A dual-clutch transmission or DCT (sometimes called a twin-clutch transmission) is effectively two manual gearboxes put together and controlled by computer.
In a conventional manual transmission there is an input shaft and an output shaft. Each one carries gear wheels, the relative size of which provide the gear ratios. Power flows in along the input shaft, across a pair of gears, and out along the output shaft.
In a dual clutch transmission there are two input shafts, one providing ‘odd’ gears (and reverse) and the other providing ‘even’ gears. Each shaft has an automatically-controlled clutch which determines whether or not it is transmitting power to the wheels.
The shaft which is not in use can independently pre-select a gear ready for the next gearchange. The computer then makes the gearchange simply by opening one clutch and closing the other.
The best-known twin-clutch transmission is Volkswagen's Direct Shift Gearbox (DSG), but this style of transmission is also used by Audi, Porsche, Ford, Bugatti, BMW, McLaren and others.
