The range of hybrid and electric vehicles is set to grow significantly in the years ahead. To achieve the targeted fuel savings and range, these vehicles must, among other things, recover as much energy as possible from braking so that this can be used for electrical propulsion. Bosch is currently developing regenerative braking systems that, in tandem with the powertrain, enable optimum recuperation. “Ideally, all braking operations will be used in their entirety for energy generation,” says Dr. Werner Struth, president of the Bosch Chassis Systems Control division. “In situations where the electric motor's brake torque is not sufficient, the hydraulic braking system provides support. This goes unnoticed by the driver.” Bosch offers two types of regenerative braking systems – solutions that interact with classic vacuum-based brake boosters and vacuum-independent systems.
With today’s vehicles, the brake booster uses a vacuum to boost the power applied by the driver. The vacuum is generated by either the internal-combustion engine or a vacuum pump. However, hybrid vehicles switch off the internal-combustion engine as often as possible and electric vehicles do not have one at all. These vehicles therefore use either an electric vacuum pump or a vacuum-independent braking system.
Vacuum-based solution: ESP® hev
The ESP® hev regenerative braking system is based on the current Generation 9 and is therefore a very cost-effective solution. In hybrid and electric vehicles with a vacuum-based brake booster, ESP® hev coordinates the electric-motor and hydraulic brake torques and controls the vacuum pump. The product is being taken into series operation in a hybrid vehicle with rear-wheel drive and front-/rear-axle brake-circuit split. With ESP® hev for this drive and brake configuration, the rear brake circuit is decoupled from the driver’s foot. An increased dead stroke of the brake pedal is used to decelerate the vehicle initially, using only regenerative power via the electric motor connected to the rear axle. For this purpose, the system requests generator torque equivalent to the pedal travel. It is limited to 0.2 g, which is around 20 percent of the maximum braking performance. This covers almost all the braking manoeuvres that occur in everyday situations. If the driver applies more pressure to the brake, additional hydraulic brake torque is generated at the front axle using the conventional method, so the vehicle is slowed down using both axles. If the regenerative brake torque available at the rear axle is not sufficient, the system uses the hydraulic modulator’s pump to generate additional braking pressure for the rear axle. These processes go unnoticed by the driver, which means there is no change to the familiar pedal response or vehicle behaviour. In addition to the version for the front-/rear-axle brake-circuit split described above, Bosch also offers a version for diagonal braking circuits.
Vacuum-independent solution: HAS hev
In future, Bosch will offer the HAS hev braking system for hybrid and electric vehicles not equipped with a vacuum-based brake booster. HAS stands for Hydraulic Actuation System and hev indicates that the product is designed for installation in hybrid electric vehicles. The system is suitable for all brake-circuit splits and drive concepts. It comprises a brake operation unit and a hydraulic actuation control module which supplement the ESP® hydraulic modulator. The brake pedal and wheel brakes are mechanically decoupled. The brake actuation unit processes the braking command, and an integrated pedal travel simulator ensures the familiar pedal feel. The braking pressure modulation system implements the braking command using the electric motor and wheel brakes. The aim is to achieve maximum recuperation while maintaining complete stability. Depending on the vehicle and system status, deceleration of up to 0.3 g can be generated using only the electric motor. If this is not sufficient, the modulation system uses the pump and high-pressure accumulator to generate additional hydraulic braking pressure for the wheel brakes.
In addition to the braking system, Bosch offers a complete portfolio for hybrid and electric vehicles. This includes electric motors for various hybrid and electric drive concepts, the corresponding power electronics, lithium-ion battery systems, and complete function development.