The Top 3 of P2
Dipl. Ing. Joachim Hoffmann
Dipl. Ing. Karl-Ludwig Kimmig
Dr.-Ing. Andreas Baumgartner
Dr.-Ing. Knut Erdmann
Dipl. Ing. Wolfgang Haas
Dipl. Ing. Philippe Wagner
Figure 1 Schematic diagram of P2 hybridization in connection with a double clutch transmission
As an add-on concept, P2 hybridization only requires average interventions in the transmission structure, and it enables modular electrification within existing drive modules. The upshot of this is that it has already been possible to combine several hybrid powertrains with a double clutch transmission in series production in recent years. The systems shown in Figure 2 are different with respect to the arrangements and designs of the three clutches required for the system. For example, the system shown on the left shows an axial arrangement of the electric motor and disconnect clutch for a dry double clutch transmission. The representation in the middle shows the Schaeffler hybrid module with the dry C0 integrated in the rotor, which is used in combination with a wet double clutch. In contrast, the system on the right depicts three wet clutches that have already been very compactly coupled with the electric motor of the hybrid module.
Figure 2 Implemented series solutions for P2 hybridization of double clutch transmissions
What all three systems have in common is that the axial length of the transmission is 80 to 120 mm longer than in a non-electrified transmission because of the hybrid module. The increase in length in a particular application is heavily dependent on the electrical output and thus on the size of the electric motor and the axial extension of the clutch system. The challenge for future solutions is to greatly reduce the additional need for installation space and, at the same time, satisfy the demands for operating life and torque capacity. Derived from this are fields of activity for optimizing the installation space in the clutch system and within the transmission.
The demands placed on the clutch system change through the electrification of the powertrain. If a high-voltage hybrid module is used in a P2 arrangement, then it is possible to use the electric motor to launch a vehicle even without significant clutch slippage. Depending on the battery capacity and battery management, the result is a considerably reduced clutch load. This is clear when the integral of the speed difference between the engine and transmission input shafts of a classic double clutch transmission is compared with that of a P2 hybrid transmission during the launch phase – Figure 3.
Figure 3 Clutch slip during vehicle launch with a combustion engine (left) and an electric motor (right)
However, this raises new challenges. For example, the thermal power loss of the electric motor needs to be taken into account when designing the clutch. Moreover, additional friction energy is produced in the CO disconnect clutch due to repeated engine starts while driving.
In order to quantify the changing requirements, Schaeffler conducted extensive measurements and simulations in the course of series development projects. The goal was to determine the cumulative friction energy registered for all three clutch types within a realistic driving cycle for each specific driving situation. Figure 4 shows the result of such measurements for a double clutch transmission with vs. without P2 hybridization.
Figure 4 Friction energy of the clutches for double clutch transmissions, with and without P2 hybridization
Based on the measurements, it is evident that the current P2 double clutch transmissions in series production have already reduced the clutch load by 50% – particularly during launch. By developing the systems further, including with respect to optimum system dimensioning, and the output of the combustion engine, the electric motor, and the battery capacity, the possibility of further reducing the friction power and the friction energy in the triple clutch system can be assumed. Friction power and friction energy are important factors when designing a clutch and directly affect the size of it. In future P2 hybrid system generations, it will be possible to accomplish further reductions in the friction energy load with an even more reduced need for installation space through smaller clutches. As with today’s non-hybridized double clutch transmissions, it is possible to employ wet and dry clutch solutions as a rule, whereby wet clutch solutions are generally used for larger and heavier vehicles, with dry solutions utilized for smaller and lighter vehicles – Figure 5.
Figure 5 Energy needed for vehicle launch with various stages of electrification of wet and dry double clutch transmissions
Figure 6 P2 hybrid module with a rotor-integrated, wet triple clutch system
The basic design concept involves the radial and axial nesting of a wet triple clutch with the corresponding actuator system as much as possible to save space, thereby achieving the required torque and cooling capacity. The magnitude of the clutch torques that are to be transferred can be specifically set via the number of plates and the available actuating force. For example, the C0 disconnect clutch was situated radially inside the C1. In turn, the C2 is positioned axially to the C1. The bearing support of the triple clutch is via the main rotor bearing in the housing, which helps give the hybrid module a high level of overall efficiency and provides it with a very compact structure.
The three clutches are actuated via standing CSCs (concentric slave cylinders) with ring pistons, positioned on the transmission side for C1 and C2 and on the engine side for C0. The force is transferred to the spinning clutch by means of engagement bearings. Since this form of actuation is practically leakage-free, the clutch actuation system works very efficiently. With the small radial dimensions of the plate pairs and optimized cooling oil flow, only minimal drag torques occur in the open clutches.
