## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission
These phases were proposed to minimize torsional oscillations, that causes the jerking, by optimally synchronizing the application of hydraulic service braking and the motor's braking torque during the brake engaging phase. As a result, the jerking was reduced by around 55% as opposed to the conventional regenerative braking that does not consider such oscillations. Next, ref. [36] presented a work carried out to optimize the gear ratios of two-speed DCT based on not only the motor's efficiency, but also the transmission efficiency. In the work, the transmission efficiency model was developed by taking into account the losses at the gear meshing, bearings, clutch and concentric shaft. Thus, different gear ratios produced different efficiency in transmitting the torque between the input and the output shafts. Based on the model, the optimum gear ratios were selected for the EV powertrain, and its performance was compared against a single-speed EV powertrain for Worldwide Harmonized Light Vehicles Test Procedure (WLTP) cycle, which showed an efficiency improvement around 10.7 to 12.1%. Regarding the transmission efficiency, research in [37] explained a possible method to improve it by modifying the tooth profile of the gears which can potentially reduce not only the loss in the gear meshing, but also the required effort for the gearshifting. In [38], on the other hand, a two-speed EV powertrain model were optimized using genetic algorithm (GA) with the objectives to achieve quick 0-100 km/h acceleration (driving performance) and minimum power consumption (efficiency) under NEDC. The type of transmission used in the model were not specifically mentioned, hence  
Figure 6: (**a**) Schematic diagram and (**b**) CAD model highlighting the important components of the novel two-speed transmission proposed by [34]. the shifting mechanism involved was unknown. However, the model included regenerative braking efficiency model. Unlike many papers that emphasized on optimizing the size of the gear ratios, the work in [38] optimized not only the gear ratios, but also the maximum motor's output torque in Nm and its maximum rotational speed in RPM. The motor's torque and RPM were optimized within the range of 150-200 Nm and 8000-12,000 RPM, respectively. In order to obtain the balance optimization results, specific weightage was given to both of the objectives: driving performance and efficiency. The results showed that it was possible to achieve a balance (compromised) solution between the driving performance (quick acceleration) and efficiency (power consumption) by optimizing the two gear ratios and the motor's maximum torque and speed. Nevertheless, different set of gear ratios, or a continuous ratio range between 1.341 and 3.050, were required to achieve the fastest possible acceleration and highest possible powertrain efficiency. This meant that, to achieve maximum performance and efficiency in a single powertrain system, the gear numbers must be higher than 2. In a discrete transmission, however, increasing the gear number must be done together with redesigning the gearshifting mechanism which leads to increased design complexity and cost. Figure 7 shows summary of the optimization work done in [38] using GA to determine the optimum gear ratios and motor's outputs for driving objectives. Based on the results, if a continuous ratio range between 1.341 to 3.050 can be provided by one transmission (like a continuously variable transmission), then all the objectives can be met, instead of opting for a compromised two gear ratios in a two-speed transmission. Another interesting work regarding EV powertrain with discrete gear transmission was presented in [40], which experimentally evaluated the performance of three different transmission ratios--6.00, 8.00 and 10.00--for one driving cycle. Among the ratios, 8.00 served as the benchmark for the results' analysis. In the experiment, the test vehicle was tested on the same track using three different one-speed transmissions, corresponding to the three ratios.