Description and Operation
System Operation
General information
The transaxle operates without interruption of the propulsive force, a main disadvantage of automated manual transaxles. Even compared with the most modern automatic transaxles, its higher efficiency is clearly noticeable.
The requirement profile for the transaxle has been designed for 'comfort' and 'functionality'.
As with conventional manual transaxles, the gear ratios are accommodated inside the transaxle in the form of gear pairs on input and output shafts.
The input shaft is split into two parts just like on the 6-speed automatic transmission (6DCT450) and comprises the hollow shaft and the core shaft.
The dry clutches, which are electronically controlled and mechanically actuated on this transmission, have been arranged in a parallel layout in order to save space. This has achieved a compact transaxle design.
The external gearshift mechanism has been carried over from the automatic transaxles.
If the selector lever is in the position null, then 1st gear and reverse gear are engaged via the null (transmission control module) .
This results in a faster response after the starting process.
Transaxle design
Schematic diagram
3
3rd, 4th and reverse gear output shaft
4
Input shaft (core shaft)
5
Input shaft (hollow shaft)
6
1st, 2nd, 5th and 6th gear output shaft
In principle, the transaxle comprises two independent gear trains.
During driving, one gear train is always positively connected and the next gear is already engaged in the other gear train (although the clutch for this gear is still open).
The input shaft is in two parts and forms the heart of the transaxle. It consists of an outer input shaft (hollow shaft) and an inner input shaft (core shaft).
The input shaft (hollow shaft) drives the even-numbered gears (2nd, 4th and 6th gear) as well as the reverse gear (via an intermediate gear).
The input shaft (core shaft) drives the odd-numbered gears ( 1st, 3rd and 5th gear).
Each of the two input shafts is connected via outer splines to a clutch disc.
Synchronizers
Single and double synchronisation are both used on this transmission.
Single synchronisation is used on gears 1, 3, 4, 5, 6 and reverse gear.
Double synchronisation is used for the 2nd gear.
Torque Path
NOTE: In the descriptions below, the shafts are shown outside their actual position for greater clarity.
9
Input shaft (hollow shaft)
10
Input shaft (core shaft)
NOTE: When looking at the illustrations, please note that the torque flows once via the input shaft (core shaft) and once via the input shaft (hollow shaft).
First gear
The torque is passed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 1 and clutch disc 1 onto the input shaft (core shaft). The input shaft (core shaft) transmits the torque to the first gear of the output shaft (1st, 2nd, 5th and 6th gear). The torque is transmitted to the differential via the output pinion.
Second gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 2 and clutch disc 2 onto the input shaft (hollow shaft). The input shaft (hollow shaft) transmits the torque to the second gear of the output shaft (1st, 2nd, 5th and 6th gear). The torque is transmitted to the differential via the output pinion.
Third gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 1 and clutch disc 1 onto the input shaft (core shaft). The input shaft (core shaft) transmits the torque to the third gear of the output shaft (3rd, 4th and reverse gear). The torque is transmitted to the differential via the output pinion.
Fourth gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 2 and clutch disc 2 onto the input shaft (hollow shaft). The input shaft (hollow shaft) transmits the torque to the fourth gear of the output shaft (3rd, 4th and reverse gear). The torque is transmitted to the differential via the output pinion.
Fifth gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 1 and clutch disc 1 onto the input shaft (core shaft). The input shaft (core shaft) transmits the torque to the fifth gear of the output shaft (1st, 2nd, 5th and 6th gear). The torque is transmitted to the differential via the output pinion.
Sixth gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 2 and clutch disc 2 onto the input shaft (hollow shaft). The input shaft (hollow shaft) transmits the torque to the sixth gear of the output shaft (1st, 2nd, 5th and 6th gear). The torque is transmitted to the differential via the output pinion.
Reverse gear
The torque is fed into the double clutch via the drive plate. From there, the torque is transferred via the driving disc, pressure plate 2 and clutch disc 2 onto the input shaft (hollow shaft). The input shaft (hollow shaft) transmits the torque to the second gear of the output shaft (1st, 2nd, 5th and 6th gear). The gear wheel for the 2nd gear has a fixed connection to the intermediate gear, The intermediate gear transmits the torque to the reverse gear wheel of the output shaft (3rd, 4th and reverse gear). The torque is transmitted to the differential via the output pinion.
Parking lock
4
Lock wheel with tooth gaps
There is a parking lock integrated in the second output shaft for safe parking of the vehicle and to prevent it from rolling away when the parking brake is not applied.
