## 4 Optimizing Power Flow in EV Powertrains
### Continuously Variable Transmission (CVT)
The results showed that, CVT was the best performer in terms of efficiency for a B-segment car, reduced the power consumption by 31.9% against the single-speed transmission, followed by the three-speed DCT (19.1%), four-speed DCT (18.2%) and the two-speed DCT (16.4%). This result highlighted the magnitude of improvement that can be gained by eliminating the losses in the hydraulic actuation system conventionally used in a CVT with metal belt. Moreover, it also emphasized the saturation in the increment of the gear numbers in a multispeed discrete transmission, which in this case, it can be observed by the reduction of the efficiency improvement between the three-speed DCT and the four-speed DCT. Thismeans that the saturation point here is at three gears, and further increases in the gear numbers will only cause significant actuation losses in the additional shifting mechanism added for the extra gear ratios. The rather low saturation point is typical for a small car (i.e., B-segment) due to its narrower range of the power required as opposed to a bigger car (i.e., E-segment). For a E-segment car, the results showed that CVT was the best performer (23.6%), followed by the four-speed DCT (15.2%), three-speed DCT (9.0%) and two-speed DCT (9.6%). This result suggested that the saturation point for E-segment EV could be higher than four gears for a multispeed discrete transmission, which is logical considering its wider range of power required as opposed to B-segment car. Based on these results, CVT seems more promising, provided that a reliable electro-mechanical actuation system can be successfully integrated in its pulleys system. To achieve this, further research works are still required, especially in the areas of the workability and durability of the electro-mechanical actuation system in the CVT, since such actuation system is still relatively new and has not been implemented previously in any commercialized CVT with metal belt. Other works discussing the application of CVT in EV powertrain were presented in [59; 60]. In [59], the potential of CVT's continuous ratio range to improved EV power consumption was assessed against the single-speed, two-speed AMT and two-speed DCT EV powertrains. In the assessment, all types of transmissions were considered to have the same constant efficiency of 97%. The assessment was conducted based on an analytical model of the motor's efficiency, and it showed that with CVT, the powertrain efficiency can be improved by about 3% for WLTP cycle against the other discrete multispeed transmissions. However, more detail analysis, particularly on the CVT's efficiency, is required, because the application of CVT conventionally involves high hydraulic pressure for clamping and ratio shifting. Thus, without optimization on the hydraulic actuation system, it is inappropriate to assume that the CVT has the same efficiency as the discrete multispeed transmission. Subsequently, in [60], the CVT was considered to be using the optimized electro-hydraulic actuation system (more compact and requiring low power for generating the belt's clamping force) and the novel single loopset belt (as opposed to the typical metal pushing V-belt, hence more compact design and reduced power losses). On top of that, the possibility of downsizing of the motor was also studied, where it was achieved through the reduction of the rotor's diameter and inertia. Nevertheless, the work did not take into account the optimization of the transmission ratios and the shifting strategy. When the powertrain was simulated under WLTC, it showed a 12.7% efficiency improvement against the EV powertrain with single speed transmission. That, however, was less than the two-speed AMT that produced a 13.5% improvement. The lower improvement was very likely caused by the hydraulic actuation system. Even though the system was optimized, the required belt's clamping force was still very high (depending on the EV motor's torque) and must be provided continuously during operation.