Part 1

Twin-Clutch Transmission

Twin-clutch transmission
In BMW vehicles with powerful motorization, a twin-clutch transmission may be in use. The twin-clutch transmission has seven gears and it enables gear shifts without tractive power interruptions.
On the twin-clutch transmission, the clutch control and gear changes are automated. An electrohydraulic control system handles clutch operation and gear shifts. The clutch pedal is no longer required.
The twin-clutch transmission combines two sub-transmissions with two clutches (designed as oil-bath clutches).
One sub-transmission with clutch is responsible for the even-numbered transmission stages (2, 4, 6). The other sub-transmission with clutch is responsible for the odd-numbered transmission stages (1, 3, 5, 7) as well as for reverse gear.
While the vehicle is being driven, 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-transmission 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 shift operation takes only a few milliseconds.
The twin-clutch transmission can be used manually in the "S" sequential mode and in the "D" automated drive mode. The gear lever switch is attached to the bus. The shift pulses are transferred electrically to the DKG control unit. With this gearshift method ("shift by wire") there is no mechanical linkage between the gear lever or shift paddles and the transmission.
"Drivelogic" (motorsport engineering) provides a total of 11 drive programs. These are used to adapt the shift characteristics of the twin-clutch transmission to the desired driving style.
The twin-clutch transmission is configured for a torque of up to 520 Newton meters and engine speeds of up to 9000 rpm.
The twin-clutch transmission weighs 89 kg (including dual-mass flywheel).

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

Arrangement of the components in the twin-clutch transmission

- Twin-clutch transmission, view from the left







- Twin-clutch transmission, 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 twin clutch.







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

GWS: Gear lever switch
The gear lever switch is configured as a control unit in its own right. The twin-clutch transmission is electronically activated via the gear lever switch. The gear lever switch is supplied with voltage via terminal 30g. The gear lever switch consists of the control electronics and the gear 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 sent across the PT-CAN and redundantly across the LIN bus to the DKG control unit (also referred to as: transmission control unit). 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 lever switch.












The functional display is shown in the gear 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 request and the gear lever 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 gear 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 gear 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 drive programs. These can be used to adapt the shift characteristics of the twin clutch transmission to the desired individual driving style.
- Five drive programs in the automated drive mode "D"
- Six drive programs in the "S" sequential mode (drive 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 lever 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 gear 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 transmission is controlled by the mechatronics module. The mechatronics module is a combination of hydraulic unit and electronics module. The mechatronics module is installed in the twin-clutch transmission. 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 transmission 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 shift pressure to shift the gearshift rods in the longitudinal axis can be equal to the system pressure. The shift pressure for the twin 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 transmission (in alphabetical order):

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

CAS: Car Access System
The Car Access System only enables the starter when the enable signal arrives from the twin-clutch transmission control unit. The following conditions 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, dynamic driving signals (e.g. wheel rotation 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 locating lights 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 tune in 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 "Powertrain-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 lever switch and the DKG control unit. The diagnosis on-CAN is connected to the junction box electronics.

INSTRUMENT PANEL: Instrument panel control unit
The selected drive positions and drive 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 gear lever knob is adapted to the lighting conditions or ambient brightness in each case. For this, the signals of the phototransistor are evaluated in the instrument panel. The brightness value calculated in the instrument panel is transferred to the gear lever 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 transmission are described:
- Functional networking
- Transmission oil cooling
- Cooling of the twin 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

Functional networking
A complex system network with distributed functions in other control units is necessary for implementation of the transmission functions. The DKG control unit is connected across data bus with these other control units.







Transmission oil cooling
For transmission oil cooling, the hydraulic fluid is always fed via the oil-to-water heat exchanger. As of a certain transmission oil temperature, the transmission oil thermostat ensures that the hydraulic fluid is additionally fed via the oil-air heat exchanger.
The oil-to-water heat exchanger is integrated on coolant side in the heater circuit of the vehicle. The coolant for the heat exchanger flows out of the extraction point on the coolant pipe between the cylinder banks directly into the oil-to-water heat exchanger. After passing through the oil-to-water heat exchanger, the coolant flows via the electric auxiliary water pump into the heat exchanger for the heating system.







The transmission oil thermostat opens or closes depending on the temperature.
- Transmission oil temperature below approx. 95 °C:
The transmission oil thermostat is closed. The bypass is open. This means the supplied hydraulic fluid is not fed via the oil air heat exchanger but rather directly via the open bypass into the return flow.
If required, the electric auxiliary water pump is requested via the DKG control unit. To achieve this, the DKG control unit sends a message across the PT-CAN. The junction box electronics receives the message and switches the auxiliary water pump on. Normally, the electrical auxiliary water pump serves the purpose of providing the necessary coolant flow rate through the heater circuit at low engine speeds.
- Transmission oil temperature above approx. 95 °C:
The transmission oil thermostat is opened. The bypass is closed. This means the hydraulic fluid is fed via the oil air-heat exchanger.







NOTE: Reduction in torque and engine-speed limitation with excessive transmission oil temperature.

If the transmission oil temperature rises above a certain limit value, the engine torque is reduced and, if required, the engine speed is limited. A Check Control message is displayed (yellow Check Control symbol). The shift program is changed to reduce the transmission oil temperature. If the transmission oil in the oil sump or in the twin clutch reaches a non-permitted excessive limit value, the engine torque is greatly reduced. A Check Control message is displayed (red Check Control symbol). The engaged gear remains engaged until the vehicle is at a standstill. With the vehicle at a standstill, the parking lock is engaged. It is only possible to continue driving once the transmission oil has cooled.