Brake Particle Emissions Research Articles

Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer

We investigated the effect of lepidocrocite-type layered titanate, which is compounded in brake pads, to reduce brake particle emissions. The dust reduction effect of titanate was evaluated using a small-scale inertial brake material friction test dynamometer. The results suggested that brake particle emissions are related to the microphysical structure of the pad surface, such as the uniformity of the friction film and secondary plateau formation, and that friction materials containing titanate contribute significantly to reducing both particle mass (PM) and particle number (PN) emissions of brake particles in both non-asbestos organic (NAO) and low-steel (LS) pads. In particular, LS pads generally have a problem of having more brake particles than NAO pads, but this study found that brake particles can be significantly reduced by compounding titanate instead of tin sulfide.

Brake Particle PN and PM Emissions of a Hybrid Light Duty Vehicle Measured on the Chassis Dynamometer

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.

Characterization of Laboratory Particulate Matter (PM) Mass Setups for Brake Emission Measurements

Vehicles’ exhaust particulate matter (PM) emissions have significantly decreased over the years. On the other hand, non-exhaust emissions, i.e., particle emissions from brakes and tires, have increased due to the increase in the vehicle fleet, traffic congestion, and the distance traveled. As a result, regulatory bodies are investigating the possibility of mitigating non-exhaust emissions. The Euro 7 proposal introduces specific emission limits for both brakes and tires for the first time in a regulation worldwide. The methodology for brake particle emissions sampling and measurement builds on the work of the Particle Measurement Programme (PMP) informal working group of the United Nations Economic Commission for Europe (UNECE). The recently adopted Global Technical Regulation (GTR) on brakes from light-duty vehicles up to 3.5 t prescribes the technical details. In this paper, we present the technical specifications for the measurements of PM. We also evaluate the penetrations for two cases with two setups for minimum and maximum particle losses. This study, using aerosol engineering calculations, estimates the maximum expected differences between the two setups, both of which are compliant with the GTR. This study also discusses the mass ratios of PM2.5 and PM10 as a function of the mass median diameters.

Interlaboratory Study on Brake Particle Emissions Part II: Particle Number Emissions

The Particle Measurement Programme (PMP) informal working group co-ordinated a global interlaboratory study (ILS) on brake wear particle emissions with the participation of 16 laboratories in 2021. Two articles present the results of the ILS: (I) particulate matter mass (PM) and (II) particle number (PN) emissions. The test matrix covered different brake systems, including ECE and NAO pad materials with grey cast iron discs and a drum brake. Regarding PN, the study measured the total particle number from approximately 10 nm to 2.5 µm (TPN). Some testing facilities measured solid particle number emissions (SPN) in parallel. The mean TPN concentrations ranged from 9.1 × 108 #/km/brake to 1.1 × 1010 #/km/brake. TPN and SPN emission levels were comparable, except for one lab that measured very high volatile particle emissions for one brake system. The minimum and maximum SPN emissions for a given brake differed by a factor of 2.5 ± 0.5, comparable to data from exhaust SPN ILS measurements. This article provides an overview of lessons learned and subsequent measures incorporated in an upcoming global technical regulation to reduce measurement variability when sampling and measuring brake particle emissions for light-duty vehicles up to 3.5 t.

Characterization of Particle Number Setups for Measuring Brake Particle Emissions and Comparison with Exhaust Setups

The stringency of vehicle exhaust emissions regulations resulted in a significant decrease in exhaust particulate matter (PM) emissions over the years. Non-exhaust particles (i.e., from brakes and tyres) account for almost half or more of road transport-induced ambient PM. Even with the internal combustion engine ban in 2035, electrified vehicles will still emit PM from brake and tyre wear. Consequently, non-exhaust PM emissions cannot decrease significantly without any regulatory measures. Because independent research carried out under different methods is not readily comparable, a Global Technical Regulation (GTR), which sets the procedures and boundaries of testing brake wear particle emissions, is currently under development. This overview describes the particle number (PN) measurement setup based on the well-known exhaust emissions PN methodology. We provide the technical requirements and the expected maximum losses. In addition, we estimate the effect of particle losses on the differences between different setups for typical size distributions observed during brake testing. Finally, we compare brake testing PN specifications to those of exhaust PN.

