DSG dual-clutch transmission

DSG dual-clutch transmission

March 13, 2009

By Guy Desjardins

The DSG gearbox, which takes its name from the German DirektSchaltGetriebe, is known commonly in English as the Direct Shift Gearbox.. This type of transmission is called DSG exclusively on Volkswagen products and by other names depending on the car maker. It is generally called DCT for Dual Clutch Transmission in English.

Many manufacturers are presently offering this transmission and it is no longer only for top-end cars or sports cars since it will soon be available on vehicles with smaller engines like the VW Golf and the Ford Focus. Audi calls it S-Tronic and for Porsche, it’s known as Porsche Doppelkupplung, or the PDK Gearbox 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 dual-clutch gearbox under the name of “Power shift”.. Nissan has been selling its own dual-clutch gearbox for a little while now, specifically on the GT-R. Dual-clutch transmissions are already very popular in Europe, since they help reduce fuel consumption and vehicle weight, but they are offered to North Americans mainly as an option to help significantly improve the performance of high-end German sports cars.

Not so new after all
The concept of the dual-clutch gearbox is not exactly new. It was invented in 1935 by French engineer Adolphe Kégresse and christened “Autoserve”, and was first installed in a Citroën Traction Avant in 1939. 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 the Volkswagen Group and its German supplier BorgWarner, that it would return in production vehicles, notably the European versions of the Golf R32 and the Audi TT. Currently, it is its gear change speed and that it simulates the experience of driving a race car that make up the primary arguments for buying it. Since 2008, a seven-speed version (DSG7) has been sold for vehicles with smaller engines mainly for better fuel economy. Since Kégresse first invented it, the biggest innovation to the dual-clutch gearbox is the use of an electronic control to anticipate the choice of the best gear.

How it works
It differs from an automatic transmission with manual mode mainly because of the presence of 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 one 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 the driver starts, the first half gearbox is engaged in first gear, while the second half gearbox is already placed in second gear. The driver accelerates in first gear. When the time comes to shift into second, clutch two intervenes to change gearboxes. The first clutch is disengaged and the second clutch shifts into second on half gearbox #2. At the same moment, half gearbox #1 pre-selects third gear. Next, when the time comes to shift into third gear, the clutch re-engages half gearbox #1 and slides into third gear, which has already been pre-selected. At this time, half gearbox #2 pre-selects fourth gear…and so on. Calculating the next logical gear is the job of the electronic control unit, and not even the transmission knows what’s coming.

When gears are changed progressively in logical ascending order, gear changes are very fast, but non-sequential shifting can mean the loss of precious seconds. Shifting from second to fourth, for example, will take longer since these two gears are on the same half gearbox. The longest reaction time will be when going from second to sixth or from sixth to second since these gears are on the same gearbox and are furthest from one another.

Sequential gear changes, in addition to being fast, help avoid power loss and eliminate lapses of acceleration. When the car is in second, third is already pre-selected by the odd half gearbox, but not yet engaged by the second 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 comfortable gear changes in no more than three or four hundredths of a second. Thus, available power is continual and smooth.

Driving with a DSG
Volkswagen’s DSG, like most twin-clutch transmissions, functions using the traditional P-R-N-D-S scale, but the difference is that this transmission operates automatically in both 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 avoid revving too high, which generates less noise and helps limit fuel consumption. In sport mode (S) however, the gearbox makes longer use of the first gears in order to keep the engine in its maximum efficiency range, or at its highest rpm.

So we get torque and maximum power the majority of the time, which is particularly nice in the case of cars with turbocharged engines, like those that German manufacturers Volkswagen and Audi have made their speciality. Although the automatic mode works wonderfully, it is possible to switch to manual mode by moving the stick to the left or to the right to control gear changes but moving the stick up or down. In certain models, it is also possible to switch to manual mode using the paddles mounted behind the steering wheel like in 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 without a doubt, the speed of changing gears which takes place in a fraction of a second. This benefit is clearly appreciated in auto racing where there are thousands of gears changes. Otherwise, for regular day-to-day use and for production cars, this feature doesn’t provide the same advantages.. In fact, for daily use, this type of gearbox mainly helps obtain better output and smoother shifting and reduces fuel consumption, while making accelerating more linear since interruption in power flow. And for driving enthusiasts, there is not doubt that the DSG gearbox brings unparalleled driving pleasure, nearing that experienced by race car drivers.

Basically, the revolutionary direct shift gearbox sets itself apart by gear changes with no lapse in propulsion. In other words, it helps change gears without a noticeable interruption in flow of power and changes gears extremely quickly, to the tune of about 8 milliseconds. For comparison’s sake, consider that the transmission used in the Ferrari Enzo needs nearly 150 milliseconds to change gears. The DSG is also ten times faster than the BMW SMG transmission, which is the fastest automatic transmission with manual mode currently on the market. If we are to trust Audi’s numbers, the gear changes are made faster than they could be 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 transmission and as exciting to drive as a standard transmission. Available on more and more affordable models, it is a tremendous success and will probably 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 provide the same sensations.

Benefits
Ultra fast shifting
Better fuel consumption
No loss of power flow
Gear changes without grabbing or jerking motion
Avoids 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