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ELECTRIC POWERTRAIN's Impact on BRAKING SYSTEM COVER STORY BRAKE SYSTEMS ELECTRIC POWERTRAIN’s IMPACT ON BRAKING SYSTEM Electrification of the powertrain has considerable effects on the chassis systems, particularly on the brake system. Adaptations are necessary, for example, to enable energy recovery to be applied. This has consequences for the system software, the modulation hardware and the pedal feel. Furthermore, brake assist systems for zero-emission vehicles require electric vacuum pumps or other non-vacuum systems. In order to achieve further synergies in the chassis, suitable interfaces need to be created between the regenerative brake system, the steering, the suspen- sion and the axles. In the following report, PSA and Bosch describe which solution has been implemented in the hybrid version of the Peugeot 3008 and which systems are to be expected in the future. 34 www.autotechreview.com AUTHORS IMPACTS ON CHASSIS DOMAIN BASICS OF REGENERATIVE BRAKING The introduction of electric powertrains The objectives of regenerative braking are has had a strong impact on the chassis to reduce CO2 emissions and to increase components of a vehicle, especially on the the energy efficiency or driving range. For braking system. Recuperative braking is a HEVs without plug-in functionality, must for electrified vehicles, either for energy recuperation during deceleration DR. MARC DUVAL-DESTIN range extension in case of electric vehicles phases is the major contribution to reduce is Research and Innovation Director (EV) or for CO2 reduction in case of the CO2 emission. Values of 20 g/km CO2 at PSA Peugeot Citroën in hybrid electric vehicles (HEV). To enable savings are reachable depending on tests Paris (France). energy recuperation during braking conditions, vehicles and drivetrain archi- phases, new and adapted braking systems tectures, amongst other parameters. are necessary. During regenerative braking phases, Regenerative braking, classical friction the electric motor is used as a generator. braking and the driver’s request for decel- The potential of energy recuperation is eration have to be harmonised. The first affected by certain motor characteristics. generation of regenerative braking sys- Especially the maximum brake torque of PROF. DR. THOMAS KROPF is Senior Vice President, Chassis Sys- tems is based on the standard Electronic the electric machine is limited. Further- tems Control at the Robert Bosch Stability Control (ESC), where a limited more, this limitation depends on the cur- GmbH in Stuttgart (Germany). electric and hydraulic brake torque gener- rent rotation speed, which is proportional ation is superimposed. Higher recupera- to the vehicle’s velocity, 1. tion performance is offered by cooperative Beyond the limitations due to braking systems, which avoid deceleration the powertrain, the availability of inconsistencies by means of automated recuperative braking is limited by the torque blending, in particular with regard charging condition of the battery and to powerful electrical machines. restrictions on vehicle stability under DR. VINCENT ABADIE To fulfil requirements for vacuum free critical driving situations. is Corporate Expert, Controlled Chassis Systems at PSA Peugeot braking, the braking architecture has to be Citroën in Paris (France). extended with hydraulic or electric boost. Hybrid and electric vehicles have thus TYPES OF REGENERATIVE BRAKING strong impacts on chassis domain: AND LIMITATIONS :: Adaptations on the braking system are needed to enable energy recuperation; The first possibility for regenerative brak- energy recuperation impacts system ing consists in simply superposing the software, modulation hardware and deceleration of the electric motor/ genera- DR. MICHAEL FAUSTEN pedal feel. tor and the conventional brake system. is Director Advanced Engineering, :: Brake assistance for Zero Emission This solution is called superimposed Chassis Systems Control at the Robert Bosch GmbH in Stuttgart (Germany). Vehicles (ZEV) requires electric regenerative braking system (SRBS). It is vacuum pump or other vacuum simple and very cost effective since it free solutions. avoids blending between friction brake :: Potentials for new synergies require and regenerative braking. The major suitable interfaces between the regen- drawback of this approach is that the drag erative brake system and functional torque of regenerative braking depends on units like a steering system, suspen- the vehicle speed and has to be limited to sions or axles. a level that is still acceptable for the driver. This applies especially for the speed range close to standstill, since the regenerative braking torque will drop to zero at very low speed. This limit has shown to be in the range of 0.1 g, hence this solution is well suited for electric