Direct-Shift Gearbox
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The Direct-Shift Gearbox (DirektSchaltGetriebe), or DSG, is an electronically controlled, twin-shaft dual-clutch manual gearbox, without a conventional clutch pedal, with full automatic, or semi-manual control. In simple terms, it is two separate manual gearboxes, contained within one housing, and working as one unit. It was designed by BorgWarner and initially licensed to Volkswagen Group (Volkswagen AG) (VWAG) (which owns the Volkswagen, Audi, SEAT, Bentley, Bugatti, Lamborghini, and Škoda brands), with support by IAV GmbH. By using two clutches, fast shifts can be achieved, and the torque converter of a conventional epicyclic automatic transmission is eliminated.
At the present time, the DSG transmission is only available in transaxle-mounted front engine, front-wheel drive or four-wheel drive. The Bugatti Veyron is a notable exception, due to its Ricardo-developed transmission. Information leaked from Porsche has that the 997 facelift to be released during 2008 will have an optional DSG-style gearbox called PDK (Porsche Doppelkupplungs).
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[edit] Introduction
The engine drives two clutch packs. The outer clutch pack drives gears 1, 3, 5 and reverse. The inner clutch pack drives gears 2, 4, and 6. Instead of a standard large dry single-plate clutch, each clutch pack is a collection of four small wet interleaved clutch plates (similar to a motorcycle wet multi-plate clutch). Due to space constraints, the two clutch assemblies are concentric. Because the alternate clutch pack's gear-sets can be pre-selected (predictive shifts taking place while the other section is in use), un-powered time while shifting is avoided because the transmission of torque is simply switched from one clutch-pack to the other. The DSG takes about 8 milliseconds to upshift[1][2]. In comparison, the sequential manual transmission (SMT) in the Enzo Ferrari takes 150 milliseconds to upshift.[1][2] The quoted time for upshifts is the time the wheels are completely non-powered.
[edit] Operation
[edit] "D" mode
When the motor vehicle is stationary, in neutral, both clutch packs are fully disengaged. When the driver has selected D for drive (after pressing the foot brake pedal), the transmission's first gear is selected on the first shaft, and the clutch prepares to engage. At the same time, the second gear is also selected, but the clutch pack for second gear remains fully disengaged. When the driver releases the brake pedal, the clutch pack for the first gear takes up the drive at the "biting point", and the vehicle moves forward. Pressing the accelerator pedal increases the speed of forward momentum. As the car accelerates, the transmission's computer determines when the second gear (which is connected to the second clutch) should be fully utilised. Depending on the vehicle speed, and amount of power being requested by the driver (full throttle or part-throttle normal driving), the DSG then upshifts. During this sequence, the DSG disengages the first clutch while engaging the second clutch (all power from the engine is now going through the second shaft), thus completing the shift sequence. This sequence happens in 8 ms, and there is practically no power loss.
Once the vehicle has shifted up to second gear, the first gear is immediately de-selected, and third gear (being on the same shaft as 1st and 5th) is pre-selected, and is pending. Once the time comes to shift, the second clutch disengages and the first clutch re-engages. This method of operation continues in the same manner up to 6th gear.
Downshifting is similar to upshifting but in reverse order. The car's computer senses the car slowing down or more power required, and thus lines up a lower gear on one of the shafts not in use, and then completes the downshift. The actual shift timings are determined by the DSG's Electronic Control Unit, or ECU, which commands a hydro-mechanical unit, and the two units combined are called a "mechatronics" unit. Because the DSG ECU uses "fuzzy logic", the operation of the DSG is said to be "adaptive", that is, the DSG will "learn" how the user drives the car, and will tailor the shift points accordingly.
In the vehicle instrument display, between the speedometer and tachometer, the available shift positions are shown, the current position of the shift lever is highlighted, and the current gear "ratio" is also displayed as a number.
Under "normal", progressive acceleration and deceleration, the DSG shifts in a "sequential" mode, i.e. under acceleration: 1 > 2 > 3 > 4 > 5 > 6, and the same sequence reversed for deceleration. However, if the car is being driven at sedate speeds, with a light throttle opening, and the accelerator pedal is then pressed fully to the floor, this activates the "kick-down" function. During kick-down, the DSG can skip gears, going from 6th gear straight down to 3rd gear.