The clutches are completely integrated in the oil/cooling circuit of the hybrid module. For efficiency reasons, care has been taken to adapt the volume flow of the oil to the cooling performance actually needed in the clutches – Figure 7. A partial flow of the respective cooling oil path is used for cooling the C0 and supplying the bearings. The oil flowing out of the outer clutch housing also flows along the rotor and partly along the stator of the electric motor, thereby contributing to the extraction of heat from there as well.
Figure 7 Oil flow inside of the rotor for the sub-clutches and the bearing positions
While designing the thermal aspects, it was possible to ensure uniform heat dissipation from all plates of the multi-plate clutches through the design of the oil flow channels. Moreover, the oil is transported away very quickly from the friction contact so that no unnecessary drag losses occur. Using rotor coo-ling, we have succeeded in keeping heat-sensitive magnetic materials within an acceptable temperature range below 150 °C, even in high load situations, thereby increasing the power density of the electric motor. Extensive flow and temperature simulations with corresponding optimizations were conducted – Figure 8 – and the layout was validated using a testing system developed by Schaeffler. In addition to ensuring the torque and thermal capacity, the focus of the testing was also on optimizing the drag torques and losses in the complete system. On the basis of many realistic test stand trials, it was possible to be demonstrated that the compact wet triple clutch system reliably meets the requirements with regard to function and operating life.
Figure 8 Flow simulation for a P2 hybrid module with an integrated wet triple clutch
As with many clutch systems, a central role is attributed to the tribological system – consisting of a friction lining, counter friction faces, and cooling oil. The functional characteristics of torque capacity, thermal robustness, NVH behavior, and open clutch drag torque are heavily influenced by the friction lining. In order to really do justice to this fact, Schaeffler has also developed a suitable lining for wet double and triple clutch systems – in addition to linings for dry clutches – and has validated it in extensive test series – Figure 9. This has resulted in very good values for both the torque capacity as well as for drag torques in the sub-clutches. The drag torques in the C0 were able to be greatly reduced yet again through the geometric design, optimization measures in the friction plate, and minimization of the cooling oil volume flow.
Figure 9 Wet lining for double and also triple clutch systems
Figure 10 Further development of the wet triple clutch
Dry Integrated Triple Clutch System
Primarily dry (i.e. air-cooled) double clutches are used for non-hybridized transmissions in the lower torque and performance classes. The question now arises as to how a dry triple clutch system optimized for the installation space can be designed in combination with a coaxial P2 hybrid system and whether a similarly high degree of integration is possible as with the rotor-integrated wet triple clutch.
The first approach is to combine a rotor-integrated C0 disconnect clutch with a radially nested double clutch, whereby the double clutch is positioned on the rotor of the electric motor – Figure 11. Compared to a dry standard double clutch, this makes it possible to save 20 to 25 mm of axial installation space. The C0 disconnect clutch actuated on the engine side is integrated in the rotor of the hybrid module and therefore has nearly no effect on the installation space.
Figure 11 Radially nested dry triple clutch
In this design, the electric motor is protected from heat and clutch abrasion from the dry triple clutch by a central sheet housing. In addition, the triple clutch unit can very easily be screwed onto the ready-mounted electric motor. This arrangement is particularly advantageous for medium electric engine outputs of around 60 kW, especially when combined with a short electric motor.
If higher electric motor outputs and thus electric motors with greater axial lengths are used, then it is usually possible for even dry triple clutches to be integrated into the rotor – Figure 12. The number of clutch friction faces varies depending on how high the required clutch torques are. This means that multi-disk clutches can be designed even for dry triple clutches, thereby allowing even more significant axial reductions in the installation space to be achieved.
Figure 12 Triple clutch integrated inside the rotor
Further Development around the Clutch
In today’s triple clutch systems that are ready for series production, two radially nested coaxial ring pistons are used for clutch actuation. The actuator system for the C1 and C2 clutches is ideally located between the main bearing of the transmission input shaft and the auxiliary shaft bearings. Due to geometric restrictions – mainly in a radial direction – caused by the position of the auxiliary shaft eyes, the ring pistons cannot be optimally integrated in many application cases. In order to compensate for the need for installation space despite this radial restriction, the outer ring cylinder can be replaced by three interconnected individual piston release mechanisms as an alternative. They can then be positioned between the auxiliary shaft eyes without having any effect on the installation space – Figure 13. An additional need for installation space of up to 30 mm can be avoided this way.
