After at least two decades of gestation, the promising dual-clutch automatic layshaft gearbox finally has surfaced in production.

Taken through the development and refinement stage by partner BorgWarner Inc., the innovative Direct Shift Gearbox (DSG) has been adopted by Volkswagen AG and Audi AG for their new high-performance cars. It is a 6-speed manual transmission, but it can be shifted manually or used like an automatic, where gearshifting is automated.

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By staggering gears on two layshafts and using two clutches, the VW/Borg-
Warner-developed 6-speed automated manual transmission nearly eliminates interruption of power flow associated with gearchanges.

Automated manual gearboxes are not new. But DSG is unique because it carries the even-numbered gears on one shaft and the odd numbered gears (plus reverse) on a second one. These shafts are partly coaxial, with the even ratios on the hollow outer one and the odds and reverse on the extended solid inner shaft.

The two sets of cogs mesh with corresponding gear clusters on layshafts on either side of the central gear groups. Both the coaxial shafts are driven by the engine, though only one transmits output torque via individual hydraulic clutches.

Under electronic management, the clutches are operated alternately. Starting the car from rest, first gear on its layshaft is engaged by a synchromesh cone, and the inner shaft is driven by clutch No. 1. Meanwhile, second gear on the "even" layshaft is "pre-engaged," ready for the upshift, though not yet driven.

For the gearchange, clutch No.1 is released and clutch No. 2 is simultaneously engaged. This odd-even sequence is followed up and down through the ratio range. The DSG design enables split-second "hot" shifts: There is no power interruption to the road wheels, as is typical with a conventional manual gearbox when the driver de-clutches to select a new gear.

Fuel economy and compactness are DSG's big advantages. Engine/transmission coupling is handled by the powered clutches, eliminating the hydraulic losses from the torque converter, a factor in conventional automatic transmissions. Mechanical efficiency of this "automatic" matches that of a manual gearbox, yet in regions of heavy automatic-transmission penetration (such as the U.S.), DSG can be sold as an automatic, presenting the best of both worlds.

DSG's six forward gears allow a wide ratio spread and the possibility of two overdrive ratios for low-rev highway cruising. It also permits maintaining a fairly constant engine speed near the engine's area of peak thermal efficiency, which also reduces emissions.

Because it is a manual transmission, the DSG also enables European manufacturers to use their longstanding investment in manual-transmission manufacturing to produce an "automatic" that is manufactured and assembled on manual-transmission production lines.

Although six speeds in a typical manual transmission dictate an elongated casing, with DSG they can fit in the same length as four gears, because the meshing gears are "stacked" in a vertical arrangement on three shaft assemblies instead of the usual two. The design makes the DSG compact enough for transverse engine/front-drive installation, such as in the VW and Audi cars - a breakthrough for 6-speed automatics.

Origin of this gearbox concept dates at least to 1981. Automotive Products Ltd. in the U.K. once produced a 4-speed prototype fitted to a Ford Fiesta, with a tiny 1.1L transverse engine considered too puny for any conventional automatic. This transmission had two clutches at opposite ends of the coaxial input shafts and full electronic control of the hydraulic shift actuators.

A brief drive of that prototype recalls impressively smooth gearchanges and AP's engineering director says the company recently demonstrated the automatic to many major auto and component manufacturers, with interest and response being "tremendous." It is not known if BorgWarner was among those companies.

In 1983, Porsche AG demonstrated its own version of the twin-shaft automated gearbox. This had five forward speeds with the two clutches placed at the front ends of the coaxial input shafts. It was adapted for the modified rear transaxle of a 944 model and later was applied to some of Porsche's competition cars.

The VW/Audi transmission updates these early developments with state-of-the-art technology. Longitudinal compactness is enhanced by the unique engine-driven dual clutch, where the central input shaft for the even-numbered gears is handled by a clutch coaxial with and encircled by a large-diameter one for the odd-numbered gears. These hydraulically operated multi-plate units run in oil fed from an integral pump serving the entire system.

Shifts and all other functions are controlled by a "Mechatronic" module located at the top of the gearbox, and itself immersed in oil at up to 284°F (140°C), held at the optimum thermal level by an external heat-exchanger. This combines an electronic management unit with a hydraulic valve chest with 12 input sensors. The unit regulates the shift selector-fork actuators and clutch solenoid valves as part of the overall program management.

Control logic optimizes shift strategies for smooth, near-instantaneous gearchanges. In the automatic mode, the driver can move from the normal D (drive) position to the S (sporty) pattern, where upshifts are retarded and downshifts advanced. For manual sequential shifting, there is a special gate on the selector lever and paddles on the steering wheel.

The DSG has a torque rating of up to 250 lb.-ft. (340 Nm), which is ample capacity for the 3.2L V-6 engine in the TT 3.2 Quattro, Audi's range-topping sports model. At 250 hp and backed by the DSG, the TT 3.2 Quattro accelerates to 62 mph (100 km/h) in 6.4 seconds and has a governed top speed of 150 mph (250 km/h).

Following the debut of the Golf R32 DSG, Volkswagen has plans for the 6-speed DSG for the Touareg V-10 this fall. The gearbox is being produced at the VW plant in Kassel, Germany, where installed capacity is 1,000 units a day.