Dual-Clutch Transmissions: What are They?
Posted Tuesday, Jul 30, 2019
One oft heard complaint of the new C8 Corvette and the 2020 Shelby GT500 is that they are not going to be offered with manual transmissions; both will come with automatic dual-clutch transmissions (DCT). But what is a DCT? It’s name includes the word automatic, so is it an automatic? Sort of. It has a clutch, actually two, so is it a straight-drive? Kind of.
To understand DCTs, we first need to look at transmissions in general. We’ll begin with straight-drives, or manuals. Obviously, the first feature of a manual that comes to mind is the clutch. To keep things simple, a clutch is a collection of plates between the engine and the transmission. When you press the clutch pedal, you disengage the clutch- which means you’re holding these plates apart. In this way, the engine can turn but the transmission will not. At this point, you’re able to select the appropriate gear from a series of helical gears arranged along a straight shaft(s) (hence the name straight-drive). Because the gears are arranged along the shaft, there are only so many that will fit without making the transmission too unwieldy. Hence, most manuals have no more than 5 or 6 “speeds.” Once the appropriate gear is selected, you release the clutch pedal, which engages the clutch; in other words, you allow the plates to press together so that the transmission turns along with the engine.
An automatic, in contrast, does not have a clutch (at least not one you control with a pedal), but instead relies upon a torque converter between the engine and transmission. Whereas the plates in a manual clutch make physical contact when engaged, the plates in a torque converter do not; instead they use hydraulic pressure to transmit power. Think of it like this: you drop a cork into the center of a bucket filled with water; you then use a spoon to stir the water while avoiding direct contact with the cork; after some time, the stirring of the water will cause the cork to begin rotating as well. But the presence of the torque converter is not the only difference, though. Instead of having a shaft comprising several gears of varying diameters, an automatic has an intermediate shaft with several gear assemblies, each housing a “sun” gear, several planetary gears, and a ring gear. This description doesn’t suffice, I know, and automatics need their own article. But the gist of it is that automatics can use these multi-geared assemblies to create several gear ratios; several modern cars have automatics with 4 gears but 10 “speeds.” Lastly, the automatic automatically selects the ratio for you.
So now we come to dual-clutch transmissions. Before, I said it was sort of an automatic and kind of a manual; let’s look at the characteristics it shares with each. Like an automatic, dual-clutch transmission select the appropriate gear for you. Like a manual, dual-clutch transmissions disengage the engine from the transmission via clutch plates, but they have two clutches as the name implies. Furthermore, they have their series of helical gears arranged along two shafts. Therein lies the dual-clutch transmissions genius: it is a hybrid of the two transmission types. Since it has clutches, it is more fuel-efficient than an automatic because of the efficiency loss of the torque converter. And since the shifts are controlled by the computer, it shifts quicker than a manual.
Well, the computer controlled shifts aren’t the only reason it shifts so quickly. Its also due to the other feature of DCTs: it’s like two manual transmissions in the same housing. Each clutch has its own shaft of helical gears, one containing the even numbered gears and one housing the odd numbers. So, not only does it shift from one ratio between 1st and 2nd gear, but it is also switching from one shaft to another. This also augments the shift speed. Let’s say you’re accelerating from a traffic light. Obviously, you’re starting in 1st. As your rpm increase, the computer guesses that you will want to continue accelerating and would hence want to eventually move into 2nd gear. So, it selects second gear on the other shaft even while you’re still in 1st. Then, at the precise moment, it disengages clutch 1 and engages clutch 2 almost simultaneously. Boom! You’re almost instantly in 2nd. As you continue to accelerate, the computer selects 3rd on the first shaft, switches clutches, and, boom again! You can see how this would help with acceleration. In fact, both the Corvette’s 8-speed DCT and the Shelby GT500’s 7-speed DCT shift in 100 milliseconds (1/10th second). That’s faster than any human with a manual. In vehicles like these where performance is paramount, why not opt for the more efficient, quicker shifting option? I understand why purists miss the opportunity to “row” through the gears, but why wouldn’t you want performance in its purest form? DCTs seem to be the best of both worlds.