Mercedes-Benz to offer new generation of 4MATIC all-wheel drive system next year integrated with 7G-DCT transmission; lighter and more efficient
18 December 2012
|Elements of the new 4MATIC. Click to enlarge.|
Starting next year, Mercedes-Benz will offer a new generation of the 4MATIC permanent all-wheel drive system with fully variable torque distribution. This completely new development is based on the front-wheel drive architecture with transversely mounted engines.
The most innovative components of the new 4MATIC include the power take-off to the rear axle which is integrated in the 7G-DCT automated dual clutch transmission and the rear-axle gear unit with integrated, electrohydraulically controlled multi-disc clutch. This set-up enables fully variable distribution of the drive torque between front and rear axle. Additional benefits of this design are a lower system weight than is available from the competition and high efficiency. Similarly to the front-wheel drive versions, all 4MATIC models also boast good energy efficiency.
The new 4MATIC will complement the new compact class and the models of the C-, E-, S-, CLS- and CL-Class to the range of SUVs and ATVs covering the GLK-, M-, GL-Class and G-Class. The future all-wheel-drive line-up from Mercedes-Benz will be introduced in the coming year by the CLA-Class and the CLA 45 AMG. While the technical basis is identical on all versions, the AMG models feature a modified power output to the rear axle.
|All-wheel drive for every vehicle class. Click to enlarge.|
Integrated power take-off unit (PTU) channels power to the rear axle. The new 4MATIC is being combined with the 7G-DCT seven-speed automated dual clutch transmission which was premiered in the B-Class. The power flux to the rear powertrain is provided by a compact power take-off unit (PTU) which is fully integrated in the main transmission and is supplied with lubricant from the latter’s oil circuit.
This configuration gives rise to substantial weight advantages in comparison to rival systems which branch off power by means of an add-on component with its own oil circuit. Together with the friction-minimized tapered roller bearings, the PTU achieves an excellent level of efficiency, Mercedes says.
The low system weight of the new 4MATIC is not attributable solely to the PTU. Other weight-stripping measures include a compact rear-axle gear unit, inductively hardened rear-axle side shafts in hollow design and weight-optimized propeller shafts. The system weight of the new 4MATIC is up to 25% lighter than the all-wheel drive versions offered by the competition.
The two-part cardan shaft is vibrationally decoupled, with an axially moving joint at the front compensating the movements of the engine-transmission unit and preventing transfer to the rear powertrain. To minimize crash-induced stress, the cardan shaft is fitted with a crash joint.
|Torque-on-demand rear-axle transmission with integrated electrohydraulic disc clutch. Click to enlarge.|
Completely new development with torque-on-demand rear axle. The electrohydraulically actuated multi-disc clutch integrated in the rear axle gear unit is responsible for fully variable torque distribution. Operating principle: when the multi-disc clutch is open, the CLA is driven almost exclusively by the front axle. When the clutch is closed, the rear axle comes into play. This means that the drive torque can be shifted in fully variable mode between front and rear axle according to the given situation (torque on demand).
The system pressure to activate the rear axle is supplied in milliseconds by the rotor-type pump which is integrated in the rear-axle gear unit. The pump is activated automatically as soon as only minimal speed differences apply between front and rear axle. Pressure control is performed by a proportioning valve under the steering of the ESP.
The most diverse parameters are taken into consideration here, such as lateral acceleration, steering angle, yaw rate, accelerator position, ABS control settings or the selected shift program of the 7G-DCT dual clutch transmission. An additional pressure regulating valve limits the drive torque at the rear axle to 750 N·m. Up to 1000 N·m are possible on the AMG version.
Intelligent control systems. The 4MATIC is activated according to the basic principle of “as often as necessary, as rarely as possible”. This means that when the underlying conditions allow, the all-wheel drive CLA models run almost exclusively in particularly economical front-wheel drive mode. As soon as the driving situation requires, drive torque is channelled to the rear axle as appropriate. The reverse process takes place just as quickly: as soon as additional drive torque is no longer necessary at the rear axle—e.g. in case of heavy braking maneuvers with ABS intervention—the rear powertrain is deactivated and torque is reduced to zero.
In case of understeering or oversteering under load, the drive torque is first of all distributed such as to stabilise the vehicle. Only if these measures fail to have a stabilizing effect do the control systems adapted to 4MATIC conditions, such as ESP or 4ETS, intervene to keep the CLA stable. With this strategy, the developers have managed to combine two key requirements defined in the performance specifications: optimum energy efficiency coupled with maximum driving dynamics and driving safety.
In normal circumstances, the drive torque distribution between front and rear axle can be set from 100:0 to 50:50. In certain driving situations, such as when exceeding the µ-jump limit (i.e., limited grip), higher levels of torque may be allocated to the rear axle to avoid unstable operating conditions. Here are some examples of torque distribution (front:rear in percent, approximate values):
- Full-load acceleration at approx. 50 km/h → 60:40
- Dynamic cornering under load → 50:50
- Exceeding the µ-jump limit → 10:90
- Heavy braking with ABS intervention → 100:00
Another basic principle of the 4MATIC system results in a further increase in driving dynamics, delivering greater driving enjoyment without compromising driving safety. In case of understeering or oversteering under load, the drive torque is first of all distributed such as to stabilise the vehicle. This ensures that traction and driving enjoyment are fully maintained in speedy driving on winding stretches of road. Only if these measures fail to have a stabilizing effect do the control systems adapted to 4MATIC conditions, such as ESP or 4ETS, intervene to keep the CLA stable.
The 4ETS electronic traction system forms part of the ESP electronic stability program and brakes the driven wheels individually when they go into spin. This provides for a high level of traction on slippery surfaces, on roads which are icy on one side or in continually changing conditions. While 4ETS is responsible for maintaining the highest possible traction in all conditions, ESP serves as the higher-level control instance to ensure optimum driving stability.
|Automated 7G-DCT dual-clutch transmission with integrated output to the rear axle. Click to enlarge.|
7G-DCT dual clutch transmission. 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 enables a comfortable but nevertheless dynamic mode of driving.
With its seven gears it offers a large gear ratio 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 transmission’s efficiency improves fuel economy by up to 9% in comparison to the CVT which has been employed in the A-Class to date, bringing fuel consumption down to a level below that obtained with a manual transmission.
At a length of 367 millimeters 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 lifts. 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 electrical 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 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 ten seconds, or after a longer delay when driving downhill or on winding roads.
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