When the floor-mounted gear selector lever is in position D, the DSG works in full automatic mode, with emphasis placed on gear shifts programmed to deliver maximum fuel economy. That means that shifts will change up and down very early in the rev-range. As an example, on the Mk5 Golf GTI, sixth gear will be engaged around 35mph, when initially using the DSG 'box (although with an "aggressive" or "sporty" driving style, the adaptive shift pattern will increase the speed at which 6th gear engages).
[edit] "S" mode
The floor selector lever also has an S position. When S is selected, "sport" mode is activated. Sport mode still functions as a full automatic mode, identical to "D" mode, but upshifts and downshifts are made much higher up the rev-range, which aids a more sporty driving manner, by utilising all of the available engine power, and also maximising engine braking.
S is also highlighted in the instrument display.
[edit] Manual (Tiptronic) mode
Additionally, the floor shift lever also has another plane of operation, for manual or Tiptronic mode, with "+" and "−". This plane is selected by moving the stick away from the driver (in vehicles with the drivers seat on the right, the lever is pushed to the left, and in left-hand drive cars, the stick is pushed to the right). When this plane is selected, the DSG can now be controlled like a manual gearbox, albeit under a sequential shift pattern.
The readout in the instrument display changes to 6 5 4 3 2 1, and just like the automatic modes, the current gear ratio is highlighted. To change up a gear, the lever is pushed forwards (against a spring pressure) towards the "+", and to change down, the lever is pulled rearwards towards the "−". The DSG box can now be operated with the gear changes being determined by the driver. This method of operation is commonly called "Tiptronic". In the interests of engine preservation, when accelerating in Manual/Tiptronic mode, the DSG will still automatically change up just before the red line, and when decelerating, it will change down automatically at very low revs. Furthermore, if the driver calls for a gear when it is not appropriate (i.e., engine speed near the red-line, and a down change is requested) the DSG will delay the change until the engine revs are at an appropriate level to cope with the requested gear.
[edit] Paddle shifters
On certain "sporty" high-powered cars (such as the Audi TT, Audi A3 (2.0T and 3.2), Golf GTI, Golf R32, Jetta GLI, Eos (2.0T and 3.2), Seat Leon FR with DSG), steering wheel-mounted paddle shifters are available. These operate in the same manner as the floor mounted shift lever when using manual mode.
The paddle shifters have two distinct advantages: the driver can safely keep both hands on the steering wheel when using the Manual/Tiptronic mode, and the driver can temporarily override either of the automatic programmes (D or S) and gain instant manual control of the DSG box. If the manual override of one of the automatic programmes (D or S) is utilised, the gearbox will "default" back to the chosen auto mode after a predetermined duration of inactivity of the paddles, or when the car becomes stationary. Alternatively, should the driver wish to revert immediately to automatic control, this can be done by holding the + paddle for at least two seconds.
[edit] Advantages and disadvantages
Advantages
- Extremely fast shift time of 8 milliseconds, when shifting to a gear ratio the transmission ECU is expecting;
- Practically no power loss, due to the use of twin clutches instead of a torque converter;
- Better fuel economy than conventional planetary geared automatic transmission (due to lack of "slip" in a torque converter);
Disadvantages
- Lengthy shift time when shifting to a gear ratio which the transmission did not anticipate (around 400ms, depending on the situation);
- Relatively expensive to manufacture;[citation needed]
- Heavier than a comparable Getrag conventional manual transmission (75 kg vs. 47.5 kg);
- Greater potential failure due to complexity;
- Torque handling capability constraints limit after-market engine tuning modifications.
[edit] Applications
Volkswagen group vehicles available in Europe with the DSG gearbox include:
- Audi: Audi have renamed the DSG "S-Tronic" in newer models.
- Bugatti:
- Bugatti Veyron 16.4 (developed by Ricardo rather than Borg Warner)
- SEAT:
- Škoda:
- Volkswagen:
[edit] Notes
- ^ a b How the Direct-Shift Gearbox works - from About.com Cars
- ^ a b Transmission | Twin-Clutch Gearbox - from AutoZine Technical School
[edit] See also
[edit] External links
official links
independent links
- Technical explanation of manual transmissions
- Audi TT 3.2 DSG review
- Informative article from Just-Auto.com
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