# Development of a Hybrid Electric Vehicle Simulation Tool with a Rule-Based Topology
## 2 Materials and Methods
### Control Strategies of the Simulation
#### 2.2.2 Rule-Based Control Strategies
As a control parameter, the SOC is divided into five levels: \"SOC_min\" is the minimum level of SOC for the battery function, \"SOC_low\" is a variable level to represent the relatively low level of SOC based on the driver's opinion for the optimization. \"SOC_initial\" is the SOC at the beginning of the simulation, and \"SOC_high\" is again a variable SOC representing a high level of SOC, which can set by the driver. \"SOC_max\" is the maximum SOC of the given battery. In principle, the drivers are able to design their own control topology by themselves in the HEV simulation tool (e.g., state-flow box or Fuzzy logic toolbox). According to Figure 3, the judgment conditions of each working state are set, and the target torque of the motor and engine can be varied upon driver request. Region I is the economic zone, in which EV mode is mostly used to minimize fuel consumption. The engine runs only when the low limitation of SOC is reached. Region II is the area where the e-driving mode is slightly reduced, depending on the SOC, and the engine runs to charge the battery when the SOC is lower than SOC_low. In Region III, the engine is operated mainly near the optimum point in the minimum fuel consumption area to manage the required torque from the wheel while the battery is charged. The EV mode can only be handled when the SOC is above SOC_high in the region. In Region IV, the vehicle runs mostly like a conventional ICE vehicle, with the highest efficiency from the engine map. The engine cannot achieve the very high torque requirements from the wheels, like in Region V, and thus the EM and the ICE are coupled to deliver very high torque in hybrid mode if the SOC is sufficient. The engine runs with the highest torque to minimize the drop of SOC in this region. In all cases, the vehicle is basically not in a stationary position, and from the figure, the parameters in black (e.g., SOC_min, SOC_initial, SOC_max) are a fixed value, while the red parameters (e.g., SOC_low, SOC_high, Te_min, Te_low, Te_ch, etc.) are varied for the optimization process (Section 3). SOC_min, SOC_low, SOC_initial, SOC_high and SOC_max, and SOC_low and SOC_high are again the flexible values for the optimization process. According to the power demand of the wheels, the operation modes have been divided by the different levels of power from the EMs, and it also depends on the number of engine OPs from single OP to multiple OPs. Figure 4 shows examples of the engine OPs for serial hybrids with single OP (the symbol with the red star) and multiple OPs (the symbols with blue stars). The control strategy for a single OP for serial hybrids is relatively simple. As long as the SOC is above the threshold value, depending on the power demand of the EM, the ICE will be at rest. The ICE with generator is switched on to charge the battery when the SOC of the battery is depleted. Both the ICE and generator run in their most efficient areas. The serial hybrids with a single engine OP are ideal for serial hybrids with large battery sizes, like range-extender hybrids. Mostly, EV mode is used until \"Pm_eff\" (Pm_eff is the calculated power of the engine at the OP to transfer the power demanded by the EM), while the engine runs to charge the battery according to the SOC. In case of high power demand from the EM, the engine and EM run mostly together in hybrid mode to avoid high battery discharge in this region. In the case of the traditional serial hybrid topology with multiple engine OPs, three engine OPs (blue symbols in Figure 5, for example) are used in the case of the serial hybrid; OP2 is the most efficient operating condition in the given engine map, and OP1 is a relatively highly efficient point with low torque and speed. OP3 is the maximum power from the engine. The ICE power command depends on the threshold value of SOC at different levels of total power demand of the EMs. The power demands of the EMs is divided across the range from \"Pm_min\" to \"Pm_OP3\".