Clutches connect or disconnect the engine shaft and the gearbox shaft. Basically, clutches consist of a flywheel, clutch disc and pressure plate to transmit the engine speed to the gearbox. Under load, the clutch disc clamps between the flywheel and pressure plate and rotates. When the clutch is depressed, the pressure springs and release levers are loosened and the rotation of the clutch disc is slowly reduced. For starting, i.e. until a minimum speed is reached, and for the shifting process, the clutch must be disengaged. Hydrodynamic torque converters transmit the rotary motion of the engine to the transmission. They are installed in automatic transmissions. A typical design consists of a housing, turbine wheel (input), guide wheel, pump wheel (output) and converter lock-up clutch. If there is a large difference in speed between the turbine and pump wheels, an oil circuit is created; the oil flows between the pump, turbine and guide wheels. The wheels, which are equipped with counter-rotating blades, add the movement of the turbine and guide wheels to the pump wheel torque. When the values are almost equal, the converter lock-up clutch engages. The resulting rigid connection of input and output shaft can be used for power transmission. Hydrodynamic torque converters are also synonymously referred to as Föttinger converters.