In a car that uses the standard paddle shifter, if the driver wants to change from one gear to another, the first step is to depress the clutch pedal. This action operates a clutch that disconnects the engine from the transmission, interrupting power to the transmission. The driver then selects a new gear using the shift lever, a process that involves moving the toothed coupling from one gear to another of a different size. A device called a "synchronizer" would make the gears mate before joining to prevent grinding. Once shifted into a new gear, the driver can release the clutch pedal, which reconnects the engine to the transmission and sends power to the wheels.
Thus, in a conventional manual transmission, there is no continuous power output from the engine to the wheels. During the shifting process, the power delivery will change from "yes" to "no" to "yes", which can cause "shift shock" or "torque interruption" phenomenon. For unskilled drivers, this phenomenon can cause vehicle occupants to feel swaying back and forth during gear changes. In contrast, a dual-clutch transmission uses two clutches but no clutch pedal. Advanced electronics and hydraulics control the clutch like a standard automatic transmission. In a dual clutch transmission, however, each clutch operates independently. One clutch controls the odd-numbered gears (1st, 3rd, 5th and reverse) and the other clutch controls the even-numbered gears (2nd, 4th and 6th). In this way, gear changes can be made without interrupting power transmission from the engine to the transmission.
The driver can also choose a fully automatic mode, which leaves all gear shifting to the computer. In this mode, the driving experience is very similar to a normal automatic car. Shift shock is reduced because the dual-clutch transmission auto part can "gradually out" of one gear and "gradually in" another. What's more, shifting is done under load, so power delivery is maintained at all times. This is made possible by the ingenious dual-shaft construction that separates odd and even gears.
Unlike ordinary one way clutches, where all gears are placed on one input shaft, dual-clutch transmission auto parts place odd-numbered gears and even-numbered gears on two input shafts. How is this achieved? The outer shaft is hollow, leaving room for nesting the inner shaft. The outer hollow shaft powers second and fourth gears, while the inner shaft powers first, third and fifth gears. One clutch controls second and fourth gears, while another separate clutch controls first, third and fifth gears. That's the trick to achieve instant gear changes and maintain continuous power transfer. A standard manual transmission cannot do this because one clutch must be used to control all odd and even gears.
Since a dual-clutch transmission is similar to an automatic, you might think that a dual-clutch transmission, like an automatic transmission, would require a torque converter to transfer engine torque from the engine to the transmission. However, a dual-clutch transmission does not require a torque converter. Current dual-clutch transmissions on the market use wet multi-plate clutches. A "wet" clutch is one in which the clutch components are soaked in lubricating fluid to reduce friction and limit heat generation. Several manufacturers are developing dual-clutch transmissions that use dry clutches, but now all production cars equipped with dual-clutch transmissions use wet clutches. Many motorcycles use a single multi-plate clutch.
Fluid acts inside the clutch piston. When the clutch is engaged, hydraulic pressure inside the piston forces a set of coil springs to disengage, pushing a series of clutch plates and friction discs against a stationary pressure plate. The friction discs have internal teeth sized and shaped to engage splines on the clutch driven drum. The moving drum is in turn connected to the gears that will receive the transmission power. Audi's dual-clutch transmission has both a small coil spring and a large diaphragm spring in its wet multi-plate clutch automatic. In order to disengage the clutch, the hydraulic pressure inside the piston is reduced. This allows the piston spring to relax, reducing the pressure on the clutch assembly and pressure plate.