Auto Electronics

Regenerative Braking Free Energy

The right hardware and control strategy allow the vehicle to recoup the braking energy rather than have it just generate heat. Using regenerative braking, fuel economy improvements range from a few percent to as high as 25% or more.

Regenerative braking is an integral part of hybrid and electric vehicles. In a micro hybrid, regenerative braking adds more than the basic stop-start system to improve mpg. However, regenerative braking is not limited to hybrids. Other vehicles can take advantage of the kinetic energy captured from deceleration.


Using the propulsion motor in hybrid, electric and plug-in hybrid electric vehicles to provide regenerative braking is a common design practice. Regenerative braking re-captures and stores part of the kinetic energy that would otherwise be lost to heat during braking. The captured energy is used to recharge the electric batteries reducing the fuel consumption in the hybrid architecture.

“One of the objectives of regeneration is to keep the battery at a state of charge that allows you to use the electric motors more often,” says Dan Milot, Chief Engineer of New Product Development for Electronic Braking Systems, TRW Automotive.

A mild hybrid with stop/start and regenerative braking can provide up to 7% fuel economy savings over a driving cycle. The contribution of regenerative braking is about half of the total savings. Regenerative braking is part of the 25 to 50% gain in fuel economy on the Chevrolet Tahoe or GMC Yukon hybrid SUV (that use a TRW system for regenerative braking) over its non-hybrid counterpart. A plug-in hybrid, like the Chevy Volt, targets a higher, 40-mile range operating exclusively on electric power without using gasoline to recharge the battery. The 40-mile range is possible, in part, from the regenerative braking energy.

Since the motor in a mild hybrid is considerably smaller than a large SUV, such as the Tahoe, the regenerative contribution can be even greater in the full hybrid. “When you get into a full hybrid or even these plug-in hybrids or you are driving an electric, obviously the fuel economy gains are larger and the dependency on regen braking to give you those fuel economy gains goes up,” says Milot.

In some mild hybrids with mild regenerative braking, carmakers may not blend the electric and hydraulic brake operation. “If it's small enough, they won't do anything with the brake systems in terms of compensating for decel changes,” said Dave Agnew, Manager, Brake Systems Engineering for North America, Continental. The carmaker simply adds the regen on top of the standard hydraulic brake system. In contrast, Continental's second-generation Regenerative Brake System (RBS), shown in Figure 1, blends the capabilities of the friction brakes and the regenerative braking from a hybrid's motor running as a generator.

Continental's first-generation regenerative brake system, called electro-hydraulic braking (EHB), was used on the Ford Escape. The new system uses a completely different architecture. “The strategy that we took with it was any brake by wire system ends up being a very complicated overall system and you also end up with very complicated components,” said Agnew. With system and component developing occurring at the same time in a safety-related system, the risk increases.

To avoid higher risk and higher component and system costs, the goal for the next-generation system was to use proven brake components as much as possible. “The vacuum booster, the master cylinder and the reservoir are all essentially current mass production parts with some modifications,” said Agnew. “The only new development parts were a vacuum pump and a pedal feel simulator, which is just a mechanical spring pack and also a cut-off device on the pedal-feel simulator.” The cut-off device is used for a fall-back mode, in case the system has an electrical failure. Even this unit uses standard components out of one of Continental's ABS units. Figure 2 shows the components in the second-generation architecture.

TRW has two systems for regenerative braking: Electronic Stability Control — Regenerative (ESC-R) and Slip Control Boost (SCB). Both systems target hybrids for regeneration and both are based on ESC and operate from the 12 V architecture. “The ESC-R is specifically designed to support a regenerative braking blending platform, vehicles with hybrid electric or electric powertrains that want to do some recapturing of energy,” said TRW's Milot. Figure 3 shows the ESC-R system architecture.

TRW's ESC-R uses a conventional booster and master cylinder, with a pedal simulator that is attached downstream from the electro-hydraulic control unit or EHCU. When the driver pushes on the pedal, the booster master cylinder pushes fluid into the simulator, so the driver gets the perception he's actually pushing into the brakes. “You measure the signals of pedal travel and pressure being generated in the simulator and then you determine how much braking you want to achieve,” explained Milot.

Some of the braking comes from the electric motors in the hybrid powertrain running as a generator. When the electric motor generates power back into the battery, it puts a load on the driveshaft. The additional braking requirements requested by the driver are provided by the hydraulic portion of the brakes.

TRW's Slip Control Boost (SCB) (Fig. 4) was originally designed and put into production on a platform that was a hybrid. However, the technology is a hydraulic boosted brake system and does not use vacuum boost. SBC also targets a conventional vehicle where the carmaker wants to replace the vacuum portion of the brake system with a hydraulic boost device.

Although the SCB system can be used on non-hybrid vehicles, it does not provide the regenerative portion of brake blending on those vehicles. In these applications, the system provides the driver a simulated pedal feel, computes the driver's deceleration intent and applies the boosted hydraulic pressure to the brakes. Future powertrains such as gas direct injection (GDI) and GDI with added turbo charging and even diesel engines have significant mechanical losses due to vacuum pumps. As a result, eliminating the use of vacuum is under serious consideration. A hydraulic boosted system like the SCB is an alternative to a vacuum-boosted brake system.

ADVICS Co., Ltd., a Japan-based joint venture between Aisin Seiki, Denso Corporation, Sumitomo ElectricIndustries and Toyota Motor Corporation, also provides regenerative braking systems to the leading hybrid manufacturer, Toyota. These systems optimize the utilization of braking energy in electric and hybrid electric vehicles by controlling the balance between hydraulic braking and regenerative braking to offer greater vehicle range.


BMW took a quite different approach to regenerative braking to provide improved fuel economy to vehicles that do not rely on electrical propulsion. With the Brake Energy Regeneration (BER) system, BMW has improved fuel efficiency by up to three percent on a standard non-hybrid vehicle.

Using a specially designed alternator as the electrical machine for generating the energy, the BER system implements a unique charging system strategy. Instead of constantly running the alternator to maintain a high level of charge in the battery, the system only charges to 80% of its capacity using power from the engine. This relieves the power requirements to drive the alternator from the engine.

When the driver applies the brakes, the Intelligent Alternator Control (IAC) is activated to provide additional energy to the battery. The alternator is also engaged in overrun conditions. The system maintains a reserve charge adequate for the power requirements while the car is idling and sufficient to start the engine under all circumstances. From the braking side, the vehicle's conventional brakes contribute whatever additional braking is needed to meet the driver's commands.

The IAC also uses an absorbent glass mat (AGM) battery to handle greater charging and discharging requirements than a conventional lead acid battery. Since its initial introduction, BMW has offered the system on vehicles in its 1 series, 3 series, 5 series, 6 series, 7 series and even the Mini Cooper.


Bosch Rexroth has a regenerative braking system that does not require a hybrid vehicle. In fact, it does not involve electrical storage. The Hydrostatic Regenerative Braking (HRB) system is intended for commercial vehicles and mobile equipment. The company says that initial measurements show that the HRB system reduces the fuel consumption in these vehicles by up to 25%. Figure 5 shows the key components in the system.

In the HRB system, braking energy is converted to hydraulic pressure and stored in a high-pressure hydraulic accumulator. When the vehicle accelerates, the stored hydraulic energy is applied to the transmission reducing the energy that the combustion engine has to provide. An electronic controller and a hydraulic valve manifold control the process.

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