## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission
A summary of the works related to the gearshifting mechanisms and the adaption of driver behaviors in [41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51] is presented in Table 3. In terms of gearshifting mechanisms, the works reviewed here mostly discussed their standalone performance in terms of jerking and shifting time, while limited discussions were carried out to evaluate their performance when integrated in a powertrain system. This means that the question on the potential improvement efficiency and driving performance in a complete powertrain system is still not properly answered. Nevertheless, with the numerous novel designs of gearshifting mechanism proposed recently by the researchers, the outlook of developing and implementing multispeed discrete transmission, especially two-speed transmission, in a commercialized EV looks promising. \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  & - & Discussing the importance of optimum & & \\  & - & gearshifting process for efficiency and driving & & \\  & - & In [44], analysis and comparison have been & - & In [44], results showed the & importance of selecting the right \\  & - & merchanisms (friction clutch and one-way clutch, among others) for efficiency and jerking & - & shifting mechanism to minimize the \\  & - & There were analyzed for three & - & jerking so that maximum efficiency \\  & - & The were analyzed for three & - & can be achieved. \\  & - & The were analyzed for three & - & In [44; 41; 42], the proposed \\  & - & The was identified for the & - & Harpoon-shift synchronizer \\  & - & In [44; 43], novel shifting mechanism named & & \\  & bilateral Harpoon-shift synchronizer was & & - & \\  & - & Using torsional spring (bended compression & & \\  & - & Using torsional spring) inside the dog gear internal groove to & & \\  & - & provide spring force to the guide ring. Other & & \\  & - & Example of similar design was described in [45]. & - & In the compromised solution \\  & - & Optimizing gearshifting to minimize shifting & (balanced between all the \\  & - & time, friction work and jerking using Legendre & & objectives), the shifting time was \\  & - & pseudospectral method. & - & recorded at 0.92 s, with the square \\  & - & Gearshifting model was simplified as 2-DOF and & & of continuous jerking measured at \\  & - & 4-DOF dynamic models based on a friction & & 0.48 (m/s\({}^{3}\))\({}^{2}\), and the friction work \\  & - & clutch and a sleeve shifting mechanism in a & - & For now, no details descriptions on \\  & - & two-speed transmission. & - & the actual actuators used for the \\  & - & & clutch and the gearshift sleeve. \\ \hline  & - & Some power was lost in the wet & \\  & - & clutch actuation. & - & High jerking at around 10 m/s\({}^{3}\), \\  & - & Evaluating wet clutch control on a dynamometer & & \\  & - & & interruption during the shifting. \\  & - & test rig in terms of shifting time and jerking. & - & Difficulty to optimize the shifting \\  & - & Based on a two-speed DCT applied in an EV & & due to the sticking characteristic of \\  & - & powertrain. & - & the wet clutch. \\  & - & Highlighting the importance of & \\  & - & precise clutch control in shifting, & \\  & - & can be potentially done by an & \\  & - & & electro-mechanical actuator. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the literature review on gearshifting mechanism of multispeed discrete transmission in EV powertrain. Overall, it can be summarized that the research works on multispeed discrete transmission for EV mainly focused on the implementation of two-speed discrete transmission which can be in the form of AMT or DCT. The two-speed design is very compact, which means the additional weight relative to the usual single-speed transmission can be kept to a minimum. Besides, the two gear ratios provide the necessary flexibility in the EV driving modes' selection for optimum efficiency and driving performance.