A parking lock needs to be installed since both clutches are opened after the engine is switched off.
The parking lock is engaged by moving the selector lever to the null position. As a result, the lock pawl (5) engages in a tooth gap on the lock wheel (4).
If the lock pawl (5) is positioned against a tooth of the lock wheel (4), then the torsion spring (3) on the actuation shaft is tensioned. If the vehicle moves, the lock pawl (5) engages in the next tooth gap on the lock wheel (4) under the effects of the releasing torsion spring (3).
Internal gearshift mechanism
Layout of the internal gearshift mechanism
2
Gear selector drum 2 with spur gear
Comments:
Controls the selector forks for 2nd/6th gear and 4th/reverse gear
5
Gear selector drum 1 with spur gear
Comments:
Controls the selector forks for 1st/5th gear as well as 3rd gear
The gears are shifted by means of two brushless DC motors, which each actuate a gear selector drum via a two-stage transmission ratio. Both of the gear selector drums are identical and each have one shift slot for moving the selector forks. As a result of using the gear selector drum principle, no additional mechanical lock is required in order to prevent more than one gear being engaged at the same time in the same sub-transmission in the event of a fault.
Layout of the gearshift system (schematic diagram)
1
Gear selector drum 2 with spur gear
2
Selector fork - reverse gear/4th gear
3
Selector fork - 3rd gear
4
Selector fork - 1st/5th gear
5
Gear selector drum 1 with spur gear
6
Selector fork - 2nd/6th gear
Each gear selector drum actuates two selector forks. The total angle of rotation of the gear selector drums is limited by means of two stops which are cast as an integral part of the transmission housing.
The angle of rotation of the gear selector drum 1 is 200°. The angle of rotation of the gear selector drum 2 is greater and measures 290°, as this gear selector drum is used to shift four gears.
Overview of the gearshift system
1
Gear selector drum 2 with spur gear
2
Selector fork - reverse gear/4th gear
3
Selector fork - 3rd gear
4
Selector fork - 1st/5th gear
5
Gear selector drum 1 with spur gear
8
Selector fork - 2nd/6th gear
The shift slot in the gear selector drum has two counter-running cams on its circumference, which are offset by 180°. A slider which is connected to the selector fork is moved via the shift slot. If the slider moves up or down on the cam then the selector fork is moved accordingly in an axial direction, and as a result either a gear is engaged or the synchroniser assembly is moved to the neutral position.
Function of the gear selector drum 1
1
Shift slot of gear selector drum 1
Comments:
In the coloured area the angle of rotation is 200°.
3
Selector fork for 3rd gear with slider
5
Selector fork for 1st/5th gear with slider
6
Lower end position (rotation angle 0°)
7
Rotation angle 10°
Comments:
The selector fork for 1st/5th gear is moved axially and 1st gear is engaged.
8
Rotation angle 55°
Comments:
Neutral position between 1st gear and 3rd gear
9
Rotation angle 100°
Comments:
The selector fork for 3rd gear is moved axially and 3rd gear is engaged.
10
Rotation angle 145°
Comments:
Neutral position between 3rd gear and 5th gear
11
Rotation angle 190°
Comments:
The selector fork for 1st/5th gear is moved axially and 5th gear is engaged.
12
Upper end position (rotation angle 200°)
Function of the gear selector drum 2
1
Selector fork for 2nd/6th gear with slider
2
Shift slot of gear selector drum 2
Comments:
In the coloured area the angle of rotation is 290°.
5
Selector fork for reverse/4th Gear with slider
6
Lower end position (rotation angle 0°)
7
Rotation angle 10°
Comments:
Selector fork for reverse/4th gear is moved in an axial direction and reverse gear is engaged.
8
Rotation angle 55°
Comments:
Neutral position between reverse gear and 2nd gear
9
Rotation angle 100°
Comments:
The selector fork for 2nd/6th gear is moved axially and 2nd gear is engaged.
10
Rotation angle 145°
Comments:
Neutral position between 2nd gear and 4th gear
11
Rotation angle 190°
Comments:
Selector fork for reverse/4th gear is moved in an axial direction and 4th gear is engaged.
12
Rotation angle 235°
Comments:
Neutral position between 4th gear and 6th gear
13
Rotation angle 280°
Comments:
The selector fork for 2nd/6th gear is moved axially and 6th gear is engaged.
