The function of the transmission clutch is to couple or decouple the movement of the engine with that of the wheels using the gear shift. When we press the clutch pedal, we move the clutch collar inwards, pressing the pads that rotate and separate the clutch disc from the engine flywheel (flywheel). This allows us to transmit motion to the gearbox and shift gears. When the clutch pedal is fully depressed, the engine is disengaged from the transmission, and therefore no force is transmitted to the drive wheels.

The engine and gearbox are engaged when the clutch pedal is not depressed and engine power is transmitted to the drive wheels. When the clutch is released, the clutch disc engages the flywheel.

The transmission clutch system is made up of a series of constituent elements that form a complete gear in which the pedal and the gearbox are the visible elements:

• Pedal.
• Bell: metal cover attached to the engine flywheel.
• Clutch disc: positioned between the flywheel on the engine side and the inner pressure plate of the clutch cover. The clutch disc is attached to the primary shaft of the gearbox using its splined sleeve. The primary shaft is supported on the crankshaft. Thus, it connects to the primary shaft of the gearbox.
• Pressure plate: metallic and in the shape of a ring gear with supports on which the pins act.
• Springs and pins: connect the bell with the pressure plate.
• Collar: formed by an axial bearing with a central hole through which the primary shaft passes.

When the clutch pedal is depressed, the thrust collar moves inward, pressing the pins that rotate the pressure plate, which releases the clutch disc by overcoming the resistance of the springs.

There are different types of clutches, depending on their characteristics. Thus, depending on the number of discs, clutches can be hydraulic (they have no discs, typical of industrial vehicles), single-disc (the single-disc clutch is the most commonly used in the automotive industry), bi-disc, and multi-disc. By type of cooling, they can be dry clutches (air-cooled, as opposed to wet clutches, oil-cooled) or wet clutches. Finally, depending on how pressure is exerted on the clutch plate for coupling with the disc, they are classified as follows:

• Spring clutch: the springs on which the pressure is applied are located on the periphery of the clutch disc.
• Diaphragm clutch: the springs are replaced by an elastic carbon steel diaphragm.
• Centrifugal clutch: typical of automatic driving, clutch engagement and disengagement are entrusted to counterweights that operate by the centrifugal force of engine rotation. Thus, it can be considered a clutch that acts automatically from a certain speed of rotation of the motor shaft.

But the most significant classification is according to the type of control:

• Mechanical friction clutch: this consists of the flywheel on which, thanks to a bronze bushing, the primary shaft of the gearbox is supported. It is the most common in passenger cars. It works utilizing liquid (hydraulic) or, more commonly, using a cable.
• Hydraulic clutch: acts as an automatic clutch between the engine and the gearbox, allowing the engine to transmit torque when it reaches a certain speed. In this case, the energy is sent by a centrifugal pump that communicates with a turbine.
• Electromagnetic or electric-assisted clutch: it takes advantage of the strength of the electromagnetic field (electromagnetic lock-up) for torque transmission.

Depending on the manufacturer, car models may be equipped with different transmission clutch systems according to the driving characteristics.