## 4 Optimizing Power Flow in EV Powertrains
### Continuously Variable Transmission (CVT)  
The main motivation of utilizing CVT in EV, identical to the multispeed discrete transmission, is to provide variable transmission ratios so that the motor can operate optimally for diverse driving conditions. In general, there are many types of CVT available for automotive application, but in this review paper, the focus will be on CVT that uses pulleys and metal belt which is the most widely used type currently in automotive. Unlike multispeed discrete transmissions, CVTs with metal belts offer a continuous ratio range, which mean more ratios are available to be chosen to suit the driving conditions. In this sense, a CVT is more flexible than any multispeed discrete transmission, hence, the motor has a much better chance to operate optimally for a longer duration of the driving. However, this type of CVT has certain limitations in terms of power loss in the metal pushing V-belt, or metal chain, used to transmit the torque between the primary pulley; connected to the motor, and the secondary pulley; connected to the vehicle's wheels. The loss is caused by a portion of motor power consumed to produce the required high clamping force to clamp the belt for the torque transmission between the pulleys. Research in [55] discussed the possibility of controlling the appropriate CVT ratio using fuzzy logic based on the motor's efficiency mapping as the reference. The fuzzy control algorithm was tested in a simulation model developed based on three driving routes differentiated in terms of road gradients. While the controller helped in enhancing the motor's efficiency throughout the routes, more detail studied are still required particularly for the ratio and clamping force actuation system of the CVT which was not explained in the paper. Subsequently, in 2017, ref. [56] suggested that a CVT, with a possibility to clamp the belt using an electro-mechanical actuation system with self-lock capability, has the potential to increase the powertrain efficiency. They explained that, unlike conventional CVT that uses engine power to generate hydraulic pressure to clamp its metal belt, such CVT eliminates the required power consumption for the clamping since the self-lock mechanism can held the clamping force. Thus, more power can be transmitted to the wheels, and its ratio can also be selected more efficiently. For an EV, this is particularly beneficial since the motor can operate with high flexibility, resulting in improved powertrain efficiency and increased driving mileage. However, to incorporate the self-locking mechanism required extensive design modifications on the CVT's pulleys as well as integration of the DC motor to actuate the mechanism accordingly. Next, refs. [57; 58] reported their research involving an evaluation of four different DCTs and a CVT applied in an EV. The four DCTs were differentiated in terms of the number of gear ratios (from single to four-speed), and the size of the gear ratios were determined based on the gradient climbing requirement (first gear), high speed driving (top gear) and the progression factor for the intermediate gears. Thus, the gear sizes, as well as the gearshifting strategy, were not optimized based on suiting any driving cycle. The ratio range of the CVT, on the other hand, was defined based on the continuous ratio range between the first and the top gears of the DCTs. In addition, the CVT was also considered to use electro-mechanical actuation system, instead of the conventional hydraulic system, to vary its ratio and to clamp the belt. Therefore, its efficiency was considered to be significantly higher as compared to the existing CVT used in any ICE vehicle. Also, the manufacturing cost for the CVT was considered to be lower than the two-speed transmission in the research. All the transmissions were then simulated based on a hybrid driving cycle established by combining FTP-75 and HWFET.