11/7/11

Overview: Volkswagen 7-speed dual-clutch gearbox

Introduction

The seven-speed double-clutch gearbox 0AM is a twin-clutch automatic transmission with dry clutches. It has a torque capacity of 250Nm, and is a development of the 02E transmission; the latter is a six-speed transmission, uses wet-plate clutches and has a torque capacity of 350Nm. The 0AM is designed for transversely-mounted engines.

Key design features include

  • Modular design of the gearbox: The clutch, mechatronic (mechanical-electronic) unit and gearbox each form one unit;
  • Dry double clutch;
  • Separate oil circuits for the mechatronic unit and mechanical gearbox, with lifetime fillings;
  • Seven gears on four shafts;
  • Oil pump driven according to demand;
  • No oil/water heat exchanger.

The instaled mass is roughly 70kg, including the clutch. The seven forward gears offer a ratio spread of 8.1:1; ratios are normally changed automatically, in normal Drive or Sport modes, though it is possible to use the transmission manually by way of the selector lever or steering-column switches.

Volkswagen 7-speed dual-clutch gearbox 0AM.

Design of the selector lever

Hall senders in the selector lever mounting register the position of the lever, ‘informing’ the mechatronic unit via the CAN bus.

The selector lever lock solenoid (N110) locks the selector lever in the ‘P’ and ‘N’ positions. The solenoid is controlled by the selector lever sensors control unit.

Selector lever. Click here for larger image.

If the selector lever is in the ‘P’ position, a switch (F319) transmits a signal to the steering column electronics control unit. The control unit requires this signal to control the ignition key withdrawal lock.

Selector lever lock solenoid: How it works

If the selector lever is set to ‘P’, the locking pin is located in the locking pin latch ‘P’. This prevents the locking lever from being moved unintentionally. After switching on the ignition and pressing the footbrake, the selector lever sensors control unit supply the selector lever lock solenoid with current. As a result of this, the locking pin is withdrawn from the locking pin latch. The selector lever can now be moved out of the ‘P’ position.

After switching on the ignition and applying the footbrake, the selector lever sensors control unit supplies the selector lever lock solenoid with current. As a result of this, the locking pin is withdrawn from the locking pin latch. The selector lever can now be moved out of the ‘P’ position.

If the selector lever is set to the ‘N’ position for longer than 2s., the control unit supplies the solenoid with current. As a result of this, the locking pin is pressed into locking pin latch. The selector lever can no longer be unintentionally moved into a gear. The locking pin is released when the footbrake is applied.

If the voltage supply to the selector lever lock solenoid fails, the selector lever can no longer be moved, because the selector lever lock remains activated in the event of a power failure. By mechanically pressing in the locking pin with a narrow object, the lock can be released and the selector lever can be ‘emergency released’ to the ‘N’ position. The vehicle can be moved again.

An ignition key withdrawal lock prevents the ignition key from being turned back to the removal position if the parking lock is not engaged. It functions electromechanically and is controlled by the steering column electronics control unit. The steering column electronics control unit detects the open switch. The ignition key withdrawal lock solenoid is not supplied with current. The compression spring in the solenoid pushes the locking pin into the release position.

Ignition key withdrawal lock, showing solenoid in yellow.

How it works

If the selector lever is in the Park position with the ignition switched off, the lever is locked in position. The ‘P’ switch F319 is open.

The selector lever is in the Park position with the ignition switched off.

If the selector lever is in the Drive position with the ignition switched on, the selector lever locked in position ‘P’ switch (F319 above) is closed. The steering column electronics control unit then supplies the ignition key withdrawal lock solenoid N376 with current. The locking pin is pushed into the locked position by the solenoid. In the locked position, the locking pin prevents the ignition key from being turned back and withdrawn. Only when the selector lever is pushed into the park position does the selector lever locked in position ‘P’ switch open, and the control unit then swtches off the current supply to the solenoid. The locking pin is then pressed back by the compression spring. The ignition key can be turned further and can be removed.

Thus, you can’t take the key out of the ignition unless the selector lever is in Park.

Ignition switched on, selector in ‘D’.

Design of the gearbox

Basic principle

In principle, the double-clutch gearbox consists of two independent gear trains, or ‘gear train halves’ as Volkswagen calls them. Click here for a schematic diagram. In terms of function, each gear train half is designed as a manual gearbox. A dry-plate clutch is assigned to each gear train half, and the clutches are controlled by the mechatronic unit. Gears 1, 3, 5 and 7 are shifted by clutch K1 (Kupplung in German) and transmit drive via gear train half 1 and output shaft 1. Gears 2, 4, 6 and reverse drive through clutch K2 and thus gear train half 2 and output shafts 2 and, for reverse, 3. One gear train half is always positively connected, except with the selector lever in Neutral. The next gear can already be engaged in the other gear train half, because the clutch for this gear is still open.

