The development of the Mercedes-Benz A-Class: The 7G-DCT gearbox - Dynamic and efficient

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Drawing on over 50 years of experience in the development and production of automatic transmissions, Mercedes-Benz ventured into new technical territory with the 7G-DCT dual clutch transmission: the new system is an automated three-shaft manual transmission consisting of two subtransmissions, each with its own clutch. Both actuation of the clutches and gear shifting take place fully automatically and without any interruption in tractive power. This allows a particularly comfortable but nevertheless dynamic mode of driving.

Thanks to its seven gears it offers an exceptionally large spread of up to 7.99. This means that a very short transmission ratio is available when moving off on an uphill slope with a high payload, for example, while during cruising the engine speed can be lowered considerably. The efficiency of this transmission leads to a nine percent improvement in fuel consumption compared to the CVT employed in the A-Class to date, and betters the consumption with a manual transmission.

At a length of 367 millimetres and a weight of 86 kilograms, the 7G-DCT is more compact and lighter than the transmissions in this torque class which have been available on the market to date. The clutches take the form of oil-cooled multi-disc clutches. The specially developed hydraulic fluid is actively cooled, thus ensuring correct functioning of the transmission even under extreme conditions, despite the comparatively low filling level of six litres.Excess engine heat can also be transferred to the transmission, which improves its efficiency. 

For the first time on this type of unit, the transmission is supplied with oil by two oil pumps – one mechanical and one electric. The electric pump maintains the oil pressure when the engine is switched off via the start/stop function. This means that the transmission is immediately operational when the engine is restarted and the vehicle can move off again without any delay. In addition, the electric pump is able to support the mechanical pump when peak loads apply, enabling a more compact and efficient design for the mechanical pump.

Another new feature is electric activation of the hydraulics for the parking lock, which is locked by mechanical means. This "park by wire" function enables the transmission selector lever to be positioned as desired: it is located in the form of a steering column lever on the right behind the steering wheel in the A-Class. In combination with the electric parking brake, additional space has thus been created in the centre console for additional stowage facilities.

Three gearshift modes are available to the driver:
ECONOMY: In this mode, the transmission performs gearshifts fully automatically and particularly comfortably. The gears are selected with due regard to a particularly economical style of driving at low revs.

SPORT: The transmission performs gearshifts fully automatically. The shift and response times are shorter, with gearshifts at higher engine speeds.

MANUAL: In this mode, the driver operates the transmission manually via shift paddles behind the steering wheel. The engine speeds at which the gears are shifted can therefore be freely chosen.
In ECO or Sport mode, the driver is still able to intervene manually in the gear-shifting process via the shift paddles. The transmission reverts to the selected automatic mode after the paddles have remained inactive for twelve seconds, or after a longer delay when driving downhill or on winding roads.

A key factor contributing to the overall efficiency and dynamism of the A-Class is the closely coordinated operation of the transmissions and engines. A continuous exchange of data between the control units ensures that the engines run at the ideal operating point at all times. The 7G-DCT transmission is manufactured at Daimler's Stuttgart-Hedelfingen plant.

Credits: Daimler AG

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