How DCT gearboxes work

Not your typical manual gearbox, but not a standard automatic shifter either, the dual-clutch transmission (DCT) is supposed to the provide advantages of both. So, why is it losing favour?

In its outward operation, the dual-clutch transmission (DCT) is so similar to a standard automatic that it is virtually indistinguishable from it. There is no clutch. You simply control the throttle and the brake, and the car figures out the rest. However, a DCT differs from your standard automatic gearbox in a number of important ways. In fact, in some ways it is more similar to a manual shifter than an auto ‘box; more of a ‘clutchless manual’ than anything else. In the world of production cars, the DCT is a relatively new phenomenon, but in the racing world it has been around far longer as the semi-automatic transmission or the sequential manual gearbox.

Before we look at the DCT, it’s probably a good idea to briefly examine the inner workings of your standard manual gearbox. When you decide to change gears with a manual gearbox, you press down on the clutch pedal. The pedal activates a clutch, which disconnects the engine from the gearbox. Power flow between the two is thus interrupted. You then select the desired gear with the shifter, which moves a toothed collar from one gear to the next.

Importantly, grinding would occur if these two weren’t moving at the same speed, so a manual gearbox has synchronisers (from here the word ‘synchro’) in it that help match these two speeds. Typically, there is a cone on the gear that slots into a groove on the collar. The gear and the collar then synchronise their speeds through the friction created where the cone and collar meet, and once they move at the same speed, the collar slides out of the way so that the actual teeth can be engaged.

As should be obvious to anyone who has ever driven a vehicle with a DCT, there is no clutch pedal, but that should not lead you to believe that there is no clutch. As its name suggests, a dual-clutch transmission actually has two. As with a typical automatic gearbox, clever hydraulics and electronics manage the swapping of cogs, but instead of your usual torque converter and bell housing, you have two clutches that make the gearbox more like a manual one than your typical auto transmission.

So, why two clutches? The first clutch is responsible for the odd gears (first, third, fifth), while the second clutch takes care of the even gears (second, fourth, sixth). While the vehicle sets off in first gear, the second clutch has already engaged second gear. When the vehicle shifts from first to second gear, it is actually swapping from one gearbox to another. When shifting from second gear to third gear, the vehicle switches back to the original gearbox.

A DCT transmission is sort of like having two gearboxes in one. The advantage is that the upcoming gear can be pre-selected, making shifts smooth and virtually instantaneous. Also, a DCT allows the driver to control upshifts, usually with the help of paddles behind the steering wheel. Simply tap the paddle, and the transmission instantly swaps cogs. A dual-clutch transmission can generally also perform something called a matched-rev downshift. When the driver selects a lower gear via the paddleshift, the clutches are disengaged and the engine is revved to provide the exact speed needed for the gear. When you downshift, you get a smooth shift without any lurching. Some DCT gearboxes allow you to skip gears when downshifting, and once again, the gear and engine speed is matched to provide a smooth and efficient shift.

The manual gearbox is quickly going extinct. They’re still used in vehicles because of affordability, but many premium manufacturers have done away with them. It’s basically impossible to buy a German luxury vehicle with a manual transmission these days. Many people have no problem with this, since an automatic gearbox is easier to live with, especially if you’re sitting in traffic every day. However, for true driving enthusiasts, the disappearance of the manual shifter is a tragedy, since an automatic gearbox doesn’t offer the same thrill and control. The DCT is supposed to be a satisfying ‘automatic’ alternative to the old-school manual gearbox. With its quick shifts and facility for the driver to take control, a DCT offers the convenience of an automatic transmission, while still providing a fun and involving driving experience for those who care about that sort of thing.

Sadly, the DCT may be going the way of the manual gearbox, too. While no one is denying the excellent performance offered by a dual-clutch transmission; cost, complexity and weight is an issue. As mentioned, a DCT is essentially two gearboxes in one, which means they are incredibly complex, and therefore pricey to build. Some manufacturers, like BMW, for example, feel that standard automatic gearboxes, like those produced by ZF, are so good that they are as good as a DCT version. Peter Quintus, vice president for sales and marketing at BMW M, said earlier this year that he believed DCT gearboxes, at least within BMW, would be gone in a few years. According to him, ZF’s eight-speed automatic transmissions are now as slight and fast as the DCT ones.