A conventional, manual gearbox synchroniser and shift unit is assigned to each gear.

Torque input

Drive the torque is transferred from the dual-mass flywheel, which is secured to the crankshaft, to the double clutch. To achieve this, the dual-mass flywheel is equipped with inner teeth. These engage in the outer teeth on the double clutch carrier ring. From there, the torque is transmitted onwards into the double clutch. See picture.

The double clutch is located in the bell housing. It consists of two conventional clutches, which are assembled to form a double clutch. The clutches are referred to as K1 and K2. Clutch K1 transfers the torque to driveshaft 1 by way of splines. From driveshaft 1, the torque for gears 1 and 3 is transferred to output shaft 1 and that for gears 5 and 7 to output shaft 2. Clutch K2 transfers the torque to driveshaft 2 via splines. It transfers the torque for gears 2 and 4 to output shaft 1 and the torque for 6th gear and reverse gear to output shaft 2. Intermediate gear R1 is used to transfer the torque to reverse gear R2 on output shaft 3. All three output shafts are connected to the differential final drive gear. See picture.

From the carrier ring, torque is transferred to the drive plate in the double clutch. To achieve this, the carrier ring and drive plate are welded together. The drive plate is mounted on driveshaft 2 as an idler gear.

If one of the two clutches is actuated, the torque is transferred from the drive plate onto the relevant clutch plate and onwards onto the corresponding driveshaft. See picture.

The clutches

Two independent, dry clutches operate in the double clutch. They each conduct the torque into one gear train half. Two clutch positions are possible:

  • When the engine is switched off or idling, both clutches are open;
  • During vehicle operation, one and only one of the two clutches is closed.

Clutch K1 conducts the torque for gears 1, 3, 5 and 7 to driveshaft 1. These two pictures show clutch K1: first not engaged, then engaged.

To actuate the clutch, the engaging lever presses the engagement bearing onto the diaphragm spring. At several relay points, this compression movement is transformed into a tension movement. As a result of this, the pressure plate is pulled onto the clutch plate and the drive plate. The torque is then transferred onto the driveshaft. The engaging lever is actuated by way of Valve 3 in gear train half 1 by the hydraulic clutch actuator for K1.

Clutch K2 conducts the torque for gears 2, 4, 6 and reverse to driveshaft 2. These two pictures show clutch K2: first not engaged, then engaged. When the engaging lever is actuated, the engagement bearing presses against the pressure plate’s diaphragm spring. As the diaphragm spring is supported by the clutch housing, the pressure plate is pressed against the drive plate and the torque is transferred onto driveshaft 2. The engaging lever is actuated via Valve 3 in gear train half 2 by the hydraulic clutch actuator for K2.

Driveshafts

The driveshafts are located in the gearbox housing. Each driveshaft is connected to a clutch via splines which transfer the engine torque onto the output shafts according to the gear which is engaged. driveshaft 2 is hollow, while driveshaft 1 runs through hollow driveshaft 2. A ball bearing, which is used to mount the driveshafts in the gearbox housing, is located on each shaft. These two pictures show the arrangement of the two shafts.

Because of to its installation position, we will describe driveshaft 2 before driveshaft 1.

Driveshaft 2.

Driveshaft 2 is a hollow shaft. It is connected to clutch K2 using splines and is used to select gears 2, 4, 6 and reverse. To record the gearbox input speed, this shaft has the gear for gearbox input speed sender 2.

Driveshafts: installation position in the gearbox.

Driveshaft 1 similarly is connected to clutch K1 with splines. It is used to select gears 1,3,5 and 7. This shaft has the impulse wheel for gearbox input speed sender 1.

Output shafts

Three output shafts are contained in the gearbox housing. Depending on the gear which is engaged, torque is transferred from the driveshafts to the output shafts. An output gear, through which the torque is passed on to the differential final drive gear, is located on each output shaft.

Output shaft 1: Installation position in the gearbox (view from the left, shown elongated). Click here for an enlarged image.

The following are located on output shaft 1:
The selector gears for gears 1, 2 and 3; the three gears are synchromeshed three-fold;
The selector gear for 4th gear; 4th gear is synchromeshed two-fold.

