What is a DCT or Dual-Clutch Transmission?

Jack R. Nerad | Mar 26, 2021

If you've done any research on current cars, you've probably seen references to dual-clutch transmissions (DCTs). And you might well have asked yourself, "What is a DCT? What is a dual-clutch transmission?" If you are young enough, you might also ask yourself, "What is a clutch?" To explain all that, let's go back in time for a minute.

There was an era when learning to drive meant learning to master the intricacies of changing gears manually while manipulating the clutch pedal, the accelerator, and the gear selector lever all at once. Though the accelerator (gas pedal) and gear selector (shifter) are familiar to virtually all current drivers, the clutch pedal might not be.

2021 Porsche 911 Carrera S Coupe Red Action

In a car with a manual transmission, the clutch disengages the engine from the transmission for a few moments so the driver can switch from one gear to another. The driver pushes the clutch pedal in to operate the clutch. When pulling away from a stop in a manual transmission-equipped car, your feet do a little dance as you engage first gear or reverse, let out the clutch pedal, and add the proper amount of acceleration with the accelerator. The procedure takes a degree of skill that some find challenging to master.

Thus, to make cars easier to drive, inventors and tinkerers have sought ways to automate the process so the machine accomplishes what the driver used to perform. 

Today, several different types of automatic transmissions do the gear-changing for us. Conventional automatics use torque converters to facilitate the gear changes. Continuously variable transmissions (CVT) use belts or chains and pulleys to manipulate the gear ratios. This article's subject – the dual-clutch transmission (DCT) – internally works similar to a manual transmission but doesn't require the driver to operate a clutch pedal.

A Short History of Dual-Clutch Transmissions - Find the best car deals!

A French engineer named Adolphe Kégresse is widely regarded as the father of the DCT. Seeking an efficient way to automate gear selection in vehicles, he came up with the concept that would develop into the dual-clutch transmission in the late 1930s, but World War II intervened and prevented him from implementing his idea.

It wasn't until the 1980s that the idea really began to take shape. As with many automotive improvements, racing was the impetus. 

Porsche developed the PDK (Porsche Doppelkupplungsgetriebe) transmission in the 1980s and used it in the highly successful 962 race cars. In 1986, a 962 with the PDK transmission won its first World Sports Prototype Championship race at Monza in Italy, and it went on to be one of the most heralded race cars of all time. 

The PDK dual-clutch transmission has since become a staple in Porsche street cars. Many other performance-oriented cars now use DCTs.

What is a DCT or Dual Clutch Transmission? - Find the best car deals!

Dual Clutch Transmission DCT Porsche PDK Cutaway

As we said earlier, in automotive terms, a clutch disengages the engine's power-providing crankshaft from the transmission, so the driver can switch from one gear to another without causing potential binding of the machinery. 

A dual-clutch transmission has – this should come as no surprise – two clutches. Why? Because a DCT is essentially two transmissions working in concert, each of which requires a clutch for the same reason a manual transmission requires a clutch. 

Further, dual-clutch transmissions do not require the driver to manually change from one gear to another using the shift lever. Instead, the gear selection process is automated so that a DCT can act as an automatic transmission. But as we'll explain, it does not select the gears in the same way that a conventional automatic transmission with a torque converter performs that task. 

The magic of the dual-clutch transmission is that it can change from one gear to another very quickly. This speedy shifting is possible because while a gear is engaged in one of the two internal transmissions in the DCT, the next gear is pre-selected in the second of the two internal transmissions and ready for immediate engagement. 

Thus, the transition from one gear to the next takes mere milliseconds, making gear changes with a DCT much quicker than with a traditional manual transmission or a conventional automatic transmission. This is especially helpful in performance and racing cars, where split seconds can be the difference between winning and losing.

How Does a Dual-Clutch Transmission Work? - Find the best car deals!

Paddle Shifter for Porsche PDK Dual Clutch Transmission

A dual-clutch gearbox has two clutches but no clutch pedal. With a traditional manual transmission, clutch engagement requires the driver to use a clutch pedal. With a DCT, electronics control clutch engagement, issuing commands to the transmission's internal hydraulics. 

