Press Release
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Multiple downshifts for fast "kickdown" acceleration
The sliding multiple downshift principle is enhanced by dispensing with sequential gearshifts when appropriate. When fast downshifts are required, the 7G-TRONIC transmission will sometimes skip gears, for example changing down from seventh gear directly into fifth, and then from fifth into third. Thus in some driving situations, only two instead of the previous four gear changes may be necessary for fast acceleration under kickdown.
At the same time smoothness is also improved. The rapidly executed, "sliding" gear changes are virtually imperceptible, particularly in the higher gears. In subjective appraisals of smoothness, around 95 percent of all gear changes went virtually unnoticed by the driver.
Converter lockup engages even in first gear
Like its predecessor, the new seven-speed automatic transmission is equipped with a torque converter lockup clutch. Under a wide range of operating conditions the lockup clutch, located in the hydrodynamic torque converter, almost completely eliminates slip between the pump and turbine. Whenever appropriate, it creates a virtually rigid connection between the engine and the transmission shaft, thus preventing power losses. On the new Mercedes-Benz 7G-TRONIC, converter lockup engages even in first gear, rather than only in the higher gears as on conventional automatic transmissions.
The lockup clutch engages with a degree of controlled slippage, which makes a further contribution to smoothness.
Based on the intelligent principle pioneered by French engineer Ravigneaux
In technical terms, the new seven-speed automatic transmission is a logical development of its predecessor, which has been giving excellent service for many years. The transmission developers at Mercedes-Benz decided to combine two simple planetary gear sets with a "Ravigneaux" gear set. Here the simple planetary gear set used before is replaced by two different planetary gear sets and sun gears operating in a single ring gear. This design, first developed by the Frenchman Paul Ravigneaux in the nineteen twenties, provides the basis for a transmission with seven forward speeds and two reverse speeds. At the same time the Stuttgart engineers have also changed the gear tooth geometry, resulting in a further reduction in power losses and noise.
As before, the actuation elements comprise hydraulically operated multiple-disc
clutches and brakes – a design which is capable of handling much higher torque, of up to 700 newton metres – than other solutions.
Despite its greater torque-carrying capacity and its two additional forward gears, the new transmission is only 41 millimetres longer than the previous version, while the diameter of the torque converter is actually 20 millimetres smaller than before. And despite the seven gears, there is virtually no difference in weight between the two transmissions – since for the first time the transmission housing is produced from magnesium, the 7G-TRONIC weighs in at just 81.5 kilograms excluding the control unit.
Shift plate and control unit form a single unit
Operation of the gearshift is the same in principle as for the previous five-speed
transmission with Touchshift. Also familiar is the mode selector for "S" (=Standard) and "C" (= Comfort) programs. Program "C" features second-gear start and lower-rpm upshifts. Future shift-by-wire technology is also catered for, with steering-wheel gear-shift buttons which allow gears to be selected quickly and conveniently.
As far as the control unit is concerned, the engineers have once again trodden new paths. It is integrated with the shift plate, which contains the maze of hydraulic passages and tubes supplying the hydraulic fluid. This means fewer cables and further enhanced reliability.
The electronic control unit is directly integrated in the oil sump, which allows it to
directly measure, using sensors, three different rotational speeds within the transmission and to directly evaluate this information. The signals for the eight solenoids which are also integrated in the control unit are likewise transmitted very directly. The solenoids regulate the working pressure, the seven actuation elements and the torque converter lockup clutch. The fully integrated control system makes a further important contribution to the exceptionally fast operation of the new transmission.
Fast data exchange with other control units
Further information is supplied to the control unit by sensors monitoring rolling resistance (uphill or downhill gradient, vehicle payload), atmospheric pressure (altitude), driver inputs and longitudinal and transverse vehicle acceleration. The transmission control unit uses this information to vary the shift points with reference to driver-specific and vehicle-specific requirements, and to calculate the required hydraulic pressures and the control signals for the torque converter lockup clutch. The transmission control unit is also linked, by databus, to the engine management system, which means that it has access at all times to all the signals and derived data currently being supplied by sensors and other control units.
Conversely, as well as receiving information, the transmission control unit also
transmits its own commands to the engine management system, for example requesting a reduction in engine power while gears are being changed, or calling for shift point changes designed to speed catalytic converter warm-up. The control unit of the Mercedes-Benz seven-speed automatic transmission operates flexibly rather than just switching between a small number of stored shift programs as in the case of previous transmissions.
This ensures optimal transmission control whatever the operating conditions and
whatever the wishes of the driver. This in turn guarantees fuel-efficient operation,
high standards of ride comfort and sporty, agile response.
