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.
Before DSG: The SMT
The twin clutch/DSG is a development of the sequential manual transmission (SMT), which is essentially a fully-automated manual transmission with a computer-controlled clutch, intended to deliver stick-shift performance with automatic convenience. The advantage of an SMT is that it uses a solid coupling (the clutch), which provides a direct connection between engine and transmission and allows 100% of the engine's power to be transmitted to the wheels. (Traditional automatics use a fluid coupling called a torque converter, which allows some slippage.) The chief drawback of the SMT is the same as that of a manual: In order to change gears, the engine and transmission must be disconnected, interrupting the flow of power.
Twin-clutch: Solving the SMT's problems
The twin-clutch transmission was designed to eliminate the lag inherent in SMTs and manuals. 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.
One advantage to both SMTs and twin-clutch/DSG transmissions is the ability to perform matched-rev downshifts. When a driver selects a lower gear, both types of transmission disengage the clutch(es) and rev the engine to the exact speed required by the selected gear. Not only does this make for a smoother downshift, but in the case of the twin-clutch transmission, it allows plenty of time for the proper gear to be pre-selected. Most (though not all) twin-clutch transmissions can skip gears when downshifting, i.e. shifting from 6th gear directly down to 3rd gear -- and because of their ability to match revs, they can do so without the lurching or surging typical of traditional automatic and manual transmissions.
Driving a car with a twin-clutch/DSG transmission
Twin-clutch equipped cars do not have a clutch pedal; the clutch is engaged and disengaged automatically. Most twin-clutch transmissions use an automatic-style shift selector with a traditional P-R-N-D or P-R-N-D-S (Sport) shift pattern. In Drive or Sport mode, the twin-clutch transmission operates like a regular automatic. In Drive, the transmission shifts to higher gears early in order to minimize engine noise and maximize fuel economy, while Sport mode holds the lower gears longer in order to keep the engine in its powerband. Sport mode also provides more aggressive downshifts with less accelerator pedal pressure, and in some cars, including most DSG-equipped VWs and Audis, engaging Sport mode also causes the car to react more aggressively to the accelerator pedal.
Most twin-clutch transmissions have a manual mode which allows manual shifting via the shift lever or paddles mounted on the steering wheel. When driven in manual mode, the clutch is still operated automatically, but the driver controls which gears are selected and when. The transmission will follow the driver's commands unless the selected gear would over-rev the engine (i.e. commanding first gear while driving 80 MPH).
Advantages of the twin-clutch/DSG transmission
The primary advantage of the twin-clutch/DSG is that it provides the same driving characteristics of a manual transmission (i.e. quicker throttle response and no drop in engine speed when the driver lifts off the accelerator) with the convenience of an automatic. However, the ability to perform near-instantaneous gearshifts gives the twin-clutch advantages over both manuals and SMTs. Volkswagen's DSG takes about 8 milliseconds to upshift. Compare that to the SMT in the Ferrari Enzo, which takes 150 ms to upshift. Instant gear shifts mean faster acceleration: According to Audi, the A3 runs 0-60 in 6.9 seconds with a 6-speed manual and 6.7 seconds with the 6-speed DSG.
Disadvantages of the twin-clutch transmission
The main limitation of the twin-clutch/DSG is the same as all geared transmissions: Because there are a fixed number of gears, and the transmission cannot always keep the engine at its best speed for maximum power or maximum fuel economy. Because of this, twin-clutch transmissions generally cannot extract as much power or fuel economy from an engine as a continuously-variable automatic transmissions (CVTs). But because twin-clutch transmissions provide a more familiar driving experience than CVTs, most drivers prefer them. And while the twin-clutch provides superior performance compared to a manual, some drivers prefer the interaction that a manual clutch pedal and gearshift provide.