Dual Clutch Transmissions (DCT)
Gearbox layouts for DCT
All three principle gearbox layouts of MT / AMT gearboxes can be enhanced and designed as a DCT gearbox. This requires that the gearbox input shaft is doubled and turned into two concentric input shafts – the one is a solid shaft, the other is a hollow shaft around the solid one, each driven by one of the clutches. The synchronizers used as shifting mechanism follow the same principle as presented for MT / AMT transmissions.
Two-shaft DCT gearbox layout
In case of a DCT gearbox based on the two-shaft principle, forward gears are achieved as usual by gearwheel pairs between the output shaft and one of the input shafts (only 1-1 gearwheel pair shown in the figure). The hollow input shaft is shorter than the solid input shaft, therefore the solid shaft is also accessible, and the gearwheel pairs between the solid input shaft and the output shaft can be located on the rear part of the gearbox.
The gearwheel pairs are distributed between the input shafts in a way that the pairs of the odd gears belong the to one shaft, the pairs of the even gears but to the other input shaft. This separates the transmission into an odd and an even sub-transmission. The sub-transmission of the current gear is called the active sub-transmission, the other is the inactive one.
While a gear is engaged in the active sub-transmission, the clutch of the inactive sub-transmission is open, and the gears of it can be shifted. It means that depending on the actual driving situation, the next higher or next lower gear can be engaged by the control unit in the inactive sub-transmission, preparing for an upshift or a downshift.
When it comes to the actual gear change, the new gear is already available, only the active clutch needs to be opened and the inactive closed at the same time, which ensures continuous drive torque from the engine also during the gear changes.
When changing the gears sequentially (one-by-one), a DCT transmission feels like an AT, but the powershifting is limited to gearshifts when the sub-transmissions are swapped. No powershifting is possible in case of double gear jumps, e.g. from gear 7 to 5.
The reverse gear is achieved using a reverse shaft just like in an MT / AMT gearbox. The reverse gear can be attached to any of the input shafts and can be part of either the odd or the even sub-transmission, as powershifting between the reverse and forward gears is not required.
The two-shaft DCT gearbox layout is used typically with longitudinally oriented engine up to 7 forward gears. In case of more gears or in case of transversal engine, the more compact three-shaft layout is used.
Three-shaft DCT gearbox layout
Converting a three-shaft gearbox into a DCT gearbox is done on the same way as for two-shaft gearboxes and three-shaft DCT gearboxes have the same advantage in terms of installation length as explained for the MT / AMT gearboxes.
The forward gears are achieved by gearwheel pairs between one of the input shafts and one of the output shafts. Here again, the hollow input shaft is shorter than the solid input shaft, therefore the solid shaft is also accessible, and the gearwheel pairs between the solid input shaft and one of the output shafts can be located on the rear part of the gearbox.
The gearwheel pairs are distributed between the input shafts in a way that the pairs of the odd gears belong the to one shaft, the pairs of the even gears but to the other input shaft.
The output shafts are functionally equivalent, which means that each input shaft can be used with any of the output shafts or with both of them, for odd or even gears as well.
The single dependency (as described for MT / AMT gearboxes) is applied also for DCT gearboxes in order to save installation space, which means that one gearwheel on one of the input shafts is connected to both output shafts. If needed, double dependency might also be applied.
The separation of the transmission into an odd and an even sub-transmission, and the swapping of the sub-transmissions is the same as described for the two-shaft DCT gearbox layout.
The principles of achieving the reverse gear are the same as described for the three-shaft MT / AMT gearbox layout, please refer to the description given there. As the powershifting is not required between the forward and the reverse gears, the reverse gear can be assigned to any of the clutches (odd or even forward gears).
Countershaft DCT gearbox layout
The dual clutch principle was introduced into the commercial vehicles keeping the countershaft gearbox layout, which was already proven in that vehicle category for MT / AMT transmissions. The transformation of a countershaft AMT into a DCT is however not quite straightforward, and such DCT gearboxes show great variation in their particular design.
The most common layout has not only dual input shafts but also dual countershafts. As usual, the two inputs shafts are concentric, there is a shorter hollow shaft around the solid input shaft.
Typically, each input shaft has only one gearwheel and drives a dedicated countershaft with a fixed ratio. The two countershafts are also concentric, just like the input shaft, the one countershaft is a solid shaft, the other is a hollow shaft around the solid one. The hollow input shaft drives the solid countershaft, and the solid input shaft drives the hollow countershaft.
Sometimes it is hard to recognize the hollow countershaft in a particular gearbox, because it might be very short with just two gearwheels right next to each other.
The countershafts can be locked together with a shifting mechanism, and additionally, the solid input shaft and the output shaft can also be locked together. This allows that both input shafts are capable of driving any of the gearwheel pairs, which increases the number of available gears, including a direct drive.
In general, this layout enables a high variety with respect to the power flow in the particular gears: through the hollow countershaft, through the solid countershaft, from one countershaft to the other or as direct-drive from the input shaft directly to the output shaft.
The reverse gear is achieved using a reverse shaft between the solid countershaft and the output shaft. Thanks to the shifting mechanism between the two countershafts, the reverse gear can be driven through both of the input shafts, which means that potentially there are two reverse gears which are powershiftable.
It is worth to mention that there is an alternative solution to achieve a DCT gearbox, using a single countershaf onlyt.
In this case the input shafts and the single countershaft work as a two-shaft DCT gearbox, but on top of that, there is a constant gear ratio from the countershaft to the output shaft. Here again, the direct coupling between the input and the output shafts enables an additional forward gear without additional gearwheels.
↑ Back to the top.
← Back to the DCT home page.