Clutch actuation system

In an internal combustion engine (ICE) powered vehicle, the engine is connected to the rest of the powertrain through a coupling device, which can be a clutch or a torque converter. One of the roles of the clutch (torque converter) is to temporarily interrupt the power flow between the engine and transmission (e.g. to allow a gear shift).

For a vehicle with manual transmission, the clutch actuation system (mechanism) is the interface between the driver and the clutch, which allows the driver to control the connection (engagement) and disconnection (disengagement) of the clutch.

In order to understand how a clutch works, read the article How a clutch works.

A clutch actuation system can be mechanical, hydraulic or electrical (by-wire). The mechanical actuation systems can be with metallic bars and rods or with metallic cable.

Compared to a mechanical clutch actuation system, a hydraulic actuation system is much more flexible and reliable. Hydraulic clutch actuation systems enable an optimal and constant pedal effort, are manufactured from much lighter materials (up to 70% reduction of weight compared to the standard clutch system command) and are much more compact.

In the diagram below we can see the main components of a hydraulic clutch actuation system.

Clutch components with actuation system

Image: Clutch components with actuation system

  1. dual mass flywheel
  2. clutch cover
  3. mechanical releaser
  4. pedal vibration damping device
  5. clutch master cylinder (CMC)
  6. plastic clutch pedal
  7. clutch slave cylinder (CSC)
  8. clutch (friction) disc

Depending on the type of actuation of the diaphragm spring, clutches are classified in:

  • push-type clutches
  • pull-type clutches
Push-type and pull-type clutch

Image: Push-type and pull-type clutch
Credit: ZF Sachs

  1. clutch case (cover)
  2. pressure plate
  3. rivet
  4. release bearing
  5. diaphragm spring (inner lever)
  6. diaphragm spring (outer lever)
  7. drive strap

In a push-type clutch, when the clutch pedal is pressed, the clutch release bearing pushes on the diaphragm spring and the pressure plate releases the clutch friction disc.

In a pull-type clutch, when the clutch pedal is pressed, the clutch release bearing pulls the diaphragm spring and the pressure plate releases the clutch friction disc.

The hydraulic actuated push-type clutch systems are widely used in passenger vehicle applications.

The clutch actuation systems must fulfill several design requirements:

  • it must assure complete clutch disengagement
  • it must be able to allow a smooth engagement and disengagement of the clutch
  • the clutch pedal force must be around 100 … 150 N, which means moderate to low pedal force is required to disengage the clutch
  • the clutch pedal stoke should be around 120 … 150 mm, which means that the driver should be able to press the clutch pedal up to its end position
  • it should have automatic clutch wear compensations mechanisms, which means that the pedal force should have the same characteristic even if the friction disc width becomes smaller
  • must be a compact system, have a light-weight design which can be assembled fast and easy
  • most of the components must be manufactured from recyclable materials
  • must be resistant to corrosion
  • must filter out the structural vibrations of the vehicle (no impact on the driver feel)

The clutch torque is regulated by the clutch pedal force. Since it is indirectly controlling the torque at the wheel, it is critical the the hydraulic clutch actuation system works seamlessly, is reliable and a long service life is guaranteed.

How does a hydraulic clutch actuation systems work

The hydraulic clutch actuation system working principle is based on Pascal’s law (also know as Pascal’s principle or the principle of transmission of fluid-pressure).

Hydraulic clutch actuation system - schematic

Image: Hydraulic clutch actuation system (pull-type) – schematic
Credit: Eaton

  1. master cylinder
  2. reservoir
  3. piston
  4. high pressure line (pipe)
  5. slave cylinder
  6. pushrod

The clutch pedal is linked directly to the piston (3) of the master cylinder (1). When the driver pushes the clutch pedal, the piston moves within the master cylinder and compresses the hydraulic fluid building up pressure. The pressure is transmitted through the high pressure pipe (4) to the slave cylinder (5). The pushrod (6) is connected to the piston of the salve cylinder. Due to the increase of pressure in the slave cylinder, the pushrod is pushed outside acting on the clutch fork, which releases the pressure plate and opens the clutch.

The hydraulic fluid used for actuation is usually brake fluid or mineral oil.

During actuation, the clutch pedal stroke R is converted (mechanical-hydraulic-mechanical) into the release bearing stroke r.

Hydraulic clutch actuation system - components

Image: Hydraulic clutch actuation system – components
Credit: Eaton

  1. master cylinder
  2. reservoir
  3. adapter
  4. hose and fitting
  5. slave cylinder (or servo air/hydraulic)
  6. (optional) air regulator
  7. housing and fork assembly
  8. clutch

The clutch master cylinder (CMC) is connected directly to the clutch pedal through the piston and pushing rod. The pushing force of the driver acts on the piston which compresses the hydraulic fluid inside the master cylinder. The mechanical force at the clutch pedal in converted into hydraulic pressure and flow transferred through the hose (pipes) into the slave cylinder and converted back into mechanical force at the clutch fork.

Clutch master cylinder

Image: Clutch master cylinder
Credit: FTE automotive

  1. clutch pipe connector
  2. position sensor connector
  3. piston rod head
  4. bayonet fitting for the pedal
  5. position sensor

Some variants of clutch master cylinders have travel sensors which send the position of the clutch pedal (piston) back to an electronic control unit (ECU).

