DSG dual-clutch transmission

The DSG gearbox, which takes its name from the German DirektSchaltGetriebe, is commonly known in English as the direct-shift gearbox. The DSG name applies exclusively to Volkswagen products, while the same type of technology goes by other names depending on the maker. In general terms, it’s called DCT for dual-clutch transmission.

With many manufacturers currently offering this type of transmission, it’s no longer reserved for the upper crust of cars and sports cars. Vehicles with smaller engines, like the VW Golf and Ford Focus, will soon be equipped with a dual-clutch transmission. Audi calls it S-Tronic, while at Porsche it’s known as Porsche Doppelkupplung (PDK for short). The term used at Mitsubishi is TC-SST for Twin Clutch Sportronic Shift Transmission. As for the Americans, Ford will soon launch a version of the DCT under the name Power Shift. Nissan has been selling its own dual-clutch gearbox for a little while now on the GT-R. Dual-clutch transmissions are already very popular in Europe, as they help reduce fuel consumption and vehicle weight. North Americans, however, get DSGs mostly as an option on high-end German sports cars as a measure to significantly improve performances.

Not so new after all

The concept behind the dual-clutch gearbox is not exactly new. It was invented in 1935 by French engineer Adolphe Kégresse. Christened Autoserve, it was first installed in a Citroën Traction Avant in 1939. Subsequently abandoned for many years, the idea momentarily resurfaced in the 1980s on the Porsche 956 and 962 C competition vehicles, as well as the Audi Sport Quattro. However, it would not be until 2003, after six years of work by Volkswagen Group and its German supplier BorgWarner, that this transmission would return to production vehicles – notably the European versions of the Golf R32 and the Audi TT. The DSG was added to improve shifting speed and give drivers that race-car feeling. Since 2008, a seven-speed version (DSG7) has been offered on vehicles with smaller engines to improve their fuel economy. Since Kégresse first invented it, the biggest innovation to the dual-clutch gearbox is the addition of an electronic control to anticipate gear selection.

How it works

The DCT differs from an automatic transmission with manual mode mainly in that it has two clutch packs. In fact, the twin-clutch gearbox is essentially made up of two gearboxes joined by a pair of clutches, one for even-numbered gears, and the other for odd-numbered gears. It’s a little like dividing a conventional transmission into two half-gearboxes – one for gears 1, 3 and 5, and the other for gears 2, 4 and 6. The best way to understand the concept is through a concrete example.

When you start driving, the half-gearbox #1 is engaged in first, while half-gearbox #2 is already in second. You accelerate in first gear, and when it comes time comes to shift into second, clutch two intervenes to switch gearboxes. The first clutch is disengaged and the second clutch shifts into second on half-gearbox #2. At the same time, half-gearbox #1 pre-selects third gear. Next, when it is time to shift into third gear, the clutch re-engages half-gearbox #1 and slides into third, which has already been pre-selected. At the same time, half-gearbox #2 pre-selects fourth gear, and so on. The electronic control unit calculates the next logical gear, and not even the transmission knows what’s coming.
When gears change progressively, in logical ascending order, shifting is very fast. Non-sequential shifting, however, can mean the loss of precious seconds. Shifting from second to fourth, for example, takes longer since these two gears are on the same half-gearbox. The longest reaction time is going from second to sixth (or from sixth to second) as these gears are on the same gearbox and are furthest from one another.

In addition to being fast, sequential gear changes help prevent loss of torque and eliminate lapses in acceleration. When the car is in second, third is already pre-selected by half-gearbox #1, but not yet engaged by the other clutch. Once the ideal shifting point is reached, the clutch associated with the second gear opens (leaves its position) while the other closes (engages) simultaneously. This overlapping process, where one clutch opens and another closes, makes for easy gear changes in just three or four hundredths of a second. Thus, available power is continual and smooth.

Porsche PDK

http://www.youtube.com/watch?v=m2bZ4R

Volkswagen DSG

http://www.youtube.com/watch?v=PH00yBaAV7s

Driving with a DSG

Volkswagen’s DSG, like most twin-clutch transmissions, uses the traditional P-R-N-D-S scale, but the difference is that it operates automatically in modes D and S – with D for comfort mode and S for a sportier, more incisive ride. In comfort mode (D), gears are engaged earlier in order to prevent revving too high. This generates less noise and helps limit fuel consumption. In sport mode (S), the gearbox makes use of the first gears longer to keep the engine in its maximum efficiency range or at its highest rpm. So, we get torque and maximum power most of the time, which is particularly nice in cars with turbocharged engines – like those that Volkswagen and Audi specialize in. Although the automatic mode works great, you can switch to manual mode by moving the stick left or right before upshifting or downshifting. On some models, you can also switch to manual mode using steering-wheel mounted paddles, just like a Formula 1 car. Use the one on the left to upshift and the one on the right to downshift.

Of the several advantages of this type of transmission, the most obvious is shifting speed, which takes just a fraction of a second. This benefit is best appreciated in auto racing, which involves thousands of gear changes. For regular day-to-day use and for production cars, this feature doesn’t provide the same advantages. For your everyday driving, this type of gearbox improves output, offers smoother shifting and reduces fuel consumption. Plus, your accelerations are more linear as there is no interruption in power flow. And for driving enthusiasts, rest assured that the DSG gearbox delivers unparalleled driving pleasure, giving you a taste of what race car drivers experience.

In short, the revolutionary direct shift gearbox sets itself apart through gear changes with no loss of torque transmission. In other words, you can change gears without a noticeable interruption in the flow of power. Then there’s the fact that shifting is super fast, to the tune of about 8 milliseconds. For comparison’s sake, consider that the Ferrari Enzo’s transmission takes nearly 150 milliseconds to change gears. The DSG is also 10 times faster than the BMW SMG transmission, which is the fastest automatic transmission with manual mode currently on the market. According to Audi’s numbers, shifting operations are even faster than if they were done manually. For example, the A3 with a 6-speed manual transmission goes from 0-100 km/h in 6.9 seconds, while it takes only 6.7 seconds with the DSG transmission.

Thus, the DSG transmission is as comfortable as an automatic gearbox and as exciting to drive as a standard. Available on an increasing number of affordable models, it is a hands-down success. You can expect it to be the best selling transmission very soon. In fact, only the CVT transmission is smoother and thriftier than the DSG, but it obviously doesn’t produce the same excitement.

Benefits

Ultra-fast shifting
Better fuel consumption
No loss of torque transmission
Gear changes without grabbing or jerking
Prevents bad gear changes therefore easier on the engine
Lighter than an automatic transmission

Drawbacks

Takes longer for gear changes on the same half-gearbox
Mechanical complexity
More costly to manufacture
Heavier than a conventional standard transmission