Control Strategies
Operating principle of the transmission.
In this transmission, the use of a dry double clutch in conjunction with an electromechanical control system means that two gears (transmission ratios) are engaged at the same time.
One of the clutches is engaged in driving mode, the other is already preselected when approaching the next gearshift with the clutch open.
Depending on the position of the accelerator pedal and the demand issued by the driver, the clutch of the previously activated gear is opened, while at the same time the clutch of the pre-selected gear is closed. As a result of this overlap in clutching, only minimal losses in tractive force are encountered during gearshifts.
Gearshift on a manual transmission
| Item | Description |
|---|---|
| 1 | Drive torque (Nm) |
| 2 | First gear |
| 3 | Second gear |
The illustration shows that in conventional manual transmissions, gearshifts result in a customary interruption of the propulsive force.
Gearshift process
| Item | Description |
|---|---|
| 1 | Drive torque (Nm) |
| 2 | First gear |
| 3 | Second gear |
With this gearshift, shown under load in the illustration, the power flow is only slightly restricted; there is constant propulsion perceptible.
Control
The TCM controls the clutch and gearshift system with the aid of the four brushless DC motors with integrated position sensors.
The information about the position of the DC motors is sent to the TCM. On the basis of this information, the TCM knows which gears are engaged and which clutch the power flow is routed through.
Gearshift Control
The gearshift control system is based on a software strategy for shift point determination which corresponds to the driving conditions and the driver input.
The TCM actuates the relevant DC motors in order to perform the automatic gearshift.
In order to be able to precisely determine the shift points on the basis of the selected driving program, the TCM receives the following information:
- Chosen transmission range
- Vehicle speed via the HS-CAN data bus
- Engine speed and torque as well as the throttle position via the HS-CAN data bus
- Engine temperature via the HS-CAN data bus
- Outside air temperature via the HS-CAN data bus for determining the viscosity of the transmission fluid when cold
- Steering angle from the steering wheel rotation sensor via the HS-CAN databus in order to avoid upshifts or downshifts during cornering.
- Information about braking interventions via the HS-CAN databus
- Input shaft speed for even and uneven gears from the respective speed sensor
Adaptive control
The TCM monitors each gearshift in order to enable smooth gearshifting under all driving conditions. To do this, the control unit controls the brushless DC motors for the clutch and gearshift system via an open-loop control system.
Adaptation of the following is performed:
- Clutch engagement points
- Clutch friction coefficient
- Individual positions of the synchronizer assembly
The adapted values are stored in the non-volatile RAM (read/write memory) of the control unit. This permits improved shifting smoothness and increases transmission service life.
Automatic mode, selector lever in the D position
The TCM adapts the shift points to match the driving conditions.
If special driving conditions are detected, the TCM switches to predefined characteristics.
Select-shift mode
The Select-Shift mode can only be activated if the selector lever is in the S position. Individual gears can be requested by operating the Select-Shift switch which is located on the side of the selector lever. Manual gearshifting will only be performed if the engine speeds do not exceed 6500 RPM or fall below predetermined values.
Moving the selector lever from N to R
The TCM only permits shifting to reverse gear if the vehicle speed is less than 14.5 km/h (10 mph).
At a vehicle speed above 14.5 km/h (10 mph), reverse gear is not engaged and the gearshift process is prevented.
Selector lever position N
In accordance with the calibration of the TCM, the following gears are engaged in the selector lever position N:
- During a shift from selector lever position R to N, 1st gear and reverse gear are engaged.
- During a shift from selector lever position D to N, 1st gear and N are engaged.
Selector lever position P
In accordance with the calibration of the TCM, 1st gear and reverse gear are engaged in the selector lever position P.
Altitude correction
Engine performance is reduced as air pressure decreases at higher altitudes. This situation is detected by the PCM.
In order to compensate for this operating situation, the TCM changes the shift points.
Speed control system
When the vehicle speed control system is switched on, a gear change can be executed by the TCM. This is done on the basis of the demand from the throttle plate position, which is controlled by the PCM.
Hill Start Assist
Hill start assist helps the driver when accelerating from a stop on an incline or side of a hill. Hill start assist supports the driver during drive-off situations on upward gradients. When the driver releases the vehicle brake, hill start assist prevents the vehicle from rolling back in the opposite direction.
Hot mode
The temperature of the clutch is calculated via a model in the TCM.
The following variables are incorporated in the calculation:
- Engine torque
- Rotational speed signals from the ISS A, ISS B, OSS and the engine speed
- Calculated torque transfer of the clutch
The Hot Mode function is used to prevent the clutch from being damaged as a result of excessively high temperatures. When hot mode is active, the clutch is engaged more quickly and the engine torque is reduced.
At a calculated clutch temperature at which the clutch lining would suffer heat damage, the following message is displayed on the message center:
- Transmission hot - stop the vehicle or accelerate
- Transmission hot - wait
- Transmission hot - wait 10 minutes.
Once the clutch has cooled down, the message "Transmission ready for operation" is displayed on the multi-function display of the instrument cluster.
At a calculated clutch temperature of more than 300°C (572°F) the clutches are disengaged.
Limp home mode
The TCM software contains functions which take control of the transmission if serious faults occur.
The fault characteristic decides which strategies are to be used.
The vehicle remains capable of restricted operation, unless there is a fault in the TCM itself or at the TR sensor.
Different measures are implemented depending on the current gear position and driving situation when the fault occurs:
- In the event of failure of an electric motor which actuates the lever actuator of the clutch, the TCM will only actuate the electric motor which is intact. If, for example, electric motor A has failed, then this path through the transmission is blocked (1st, 3rd and 5th gear). The TCM then only actuates electric motor B. This actuates the clutch of reverse, 2nd, 4th and 6th gear via the lever actuator.
- In the event of failure of the gearshift system or the rotational speed sensors, the fault reactions of the system range from the blocking of individual gears or the blocking of an entire path through the transmission (even/odd gears) to only permitting the vehicle to be driven in the currently engaged gear.
- In limp home mode, a text message is displayed in the instrument cluster and/or the MIL and/or the transmission warning light comes on (depending on the fault type).
When the engine is started again (ignition switched off for approximately 15 seconds), a self-test is performed in order to check whether there are any faults in the system. If the fault is still present, limp home mode is reactivated. If the fault is no longer present then it is no longer displayed on the instrument cluster and the MIL and/or the transmission control light will be off. However, the fault remains stored in the TCM.
In the event of a fault, it is recommended to continue driving so long as it is necessary and to look for the nearest workshop or to park the vehicle in a safe location.