Technical data for the clutch master cylinder

Credit: FTE automotive

Operating pressure [bar]< 50
Vacuum resistance [mbar]< 2
Temperature range [°C]-40 … 130
Peak temperature [°C]150
Diameter range [mm]15.87 … 38.1
Stroke range [mm]< 45
Operating mediumbrake fluid or mineral oil

The pressure build up in the master cylinder is transferred through the pipes (hoses) to the clutch slave cylinder (CSC).

Clutch slave cylinder

Image: Clutch slave cylinder
Credit: FTE automotive

One requirements of the pipe/hose assembly is to filter out the external vibrations, in order to ensure a comfortable operation of the clutch pedal. For this reason, the clutch pipes are equipped with dampening components such as frequency modulators or vibration dampers.

Clutch pipe-hose assembly

Image: Clutch pipe-hose assembly
Credit: FTE automotive

  1. frequency modulator (compact design)
  2. connector
  3. frequency modulator

Technical data pipe-hose assembly

Credit: FTE automotive

Operating pressure [bar]< 50
Vacuum resistance [mbar]< 2
Temperature range [°C]-40 … 130
Peak temperature [°C]160
Outer diameter of tube [mm]4.75 or 6
Inner diameter of tube [mm]3.2 or 6
Operating mediumBrake fluid or mineral oil

Technical data plastic pipe

Credit: FTE automotive

Operating pressure [bar]< 50
Vacuum resistance [mbar]< 2
Temperature range [°C]-40 … 130
Peak temperature [°C]160
Outer diameter [mm]8
Wall thickness [mm]2.15
Operating mediumBrake fluid or mineral oil

The clutch slave cylinder receives the hydraulic energy (pressure and flow) from the master cylinder and converts it back to mechanical force. The pressure inside the slave cylinder pushes the piston out, which is acting on the clutch fork, disengaging the clutch.

When the driver releases the clutch pedal, the pressure inside the master cylinder and slave cylinder decreases and allows the diaphragm spring to push back (in case of a push-type clutch), through the clutch fork, the piston/pushrod into the slave cylinder.

The clutch actuation system is static, relative to the body of the vehicle. The clutch pressure plate and diaphragm spring are rotating together with the crankshaft of the internal combustion engine. The clutch release device has to provide the link between a static element (slave cylinder piston/pushrod) and a moving element (diaphragm spring). This requirement can be achieved either by using a release bearing together with a clutch fork or using a concentric slave cylinder.

Clutch concentric slave cylinder

Image: Clutch concentric slave cylinder
Credit: FTE automotive

Concentric slave cylinders contain also the clutch release bearing. With this assembly there is no need for a clutch fork, the slave cylinder being mounted concentric with the clutch diaphragm spring.

Technical data clutch slave cylinder

Credit: FTE automotive

Operating pressure [bar]< 50
Vacuum resistance [mbar]< 2
Temperature range [°C]-40 … 120
Peak temperature [°C]150
Diameter range [mm]15.87 … 38.1
Operating mediumbrake fluid or mineral oil

Technical data concentric slave cylinder

Credit: FTE automotive

Operating pressure [bar]< 50
Vacuum resistance [mbar]< 2
Temperature range [°C]-40 … 180
Peak temperature [°C]200
Max. release load [N]< 7000
Operating mediumbrake fluid or mineral oil

Clutch-by-wire actuation systems

Having a driver independent control of the clutch brings some opportunities in terms of vehicle fuel economy improvement and reduced exhaust gas emissions. These improvements can be achieved when the vehicle enters a Coasting state.

Vehicle Coasting (also called Sailing) means that the engine is decoupled from the rest of the powertrain and the vehicle is moving due to its kinetic energy (inertia). A vehicle can perform two types of Coasting functions:

  • Idle Coasting: when the engine is disconnected from the powertrain but kept at idle speed
  • Off Coasting: when the engine is disconnected from the powertrain and stopped

The Off Coasting scenario gives the most fuel economy improvement but it can affect the driveability of the vehicle in terms of time taken to accelerate the vehicle after a Coasting event.

Coasting can be easily obtained on vehicles with automated manual transmissions (AMT), double clutch transmissions (DCT) or automatic transmissions (AT), thanks to the electronic control of the clutches.

On a vehicle with a manual transmission (MT) in order to enable Coasting, the clutch needs to be controlled independently of the driver intention.

Schaeffler has developed a number of intelligent clutch actuation systems for vehicles with manual transmissions, which automatically disengages the clutch and allows the vehicle to enter Coasting.

Clutch-by-wire (E-Clutch)

Image: Clutch-by-wire (E-Clutch)
Credit: Schaeffler

In the clutch-by-wire concept there is no mechanical or hydraulic connection between the clutch pedal and the clutch release system. In order to maintain the same behavior, relative to the driver (receive an opposing force when pressing the clutch pedal), a pedal force adjuster is integrated in the clutch pedal.

On the clutch side, the slave cylinder is replaced by an electronic hydraulic actuator, which generates the required pressure for clutch position control.

The clutch pedal system contains also a travel sensor, which sends the clutch pedal position to the clutch actuator. Based on this information, the clutch actuator regulates the hydraulic pressure and thus the opening/closing of the clutch.

The clutch-by-wire systems can also accommodate the state of the clutch to the driving conditions with very high dynamic requirements, such as rapid gear shifting or emergency braking. Clutch-by-wire systems can also include other options, such as a stall prevention function or driver-assist functions for relieving stress in stop-start traffic situations.

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