The twin-clutch transmission, also known as the Direct Shift Gearbox (DSG) or dual-clutch transmission, is an automated transmission that can change gears faster than any other geared transmission. Twin-clutch transmissions deliver more power and better control than a traditional automatic transmission and faster performance than a manual transmission. Originally marketed by Volkswagen as the DSG and Audi as the S-Tronic, twin-clutch transmissions are now being offered by several automakers, including Nissan, Mitsubishi, BMW and Porsche.
The twin-clutch transmission is essentially two separate transmissions with a pair of clutches between them. One transmission provides odd-numbered speeds (ie first, third and fifth gear), the other provides even-numbered speeds (second, fourth and sixth). When the car starts out, the "odd" gearbox is in first gear and the "even" gearbox is in second gear. The clutch engages the odd gearbox and the car starts out in first gear. When it's time to change gears, the transmission simply uses the clutches to switch from the odd gearbox to the even gearbox, for a near-instant change to second gear. The odd gearbox immediately pre-selects third gear. At the next change the transmission swaps gearboxes again, engaging third gear, and the even gearbox pre-selects fourth gear. The twin-clutch transmission's computerized controller calculates the next likely gearchange based on speed and driver behavoior and has the "idle" gearbox pre-select that gear.