Dual clutch transmissions (DSG) shift quickly and efficiently. But how does the transmission work? We explain the technology, the controls and the advantages and disadvantages of the automatic transmission.
Let us begin with the name confusion surrounding the double-clutch transmission. Volkswagen calls it DSG direct shift gearbox, Audi says S Tronic, Power Shift at Ford, DCT at Mercedes and Kia, PDK at Porsche and TCT at Alfa, to name just a few. What is meant is always the same, a mechanical manual transmission with two clutches consisting of two partial transmissions, which can be shifted both automatically and often by a paddle on the steering wheel.
Always two gears
The basic principle of the dual-clutch transmission is very simple: a six or seven-speed transmission is divided into two work areas. One gearbox is responsible for the odd gears, the other for the even gears. Each of the two sub-transmissions is connected to the flywheel and thus the crankshaft of the engine via its own clutch, either a dry clutch for smaller engines or a multi-disc multi-plate clutch running in an oil bath. Although it can cope with significantly higher torques beyond 250 Nm, it has a somewhat poorer efficiency.
The secret of quick gear changes is that there are always two gears, one is active, the other is in standby mode. If you drive through a town in fourth gear, for example, the fifth stage is already engaged in the second sub-transmission but runs empty with the opened clutch. If the town sign has passed and the driver accelerates, the control electronics cause the clutch for partial transmission 1 to be opened in a split second and the other to be closed at the same time. Now fifth gear gets the torque of the engine on its gears, can convert it and pass it on to the drive shafts. The fourth gear now has a break.
Shift without interruption of traction
The nice thing about the DSG is the lack of interruption in tractive power when shifting because the clutch processes are run so quickly that they can only really be felt under load. So the process of accelerating is continuous.
The control unit ensures that the correct gear is selected based on a large amount of information that is supplied by sensors. When fuel-saving driving, the next higher gear is engaged early, if it is to go ahead vigorously, the engine may turn higher, the gearshifts take place later. The control can also skip gears in certain driving situations. This applies both when accelerating and vice versa when decelerating, where the transmission shifts down automatically.
The dual-clutch gearbox is different: it costs about half the cost and works on a completely different principle. The automatic system recognizes which gear needs to be selected next – when accelerating, of course, this is the next higher. This gear is already engaged in parallel. The power flow is interrupted by a second clutch.
When the best time for the switch has come, the second clutch opens and thus establishes the frictional connection without interruption. The dual clutch transmission thus moves the actual shift point into a period in which the duration is less important. With the DSG and PDK, this makes it possible to change gear without interrupting traction . This applies to both sports cars, small cars and even tractors in agriculture. Thanks to the ideal gearshift timing, the direct shift gearbox reduces consumption and at the same time improves the vehicle’s acceleration values. In addition, more gear steps can be integrated, with the PDK from Porsche there are at least 7.
CONCLUSION
Dual-clutch transmissions are real alternatives to the expensive automatic converter. They shift extremely quickly and help save fuel. A disadvantage compared to the converter is the lower starting comfort and the higher temperature sensitivity – this applies especially to gearboxes with dry-running clutches.
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