Output shaft 2: Installation position in the gearbox (view from the left, shown elongated). Click here for an enlarged image.

The following are located on output shaft 2:
The two-fold synchromeshed selector gears for gears 5, 6 and 7;
The intermediate gears R gear 1 and R gear 2 for reverse.

Output shaft 3: Installation position in the gearbox (view from the left, shown elongated).
Output shaft 3. Parking lock gear near left end.

The following are located on output shaft 3:
The singly-synchromeshed selector gear for reverse;
The parking lock gear.

Differential

The differential transfers the torque onwards to the vehicle’s wheels via the driveshafts.

Differential. Click here for a cutaway image.

Parking lock

A parking lock similar to those fited to conventional automatic gearboxes is integrated into the double-clutch gearbox. The locking pin is engaged purely mechanically by means of a Bowden cable between the selector lever and the parking lock lever on the gearbox. The Bowden cable is used exclusively to actuate the parking lock. This picture shows the layout of the parking lock.

When the parking lock is not applied, the cone of the actuation pin lies against the clamping device and the locking pin. The parking lock is held in the non-actuated position by a locking device. By actuating the parking lock, the cone of the actuation pin is pressed against the clamping device and the locking pin. As the clamping device is stationary, the locking pin moves down. If it encounters a tooth on the parking lock gear, the pre-tensioning spring is tensioned. The actuation pin is held in this position by the locking device. If the vehicle continues to move, the parking lock gear also rotates. As the actuation pin is pre-tensioned, it automatically pushes the locking pin into the next tooth space on the parking lock gear. The following sequence of images show, in order: the parking lock not actuated; the parking lock actuated, but with the locking pin not yet engaged as it’s sitting on top of a gear tooth; and finally the parking lock actuated and the locking pin engaged between two gear teeth.

Parking lock not actuated (selector lever not in ‘P’ position).
Parking lock actuated (selector lever in ‘P’ position), locking pin not engaged.
Parking lock actuated (selector lever in ‘P’ position), locking pin engaged.

Gear synchronisation

A balked synchromesh with locking pieces is used in the case of all gears to synchronise the different speeds when changing gear. Depending on the shifting load, the gears are synchronised one- to three-fold.

Gear Synchromesh Synchroniser
ring material
I, I, III 3x Brass with
molybdenum coating
IV 2x
V, VI, VII, R 1x

These pictures show the synchromesh design for 2nd, 4th and reverse gears.

Power transmission in the gears

Torque is transmitted into the gearbox by way of either clutch K1 or K2. Each clutch drives a driveshaft. Driveshaft 1 is driven by clutch K1 and driveshaft 2 is driven by clutch K2. Power is transmitted to the differential by:

  • output shaft 1 for gears 1, 2, 3, and 4;
  • output shaft 2 for gears 5, 6 and 7, and;
  • output shaft 3 for reverse gear and the parking lock.

First gear
Clutch K1
Driveshaft 1
Output shaft 1
Differential

Second gear
Clutch K2
Driveshaft 2
Output shaft 1
Differential

Third gear
Clutch K1
Driveshaft 1
Output shaft 1
Differential

Fourth gear
Clutch K2
Driveshaft 2
Output shaft 1
Differential

Fifth gear
Clutch K1
Driveshaft 1
Output shaft 2
Differential

Sixth gear
Clutch K2
Driveshaft 2
Output shaft 2
Differential

Seventh gear
Clutch K1
Driveshaft 1
Output shaft 2
Differential

Reverse gear
Clutch K2
Driveshaft 2
Output shaft 3
Differential

The change in rotational direction for reverse gear is carried out by output shaft 3.

Mechatronic module

The mechatronic unit is the central gearbox control unit. Within it, the electronic control unit and the electrohydraulic control unit are combined to form one component. The mechatronic unit is flanged onto the gearbox, and is an autonomous unit. It has a separate oil circuit, which is independent of the oil circuit for the gearbox. The advantages of this autonomous, compact unit are:

  • Apart from one sensor, all sensors and actuators are contained in the mechatronic unit.
  • The hydraulic fluid is specifically adapted to the requirements of the mechatronic unit.
  • Due to the separate oil circuit, no foreign material from the mechanical gearbox enters into the mechatronic unit.
  • Good low-temperature behaviour, as no compromise has to be made with the requirements of the gearbox in terms of the viscosity behaviour of the oil.
Mechatronic unit. Click here for detailed images showing the location of sensors.

