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EuroBrake 2022: Environmental impact of brake wear particulate emissions session overview

The Environmental impact of brake wear particulate emissions session will take place on Tuesday May 17th and will be chaired by Theodoros Grigoratos of The European Commission JRC and Katharina Kolbeck of BMW Group.

Topics and speakers for the session include:

The effects of corrosion on particle emissions from a grey cast iron brake disc

Ishmaeel Ghouri, University of Leeds

The automotive industry continually strives to increase efficiency and reduce emissions through a variety of methods. Such approaches typically involve attaining technological advancements in engine performance, enhancing aerodynamic efficiency, or reducing vehicle weight. Despite numerous effects, one area that appears to be largely overlooked is the effect of the brake system on environmental emission. A key by-product of the braking mechanism is the emission of particles from the brakes into the environment. These particles not only cause pollution but also pose possible risks to human health.

No current legislation exists in terms of limiting the quantity or type of brake emissions produced, despite there being stringent legislation in place with regards to exhaust emission. It is speculated that friction brakes will become one of the dominant sources of particulate emissions because of the rising number of electric vehicles on the road each year. Electric vehicles still require friction brakes to supplement regenerative braking and it is likely that these will continue to use the traditional grey cast iron brake rotor, which is both heavy and exhibits poor corrosion resistance, and is therefore likely to contribute significantly to brake wear emissions.

To understand the inter-relation between brake rotor corrosion and particulate emission, this study concentrates on quantifying such emissions from a conventional grey cast iron friction brake both before and after exposure to a corrosive environment.

The ‘drag braking’ duty cycle was chosen for this study, as this produces near steady-state conditions at the friction interface. Test were at a constant speed of 150 rpm at three different brake hydraulic pressures, 5 10 and 15 bars. As for the brake pad material, OEM-recommended brake pad materials are used. The duration of each test was 90 minutes, and each test was repeated three times. 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+). After the three repeated test cycles have been completed, the brake wear particles captured by the ELPI+ were examined as described below.

The corrosion test consisted of exposing the brake rotor to a corrosive environment in a salt spray chamber. The salt spray conditions were based on the ASTM B117-11 standard, under 96 hours of exposure. The corroded brake disc then underwent the drag brake duty cycles which as before were repeated three times for each pressure condition. Similarly, to the non-corroded tests, the brake wear particles were collected from the ELPI+ and later examined.

The post-test analysis consisted of using different microscopy techniques to investigate the topography and composition of the brake wear particles. Gravimetric wear measurement methods were incorporated into the post-test protocol. Advanced characterisation techniques such as energy-dispersive X-ray spectroscopy (EDX) and secondary electron microscope (SEM) were used to characterise the surfaces of the brake pads and to characterise and identify the elements of the brake wear particles collected from the ELPI+ before and after the corrosion cycles.

Influence of the driving behaviour on the non-exhaust brake emissions

Matteo Federici, Brembo S.p.A

With the increasing interest in topics related to the environmental pollution by the airborne particulate matter (PM), several strategies have been implemented to reduce the emission contribution from road transport. Due to the worldwide restrictions on the tailpipe emissions, in recent years, an increased concern on the non-exhaust emissions has been observed. The wear of the braking components, clutches and tires have been identified as the major sources of non-exhaust emissions from road transport. The present research focuses on the reduction of the emissions coming from the wear of the braking components and it shows a new approach that can be applied in parallel to the development of novel materials and advanced brake control strategies; this is based on the correlation between driving and braking behaviour of drivers; this correlation could be exploited to lower the non-exhaust brake emissions.

To understand the influence of the driving behaviour on the emissions, the braking system of a C-segment passenger car was tested in a dynamometric bench. The test has been carried out following the informal brake protocol GRPE-81-12 published by the Particle Measurement Programme Informal Working Group (PMP-IWG). The cycle used as a reference for measuring emissions was the WLTP-brake cycle. A modified version of this cycle was generated to simulate a more conservative driving style. Some stops of the reference WLTP-brake cycle have been identified, according to different criteria and then modified leading to a cycle with a lower average vehicle speed and deceleration. These modifications led to a testing cycle roughly 17 minutes longer, with the same vehicle mileage and to lower disc temperatures with respect to the reference WLTP-brake. The modifications resulted in a significant decrease in both PM10, and PN10, decreased of the 48% and 53% respectively, with respect the value obtained with the reference WLTP-brake cycle.

Next evolutions in the development of brake emission testing

Christof Danner, AVL List GmbH

The first approach towards a regulation of Brake Emissions by PMP (Particle Measurement Program) under the United Nations Economic Commission for Europe (UNECE) is ongoing – with a dedicated cycle, defined indicators to be measured and a procedure to be confirmed within a Round Robin end of 2021 (results / feedback will be available for the paper then).

Next big steps shall then be the consideration of xEVs with their capability to recuperate (TF4) and thus reduce the use of the friction brake, which shall be included in the first draft of an upcoming legal regulation. This will subsequently increase the complexity of the measuring procedures as most likely a full vehicle simulation will be required including recuperation strategies.

In the end all measures and vehicle systems that contribute to a reduction of Brake Emission shall be considered, which means a huge peak of development on this topic with a respective high demand on test bench capacity. Affected components will not only be software and functions, but also new brake system hardware like Electromechanical Brakes, that shall also reduce emissions by e.g., “true zero drag” or better controllability for brake blending. Adapted simulation will be introduced and optimized to enable respective measuring at the test rigs.

As the availability of brake test rigs is limited the industry is requested to flexibly provide affordable testing solutions to cover this accumulation in testing. The test rigs themselves might evolve from inertia dynos to inertia simulation – increasing the degrees of freedom.

After the actual focus on – also electrified – passenger cars and Light commercial vehicles up to 3,5 tons also trucks will be considered. The topic is quite new, activities are just ongoing and will influence development and legislation soon.

Investigations on recuperation and operation strategies of a battery electric vehicle under real world conditions as a basis for future brake wear particle emission measurements

Christopher Hamatschek, TU Ilmeanu

Regenerative braking enables the conversion of kinetic energy into electrical energy, which increases the range of electric vehicles. Regenerative braking greatly reduces the need to use friction brakes. Depending on the powertrain architecture (mild HEV, PHEV, BEV), the operating conditions of the friction brake vary significantly. The decision to what degree the vehicle speed is reduced by regenerative braking also depends on many other parameters, like the state of charge, the steering angle, or the driving style.

Nevertheless, the use of regenerative braking is expected to make a significant contribution to the reduction of total non-exhaust PM. The amount of this contribution is still largely unknown, especially for real world driving conditions. In this study, the regenerative braking performance of a battery electric vehicle is analysed under variation of different pre-programmed recuperation modes during on-road tests.

For data acquisition, the vehicle is equipped with numerous sensors and data acquisition systems. The collected data can be used to map the characteristics of the recuperation modes. The anomalies and effects detected during data acquisition represent the challenges that exist in the simulation of regenerative braking on the inertia dynamometer and especially for potential legislative procedures.

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