## 2 Rule-Based Strategies
### CD-CS Strategy and Blended Strategy  
The limited charge/discharge capability and low battery capacity/power of conventional HEVs leads to a small operating area of battery SOC; therefore, the battery SOC of the conventional HEV has  
Figure 2: Depiction of the main research areas for rule-based strategies. a certain convergence. In other words, the final value of SOC should be same as its initial value over the whole driving cycle, which is referred to as charge sustaining (CS) mode. In CS mode, vehicles are powered by an electric machine, engine, or both to maintain the battery SOC. Plug-in hybrid electric vehicles (PHEVs) can be regarded as a combination of an electric vehicle (EV) and a conventional HEV, which have the all-electric capability of an EV and the extended range capability of an HEV. Due to the ability to charge via an external grid, the EMS of PHEVs increases charge depleting (CD) mode. In CD mode, the electric machine is primarily used to power the vehicle with a net decrease in battery SOC, while the engine turns on when the electric machine cannot provide the required power or SOC drops too low. Based on the above characteristics, the charge depleting-charge sustaining (CD-CS) strategy, as the simplest method, is first proposed to generate an optimal SOC trajectory. This strategy operates in an all-electric mode in its initial stage, following by CS mode when the battery SOC decreases to a predefined low threshold along with an increase in driving distance. It is generally desirable that the battery reach its depleting limit by the end of the given driving distance. Furthermore, simulation results indicate that the CD-CS strategy, along with electric assistance, is more effective in PHEV than in HEV that have the same battery energy capacity as PHEV [12]. A CD control strategy for a generic parallel PHEV (SUV) was investigated in [34], which uses electric power to drive the vehicle until the power demand reaches a preset threshold, and then the engine turns on to meet the desired power requirements with the assistance of the motor. Meanwhile, the mechanical power of the motor remains constant from when the engine turns on until the end of the drive cycle. With the development of intelligent transportation systems (ITSs), geographical information systems (GISs) and global positioning systems (GPSs), the trip information can be obtained in advance for a certain route. Therefore, if the basic trip information can be acquired in advance, the CD strategy would slightly increase the fuel economy of a PHEV compared with the AER strategy [35]. Although the CD-CS strategy is characterized by its simplicity, ease of implementation, and allowing HEVs to operate as an \"electric vehicle\", it is far from obtaining the optimal fuel economy of energy management for HEVs. To obtain relatively satisfying fuel economy, the CD-CS strategy should equip a relatively large battery to meet power performance requirements in CD mode over the whole driving cycle, which will increase the vehicle cost. Generally speaking, sufficient CD operations may lead to more electric loss and a sharp decrease in engine efficiency under high power demand, while inadequate CD operations may not well achieve an improvement of fuel economy and make full use of EMS. To further reduce fuel consumption, the blended strategy with gradual battery depletion in a blended mode (BM) is implemented for an optimal EMS of HEVs. Engine and motors would operate in coordination, causing the electricity to not be completely consumed until the end. A blended-mode HEV can achieve cruising with low electric drive. Simultaneously, it can moderately accelerate at low to moderate vehicle speeds in electric mode. Under moderate to high vehicle speeds, the engine will turn on to meet power/torque requirements, and the motor will serve as auxiliary power to provide more power/torque when the output power of engine cannot satisfy power demand.