Since kinetic energy is the energy of motion, you could probably guess that cars create lots of it. Capturing some of that kinetic energy for the sake of fuel efficiency in a hybrid car is a little tricky, but regenerative braking is one common method employed by many automakers.
On a non-hybrid car during a routine stop, mechanical braking slows and then stops the vehicle. For instance, if your vehicle has disc brakes, the brake pads clamp down on a rotor to stop the car. If your car has drum brakes, the brake shoe pushes the brake lining material outward toward the brake drum surface to slow or stop the car. In both cases, most of the kinetic energy in the spinning wheels is absorbed by the pads or the drums, which creates heat.
On a hybrid car that uses regenerative braking, the electric motor is used to slow the car. When the motor is operating in this mode, it acts as a generator to recover the rotational kinetic energy at the wheels, convert it into energy and store it in the car's batteries. When the driver of the hybrid car takes his or her foot off of the accelerator pedal, the resistance provided by the generator slows the car first and then the mechanical brake pads can be applied to finish the job. Of course, the mechanical brake pads can also be engaged immediately in an emergency braking scenario.
The car uses the energy stored in the battery to power the electric motor which drives the car at low speeds. Depending on the type of hybrid, the electric motor can either work alone to move the car or it can work in concert with the car's gasoline-powered engine. So regenerative braking, coupled with eco-friendly driving techniques like slow starts and slower overall vehicle speeds, is an important feature on some of some of the most fuel-efficient vehicles on the road today.
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