Each of the two clutches is associated with one or the other of the two internal transmissions. Most often, one of those transmissions houses the odd-numbered gears, with the other controlling the even-numbered gears. (One of the transmissions also houses the reverse gear for backing up.)

While driving a car with a DCT, depending on the selected gear, one of the internal transmissions is engaged with the engine and powering the car while the other is disengaged from the engine but with the next gear pre-selected for use. The change between gears is swift and occurs without interrupting the torque flow through the driveline.

Two Modes — Automatic and Manual  - Find the best car deals!

In the typical DCT, when driving automatic mode, the transmission will pre-select the next gear and then perform all the actions necessary to change between gears. The DCT will actuate and release the two clutches as necessary to upshift during acceleration and downshift during deceleration. Gear changes are sequential, which means in numerical order, up and down. However, a DCT can progress through its full range of gears quickly because of the dual-clutch gear pre-selection process.

Beyond automatic operation, a modern DCT can also operate as a clutch-less manual. That means that the driver can select the next gear manually, usually using levers (paddle shifters) on the steering wheel. The driver does not need to use a clutch pedal or actuate a clutch in any way. The transmission's electronics and hydraulics automatically engage both internal clutches. Manual shifting enables very rapid up-and-down gear changes and gives a driver complete control over transmission behavior.

How Does a DCT Differ From Other Automatic Transmissions? - Find the best car deals!

Dual Clutch Transmission DCT Paddle Shifters Porsche 718 Boxster

Conventional automatic transmissions use a torque converter to transfer the engine's power to the transmission and onward to the drive wheels. Essentially a hydraulic torque transfer device, a torque converter enables power delivery but can also decouple power from the transmission when the engine is operating at very low engine speeds. This allows drivers to stop and sit idle without shifting the car out of gear. In a DCT, the two clutches perform the coupling and uncoupling action. 

Continuously variable transmissions (CVTs) are another type of automatic transmission, and they are growing increasingly popular. A CVT uses a moving pulley system in conjunction with a belt or chain to adjust the gear ratio across a wide range. A CVT transmission does not involve the discrete changing of gears from one to another as in a conventional automatic or a DCT. Instead, it can vary the drive ratio based on power requirements. 

CVTs typically offer excellent fuel efficiency, but they have characteristics that some drivers find objectionable. To combat this, automakers commonly program specific CVT operation ratios, making acceleration feel and sound more like that of a conventional automatic transmission. Some even offer paddle shifters to give the driver control over those pre-programmed ratios, such as in modern Subaru models.

In contrast to a CVT, a dual-clutch transmission changes gears like a conventional automatic transmission, but it does it more quickly and with much less power loss in the transition. A DCT also feels and sounds more satisfying to the ears of a driving enthusiast.

Why Some People Dislike Dual-Clutch Transmissions - Find the best car deals!

Like CVTs, DCTs typically deliver good to excellent fuel economy versus other types of transmissions, but they do have a few downsides. 

  • Some drivers might find the quick shifts with some DCTs to be abrupt and uncomfortable
  • When shifting between reverse and drive, a DCT can sometimes feel hesitant 
  • A similar hesitation is sometimes evident during initial acceleration from a stop
  • Dual-clutch transmissions are typically not as adept at creeping forward at extremely low speeds when at a stoplight or inching forward into a parking place

For these reasons, many car makers prefer to use traditional torque-converter automatic transmissions or CVTs in their vehicles instead of DCTs. But for performance vehicles, dual-clutch transmissions remain a popular choice.

What Cars Use Dual-Clutch Transmissions? - Find the best car deals!

Dual Clutch Transmission DCT Porsche PDK Shifter

Several current performance cars offer dual-clutch transmissions. Among them are the Audi R8, Ford Mustang Shelby GT500, Nissan GT-R, and Porsche 718 and 911. But DCTs are not limited to high-end sports cars. You'll find them in the Kia Forte GT and Volkswagen GTI, for example. One of the strangest uses of the technology is in the Hyundai Ioniq Hybrid and Plug-in Hybrid models.

Advancements in conventional automatic transmission technology seem to be threatening the future popularity of dual-clutch transmissions. Some predict that fewer and fewer models will offer DCT technology going forward. But those cars that currently have them profit from their ability to make rapid gear changes with minimal loss of torque.

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