How the Dual Clutch Transmission Works
September 24, 2010
The DCT is in effect two gearboxes operating in parallel. Each has its own clutch. One gearbox houses first, third, fifth and, if fitted, seventh gears; the other carries second, fourth, sixth and reverse. While the drive is going through one half of the gearbox – in third gear, for instance – the other half of the box preselects the next expected ratio – fourth, in this case – ready for the first clutch to disengage and the second to engage in a precisely-timed sequence.
The result is a very fast but completely smooth gearshift, free from any interruption in torque flow to the driving wheels.
In practice, packaging constraints within most vehicles dictate that the two gearboxes, and often clutches, are configured concentrically using a system of hollow shafts. Shifts can take place either manually, via Formula-1-style shift paddles on the steering wheel, or under automatic control when the driver places the shift selector in Drive.
In the latter case, careful programming of the shift points and throttle openings allows an improvement in fuel consumption of up to 15 percent compared with a conventional planetary automatic, thanks in part to the elimination of the energy-sapping torque converter.
In terms of mechanical efficiency, a dual clutch transmission will never quite be able to match the theoretical efficiency of a standard six-speed manual transmission, as the necessary hydraulic systems will inevitably absorb some energy, especially on a wet clutch DCT. Manuals have no such systems.
But where DCTs really score is in their wide range of abilities. Not only do they combine fuel economy with smoothness and driving refinement to match even the best automatics, they also provide the rapid responses and snappy shifts that sporting drivers demand.
Never before has a single transmission type been able to satisfy so many buyer groups at the same time. This is good news for both automakers and their customers: factory processes are simplified by having just one basic transmission shape to install, and buyers no longer have to make the tricky decision whether to go for an automatic or a manual because the DCT gives them both in the same vehicle.
© 2013, The Lubrizol Corporation