According to estimations 21% of total traffic related PM10 emissions in urban environments originate from airborne brake wear particles (Grigoratos and Martini, 2015). One approach to identify potential reduction schemes is the development of a better understanding of the formation processes of brake wear particle emissions. Due to the complexity of tribological processes in the friction layer between brake disc and pad it is quite challenging to derive a white box model of the particle emission behavior. Therefore, the approach chosen in this work is the development of an empirical mathematical model by measuring emission maps and quantify the particle emissions in dependency of the influencing parameters brake pressure, vehicle speed, disc temperature and load history. The experimental setup used for the measurements consists of a sampling enclosure on a brake dynamometer which was published and validated in previous publications (Asbach et al. 2018, Niemann et al. 2018). Particle size distributions from 5.6 nm to 560 nm are measured by an electrometer based measurement technique (TSI FMPS) and from 300 nm to 10 µm by an optical measurement technique (TSI OPS).
The influencing parameters are varied by several design-of-experiments using drag braking events. Temperature’s influence on the emission of coarse and ultrafine particles was investigated in one pressure-velocity-operating point of the brake and shows up a hysteresis behavior. As known from several studies (e.g. Ostermeyer et al. 2018) the tribological properties of friction brakes are time variant. This behavior is often called “memory effect”. The memory effect or load history was investigated by varying the brake pressure and speed stepwise. The total number of coarse particles emitted during one drag brake event is affected by the load history with a factor of approximately up to 3.
The rotational speed of the disc was identified as a dominant influencing parameter for the emission of coarse particle fraction measured by the OPS with a monotonous increase of the number concentration of approximately up to N ~ v³. Brake pressure influences the measured concentration in a way that is not monotonous and to a smaller degree than speed.
In order to investigate the transferability of the results gained in drag braking events to the emission behavior in a dynamic WLTP cycle an emission model was derived based on the knowledge gained from the grey box model. The emission map uses brake pressure and velocity to predict the emitted number concentration of the coarse fraction in a WLTP cycle. The agreement between measured and simulated emission factors was within 20%.