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What is dual clutch transmission?

The dual clutch transmission transmits power to the driving wheels more efficiently and in a more compact system than a conventional torque converter transmission. In future, it is also expected to make highly automated driving possible.

Efficient mobility due to dual clutch transmission technology

Smooth, jerk-free gear shifts are an important comfort element for car makers. The development of the dual clutch transmission has made this possible in modern vehicles. As a result, car manufacturers are increasingly focusing on automatic transmissions. These transmissions are also extremely important when it comes to the issue of autonomous driving. After all, the electronic systems for autonomous driving functions must also perform gear shifts (shift by wire) to enable the driver to be completely alleviated from the task of driving the vehicle. However, fully automatic gear shifts without any interruption in power delivery means that two automated sub-transmissions are required. These are connected to the power unit via clutches and transmit the engine torque.

Dual clutch transmissions usually use two wet multi-plate clutches. The plates are coated in order to increase the friction coefficients and to improve temperature stability. Because this clutch design has drag torque when the clutch is disengaged, efficiency is reduced by around three to four percent. Therefore, for engines with a maximum torque of around 250 Nm, vehicle developers also use single-plate dry clutches. Force is transmitted by a clutch plate that is pressed to the flywheel of the engine by several springs with a pressure plate.

Dual clutch transmission with shorter design

A further difference compared to conventional dual clutch transmissions is the way in which the clutches are arranged. Whereas some manufacturers arrange clutches of the same diameter one behind the other, the designers at Volkswagen, for example, use different plate diameters for their DSG direct-shift gearbox. This makes it possible to interleave the two clutches and to shorten the length of the gearbox as a whole. The gearshift function is maintained, however. While one clutch closes, the other opens.

The gearshifts are controlled by the individual gearshift programs that are determined by the electronic transmission control unit (TCU) on the basis of engine speed and road speed. The gear shifts can, however, also be activated by the driver via the accelerator pedal or the gear selector lever. The actuators integrated into the gearbox actuate both the sub-transmissions and the clutches. The arrangement that one sub-transmission transmits the even gears while the other the uneven gears, and the way in which reverse gear is shifted, are determined according to the transmission concept. With regard to the gear wheels (gear set principle), the sub-transmission is no different from a manual gearbox. Both sub-transmissions also transmit their torque to a joint output shaft. Also for that reason, dual clutch transmissions are suitable for vehicles with a start-stop system, as they can be relatively easily combined with each other.

Dual clutch transmissions on the advance

In spite of the advantages offered by dual clutch transmissions, only 20 percent of cars on the German market have an automatic transmission. Only around twelve percent of cars have a dual clutch transmission. However, against the background of highly automated driving and a further tightening of emissions legislation, the industry is expecting manual transmissions to be phased out. Depending on the engine configuration (front transverse or front longitudinal), dual clutch transmissions have further advantages over torque converter transmissions with regard to installation space and weight.

Whereas dual clutch transmissions are available today for almost all compact and mid-size vehicles from the manufacturers Volkswagen (with the largest range for Seat, Skoda, Audi), Porsche, Ford, Mercedes and Volvo, DCTs are only rarely installed in SUVs and in luxury-class vehicles. However, future generations of automatic transmissions will also be suitable for fast-accelerating sports cars and for vans. A further trend is the electrification of the powertrain with the use of electric motors as a source of driving power.

Dual clutch transmissions for hybrid drive systems

Although vehicles with an all-electric powertrain do not require a dual clutch transmission, hybrid vehicles need special gearbox designs. For example, the supplier ZF and car maker Porsche have collaborated to develop an eight-speed dual clutch transmission (8DT). Conventional DCTs have six or seven gears. The special feature of the eight-speed DCT is that it is a modular hybrid transmission system. The 8DT is suitable for vehicles with a standard, hybrid and all-wheel hybrid powertrain. It can handle a maximum torque of 1,000 Nm.

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Dipl.-Ing. Dietmar Bichler
Bertrandt AG

Dipl.-Ing. Kurt Blumenröder

Dr.-Ing. Joachim Damasky

Prof. Dr.-Ing. Lutz Eckstein
RWTH Aachen, WKM

Dr.-Ing. Ulrich Eichhorn
Volkswagen AG

Dr. rer. nat. Andreas Eilemann
Mahle Behr GmbH & Co. KG

Dipl.-Ing. Klaus Fröhlich

Prof. Dr.-Ing. Burkhard Göschel
Burkhard Goeschel Consultancy

Prof. Dr.-Ing. Peter Gutzmer
Schaeffler AG

Dr.-Ing. Markus Heyn
Robert Bosch GmbH

Dr.-Ing. Carsten Intra
MAN Truck & Bus AG

Prof. Dr.-Ing. Pim van der Jagt
Ford-Forschungszentrum Aachen GmbH

Dr.-Ing. Stefan Knirsch
Audi AG

Dipl.-Ing. Ralph Lauxmann
Continental Teves AG & Co. oHG

Dipl.-Ing. (BA) Joachim Mathes
Valeo Schalter und Sensoren GmbH

Dr.-Ing. Harald Naunheimer
ZF Friedrichshafen AG

Dipl.-Ing. Jörg Ohlsen
Edag GmbH & Co. KGaA

Prof. Dr. Dipl.-Ing. Peter Pfeffer
Hochschule München

Prof. Dr.-Ing. Rodolfo Schöneburg

Dipl.-Ing. (FH) Dipl.-Wirtsch.-Ing. (FH) Wolfgang Schwenk
Adam Opel AG

Dr. Michael Steiner
Dr. Ing. h.c. F. Porsche AG

Prof. Dr.-Ing. Thomas Weber
Daimler AG

Dr.-Ing. Christian Wiehen
Wabco GmbH

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