*2.1.2. Electric drives for extended-range/hybrid electric powertrain with participation of engines*

In a series HEV (SHEV), the internal combustion engine (ICE) is the main power source that converts the original energy in gasoline to mechanical power. The electric motor is actuated by the electricity generated by the generator or electricity stored in the battery, or both. The vehicle speed does not affect the engine speed because the engine and the wheels are decou‐ pled. Thus, the engine can operate at its optimum speed to achieve the best fuel economy and be placed more flexibly.

A parallel hybrid electric vehicle (PHEV) adds a mechanical link (such as a clutch, belts, pulleys, and gears) between the engine and the final drive, so the engine can drive the wheels directly. The mechanical coupling adjusts the torque from the engine and the motor depending on the operation mode. The parallel hybrid electric vehicle has two propulsion devices, which can be used in motor-alone mode, combined power mode, engine-alone mode, power split mode, stationary charging mode, regenerative braking mode.

The series–parallel HEV can be operated as a series or parallel HEV. In comparison to a series HEV, the engine is coupled to the final drive through a mechanism, so the engine can deliver power to the final drive directly. In comparison to a parallel HEV, the extra mechanical output of the engine can be converted to electricity directly using the generator. The more complicated configuration brings out more flexible control strategy, better optimization of fuel economy and driveability, as well as higher cost.