14
Upper end position (rotation angle 290°)
Double clutch system
2
Electromechanical lever actuator 1
4
Electromechanical lever actuator 2
The clutch unit is connected to the two input shafts of the transaxle and attached to the drive plate with nuts. The nuts need to be removed from the drive plate if the transaxle is removed.
Clutch unit
Sectional view
7
Input shaft (hollow shaft) hub
8
Input shaft (core shaft) hub
9
Bearings of the driving disc
The torque is transmitted in each case via a clutch disc, with a parallel layout for the two sub-transmissions. For safety reasons the double clutch is designed to be open in the rest state. This type of clutch is referred to as a so-called "active clutch". On an active clutch, the contact pressure is zero if no force or only a small force is applied at the lever springs.
The clutches are equipped with an internal travel-controlled wear adjustment system in order to keep the necessary actuator travel paths and therefore the required packaging space within tight limits.
In order to dampen the torsional vibrations, torsional vibration dampers are integrated in the clutch discs.
The driving disc of the double clutch is mounted on the input shaft (hollow shaft) of the transmission.
Schematic diagram of the open and closed clutch
A
Clutch in the rest state (open)
6
Input shaft (core shaft)
7
Input shaft (hollow shaft)
The two lever springs open the clutches in the rest state. They are closed through actuation of the relevant engaging bearing, which acts on the corresponding lever spring. By pressing the lever springs, the relevant pressure plate is pressed against the clutch disc and the driving disc.
Engaging unit
Sectional view
1
Engaging bearing 2
Comments:
Actuates the lever spring 2 of the 2nd clutch
2
Engaging bearing 1
Comments:
Actuates the lever spring 1 of the 1st clutch
The two engaging bearings are accommodated by the guide sleeve in such a way that they can be moved independently of each other. The sliding sleeves are slotted for this purpose and engage in segments in each other. The compensating element is used to compensate for any offset to the actuating levers of the electromechanical lever actuators.
The two engaging bearings are each equipped with a hardened engaging disc. This lies loose on the engaging bearing and transmits the axial forces.
Electromechanical lever actuator
1
Brushless direct current motor
The force required to close the clutches is largely generated by a compression spring via the mechanical system of the lever actuator. This force acts on the outer end of the engaging lever. This has the form of a rocker. The rollers form the central point of contact of the engagement lever.
The brushless direct current motors are bolted directly onto the transmission bell housing. The electric motor drives the threaded bar of the ball screw via gear teeth. Through rotation of the threaded rod, the recirculating ball nuts and thus the rollers are moved in an axial direction. Due to the axial movement of the rollers, the central support point of the engaging lever is displaced, as a result of which the leverage is altered.
Function of the electromechanical lever actuator
A
Clutches open
Comments:
Brushless DC motor de-energised
B
Clutch 2 closed
Comments:
Brushless DC motor energised
When the electric motor is de-energised the clutch is open. In order to close the clutch, the electric motor is actuated by the null. As a result of the rotation of the ball screw, the roller is moved downwards via the recirculating ball nut. Due to this axial movement of the rollers, the central support point of the engaging lever is displaced, as a result of which the leverage is altered. The change in leverage in turn causes the force which acts via the engaging lever and the engaging bearing on the lever spring of the clutch to be increased. As a result, the engaging lever and the engaging bearing are lifted up. The engaging bearing presses against the lever spring and the clutch is pressed into the closed position.
In order to hold the clutch in the closed position, a holding current is applied to the electric motor.
As soon as the holding current is switched off by the null, the lever springs will slacken and the clutch will open. Through the release of the lever springs, the engaging bearing and the engaging lever are rotated back. When the engaging lever is rotated back, the shape of the engaging lever ensures that the rollers return to their starting position.
null
Exploded view of the null
5
Stator coils (electric motor 1)
6
Bearings of the electric motors
7
Rotor with magnetic poles (electric motor 1)
9
Rotor with magnetic poles (electric motor 2)
10
Stator coils (electric motor 2)
11
Control unit connector plug
The control unit and the two brushless DC motors for changing gears are integrated in the null. The primary function of the null is to collect the incoming signals from the sensors, evaluate these signals and control the actuators accordingly. In service, the null
can only be replaced as a complete unit.
Control Strategies
Operating principle of the transaxle.
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 multi-plate 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 transaxle
The illustration shows that in conventional manual transaxles, gearshifts result in a customary interruption of the propulsive force.
