Dedicated Hybrid Transmission
Plug-in hybrid vehicles combine local emissions-free driving with low consumption in hybrid operation and a high degree of driving pleasure. In addition to this, more stringent legislative requirements will see increases in the battery capacity and the electrical output as performance increases. This results in greater challenges with respect to spatial integration and the overall design of these types of drive trains. The greater costs of the electrical components make it necessary to take every opportunity to simplify the technical design.
Figure 1 Function of dedicated hybrid transmissions with one or two electric motors
Many drive structures can include more than one electric motor and thus provide a range of different ways of meeting the specific requirements. As with concepts with only one electric motor, the basic question arises of whether the required comfort is achieved with continuously variable operation, for example by power split or serial functionality. As the high levels of efficiency in plug-in hybrids must be maintained when operating the internal combustion engine, this requires a corresponding gear ratio spread.
Two structures, each with only one electric motor, are selected for this investigation in order to reduce the complexity of the system.
The first concept is based on a CVT which is characterized by high comfort levels and good dynamics; the reverse planetary gear set and the associated shifting elements are dispensed with here. The second concept is based on an automated manual transmission and is intended above all for markets such as China and Europe where stepped transmissions reach significant volumes.
Figure 2 Hyundai Motors CVT transmission with the Schaeffler 05 chain
The basic characteristics of continuously variable force transmission are particularly well suited to the power delivery of an electric motor whose operating points can also be freely selected over a wide range of speeds. In addition to this, the CVT also allows the internal combustion engine to operate efficiently.
Continuously variable transmissions which incorporate an additional electric motor on the engine side are already in volume production. However, these transmissions use a planetary gear set for driving in reverse. In the future, it will be possible to achieve reverse driving purely via the electric motor due to increasing torques in the electric drive and greater battery capacity. This dispenses with the planetary gear set, the actuator required to shift the gear set and a clutch. The characteristics of the electric motor allow the torque converter to be removed, too. All other operations correspond to those of a classic P2 hybridization, see Figure 3.
Figure 3 Operating modes of the DH CVT with electric reverse gear
Figure 4 Cross-section and construction of the DH CVT
Figure 5 Comparison of the of the output requirements of a standard hydraulic unit with EPA actuation technology
Figure 6 DH CVT hydraulic unit with innovative EPA actuator technology
Figure 7Figure 7
Figure 7 Tip-in maneuver. Left: Time dependency of variator ratio and torque. Right: Output uptake for both EPA actuators (dotted line) and maximum continuous mechanical output for the electric motors The previous section shows that the simulation tools developed by Schaeffler allow an optimum design of the pump size, the motors and the actuator electronics to be found for the specific customer requirements. The compact EPA enables such highly efficient CVT-actuation concepts.
Figure 8 Cross-section and design of the DH ST 6+2 with six combustion engine and two electric gears
Figure 9 Transmission schematic for a hybrid transmission with six combustion engine and two electric gears
This combination of a single clutch and three shift elements allows further drive conditions and power flows to be achieved, for example a purely electric reverse gear. Gear selection is via a single actuator which can also operate the parking lock.
A significant benefit of this design is that the tractive force can be augmented by the electric motor during combustion engine shifts. This structure provides purely electric driving similar to existing purely battery-operated electric applications. After switching modes from electric to hybrid driving and the associated internal combustion engine start-up, the electric motor can be shifted to second gear. This allows comfortable shifting in the upper gears due to the augmentation of the tractive force. At high driving speeds, the six gears used with the internal combustion engine and the second electric gear provide favorable consumption and noise reduction.
When designing the transmission, it was assumed that the target maximum speed is greater than 200 km/h and that electric driving up to 140 km/h should also be possible. If the acceleration target and, if necessary, the requirement for shift comfort with a high accelerator pedal position are reduced, smaller electric motors can be used. A value-oriented variant with a peak electric output of, for example, 100 kW (maximum torque 170 Nm) is feasible in addition to the performance variant of the transmission, see Figure 10.
Figure 10 Axle torque as a function of the driving speed for two variants of the electric motor (147 and 100 kW) as well as two different ratio stages in the electric gears
There is a basic assumption that plug-in hybrid vehicles will cover significant distances in electric mode, given sufficient battery capacity. Beyond a certain driving speed or a required driving torque, the internal combustion engine can be started dependent on the battery charge. This is achieved by pre-selecting a suitable gear and closing the friction clutch. A pinion starter is therefore not required, see Figure 11. As soon as the internal combustion engine achieves driving torque, the electric motor can be shifted up. Moreover, the shift point from first to second electric gear can be adjusted depending on the load condition.
Figure 11 Internal combustion engine gear selection and start-up process
Further arrangements of the DH ST solution are feasible. A coaxial electric motor could be used or the number of gears for the internal combustion engine reduced to three or even to the point where the clutch is no longer required as far as the vehicle launch is achieved exclusively via the electric motor.
Figure 12 Comparison of the fuel efficiency of various hybrid drives in charge sustaining mode
Figure 13 Criteria with allocation by market and segment for the DH CVT and DH ST
Dedicated hybrid drives offer considerable potential to further increase the efficiency and driving dynamics of plug-in hybrid vehicles. Focusing on the use in hybrid vehicles allows system characteristics to be improved whilst simultaneously reducing the overall complexity of the drive train.
The hybrid transmissions presented here based on a CVT represent the logical further development of the current P2 arrangement. The mechanical reverse gear, which is usually achieved with a planetary gear set, is dispensed with completely. A new concept in actuator technology based on separate pump actuators provides further significant reductions in the hydraulic losses and therefore high levels of efficiency.
Schaeffler’s dedicated hybrid transmission based on an automated manual transmission takes the concept of a transmission a step further by offering a very compact design with six ratio steps for the internal combustion engine and two gears for the electric motor. The mechanical power path allows a very high degree of overall efficiency in the drive train.
Both the concepts discussed here, the DH CVT and the DH ST 6+2, are by no means the final say in the field of hybrid transmission technology. They open up an attractive field of collaborative work in the development of drive trains.
The low overall quantities of hybrid vehicles have lead to the electric drive being primarily a P2 arrangement between the internal combustion engine and the transmission and the classic drive train components remaining largely the same. The expectation of increasing quantities means that optimization of the overall system is becoming a priority . This also includes the option of simplifying the mechanical transmission, possibly by removing the reverse gear and integrating at least one of the electric motors into the transmission to take over this function completely. These transmission concepts are examples of Dedicated Hybrid Transmissions (DHT) .
Data maps are also used here as there was a very high emphasis on the accuracy of the model in these simulations. So, for example, a hybridized double clutch transmission with wet-running multi-disk clutches from a volume application with actuator technology optimized for efficiency  is measured on a test rig and the data map is subsequently scaled. The shifting strategies for various wet-running double clutch transmissions are also already known from comprehensive vehicle tests. The model is also compared to these strategies.
 Shinbori, I.: Transmission which has built-in electric motor for hybrid electric vehicle. In: Conference Report 12th Internationales CTI Symposium Automotive Transmissions, HEV and EV Drives, Berlin, 2013