The German auto giant has developed a 7-speed auto transmission based on its DSG technology platform

Volkswagen launched the Passat in India with a 6 speed DSG (auto) transmission. It received quite an amount of attention from the media for its brilliance and technology. Volkswagen has now come up with a 7-speed DSG (auto) transmission for high-volume production - starting in the Golf.

For many car drivers this could signify the final turn away from conventional transmissions because the new DSG can do everything better than a manual gearbox. It is more fuel efficient, sporty and comfortable. Making the revolutionary transmission technology available for smaller engines that develop up to 250 Nm, the 7-speed DSG uses the same technology as the 6-speed DSG except that there is no "wet" clutch in the new transmission. The two computer controlled clutches in the 7-speed DSG are dry. The brains of Volkswagen's dual clutch tranny lie in its mechatronics. As a control centre it regulates shifting processes that are as fast as they are complex. In its basic configuration, the mechatronics consist of a control module and the so-called control valve assembly with individual sensors (sensor: converts physical values into electrical values) and actuators (actuator: this converts electrical signals into actuating motions).

Specifically, the mechatronics module acquires and manages the data for controlling clutches, the individual gears, pressures and various safety stages. In addition, modulator valves, switching valves and a large number of hydraulic valve spools are used. The transmission data is exchanged between the control module and the vehicle's electrical system via a plug-in communications interface; in the reverse direction, information flows from the vehicle and engine via this interface to the transmission's computer.

While the mechatronics of the 6-speed DSG utilise the transmission's oil circulation, the mechatronics of the 7-speed DSG are laid out as an autonomous unit with oil circulation separate from the transmission. This yields a number of advantages. The hydraulic fluid can be specially tuned to the needs of the mechatronics while normal oil is used for the transmission that is similar to the oil used in conventional manual transmissions. Properties of the mechatronics are very good at low temperatures, since no compromises need to be made in terms of oil viscosity. The high purity of the hydraulic oil enables the use of so-called cartridge valves with very small gap dimensions. This significantly reduces leakage quantities, and use of an electrically-driven pump is economical.

Compared to open hydraulics, the pressure level can be elevated, the sizes of actuators can be reduced due to the higher performance density, and the gross weight of the transmission is reduced. The mechatronics can be fully assembled and fully tested outside of the transmission. It is possible to actuate the dual clutch and shift gears independent of the combustion engine. This fulfills a precondition for a hybrid drive with start/stop function.

Besides an intelligent electro-hydraulic transmission control (mechatronics), there are two clutches as well as two drive shafts and three final drive shafts. This network makes it possible to continually "lie in wait" to become active at the next higher driving level and lightning-fast.

For example, while the car is driven in sixth gear, the seventh gear is already engaged, but is not yet "active". As soon as the ideal shifting point has been reached, the clutch responsible for the sixth gear automatically opens, while the other one closes, and this "pre-shifts" to the seventh gear. This results in an overlap between opening and closing of the two clutches, which leads to the comfortable shifting described above. The entire shifting process is completed within just a few hundredths of a second.

The dual clutch consists of two dry friction clutches - similar to those of manual transmissions - and a central plate. It transfers the torque to the drive shaft via the relevant clutch. In general, the dry dual clutch assumes somewhat larger dimensions than the wet running version. This is due to the layout of the two clutches and the material usage required absorbing the frictional energy. Since the dual clutch is larger, the rest of the transmission was designed to be that much more compact, so that it could be used even on small cars the size of the Polo.

Interestingly, the two drive shafts are arranged centrally on a single axis. Located on a common axis, their design assumes a key function. The outer driveshaft II was designed as a hollow shaft, in which driveshaft I is integrated as a solid shaft. The two shafts are mounted concentrically, one below the other, on needle bearings. Assigned to drive shaft I are the odd gears (1, 3, 5 and 7), while the even gears (2, 4, and 6) and reverse gear reside on drive shaft II. The distribution of the "1" and "R" gears permits quick changes between forward and reverse in parking maneuvers, simply by controlling two clutches. The clutches are controlled by hydraulics, and solenoid valves regulate the contact pressures of the two clutches. The solenoid valves are directly connected to a mechatronics module that coordinates everything.