## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission
Nevertheless, for the actual application of these strategies, an additional system is required to activate the suitable strategy. In this case, a gradient detection system is required so that the road gradient can be measured to activate the proposed gearshifting strategy. Therefore, further comparisons between the proposed strategy and the conventional strategy should be carried out on more driving cycles (instead of just NEDC and ECE Extra Urban) to provide clearer picture on the importance of implementing two different gearshifting strategies. Figure 5 presents a graphical summary of the work performed by [33]. In short, it can be concluded from the work, that highly flexible gear shifting strategy is crucial in optimizing EV driving mileage, and such flexibility is possible with the optimized multispeed transmission in the EV powertrain. Other works involving the optimization of multispeed discrete transmission in EV can be found in [34; 35; 36; 37; 38; 39]. However, unlike [33; 34] they presented the optimization of two-speed transmission in an electric truck subjected to specific gradient route without heavy traffics, and the motor's efficiency mapping also included regenerative braking efficiency. The two-speed transmission mainly consisted of two planetary gearsets with two brakes to select the desired gear ratios. The brakes were actuated using a DC motor through a worm gear and worm wheel (Figure 6). The worm wheel was designed with an inner spiral guide, allowing it to convert its rotation about the axis of the motor's shaft into an axial movement. Depending on its rotational direction, the worm wheel, at one time, could press and lock either the first brake (engaging gear 1) or the secondary brake (engaging gear 2), accordingly. The application of worm gear here provides an advantage in terms of big torque multiplication, which leads to the possibility of using compact motor to engage the brakes. However, the worm gear is more vulnerable to tear and wear than the usual gear wheels, which means frequent gear shifting here will very likely lead to high maintenance cost. The shifting strategy used here, in contrast to [33] that took into account the road gradient, involved only throttle position and motor speed as the parameters and the driving cycle was designed to reflect an operation in an iron mine. Apart from vehicle speed, the studied driving cycle also took into account change in the weight, considering the delivery of iron ore, and also drastic change of gradient, considering the geography of the mine. Based on the aforementioned shifting strategy and driving cycle, the two ratios of the transmission were optimized for efficiency and acceleration using particle  
Figure 5: Summary of the work done in [33]. swarm optimization (PSO). The results showed that, when compared against one-speed transmission, the optimized two-speed transmission managed to reduce the overall power consumption by 6.1%, contributed by efficiency motor's operation during driving and regenerative braking, but the gain in acceleration was very minimal. In terms of shifting strategy, for the future study, it is interesting to evaluate if there will be any efficiency improvement if the same strategy as described in [33] (consider road gradient as parameter for gearshifting) is to be implemented here in [34] (do not consider road gradient as parameter for gearshifting). The efficiency difference between them is crucial for evaluating the viability of considering the road gradient in the gearshifting strategy. Another study involving regenerative braking for a two-speed transmission was described in [35], where a two-speed planetary gearset was used. Here, the main objective, instead of maximum efficiency, was to minimize jerking during the braking. The regenerative braking procedure was divided into three phases: driving phase, brake engaging phase, and braking phase.