Statistical Assessment and Temperature Study from the Interlaboratory Application of the WLTP–Brake Cycle

The relative contribution of brake emissions to traffic-induced ambient Particulate Matter (PM) concentrations has increased over the last decade. Nowadays, vehicles’ brakes are recognised as an important source of non-exhaust emissions. Up to now, no standardised method for measuring brake particle emissions exists. For that reason, the Particle Measurement Programme (PMP) group has been working on the development of a commonly accepted method for sampling and measuring brake particle emissions. The applied braking cycle is an integral part of the overall methodology. In this article, we present the results of an interlaboratory study exploring the capacity of existing dynamometer setups to accurately execute the novel Worldwide Harmonised Light-Duty Vehicles Test Procedure (WLTP)–brake cycle. The measurements took place at eight locations in Europe and the United States. Having several dynamometers available enabled the coordination and execution of the intended exercise, to determine the sources of variability and provide recommendations for the correct application of the WLTP–brake cycle on the dyno. A systematic testing schedule was applied, followed by a thorough statistical analysis of the essential parameters according to the ISO 5725 standards series. The application of different control programmes influenced the correct replication of the cycle. Speed control turned out to be more accurate and precise than deceleration control. A crucial output of this interlaboratory study was the quantification of standard deviations for repeatability (between repeats), sample effect (between tests), laboratory effect (between facilities), and total reproducibility. Three critical aspects of the statistical analysis were: (i) The use of methods for heterogeneous materials; (ii) robust algorithms to reduce the artificial increase in variability from values with significant deviation from the normal distribution; and (iii) the reliance on the graphical representation of results for ease of understanding. Even if the study of brake emissions remained out of the scope of the current exercise, useful conclusions are drawn from the analysis of the temperature profile of the WLTP–brake cycle. Urban braking events are generally correlated to lower disc temperature. Other parameters affecting the brake temperature profile include the correct application of soak times, the temperature measurement method, the proper conditioning of incoming cooling air and the adjustment of the cooling airspeed.

Impacts on Brake Particle Emission Testing

The presented article picks out brake particle emission testing as a central theme. Those emissions are part of the so-called non-exhaust emissions, which play an increasing role for particle emissions from transportation. The authors propose a laboratory test setup by using a brake dynamometer and a constant volume sampling approach to determine the emissions in regard to the particle number concentration. Several impacts were investigated while the same test cycle (novel worldwide harmonized light vehicles test procedure (novel-WLTP)) was applied. In a first item, the importance of the bedding process was investigated and it is shown that friction couples without bedding emit much more particles. Furthermore, the efforts for reaching a bedded friction state are discussed. Additionally, the impact of brake lining compositions is investigated and shows that NAO concepts own crucial advantages in terms of brake particle emissions. Another impact, the vehicle weight and inertia, respectively, shows how important lightweight measures and brake cooling improvements are. Finally, the role of the load profile is discussed, which shows the importance of driving parameters like vehicle speed and reservoir dynamics. The authors show that, under urban driving conditions, extreme low particle emissions are detected. Furthermore, it is explained that off-brake emissions can play a relevant role in regard to brake particle emissions.

