Twin Clutch / Direct Shift Gearbox (DSG)
What it is, how it works
By
Aaron Gold, About.com
Filed In:- New Technology
DSG diagram showing seperate transmissions and twin clutches
© Volkswagen of America
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The twin-clutch transmission, also known as the Direct Shift Gearbox (DSG), R-Tronic or S-Tronic, is an automated transmissin that can change gears faster than any other geared transmission. Twin-clutch transmissions can be shifted either manually or automatically. They deliver more power and better control than a traditional automatic transmission and faster performance than a manual transmission. Originally markted by Volkswagen and Audi as the DSG and R/S-Tronic transmission, twin-cluch transmissions are now being offered by other automakers.
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. Before we delve into the workings of the DSG, let's start with an explanation of the SMT.
SMT: The control of a manual with the ease of an automatic
Though it has appeared on a few passenger cars, the SMT is widely used in certain forms of racing and is featured on some exotic high-performance cars.
Cars with SMTs have a manual transmission with no clutch pedal; the clutch is automatically engaged. Most SMTs can be shifted automatically or manually, the latter using a shift lever or paddles on the steering wheel. The advantage to an SMT is that it uses a solid coupling, as opposed to a fluid coupling (torque converter) as used in a traditional automatic (including Tiptronic- or manumatic-style transmissions). As with a manual transmission, the SMT provides a direct connection between engine and transmission, allowing 100 percent of the engine's power to be transmitted to the wheels. The SMT provides more immediate response and ensures that the engine RPMs do not drop when the driver lifts off the accelerator (as happens with an automatic), giving her more precise control over power output.
SMTs can also perform matched-rev (or double-clutch) downshifts: When the driver downshifts, the SMT disengages the clutch, shifts to neutral, and re-engages the clutch. The SMT calculates what the engine RPMs will be in the next-lowest gear based on the current road speed, and revs the engine (and, since the clutch is engaged, the transmission) to that speed. It then disengages the clutch, shifts into the lower gear, and re-engages the clutch. The result is a smooth change with no jerk and no sudden deceleration.
SMTs have one major disadvantage, same as a manual: Power must be interrupted while the transmission changes gears.
Twin clutch: An SMT without the lag
The twin-clutch transmission all but eliminates the lag inherent in SMTs. The twin-clutch is essentially two gearboxes with a pair of clutches between them, one for odd gears, one for even gears. When the driver starts out, transmission #1 is in first gear and transmission #2 is in second. The clutch engages and the car starts out in first. When it's time to change gears, the transmission uses the clutches to swap transmissions. The #1 transmission immediately shifts to third gear. At the next change the transmission swaps transmissions again, and #2 shifts to fourth. The twin-clutch transmission's computerized controller calculates the next likely gearchange and shifts the "idle" transmission into that gear. The advantage is the speed of gearchanges: Volkswagen's DSG takes about 8 milliseconds to upshift. Compare that to the SMT in the Ferrari Enzo, which takes 150 ms to upshift. It's also significantly faster than a human: 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. Like the SMT, the DSG performs double-clutch downshifts and can skip gears (i.e. downshifting from 6th directly to 4th, 3rd, etc).
Driving with a twin-clutch transmission
Most twin-clutch transmissions, incliding Volkswagen's DSG, use a traditional P-R-N-D-S shift pattern. In the case of Volkswagen's DSG, the transmission can shift automatically in either normal (D) or Sport (S) modes. In Normal mode, the DSG shifts to the higher gears early in order to minimize engine noise and maximize fuel economy. In Sport mode the transmission holds the lower gears longer in order to keep the engine in its powerband. This is especially useful for turbocharged cars like
Audi A3,
Volkswagen GTI and
VW Jetta GLI, since the turbocharger is only active at higher RPMs. Sport mode also provides more aggressive downshifts with slight accelerator pedal pressure. Manual mode is engaged by either sliding the shift lever to the side or pulling one of the paddles on the steering wheel. Once in manual mode, shifts are made by moving the shifter fore and aft or using the paddles (which are marked " " and "-"). If manual mode was engaged with the paddles, pulling and holding the upshift paddle returns the transmission to automatic mode, allowing the driver to quickly downshift to a lower gear for passing and then hand control back over to the transmission.