Hybrid and Electric Drives
Figure 1 Series launches for electrical drives in 2017 to 2019
In the past few years, a large number of concepts have been developed in the automobile industry for hybrid and electric drivetrains. The driving force behind this is mainly CO₂ legislation and type authorization, which mandate compliance with limiting values for pollutant emissions. The gradual introduction of the assessment criteria relevant for this, including the new consumption cycles, started on September 1, 2017 (WLTC and RDE for new vehicle types). Beginning on September 1, 2019, it will be necessary for all new vehicles sold to demonstrate compliance with the pollutant limiting values on the road as well (Euro 6d TEMP, RDE).
The difficult task for the OEMs and the suppliers has to do with defining just the right fleet mix, while at the same time living up to customer demands for price and performance, albeit customer preferences vary greatly around the world. In addition, it is necessary to intelligently use the limited resources available for development.
Based on current global and regional forecasts relating to technology development, the transformation of the energy chain, infrastructure, and the availability of resources, it must be assumed that there will continue to be a mix of different powertrain technologies tailor-made for certain usage profiles for the time being. The percentage of purely electric powertrains will be increasing, with this growth heavily dependent on the development of mobile energy storage systems and the associated energy chain and infrastructure. The solution space with relation to the drive unit is limited to electric axles, related near-wheel drives, and drives that are directly integrated in the wheel (wheel-hub drives). The area of digitalization and the development of autonomous driving represent another driving force in this area. New mobility concepts for urban areas are leading to new vehicle concepts where mobility is being offered as a service in specially tailored business models. Drives that enable optimum use of space and a high degree of maneuverability, such as the wheel-hub drive, are taking center stage.
Hybridization leads to the development of many more versions. These powertrains can be systematized on the basis of the architecture and topology and described with respect to the possible functions. The degree of hybridization has become established in everyday language (micro, mild, full, and plug-in hybrids, along with purely electric drives), and can be characterized based on the possible functions. Parallel, serial, and power-split architectures can be differentiated on the basis of the energy flow. Selecting which system is suitable ultimately depends on what functions are desired at the vehicle level (driving dynamics, driving performance).
Parallel architectures have also become established alongside the large number of power-split hybrid transmissions on the market (e.g. Toyota Prius). In addition, there are also structures that enable parallel or serial operation (e.g. Mitsubishi Outlander). Parallel structures are advantageous if
• the powertrain is primarily designed for recovering kinetic energy (mild hybrid).
• only one electric drive is to be used.
• the basic transmission is to be incorporated unchanged.
• a large percentage of the driving profile involves high velocities.
• a high level of functional redundancy is required (full function when the battery is low).
When electrical power is sufficient and redundancy limited, the transmission structure is not as demanding and will therefore be more cost-effective. Another advantage can be gained if it is possible to limit the operating range of the combustion engine. This can be achieved by the following operating modes:
• Stepless operation (eCVT, power split)
• Serial operation
• Limited driving range for the combustion engine (mechanical drive through of the combustion engine beginning at a limit velocity up to a top velocity)
• Purely electric operation in the lower velocity range
The best results can be obtained by combining the operating modes, which – in addition to special transmission structures – also requires dedicated designs of the combustion engine and electric drives. In these specially tailored powertrains, the operating strategy takes on a significant role in coordinating performance, consumption, drivability, and acoustic behavior.
It is primarily with parallel structures that different topologies come to bear with regard to the position at which the output of the electric drive is coupled to the powertrain (PO to P4) – Figure 2.
Figure 2 Installation position of the electric drive in hybrid vehicles
When other powertrain functions such as all-wheel drive are considered along with the pure vehicle driving function, then the question of architecture involves the function level as well. All hybrid functions, including electric driving, can be implemented using a P4 topology (electric axle). When combined with a combustion engine on the other axle, however, there is the possibility of all-wheel drive. If it is not acceptable for this all-wheel function to be lost when the battery is empty, then it will be necessary to combine the electric axle with a second electric drive, which in this case can work as a generator. Limited output when the battery is empty is conceivable in many applications, resulting in the possibilities shown in Figure 3.
