We have witnessed a tremendous rise in applications of twin clutch transmission in recent years. Manufacturers may call such technology by different names such as DSG (VW), Powershift (Ford), S Tronic (Audi), PDK (Porsche). I won't be surprised if torque converter equipped transmission makes it to the museum in 10 years' time. Let's take a detailed look at the future of automatic transmission.
Twin-clutch gearbox was first put into production by BorgWarner, which calls it "DualTronic". It was first used in Audi TT 3.2 in the name "DSG" (Direct-Shift Gearbox). Like automated manual gearbox, BorgWarner’s DualTronic can operate as a semi-automatic, where the driver changes gears via buttons, paddles or conventional shifter. There is no clutch pedal, because the clutch is automatic while the gearshift is implemented by electro-hydraulic actuators. For relax driving, there is also a full automatic mode, where computer determines which gear to be selected.
Unlike conventional gearboxes, DualTronic uses 2 clutches - one clutch connects to the odd gears (1st, 3rd and 5th) while another clutch connects to even gears (2nd, 4th and 6th). This enable it to shift far smoother and faster than conventional gearbox.
When a driver wants to change gear in a conventional gearbox, he presses down the clutch pedal, thus the engine is disconnected from the gearbox. During this period, no power is transmitted to the gearbox and the driver can shift gears. When it is done, he engage the clutch again and power is again transmitted to the gearbox. The power delivery change from ON to OFF to ON during gearshift. How smooth the change depends on how skillful the driver cooperate the clutch and throttle. Automated gearbox like Ferrari F1 is similar. The only difference is that the clutch and gearshift are operated by computer via hydraulic actuators. The ON-OFF-ON power delivery still exist. In contrast, an automatic transmission with torque converter does not has this problem.
Twin-clutch gearbox can overcome the ON-OFF-ON problem, thanks to the twin-clutch design which enable it to "pre-select" the next gear. Assuming the car is accelerating at 2nd gear, the clutch controlling the even gears is now engaged while another clutch is disengaged. From the data taken at throttle position and rev counter, the computer predicts that the driver will upshift, thus it will connect the 3rd gear. Because at this moment the clutch for odd gears is disengaged, the pre-selection of 3rd will not affect the 2nd gear currently running. When the driver touches the gear-shift paddle, computer signals the even-gear clutch to disengage and simultaneously the odd-gear clutch to engage. In this way, gear is changed from 2nd to 3rd instantaneously, without any OFF period or delay.Hence, power delivery is smooth and uninterrupted.
Pre-selection of gears quicken the shift a lot. Upshift takes just 8ms but downshift is less impressive as the gearbox need to wait for the throttle blip to match gearbox speed with the engine speed. Changing down a few gears could be more complicated. The most complicated is from 6th to 2nd (both are controlled by the same clutch while the distance between the two gears is the longest). It needs to change to 5th (controlled by another clutch) temporarily before 2nd is selected. The whole process takes 900ms.
However, twin clutch designs are not without it's drawbacks. They are more complex in design and hence consumers may have a concern over the long term reliability. In addition, in some applications such as the 7 speed DSG in the VW Jetta, there is a limit to the amount of torque it can handle. This certainly does not go down well with tuners!