Comparison of Particulate Matter and Number Emissions from a Floating and a Fixed Caliper Brake System of the Same Lining Formulation

The particulate emissions of two brake systems were characterized in a dilution tunnel optimized for PM10 measurements. The larger of them employed a fixed caliper (FXC) and the smaller one a floating caliper (FLC). Both used ECE brake pads of the same lining formulation. Measured properties included gravimetric PM2.5 and PM10, Particle Number (PN) concentrations of both untreated and thermally treated (according to exhaust PN regulation) particles using Condensation Particle Counters (CPCs) having 23 and 10 nm cut-off sizes, and an Optical Particle Sizer (OPS). The brakes were tested over a section (trip-10) novel test cycle developed from the database of the Worldwide harmonized Light-Duty vehicles Test Procedure (WLTP). A series of trip-10 tests were performed starting from unconditioned pads, to characterize the evolution of emissions until their stabilization. Selected tests were also performed over a short version of the Los Angeles City Cycle. PM2.5 emissions of burnished pads averaged at 2.6 mg/km/brake and 4.1 mg/km/brake for the FLC and the FXC system, respectively. A large fraction of the airborne PM was found to be larger than 2.5 μm, leading to 2.7 times higher PM10 emissions. Therefore, proper PM10 measurements will require careful considerations on the operating tunnel parameters to minimize particle losses. The FXC system yielded ~50% higher PM and two times higher PN emissions despite the 22% lower disc temperatures measured with an embedded thermocouple. No indication of volatile particle formation was observed, with more than 80% of total PN having an optical diameter larger than 300 nm. The results illustrate the challenges associated with the use of disc temperature measurements for the control of a representative braking procedure.