Automatic coupling system

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An automatic clutch system (AKS), English Automatic Clutch System (ACS) or e-Clutch , is a self- coupling system for motor vehicle transmission , wherein the opening (disengaging) and closing the coupling (engagement) by sensor signals triggered and auxiliary drives is made. Since the driver does not have to engage the clutch, a clutch pedal is no longer necessary.

A clutch disc, the main separator between the flywheel and the transmission

Components of the coupling system

Circuit release

The driver's intention to trigger the gearshift is detected by a sensor ( rotary potentiometer ) on the gearshift lever .

Further sensor signals influencing the control process are:

Gear recognition

It is recorded by two non-contact angle sensors on the shift linkage in the transmission . In addition to the sensor signals for the intention to change gear and gear recognition, the control unit receives signals via the CAN-BUS in front of the control units of the engine control and the ABS / ASR and ESP control .

Mode of action

In order to record the respective system status, the control unit processes the input signals from the sensors under software control and transmits the output signals to the actuating devices (actuators). The clutch is opened or closed according to their signals.

Start up

From the from the sensors the system is started further given input signals and the information contained in the software maps the control unit calculates the optimum slip for the start.

Gear change

The sensor on the shift lever reports the driver's intention to shift . The control unit generates pressure in the master cylinder via an electric motor with a worm gear. This pressure opens the clutch via the hydraulic central slave cylinder (slave cylinder).

After shifting, the gear detection sensors report which gear has been shifted. Now the control unit sends a signal to the electric motor with worm gear, which causes the clutch to close with a defined slip.

The accelerator pedal does not necessarily have to be taken back when changing gears. The injection quantity is automatically reduced via the control unit and then increased again.

Normal driving

In order to dampen torsional vibrations, the control unit calculates the difference from the signals for the engine speed and the transmission input speed , so that a controlled slip can be set if necessary.

Load change

When the accelerator pedal is pressed abruptly, the vehicle's rocking ( bonanza effect ) is kept within limits because the clutch opens briefly. This enables jerk-free acceleration.

Shift down on a slippery road surface

The signal from the locking drive wheels is processed by the control unit in such a way that the clutch opens at the start of locking and releases the wheels.


  • No clutch pedal available
  • Favorable wear method for clutch lining and release bearing
  • No “stalling” of the engine when starting and braking
  • Torsional vibrations of the engine are dampened by slip in the clutch.
  • Less disruptive load change reactions.


A completely mechanical solution was the “ Saxomat ” developed by Fichtel & Sachs in the mid-1950s . He was as an extra z. B. in the DKW F93 , the VW Beetle and the Taunus 12 M ("Weltkugeltaunus") offered by Ford. From 1959 the system was available under the name "Olymat" in the Opel Rekord P1 . At Daimler-Benz there was also a similar automatic clutch from Fichtel & Sachs called Hydrak , which was available from August 1957 in the Mercedes 219 sedan for an extra charge of DM 450  , but only for the six-cylinder models.

The Trabant 601 H ( Hycomat ) was produced with a similar system from 1965 to 1990 . The clutch was operated electro-hydraulically, but there was also an "emergency pedal".

While a centrifugal-controlled starting clutch was built into the Saxomat / Olymat , "Hydrak" had a hydraulic starting clutch that was technically more complex and somewhat more comfortable. An additional mechanical dry clutch was available for the gear changes. The disengagement process activated by the negative pressure of the engine is activated electrically by touching the shift lever.


Examples of vehicles with the electronic semi-automatic transmission described here are the Renault Twingo "Easy", Golf Ecomatic and the Mercedes-Benz A-Class . Examples of designations of this type of coupling systems are e.g. B. the AKS (automatic clutch system), EKS (electronic clutch system) or the EKM (electronic clutch management).

In the Smart Fortwo BR 450/451, on the other hand, there is an automated manual transmission , where both clutch operation and gear changes can be automated. In the soft tip version, the gear change function is only severely restricted by software, but it still works at the end of the speed range of each gear.

In vehicles with automatic transmissions , manual gear selection is sometimes referred to as a semi-automatic mode. The shift request (upshift or downshift) of the transmission electronics z. B. communicated via steering wheel buttons, but clutching and shifting is ultimately handled by the (fully) automatic transmission (e.g. Tiptronic ).


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  • Lexus SC Workshop Manual.
  • Europe teaching material: specialist knowledge of automotive engineering. 2001, ISBN 3-8085-2067-1 , p. 392.
  • Bosch : Automotive paperback. 22nd edition, p. 516.