Dual Clutch Transmissions

Most drivers know that cars come with two basic transmission types- manuals, in which the driver changes gears by depressing a clutch pedal and using a stick shift, and automatics, which shifts using clutches, a torque converter and sets of planetary gears. But there's also something in between that offers the best of both worlds- the dual-clutch transmission- also called the semi-automatic transmission, the "clutchless" manual transmission and the automated manual transmission.




In the world of race cars, semi-automatic transmissions, such as the sequential manual gear box (SMG), have been a staple for years. But in the world of production vehicles, it's a relatively new technology- one that is being defined by a very specific design known as the dual-clutch, or direct-shift, gearbox.


The man who invented the dual-clutch gearbox was a pioneer in automotive engineering. Adolphe Kégresse is best known for developing the half-track- a type of vehicle equipped with s rubber treads allowing it to drive off-road over various forms of terrain. In 1939, he conceived the idea for a dual-clutch gearbox, which he hoped to use on a Citroën "Traction" vehicle. Unfortunately, adverse business circumstances prevented further development.


Adolphe Kégresse


Half-track (with rubber treads)

Both Audi and Porsche picked up on the dual-clutch concept, although its use was limited at first to race cars. The 956 and 962C racecars included the Porsche Dual Klutch, or PDK. In 1986, a Porsche 962 won the Monza 1000 Kilometer World Sports Prototype Championship race- the first win for a car equipped with the PDK semi-automatic paddle-shifted transmission. Audi also made history in 1985 when a Sport Quattro S1 rally car, equipped with dual-clutch transmission, won the Pikes Peak Hill Climb.


Porsche 962


Sport Quattro S1

Commercialization of the dual-clutch transmission, however, has not been feasible until recently. Volkswagen has been a pioneer in dual-clutch transmissions, licensing BorgWarner's DualTronic technology. As with many new technologies, most car makers have decided to market DCTs under their own trademarked brand names: DSG (VW), EcoShift (Hyundai), PDK(Porsche), PowerShift (Ford, Volvo), S-Tronic (Audi), SpeedShift (Mercedes), and TC-SST (Mitsubishi). For some companies, most notably Volkswagen and Ford, DCTs are available on mainstream vehicles. (Note- Nissan, BMW and Mitsubishi restricts usage to high-performance models).

Although dual clutch transmissions (DCTs) are generally employed as replacements for traditional automatic gearboxes, their mechanical workings actually bear more in common with the traditional manual transmission. Changing from one clutch to another takes between an astounding 8-200 milliseconds- that's considerably faster than the half second or more required by most manual drivers to change gears. Even the legendary Ronnie Sox, “Mr. 4-Speed,” would be smoked by this new technology. By wasting less time between gears, DCTs are often able to out-sprint their manual equivalents. The downside with fast-shifting DCTs is that at lower speeds they tend to jerk and lurch around. In tight parking spaces, it can be a little frightening to suddenly lunge forward, even if it's just a few inches when you're feathering the throttle.

A dual-clutch transmission offers the function of two manual gearboxes in one. Although it uses two clutches, it has no clutch pedal. Sophisticated electronics and hydraulics control the clutches, just as they do in a standard automatic transmission. In a DCT, however, the clutches operate independently. One clutch controls the odd gears (first, third, fifth and reverse), while the other controls the even gears (second, fourth and sixth). Using this arrangement, gears can be changed without interrupting the power flow from the engine to the transmission.
Commercialization of the dual-clutch transmission, however, has not been feasible until recently. Volkswagen has been a pioneer in dual-clutch transmissions, licensing BorgWarner's DualTronic technology. As with many new technologies, most car makers have decided to market DCTs under their own trademarked brand names: DSG (VW), EcoShift (Hyundai), PDK(Porsche), PowerShift (Ford, Volvo), S-Tronic (Audi), SpeedShift (Mercedes), and TC-SST (Mitsubishi). For some companies, most notably Volkswagen and Ford, DCTs are available on mainstream vehicles. (Note- Nissan, BMW and Mitsubishi restricts usage to high-performance models).

A two-part transmission shaft is at the heart of a DCT. The outer shaft is hollowed out, making room for an inner shaft, which is nested inside. The outer hollow shaft feeds second and fourth gears, while the inner shaft feeds first, third and fifth. The diagram below shows this arrangement for a typical five-speed DCT. Notice that one clutch controls second and fourth gears, while another, independent clutch controls first, third and fifth gears. That's the trick that allows lightning-fast gear changes and keeps power delivery constant. A standard manual transmission can't do this because it must use one clutch for all odd and even gears.



There are two types of dual-clutch transmission on the market- dry and wet clutch. Several manufacturers are developing DCTs that use dry clutches, like those usually associated with manual transmissions. Although dry clutches are more efficient, they are restricted in the amount of power and torque they can handle. As a result, all production vehicles equipped with DCTs, today, use the wet version. Wet clutch models are so called because the clutch is bathed in a sea of oil and this type is often found in high-performance cars.

Because a dual-clutch transmission is similar to an automatic, you might think that it requires a torque converter which is how an automatic transfers engine torque from the engine to the transmission. DCTs, however, don't require torque converters. Instead, DCTs currently on the market use wet multi-plate clutches. A "wet" clutch bathes the clutch components in lubricating fluid to reduce friction and limit the production of heat. When the clutch is engaged, pressure inside the piston forces a set of coil springs apart, which pushes a series of stacked clutch plates and friction discs against a fixed pressure plate. The friction discs have internal teeth that are sized and shaped to mesh with splines on the clutch drum. In turn, the drum is connected to the gear set that will receive the transfer force. To disengage the clutch, fluid pressure inside the piston is reduced. This allows the piston springs to relax, which eases pressure on the clutch pack and pressure plate.



With upshifts taking a mere 8 milliseconds, many feel that the DCT offers the most dynamic acceleration for vehicles. It certainly offers smooth acceleration by eliminating the shift shock that accompanies gearshifts in manual transmissions and even some automatics. Best of all, it affords drivers the luxury of choosing whether they prefer to control the shifting or let the computer do all of the work.

Perhaps the greatest advantage of a DCT is improved fuel economy. Because power flow from the engine to the transmission is not interrupted, fuel efficiency increases dramatically. Some experts say that a 6-speed DCT can deliver up to a 10% increase in relative fuel efficiency, when compared to a conventional 5-speed automatic.

Many car manufacturers are interested in DCT technology. However, some automakers are wary of the additional costs associated with modifying production lines to accommodate a new type of transmission. This could initially drive up the costs of cars outfitted with DCTs, which might discourage cost-conscious buyers.

In addition, manufacturers are already investing heavily in alternate transmission technologies. One of the most notable is the continuously variable transmission or CVT (see previous article). A CVT is a type of automatic transmission that uses a moving pulley system and a belt or chain to infinitely adjust the gear ratio across a wide range. CVTs also reduce shift shock and increase fuel efficiency significantly. But CVTs can't handle the high torque demands of performance cars.

DCTs don't have such issues and are ideal for high-performance vehicles. In Europe, where manual transmissions are preferred because of their performance and fuel efficiency, some predict that DCTs will capture 25% of the market.