Seamless-Shift Two-Speed eAxle with Torque Vectoring Lastschaltfähige Zweigang-eAchse mit Torque Vectoring

Abstract

Most of the electrical axles for BEV and PHEV on the market today are using single speed gear trains. But a fixed ratio usually requires a compromise between an adequate launch torque and the maximum vehicle speed the eAxle can support. Most eAxles in “through the road hybrids” are disconnecting the electric motor before reaching the maximum vehicle speed. But with the trend to smaller combustion motors and more powerful electric machines, disconnecting the electric drive is less of an option. As a logical consequence, there is an increasing demand for multi-speed gearboxes, which enables considerable improvements in performance, comfort and efficiency of PHEV and BEV. VDI-Berichte Nr. 2328, 2018 255 https://doi.org/10.51202/9783181023280-I-255 Generiert durch IP 54.70.40.11 , am 13.05.2021, 23:17:17. Das Erstellen und Weitergeben von Kopien dieses PDFs ist nicht zulässig. GKN Driveline has taken up this trend and developed the eTwinsterX, a new type of very compact electrical drive unit that features a two-speed gearbox with seamless torque-shift capability as well as a full-fledged torque vectoring function. These features enable a significant increase in driving comfort and safety as well as energy efficiency, as it can also recuperate at high vehicle speeds. In particular, the operation of the eMotors in an efficiency-optimized region is much easier to realize due to the speed-speed shifting functionality. Introduction Today’s trends in power train development are very much focused on electrification due to increased environmental and legislative pressures. In particular the aimed reduction of emissions like CO2 and newly also local emissions like nitrogen oxide and fine dust forcing a change in the way of transportation. Meeting these market and legal requirements, GKN as a supplier of conventional and electrified power trains has focused on the continuous improvement of efficiency, performance and handling of electric and hybrid vehicles. GKN supports this goal with a broad portfolio of electric transmissions and axles, and consistently promotes the development of eDrive systems that are both more efficient and increasingly powerful. In particular, plug-in hybrids are a smart and flexible solution for combining the customer demands of high performance and legislative demands of reduced emissions. Excellent fuel consumption can generally be achieved with a hybrid drive system whereby short distances can be driven fully electrical. A simple way for such kind of electrification represents the P4 hybrid vehicle architecture that utilized two independent drivetrains in one vehicle: a standard combustion engine driven axle and an eMotor driven eAxle. In addition to a high degree of flexibility for the OEM in terms of modularity, this architecture also provides an easy way to realize an all-wheel drive (AWD) vehicle. However, compared with an equivalent AWD vehicle only with a conventional combustion engine, the driving dynamics of a hybrid vehicle is challenging due to a higher system weight as a result of its battery, electric motor, inverter etc. that leads to noticeable understeering behavior. Ultimately the traction of these vehicles is also limited by the commonly used open differentials [1]. Starting with the GKN eTwinster in 2015 [2], these typical disadvantages of an eAxle could be overcome by a torque vectoring system utilizing a controlled Twinster clutch system well known from GKN’s AWD systems like the Land Rover Evoque Active Drive, Opel Insignia or Ford Focus RS [3]. The Twinster concept inherently allows diff-locking, disconnect, torque -limiter and torque vectoring functions. GKN has demonstrated a significant improvement of agility, traction and stability in a premium SUV with a P4 hybrid vehicle architecture using a 60kW VDI-Berichte Nr. 2328, 2018 256 https://doi.org/10.51202/9783181023280-I-255 Generiert durch IP 54.70.40.11 , am 13.05.2021, 23:17:17. Das Erstellen und Weitergeben von Kopien dieses PDFs ist nicht zulässig. eTwinster with 1.200 Nm max wheel torque. However, it was as well obvious that for a 2.5 t premium SUV significantly more power and axle torque would be desirable. Additionally, the fixed ratio eAxle is also forced to disconnect latest at a speed of approximately 150 km/h what avoids an electrical support as well as recuperation in high speed regions. This will not be acceptable for future high performance hybrids with downsized combustion engines for better fuel efficiency. The newly developed eTwinsterX is overcoming the mentioned drawbacks of the low power eTwinster concept like speed and torque limit. This innovative 2-speed seamless-shift eAxle enables high launch torque in first gear and electric assistance up to top speed in second gear. The gear shift concept enables a high shifting comfort without interruption of traction and exhibit a high dynamic switching for good driving performance and, at the same time, high efficiency. In the following chapters, the development targets will be discussed and the gearbox design proposed. Detailed explanation of the gear shifting concept will be given and the efforts of the gear shifting functionality combined with a suitable shift strategy will be proposed by means of simulation results. Finally, an outlook for the application and integration in a demonstrator vehicle will be given. Development Targets Two key development targets for eTwinsterX gearbox resulting from experiences gained in the former eTwinster projects and also from customer discussions were defined as follows: 1) High torque output in lower speed range including torque vectoring functionality for realizing on the one hand full traction support as required for typical 4WD vehicles as well as on the other hand full vehicle dynamics support, in this case 3,500 Nm eAxle torque with a maximum vectoring torque of 2,000 Nm. 2) Availability across the entire speed range up the maximum vehicle speed, in this case up to a top speed of 250 km/h with the option but not the need of disconnecting. Using a two-speed gearbox, it was possible to utilize an eMotor with a rated peak power of only 120 kW with a maximum torque of 210 Nm and a maximum speed of 18,000 min-1 (see Fig. 1a). The gear ratios are derived for meeting the listed key targets, what results in an axle torque characteristic as shown in Fig. 1b. For providing a high driving comfort, the shifting functionality should exhibit a seamless torque-shift (no torque interruption) as in principle shown in [4]. The actuation of the torque vectoring system as well as the shifting system should be realized hydraulically by a single actuation system to enable a tight package and low power consumptions. To achieve a high level of efficiency, an actively controlled dry sump lubrication VDI-Berichte Nr. 2328, 2018 257 https://doi.org/10.51202/9783181023280-I-255 Generiert durch IP 54.70.40.11 , am 13.05.2021, 23:17:17. Das Erstellen und Weitergeben von Kopien dieses PDFs ist nicht zulässig. should also be used as already introduced with the basic eTwinster [2]. Additionally, the aspects of a generally small design package and light weight and high overall system efficiency should also be considered and high prioritized. The requirements are summarized in Tab. 1. Fig. 1: a) eMotor characteristic with peak and nominal power and b) expected eAxle performance for first and second gear. Table 1: Summarized key specifications for the eTwinterX development. VDI-Berichte Nr. 2328, 2018 258 https://doi.org/10.51202/9783181023280-I-255 Generiert durch IP 54.70.40.11 , am 13.05.2021, 23:17:17. Das Erstellen und Weitergeben von Kopien dieses PDFs ist nicht zulässig. Modular Gearbox Design Approach The design approach of the eTwinsterX is shown in Fig. 2 and follows GKN’s strategy of utilizing a most modular concept with GKN-standardised system elements. These modules exhibit clear defined interfaces and a high level of development in terms of hardware and software. This strategy makes it possible to robustly develop and create a complex system and allows a most flexible possibility to react on widely different customer demands with separately proven and tested submodules. In general, the eTwinsterX system can be separated in four main