Introduction

Increasing levels of particulate matter in urban areas is a growing global concern due to the negative impact on human health. Accordingly, the next EURO 7 emissions standard will extend its scope to regulate non-exhaust emissions which significantly contribute to traffic-related PM10 emissions. 

As overload is a common drawback of dynamometers, tribometers are emerging as a rapid and cost-effective alternative. However, they do have limitations; although they simulate kinematics through programmed deceleration rates, they do not dissipate energy. 

This study analyses the critical factor of energy dissipation when replicating a dynamometer test in a tribometer, focusing on friction, wear, and particle emission tests.

Methodology 

The Bruker UMT Tribolab with a pin-on-disc configuration was used for this study. The setup involved a rotating disc and a vertical displacement actuator applying force. A Low Metallic (LM) material pad (10x10x7mm) interacted with discs of Grey Cast Iron (GCI) and Chrome Plating (both 95mm diameter) at a 25mm effective radius. 

Pad wear analysis involved pre- and post-test weighing alongside actuator displacement tracking, while disc wear was measured using the Sensofar confocal profilometer. Particle emissions were measured in an enclosed setup, equipped with an airflow pump, a HEPA filter, an anemometer to ensure isokinetic conditions, and an Optical Particle Sizer (OPS) for particle measurement.

Tests were conducted at constant pressure and velocity (PV) for each disc material (3 repetitions) after a previous bedding. Subsequently, the pressure applied to the GCI disc was adjusted based on the coefficient of friction (CoF) with the aim of comparing both materials at equal energy dissipation. 

This adjustment mirrors real-world dynamics where brake pedal force is dynamically regulated to achieve the desired friction force for optimal deceleration.

Results 

The experimental results achieved consistent repeatability between tests for both the GCI and Chrome plating, particularly regarding the CoF. Interestingly, the Chrome plating discs exhibited nearly half the number of particle emissions than the GCI. In particular, the GCI displayed significantly higher particle number concentration in the 0.3-1 micron range, whereas the Chrome plating reported more uniform values. Despite these differences, both materials yielded similar particle mass results, emitting a higher number of particles in the 3-10 micron range for the coated discs. A comparison between PV constant tests and dissipated energy tests indicated elevated pad wear levels with equalising energies due to the increase in contact pressure. Nevertheless, this phenomenon was not reflected in the emission results. Similarly, an increase in normal force on the GCI disc led to less disc wear. However, the wear of the coated disc was lower than that of the GCI by an order of magnitude.

Limitations 

This study is limited by its focus on testing under constant pressure and speed conditions, which may not accurately replicate real-world vehicle braking scenarios due to the absence of actual braking and the continuous nature of the process. Additionally, the Optical Particle Sizer (OPS) utilized in this research solely detects particles within the 0.3-10 micron range. While it assigns an aerodynamic diameter to each particle and estimates density to report data in mass, it lacks the capability to filter particles by size and collect them for further analysis. Incorporating a sensor for ultrafine particles and an impactor to filter particles by size and mass would provide a more comprehensive understanding of emissions.

What is new in this paper?

The literature lacks consensus on tribometer measurement procedures. To address this gap, the contribution of the present work is addressing a critical factor in replicating a dynamometer bench brake test on a tribometer. The article compares friction, wear, and emissions of two disc materials to assess the impact of dissipated energy. The challenges and considerations in transitioning from dynamometer to tribometer testing are also highlighted.

Conclusion 

This study highlights the importance of equal energy dissipation in tribometer testing to ensure faithful reproduction and fair comparison. A comparison of friction, wear, and emissions between GCI and Chrome plating reveals notable differences. While the coated disc exhibits superior performance in particle size and number, the focus on mass evaluation—emphasised in Euro 7—indicates similar performance between the two materials. This thorough approach enhances the reliability and consistency of tribometer-based tests, offering valuable insights into crucial factors for replicating dynamometer bench brake tests.

Mr. Fabian Limmer, University of Leeds, UNITED KINGDOM

Prof. David Barton, University of Leeds, UNITED KINGDOM

Dr. Carl Gilkeson, University of Leeds, UNITED KINGDOM

Dr. Peter Brooks, University of Leeds, UNITED KINGDOM

Dr. Shahriar Kosarieh, University of Leeds, UNITED KINGDOM

The brake industry is currently on the search for lighter, corrosion-resistant and more eco-friendly brake systems. Apart from health and environmental issues, the main drivers for this development are the changing load profiles arising from the megatrends of electrification and autonomous driving. As the brake disc and brake pad together represent a tribological system, both components must be adjusted in order to achieve optimal functionality.

Testing of brake friction couples, however, is usually a very costly, energy and time-consuming process, that only allows for a very limited range of material concepts to be considered. This is where testing friction materials on a small-scale level has great advantages because much time and money can potentially be saved in sample generation, testing and post-test analysis compared with full-scale testing.

A novel small-scale test bench has been developed at the University of Leeds which aims to screen friction materials under realistic braking conditions. The foundation of the setup is the Bruker UMT TriboLab tribometer operating in a modified pin-on-disc type configuration. Popular full-scale cycles such as the WLTP based real-world driving cycle have been implemented to replicate current everyday driving scenarios as well as custom cycles that aim to simulate possible future load profiles. A full enclosure around the friction couple has been designed using CFD to allow for controlled airflow and subsequent wear debris capture and analysis. The wear particles generated during braking operation are sampled under isokinetic conditions using the well-known Dekati ELPI+ instrument.

The paper will report on the scaling approach used to design the test bench and the conversion of the WLTP based real-world driving cycle to a non-inertial system. Details of the CFD analysis as well as preliminary test results will also be presented.