## 4 Optimizing Power Flow in EV Powertrains
### Multi-Motor Configuration  
The idea of optimizing EV powertrains using a multi-motor configuration involves properly distributing the driving loads, typically between two motors or four motors, so that they can operate optimally under various driving conditions. Additionally, in some situations, these motors can also provide boosting power for faster acceleration and higher top speed. As a result, the capacity of the motors can be reduced without compromising the driving performance and this leads to potentially significant cost savings. There are three common types of multi-motor configuration studied for EV; the first one is two-motors configuration where both motors are connected to a transmission before the wheels, the second is the two-motors configuration where both motors are directly coupled to the wheels, and finally, the four-motors configuration where all motors are directly coupled to the wheels (Figure 11). In the two-motors configuration where two motors are connected to a transmission before the wheels, allowing torque and speed couplings between the motors is crucial to suit diverse driving conditions. As shown in Figure 12, torque coupling means the torque from the ICE and the motor is combined through direct gearwheel, resulting in the shared torque requirement between them at the wheels. Equations describing the output torque and speed of the coupling are shown in Equations (1) and (2). Speed coupling, on the other hand, means the speed of the ICE is combined with the motor's speed through planetary gearset, resulting in higher speed, hence higher power, at the wheels (Figure 13). Equations (3) and (4) explain the relationship between the motors' inputs and the coupling's outputs. Torque coupling is generally useful for start-stop condition, while  
Figure 11: (**a**) Two-motors configuration where both motors connected to a transmission before the wheels, (**b**) both motors connected directly to the wheels, and (**c**) four-motors configuration where all motors directly connected to the wheels. speed coupling is usually applied to achieve fast acceleration and high-speed driving. Latest examples for the optimization of HEV powertrain configurations can be read in [71; 72; 73]. In [PERSON] et al. [71], various HEV powertrain configurations based on CVT with metal belts and discrete gear automatic transmissions as the torque coupling, and a planetary gearset as the speed coupling, were generated using a fundamental matrix. In the work, feasible driving modes of these configurations were determined using an adjacency matrix, and based on these modes, the powertrain configurations were evaluated and compared against the benchmark configuration (Figure 14a) in terms of 0-100 km/h acceleration time and average power consumption under WLTC. The results demonstrated that the best configuration, as depicted in Figure 14b, managed to reduce the acceleration time and the average power consumption by 8.7% and 12.2%, respectively. Such improvements were possible because of the flexible driving modes provided by the proposed configuration that resulted in the reduced ICE power required for some driving conditions (due to the planetary gearset at the motor's output), and more efficient regenerative braking (due to the several torque coupling possibilities at clutch C3 and C2). Results in [71] are also consistent with those discussed in [73], where it was found that the combination of CVT and planetary gearset is crucial to optimize the HEV's efficiency for various vehicle speeds. \[T_{out} =\frac{R_{out}}{R_{motor1}}T_{motor1}+\frac{R_{out}}{R_{motor2}}T_{ motor2} \tag{1}\] \[\omega_{out} =\frac{R_{motor1}}{R_{out}}\omega_{motor1}=\frac{R_{motor2}}{R_{ out}}\omega_{motor2}\] (2) \[T_{out} =\frac{2R_{carrier}}{R_{sun}}T_{motor1}=\frac{2R_{carrier}}{R_{ ring}}T_{motor2}\] (3) \[\omega_{out} =\frac{R_{sun}}{2R_{carrier}}\omega_{motor1}+\frac{R_{ring}}{2R_{ carrier}}\omega_{motor2} \tag{4}\]  
Figure 12: Torque coupling realized through direct gearwheel meshing.