Dual clutch control, basic function

Overview

This document contains information on:

 General

 Function requirements

 Function

General

The electrohydraulically actuated dual clutch contains two multidisk clutches.

The multidisk clutches shift their respectively assigned sub-transmission.

Function requirements

 Ignition ON (circuit 15)

 Engine in operation

 Transmission control not in limp-home mode

P27.20-2143-76

Function

Dual clutch control

If the regulation valves of clutch K1 and K2 are not actuated, both clutches are disengaged and there is no power transmission from the engine to the internal shaft or hollow shaft (8, 9).

To actuate the respective clutch, the affected clutch regulation valve or the dual clutch actuator is actuated. The clutch oil pressure is reduced through the control edge of the emergency shift mode shift valve. To achieve, for  example, power flow in 1st gear via clutch K1, oil is supplied to the K1 clutch pressure chamber (11) via the oil duct of the clutch hub (10) by means of a rotary inlet. The pressure buildup in the K1 clutch pressure chamber (11) pushes the K1 clutch piston (13) against the K1 clutch pack. The K1 clutch pack consists of the K1 clutch outer plates (18) and the K1 clutch inner plates (17). A frictional connection then exists from the engine to the internal shaft (9) over the clutch pack of clutch K1. The internal shaft (9) is positively connected to the K1 clutch inner plate carrier (3) via a ring gear profile.

A part of the transmission oil is also routed to the K2 clutch pack through a coaxial bore (oil duct) in the clutch hub (10). The clutch pack consists of the K2 clutch outer plates (4) and the K2 clutch inner plates (5). The transmission oil flows through clutch K2, which is not frictionally-connected, to clutch K1. There, it absorbs the waste heat at the inner and outer plates (17, 18). At high engine speeds, the pressure is increased through the rotation of the dual clutch on the K1 clutch piston (13).

 The pressure increase results from the increase in centrifugal forces. This effect is referred to a "dynamic pressure buildup" and is undesirable as it increases the contact pressure unnecessarily, making controlled pressure buildup and pressure reduction difficult. To counteract the dynamic pressure buildup, a centrifugal force compensation chamber with return compression spring package is located on the opposite side of the pistons (13, 15). At higher rotational speeds, dynamic pressure compensation takes place here in order to ensure a more precisely controlled and more comfortable shift operation.

 Centrifugal force compensation
The centrifugal forces induce the pressure in the respective pressure chamber to increase according to the rpm increase of the combustion engine or dual clutch.
In order to counteract this effect, a dynamic pressure compensation takes place in the corresponding centrifugal force compensation chambers. This acts as "back pressure", reduces the contact pressure of the clutch packs and thus allows more precisely controlled actuation of the clutch operating system.

Dual clutch power-flow change

The power flow changes through a so-called overlap gearshift system operation. During overlap gearshift system operation, torque transfer is reduced through the K1 clutch proportionate to the increase at the K2 clutch. During torque transfer the combustion engine drive torque is reduced, within a defined time window, by the combustion engine control unit.