What are the key components of a hybrid electric vehicle (HEV) powertrain, and how does it operate to improve fuel efficiency and reduce emissions compared to traditional internal combustion engine vehicles?

A hybrid electric vehicle (HEV) powertrain typically includes an internal combustion engine, an electric motor, a battery pack, and a power control unit. During operation, the vehicle can utilize either the internal combustion engine, the electric motor, or both in combination, depending on driving conditions and power demands. The hybrid system allows for regenerative braking to recharge the battery, and the electric motor can assist the engine during acceleration, reducing fuel consumption and emissions.