Regenerative Braking

When you hit the brakes, the car's kinetic energy is converted to heat through friction—throwing away the energy that was previously used to accelerate the car. In city driving, about 30 percent of a typical car's engine output is lost to braking. This proportion drops to almost zero in highway driving, where braking is much less frequent.

Have you ever followed a large truck down a long hill and smelled the acrid smoke from overheated brakes?

The heat that causes parts of a truck's brake system to melt and create smoke comes from friction. Traditional brake systems grip metal disks or drums, using friction to slow or stop the rotating wheels of a vehicle. The friction of the brakes resists the forward momentum of the whole vehicle, and that friction creates heat.

In order for something to heat up it takes energy. The energy that heats up a truck's brake system comes from its momentum, speed, and mass. Where does a truck's momentum come from? It comes from fuel. Traditional brake systems, like those on large trucks, waste energy by converting forward momentum into heat.

One of the energy efficiency advantages of hybrid-electric technology over traditional drivetrains is regenerative braking.

A hybrid-electric vehicle uses an electric motor to create torque to drive its wheels. Interestingly, electric motors can be designed to be virtually identical to electric generators. This means an electric motor can either use electricity to create torque, or reverse the process to use torque to create electricity.

This "reversability" of electric motors is very different from the internal combustion motors in most cars. Can you imagine turning the wheels of a typical car backwards and having gasoline pour into the tank? Essentially this is what happens when you put your foot on the brake of a Toyota Prius or Honda Insight hybrid-electric vehicle.

When a hybrid-electric vehicle is approaching a stop light, it does not create friction and useless heat in order to slow down. Instead it reverses its electric motor turning it into an electric generator, creating electricity which is fed back into a battery and stored for when the light turns green. In fact any time a hybrid-electric vehicle slows down, lifting the accelerator or application of the "brake" causes the system to use the vehicle's momentum to generate electricity.

Hybrid-electric vehicles with regenerative braking can save a great deal of energy when compared to traditional cars, especially in "stop-and-go" driving situations.