## 5 Comparison and Future Works Related to the Methods for Optimizing Power Flow in EV Powertrains
### Key Areas for Future Research Works  
The latest research works on two-speed transmission for application in EV powertrains reviewed in this paper mostly focused on optimizing the gear ratios for powertrain \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Methods** & **Advantages** & **Disadvantages** \\ \hline  & - & The gear ratios can be optimized for different driving conditions, more flexible than the single-speed transmission commonly used in EV. & - & Limited gear ratios, which limits the flexibility for more diverse and demanding driving conditions. Also prevent the possible application in wider vehicle segments. \\ Multispeed discrete transmission & - & Possibility to optimize the size and capacity of the motor and the battery for improved sustainability, production and ownership cost. & - & Torque interruption and jerking during gearshifting. \\  & - & Share significant number of components similarity with the ICE vehicles, hence possible to reuse the existing production facilities and process. & - & Extra maintenance cost as compared to the single-speed transmission. \\ \hline  & - & It is capable of providing continuous ratio range, hence greater flexibility than any multispeed discrete transmission. & - & The high requirement for the metal belt’s operation and maintenance cost involving higher hydraulic pressure than AMT and DCT for maintaining its clamping force and ratio. \\ Continuously variable transmission (CVT) & - & Share significant number of components similarity with the ICE vehicles, hence possible to reuse the existing production facilities and process. & - & Inevitable micro slippage between the belt’s components, thus higher transmission loss than AMT and DCT. \\  & - & Presents the possibility for application across wider vehicle segments, thanks to the greater flexibility. & - & Slightly costlier maintenance AMT and DCT based on the usage in ICE vehicles. \\ \hline  & - & Possible to be implemented with multispeed transmission for much greater flexibility. & - & Complexity in mechanical design to allow the speed and torque couplings between the motors (for two-motor configuration). Even higher complexity when involved multispeed transmission. Complexity in controls, especially for four-motor configuration that needs precise coordination between all motors at the wheels. Even higher complexity when involved multispeed transmission. The production sustainability due to the high number of motors involved. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison between the application of multispeed discrete transmission, CVT and multi-motor configuration in EV powertrain. efficiency and performance. Besides, these works also emphasized the proper gearshifting strategy, which should be formulated accurately by taking into account the road conditions and driver's input so that a balance between the powertrain efficiency and performance can be realized. This was consistent with the research trends previously discussed and reviewed in [89; 90]. However, these works still did not sufficiently discuss the control algorithm of the gearshifting mechanism in detail. Examples of works on this topic can be read in [41; 42; 43; 47; 48], though these works evaluated only on the jerking of the mechanism alone without being integrated with the multispeed transmission inside the powertrain. Besides, they also did not assess the effect in terms of transmission efficiency and actuation power consumption. Therefore, in the future, works in the control of the gearshifting mechanism are expected to be intensified. On the CVT with metal belt for EV powertrains, most of the previous related literatures focused on analyzing and comparing its efficiency and performance against multispeed discrete transmissions and multi-motor configurations. However, due to the application of hydraulic actuation system, the CVT suffered significant power losses, which prompted some scholars to study the practicality of replacing it with an electro-mechanical actuation system.