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.

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