Brake wear is a major source of non-exhaust particle emissions from vehicles, with increasing relative contribution due to minimization of exhaust particles. Their importance is acknowledged in the proposed Euro 7 regulation, which includes emission limits for brake emissions. Brake system testing in the laboratory is performed following the relevant PMP procedure, while a system and procedure for vehicle testing are not yet standardized. The current work aims at developing and evaluating a measuring system of brake particle emissions from passenger cars, and implementing it on chassis dyno testing for preliminary emissions characterization under varying driving conditions. 

Following an extensive literature review, a semi-enclosed sampling concept was adopted with a cone-type sampler. Three different configurations of the measuring system were examined during chassis dyno testing, so as to conclude to the preferred layout for on-road testing that will follow.

Laboratory measurements were conducted on a passenger car with the brake particle sampling system attached to a front wheel. The vehicle followed various velocity and road grade profiles, including WLTC, Trip 10, 20-min part of LACT, real-world profile with regular driving, downhill driving with severe braking.

Temperature was monitored on the brake discs and pads. Particle mass, as well as number at various cut-off diameters (23, 10, 2.5 nm) and size distribution of both solid and total particles were measured. The matrix of measuring equipment included various AVL and TSI CPC’s, AVL APC, TSI EEPS, Dekati ELPI and DGI. In particular cases, gaseous emissions were also recorded.

Although that the semi-enclosed system could presumably limit the physical cooling of the brake system, minimal impact was observed on disc and pad temperature, which varied from 90°C in the WLTC to above 350°C in downhill driving with severe braking.

Similar results, in terms of particle emissions, were achieved with the three configurations of the system, differing in the length of the sampling hoses, ensuring that no significant particle loss occurs along the system.

Higher temperature, resulting from harder braking, led to increased particle emissions, particularly in the lower size range (10 nm). These effects were further intensified when using brand new pads (without any bedding), where instantaneous spikes were higher by 3 orders of magnitude compared to used pads, while hydrocarbon emissions were also recorded. Depending on the testing conditions, bi- and tri-modal size distributions were observed, with the ultrafine particles increasing significantly at higher temperatures.

The limitations of the current study lay at two levels: the measuring equipment and the testing conditions.

In terms of the former, the instrumentation used in the previous testing activities covered partially larger particles, i.e., with diameter greater than 1 μm. Additional measuring equipment is applied in the ongoing and future experimental campaigns.

In the context of this study, only lab measurements, yet extensive, were conducted, in order to gain confidence in the complete sampling system. On-road testing is scheduled and will be included in the following campaigns.

Building on the long experience of the present research group in the characterization and analysis of particle emissions, covering both exhaust and non-exhaust ones, the current paper sets the basis for further investigations and comprehensive analyses.

The brake particle sampling system was extensively evaluated, concluding to the preferred configuration for on-road testing that will follow. At the same time, lab testing on the chassis dyno ranged from mild to severe braking, stressing the friction components at very high temperatures and providing useful insight concerning brake particle emissions.

A sampling and measuring system of brake particle emissions from passenger cars was developed and evaluated under controlled laboratory conditions. The system did not have any significant impact on brake temperature, ensuring that the measured emissions are representative of the actual ones. In addition, the strong influence of temperature, even exceeding 350°C, on particle emissions was confirmed, while the brand-new pads led to significantly higher emissions during their first application. Finally, size distribution recordings allowed for a more detailed analysis of the physical characteristics of the emitted particles.

The new Euro 7 standard is set to be in place by 2025, which will be the first legislation that will cap the emissions produced by a brake system. This has caused brake manufacturers to find alternative solutions to reduce the emissions generated from conventional grey cast iron (GCI) friction brakes. With electric vehicles (EVs) becoming the future of modern vehicles, their regenerative braking system will cause friction brakes not to be used as frequently as for an internal combustion engine vehicle. This may lead to a build-up of corrosion on the friction surface that may not only affect the performance and service life of the brakes but also increase wear particle emissions when braking. Plasma electrolytic oxidation (PEO) ceramic-coated aluminium alloy rotors could be an alternative solution to reduce the risk of corrosion failure, produce lower brake emissions and also improve the energy efficiency of the EV by reducing its un-sprung mass. To understand the interrelation between brake rotor corrosion and particulate emission, this study concentrates on quantifying wear particles from a PEO-coated Al6082 brake rotor, both before and after exposure to salt fog corrosion.

A ‘drag braking’ duty cycle was chosen for this dynamometer study, as this produces near steady-state conditions at the friction interface. Each test was run at a constant speed of 150 rpm and three different brake hydraulic pressures: 5, 7.5 and 10 bars for 90 minutes each. The brake pad material used was a commercial glass-reinforced non-abrasive pad, chosen to be compatible with the PEO-Al rotor surface. Tests were conducted within an enclosed chamber on the Leeds brake dynamometer and airborne emissions were sampled using a Dekati electrical low-pressure cascade impactor (ELPI+). A salt spray chamber was used to mimic a corrosive environment, the ASTM-B117 standard was the chosen test protocol.

A comparison was made between corresponding emissions from a conventional grey cast iron (GCI) rotor subjected to identical corrosion and brake test cycles. Overall the emissions from the uncorroded PEO-Al rotor were lower than those from the uncorroded GCI in the low pressure tests. The corrosion cycle did not have a significant impact on the friction performance or wear emissions of the PEO-Al rotor, unlike the GCI rotor where there was a 30-fold increase in wear emissions following corrosion.

Ricardo was contracted by the UK Department for Transport (DfT), to develop a “proof-of-concept” system for measuring mass and number of non-exhaust emissions (NEE) of particles, under real-world driving conditions. The project is planned to comprise three phases. The initial, recently completed phase, saw the development of on-board approaches to sample and measure brake (and also tyre wear) particles from light-duty vehicles. Brake wear particles were sampled from a Volkswagen Caddy van, extracted from a fixed volume created by enclosing the pad and disc. Three different enclosure designs were developed. Particles produced by braking events were continuously transported, near-isokinetically, from the sampling point to a sample tunnel, where measurements of number-weighted particle size distributions were determined by two Dekati ELPI+ systems. ELPI+ systems function by separating particles into different size ranges according to their inertial properties. Particles are charged at the ELPI+ inlet. Charges carried by the particles are transferred to electrometers associated with each size range, allowing real-time size distribution and number concentration data to be acquired. One ELPI+ was heated, with the other at ambient temperature, further allowing discrimination between particle volatilities. A Dekati eFilter based upon diffusion charging was used to provide a cumulative PM mass sample and a real-time mass signal. Filters collected were subjected to basic chemical analyses. To avoid particle losses from the brake enclosure, and to manage temperatures, a constant stream of HEPA filtered air was supplied to move particles to the sample tunnel, this also providing positive pressure within the enclosure and preventing particle ingress from ambient air. Consequently, background particle levels were negligible. Measurements were made on the chassis dynamometer from a bespoke drive cycle constructed to produce high particle emissions, from road tests in an urban area, and from repeated braking events on a test track. Results showed strong signals of particle release from brakes in response to braking pressure signals and road speed data recorded from the vehicle On-Board Diagnostics, with both solid and volatile particles produced. Particle number emissions from brakes were of a similar order to published values from brake dynamometer testing, while mass emissions sampling efficiency appeared to be consistent with collecting PM2.5.