Figure 13 Further development of the double actuator system for C1 and C2 (3-piston actuator system)
Figure 14 Arrangement variations for the centrifugal pendulum absorber in the P2 hybrid system
Figure 15 Axially parallel arrangement of a P2 system with a triple wet clutch
The axially parallel arrangement poses special challenges for the mechanical design, particularly with regard to the bearing concept for the clutches and the rotor.
Synchronization Devices with a Minimal Overall Axial Length
The concept, which has since been developed much further, makes it possible to reduce the need for axial installation space by up to 10 mm in each synchronization direction – Figure 16. This reduces the overall length by approx. 20 mm in a typical three-shaft transmission setup with two synchronization units arranged axially in a row. A key prerequisite for reducing the overall length is the production technology for producing gearing with a small module. Gearing for synchronization devices is produced through machining – similar to larger gearwheels in transmissions. In contrast, Schaeffler is relying on sheet-metal-formed gearing, which makes it possible to increase the number of teeth by reducing the module from 2 down to 1 mm. Using a larger number of teeth makes it possible to achieve an identical torque capacity despite the fact that the teeth are smaller. This measure alone allows the overall axial length to be shortened by approx. 6 mm per synchronization device, since the smaller teeth lead to shorter axial point lengths with the given point angles.
Figure 16 Optimizing installation space by using shorter synchronization units, taking the Schaeffler Short Synchro as an example
Even though smaller gearing modules are used, relevant parameters are staying the same: width of the friction cones, load on the gears, the clearance for drag torque, and wear reserve. Depending on the application, the installation space gained can also be used for designing gearing for higher torques.
A further reduction of around 3 mm can be achieved with a new pressure piece design for the sliding sleeve. A radially flat pressure piece makes it possible to decrease the wall thickness of the sleeve carrier at the bar accordingly. This allows the cone surfaces of the synchronization rings that are decisive for the friction performance to remain the same. The power loss due to the friction is not greater compared to today’s synchronization devices.
The shorter axial length additionally reduces the shift travel by approx. 2 mm. This is equivalent to a shift travel reduction of up to 25%, allowing for a corresponding dynamic design. In vehicle tests, the shift quality was evaluated positively even for manual shifting. Another secondary effect is the weight savings obtained. If the axial installation space gained is used for shorter shafts and a correspondingly smaller housing, the weight savings add up to 3.5 kg.
Rotary Actuated Gearshifts
Another possibility for greatly reducing installation space involves innovative rotary actuated transmission shifting, in which the entire axial movement of the sliding sleeve takes place inside the gearwheel contour, meaning that there is no longer any need for external installation space. In transmissions with large gearwheels, such as when the gearwheel of first gear is situated directly next to the gearwheel of third gear, it is possible for such a system to be arranged completely under the gearing. This shrinks the idler gear distance significantly so that another 5 to 10 mm of axial installation space can be saved at each synchronization unit.
The module shown in Figure 17 shows such a shifting system. Actuation merely involves a swiveling rotation by an actuation wheel. The sliding sleeve on the inside is moved axially by a ring nut over a sliding surface. The ring nut has three slide cams and is turned from the outside via the actuation wheel. A threaded sleeve fixed to the housing absorbs the axial forces.
Figure 17 Synchronization unit with a rotary actuation under the gearwheels (Schaeffler Ultra Short Synchro)
Another advantage is that the actuator for shifting – preferable in P2 double clutch transmissions – can be used in a direct acting actuating motor. In this way, shift rods and other actuation elements can be eliminated, thereby reducing the weight even further.
Currently, these ultra-compact synchronization units are at an advanced stage of development. However, initial tests with prototypes show considerable potential for this technology, which may be used in completely newly developed transmissions.
In order to maintain the given installation space with P2 hybridization of double clutch transmissions, Schaeffler – as shown in the statements above – is making a large number of product innovations and further development solutions available. Many of the product ideas in the P2 hybrid module and in the transmission part can be combined in such a way that the overall transmission length can remain nearly the same compared to existing double clutch transmissions, despite the powerful P2 hybridization – Figure 18.
Figure 18 Measures for reducing the overall transmission length of P2 hybrid double clutch transmissions
Thanks to the combination of dynamics, efficiency, and comfort, double clutch transmissions are continuing to gain ground on the global transmission market. The systems designed so far are proving to be very suitable for electrification through the integration of P2 hybrid modules. However, the fact that the hybrid module is situated between the combustion engine and the transmission means that the powertrain is significantly longer. Schaeffler has developed a large number of product innovations for reducing the axial installation space, which will be able to be implemented in future series applications. An important basis for reducing the installation space has to do with the fact that P2 hybridization and a powerful battery reduce the maximum and average loads for the clutch system to less than 50 % compared to conventional double clutch transmissions since vehicle launches are largely carried out under electric power.
