Effect of pad material and regenerative braking on brake wear emissions

Non-exhaust sources, including brake, tire and road wear, have been identified as a major source of current traffic-related particle emissions [ 1 ]. Brake wear particle characteristics and quantity depend on factors such as vehicle properties, brake material composition and driving characteristics. Recent studies have been conducted to analyze effect of these topics. For instance, composition and structure of brake wear particles has been analyzed [2] as well as the effect of regenerative braking on particle emissions [3], which is important topic due increasing number of hybrid and electric vehicles on roads.
In our study, a set of brake pads, from different manufactures, was purchased from local spare part suppliers. In total of 7 pads were tested with X-ray fluorescence (Olympus Vanta VMR) to obtain their elemental composition. Two of the pads were selected for testing in a dynamometer bench: original pads and pads with low Cu and Fe content.
Brake particle emissions were tested in a measurement campaign using a dynamometer with WLTP brake cycle and with repetitive from constant speed to stop cycle. Increased mass of the vehicle was simulated by increasing the brake torque demand and effect of regeneration by decreasing the torque demand. Several type of particle properties were measured, for instance PM10, PN10 and size distribution.
There were significant differences in the chemical composition of the analyzed pads. Light elements (LE<Mg) were the most abundant and could not be specified with the XRF. Amount of Fe (0.7-35%), Cu (0-1 1%), Zn (0-10%) and Ba (0.1-30%) varied significantly between the pads. Mg, Al, Si and S were detected from all the pads.
The brake pads with low Cu and Fe content were consistently found to produce lower (-30...-90%) PM and PN emissions than the original pads. Regenerative braking reduced PM10and PN 10 emissions significantly in most cases, often more than -50%. However, in some cases, relatively high PN 10 concentrations were observed during regenerative braking although the brake pressures and resulting torques were low compared to normal braking. Also during heavy braking, where the share of the regeneration is low, the particle concentrations were close to conventional braking.