Part I

Twin-Clutch Gearbox

Twin-clutch gearbox
In BMW vehicles with powerful motorization, a twin-clutch gearbox may be in use. The twin-clutch gearbox has seven gears and it enables gearshifts without tractive power interruptions.
On the twin-clutch gearbox, the clutch control and gear changes are automated. An electrohydraulic control system handles clutch operation and gearshifts. The clutch pedal is no longer required.
The twin-clutch gearbox combines two sub-gearboxes with two clutches (designed as oil-bath clutches). One sub-gearbox with clutch is responsible for the even-numbered transmission stages (2, 4, 6). The other sub-gearbox with clutch is responsible for the odd-numbered transmission stages (1, 3, 5, 7) as well as for reverse gear.
While driving, one of the two clutches is closed. With each gear change, one clutch opens parallel to the other clutch closing. This achieves jolt-free and rapid gear changes without tractive power interruptions.
The use of two clutches enables a very rapid gear change without tractive power interruptions. Reason: Because the electronic transmission control preselects and prepares the next gear with the ideal ratio for the current engine speed and vehicle speed. Example: The vehicle is accelerating in 3rd gear. In the sub-gearbox with the even-numbered transmission stages, the gear required to continue the acceleration, in this case 4th gear, is already engaged. If the clutch responsible for 4th gear now closes, the clutch for the 3rd gear opens simultaneously. This gear shift operation takes only a few milliseconds.
The twin-clutch gearbox can be used manually in the "S" sequential mode and in the "D" automated drive mode.
The gear selector switch is a bus user. The shift pulses are transferred electrically to the DKG control unit. With this gearshift method ( shift by wire ) there is no mechanical connection between the gearshift lever or shift paddles and the transmission.
"Drivelogic" (motorsport engineering) provides a total of 11 driving programs. These are used to adapt the shift characteristics of the twin-clutch gearbox to the desired driving style.
The twin-clutch gearbox is configured for a torque of up to 520 Newton meters and engine speeds of up to 9000 rpm.
The twin-clutch gearbox weighs 89 kg (including dual-mass flywheel).

Brief component description
The twin-clutch gearbox essentially consists of the following components:

Arrangement of the components in the twin-clutch gearbox
- Twin-clutch gearbox, view from the left







- Twin-clutch gearbox, view from rear right







Schematic structure with sensors
The signals from the sensors are transferred directly to the DKG control unit. In addition, the oil sump temperature is calculated in the DKG control unit using a complex temperature model. For the plausibility check, the calculated temperature is checked against the temperature registered by the temperature sensors in the DKG control unit as well as the sensor for cooling oil temperature in the dual clutch.







The two sensors that measure the transmission input speed and cooling oil temperature on the dual clutch are grouped in the combination sensor for the twin-clutch gearbox.

GWS: Gear lever switch
The gear selector switch is configured as a control unit in its own right. The twin-clutch gearbox is electronically activated via the gear selector switch. The gear selector switch is supplied with voltage via terminal 30g. The gear selector switch consists of the control electronics and the gearshift lever. 14 hall effect sensors for non-contacting detection of the gear lever position are integrated in the control electronics circuit. The gear lever position is transferred to the twin-clutch gearbox (DKG) via PT-CAN and redundantly via a local interconnect network bus. This means that data are sent to the DKG control unit even if one of the two transmission lines fails. The moved program switch for "Drivelogic" is connected directly to the control electronics circuit of the gear selector switch.











The functional display is shown in the gearshift lever. The functional display is shown as a shift pattern. The drive position engaged is displayed in the functional display regardless of the gear lever position. The engaged drive position is only shown in the shift pattern when the electronic transmission control (DKG control unit) has implemented the driver's choice and the gear selector switch has confirmed this state via a message across the bus. The functional display is active as long as the bus communication on the PT CAN or LIN bus is active.

The gearshift lever is monostable, i.e., it always returns to its initial setting. The change to the "R" gear lever position is bi-stable, i.e., the gearshift lever remains in the selected gear lever position. A shift from the gear lever position "R" into the automated drive mode "D" or the sequential mode "S" can only be carried out manually.

Program switch for "Drivelogic" (motorsport engineering)
"Drivelogic" provides various driving programs. These can be used to adapt the shift characteristics of the twin-clutch gearbox to the desired individual driving style.
- Five driving programs in the automated drive mode "D"
- Six driving programs in the "S" sequential mode (driving program six only sequential with DSC switched off or with launch control).

The signals of the program switch for "Drivelogic" are transferred by hard-wired lines to the control electronics circuit of the gear selector switch.
The forward direction of movement of the selector lever towards R and to the left towards M/S must be perceptibly blocked or released depending on its initial position and the status of the release button.

NOTE: BMW AG vehicles have a SPORT button.

