Brake particle emissions number (PN) and mass (PM) of a light-duty hybrid-electric vehicle have been assessed under realistic driving patterns on a chassis dynamometer. Therefore, the front-right disc brake was enclosed in a specifically designed casing featuring controlled high scavenging air ventilation. The WLTC cycle was chosen for most measurements. Different scavenging flow rates have been tested assessing their influence on the measured particles as well as on the temperature of the braking friction partners. Particle transport efficiencies have been assessed revealing scavenging flow rates with losses below 10%. During the performed cycle, most brake particle emissions occurred during braking. There were also isolated emission peaks during periods with no brakes in use, especially during vehicle accelerations. Sequential WLTC cycles showed a continuous decrease in the measured PN and PM emissions; however, size-number and size-mass distributions have been very similar. The measured PN emission factors (>23 nm) at the right front wheel over the WLTC cycle lie at 5.0 × 10¹⁰ 1/km, whereas the PM emission factor lies at 3.71 mg/km for PM < 12 µm and 1.58 mg/km for PM < 2.5 µm. These values need to roughly triple in order to obtain the brake particle emission of all four brakes and wheels of the entire vehicle. Thus, the brake PN emissions factors have been in the same order of magnitude as the tailpipe PN of a Euro 6 light-duty vehicle equipped with a particle filter. Finally, differences between brake particle emissions in hybrid and all-electric operating modes have been assessed by a series of specific measurements, demonstrating the potential of all-electric vehicle operation in reducing brake particles by a factor of two.

The present work focuses on a detailed analysis of new and used braking friction partners (discs and pads) in order to provide a comprehensive characterization of the source of the airborne particles formed during braking. Scanning electron microscopy (SEM) combined with energy dispersive X-Ray analysis (EDX) was applied to investigate the new brake disc and new and used brake pad components of a passenger vehicle. The pads include at least 21 different substances, involving carbonaceous particles, oxides, sulfides, sulfates and silicates of Al, Si, K, Ca, Ti, Fe, Zr, Sn, less Mg, Ba, Na and, rarely, Bi and Zn, as well as K-titanate. Aramid and phenolic resin are also present, enriched toward the metal interface. The size of the pad constituents extends over a very wide range, from hundreds of µm to a few µm, and goes down to hundreds of nm and, rarely, tens of nanometers. Carbonaceous particles with sizes down to a few tens of nanometers occupy ca. 16% of the total of the pad constituents. Abundant Zr-bearing phases, as well as various other phases involving S, Ca, Mg, Si, Ti and, to a lower extent, Ba and Fe in different combinations, constitute the pad main matrix.

Many countries worldwide have introduced a limit for solid particles larger than 23 nm for the type approval of vehicles before their circulation in the market. However, for some vehicles, in particular for port fuel injection engines (gasoline and gas engines) a high fraction of particles resides below 23 nm. For this reason, a methodology for counting solid particles larger than 10 nm was developed in the Particle Measurement Programme (PMP) group of the United Nations Economic Commission for Europe (UNECE). There are no studies assessing the reproducibility of the new methodology across different laboratories. In this study we compared the reproducibility of the new 10 nm methodology to the current 23 nm methodology. A light-duty gasoline direct injection vehicle and two reference solid particle number measurement systems were circulated in seven European and two Asian laboratories which were also measuring with their own systems fulfilling the current 23 nm methodology. The hot and cold start emission of the vehicle covered a range of 1 to 15 × 10¹² #/km with the ratio of sub-23 nm particles to the >23 nm emissions being 10–50%. In most cases the differences between the three measurement systems were ±10%. In general, the reproducibility of the new methodology was at the same levels (around 14%) as with the current methodology (on average 17%).