On-road vehicle measurements of brake wear particle emissions

Brake wear emissions are investigated during on-road driving with a midsize passenger car on a closed test track. A novel sampling system is designed that aims at monitoring the entire aspiration of brake wear particles. The wear particles are collected by a cone-shaped sampler, which is attached at the outer side of the wheel rim. Thus, the air flow direction penetrating the brake assembly from the vehicle underbody to the vehicle outside is preserved. For analysis, the wear aerosol is routed to the trunk of the car. In addition to the emission measurements, the setup flow is monitored, which enables quantification of the acquired emission data. A 3 h subsection of the Los Angeles City Traffic (LACT) cycle, representative for realistic driving behavior, is used as test cycle. For two different brake materials, PM10 emission factors are ranging from 1.4 mg km−1 brake−1 to 2.1 mg km−1 brake−1, while one material is found to be 18% less emissive. Due to high brake disc temperatures exceeding 170 °C, high particle number emissions occur through ultrafine particle generation. The unrealistic temperatures are caused by the limited brake cooling in the semi-closed measurement setup. In contrast, the reference brake temperature does not exceed 153 °C during the same test, thus ultrafine brake particle emissions are not expected during normal driving. Furthermore, well run-in brake material shows emission factors near the lower measurement limit at the unrealistic temperatures, suggesting that ultrafine particle emissions are characteristic for brand-new materials in combination with high brake temperatures observed due to the semi-closed housing of the measurement setup.

A proposed driving cycle for brake emissions investigation for test stand

Particulate matter emission factors from vehicle brakes are difficult to assess directly from the field. Moreover, there is a lack of a standardized cycle and test stand for evaluating brake emissions. For these reasons, a test cycle was developed from real driving data collected from a car. This new test cycle was implemented on an inertia disc brake dynamometer appositely designed for brake particle emission studies. Results reveal that, for the brake system used as an example, the obtained emission factors for the urban driving conditions studied are comparable to EURO 6 regulations in terms of particle number and comparable to EURO 4 levels in terms of mass with brake emission factors equal to 4.37–6.46 × 1011 particles/km and 44–48 mg/km, respectively.

Study of Brake Wear Particle Emissions: Impact of Braking and Cruising Conditions.

A novel measurement setup is designed, constructed, and validated by theoretical simulations and by experiments enabling sensitive and loss-free brake particle emission investigations. With the goal to simulate realistic driving, a 3 h subsection of the Los Angeles City Traffic (LACT) cycle is selected as test cycle. The tests are performed with the front brake of a midsize passenger vehicle under both static laboratory and more dynamic realistic conditions that include parasitic drag and vehicle brake temperatures (advanced vehicle simulations). A PM10 emission factor of around 4.6 mg km-1 brake-1 is determined. During five cycle runs the emission factor in terms of particle number decreases by 1 order of magnitude. This decrease is accompanied by a shift of the critical brake temperature Tcrit, at which ultrafine particle emissions occur, from 140 to 170 °C. Investigations with advanced vehicle simulations generate brake temperatures below Tcrit and consequently do not show ultrafine particle emissions above background level. A particle number emission factor of approximately 4.9 × 1010 km-1 brake-1 is estimated for realistic vehicle brake temperatures. Particle formation during cruising is clearly identified. The brake drag is estimated to contribute about 34% to the total airborne particle mass emission.

Brake Particle Emission Measurements - Testing Method and Results

Brake Particle Emission Measurements - Testing Method and Results

Towards the ranking of airborne particle emissions from car brakes – a system approach

Airborne particulate matter emitted from motor vehicle brakes is a contributor to urban air quality. Therefore, a method to rank brake pairs (pads and rotors) with respect to their particle emission factors in a reliable way is needed to develop a low-emission disc brake. A novel inertial disc brake dynamometer designed for brake particle emission studies, a modified SAE J 2707 cycle, an electrical low-pressure cascade impactor and a filter are used to test five different pad materials against cast-iron rotors. By changing only the pad materials, it is shown that the differences between the mass emission factor and the number emission factor of the the worst brake pair and those of the best brake pair decreases by more than four times and 19 times respectively. Furthermore, the results show that the material combination ranked the best in terms of the mass emission factor is ranked the worst in terms of the number emission factor. The results reveal that this combination of a test stand, a test cycle and particle instruments can discriminate between different brake pair materials in a reliable way in the case of the mass emission factors while more research has to be carried out in the case of the number emission factors.