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The revolutionary VW-Audi double clutch transmission

by François Dovat

Figure 1: The new "Direct Shift Gearbox" (DSG) of VW and Audi bears some 350 Nm and is manufactured in the Kassel transmissions plant which is geared to produce up to 1000 units a day. It is available since mid-2003 on the Audi TT 3.2 and on the Golf R32, both equipped with the 15° V6 of respectively 250 / 241 hp and 320 Nm. Several other vehicles of the VAG group will also be available with this transmission, including the new Golf V with the latest 16 valves 140 hp TDI.

Figure 2: The new VW Golf 5

In order to avoid any tractive power interruption during gearshifts, two clutches can be arranged upstream of a gearbox – preferably automated or sequential. These clutches are connected by concentric shafts to conventional synchro shift slider-sleeves units, the hollow outer shaft driving per example the slider-sleeves of the even ratios, (2nd, 4th, 6th ) while the inner shaft drives the ones of the odd ratios ( 1st, 3rd, 5th ). Thus the control system can pre-select the next gear while the engine is in traction. The shift is carried out under load simply by disengaging a clutch while simultaneously engaging the other.

The first model of a double clutch gearbox seems due to French engineer Adolphe Kégresse – who was at 25 years old the technical director of the garages of the Tsar Nicolas II and who invented and developed the endless conveyor belt which was the forerunner of the caterpillar tracks. The said gearbox was conceived for the legendary Citroen "Traction" but it was not produced because of adverse circumstances. Years later, Automotive Products also conceived a double clutch transmission.

Figure 4: The Kégresse twin clutch hydro-mechanical transmission intended for the "Traction" Citroen. (Picture Science & Vie).

Figure 5: Basic scheme of a 5-speed twin clutch cogbox.

Figure 6: Porsche - Audi PDK (1985).

Figure 7: Automotive Products proposed a different design.

A layout similar to the one of Kégresse was successfully raced by Porsche and Audi (PDK 5-speed). If a hydrokinetic torque converter or a hydraulic clutch had taken place between the crankshaft and such a gearbox, its behavior would have been practically identical to that of a conventional planetary automatic transmission. But, although subjected to intensive testing at ZF, the giant tier1 of Friedrichshafen had found no practical advantage in the twin clutch design over common epicyclical transmissions and did not launch the concept, arguing also of the impossibility to skip a gear, i.e. to shift directly from an odd gear to an even one or conversely.

Figure 8: Audi-VW DSG

Together with a dual-mass flywheel, the twin multi-disk wet clutch is known to be perfectly smooth and it can mimic the creep provided by a torque converter. In D or S (S standing for "sport") position, the operation is quite similar to that of a conventional automatic transmission. But the driver is also offered a sequential mode controlled by two paddles behind the steering wheel, or alternatively by the central console shifter just as on the Tiptronic and Multitronic. Racing launches are possible, thanks to the well cooled outer multi-disk clutch and to the powertrain's electronic regulation: simultaneously hammer down on both the accelerator and brake pedals and then release the brakes. On downshifts, the engine is automatically accelerated to the matching revs by the ECU, supposedly to avoid unnecessary clutch slippage.

In view to get a compact and short unit, it is designed on a 3 shafts layout, just like the basic manual 6-speed O2 M 350 from which it was developed from. On the Golf R32 both manual and optional DSG transmissions encompass the same ratios as well as several common parts. Due to its more elaborate construction and automatic control system, the DSG brings an extra weight of 30 kg for a total of 90 kg. The twin clutches are concentric and electro-hydraulically activated, controlled by a "Mechatronic module" placed on the transmission's casing. The signals coming from 10 individual sensors are centralized there and the data for actuation are calculated according to the actual driving conditions recorded by the CAN bus. The pressure applied to the clutches and the position of the synchro-sliders-sleeves units are managed according to the situation by shift-valves and slider-valves controlled by solenoids. Electronics also work out which gear is to be pre-selected and manages the oil circuit via 6 automatically controlled governors and 5 on/off valves. It remains to be checked if the pre-selected gear always corresponds to the driver's desires!

It is possible to downshift from 6th into 2nd in 0.9 second, the transmission shifting first to 5th then to 2nd. Of course the shift from an even ratio to an odd or conversely is done without delay after pre-selection.

The oiling circuit comprises an oil cooler. The average fuel consumption, given to 9.8 l/100km, is slightly reduced compared to a manual cogbox while it is strongly diminished compared to a conventional automatic torque converter transmission. The maximum speed of the Audi TT V6 is limited to 250 km/h. The Golf R32 can reach 247 km/h and the optionally DSG equipped sprints from 0 to 100 km/h in 6.4 seconds, faster than the manual version which needs 6.6 seconds.

Figure 9: Basic principle (picture LuK)

(© François Dovat)

Technical files

Idée & conception © 1999-2011 van Damme Stéphane.

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