Investigations on the Deposition Behaviour of Brake Wear Particles on the Wheel Surface 2021-01-1301

The deposition behavior of brake wear particles on the surface of a wheel and the mechanisms on it have not been fully understood. In addition, the proportion of brake wear particles deposited on the wheel surface compared to the total emitted particles is almost unknown. This information is necessary to evaluate the number- and mass-related emission factors measured on the inertia dynamometer and to compare them with on-road and vehicle-related emission behaviour.

The aim of this study is to clarify the deposition behavior of brake particles on the wheel surface. First, the real deposition behaviour is determined in on-road tests. For particle sampling, collection pads are adapted at different positions of a front and rear axle wheel. In addition to a Real Driving Emissions (RDE)-compliant test cycle, tests are performed in urban, rural and motorway sections to evaluate speed-dependent influences. Microscopic analysis is used to determine the particle number concentration and particle size distribution of the samples. Another focus is the analysis of particle deposition behaviour on the wheel surface along inertial dynamometer tests with Economic Commission for Europe (ECE) and Non-Asbestos Organic (NAO)-brake pads. For this purpose, a special setup is used which allows the simulation of realistic flow conditions. In addition to simulating the RDE-compliant test cycle, the novel Worldwide harmonized Light-Duty vehicles Test Procedure (WLTP)-brake cycle is used to ensure real world driving and braking conditions. Finally, simulative investigations on the basis of a transient Computational Fluid Dynamics (CFD) model allow the particle motion to be evaluated under consideration of geometry and flow conditions. By implementing a physical deposition model, it is possible to quantify particle deposition as a function of particle size and material properties. The investigations have shown that < 15 % of the total wear mass deposits on the surface of the wheel. In addition, a correlation with the driving speed could be demonstrated. The highest relative proportions could be determined in the urban and motorway sections, the lowest in the city section. In addition, a clear correlation could be established between pad composition, particle size and deposition position.