The mechatronic unit’s electronic control unit is the central gearbox control unit. All sensor signals and all signals from other control units come together here, and all actions are performed and monitored by it. Eleven sensors are integrated into the electronic control unit; only the gearbox input speed sender is located outside of the control unit.

The electronic control unit hydraulically controls and regulates eight solenoid valves. These shift the seven gears and actuate the clutches.

The ECU ‘learns’ the positions of the clutches and the positions of the gear selectors when a gear is engaged, and takes what it has learnt into account for future operation of these components.

To see the location of the sensors, take a look at the popup.

Electrohydraulic control unit

The electrohydraulic control unit is integrated into the mechatronic module. It generates the oil pressure which is required to shift the gears and to actuate the clutches. The oil pressure is generated by the hydraulic pump’s motor. An oil pressure accumulator ensures that sufficient oil pressure is always available at the solenoid valves. For a detailed image of the electrohydraulic unit, click here.

Oil circuit

The double-clutch gearbox operates with two independent oil circuits using two different oils. These are respectively for the gearbox (shown here in brown) and for the mechatronic module (shown in beige).

DSG: Twin oil circuits.

The oil supply to the shafts and gears of the mechanical gearbox is carried out in the same way as for a normal manual gearbox. The oil supply for the mechatronic unit is separate from that for the gearbox; an oil pump delivers the oil on demand at the pressure required to enable the hydraulic mechatronic unit components to function. Click here for an oil circuit flow chart.

The hydraulic pump, which is electric, unit is located in the mechatronic module along with its associated electric motor. The motor is a brushless DC item. It is actuated and its output is controlled by the mechatronic unit’s electronic control unit. The hydraulic pump is a gear pump. It intakes the hydraulic oil and pumps it into the oil circuit at a pressure of roughly 70 bar. The hydraulic oil is pumped from the intake side to the pressure side between the walls of the pump housing and the tooth gaps.

Mechatronic unit: Oil pump.

Like conventional, smaller DC electric motors, the brushless DC motor consists of a stator and a rotor. But while in the conventional motor the stator consists of permanent magnets and the rotor of electromagnets, the opposite is true in the case of the brushless DC motor. The rotor consists of six permanent magnet pairs and the stator of six electromagnet pairs.

In the conventional DC motor, commutation (current direction change-over) takes place via ring contacts. Commutation in the brushless DC motor is carried out by the mechatronic unit’s electronic control unit and is therefore contact-free. The stator coils are actuated in such a way that a rotating magnetic field occurs in the coils. The rotor follows this magnetic field and is therefore caused to rotate. Thanks to contact-free commutation, the DC motor runs entirely wear-free, with the exception of bearing wear. Click here for detailed images.

Oil circuit, hydraulic system

Task and function of the solenoid valves in the oil circuit

The gear train half pressure control solenoid valves control the oil pressure for gear train halves 1 and 2. If a fault is detected in a gear train half, the pressure control solenoid valve can shut off the corresponding gear train half.

The gear selector solenoid valves control the volume of oil to the gear selectors. Each gear selector shifts two gears. If no gear is engaged, the gear selectors are held in the neutral position by oil pressure. In selector lever position ‘P’ and when the ignition is switched off, first gear and reverse gear are engaged simultaneously, because there is no oil pressure to hold the clutches open.

The clutch actuator solenoid valves control the volume of oil to the two clutch actuators. When not supplied with current, the solenoid valves and the clutches are open.

Shifting the gears

As in the case with conventional manual gearboxes, the gears are shifted using selector forks. Each selector fork shifts two gears. The selector forks are mounted on both sides in the gearbox housing. Click here to see a cutaway image.

When changing gears, the selector forks are moved by the gear selectors integrated into the mechatronic unit. The gear selector piston is connected to the selector fork. To change gears, oil pressure is applied to the gear selector piston, which moves it. When it moves, it also moves the selector fork and the sliding sleeve. The sliding sleeve actuates the synchronising hub and the gear is engaged. This is shown in the second image on the popup.

By way of the permanent magnet and the gear selector movement sensor, the mechatronic unit detects the new position of the selector fork.

As in the case of the direct shift gearbox 02E (the wet-plate high-capacity unit), the selector forks are actuated hydraulically. To change the gears, the mecha­tronic electronic control unit actuates the corresponding gear selector solenoid valve.

How it works

Changing to first gear is shown here as an example. Click here for a sequence of images.

Initially, the gear selector piston is held in neutral position ‘N’ by the oil pressure which is controlled by the gear selector solenoid valve for gears 1 and 3. No gear is engaged.