By taking these boundary conditions into account, it has been possible to develop and test highly integrated wet and dry triple clutch systems, in which the clutches are located almost completely within the installation space of the electric motor’s rotor. To actuate the three clutches, compact, highly integrated actuating pistons needed to be developed, with a three-piston engagement device being used for the first time as well.
Other measures for reducing the installation space can be implemented within the transmission itself. For this purpose, Schaeffler has developed technologically new synchronization units that enable double clutch transmissions to be built that are 15 to 20 mm shorter. Since the various solutions for clutch, actuators, and transmission can be combined almost at will, a total installation space reduction of up to 100 mm can be achieved. In this way, P2-hybridized double clutch transmissions are able to preserve common vehicle installation space dimensions.
By introducing double clutch transmissions, it has not only been possible to increase driving dynamics and comfort in the last ten years, but also to make great strides in the efficiency of modern vehicles. In order to further reduce climate-damaging CO₂ emissions and achieve future limiting values, however, it will be necessary to slash fuel consumption levels even more. A key concept for this is P2 hybridization combined with a double clutch transmission . In P2 hybridization, a powerful electric motor is installed (coaxially or even axially parallel) between the combustion engine and the transmission – Figure 1. In addition, an automated C0 disconnect clutch is needed so that the electric motor can also be used independently of the combustion engine for pure electric driving.
As explained in detail in , the particular advantage of combining a double clutch transmission with a P2 hybrid system rather than a P2.5 hybrid system with fixed coupling of the electric motor and a sub-transmission is that the gear ratio function of the double clutch transmission can be used in all operating modes (purely electric driving, hybrid mode, and battery charging). This results in a high level of overall energy efficiency. It is also possible to drive in any situation without interrupting the tractive force, making for extremely comfortable driving.
Schaeffler has developed a wet triple clutch system to series-production readiness according to the above-mentioned requirements with the C0, C1, and C2 clutches. It can be integrated almost completely in the rotor of the electric motor, thereby fitting in the latter’s overall length . The complete system consisting of the hybrid module, triple clutch, and transmission is very compact. For this reason, in comparison to non-rotor-integrated solutions, the axial transmission length can be reduced by 50 to 70 mm – Figure 6.
Since even lower clutch loads are to be expected in future applications, it will be possible to raise the design values of the friction system and shrink the clutch packages even more. The two-layer Schaeffler wet friction facing, which is still in the developmental stage, will support this potential for optimization through its enhanced performance . In this way, only the length of the electric motor will ultimately be decisive for the axial installation space required. Figure 10 depicts such concepts, including a version with hydraulic rotary transmissions on the right. Other innovative approaches for the next generation of wet triple clutches are installation space optimized needle roller bearings in the actuation system, a low hysteresis actuator piston, a rotor bearing arrangement that is very resistant to tilting, and low mass inertias on the output side. The installation space required by the hybrid module is reduced to a minimum when combined with the latest compact electric motor developed.
Due to the electric driving elements NVH performance is even more important in vehicles equipped with a high-voltage hybrid system. As shown in , the phase current variation of the e-machine can be used to optimize the vibration behavior of the entire powertrain. Depending on the vehicle class, the customer engine used, and the demand for comfort, it may make sense to integrate a centrifugal pendulum absorber (CPA) as an additional damping element. As an alternative to the classic arrangement at the dual-mass flywheel, position a in Figure 14, an installation position at the rotor/clutch flange on the transmission side, position b in Figure 14, is also possible. Since installation space needs to be provided at this point for the clutch actuator anyway, such a measure may represent another contribution towards reducing the overall axial length.
With regard to vibration-damping properties, both installation positions have specific advantages and disadvantages, depending on the driving situation. The arrangement on the transmission side is mainly advantageous when the combustion engine is started via the electric motor of the hybrid module (drag start), while the arrangement on the engine side exhibits the biggest advantages when the combustion engine is engaged and running at a low engine speed.
The P2 hybrid systems with a triple clutch that are described above are all arranged coaxially around the transmission input shaft. Depending on the size and capacity of the electric motor, an axially parallel arrangement of the electric motor may be advantageous – particularly for passenger cars with a front-transverse powertrain. Schaeffler has also additionally developed axially parallel P2 hybrid modules  that are well suited to being combined with a double clutch transmission. Figure 15 shows a possibility for further reducing the axial installation space needed for the hybridization through radial nesting of wet plate clutches.
If the transmission structure and number of gears in an electrified double clutch transmission remain the same, then the options for reducing the transmission length itself will be limited. The short synchronization devices already introduced by Schaeffler back in 2014 offer a solution for this .