Final answer:
The charge-depleting mode in hybrid and plug-in hybrid vehicles is when the vehicle uses the stored electrical energy until it reaches a low state of charge, after which it may switch to a charge-sustaining mode that uses the internal combustion engine.
Step-by-step explanation:
The charge-depleting operating mode, often referred to as CD mode, is a functional state in hybrid and plug-in hybrid electric vehicles (PHEVs) during which the vehicle primarily runs on electrical energy stored in the battery. In this mode, the vehicle utilizes the electric charge until it reaches a predetermined lower state of charge, at which point it may switch to a charge-sustaining (CS) mode. In CS mode, the internal combustion engine (ICE) generates power to maintain the battery's state of charge. The transition between CD and CS modes is managed by the vehicle's powertrain control system.
The primary goal of charge-depleting mode is to maximize the use of the electric drive and to minimize the use of the internal combustion engine, thus reducing fuel consumption and emissions. This mode is particularly beneficial in urban environments where frequent stop-and-go driving can be efficiently managed by the electric motor.
A PHEV begins its journey in charge-depleting mode if the battery is sufficiently charged. Once the battery's state of charge drops to the predetermined threshold, the vehicle's system switches to charge-sustaining mode to preserve the remaining charge for later use or for conditions that are more efficient for electric drive.