## 4 Optimizing Power Flow in EV Powertrains
### Multi-Motor Configuration
Outcomes in [71; 72; 73] highlight the importance of a proper strategy for torque and speed coupling between two or more motors that can lead to flexible driving modes for an EV. Thus, to evaluate its implementation in EV [74; 75; 76; 77], studied the effects of using multi-motor configurations in EV powertrain on the power consumption based on several driving cycles. In [74], the same two motors were used in a two-motor EV powertrain configuration, and the powertrain was evaluated based on three different torque distribution strategies between the motors, where the final strategy was optimized using adaptive non-linear PSO. In [75], on the contrary, two motors with different maximum torque were used where they were connected through planetary gearest. Here, only speed coupling is possible, and these motors were controlled using a combination of speed feedback control strategy and torque feedforward control strategy to minimize jerking during the shifting of the driving modes. Other papers describing the implementation of dual motor configuration with planetary gearest can be read in [76; 77], and because they allowed only speed coupling, the flexibility in terms of driving modes was limited. Meanwhile, in [78], three configurations of two motors EV powertrain without multi-tipeed transmission were considered; configuration for torque coupling (Figure 15a), configuration for speed coupling (Figure 15b), and a configuration for both torque and speed couplings (Figure 15c). Therefore, this configuration offered significant improvement in driving modes flexibility. In the first configuration, a single planetary gearest was used, in which the first motor was directly connected to the gearest's sun gear, while the second  
Figure 14: (**a**) The benchmark and (**b**) the proposed HEV powertrain configuration described in [71]. Figure 13: Speed coupling realized through planetary gearest. motor was connected to the same sun gear through a clutch. For the second configuration, two planetary gearsets were used with a brake at their ring gears. The first motor was meshed to the sun gear of the first planetary gearset, and the planet carrier here was rotatable. The second motor, on the contrary, was connected to the second planetary gearset's sun gear, where its planet carrier was fixed to the casing. The engagement of the brake on the ring gears of both gearsets was used to control the speed coupling in the configuration. Finally, the third configuration was essentially a heavily redesigned second configuration, that now has a clutch between the sun gears, and the rotation of the second planet carrier was controllable through another brake. As a result, the third configuration was more flexible, thanks to its capability to provide both torque (through the clutch between the sun gears) and speed couplings (through the brake at the ring gears). The benchmark in the study was a single motor EV powertrain with single-speed transmission. The gear ratios of all these powertrain configurations were optimized using non dominated sorting GA (NSGA-II) for optimum efficiency under UDDS, HWFET and NEDC. Compared to the single motor EV powertrain with single-speed transmission, in average, the single motor with two-speed transmission was about 2% more efficient, while the first, second and third two motors EV powertrain configurations were 5.77, 5.57 and 6.40% more efficient, respectively. It was interesting to note here, that the efficiency of the first two motors configuration (capable of torque coupling only) and the second configuration (capable of speed coupling only) were pretty much the same, although, in terms of mechanical system, the latter was significantly more complex than the former due to the application of two planetary gearsets. Next, for the second configuration with speed coupling and torque coupling options, the design was much more complex since it required three actuators for the two brakes and a clutch.