums via a train of two intermediate gears on each side. The shift drums are identical, with a single groove, and the shift forks are moved by the motion of the shift drum turning. This arrangement means that there is no need for an interlock system to prevent two gears from being engaged at once. All shifts are fully torque-supported, even multiple downshifts as far as sixth to second.

Development Objectives

Prime among Getrag’s development aims for the second-generation DCT was the minimization of energy losses so as to guarantee CO2 reduction targets. The use of low-inertia, brushless DC motors for clutch and shift actuation is part of that, as is the low average current consumption under a wide range of driving and shifting conditions. Getrag’s own drive cycle, which is considerably more extensive than the official European equivalent, shows an average power consumption of 15 watts; this excludes the current drain from the microcontroller.

The non-hydraulic actuation system fulfills another design objective – enabling start-stop and hybridization without any need for hardware changes. Development work continues on the integration of such systems.

Getrag’s claims of superior efficiency appear amply supported by Renault’s experience of a 17 percent reduction in fuel consumption, at last bringing parity in CO₂ emissions for consumers choosing the automatic rather than the manual transmission option.

The fact that Getrag is itself tooling up for a million units of annual production is further evidence of the auto industry’s confidence that DCTs, especially in their new and more efficient second generation, dry-clutch form, are the transmission of choice for carmakers striving to match – or even undercut – stringent upcoming legislative limits such as the EU’s 130 grams per kilometer fleet average CO₂ emissions requirement.

6DCT250 Transmission Specification
Max. Torque Capacity	240 - 280 Nm
Weight (dry)	72 -82 kg (incl. damper/DMF)
Installation Length	350 - 400 mm
Center Distance: Input shaft to differential	183 - 205 mm
Oil	1.7 - 1.9 l
Ratio 1st Gear	15 - 18.5
Ratio 6th Gear	2.1 - 4
Gear Ratio Spread	max 7.2
Synchronization
1st and 2nd Gear	Dual cone
3rd and 4th Gear	Single cone
5th and 6th Gear	Single cone
Control Unit	Integrated (only one plug to the vehicle)
Parking Lock	Mechanically actuated and locked
Shifter System	Electromechanical actuation of the dry clutch and shifting system

Story Filed: 5/03/2010
By Tony Lewin, managing editor DCTfacts.com