Low Carbon Technology Project

Workstream 8

Vehicle Dynamics and Traction control for

Maximum Energy Recovery

Phil Barber

CENEX Technical review 19th 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

• Modelling and simulation

• Friction Brake re-design

• Prototype vehicle validation

(Rheostatic Braking)

• State of the Art review

(Competitor Benchmarking)

• Legislation

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

Toyota Auris

Honda Civic

Porsche Cayenne

These have differing Hybrid

Architectures and are from

Various Market Sectors

Subjective Ratings

Pedal Smoothness/Progression

Light Braking Confidence

Vehicle 1 2 3

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.

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.

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.

Matlab Control &

Dymola Plant Model

Carmaker Interface

Brakes Modelling

• 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.

• 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.

Simulation has been used extensively to examine the consequences

on vehicle stability of regenerative braking control algorithms.

•Full hydraulic braking system

•Pedal assembly

•Brake booster

•Master cylinder

•Lines

•Modulator

•Calipers

•Car model

Hydraulic models

Brakes Modelling

Dymola Brakes Plant Model Correlation Data

• 1st 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.

• Real-time simulation of LCVTP full vehicle model

implemented on CarMaker XPack4 HIL Platform

Simulation & Modelling

Hardware-In-Loop

• 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.

1 A 2000 Kg car from 130mph at 9 m/s2

• 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.

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.