Valve 4 in gear train half 1 controls the oil pressure in gear train half 1.

To change to 1st gear, the gear selector valve increases the oil pressure in the left piston chamber. As a result of this, the gear selector piston is pushed to the right. As the selector fork and the sliding sleeve are connected to the gear selector piston, they also move to the right. Due to the sliding sleeve’s movement, first gear is engaged.

Clutch actuators

Clutches K1 and K2 are actuated hydraulically. To achieve this, the mechatronic unit contains a clutch actuator for each clutch. A clutch actuator consists of a clutch actuator cylinder and a clutch actuator piston. The piston actuates the clutch engaging lever. The clutch actuator piston is equipped with a permanent magnet, which is required by the clutch travel sender to detect the piston position. To prevent detection of the piston position from being impaired, the actuator cylinder and the actuator piston must not be magnetic.

To actuate the clutches, the mechatronic electronic control unit actuates the relevant solenoid valve: valve 3 in gear train half 1 for clutch K1 and valve 3 in gear train half 2 for clutch K2.

Actuation of K1 is shown in the popup as an example. When the clutch is not actuated, the clutch actuator piston is in the resting position. The solenoid valve is open in the return flow direction. The oil pressure from the gear train half pressure control valve flows into the mechatronic unit’s oil reservoir.

If clutch K1 is to be actuated, the relevant solenoid valve is actuated by the electronic control unit. When actuated, it opens the oil channel to the clutch actuator, and oil pressure is built up at the rear of the clutch actuator piston. The piston moves and thereby actuates the K1 clutch engaging lever. Clutch K1 is engaged. The control unit receives a signal regarding the precise position of the clutch from clutch travel sender 1. Clutch slip, the speed difference between the gearbox input speed and driveshaft speed, is achieved by solenoid valve N435 by controlling the oil pressure between the clutch actuator and the return flow.

Gearbox management system

For a schematic overview of the system, click here.

The selector lever communicates with the dashboard display by way of the CAN (control area network). The sensors are clutch travel sender 1 (G617) and clutch travel sender 2 (G618). The clutch travel senders are located in the mechatronic unit above the clutch actuators. The double clutch control system requires reliable and precise recording of the current clutch actuation status. For this reason, contact-free sensor technology is used to record clutch travel. Contact-free position recording increases the reliability of the sensor functions: errors caused by wear and vibration is avoided.

The control unit requires these signals to control the clutch actuators.

If clutch travel sender 1 fails, gearbox path 1 is shut off. Gears 1, 3, 5 and 7 can no longer be engaged. If clutch travel sender 2 fails, gears 2, 4, 6 and reverse can no longer be engaged.

A clutch travel sender consists of:

  • An iron core, around which the primary coil is wound;
  • Two secondary evaluation coils;
  • A permanent magnet, which is located on the clutch actuator piston; and
  • The sensor electronics

An alternating voltage is applied to the primary coil. As a result of this, a magnetic field is built up around the iron core. If the clutch is actuated, the clutch actuator piston moves through the magnetic field with the permanent magnet. Due to the permenant magnet’s movement, a voltage is induced in the secondary evaluation coils. The level of the voltage induced in the left and right evaluation coils is dependent on the position of the permanent magnet. From the level of the voltage in the left and right evaluation coils, the sensor electronics detect the position of the permanent magnet and therefore the position of the clutch actuator piston.

Tiptronic

In Tiptronic mode, the gears can also be shifted up and down using the steering wheel switches. If the Tiptronic switches on the steering wheel are actuated in automatic mode, the gearbox control system switches to Tiptronic mode. If the Tiptronic switches on the steering wheel are not used for a certain time, the gearbox control system automatically switches back to automatic mode.

The Tiptronic shifting strategy

  • Automatic upshifting on reaching the maximum engine speed;
  • Automatic downshifting on falling below the minimum engine speed;
  • Kick-down downshifting.

On the road

We have found that the DSG gearbox seems to vary somewhat from car-to-car. In some cases it can waste revs or be sluggish to change down, while other installations seem almost to read the driver’s mind. Pulling away from rest can be a weakness on some cars, with the sense that the clutch is not engaged quite gently and progressively enough. Could it be that tiny engineering variations between cars are responsible for this disparity in behaviour? We don’t know. But significantly, gearchanging is always smooth, there is never any wild hunting through the gears, and the driver does, after all, have the facility to use the manual controls if a twisting road demands a little more foresight than the machinery can manage. As this technology is developed further, we can’t help wondering if manual gearboxes will become the preserve of only the very cheapest models.

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