New Mercedes Compact Car Design Centered Around DCT

By Tony Lewin, managing editor DCTfacts.com

Mercedes-Benz provided a major stamp of approval for dual clutch transmissions with the launch of its new B-Class mid-sized hatchback at the Frankfurt auto show in September. The standard automatic transmission option for this model is an all-new seven-speed DCT, selected after exhaustive evaluation of many other transmission options. The outgoing B-Class used a push-belt CVT system supplied by Bosch-Van Doorne; the smaller A Class model range used exactly the same mechanical elements as the B-Class, so its replacement, due next year, will have exactly the same read-across of components.

In contrast to the specialized DCT-equipped SLS supercar at the top of the Mercedes-Benz range, the B- and A-Class models will be major sellers around the world. Their sales numbers are likely to help reinforce the rising global acceptance of DCT transmission systems.

Mercedes often boasts of a significant number of innovations when it launches a major new luxury car. Surprisingly, considering the compact dimensions of the new B-Class, Mercedes-Benz R&D director Dr. Thomas Weber is at pains to point out that “. . . no model change in the history of Mercedes-Benz has ever seen so many new developments introduced in one fell swoop."

Headlining the list of innovations – apart, of course, from the all-new architecture and styling – are new gasoline and diesel engines, new six-speed manual and seven-speed dual clutch automatic transmissions, new dual-pinion electromechanical power steering, new suspension systems and a host of electronic innovations headlined by radar-based collision prevention assistance, claimed to be the first in the compact class.

Gearbox Design

With the gasoline and diesel powerplants now sitting more upright in the engine bay (in contrast to the old model, where the motors were steeply reclined), the new gearboxes can now be more conventional in their layout. The six-speed manual and seven-speed DCT are closely related in their design, says Mercedes, both adopting a three-shaft layout for maximum compactness and sharing several components, as well as being made in the same production plant.

Just 345 mm in axial length, the manual has a dry weight of 46 kg and its six ratios span a range of 6.7; clutch action is hydraulic and the gear selectors are cable operated. An integrated magnet on the internal shift shaft is monitored by a Hall sensor to provide the neutral detection required to trigger the engine stop-start and, in the interests of reducing drag torque, the third, fourth and reverse gears on the top shaft do not run in the oil bath. Mercedes also says that this aids gear shifting at low temperatures.

As befits for the premiere of an all-new transmission type within the Mercedes compact car range, the company makes bold claims for the 7G-DCT in the B-Class. “This transmission is extremely compact, extremely variable with regard to adaptation of the engine speed thanks to seven gears,” says the announcement. It “. . . incorporates an electric oil pump for start/stop capability, shifts gear without any interruptions in tractive power and combines the comfort of an automatic with the efficiency of a manual transmission.”