A double-clutch system, also known as Direct Shift Gearboxes or DSG transmissions, are essentially manual transmissions that are fully automated.
Like conventional manual transmissions, all the variants of DSG transmissions use solid steel gears that are engaged in different ratios to produce either six or seven speeds, depending on the transmission variant.
The most important differences between DSG and conventional manual transmissions are that DSG transmissions use two clutches that are incorporated into a single assembly, as opposed to a conventional manual transmission that uses only one clutch, and that on some DSG variants, the clutch assembly runs in oil similar to most motor cycle clutches.
In terms of operation, DSG transmissions are highly sophisticated systems that depend on hydraulic actuators to operate both clutches alternatively, as well as other actuators that actually engage and disengage the gears, with the whole being controlled by a control module known as a “mechatronic” unit. In simple terms, mechatronic units can be described as a sometimes-rocky or even unhappy marriage between hydraulic and electronic systems that are intended to work in conjunction with each other to control the transmission, using input data from a wide variety of engine and other sensors when the transmission is in automatic mode.
As a practical matter though, when the transmission is in neutral both clutches are disengaged, but when a gear is selected, the control system selects both first and second gears, and engages the clutch that operates first gear, while keeping the clutch that operates second gear disengaged.
The transmission will now remain in first gear until either the control system initiates a shift to second gear, or the driver initiates a shift to second gear manually. Since second gear is already selected but kept in “reserve” by the transmission’s control system, the system accomplishes the actual shift to second gear simply by disengaging the one clutch while engaging the other.
This arrangement also works for the other gears; when first gear is deselected (and the transmission is in second gear), third gear is selected, but it is kept in reserve by the control system. In this way, there are always two gears selected, but to make the system work, the odd-numbered gears (1, 3, 5, and 7) are operated by one clutch, while the even-numbered gears (2, 4, and 6) are operated by the other clutch. In most variants, reverse gear is also operated by the clutch that operates the odd-numbered gears.