Influence of Powertrain Electrification on Brake Wear Particle Emissions

The upcoming Euro 7 regulation encompasses limitations for nonexhaust emissions introducing a new test procedure (UN-GTR No. 24) and a new driving cycle (WLTP brake) for brake emission measurement. Brake PM10-limits of 7mg/kmV for ICV and hybrid vehicles as well as 3mg/kmV for fully electric vehicles have been defined. Measurements based on standard friction materials (cast iron discs) have shown that these limits can hardly be complied with under current circumstances. This calls for action such as the development of new technologies (coated discs, filtration systems).Regenerative braking has been identified as one of the greatest potentials for the reduction of particle emissions. Depending on the degree of electrification of the vehicle, a large proportion of the deceleration force can be shifted to the electric motor and must no longer be applied by the friction brakes. Consequently, brake wear and PM10 can be heavily reduced. However, the implementation of the individual generator characteristics into the test bench control systems represents a significant increase in testing effort. The proposed UNGTR No.-24 approach is based on the application of friction braking share coefficients (c-coef.) to the emission factors of a standard full friction brake-test (FFB) depending on the degree of electrification (e.g. NOVC-HEV-C1, OVCHEV, PEV). This bears the risk that the real emission characteristics are not represented correctly.Within this study the influence of powertrain electrification on brake wear particle emissions is investigated. Therefore, exemplary generator characteristics of NOVC-HEVC1, OVC-HEV and PEV were implemented into an UN-GTR No. 24 compliant brake dynamometer setup. The resulting emission characteristics were evaluated for two different brake systems. In addition, on- and offbrake emissions were differentiated. The results provide insights into the interaction between friction work and emission generation over the course of the WLTP brake cycle. Results show a considerable deviation of emissions factors from the proposed method (c-coef.). Ultimately, an alternative approach for the implementation of regenerative braking should be considered in the future since it clearly gives a more realistic picture of the emissions process. However, first the UN-GTR No. 24 quality criteria need to be adopted (e.g. friction work, number of brake applications etc.).