## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission
Nevertheless, unlike [27; 28] focused on maximizing the efficiency during city driving with the speed below 60 km/h and frequent starts-stops. Hence, the results are applicable only for a very specific city driving cycle. Moreover, no details on the gearshifting mechanism are provided, which means that further study to evaluate the jerking during gearshifting is required. This is particularly very important since the driving cycle studied here involves frequent start-stops. Finally, ref. [28] also considered the application of CVT with metal belt, which they found out that was less desirable due to the significant power losses in the belt. This is expected because of the high hydraulic pressure requirement to clamp the belt, especially since higher torque is required to move the bus as opposed to the passenger cars. Therefore, using such CVT in heavy vehicles, like a bus, is less practical as compared to using it in passenger cars. Another study covering the implementation of multispeed discrete transmission in an electric bus was described in [29] where a four-speed automated manual transmission (AMT) was used. In the transmission, two DC electric motors were used for gearshifting, where one motor was used to select gear 1 and gear 2, while another motor was used to select gear 3 and gear 4. Because of this configuration, the shifting from gear 2 to gear 3 required sequential operation of both motors, thus it is expected to take longer time and higher actuation power than the shifting of gear 1 to 2 as well as gear 3 to 4. The shifting performance was evaluated experimentally on a test bench in terms of efficiency and shifting time. Based on that, a complete powertrain model for the electric bus was developed so that the optimized gearshifting strategy (defined as the optimal gearshifting points with respect to vehicle speed and throttle's opening) can be determined for minimum power consumption and minimum shifting frequency. The optimized gearshifting strategy was formulated based on the actual Beijing city driving data representing two driving conditions; high urgency driving with frequent acceleration and higher average speed, and, gentle driving with less frequent acceleration and lower average speed. Then, the four-speed AMT with the optimized gearshifting strategy was tested on a dynamometer to gauge its workability. Unlike the works in [27; 28; 29] provided the details of gearshifting mechanism in the AMT, hence analysis on the shifting time can be done realistically. However, further improvement in the shifting time here to match the DCT's performance is challenging due to the operation of two DC motors in the mechanism. This means that the shifting time can be minimized only if the number of motors can be reduced, which is possible only with the reduction of the number of gears. Therefore, the next area that can be focused on in [29] is the optimization of the gear ratios so that the possibility to reduce the number of gears can be explored. Besides, study on the jerking during gearshifting can also be carried out here thanks to the availability of the AMT's prototype. Subsequently, research works in [30; 31; 32] described the working principle of two-speed transmissions using planetary gearset for application in an EV powertrain. In terms of the planetary gearset design, the transmission was similar to a conventional automatic transmission for ICE vehicles. However, in terms of actuation system for its clutches and brakes, the proposed one used electro-mechanical actuation system that featured DC motor and a screw nut system. The significant benefit of using the screw nut system is that it provides self-locking mechanism, hence the desired gear can be maintained without exerting continuous hydraulic pressure on the clutches and brakes. This will improve the transmission's efficiency since no power is required to generate the needed pressure. The challenge, however, is the complexity to integrate the design of the screw nut system with the clutches and brakes.