A direct-shift gearbox (DSG) is an electronically controlled dual-clutch multiple-shaft manual gearbox, in a transaxle design – without a conventional clutch pedal, and with full automatic, or semi-manual control. In DSG, two separate manual gearboxes (and clutches), are contained within one housing, and work as one unit. By using two independent clutches, a DSG can achieve faster shift times, and eliminates the torque converter of a conventional epicyclic automatic transmission.
This transmission system has been licensed to Volkswagen and is an effective system of transmission.
Whilst the motor vehicle is stationary and in neutral (N), the driver can select D for "drive" (after first pressing the foot brake pedal). The transmission's reverse gear is selected on the first shaft K1, and the outer clutch K2 engages at the start of the 'bite point'. At the same time, on the alternate gear shaft, the reverse gear clutch K1 is also selected (pre-selected), as the gearbox doesn't know whether the driver wants to go forward or reverse. The clutch pack for second gear (K2) gets ready to engage. When the driver releases the brake pedal, the K2 clutch pack increases the clamping force, allowing the second gear to take up the drive through an increase of the 'bite point', and thereby transferring the torque from the engine through the transmission to the drive shafts and road wheels, causing the vehicle to move forward. Depressing the accelerator pedal engages the clutch and causes an increase of forward vehicle speed. Pressing the throttle pedal to the floor (hard acceleration) will cause the gearbox to "kick down" to first gear to provide the acceleration associated with first, although there will be a slight hesitation while the gearbox deselects second gear and selects first gear. As the vehicle accelerates, the transmission's computer determines when the second gear (which is connected to the second clutch) should be fully used. Depending on the vehicle speed and amount of engine power being requested by the driver (determined by the position of the throttle pedal), the DSG then up-shifts. During this sequence, the DSG disengages the first outer clutch whilst simultaneously engaging the second inner clutch (all power from the engine is now going through the second shaft), thus completing the shift sequence. This sequence happens in 8 milliseconds (aided by pre-selection), and can happen even with full throttle opening, and as a result, there is virtually no power loss.
Once the vehicle has completed the shift 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 into 3rd, the second clutch disengages and the first clutch re-engages. This method of operation continues in the same manner for the remaining forward gears. Downshifting is similar to up-shifting but in reverse order, and is slower, at 600 milliseconds, due to the engine's Electronic Control Unit, or ECU, needing to 'blip' the throttle so that the engine crankshaft speed can match the appropriate gear shaft speed. The car's computer senses the car slowing down, or more power required (during acceleration), and thus engages a lower gear on the shaft not in use, and then completes the downshift. The actual shift points are determined by the DSG's transmission ECU, which commands a hydro-mechanical unit. The transmission ECU, combined with the hydro-mechanical unit, are collectively called a "mechatronics" unit or module. Because the DSG's 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 progressively tailor the shift points accordingly to suit the habits of the driver.
In the vehicle instrument display, between the speedometer and tachometer, the available shift-lever positions are shown, the current position of the shift-lever is highlighted (emboldened), and the current gear ratio in use is also displayed as a number.
Under "normal", progressive and linear acceleration and deceleration, the DSG shifts in a "sequential" manner, i.e. under acceleration: 1st > 2nd > 3rd > 4th > 5th > 6th; and the same sequence reversed for deceleration. However, the DSG can also skip the normal sequential method, by 'missing out' adjacent gears, and shift two or more gears. This is most apparent if the car is being driven at sedate speeds in one of the higher gears with a light throttle opening, and the accelerator pedal is then pressed down, engaging the "kick-down" function. During kick-down, the DSG will skip gears, shifting directly to the most appropriate gear depending on speed and throttle opening. This kick-down may be engaged by any increased accelerator pedal opening, and is completely independent of the additional resistance to be found when the pedal is pressed fully to the floor, which will activate a similar kick-down function when in Manual operation mode. The seven-speed unit in the 2007 Audi variants will not automatically shift to 6th gear; rather, it stays at 5th to keep power available at a high RPM while cruising. 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 Volkswagen Golf Mk5 GTI, sixth gear will be engaged around 52 km/h (32 mph), when initially using the DSG transmission with the 'default' ECU adaptation - although with an "aggressive" or "sporty" driving style, the adaptive shift pattern will increase the vehicle speed at which sixth gear engages.
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