Shift paddles on steering wheel
In addition to conventional gear selection using the gearshift lever, the gears can also be selected using the 2 shift paddles on the steering wheel.
- Upshift: pull the right shift paddle
- Downshift: pull the left shift paddle

Mechatronics module
The twin-clutch gearbox is controlled by the mechatronics module. The mechatronics module is a combination hydraulic unit and electronics module. The mechatronics module is installed in the twin-clutch gearbox.
The hydraulic unit is the hydraulic part of the transmission control unit. The hydraulic unit contains, for example, the electronic pressure control valves and shift valves for control of the hydraulic shift cylinders. The electronic pressure control valves convert an electric current specified by the DKG control unit into a proportional hydraulic pressure. The electronic pressure control valves actuate the valves belonging to the shift elements. The four gearshift rods are shifted in a longitudinal direction by the hydraulic shift cylinders in the hydraulic unit.
The electronics module contains the DKG control unit, the sensors as well as the transmission-internal electrical connections.







Oil pump
The oil pump in the twin-clutch gearbox builds up the hydraulic pressure required with the engine running. The oil pump is meshed with the transmission input shaft via a gear. The hydraulic pressure is set via a regulated control valve on a load- and function-related basis. A pressure limiting valve in the oil pump prevents excessive hydraulic pressure in the hydraulic system.
Normally, system pressure lies between 5 and 20 bar. In exceptional cases, system pressure can be increased to up to 30 bar. The maximum shifting pressure to shift the gearshift rods in the longitudinal axis can be equal to the system pressure. The shifting pressure for the dual clutch or for both clutches is limited to 18 bar.
The following control units and components are involved in the function of the twin-clutch gearbox (in alphabetical order):

Brake light switch
The signal of the brake light switch is processed in the Digital Engine Electronics (DME). The Digital Engine Electronics send the signals as a CAN message on the CAN bus. The signal of the brake light switch is used for the following processes:
- start enable
- detection of braking
- adaptive transmission control unit
- for driving off on inclines

CAS: Car Access System
The Car Access System only enables the starter motor when the enable signal arrives from the twin-clutch gearbox control unit. The following preconditions must be met for a start enable to take place:
- The automatic parking lock must be engaged.
- The DKG control unit enables the start when the brake pedal is pressed.

The Car Access System also delivers input signals relating to the terminal voltage, e.g. terminal 15 wake-up. Switching on terminal 15 activates (wakens) the control units on the PT CAN via the wake-up line (terminal 15 wake-up).

CID: Central information display
Detailed information about the Check Control messages in the instrument panel is displayed on the Central Information Display. The instrument panel controls these messages on the K CAN (body CAN). In addition to the visual display, the Check Control also alerts the driver with acoustic signals.

DSC: Dynamic Stability Control
The Dynamic Stability Control optimizes driving stability when starting and accelerating, and optimizes traction. Within the limitations of the laws of physics, Dynamic Stability Control compensates for driving conditions such as oversteering/understeering. As part of this process, dynamics driving signals (e.g. wheel speed, lateral acceleration) are sent to the DSC control unit. The Dynamic Stability Control sends the processed signals for cornering and acceleration detection to the DKG control unit.

Accelerator pedal module
The signal of the accelerator pedal module (driver's choice) is required for permanent checking of each driving condition (evaluation via DME).

FRM: Footwell module
Based on messages from the DKG control unit to the footwell module, the footwell module switches the reversing light on or off, as the case may be. The footwell module supplies, among other things, the locator lighting of the program switch for "Drivelogic". The footwell module also evaluates the status of the hall effect sensors in the door contacts. The status is sent as a message on the CAN bus and received by the DKG control unit. If the driver's door is opened with the engine running, visual as well as acoustic signals draw attention to the position of the transmission.

JBE: Junction Box Electronics
Junction Box electronics is the data interface (= gateway) between the K CAN and PT-CAN (K-CAN stands for "body CAN"; PT-CAN stands for "Drive train-CAN"). The junction box consists of the junction box electronics and the power distribution box. The power distribution box provides power, amongst other things, for the gear selector switch and the DKG control unit. The diagnosis on-CAN is connected to the junction box electronics.

Instrument panel: Instrument panel
The selected drive positions and driving programs are displayed in the instrument panel by means of a CAN message from the DKG control unit.
The brightness of the functional display in the gearshift lever knob is adapted to the lighting conditions or ambient brightness in each case. For this, the signals of the photo-transistor are evaluated in the instrument panel. The brightness value calculated in the instrument panel is transferred to the gear selector switch via the Controller Area Network bus.
Dimming can be set individually via a menu in the instrument panel. When the vehicle lights are switched on, this individual specification and the phototransistor are used to control the dimming. When the vehicle lights are switched off, dimming is controlled by the phototransistor alone.

System functions
The following system functions of the twin-clutch gearbox are described:
- Functional networking
- Transmission oil cooling
- Cooling of the dual clutch
- Engine start
- Overlap control
- Drivelogic
- Driving in gear lever position "D" (automated drive mode)
- Launch control (acceleration assistant)
- Wheel-slip monitoring
- Driving off on inclines
- Low Speed Assistant
- Vehicle at a standstill
- Automatic parking lock
- Emergency program