## 4 Optimizing Power Flow in EV Powertrains
### Multi-Motor Configuration
This was to avoid gear hunting, which not only affected the powertrain efficiency, but was also detrimental to the driving comfort. Next, in [81], the objective was to determine and compare the performance of the configuration against single motor configuration with four-speed AMT. The study started by generating two optimized configurations of two-motors-four-speed-AMT EV powertrain, where the optimization objectives were to obtain minimum operating cost (defined as minimum total power consumption for the aforementioned driving cycle) and high driving performance (defined as the minimum acceleration time for 0-40 km/h). The control variables of the optimization were motor scale factor; expressed as the ratio of motor 1's power divided by the power of motor 2, and the four gear ratios of the AMT. The optimum combinations of the variables to meet the optimization objectives were determined using NSGA-II. As a result, three powertrain configurations were finalized; Configuration 1 that consisted of one motor configuration and two optimized gear ratios, Configuration 2 that consisted of two motors configuration with the motor scale factor of 0.42 and four optimized gear ratios, and Configuration 3 that consisted of two motors configuration with the motor scale factor of 1.00 and four optimized gear ratios. All of them achieved the same acceleration time of 8.5 s, but in terms of power consumption, Configuration 1 recorded the worst at 7.48 kWh, while the second and third configurations managed to improve over the first one by 4.82% and 5.08%, respectively. Next, the optimum driving strategy was formulated for each configuration in terms of shifting schedule and motors coupling modes so that the driving efficiency and performance can be improved further. The simulation results of the three powertrain configurations plus the optimal driving strategy showed that, both Configurations 2 and 3 allowed the motors to operate with at least 85% efficiency rate for 65% of the driving cycle. As a comparison, in Configuration 1, the motor was allowed to operate at the same efficiency rate only for about 32% of the same driving cycle. Because of that, the total power consumptions obtained for Configurations 2 and 3 were lower than Configuration 1, at 7.219 kWh and 7.216 kWh, against 7.627 kWh, respectively. Nevertheless, it must be mentioned that in Configuration 1, the gear shifting occurred only 46 times during the cycle, which was significantly lower than 84 and 80 each for Configurations 2 and 3. These findings indicated that the one motor EV powertrain configuration with two-speed transmission is potentially advantageous in terms of overall cost (production, operation and maintenance costs) than those two motors configurations, even though it performed the worst in terms of efficiency. Not only that, the acceleration time was also the same for all configurations, and this reinforce Configuration 1 as the overall best choice as opposed to the other two. Therefore, further study on the operation cost can be carried out in the future by relating the data of the gearshifting frequency and the wear and tear of the shifting mechanism. Next, the shifting performance especially in terms of jerking must also be evaluated for all the configurations so that more aspects can be compared to determine the overall best configuration between the three. Other works describing the application of multi-motor configurations with multispeed transmission can also be read in [82], which reviewed the methodologies of the multi-motor configurations, and [83], which presented an optimization of gear ratios and torque distribution of two-motor EV powertrain configuration with two-speed transmission using a surrogated model developed based on an effective adaptive sampling method. Apart from using two motors powertrain configurations, there were also other studies performed to evaluate the application of four motors in the EV powertrain. For instance, in [84; 85], four motors were used with each of them assembled in the EV's four wheels (Figure 17).