## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission
Besides, the system also has to handle very high thrust force between the pulley (rotating based on motor's power) and the screw (rotating only during ratio shifting to axially move the pulley). If not properly optimized, this will lead to excessive tear and wear in the screw nut system, and also power loss in the thrust bearing. In the research works conducted by [30; 31; 32], the focus was to minimize the jerking by properly implementing various gearshifting strategies with different objectives; first, to maintain a constant transmission input torque, second, to maintain a constant transmission output torque, and third, to maintain a semi-constant transmission output torque. In terms of efficiency analysis, however, no results and comparison were presented between the proposed transmission and the typical single speed transmission in an EV. A summary of the works described in [27; 28; 29; 30; 31] is presented in Table 1, highlighting the potential of multispeed discrete transmissions in improving the efficiency of EV powertrains. However, these works still insufficiently discussed the topic of gear ratio optimization which is crucial to achieve not only powertrain efficiency, but also desirable driving performance. In addition, details on the gearshifting mechanism were also rarely provided, which means that analyses of the jerking and actuation power consumption during gearshifting are still lacking. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Literature, Year** & \multicolumn{2}{c}{**Summary of the Works**} & \multicolumn{2}{c}{**Significant Findings**} \\ \hline \multirow{8}{*}{[PERSON] et al., 2021 [27]} & - & Powertrain model of a passenger car was developed and then run using single-speed transmission under three driving cycles & - & Improved efficiency by 1.7 to 2.4\% for two-speed transmission against single-speed transmission. \\  & (NEDC, WLTC and FTP-75). & - & Details on the shifting mechanism were not presented. \\ \cline{1-1}  & - & Simulation results for WLTC were used to determine the size of two gear ratios for efficiency. Then, the powertrain model was & - & Driving performance (acceleration rate, top speed) were not considered. \\ \cline{1-1}  & & rerun using the determined two gear ratios for the same driving cycles. & - & Driving performance (acceleration rate, top speed) were not considered. \\ \hline \multirow{8}{*}{[PERSON] et al., 2020 [28]} & - & Powertrain model of an electric bus was developed for running in a specific city driving cycle. Data for the driving cycle were obtained experimentally. & - & Improved efficiency by 2-3.2\% for two-speed DCT against single-speed transmission. \\ \cline{1-1}  & - & The two gear ratios were determined using exhaustive search algorithm in the & - & CVT suffered from belt losses, ultimately no gain in the power consumption. \\ \cline{1-1}  & - & Powertrain model was run using single speed transmission, two speed DCT and CVT. & - & Driving performance (acceleration rate, top speed) were not considered. \\ \cline{1-1}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the literature review on efficiency evaluation of using two-speed discrete transmission in an EV powertrain. Optimizing the best two-speed gear ratios, however, is not straightforward due to its multi-objective nature. For instance, the best ratios should be able to achieve the desirable driving performance (in terms of acceleration rate and top speed), and minimum power consumption. These objectives consistently contradict each other, and they are influenced by diverse parameters like the road gradient and instantaneous vehicle speed. Thus, advanced optimization techniques are required, for instance, a work by [33] focused on the optimization of two gear ratios based on specific gearshift scheduling strategy that took into account three parameters; vehicle speed, vehicle acceleration and road gradient. As a comparison, the usual parameter used for gearshift scheduling is the throttle position.