Gearshift process on the 6DCT250 transmission
With this gearshift, shown under load in the illustration, the power flow is only slightly restricted; there is constant propulsion perceptible.
Control
The gearshift functions of the 6DCT250 can be divided into a clutch system and a gearshift system. The null controls the clutch and gearshift system with the aid of the four brushless DC motors with integrated position sensors.
The corresponding DC motor is actuated both during clutching and during the gearshift. The information about the position of the DC motors is sent via the integrated position sensors to the null. On the basis of this information, the null 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 null 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 null receives the following information:
- Chosen transmission range
- Vehicle speed via the HS-null (controller area network) data bus
- Engine speed and torque as well as the throttle position via the HS-null data bus
- Engine temperature via the HS-null data bus
- Outside air temperature via the HS-null data bus for determining the viscosity of the transmission fluid when cold
- Steering angle from the steering wheel rotation sensor via the HS-null databus in order to avoid upshifts or downshifts during cornering.
- Information about braking interventions via the HS-null databus
- Input shaft speed for even and uneven gears from the respective speed sensor
Adaptive control
The null 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.
The adapted values are stored in the non-volatile RAM (read/write memory) of the control unit. This permits improved shifting smoothness and increases transaxle service life.
Automatic mode, selector lever in the null position
The null adapts the shift points to match the driving conditions.
If special driving conditions are detected, the null switches to predefined characteristics.
Sports mode, selector lever in the null position
In this mode the null switches to another set of characteristic curves. These characteristic curves for control of the gear changes are adapted to sporting calculations (e.g. gear change at higher engine speed).
Select-shift mode
The Select-Shift mode can only be activated if the selector lever is in the null position. Individual gears can be engaged by operating the Select-Shift switch which is located on the side of the selector lever.
NOTE: Manual gearshifting can only be performed if the engine speeds do not exceed or fall below predetermined values.
If the vehicle speed decreases so that the engine speed falls below the lower limit, the null causes the transaxle to shift to a lower gear.
When shifting down, the null checks whether the upper engine speed limit is exceeded. If the speed limit is exceeded, the null prevents the gearshift.
If an engine speed of 6,500 rpm is exceeded under acceleration then the system automatically shifts up to the next higher gear.
Moving the selector lever from null to null
The null only permits shifting to reverse gear if the vehicle speed is less than 14.5 km/h.
At a vehicle speed above 14.5 km/h, reverse gear is not engaged and the gearshift process is hence prevented.
Selector lever position null
In accordance with the calibration of the null, the following gears are engaged in the selector lever position null:
- During a shift from selector lever position null to null, 1st gear and reverse gear are engaged.
- During a shift from selector lever position null to null, 1st gear and null are engaged.
Selector lever position null
In accordance with the calibration of the null, 1st gear and reverse gear are engaged in the selector lever position null.
Altitude correction
Engine performance is reduced as air pressure decreases at higher altitudes. This situation is detected by the null (powertrain control module).
In order to compensate for this operating situation, the null changes the shift points.
Speed control system
When the vehicle speed control system is switched on, a gear change can be executed by the null. This is done on the basis of the demand from the throttle plate position, which is controlled by the null.
Rollback prevention function
NOTE: This function is only activated if the vehicle is equipped with hill start control.
If the vehicle is stopped while driving uphill, then 1st gear is pre-selected in 'P' and 'N'. The pressure in the brake system is held until the engine torque is sufficient to move the vehicle uphill.
Hot mode
The temperature of the clutch is calculated via a model in the null.
The Hot Mode function is used to prevent the clutch from being damaged as a result of excessively high temperatures. Here, 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 multi-function display of the instrument cluster:
- Transmission hot - stop the vehicle or accelerate
- Transmission hot - wait
- Transmission hot - wait 10 mins.
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 the clutches are disengaged.
Limp home mode
The null 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 null itself or at the null (transmission range) sensor.
NOTE: If the null sensor is defective, both clutches are disengaged and it is no longer possible to continue driving. In the event of failure of the null sensor, it is no longer possible to start the vehicle, or the transmission is fixed in the null position, as a result of which it is impossible to continue driving.
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 null will only actuate the electric motor which is intact. If, for example, electric motor 1 has failed, then this path through the transmission is blocked (1st, 3rd and 5th gear). The null then only actuates electric motor 2. 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 null (malfunction indicator lamp) 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 null and/or the transmission control light will be off. However, the fault remains stored in the null.
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.
