How a Twin-Clutch Transmission Works

Old Idea, New Application

While dual-clutch transmissions are only now becoming popular, the technology actually comes from the 1930s, when the first patents for such a system were filed. However, the idea was not practical until useable computer controls for the clutches and shift forks started appearing in the 1980s.

One of the first widespread uses was in the Porsche 956 and 962 endurance racer program. The brand’s so-called PDK transmission—that’s short for Porsche Doppelkupplung—first appeared in 1983 and helped the company score a big win at Monza in 1986.

At first, the cars had a standard shift lever that featured a back-and-forth action like a traditional sequential manual gearbox. A manual clutch pedal was used to start and stop the car. While the transmission still needed manual inputs, the dual-clutch technology reduced the shift time and softened the shift shocks. The goal was reliability and shift speed rather than automation. Later 962s got the now-standard pushbutton shifters on the steering wheel.

After a few years, Porsche dropped the PDK along with the whole 956/962 program. Audi also dabbled in the technology, as it briefly appeared in their mid-’80s Sport quattro S1 racer. 

The ideas behind the twin-clutch transmission appeared to die. Then, in the first part of this decade, manufacturers started to look to vendors like BorgWarner and Getrag for a new way to simultaneously increase performance and fuel economy. 

The new wave of automatic-clutch manuals first showed up in Europe, with the American debut coming for the 2003 model year: The V6-equipped Audi TT as well as the VW Passat both featured the VW/BorgWarner DSG dual-clutch transmission. VW and Audi soon fitted twin-clutch transmissions to several models in their lineups, including the Golf R32.

After VW and Audi, a few European makes dabbled in dual-clutch technology in 2007 and 2008, though nothing new came to the American market. Finally, the Mitsubishi Lancer Evolution MR and Nissan GT-R appeared with high-performance, dual-clutch transmissions for 2008; no manual was available for either model. 

A box for a dual-clutch transmission appeared on the BMW M3’s order form partway through 2009, and Porsche’s resurrected PDK is now ready for the masses; it can now be ordered on their Boxster, Cayman and 911 models, and company officials expect about 80 percent of their customers to make the selection. 

Ford, Volvo, Mercedes and Fiat all have models featuring dual-clutch transmissions; they’re either available in other markets or planned for sale here in the next few years. Honda recently announced that a dual-clutch transmission will be available on some of their large-displacement sport bikes starting in 2010. As fuel economy regulations become even more stringent, expect to see these wonders of mechanized shifting on more and more cars in all kinds of segments.

Don’t Call It an Automatic

As the name suggests, dual-clutch transmissions feature two separate clutches plus two separate mainshafts, each fitted with synchronizers and gears. Basically, they’re constructed like parallel manual transmissions but with an automatic clutch for each unit. Most designs place the clutches and input shafts in a concentric configuration; gears on each input shaft drive their equivalent mainshaft gears.

One transmission shaft and clutch carries the odd-numbered gears, while the other handles the even gears plus reverse. The two mainshafts drive a single output shaft through a gear on each shaft.

Since there are two clutches packed into the space usually reserved for one traditional dry-plate clutch, most dual-clutch transmissions use wet multi-plate clutches like those found in motorcycles.

To keep these clutches cool, the small clutch packs are bathed in the transmission oil supply. However, some newer designs use dry multi-plate clutches to save weight and keep the oil cleaner. 

Unlike the single-plate clutch found in a manual transmission, these multi-plate clutches are normally disengaged. They’re engaged by hydraulic pressure. As a result, when the engine isn’t running—and therefore when there isn’t any hydraulic pressure—both clutches are disengaged. This serves as a sort of fail-safe system, as a car can at least be towed or pushed if the engine or transmission fails.

The shifting is actuated by hydraulics, working through forks and synchronizers that look just like the ones found in conventional manual transmissions. These parts are actually smaller than their manual transmission counterparts, however, because they don’t have to cope with ham-fisted drivers. Solenoids—like those found in traditional automatics—and an electronic control module—often incorporated into the transmission case—take care of the actual shifting. 

All of this shifting, clutching and control requires a good hydraulic supply, and dual-clutch transmissions feature large, high-pressure oil pumps. They also have coolers—larger than those found in traditional manual transmissions—to dissipate the heat generated by the pump and wet clutches.

Balance of Power

This color-coded illustration shows the first-gear engagement path (in red) as well as the pending second-gear shift (in green) in an Audi gearbox.

Relatively speaking, each shift takes a long time with a standard manual transmission. We may think that we’re Johnny Gearbanger, but there’s a lot going on.

