Development of highly functional dual-clutch transmissions for future commercial vehicles

Total cost of ownership (TCO) will remain the major driver for developments of powertrains for future commercial vehicles (CV). Simulations show that fuel consumption, as a key driver for TCO, may be reduced by powertrains with down speeded combustion engines (ICE) [1]. Adversely such powertrains require a higher shift frequency because of a lack in torque reserve in different driving situations. To prevent vehicle speed losses during shifting it is advantageously if the shifts are carried out with a continuous power flow from the transmission input side to the output side. Such power shifts may be realized with automatic transmissions based on planetary gear sets (AT) or dual-clutch transmissions (DCT).

Further fuel efficiency benefits may be tapped by the hybridization of the powertrain [2]. Such a powertrain hybridization may easily be realized with an electric motor which is placed between the ICE and the transmission. With this implementation of an electric motor the whole powertrain unit would be lengthened which is not preferable. Therefore transmissions with a short axial length are essential. Previously, IAV presented a methodology to determine well suited transmission solutions for one or several applications [1]. This methodology is applied to an exemplary heavy-duty application with offroad usage. Some rough requirements for such an investigation are shown in Figure 1.

Figure 1 Requirements definition for an exemplary heavy-duty application

Requirements for power shiftable concepts:

  • 1st concept: 9-speed AT with min. 2 reverse speeds - Total ratio range (gear set): jtotal ≈ 13 - Torque multiplication from torque converter increases torque range of transmission
  • 2nd concept: 12-speed DCT with min. 2 reverse speeds - Total ratio range: jtotal ≈ 17 - Comparable launch capability is realized by increased ratio range of the gear set
  • High gear set efficiency

In a first investigation a new highly flexible AT could be identified, Figure 2. This AT realizes 9 well stepped forward speeds and 2 reverse speeds. The structural characteristics enable a compact design which allows a modular transmission system with a 9 and 6-speed variant that uses a high share of common parts.

Figure 2 One result of a first concept investigation for an automatic transmission based on planetary gear sets (AT)

Result of 1st concept investigation: 9-speed AT

  • 9-speed AT which fulfills the ratio requirements
  • Low torque loads at gear set and shift elements
  • Use of dog clutches for increasing efficiency and reducing design space
  • Design boundaries: - Max. input torque: 3,500 Nm - Max. input speed: 2,000 rpm
  • Length approximately 980 mm

Development of DCT for future CV

For realizing the highest possible development safety it is necessary to investigate more than one trans-mission technology. Therefore, this paper extends the previous investigations to DCT.

DCT are widespread in passenger cars of several OEMs. For CV there are only two series applications, one for the light-duty-vehicle Fuso Canter and a second one for the heavy-duty-vehicle Volvo FH. The basic concept of both transmissions is different. The light-duty 6-speed DCT is based on a one countershaft layout without a range group, Figure 3 upper left corner w/o range group. The heavy-duty 12-speed DCT is also based on a one countershaft layout but it uses the main concept of heavy-duty transmissions based on helical gear sets with a split-/main group combined with a range group. As the range group is of a conventional design with synchronizers, the transmission is not fully power shiftable through all forward speeds. IAV presented a fully power shiftable concept with 11 forward speeds already in 2009, Figure 3 lower left corner. This concept is a possible solution if full power shiftability is required. With the 7 gear set levels it may be challenging to realize a design for high input torques which is comparable with regard to the axial length to the current state of the art.

Figure 3 Possible DCT layouts

Another option to realize a CV-DCT is to base the concept on a two countershaft layout, Figure 3 upper right corner. With this arrangement the separate gear level for the reverse speed can be changed to a design based on meshing countershafts which shortens the gear set by one level. To identify a suitable solution for heavy-duty CV the computer aided transmission synthesis calculation program was used. Search areas with up to 4 gear set levels and up to 8 synchronizers were investigated. With such configurations a base 6-speed DCT may be derived which is multiplied with a range group. The design space which may be saved by the approach for a more compact base DCT may be used for either a power shiftable range group or for the integration of an electric motor in a P2 arrangement.

Authors of the article

Dr.-Ing. Jörg Müller
Head of Department, Transmission, E-Motor & Hybrid

Dipl.-Ing. Rico Resch
Project Manager, Transmission Synthesis & Concepts

Erik Schreiterer M.Sc.
Development Engineer, Transmission Synthesis & Concepts