Component Description
Double clutch with travel-controlled wear adjustment
As a result of wear to the clutch discs, the positions of the lever springs change, and this in turn alters the characteristic curves of the contact pressure and disengaging force. The consequence of this would be increased loads on the brushless DC motors of the electromechanical lever actuators.
In order to keep the position of the lever springs and therefore the contact pressure and disengaging force more or less constant, the double clutch has a travel-controlled wear adjustment mechanism.
Sectional view of clutch 2 of the double clutch
4
Adjustment roller spring
5
Adjustment tension spring
The adjustment of the clutch is triggered if - as a result of wear to the clutch lining - the lever spring for generating a specific contact pressure is pressed through further in the direction towards the engine. As a result of the additional travel, the clamping spring lifts off the ramp ring. As a result of the pre-loaded adjustment roller spring, the ramp ring is rotated on the ramp until the clearance between the clamping spring and the ramp ring has been compensated for. If the clutch is then fully opened (i.e. disengaged) again as a result of a gearshift process, the lever spring moves into a new position due to the rotation of the ramp ring, and this creates an air gap between the lever spring and the adjustment ramp ring. As a result of the adjustment ramp ring, which is also spring-loaded, it is then rotated until it abuts against the lever spring. The adjustment process is then complete.
Flow chart
2
null (body control module) 3
Electric motors in the
nullComments:
actuate the shift shafts
7
null sensor 1 of the input shaft (core shaft)
Comments:
Additional detection of the direction of rotation
8
null sensor 2 of the input shaft (hollow shaft)
11
Hall sensors of the electric motors 1 and 2
12
Hall sensors of the electric motors in the
null15
null (anti-lock brake system) 16
Steering wheel rotation sensor
Function of the electronic components
null sensor 1
The null sensor 1 is attached to the transmission housing. The spacer washer provides a defined distance between the sensor and the transmitter wheel. The spacer washer has a thickness of 3.5 mm ± 0.05 mm.
The sensor detects the rotational speed via the third gear wheel of the input shaft (core shaft (odd gears)). It is a magneto-resistive sensor which measures the rotational speed and the direction of rotation. The signal is processed via internal electronics and forwarded to the null.
null sensor 2
The null sensor 2 is attached to the transmission housing. The spacer washer provides a defined distance between the sensor and the transmitter wheel. The spacer washer has a thickness of 3.2 mm ± 0.05 mm.
The sensor detects the rotational speed via the fourth gear wheel of the input shaft (hollow shaft (even gears)). It is a magneto-resistive sensor which only measures the rotational speed. The signal is processed via internal electronics and forwarded to the null.
The null sensor
The null sensor is attached to the transmission housing.
The sensor detects the rotational speed via a transmitter wheel which is attached to the differential. It is a magneto-resistive sensor which only measures the rotational speed. The rotational speed signal is processed via internal electronics and forwarded to the null.
The null sensor
The null sensor is attached to the transmission housing. A cover plate is attached to the actuation shaft on both sides of the null sensor. Operation of the selector lever rotates the shaft and therefore also the cover plate.
The null sensor is a double contactless inductive sensor. It functions essentially in a similar way to a transformer. First, the incoming DC voltage is converted into AC voltage. The cover plates are moved when the selector lever is moved. This causes the magnetic fields to change and the induced AC voltages in the secondary coils therefore also change. These voltage changes are evaluated in the integrated electronics and converted into two digital signals (null (pulse width modulation) signals).
The null sensor is hard-wired to the null. On the basis of the two null signals, the null detects the position of the selector lever.
Electric motors
1
Electric motor 1
Comments:
Actuates the clutch for 1st, 3rd and 5th gear via the electromechanical lever actuator
3
Electric motor 1 in the
nullComments:
Controls the selector forks for 1st/5th gear and 3rd gear via the gear selector drum 1
4
Electric motor 2
Comments:
Actuates the clutch for 2nd, 4th, 6th and reverse gear via the electromechanical lever actuator
All electric motors are designed as brushless DC motors.
The stator coils are actuated by the electronics in the null in such a way that a circumferential magnetic field is generated. The rotor runs behind the magnetic field. Via the Hall sensors, the null receives information on the position of the rotor and calculates by how many rotations the motor has been turned. This information is required by the null to actuate the shift forks in accordance with the specified torque angle and to actuate the clutches. A detailed description of the function of the brushless DC motor can be found in the e-Learning module "Sensors and Actuators" (TC 401 2071C).
