Part II

Twin-Clutch Gearbox

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 reverse 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 receive 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.







NOTICE: Torque reduction 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 dual 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 vehicle 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.

Cooling of the dual clutch
To cool the dual clutch, hydraulic fluid from the oil sump is sprayed onto the dual clutch. The sensor for the cooling oil temperature on the dual clutch is hit by the spray jet of the cooling oil. The temperature signal is sent to the DKG control unit.
The heat management determines the current cooling requirement and regulates the cooling system accordingly.
To achieve this, the DKG control unit activates a solenoid valve.
The solenoid valve has 3 positions:
- Position with minimum flow to dual clutch (no current)
- Opened position
- Closed position

If the oil sump temperature is implausible in relation to the temperature of the cooling oil in the dual clutch, a Check Control message is displayed.

Engine start
Regardless of the drive position and gear in which the engine is shut down, the engine can only be started when the brake is operated. The parking lock is always engaged when the engine is started. If the engine is started in gear lever position "R", the symbol for the monostable position in the functional display in the gearshift lever flashes.
When the vehicle is unlocked, the DKG control unit is wakened by the Car Access System (CAS).

Overlap control
When a gearshift is triggered in the sequential mode "S" or in the automated drive mode "D", the DKG control unit activates the corresponding solenoid valves. Simultaneously, the DKG control unit sends a message to the DME control unit indicating that a gearshift operation is being initiated. The engine torque is reduced. During the gearshift, the engine speed is raised if required to make clutch engagement faster and more comfortable.
The DKG control unit activates the dual clutch, the solenoid valves and the hydraulic shift cylinders. The shift cylinders execute the longitudinal movement (forward or backward movement) of the corresponding gearshift rods. The desired gear is engaged.
The overlap control ensures optimized shift characteristics as follows: in the overlap control, both clutches are used simultaneously. In the engaged clutch the pressure is reduced and simultaneously pressure is built up in the clutch to be engaged. The pressure reduction or pressure build-up is maintained until a synchronized speed is reached. Then, the hydraulic pressure for the clutch to be engaged can be built up in full.
After the gearshift operation, the current to the solenoid valve is switched off.
The entire gearshift operation runs depending on the pedal sensor position, engine speed, driving speed and other control variables (e.g. engine oil temperature, selected driving program).
Gearshifts that would lead to impermissibly high or low engine speeds are not executed.

Drivelogic (motorsport engineering only)
"Drivelogic" provides a total of 11 different driving programs.
- In the automated drive mode "D" (automatic operation), there is a selection of 5 driving programs (from "relaxed" to "sports"). In driving program 1, for example, 2nd gear is selected for driving off. The clutch works particularly sensitively. This makes driving off easier on winter roads.
- In the sequential mode "S" (manual operation), there is a selection of 6 driving programs (from "balanced dynamics" to "very sporty"). Driving program 6 is only possible when DSC is switched off and it is a prerequisite for launch control (acceleration assistant).

As a general principle, the driving programs differ with regard to the preselected shifting time: the higher the driving program, engine speed and load, the shorter the shifting time.

Driving in gear lever position "D" (automated drive mode)
- Constant-speed driving
The DKG control unit registers continuous changes to the pedal sensor position (accelerate and decelerate). The DKG control unit the continuous change between neighboring gears that this causes (back and forth, e.g. on a journey on the motorway with continuously changing speed).

- Suppressing upshifts
The DKG control unit uses the pedal sensor position to detect rapid closure of the throttle, e.g. before braking. The DKG control unit suppresses the upshift. This means that the engine braking effect is retained and it supports braking.

- Manual intervention in the automated drive mode "D"
If the gearshift lever is operated towards upshift or downshift or if the shift paddles are operated in the automated drive mode "D", there is an automatic switch into the sequential mode "S". The automated drive mode "D" must be reselected.

- Driving uphill
The DKG control unit uses the signals of the following components to detect a hill ascent:
- Longitudinal acceleration sensor
- Wheel speed sensors
- Pedal sensor position

The DKG control unit prevents unwanted upshifts and disruptive pendulum shifts.

- Downhill driving
The DKG control unit uses the signals of the following components to detect a hill descent:
- Throttle potentiometers (load signal)
- Wheel-speed sensors (driving speed)
- Brake light switch.

If driving speed increases, the DKG control unit to the next-lowest gear. This means that the engine braking effect is retained or increased.

- Braking deceleration and automatic downshifting
The DKG control unit uses the signals of the following components to detect braking deceleration:
- Brake lamp switch
- Wheel speed sensors
- Pedal sensor position

To maintain the engine braking effect, the DKG control unit shifts down according to the driving speed. Here, a number of gears can be skipped. For renewed acceleration, the right gear is selected automatically.