Figure 3 Functional scope of different P4 solutions
An electric axle has additional functional advantages. If a high level of drive-away torque and high final velocities are to be offered at the same time – such as in an SUV – then a shiftable two-gear axle can be used. Another option is the targeted distribution of the torque onto the wheels, thereby improving the driving dynamics considerably (“torque vectoring”).
Combining the degree of hybridization and the topology already results in a total of 16 options. The complexity at the system level increases considerably if the transmission versions (serial and power split) provided in a drive module are also considered – Figure 4.
Figure 4 Combination of architecture, topology, and transmission options in a hybrid powertrain
In order to comply with the development goals with regard to function, cost, and time despite the high level of complexity and associated expense, automobile manufacturers have to rely on suppliers that have considerable system expertise going beyond their own scope of delivery. At Schaeffler, this system expertise involves all of the powertrain components mentioned (combustion engine, transmission, and electric transmission) and factors in the numerous interactions in the mechanical, electrical, information-technology-related, and thermally interconnected subsystems. The prerequisite for this is competence in incorporating and constantly optimizing simulation tools and test results from test stands or vehicle tests along the entire development chain.
A particular challenge for Schaeffler involves adding competence in electrical tractive drives to its already well-developed powertrain expertise and the know-how that it has built up over the course of decades in combustion engines and transmissions.
It is planned for the future product portfolio to offer electric axles, hybrid modules, wheel-hub drives, and even dedicated hybrid transmissions. The scope of services will include the entire system, i.e. even the electric machine, the power electronics, and the software. The resulting requirements for the electric machines and power electronics are therefore very different – Figure 5.
Figure 5 Platform approach for e-machines and power electronics in various applications
An ideal situation with regard to driving performance and consumption can only be achieved if the gear ratios, the electric motor, and the power electronics are all perfectly balanced. At the same time, the requirements for installation space, weight, and comfort need to be met. This is why Schaeffler has been consistently building up its development competence for around ten years in all relevant engineering disciplines.
In order to be able to make reliable decisions for the large number of individual influencing parameters, continuous model building and simulation is absolutely necessary in all key development steps. This becomes clear based on the supposedly simple question of how a PSM machine needs to be dimensioned in order to fulfill the requirements specification indicated in the requirements specification for a certain application. The design has proven to be a complex optimization problem, since in part opposing effects need to be balanced out, while boundary conditions such as minimal design voltage, maximum admissible current, available installation space, and the performance capacity of the cooling circuit are firmly defined through the application:
• The maximum torque needs to be achieved at the maximum admissible current. Since the installation space is limited, there is only a little leeway for the mechanical parameters such as the air gap diameter. Thus, the only remaining parameter is the surface force density, which is accompanied by a correspondingly high electromagnetic force (emf).
• This emf needs to be counteracted through field weakening in order to achieve the necessary maximum speed, whereby the output in the entire speed range is to remain at a high level. The optimum field weakening capacity often requires concessions to the maximum torque at the same level of power.
• In the case of a fault (an open gate at maximum speed or a phase short circuit), a high emf can also lead to critical conditions (overvoltage, overcurrent). This is only admissible if these conditions can be reliably intercepted by the power electronics (e.g. active short circuit and thermal stability with short-circuit current).
• The various losses are affected by the machine design as well (losses through ohmic resistance, hysteresis, current displacement effects, harmonics, etc.). The range of the good level of efficiency (the “sweet spot”) and its maximum value can be affected by this and harmonized with the load cycle within the physical limits.
• Limitations with this mainly electromagnetic optimization are due to the mechanical rigidity – including that of the rotor – and the heat balance. Special designs for the local reduction of mechanical tension in the pockets of embedded magnets or for winding cooling may cause the performance parameters to increase considerably.
• The characteristics of magnetic poles automatically lead to force effects that manifest as harmonic impulses in a spinning drive. At worst, they can result in an NVH problem. For this reason, the environment needs to be analyzed with regard to its transfer paths and natural modes. However, the impulses can also be affected by special design measures (a favorable pole/groove ratio, randomized groove opening width, rotor offset, etc.). The retroactive effect on the performance parameters needs to be checked here as well.
• The material quantity and quality not only affect the performance parameters, but unfortunately the costs as well.
