## 4 Optimizing Power Flow in EV Powertrains
### Continuously Variable Transmission (CVT)
Hence, continuous power to generate the clamping force, albeit lower thanks to the optimization, was still needed. The AMT, on the contrary, used geartrain to transmit the power, hence no requirement for the belt's clamping force. This means that the continuous power for the clamping force was eliminated entirely. This situation also affected the power flow in the powertrain, which can compromise the driving performance and this can be observed in the 0-100 km/h acceleration time, where the AMT and the single speed transmission yielded 6.9 s, while the CVT achieved 7.4 s. Based on the results, it appeared that AMT is the better transmission for EV than the CVT, although it must be noted that with the latter, it is possible to eliminate torque interruption during ratio shifting. Research work described in [61] explained optimization and discretization of the CVT ratios so that optimum power consumption can be realized with as minimum shifting as possible. The CVT featured an electro-hydraulic actuation system, where an electric pump was used to precisely control the required hydraulic pressure for clamping and ratio shifting (Figure 9). The rationale of discretizing the ratios was to avoid too frequent shifting would lead to uncomfortable driving due to jerking, as well as power losses in the hydraulic actuation system. The discretizing process started by first establishing the appropriate number of ratios based on the relation between the energy cost and the ratio number. Hence, the number of ratios was set at four, and the ratio sizes were determined through an equal ratio series method. Then, these ratios were optimized using GA for optimum efficiency when undergoing a combined driving cycle that comprised of UDDS, NYCC and HWFET. For the driving cycle simulation, three ratio shifting strategy models were employed; first, continuously variable ratio shifting strategy, where the best ratio was selected continuously during driving for maximum efficiency, second, the discrete ratio shifting strategy based on the ratios established through the equal ratio series method, and third, the discrete ratio shifting strategy based on the ratios optimized through GA. Comparison between the results confirmed that the third strategy performed the best, with the minimum total power consumption and average jerking, measured at 8.10 kWh and 4.32 m/s\({}^{3}\), respectively. The first and second strategies, meanwhile, recorded 8.16 kWh and 5.35 m/s\({}^{3}\), as well as 8.69 kWh and 4.65 m/s\({}^{3}\), respectively. To summarize, the work reported in [61] highlighted two very important findings. Firstly, CVT provides a continuous ratio range, hence the ratios can be discretized and optimized to suit diverse vehicle parameters, which means the same CVT can be implemented for several type of EVs for optimum driving performance and efficiency. Secondly, high ratio number presents better flexibility for motor's operation, but it also leads to complicated shifting logic which will cause too frequent shifting, resulting in the power losses in the actuation and compromised driving comfort. This presented an opportunity to apply the same CVT with different sets of discretized ratios to suit the requirements of diverse EV segments, which can contribute in terms of cost reduction in the transmission production. One of the latest works describing the application of CVT in an EV powertrain can be accessed in [62]. This work presented an optimization of the EV ownership cost by taking into account the components' cost and the electricity cost for all the components involved in the powertrain; battery, motor (with inverter), and, CVT. The optimization was carried out using convex programming design optimization, with the targets to minimize the cost; by a means of optimizing the size of the motor and battery, without compromising the driving performance in terms of 0-100 km/h acceleration time (at most 11 s), top speed of 165 km/h and gradeability of 30%.