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EB2021-IBC-006

Paper + Video + Slides

Abstract

Mr. Lennart Guckes, TU Darmstadt Institute of Automotive Engineering (FZD), GERMANY

Prof. Dr. Hermann Winner, TU Darmstadt Institute of Automotive Engineering (FZD), GERMANY

Dr.-Ing. Jens Hoffmann, Continental Teves AG & Co. oHG, GERMANY

Mr. Sébastien Pla, Continental Teves AG & Co. oHG, GERMANY


During the development toward autonomous and electrified vehicles with low emissions, many visions for future mobility concepts arise, one of them being autonomous shuttles for urban areas. Most publications concerning these concepts focus on control and software while in this paper the change of requirements for wheel brakes is examined.

The performance of wheel brakes for todays passenger cars is currently tested under different worst-case assumptions regarding area of operation and highest possible load resulting from human operation. Considering the capabilities of autonomous shuttles like autonomous driving and the availability of regenerative braking, these assumptions need to be reevaluated. This also includes comfort and lifetime requirements regarding wheel brakes for these concepts and takes in perspective that for an autonomous shuttle a certain area of operation is defined in their operational design domain (ODD) as well as a lower maximum velocity.


To do so, different autonomous shuttle concepts are aggregated as well as their respective hardware and tech specs. To gather system requirements for the braking system of an autonomous shuttle a stakeholder analysis is performed, highlighting the underlying business model, driving tasks and passenger types as well as their needs and wishes. The shift in requirements is derived in comparison to conventional wheel brakes for cars. Usual performance tests for conventional wheel brakes for passenger cars are semantically analyzed to discuss their relevance and transferred into new performance tests for the given vehicle class.


Three test scenarios are created, the first one being the “Emergency Braking Test”, which consists of two consecutive emergency brakings. Secondly a “Standard Operation Test” which consists of ten consecutive, comfortable accelerations and decelerations for passenger pickup and transport. Lastly, a “Hill Descent Test” on a long descent in the area of operation of the shuttle, like in the demanding urban topology of San Francisco. Based on the scenarios different availability levels of regenerative braking power are considered.


Based on the developed test cycles a comparison is drawn for power and energy dissipation demand and the corresponding torques needed for an example vehicle under various levels of available regenerative braking power. While power and energy dissipation have decreased heavily, the torque demand is still as high as needed for a conventional vehicle.


The changed requirements open up new possibilities for suitable braking concepts for autonomous shuttles. This may also reduce brake emissions depending on the chosen concepts.

EuroBrake 2021

BCN

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EB2021-EBS-004

Paper + Video + Slides

Abstract

Mr. Hartmut Niemann, Technical University Darmstadt, GERMANY

Prof. Dr. Hermann Winner, Technical University Darmstadt, GERMANY

Dr.-Ing. Christof Asbach, Institute of Energy and Environmental Technology, GERMANY

Mr. Heinz Kaminski, Institute of Energy and Environmental Technology, GERMANY

Mr. Georg Frentz, Daimler AG, GERMANY


According to estimations, brake wear particle emissions contribute with a share of up to 21 % to the traffic related PM10-emissions in urban environments. Depending on the brake system, a significant proportion of those emissions occurs under cruising conditions with released brake. During a WLTP exhaust cycle with two sliding calipers used in this study 16 % respectively 30 % of the total PM10-emissions occur during off-brake-phases.


The influencing parameters and formation processes of those off-brake-emissions are subject of current research to evaluate potential reduction approaches. A residual brake torque due to a disc-pad contact in the off-brake-phases as well as an air flow in the air gap between disc and pad are discussed as potential causes for the generation of those emissions. In the current state of research it is neither known how off-brake-emissions can be influenced during operation of the brake nor how a potential off-brake-emission reduction would affect dust release during the brake events and emission factors for whole test cycles.


An experimental setup for the independent adjustment of the air gap and retraction force on the brake pads is used to investigate the influence on PM10-emissions during WLTP cycle with two different sliding calipers. Therefore, the air gap is varied between 0.17 mm and 5 mm and PM10-emission factors are compared to the reference brake systems without pad retraction.


The share of off-brake emissions to the overall PM10-emission factor during the off-brake-phase is reduced from 16 % respectively 30 % to 3 % during a WLTP cycle with pad retraction. Associated with this decrease during the off-brake phases, an increase of the emission during the on-brake-phases occurred with pad retraction. During the performed WLTP the pad retraction leads to a decrease of the PM10-emission factor by 4.5 % for the first sliding caliper, which was smaller than standard deviation and by 6.0 % for the second sliding caliper, which was slightly higher than the standard deviation of the emission factor. No significant variance of the PM10-emission factor between 0.17 mm and 5 mm air gap could be determined. Instead a binary behavior with and without retraction was observed.


Based on literature and the observations in this study, two mechanisms remain as possible origin for the major share of brake wear emission during off-brake-phases. A residual torque combined with a reservoir behavior as well as an airflow through the air gap between disc and pad smaller than 0.17 mm could be the causal mechanisms for off-brake-emissions. A residual torque as a source for contious particle generations and centrifugal forces acting on adhesive particles on the disc were excluded due to findings gained in the pad retraction experiments respectively by the observations described in the state of research.

EuroBrake 2021

BEM2

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Germany

Professorship for automotive engineering at TU Darmstadt; Head of the Institute of Automotive Engineering (FZD) in the department of Mechanical and Process Engineering at TU Darmstadt

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Prof. Dr. Hermann Winner

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