machines up to 20 kW power (mild hybrids). For vehicles with stronger electric engines (strong hybrids and EV) regenerative drag torque of more than 0.2 g can be generated, 2. Due to the restric- tions to 0.1 g drag torque, a SRBS cannot 1 Generator characteristics make use of the regenerative potentials of autotechreview April 2015 Volume 4 | Issue 4 35 COVER STORY BRAKE SYSTEMS (and low drag brakes) are used for HEV and EV vehicle, which can impact the entire chassis tuning. CO2 CYCLES AND RECUPERATION Regenerative efficiency is the proportion of the brake energy that can be recuper- 2 Deceleration characteristics ated by a specific regenerative braking of electric motors system. Its value depends on the under- lying driving cycle. An increased number these powertrains. tem developed with Bosch. The system of acceleration and deceleration events Here, the adequate approach is cooper- uses on a standard ESP and is addition- within a cycle increase the potential ative regenerative braking system (CRBS), ally equipped with a pedal travel sensor, amount of kinetic energy that can be which allows a high level of recuperation a special actuation topology with recuperated. Furthermore, higher decel- without variation of deceleration over the increased dead stroke and a special con- eration levels render a driving cycle more complete speed range down to standstill, trol software. suitable for demonstrating the regenera- 3. This is achieved by blending friction tive efficiency of a particular regenerative braking and recuperative braking for braking system. speed independent constant deceleration OTHER CHASSIS IMPACTS Not all driving cycles represent realistic at a given brake pedal position. This solu- acceleration profiles. Today, several driv- tion is suited for high power electric The technology of HEV and EV vehicles ing cycles for homologation are used machines. CRBS imply additional costs for has multiple impacts and constraints on worldwide, including the NEDC (New hydraulic blending and full or partial other chassis components, depending on European Driving Cycle), FTP (Federal decoupling of the brake pedal from the the powertrain architecture. The location Test Procedure) or JP (different modes). brake circuits. of an electric motor on the rear axle NEDC and HYZEM (Hybrid Technol- implies an axle evolution in order to sup- ogy Approaching Efficient Zero Emission port the electric motor. As well, start/stop Mobility) cycles are represented in (4). REGENERATIVE BRAKING ON THE has to be considered in the control and The NEDC is characterised by low decel- PEUGEOT 3008 HYBRID hardware of steering and braking assis- eration values (maximum 0.14 g). On the tance systems since mechanical power other hand, HYZEM features higher The hybrid architecture of the Peugeot and vacuum are not permanently availa- deceleration levels and more braking 3008 is based on an electric machine ble. This leads to application of an electric events. Therefore, HYZEM offers an linked to the rear wheels. The supervi- power steering (EPS) and discards brake- increased recuperation potential com- sion control unit (PTMU), the batteries, boost solutions, which are based on pared to NEDC. the electric motor and the power elec- mechanical vacuum pumps. Otherwise re- tronics are implanted on the rear axle. start of the ICE might be necessary in case The front axle is powered with an inter- of vacuum drop due to braking in engine REGENERATIVE BRAKING SYSTEMS nal combustion engine with start/stop off mode. function. The electric motor provides the The tyre conception and tuning has to Brake system requirements largely recuperation during deceleration phases be modified as well. The particular torque depend on the underlying vehicle con- (engine drag and braking) in order to profile of the electric motor, which is dif- cepts, platform strategies and additional charge the Ni-MH batteries. ferent from an ICE, can induce some mod- functions. The lack of vacuum due to The recuperative braking system ification to the wear performance. At the ICE with low or no vacuum, start/stop (SRBS) is realised by an ESP w/RBC sys- same time, low rolling resistance tyres and coasting requirements, fuel con- 3 Limited deceleration variation (left, green line) and constant deceleration (right, blue line) in case of cooperative regenerative braking system 36 www.autotechreview.com 4 NEDC (top) and HYZEM (bottom) deceleration profiles sumption reduction for HEV, and driving range extensions for EV require regenera- tive brake systems with actuation inde- pendent from engine vacuum. Platform strategies featuring mixed drivetrain concepts (e.g. ICE and HEV or EV) in one platform demand flexible brake system configurations and scalabil- ity without on-cost for modularity. For additional safety
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