In a traditional car, when the driver applies the brakes, the kinetic energy from the vehicle’s forward motion converts into heat through the friction of the brake pads against a disc or a drum. That heat dissipates into the air, a waste of otherwise useful energy.
Furthermore, modern cars are loaded with electronics, from infotainment systems to advanced driver assists. They need electricity and lots of it. With a traditional powertrain, a belt-driven alternator supplies that juice. It comes from the gas engine that turns the belt to spin the alternator. That belt-driven alternator reduces the engine’s efficiency.
A regenerative braking system is designed to capture the kinetic energy traditionally lost during coasting and braking. It also reduces the need for alternator-generated power. That in turn makes a vehicle more efficient.
Regenerative braking systems are common in hybrid and electric vehicles (EVs), though they are not exclusive to them. Mazda recently offered one with its gasoline-fueled 2.5-liter 4-cylinder engine to power onboard electronics and improve fuel economy. This extra braking reduces the load on the engine and makes the car feel more powerful. It also improves mileage.
Most regenerative braking systems rely on an electric motor that turns into a generator when the vehicle is coasting or stopping. This is why they’re most common in hybrid and EVs. In generator mode, the electric motor captures the otherwise lost kinetic energy, converting it into electricity and storing it in the powertrain’s battery.
When the electric motor operates as a generator to capture kinetic energy, it transforms it into electricity to keeps the battery charged and ready to assist with acceleration. It also serves to power accessories when the automatic stop/start system turns off the gasoline engine. At lower speeds, the hybrid can use this power for longer periods of time in EV mode, further reducing the use of the gasoline engine.
Regenerative braking helps to preserve battery life in pure EVs. It can extend the driving range or reduce the amount of necessary recharging. On all vehicles, regenerative braking reduces the amount of hydro-mechanical braking a driver performs, extending brake life and reducing brake dust.
Early regenerative braking systems felt funny to a driver. The added friction in the drivetrain produced more slowing than normal when lifting off the accelerator pedal. The pedal at times could feel like an on/off switch during brake application. This non-linear feel makes it hard to modulate smooth braking.
Newer systems feel more natural to a driver. In hybrids, blended regenerative braking systems improve pedal feel from behind the wheel, making the cars more enjoyable to drive. In EVs, drivers may be able to calibrate how aggressively the regenerative braking works. Some EVs, like the Chevrolet Bolt EV and the Nissan Leaf, provide one-pedal driving because the highest setting can bring the car to a full stop. A careful driver who plans in advance doesn’t need to use the brake pedal at all.
Regardless of the type of vehicle in which they’re used, the point of regenerative braking systems is to increase efficiency and reduce carbon dioxide emissions. With a hybrid, regenerative brakes improve gas mileage and reduce the amount of gasoline the vehicle burns. With an EV, battery preservation reduces electricity consumption. If your local utility relies on fossil fuels to create electricity, using less of it reduces carbon dioxide emissions.
Clearly, aside from odd pedal feel when the driver presses on the brakes, there is no downside to a regenerative braking system. And in the latest hybrids and electrics, improvements in feel is making these systems as seamless as conventional brakes.
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