Final answer:
The feature that allows a hybrid Electric Vehicle's engine to start without warning is known as an automatic start-stop system. It's part of the vehicle's design to improve fuel efficiency and is supported by regenerative braking technology.
Step-by-step explanation:
The feature in a hybrid Electric Vehicle that allows the engine to start and run without warning is commonly known as an automatic start-stop system. This system is designed to improve fuel efficiency by shutting down the internal combustion engine when the vehicle comes to a stop and restarting it when the accelerator is pressed. These hybrid systems can also employ regenerative braking technology, where the electric motor functions as a generator during braking, converting mechanical energy (gravitational potential energy) into electrical energy to recharge the battery. The battery's life is prolonged, and overall vehicle emissions are reduced, making this feature particularly beneficial for both economy and environmental impact.