Launch control (motorsport engineering only)
Launch control enables maximum acceleration from a vehicle standstill on a par with motor racing standards.
When launch control is active, the vehicle automatically shifts up even in sequential mode "S" (manual operation) as long as the accelerator pedal is in kick-down position. This prevents the engine from reaching critical speeds.
After launch control, the vehicle must travel over a certain distance before another launch control is permitted.

CAUTION: Launch control leads to increased wear of components.

When running in a new car, do not accelerate the car with launch control. Frequent use of launch control leads to increased wear of components.

Wheel-slip monitoring
While the clutch is being engaged or on load reversal, excessive wheel slip can arise (e.g. on wet or icy roadways). To stabiles the vehicle, the clutch is disengaged again. This process takes place as follows:
- Opening of the corresponding clutch after detected wheel slip between the wheels on the same side on the rear axle and front axle. This counteracts vehicle swerving.
- Subsequently, gentle engagement of the clutch with continued analysis of the wheel slip.
- If required, periodical opening and closing of the clutch until no more wheel slip is detected with the clutch closed.

Driving off on inclines
To make it easier to drive off on inclines, the Dynamic Stability Control (DSC) is equipped with special software. (On an upward stretch of road, the vehicle would roll away backwards on releasing the brake.) Driving off on inclines is made easier by an automatic brake intervention. The function can be used both in the sequential mode "S" and in the automated drive mode "D" (driving forwards and reversing).
All that needs to be done is to press the brake pedal with the vehicle stationary. After releasing the brake, it is possible to drive off within approx. 0.8 seconds without rolling away in an uncontrolled manner.

Low Speed Assistant
When the Low Speed Assistant is activated, the vehicle moves at minimum speed. The engine speed is regulated automatically.
- To activate the Low Speed Assistant: Engage a drive position and briefly touch the accelerator pedal
- To deactivate the Low Speed Assistant: Brake the vehicle to a full standstill.

Vehicle standstill
The DKG control unit detects vehicle standstill via the pedal sensor position, the wheel speeds as well as the engine speed. The clutches detach. There is an automatic shift to 1st gear (in the sequential mode "S" and in the automated drive mode "D").
If the driver's door is opened with the engine running, an acoustic signal draws attention to the position of the transmission. At the same time, a visual warning appears in the LC display of the instrument panel.

Automatic parking lock
The twin-clutch gearbox is equipped with an automatic parking lock. The automatic parking lock is required, as with the engine OFF there is no power transmission between the engine and transmission or the drive gears (reason: The hydraulic pump is driven mechanically and at static pressure the clutches do not adhere).
The automatic parking lock is always engaged automatically after engine OFF. Exception: gear lever position "N". In gear lever position "N", the automatic parking lock is engaged after engine OFF under the following preconditions:
- Withdrawal of the remote key from the insertion slot (vehicles without Comfort Access)
- No ID transmitter in the insertion slot (vehicles with Comfort Access)
- 30 minutes after engine OFF
The Car Access System (CAS) uses terminal 30g to effect a time-controlled shutdown of the control unit and components with voltage supply as follows: Approximately 30 minutes after terminal R is switched OFF, the voltage supply for this component is switched off. This prevents the battery from being subjected to excess load by the electrical consumers for too long when the vehicle is in standstill.

CAUTION: Stopping the engine in a car wash.

In a car wash, gear lever position "N" must be engaged actively before stopping the engine. The remote key must remain in the insertion slot. If the vehicle is equipped with the optional equipment "Comfort Access", the ID transmitter must remain in the insertion slot or be inserted in the slot.

Emergency program
In the emergency program, the vehicle can still be driven with certain restrictions. If the voltage supply fails, the currently engaged gear remains active and the corresponding clutch is closed. The clutch only opens when a certain minimum speed of the transmission input shaft is not reached.
Depending on the type of transmission fault, the vehicle can no longer be driven after coming to a standstill.

Notes for Service department

General notes

Emergency release of the automatic parking lock
If the parking lock can no longer be released, mechanical emergency release is possible. This allows the vehicle to be pushed or pulled in case of emergency. The procedure for towing is described in the Owner's Handbook.
Releasing the automatic parking lock using the emergency release process can lead to a fault entry in the DKG control unit. The procedure for releasing the automatic parking lock is described in the Owner's Handbook.

NOTICE: Engine start by tow-starting is not possible.

Tow-starting the vehicle is not possible. With the engine off, no hydraulic pressure is built up in the transmission (clutches not adherent).
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.