How Regenerative Braking Works

Regenerative braking is something that is becoming a modern standard for vehicles of all descriptions as what was before just wasted energy used to slow a vehicle down is now being captured for re-use by the vehicle, making it more efficient.
image courtesy of Toyota
 

by Aaron Turpen

Overview

In general, regenerative braking is the capture of momentum from slowing the vehicle down so that the energy in the vehicle's momentum is not lost. Electric vehicles, such as hybrids, usually capture the regenerative braking energy as electricity for re-use by the drive train. Other vehicles capture the regenerative braking as a kinetic force to be released later to assist the powertrain or other vehicle components.

In all regenerative braking systems, the kinetic force (momentum reduction) of slowing the vehicle down is captured for storage or re-use.

Electrical Storage of Braking

The most common application of regenerative braking in passenger vehicles is to convert momentum loss to electricity. This is often a feature of electric, hybrid-electric, and similar vehicles. The energy stored as electricity is then returned to the drive train during acceleration or is used for other vehicle systems such as climate controls and other electrical components.

The Toyota Prius, which recently underwent a recall for its regenerative braking system, has its regen braking as part of its Hybrid Synergy Drive. In this setup, the motors which propel the Prius are turned in reverse during braking and become generators, returning power to the batteries. For most electric vehicles and hybrids, this is the most common way to recapture braking power.

In most of these types of vehicles, all vehicular controls are electronic, including brakes. In the Prius, for instance, the amount of brake pedal pressure denotes how much braking power is manual (physical) versus how much is recaptured (electronic). The harder the driver presses the brake pedal, the more manual effort is used to stop the car. Most other hybrids and electrics work in a similar fashion, using software to control the amount of braking that is regenerative and how much is manual. These are carefully calibrated for safety reasons.

Most of these vehicles capture braking power only from one axle rather than from both. This is also due to safety requirements. This allows for redundancy so that if the electronic braking fails, the manual brakes still control one of the axles so the vehicle can be stopped.

Mechanical Storage of Braking

The other most common type of regenerative braking is mechanical or kinetic. The two schools of thought in this system are to use flywheels or compressed fluids.

Flywheels are being tested in race cars, as the duration of the storage is short-lived and race vehicles gain and lose speed quickly during a race. The most well-known of the flywheel storage systems is the KERS (Kinetic Energy Recovery System) setup. These were tested in Formula One racing and are now being tested in Gran Tourismo racing.

In the KERS method, energy from slowing the vehicle is captured through the drive train and stored in a revolving flywheel. The flywheel spins faster and faster as the energy is introduced. When the driver accelerates, the flywheel slows, sending its energy back into the drive train to help propel the vehicle. The KERS system being used in GT racing is capable of rotating at up to 40,000 rotations per minute and can store energy for up to 8 seconds.

In the compressed fluid method, energy from the slowing vehicle is sent through the drive train and used to pressurize a fluid. Most often, this is used in heavy vehicles to compress hydraulic fluid. This form of regenerative braking has a longer storage time than KERS, being roughly equivalent to electrical storage in a battery.

When the vehicle accelerates again, the compressed hydraulic fluid is released back through the drive train to help propel the vehicle, taking a load from the engine. This system has the added advantage of being able to be retrofitted to a vehicle after the factory.

In Mass Transit

The idea of regenerative braking was pioneered in mass transit systems. Many modern mass transit trains and trollies are electric and thus were prime experimental platforms to test regenerative braking with. Most electrical trains do not use manual brakes, instead relying on what are called traction motors. These are electric motors which provide resistance to the wheels turning, slowing them down.

By capturing the energy in these traction motors, engineers were able to build regenerative braking into existing train systems. Most of the electrical regeneration technologies used today were started on rail systems.

Regenerative Braking in the Future

As designers and engineers perfect regenerative braking systems, they will become more and more common. All vehicles in motion can benefit from utilizing regeneration to recapture energy that would otherwise be lost.

Future technologies in regen brakes include new types of motors which are more efficient as generators, new drive train designs which are built with regenerative braking in mind, and electric systems which are less prone to energy loss.

Of course, problems are expected as any new technology is perfected, but few future technologies have more potential for improving vehicle efficiency than does regenerative braking.

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