Results of particulate matter measurements at a brake test bench
Creators: name: Obando, Daniel
name: Vogt, Ulrich
name: Straub, Dieter
name: Düring, Ingo
name: Duran Mantilla, Juan Mateo
name: Michael, Sabrina
Publisher: name: Zenodo
Year: 2024
Subjects: subject: Air quality
subject: railway transport
subject: brake emissions
subject: PM - particulate matter
subject: Tribology

description: Abstract

Rail transport is known for its efficient response to transportation demands and considered eco-friendly compared to other means of transportation like road traffic. However, it still poses environmental challenges, particularly in terms of air quality. The emissions from rail traffic can be divided into exhaust and non-exhaust, while the first ones have been largely investigated, the non-exhaust emissions remain relatively less explored.

The objective of this research is to tackle this deficiency by delving into the measurement and analysis of particle emissions generated during train braking processes. Specifically with focus on the size distribution and ultrafine particles (UFP).

 Considering the standardized protocol 541-4 from the UIC (International Union of Railways), a measurement program was developed to investigate braking profiles and stop braking in freight traffic at a brake test bench. The measurement program included ten cycles, seven for full-stop and three for regulation braking. Different parameters like train load, contact force, initial speed and temperature were considered. The program was developed to cover scenarios and braking conditions as close to reality as possible. Organic composite brake pads of type C810 with an arrangement of two blocks per wheel were utilized. 

 Regarding the measurement setup, the devices were installed at the exhaust dust of the test bench. For the particle size distribution an aerosol spectrometer (GRIMM Model 1.108) with 15 size channels from 0,3 to 20 µm was utilized and for the UFP a diffusion size classifier (Testo DISCmini) with a measurement range between 10 and 700 nm was used. Further parameters like temperature, relative humidity, flow rate, braking time and distance were also measured.

 The particle size distribution exhibited a unimodal behavior with a clear peak in the smallest channels, it means between 0,3 µm and 0,4 µm and more than 90 % of the emitted particles had a grain size smaller than 0,7 µm for all the cycles. According to the Pearson correlation, for the emission of particles <1 µm the most relevant parameter was the initial speed, while between 1 µm and 5 µm was the contact force. The train load remained the least important factor in particle number emission.

 In the case of UFP, the number of particles increases exponentially while increasing the initial speed varying from 1E+04 to 1E+06 particles/cm3 and at the same time, the average particle size decreases, falling from around 60 nm to 45 nm. This confirms that the initial speed was the most critical operating variable for producing particles in this range.

 As a first approach, a device that measures the number of particles and an average particle size in the range of UFP was used, however, according to the results obtained, a device, which generates a particle size distribution in the range of UFP would be more suitable.

 Further braking conditions like initial and final speeds, contact force, train loads, initial temperature, as well as different configurations, brake pads and wheels should also be studied. Brake discs were not tested, and each cycle was carried out only once, which means, that repeatability could not be checked. 

Apart from the fact that there is very little literature on emissions of air pollutants from railway transport, the size distribution of particulate matter has been little studied, with most of the publications being on mass-based particle emissions.

 This study offers a first approach to this aspect, also including the range of UFP, which is gaining more importance considering its influence on human health and in which units based on mass are less relevant than number of particles.

 The analysis of the measurement data on the test bench demonstrated the correlation between speed and contact force with particle emissions, especially in fine and ultrafine grain sizes. Contrary to what was expected, although the train load has an influence on particle number emissions, this is overshadowed by the parameters mentioned above.

 This study emphasizes the importance for exploration of methods to reduce particulate emissions during braking cycles, especially in the range of fine and ultrafine particles. The results will be considered for the dispersion model and the toxicological risk analysis and it is expected to present them to the scientific community in the future.