Low Carbon Technology Project Workstream 8 Vehicle Dynamics and Traction control for Maximum Energy Recovery Phil Barber CENEX Technical review 19 th May 2011
Overview of WS8 Workstream 8 was set up to investigate and develop Industrial methods needed to ensure the successful introduction of braking systems that provide the optimal energy recovery from road vehicles whilst maintaining the high levels of braking and stability control performance. Specifically, studies of State of the Art review (Competitor Benchmarking) Legislation Modelling and simulation Friction Brake re-design Prototype vehicle validation (Rheostatic Braking)
State of the Art Review Three hybrid vehicles were chosen, each having conventional counterparts. The aims : 1. Understand the market acceptability of current hybrid vehicles. 2. Analyse the variability during brake stops. 3. Study the issues around driveline and driveability.
Vehicles Tested at MIRA Honda Civic Porsche Cayenne Toyota Auris These have differing Hybrid Architectures and are from Various Market Sectors
Subjective Ratings Pedal Smoothness/Progression Vehicle 1 2 3 Light Braking Confidence Vehicle 1 2 3
Test Results Left - Standard Vehicle The red line indicates a constant pedal input yielding a near constant deceleration (pink line) Right - Hybrid Vehicle The brake pressure (purple line) and motor torque are blended. For a constant pedal input there is a variation in the deceleration noted.
Test Results The hybrid vehicle had inconsistent brake responses. The two plots show the similar pedal input (red line) but different decelerations (pink line).
Legislative Requirements for Regenerative Brake Systems. EC/UNECE Brake Approvals. In Europe the brake systems on new vehicles are required to be type approved to one of the following: European Directive 71/320/EEC as amended (last amendment 98/12/EC) or ECE Regulation 13H for Cars (M1) and optionally Light Commercials (N1) or ECE Regulation 13.11 for virtually all other vehicles The Braking Directive was last revised before the widespread use of hybrid and electric vehicles and, as a consequence, does not include the specific requirements that are now included in ECE Regulation 13H and 13.11. A vehicle manufacturer is therefore able to sell vehicles that comply with the Directive but not with the complex demands of the later regulations. Types of Regen Systems in Reg 13H and 13.11 The regulation divides regenerative brake systems into 3 different types: Category A The electric regen system is not part of the brake system. Typically this means regen when the throttle is released. Category B Non-Phased The electric regen system is part of the brake system. This means that regen commences or is increased when the brake is applied. Electric regen force starts to be developed at the same time as or slightly after the conventional friction brakes. Category B Phased The electric regen system is part of the brake system. Typically this means that regen forces can be developed ahead of any braking from the conventional friction brakes. This system allows the maximum amount of regen energy to be recovered. Most regen brake systems operate both as a Category A system and Category B system under different conditions.
Legislative Requirements for Regenerative Brake Systems. Specific Requirements for all Category B Phased Systems The following rules apply to Category B systems with phased regen braking and are far reaching in terms of brake system design. They are extracted directly from the regulation: 5.2.7. In the case of vehicles equipped with electric regenerative braking systems of category B, the braking input from other sources of braking, may be suitably phased to allow the electric regenerative braking system alone to be applied, provided that both the following conditions are met: 5.2.7.1. Intrinsic variations in the torque output of the electrical regenerative braking system (e.g. as a result of changes in the electric state of charge in the traction batteries) are automatically compensated by appropriate variation in the phasing relationship as long as the requirements of one of the following annexes to this Regulation are satisfied: (Note quotes paragraphs requiring ABS to still function correctly) 5.2.7.2. Wherever necessary, to ensure that braking rate remains related to the driver's braking demand, having regard to the available tyre/road adhesion, braking shall automatically be caused to act on all wheels of the vehicle. A footnote states that the Authority, which is to grant approval, shall have the right to check the service braking system by additional vehicle test procedures.