The optimization loops needed for this combine calculations of the magnetic field simulation, the electrical design, the mechanical modeling, CFD calculations, and the heat flow simulation. Moreover, a large number of verified material characteristics are required in order to parameterize calculation models. The models devised and verified during the development process, such as with respect to thermal behavior, are transferred via model reduction to network models, which are ultimately employed for controlling the machine. The subsequent metrological validation is not only used for safeguarding, but also for continuously improving the model – Figure 6.
Figure 6 Continuous development of a thermal model for tractive motorsN
Figure 7 Factors influencing the increase in power density in an electric axle drive
Considerable improvements are achieved in the four areas shown:
• The consistent optimization of the electric machine with regard to the power density and thermal stability.
• Integrated power electronics on the basis of sintered IGBTs and sophisticated, AUTOSAR-compatible control technology.
• A transmission concept that saves on installation space and makes it possible to simply adapt the gear ratio to the particular application concerned.
• A developmental approach in which key system properties such as electromagnetic compatibility, acoustics, and cooling are regarded in their entirety.
The consistent utilization of the latest technologies, optimum balancing of the subsystems, and further developed mechatronic integration in the current development stage of the electric axle have enabled the torque and power output to be doubled while cutting the weight by 15%.
The takeover of Compact Dynamics, completed in December 2017, has made it possible to broaden design expertise in the field of electric drives even further. Compact Dynamics has established itself as a successful development partner, such as for the drives used to equip the vehicles of the Audi Sport ABT Schaeffler Formula E team.
Not only the technical challenges need to be mastered, but also the commercial and scheduling challenges. It is only on the basis of flexible modular approaches that it is possible to individually tailor the solutions to various usage profiles while simultaneously shortening the development times. For this reason, Schaeffler has defined a technology module for the important subsystems. The underlying modularization allows for a high degree of solution flexibility despite standardization.
The second-generation hybrid modules in series production today will be supplemented in upcoming years by integrated launch elements (generation 3) and integrated power electronics. Dedicated hybrid transmissions represent an even greater degree of integration, with the electric drive taking on part of the transmission function. Transmissions for electric axle drives with one or two gears are already in production or are awaiting the start of series production soon. In the future, Schaeffler will also be developing and supplying axle drives along with the electric machine and power electronics – Figure 8.
Figure 8 Product road map for electric drives.
 Eckenfels, Th.: 48 V Hybridization – A Smart Upgrade for the Powertrain. 11. Schaeffler Kolloquium, Baden-Baden, 2018
 Pfund, Th.: The Schaeffler eDrive Plattform – Modular and Highly Integrated. 11. Schaeffler Kolloquium, Baden-Baden, 2018
 Kinigadner, A.: Dedicated Hybrid Transmission – How the Transmission Becomes a Powertrain. 11. Schaeffler Kolloquium, Baden-Baden, 2018
The field of e-mobility can look back on many years of expertise in the development and production of components for electrified drivetrains. The first concept vehicle was already built back in 2002. At the 9th Schaeffler Symposium in 2010, sophisticated solutions for mild and full hybrid drives were introduced . Series production for hybrid components began in 2013. In the following years, the company focused on developing its expertise further, with the result that, at the 10th Symposium in 2014, the first complete solutions were presented, such as for electric axle and wheel-hub drives along with roadworthy prototypes [2, 3]. The technical competence gained in this manner has led to several series projects for hybrid modules and electric axle drives, with production launches between 2017 and 2019 – Figure 1. In the future, with China, Europe, and the US, all of the large markets for electrified propulsion will be served by our development and production. At the start of 2018, these activities were bundled in the new corporate unit of E-Mobility.
In order to be able to handle the wide variety of market requirements at an acceptable cost, Schaeffler has built up a development platform for electric drives . The platform is just as suitable for electric axle drives as it is for hybrid modules, wheel-hub drives, and dedicated hybrid transmissions. However, the modular approach does not only include the electric components of the electric drive. Instead, modular concepts for clutches (for example, K0 or the triple clutch in the P2 hybrid modules), actuators, and mechanical transmissions have also been planned and developed. Figure 7 illustrates the application of these modular systems on an electric axle drive.