## 5 Comparison and Future Works Related to the Methods for Optimizing Power Flow in EV Powertrains
### Comparison of the Methods  
Among the three methods, multispeed discrete transmission is the most common one studied by scholars. Within this class, the two-speed transmission is the most popular, which can be described in the form of either AMT or DCT. Most of the works related to the application of two-speed transmission reviewed here involved optimization of the gear ratios and shifting strategy to achieve optimum power consumption and driving performance, in terms of acceleration rate and top speed [33; 34; 35; 36; 37; 38; 39; 40]. As a result, the EV powertrain becomes more efficient and more capable, and this opens up the possibility of optimizing the size and capacity of the motor and the battery. Such a possibility is beneficial for production sustainability, because the usage of heavy materials for motors and batteries can be reduced. At the same time, since the two-speed transmission is very compact and shares significant degree of similarity with the traditional one in ICE vehicles, the existing transmission manufacturing process can also be utilized which will be cost-effective for total EV production cost. The significant challenges, however, are the jerking in the gearshifting mechanism, the limited flexibility and the additional maintenance cost for the two-speed transmission. Similar to multispeed discrete transmission, CVTs with metal belts also involve providing multiple ratios for optimum efficiency and driving performance in an EV powertrain. However, unlike a multispeed discrete transmission, CVTs are capable of providing a continuous ratio range, which addresses the limited flexibility problem faced by the multispeed discrete transmission approach. This presents an opportunity to implement it for diverse driving conditions and various vehicle segments. At the same time, it is also beneficial in terms of production sustainability and technology migration, because of the possibility to reuse the existing manufacturing processes and facilities. This is because the metal belt-based CVT for EV shares a significant number of common components with the existing ones used in ICE vehicles. The main challenges for this type of CVT, however, are its metal belt's operation and maintenance cost. The metal belt's operation requires high hydraulic pressure, significantly higher than the requirement for AMT and DCT, for maintaining its clamping force and ratio. In addition, the belt's operation also inevitably involves micro slippage between its components. These two factors cause transmission loss which is higher than that suffered in AMT and DCT. Also, these factors require slightly costlier maintenance than the other two types of discrete transmission commonly studied for EV application. The last method reviewed here is the implementation of multi-motor configurations, which can be divided into two typical approaches: two-motor configurations and four-motor configurations. For the two-motor configuration, the possibility for both torque and speed couplings certainly leads to flexibility for various driving modes. However, in some cases, providing either speed coupling or torque coupling can already be sufficient to achieve higher powertrain efficiency than the application of two-speed discrete transmission. The two-motor configuration can also be applied with a multispeed transmission, which can be particularly useful for optimizing the power consumption of heavy vehicles such as trucks and busses. For the four-motor configuration, it allows even more diversity in the driving modes than the two-motor configuration, thanks to the possibility to distributetraction to each wheel independently. This presents an opportunity to optimize not only the powertrain efficiency, but also the driving dynamic of the EV (which is not possible for both the multispeed discrete transmission and CVT) and the motors' capacity and size.