## 4 Optimizing Power Flow in EV Powertrains  
Ensuring that a single motor EV powertrain can operate optimally for various driving conditions, especially when the vehicle is travelling at high speed and low load, is very challenging and because of that, their efficiency normally falls to only around 60% from about 90% for the best-case scenario [23]. One of the possibilities to avoid this is by allowing flexible power flow configurations in the powertrain. Studies [24; 25; 26] support this argument, where it is found that the powertrain efficiency and driving performance (in terms of acceleration time and comfort) can be optimized for the full EV driving experience if the driving loads can be properly distributed to two electric motors in the powertrain configuration with different transmission ratios. In the study, the possibility of implementing different hybrid EV (HEV) powertrain configurations was evaluated, and the configuration was defined in terms of coupling between the motors to the ICE, and also in terms of different transmission ratios. When the loads are properly distributed, the motors' speed can be reduced drastically during high vehicle speed, and this contributes to increasing the powertrain efficiency while ensuring the acceleration can be performed smoothly. Hence, it can be summarized here, that, flexible motor's power flow, optimized powertrain components and control are the key to optimize EV powertrains, and this can be achieved by optimizing multi-motor configurations, or by implementing multispeed transmission in the EV powertrain. Figure 2: Various methods recently proposed to optimize EV powertrains’ performance. In terms of design complexity, the multispeed transmission in an EV should be less complicated than the one used in the existing ICE-powered vehicles. This is because of several factors; most notably the requirement of moving-off elements in the conventional ICE vehicles. Generally, because of the ICE idling speed condition, a moving-off element; like a dry friction clutch, or, torque converter, is required to facilitate the vehicle's start-stop condition. For an EV, however, because of the availability of the motor's torque from as low as 0 RPM, the implementation of moving-off elements is no longer required. On top of that, the elimination of moving-off elements also opens up the chance to implement a much simpler transmission control algorithm, since now it is no longer necessary to control the moving-off element to achieve desirable driving comfort during start-stop conditions (Figure 3). As a result, only ratio shifting control is required in an EV, although, if a discrete multispeed transmission is used, then a clutch or brake system is still required for the shifting. This is contrary to the conventional ICE vehicles, where it is absolutely critical to optimize both moving-off control and ratio shifting control. In this paper, the works related to the implementation of multi-speed transmission in EV are divided into two categories: multispeed discrete transmissions and continuously variable transmissions (CVTs). In addition, the possibilities of implementing multi-motor configurations are also reviewed here, since this approach can also lead to optimization of the motor operation for various driving conditions, which according to some scholars [25], is more effective than the implementation of multi-speed transmissions. This paper focuses on reviewing research works published from 2018 until early 2022. Therefore, by using keywords \"multispeed transmission electric vehicle\", \"continuously variable transmission electric vehicle\", \"multi motors electric vehicle\" and \"electric vehicle powertrain\" in the ScienceDirect and Scopus databases, 60 references have been identified and shortlisted as related to the topic of optimizing the power flow in EV powertrains. Among them, 24 papers describe work related to multispeed discrete transmissions, while 13 and 15 papers cover work on CVTs and multi-motor configurations, respectively.