Technology in detail: The permanent all-wheel-drive system 4MATIC

Muamer Hodzic December 8, 2008

The 4MATIC drive system in the GLK is among the most powerful all-wheel-drive systems there is; its control systems break new ground. Advantages over other systems with a transversely installed drive unit are achieved thanks to the basic compact, light and friction-loss-optimised design with its longitudinally positioned engine and integrated main and transfer case. Fuel consumption,
for example, is at the same level as that of a comparable, conventionally driven vehicle, whilst noise and vibration levels are on a par with those in higher vehicle categories.

In cooperation with the dynamic handling control systems ESP, acceleration skid control ASR and 4ETS, the basic 45:55 percent torque split between the front and rear axle ensures assured and predictable handling. When fine-tuning the control systems the engineers at the Mercedes-Benz Technology Centre placed particular importance on defined understeer characteristics, as displayed by all GLK models even in the most varied of road conditions: on dry roads, in rain, snow, ice or on unsurfaced roads.

A double-disc clutch achieving a basic locking torque of approximately 50 Nm between the front and rear axle is fitted to the central differential. The clutch pack is permanently preloaded via a plate spring. In the case of spinning wheels on one of the two axles, a friction torque is transmitted to the axle spinning more slowly by relative movement of the discs. A sustained improvement to traction and directional stability of the vehicle are achieved through this principle of variable torque shift. Particularly in the case of low coefficients of friction between the tyres and the road surface the traction and stabilising effect of this so-called pre-lock clutch is clearly noticeable. Adaptive Brake: state-of-the-art brake control system enables shortest stopping distances – even in wintry road conditions

The newly developed ADAPTIVE BRAKE control system encompasses the following basic functions: the anti-lock braking system ABS, acceleration skid control ASR and yaw moment control. ABS and ASR record and control the driving situation longitudinally, the yaw moment control the lateral dynamics.
If ADAPTIVE BRAKE diagnoses critical operating conditions, within the limits of the laws of physics traction and directional stability are retained or reinstated through precise brake actuation and drive torque control.

New additional functions make ADAPTIVE BRAKE even safer and more convenient. Hill Start Assist prevents the vehicle from rolling unintentionally in the opposite direction of travel wished by the driver. A function known as priming has also been added. If the driver quickly removes his or her foot from the accelerator, the system prepares for possible panic braking by placing the brake linings on the brake discs using little pressure. Through the indirect pressure build-up when operating the brake pedal this will then shorten the stopping distance if a full brake application does occur. Thanks to ADAPTIVE BRAKE’s ability to produce even the slightest amounts of brake pressure precisely, it is possible to remove the film of water which forms on the brake discs in wet conditions through brief, light brake actuation. This reduces even further the brake response time, and thus the stopping distance, in the wet. Intervention takes place as soon as the windscreen wiper has reached a certain number of wipe cycles and the driver has not braked of his or her own accord in the meantime.

The GLK is fitted with adaptive brake lights as standard. This system warns following traffic that there is a critical braking situation by initiating flashing of the brake lights if the vehicle’s rate of deceleration is high. In the case of emergency braking from a speed of over 70 km/h the hazard warning lights are automatically activated when the vehicle comes to standstill.

The “off-road” switch in the centre console helps the GLK-Class to make good progress on rough terrain if the equipment line with the “off-road engineering package” is selected. At the touch of a button a special drive program comes into operation; it varies the 7G-TRONIC’s shift points, “softens” the accelerator characteristic and activates the ESP® functions with off-road ABS and a special off-road 4ETS. A further switch turns on the DSR (Downhill Speed Regulation) which automatically keeps to a pre-selected speed between 4 and 18 km/h on steep downhill inclines. The driver can vary the speed at any time using the cruise control lever.

In addition to a tyre pressure loss warning system the GLK’s Electronic Stability Program ESP® comprises vehicle/trailer stabilisation as standard. This diffuses critical operating conditions through precise brake actuation as they arise. The maximum towing capacity is 2000 kilograms.

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