The VAG Direct Shift Gearbox - sometimes called the S-Tronic in the Audi range – is the world's first production dual clutch semi automatic gearbox. The system was developed for Audi by BorgWarner for use in the companies Audi TT 3.2V6, and was so well received it is now used in much of the Volkswagen Audi Group range.
The DSG is often compared to F1 style automated or robotic manual gearboxes, but differs in some very important ways.
The DSG gearbox is, however, not a new invention. The system was first designed by Andolphe Kégresse just before the second world war but because of the lack of technology, not to mention the war, he never produced a working version of the Dual Clutch Gearbox (DCG). The system was used by another German car company in the 80's though: Porsche used the PDK (Porsche Doppelkupplungs) system in their 956 and 962 Le Mans race cars, and Audi used the same system in the Sport Quatro S1. Porsche and Audi have a long history of technology sharing.
Both of these uses eventually faded, primarily because the computing technology that allows the system to work so effectively did not prove to be reliable enough in the tough and demanding world of the race car.
To understand how revolutionary the DSG gearbox is, first I will explain the F1 style systems. These differ in name depending on the manufacturer; Ferrari call theirs the F1 system, and BMW call theirs the SMG system.
These gearboxes all have one thing in common: they are just manual gearboxes, with a manual clutch that is operated by pneumatics. This method has many of the benefits the DSG box does. The gearbox itself weighs less: as the gear change is accurate, and no human mistakes can be made, the materials can be less hardy. They are also far more performant than the manual equivalent, both the BMW SMG II and the Ferrari F1 systems change cogs in around 80 milliseconds. This means that, while the gear is being shifted, all the power from the engine is waisted and a feeling of on-off-on is felt in the car as the pneumatics shift from in gear, to clutched, and then back into gear. The smoothness of this in the car depends on the software controlling it: for the fastest changes it can feel quite 'bumpy', similar to a bad gear change in a manual car. The only way of releaving this issue in the past was to fit a torque converter, this is the way that a standard auto box does it, and why it feels far smoother than any of the manual gearboxes produced. A torque converter, however, is very wasteful in terms of power, and usually very heavy.
The DSG box takes the basic idea of pneumatic clutched boxes a step further. The heart of the DSG comes from the fact it has two clutches. The basic idea being that the gearbox can then have two gears engaged at any one time, one driving the car, the other waiting to take over.
The clutch arrangement is setup for odds and evens, where clutch 1 operates the 1-3-5 gears and clutch 2 operates 2-4-6.
DSG works by allowing the software to decide what gear the car will need next, and then pre-selects it. If you are accelerating, it will have the next gear up, and likewise if decelerating, the next gear down.
The system can then watch for a change signal – this is either done by clicking the corresponding paddle by the driver, or if in full auto mode the computer – and the gearbox can then tell the currently disengaged clutch (the one driving) to engage, and at the same time tells the engaged clutch (the pre-selected gear) to disengage. In this way the driver and passengers do not experience the on-off-on feeling associated with manual gearboxes. Instead, the gear change feels much more like that of an automatic gearbox.
However, because the change happens so quickly – around 8 milliseconds – the engine can maintain drive and the losses involved in the gear change are much reduced. This can be seen visually in the video below.
The video clearly shows the benefits of the DSG vs a very quick-changing manual driver.
The DSG gearbox is obviously controlled by computer, and this adds other benefits as well, even over race-going Sequential Manual Transmisions.
The computer in the DSG box monitors many of the cars sensors, these can be RPM, speed, angle of steering input, amount of wheel spin, braking and g-forces. It can then use these inputs to make decisions on which gear the car has to be in at any one moment.
For example, if you are approaching a tight corner you may want to change down two or even three gears. The computer sees that you are braking heavily, and that steering input is being applied, and can then change down either more quickly, or skip gears altogether and shift from sixth into third in one step. This allows the driver to keep the RPM in the best range for drivability. Likewise, if driving in snow or mud, the computer sees that there is a lot of wheel spin at low speeds, it can then shift up into a higher gear to allow the wheel spin to be controlled.
The gearbox also keeps the F1 style gearbox's benefits. Because the computer will change gear very precisely, and not over stress the components, the weight of the whole unit is not much more than a conventional gearbox and clutch. This is because the parts can be made to much tighter tolerances, but still manage the same life expectancy as the manual counterparts.
There is a downside to the gearbox though: it cannot be used in races. This is because it changes gear so quickly, and the loss of drive is so minute, that the gearbox gets classed as a Constantly Variable Transmision. The FIA and other governing bodies outlawed this in the early 80s. However, with the speed that some of the current Formula One cars can now change, this may be altered. The current Honda gearbox is called the 'Lossless' gearbox after all, but they did prove that it reduces engine power and its use was therefore allowed.
The crowning achievement of the DSG is the English-made, Ricardo Company's seven speed DSG, for use in the Bugatti Veyron. That particular car is worthy of its own – more detailed – article, so that will have to wait.
So, next time you drive a DSG car just think: You can change gear faster than even the Formula One and Indy drivers.
Special thanks to Audi for the images and the link to the video demonstrating the gearbox in action.