Mercedes-Benz CLA – Transmission
Shifting comfort combined with efficiency
The 7G-DCT dual clutch transmission and the six-speed manual transmission enable a driving style that is both comfortable and dynamic. Both transmissions are extremely compact and share a series of components with each other.
The 7G-DCT dual clutch transmission is extremely compact, extremely variable with regard to adaptation of the engine speed thanks to seven gears, 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. The CLA 220 CDI is equipped as standard with the 7G-DCT transmission, it is also optionally available for the CLA 200 and CLA 180.
The manual transmission, designed along similarly compact lines as a three-shaft transmission, is a close relative of the DCT. Extremely easy gear shifting, low shift forces, low internal friction and a low weight are among its most important characteristics.
The 7G-DCT dual clutch transmission: dynamic and efficient
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.
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 in 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 CLA-Class. In combination with the electric parking brake, additional space has thus been created in the centre console for additional stowage facilities.
Three gearshift programs 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. The gear selected is displayed in the instrument cluster and a shift recommendation is made for a fuel-saving driving style.
A key factor contributing to the overall efficiency and dynamism of the CLA-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 six-speed manual transmission: convenient manual gear-shifting
The three-shaft transmission is also very compact (length 345 mm) and light (dry weight 46 kg) and incorporates a number of special features for particularly pleasant gearshifting. An integrated magnet on the gearshift shaft is detected by a Hall sensor. The idle position is identified in this way, making the start/stop function possible. The signal for reverse gear activates the reversing lights.
The large spread of 6.7 allows a reduction in engine speed while at the same time ensuring that sufficient tractive power is available when moving off with a fully laden CLA-Class and trailer up to a gross weight of 3.4 tonnes. The clutch is operated hydraulically and the gears are actuated via cables.
The overhead camshaft with third and fourth gears and the reverse gear do not run in the oil bath. This reduces drag torque, thus facilitating gear shifting at low temperatures in particular. The three-cone synchronisation of the first two gears serves the same purpose, while the following gears are provided with two-cone synchronisation. The weight-optimised cast aluminium shift forks are installed on anti-friction bearings on the shift rods, in order to reduce the shift forces.