Direct-Shift Gearbox
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http://en.wikipedia.org/wiki/Direct-Shift_Gearbox

Direct-Shift Gearbox (Direkt-Schalt-Getriebe (German)[1]), or DSG, is a Volkswagen Group developed electronically controlled multiple-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 (which owns the Volkswagen Passenger Cars, Audi, SEAT, Škoda, Lamborghini, Bentley, Bugatti, and Volkswagen Commercial Vehicles brands), with support by IAV GmbH. By using two independent clutches, faster shift times can be achieved, and the traditional torque converter of a conventional epicyclic automatic transmission is eliminated.

At the time of launch, and for the first few years of production, the DSG transmission was only available in transaxle-mounted front engine, front-wheel drive or four-wheel drive vehicle layouts. As of 2009, a revised version of the DSG is scheduled to be fitted in certain longitudinally mounted front engine Audis. The Bugatti Veyron is a notable exception, due to its Ricardo-developed transmission.

The first DSG gearbox that went into production had six speeds and wet/submerged clutch packs (internal VW code: DQ250, parts code prefix: 02E), and the 2-wheel drive version weighed 80kg. An additional new lighter (70kg) seven speed DSG transaxle (0AM)[2] uses twin single-plate dry clutches (of similar diameter), is used in smaller cars with lower torque outputs such as the latest Volkswagen Polo and the new Seat Ibiza, due to it having a maximum torque handling capacity of 250Nm.
Introduction

The internal combustion engine drives two clutch packs. The outer clutch pack drives gears 1, 3, 5 and reverse - the outer clutch pack has a larger diameter compared to the inner clutch, and can therefore handle greater torque loadings. The inner clutch pack drives gears 2, 4, and 6. Instead of a standard large dry single-plate clutch, each clutch pack for the 6-speed DSG 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. This means that the DSG takes only about 8 milliseconds to upshift[3][4]. In comparison, the sequential manual transmission (SMT) in the Ferrari Enzo takes 150 milliseconds to upshift.[3][4] The quoted time for upshifts is the time the wheels are completely non-powered.
Operation
"P"

P position of the floor-mounted gear shift lever means that the transmission is set in "Park". Both clutch packs are fully disengaged, all gear-sets are disengaged, and a solid mechanical transmission 'lock' is applied to the crown wheel of the DSG's internal differential. This position must only be used when the motor vehicle is stationary. Furthermore, this is the postion which must be set before being able to remove the vehicle ignition key.
"N"

N position of the floor-mounted shift lever means that the transmission is in "neutral". Just like in P above, both clutch packs and all gear-sets are fully disengaged; however, the parking lock is disengaged. This position should be used when the motor vehicle is stationary for any period of time, such as red traffic lights, or waiting in a queue of stationary traffic. The DSG should not be kept in any of the other active gear modes, and be held in a stationary position on the footbrake for periods longer than a brief period - due to the clutches being held on the bite point.
"D" mode

Whilst the motor vehicle is stationary and in neutral (N), the driver can select D for drive (after pressing the foot brake pedal). The transmission's first gear is selected on the first shaft, and the outer 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 foot brake pedal, the clutch pack for the first gear takes up the drive at the "biting point", transferring the torque from the engine through the transmission to the driveshafts - and the vehicle moves forward. Pressing the throttle / accelerator pedal increases forward vehicle speed. As the vehicle 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 the shaft 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 2nd gear (where conditions permit).

When the floor-mounted gear selector lever is in position D, the DSG works in fully 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 Golf Mk5 GTI, sixth gear will be engaged around 33mph, when initially using the DSG 'box with the 'default' ECU adaptation, although with an "aggressive" or "sporty" driving style, the adaptive shift pattern will increase the speed at which 6th gear engages.
"S" mode

The floor selector lever also has an S position. When S is selected, "sport" mode is activated in the DSG. Sport mode still functions as a fully automatic mode, identical in operation to "D" mode, but upshifts and downshifts are made much higher up the engine rev-range. This aids a more sporty driving manner, by utilising considerably more of the available engine power, and also maximising engine braking. However, this mode does have a detrimental effect on the vehicle fuel consumption, when compared to D mode. This mode may not be ideal to use when road conditions are very slippery, due to ice, snow or torrential rain - because loss of tyre traction may be experienced (wheel spin during acceleration, and wheel locking during downshifts at high engine rpms under closed throttle).

S is also highlighted in the instrument display, and like D mode, the currently used gear ratio is also displayed as a number.

Manual mode

Additionally, the floor shift lever also has another plane of operation, for manual mode, with spring-loaded "+" and "−" positions. This plane is selected by moving the stick away from the driver (in vehicles with the driver's 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 in "D" mode only. 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 currently used 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 (primarily) 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 redline, and when decelerating, it will change down automatically at very low revs, just before the engine idle speed (tickover). Furthermore, if the driver calls for a gear when it is not appropriate (i.e., engine speed near the redline, 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.
Paddle shifters

On certain "sporty", or high-powered cars, such as the Audi TT, Audi A3 (2.0T and 3.2), Golf GTI, Golf R32, Passat R36, Scirocco GT, Jetta GLI, Volkswagen Eos (2.0T and 3.2), SEAT Leon FR, Skoda Superb - 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 immediately manually override either of the automatic programmes (D or S) on a temporary basis, and gain instant manual control of the DSG box. If the manual override of one of the automatic programmes (D or S) is utilised intermittently, the gearbox will "default" back to the previously selected automatic 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.
"R"

R position of the floor-mounted shift lever means that the transmission is in "reverse". This functions in a similar way to D, but there is just one 'reverse gear'.

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" found in a torque converter) and for many models even than the manual-shift version;

Disadvantages

    * Relatively lengthy shift time when shifting to a gear ratio which the transmission ECU 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 for failure due to complexity;
    * Torque handling capability constraints limit after-market engine tuning modifications.