Legislative Requirements for Regenerative Brake Systems. The Implications The implications for Cat B systems with phased braked are significant. Phased braking means that regenerative braking (under the control of the service brake system) can occur alone ahead of friction braking. Paragraph 5.2.7.1 means that the braking system must automatically compensate for different states of battery charge. The driver must be given the same response to a braking demand no matter what the state of charge of the batteries. Paragraph 5.2.7.2 means that if the axle with the regen braking system hits a patch of ice, say, the friction brakes on the other axle must be automatically applied to an appropriate level such that the driver s expected deceleration is maintained (up until the limit of adhesion on both axles). These requirements have major implications for the brake actuation system. It is no longer possible to use a conventional system. Power brake systems have been used by most vehicle systems designed to meet these criteria. A few manufacturers have used increased functionality in the stability control module, along with extra valves and a pedal simulator. Therefore to ensure maximum regen effectiveness on electric and Hybrid vehicles complex brake control systems have to be used. This also means that brake homologation requires additional complex work including analysis as well as tests.
Brakes Modelling Matlab Control & Dymola Plant Model Simulation has been used extensively to examine the consequences on vehicle stability of regenerative braking control algorithms. Impact of brake system architecture on potential energy recovery & vehicle stability. Critical interactions during ABS/ESP events. Effects of stochastic signal propagation delay over CAN. Carmaker Interface Simulation platform extended from existing longitudinal forward dynamic HEV model WARPSTAR 2+. Powertrain & brakes model implemented in acausal physical modelling package Dymola & embedded into MATLAB/Simulink. Simulink model integrated with IPG CarMaker suspension, tyre & driver + environment models.
Hydraulic models Full hydraulic braking system Pedal assembly Brake booster Master cylinder Lines Modulator Calipers Car model
Brakes Modelling Dymola Brakes Plant Model Correlation Data 1 st order hydraulic brake models implemented in Dymola incl. Electro-hydraulic regen & ABS modulation. Reduced models derived from more complex AmeSim implementations - suitable for real time application. Model output correlated with experimental data.
Simulation & Modelling Hardware-In-Loop Real-time simulation of LCVTP full vehicle model implemented on CarMaker XPack4 HIL Platform VSC & BTAC implemented on DSpace Micro-Autobox with CAN interface. Platform developed to study impact of signal propagation delay & fault injection.
LM Cranfield Thermal model for Friction Brake re-design Heat input Cooling Energy balance
Friction Brake Redesign Different drive-cycles to represent target EV usage are being analysed, to characterise braking event frequency and severity. This analysis will be used to determine the change on friction brake duty cycle that could result from regenerative braking. FTP-75 Drive Cycle Artemis Urban Drive Cycle Real world performance targets and requirements for brake torque apportioning between regenerative and friction braking systems can be determined.
Vehicle Validation Develop regenerative braking control on low mu surfaces Provide maximum energy capture whilst retaining stability Protect the battery by using a dump resistor
Regenerative Stability Control Regenerative actuation must be rapid to control wheel slip. Regenerative control will need to co-ordinate with electronic differential control to increase energy capture whilst retaining vehicle stability.
Vehicle Validation Currently the battery cannot absorb the energy potentially available from regenerative braking. Limo Green Battery absorption capacity is about 40KW Limo Green Generator capacity is 150KW An emergency stop from 130 mph is 1 MW! 1 To prove the regenerative braking stability control algorithms, a 100KW water cooled resistor will be used to allow testing at high regeneration power. 1 A 2000 Kg car from 130mph at 9 m/s 2
Conclusions The majority of brake events in a vehicle s lifetime could theoretically be performed through use of regenerative braking. Although the friction brake / stability control system cannot be completely replaced, this raises the question of how it could be redesigned and optimised to take advantage of regenerative braking, and engineered to better suit the real-life use seen in an EV powertrain. Suitability of different powertain configurations is a key factor in determining regenerative braking capabilities. On a front wheel drive vehicle almost all of the braking seen in typical city-driving could conceivably be regenerative. On Going Work: Consolidation of the modelling toolset to allow the quantitative assessment of different braking architectures. Validation of the stability control concepts in high energy regenerative braking scenarios.