## 4 Results and Discussion
### Numerical Analysis of Characteristic of SOC and Current Balance with IM240 Mode  
Driving torque and battery capacity were applied as variables to the CRUISE model developed in this study. The numerical analysis under the IM240 driving mode yielded the following results for SOC characteristics and current balance. Figure 9a shows the SOC drop between Phase 1 and Phase 2 for the three engine-torque cases listed in Table 4. This figure illustrates the difference in the battery SOC values between Phase 1 and Phase 2 in the IM240 mode. We note that during Phase 1, the SOC drop is lower than that during Phase 2. During Phase 1, the maximum speed does not reach 60 km/h, and speeds between 30 and 40 km/h account for approximately half of Phase 1. Therefore, the influence of motor driving is dominant, and the effect of reduced engine power is considered to be insignificant compared to that during Phase 2. At low speeds, engine operation is negligible, and the motor operates primarily. The difference in SOC drop was not significantly caused by the energy recovered by the regenerative braking of the drive motor and the advantage of the motor, which is more efficient than the engine. In Phase 2, the proportion of high-speed driving is higher than in Phase 1, and the engine-operating period is longer. The SOC difference is large at the end of the driving mode due to the increased amount of charging current under the high-torque condition of the engine. The driving force is generated by the motor at speeds up to 30 km/h, but at higher speeds, the engine is also used as a power source. In Case 1, the torque of the engine is relatively high, so the amount of electric energy charged by the engine is higher than in other conditions. The lower the engine torque, the lower the amount of power generated, the lower the charging, and the SOC drop gradually decreases. Figure 9b shows the SOC drop for the three cases of battery-capacity listed in Table 5 for Phase 1 and Phase 2 in the IM240 mode. As the battery capacity decreases, the SOC drop gradually increases. In particular, Phase 2 consists of a high proportion of high-speed driving, and the increase in SOC drop is larger than in Phase 1. As the engine torque decreases, the SOC drop increase compared to Case 1. As a result, the difference in SOC drop gradually increases due to the reduction in engine torque during Phase 2, which is greatly affected by engine torque. When the capacity of the battery is high, the electrical energy generated by the engine can be smoothly charged to 50% SOC. For efficient battery use, the SOC is charged to near 50%. Therefore, if the battery capacity is small, it is charged to 50% quickly, and the extra electricity cannot be charged. For this reason, the difference in SOC drop during Phase 2 occurs because the change in torque is more significant than the change in battery capacity. Figure 10a shows the SOC drop after completing of the entire driving cycle in the IM240 and FTP-75 modes. The FTP-75 mode has a longer driving time (approximately eight times longer) and a higher proportion of high-speed driving than the IM240 mode. Consequently, the load of the drive motor increases as engine power decreases, which increases the consumption of electrical energy. Therefore, the SOC drop is very sharp during the FTP-75 mode. The amount of electrical energy charged with lower levels of engine torque is predicted to decrease, and the SOC drop is more likely to occur as mileage increases. To maintain a constant SOC under conditions when the engine torque is decreasing, the fuel efficiency can be improved by applying a strategy of artificially operating the engine at optimum efficiency. The SOC drop due to a reduction in engine torque is expected to increase as mileage increases. To maintain the battery SOC constant, the length of time when the engine operates as a generator must be increased. Figure 10b shows the SOC drop due to a reduction in the lithium-ion battery capacity during the IM240 and FTP-75 modes.