## 4 Optimizing Power Flow in EV Powertrains
### Multispeed Discrete Transmission  
The main motivation of applying multispeed discrete transmission in an EV, similar to ICE vehicles, is to provide the most suitable gear ratio in the powertrain so that the motor can operate efficiently for diverse driving conditions. Since EV motors typically are capable of producing high torque output from very low RPM, the number of gears for an EV is expected to be very minimal, as few as two gears, as opposed to the ones used in the ICE vehicle. Latest works done to evaluate the efficiency difference between the EV powertrain with single-speed transmission and two-speed transmission were described in [27; 28]. In [27], the comparison was conducted using simulation model-based estimation by taking into account vehicle parameters, reference motor's data and three driving cycles, namely the New European Driving Cycle (NEDC), Worldwide Light Duty Test Cycle (WLTC) and US Environmental Protection Agency (EPA) Federal Test Procedure for city driving (FTP-75). Firstly, the simulation model was run using an EV powertrain with a single speed transmission for the three driving cycles. Thereafter, the simulation results of the WLTC were used to determine the appropriate size of two gear ratios for improving the power consumption, and then the model was rerun using the newly determined ratios. Using the WLTC results, instead of NEDC and FTP-75, was logical, considering that it is the most power demanding cycle that covers diverse driving phases of urban, suburban, rural and highway scenarios. Besides, it also saves a significant amount of work and computing time as opposed to using the results from all three driving cycles. The comparison results showed that efficiency improvements were measured at the range of 1.7 to 2.4% with the two-speed transmission for the three driving cycles. It must be highlighted, however, that [27] emphasized on obtaining the gear ratios for powertrain efficiency only, without consideration of the driving performance. In terms of driving performance, the typical target is to achieve fast acceleration with minimum jerking, which leads to contradictory requirements between this and achieving maximum powertrain efficiency. Besides, the gearshifting model must also be incorporated into the powertrain simulation model to allow a realistic evaluation of the jerking. Finally, an advanced optimization method must be implemented in the model for optimizing the gear ratios for powertrain efficiency and fast acceleration with minimum jerking. Figure 4: The number of shortlisted references related to the methods used for optimizing the power flow in an EV powertrain. In the subsequent work by [28], the comparison was conducted using an electric bus model that runs in a specific city driving cycle with a two-speed dual clutch transmission (DCT). A DCT allows fast gearshifting thanks to its capability to pre-select the next gear before the shifting is done by the engagement of the second clutch. Such capability is not available for a single clutch automated manual transmission (AMT) and a conventional automatic transmission. The driving cycle, on the other hand, was obtained based on an existing bus route in Espoo, Finland. The powertrain model took into account not only the vehicle parameters and the motor's data, but also the efficiency mapping of the inverter. Based on the model, an exhaustive search algorithm was implemented to determine the size of the two gear ratios for optimum efficiency. The results proved that, first, the efficiency gain was in the range of 2 to 3.2%, which is consistent with the findings in [27], secondly, the application of two-speed transmission opened up the option to use a more cost-effective motor with a narrow high efficiency range, and, lastly, further studies are still required to evaluate the application particularly in terms of maintenance cost, to assess how much higher the cost will be as compared to single-speed transmission.