Research Questions / Objective Brake wear particles are recognized as one of the dominant sources of road transport particulate matter emissions and are linked to adverse health effects and environmental impact. The UNECE mandated the Particle Measurement Program to address this issue, which successfully developed a harmonized sampling and measurement methodology for the investigation of brake wear particles on a brake dynamometer (dyno). Even though the brake dyno approach with tightly controlled test conditions offers good reproducibility, changing vehicle and surrounding conditions makes real-driving emission (RDE) measurements a necessity to gain a deeper understanding of the actual emission behavior. Methodology Here we show a solution for measuring real-world brake emissions of a standard passenger vehicle on public roads. The vehicle is equipped with our minimal invasive brake wear sampling system, which does not affect the brake temperature, and particle instrumentation adapted for on-board measurement, while closely following the harmonized methodology of brake dynos. We connect on-board vehicle data such as brake pressure and actuation, acceleration, and speed profile with our external sensors for braking temperature, and ambient conditions (humidity, temperature, and pressure), with the measurement instrumentation data for particle number and mass. We can close the knowledge gaps in the correlation between brake dyno and on-road testing with the vehicle fully equipped for on-board brake emission measurement. In conjunction with the recorded GPS data, we can identify emission hotspots that vary depending on the geographic location. This allows us to create city emission maps, including random and worst-case scenarios. Results We found that PM10 vehicle brake emissions measured on-road exceeded the European PM10 exhaust emission limit by a factor of 5 to 8, following the existing RDE testing specifications for exhaust emissions. PM and PN emissions measured on a test track were ~20 % higher than levels determined in a previous campaign on a brake dyno with a similar driving cycle and system setup. We verified that the temperature of the brake under testing was within ± 2° C compared to the brake without particle sampling. Limitations of this study Although we are closely following the UNGTR for brake emission measurements on a brake dyno, there are some requirements we cannot adhere to. Due to the space restrictions of the vehicle, the brake enclosure and sampling line deviate from the design on the test bench. A major part of this method's development effort and know-how is put into the design and function of the RDE enclosure. Moreover, RDE exhaust requirements are not fully suitable for RDE brake emission testing, since the requirements are based on acceleration instead of deceleration rates for braking. For future RDE brake emission tests, the exhaust standards should be adopted to the specifications of braking requirements. What is new in this paper? In comparison to previous publications on this project, this paper presents a novel fully equipped vehicle setup needed for valid RDE brake wear measurement. This includes on-board diagnostic data, external sensors, and particle instrumentation, including a CPC-based mobile particle counter. Furthermore, we now gained permission for conducting RDE emission test cycles on public roads in Austria. Conclusion These results demonstrate the need for a worldwide harmonized test procedure for brake emission measurement in Euro 7. We envision RDE testing as complementary to, rather than a substitute for dynamometer testing to enhance correlation to real-world emission behavior. With our work, we want to contribute to advancing technical progress in real-world brake emission measurement, empowering the European Commission to adopt delegated acts.
Dipl.-Ing. Michael Peter Huber, Research Engineer, TU Graz; Prof. Dr.-Ing. Peter Fischer, Head of the Institute of Automotive Engineering, TU Graz, Institute of Automotive Engineering; Dr.-Ing. Gerald Steiner, Department Manager, AVL List GmbH