To go from one gear to the next, the driver first has to depress the clutch pedal to disengage the transmission. Then the lever is used to move the shift forks. The synchronizers delay the shift until the gear and shaft are moving at the same speed before the gear is finally selected. The whole cycle can take up to a second depending on the driver and transmission. 

With a dual-clutch transmission, the parallel-transmission design allows two gears to be selected simultaneously—this is the key to the rapid gear changes. When the transmission control unit or the driver selects a particular gear, the next logical gear is also engaged on the alternate shaft—a higher gear if the car is accelerating, or lower if it’s decelerating. Since both of those gears are on the non-driven shaft, they can be selected in advance.

When it comes time for the next shift, the two clutches hand off the engine torque from one to the other. The process happens much more quickly than with a traditional manual transmission. Some manufacturers, like BMW, boast of sub-10-millisecond shifts—that’s 1/100 of a second.

Like a traditional automatic, the timing of the two clutches’ engagements determines the shift speed and quality. Lots of overlap and slow engagements produce butter-soft shifts for everyday driving at part throttle; however, this method generates more heat and wear in the clutches. Whack open the throttle, and the transmission control will snap each clutch on and off with minimal overlap, reducing wear and shortening the time between shifts.

Smart controls always try to anticipate the next gear, actually learning from mistakes and creating a database of driving situations. Yes, they can learn based on the driver’s actions.

Most dual-clutch transmissions also offer different shift modes, such as “normal” for regular driving and a “sport” or “supersport” setting for more aggressive performance. Depending on the car, the different shift modes impact shift quality, shift points and even throttle-to-pedal mapping.

One of the coolest features of a dual-clutch transmission is the automatic rev matching used during downshifts. To save wear on the synchros, the engine control unit blips the throttle after disengaging the upper clutch and before shifting into the lower gear. Most dual-clutch cars will also skip gears as needed on downshifts.

These big downshifts can also happen in a blink of an eye. Porsche claims a sixth-to-second downshift time of four-tenths of a second on cars equipped with the PDK Sport Chrono package. (The normal PDK transmission needs a whole half-second to perform the same feat.)

The Flip Side

So what’s the downside? The biggest detraction we’ve found with most dual-clutch transmissions is their weight; double the shafts, double the clutches and a high-pressure hydraulic control system are all necessary to make a dual-clutch a reality. Newer transmissions with dry clutches from VW have the potential to be a little lighter, but time will tell.

The new Porsche 911 Carrera S, for example, picks up 88 pounds when equipped with the PDK transmission. However, the driveline efficiency beats the weight gain, as the PDK-equipped car can reach 60 mph faster than one fitted with a traditional manual transmission: 4.3 vs. 4.5 seconds. (BMW shows a similar performance advantage for their M Double Clutch Transmission, as the unit shaves the M3’s zero-to-60 times from 4.7 seconds to 4.5.)

There have also been some questions regarding the dual-clutch transmission’s ability to handle power. Current models appear to be strong enough for the engines they’re paired with, but there isn’t much of a cushion regarding their ultimate ratings. Some, like the ones bolted in the latest Evo, have seen their torque limits exceeded with rather expensive results. 

Hard launches have also presented some problems. In theory the transmission control should be able to regulate slip far better than a human driver, thus providing the most traction possible. Computer controls should also be able to keep the torque load on the engine high enough to maximize boost at the same time for an impressive, asphalt-tearing launch.

So-called launch modes have also been the focus of controversy in the GT-R and Evo, among others. Some manufacturers have attempted to deny warranty claims after use. Revisions that neuter the launch mode’s ferocity have also been seen.

Finally, there’s the cost issue. This technology doesn’t come free. The twin-clutch Lancer Evolution MR comes with several upgrades but retails for $39,710, a $5400 bump over the standard Evolution GSR. Porsche gets an additional $3420 to put a PDK in a Boxster, while BMW charges $2900 to put the futuristic tranny in their M3. Adding one to a VW Jetta TDI tacks $1100 to the final tally.

Robots Are Everywhere

Dual-clutch transmissions are still fairly new on street cars, but the first wave has produced impressive results. Many people expect this technology to increase its market share in the future. It’s a win-win situation for 99 percent of the motoring public.

We know that there will be some kicking and screaming from the hardcore set—our readers, basically—but these dual-clutch transmissions are the closest thing yet to a replacement for the old-school clutch pedal. Already, cars like the Nissan GT-R are being sold without a traditional manual option, and it’s just a matter of time before more manufacturers follow in their footsteps. 

No matter what happens to the market, future dual-clutch transmissions are likely to be stronger, lighter, less expensive and more efficient than the current ones.