How Does a Dual-Clutch Transmission Work?

They also possess almost all of the characteristics of a conventional torque-converter automatic transmission that does away with manually actuating a clutch and stick shift, which bodes well for easier daily driving. But how does it do all this? What goes on inside a dual-clutch gearbox to give drivers the best of both worlds? One of the best ways to take an inside look is to examine one of the most well-regarded DCTs, ever: Porsche's PDK.

Here's a quick German lesson: PDK stands for Porsche Doppelkupplung, or more specifically, because Germans love adding words onto other words to make longer words, the gearbox is officially called Porsche Doppelkupplungsgetriebe. This translates to Porsche Double (or Dual)-Clutch Gearbox. Several other automakers use similar DCTs, such as their fellow German manufacturer Volkswagen with its Direct Shift Gearbox (DSG), or BMW, which it simply calls the DCT. To be a tad pedantic, the BMW DCT is the only unit of the three that is a true transmission, meaning it is a gearbox that's separated from its differential. The DSG and PDK are technically transaxles.

A double-clutch gearbox is exactly what it sounds like: a transmission with two clutches. The box has two clutches because it is essentially two gearboxes in one. Inside a seven-speed PDK, there are two separate driveshafts sitting on top of each other, with one clutch controlling each. On one driveshaft, you’ve got first, third, fifth, and seventh gear. On the other, you’ve got second, fourth, and sixth gear. When a particular gear is engaged by the shift fork in one gearbox, the next gear is already pre-selected in the other gearbox, connected to its clutch, but not to the engine’s driveshaft, ready to engage in the blink of an eye. When you shift between gears, within milliseconds one clutch opens and releases one gearbox, while the other clutch closes, engaging the other. 

The PDK and most other dual clutches found in enthusiast-centric vehicles use wet clutches, meaning they're bathed in the gearbox's fluid. Because of this, they stay cool and lubricated, so they wear significantly less than a dry clutch, which is what is found in a conventional manual transmission or dry dual-clutch transmission. In the sense of OEM power from the factory, theoretically wet clutches can (keyword: can) withstand higher horsepower and last much longer than their dry counterparts.

The PDK clutches are clutch packs, as well, not single clutch plates like you find in a conventional manual transmission. Porsche says that there is no service interval for replacing them—according to the service manual for a 2009-2016 991 Porsche GT3 with PDK, only the clutch fluid needs to be changed every 48,000 miles. Generally, it seems like enthusiasts and specialty shops recommend a VW DSG's interval to be 30,000 to 40,000 miles. However, like most components of a car that see high heat and mechanical movement, your mileage may vary. Shortening this interval for track duty is never a bad idea, and there's endless discussion on Porsche forums regarding whether a PDK's clutches are indeed lifetime components.

One major item that has a propensity to wear out in a DCT is the flywheel, which is a dual-mass unit. If driven poorly, they can wear and go out of balance, which makes for increased vibration, harsh shifting, or even total failure. Replacing the flywheel is a complex (and therefore time consuming) job that shops usually charge a lot of money to perform. Otherwise, regular fluid and filter servicing, as well as not being too harsh on the drivetrain, can lead to long clutch pack and flywheel life.

DCTs are also tunable, meaning you can program them how to react. They can be tuned to increase or decrease shift speeds, hold gears longer or shorter, pinpoint where shifts occur on the rev band, as well stand up to more power by increasing the hydraulic pressure that goes to the clutches to actuate them. Tunes also offer the ability to remove automatic downshifting which some DCTs feature from the factory, meaning you truly have complete manual control over the gearbox.

Although DCTs are generally smooth-operating units, they do have some downsides. Some take longer than others to engage the reverse gear, which makes three-point turns, or pulling an Austin Powers in tight quarters, a bit of a pain. From my own experience, this was one of the few minorly annoying things about the BMW M2 CS. Dual-clutch transmissions are also heavier than their manual counterparts, which might be a concern for those seeking the lightest chassis, or most ideal weight balance possible, for track work.

For as smooth as DCTs are, they can also be jerky. Slight or hesitant inputs with the gas pedal can confuse the DCT's computer, making it twitchy in the way it selects and engages gears. Though, that's not necessarily a downside, and usually more of a user error. Like any transmission design, there's a unique method to its operation.

Certain other inputs can cause premature wear in a DCT. Creeping forward, otherwise known as inching forward, with your foot off of the gas, increases wear on the clutch that engages first gear. This activity increases slipping the clutch to make a smooth transition, therefore wearing it out ever so slightly more. For those who want to do everything they can to maximize the life of the components in their DCT, it's a good idea to avoid creeping. Some modern DCTs, like the unit found in the new Hyundai Elantra N, have a setting that electronically prevents creeping, meaning you have to put your foot on the gas to advance forward.

Based on experience and popular searches on the internet, we've compiled a list of frequently asked questions about dual-clutch transmissions, answers included.

A. This is up to the motorist's preferences, but for more performance, control over the revs, and just overall enthusiasm, DCT is the way to go.

A. It's an automated manual transmission, meaning the components are basically of a manual transmission, but they're actuated automatically.

A. Its purpose is to have very short shift times, which boosts performance and fuel economy.

A. They can be jerky, as well as expensive to service and replace. They're also heavy, and some manufacturers' have been the source of several class action lawsuits.

A. This is a DCT that doesn't utilize a clutch that's bathed in fluid, hence calling it dry. Dry clutch DCTs tend to yield better fuel economy, though don't stand up to heat or torque as well as their wet counterparts. They're also cheaper to produce, which is why they're found in some cars' non-performance trims.

European parts supplier FCP Euro has great visual resources for learning more about DCTs. Check out this blog post of theirs, or take a gander at the video below. TransmissionDigest.com also features a very thorough explanation of the inner workings of both wet and dry dual-clutch transmissions.