Source: University of Leeds
Introduction:
Leeds, United Kingdom – In recent years brake dust has risen to be one of the most important issues facing the braking fraternity. It is a global issue that exceeds the concerns of vehicle quality or risks to the environment and now extends to include threats to human health. It is a problem that will not go away and will indeed increase as the general public becomes more aware of the health issues. As such, OEMs and their supply chain have now begun to focus their attention towards addressing the increasing concerns of total vehicle emissions.
Cars are perceived as the main problem, not only because of the number of units on the road but because of individual ownership and consumer impact. Regardless, trucks and rail cannot be disregarded as “not an issue” simply because of ownership. Currently there are around 1000 million cars on the road worldwide and about 350million commercial vehicles (light to heavy). The number differences become less relevant when mileage and size is considered. Typically, commercial vehicles possibly travel 10 times more in a year than cars. In addition, the brakes on heavy commercial vehicles have pads and discs with 8 times greater mass and possibly 3 times the number of brakes. Therefore, at face value, the gap between cars and trucks may not be as it seems although ownership and buying power remains a prominent consideration by OEMs which influences research direction.
The comparisons of such data draw commercial vehicles firmly into the debate and should be regarded as equally important when non-exhaust emissions are considered. Rail is a special case as main braking is electrical resistance dissipation or regenerative. Regardless, it should also feature as an area for concern, particularly for underground systems. This is raised because friction braking on the underground is predictable and always at the same point – just before a station stop. The dust accumulates at that point and the tunnelling effect of the next train pushes the dust towards the commuters who are in a confined space so cannot avoid being exposed to that dust. Following the first successful litigation case the efforts to resolve the problems of harmful brake dust will increase exponentially.
The UK Government recently issued a “Call for evidence on brake, tyre and road surface wear”. An “Air Quality Expert Group” was formed and the Department for Environment, Food and Rural Affairs (DEFRA) were charged with coordinating the responses. They produced a final report summarizing their findings [1] which includes an extensive listing of research papers. One observation within the recommendations for consideration says, “In contrast to vehicle exhaust emissions, road-traffic non-exhaust emissions are currently subject to almost no type approvals and regulations”. It is not beyond the imagination to envisage that such regulation will be forthcoming in the very near future.
Brake dust research in the UK
The University of Leedsin the UK have completed extensive brake research for a considerable number of years, embracing diverse investigations from the understanding and control of brake noise & vibration (NVH) to examining the viability of plasma coated aluminium brake discs. Their collaborating partners include international OEMs and their supply chains – from cars to heavy duty trucking. Most recently they have entered the arena of rail and are major players in a multi-million pound investment that will develop a “Centre for high speed rail research” at Leeds – which will not only include the dynamics of high-speed trains but chassis research, including brakes. Their experiences in braking research is diverse, extensive and unparalleled.
For a considerable time, Leeds have recognised the problems of brake dust emissions and moved recent research efforts towards developing a brake dust capture and analysis system [2]. It is intended that the planned research will contribute positively to other ongoing international research such as Horizon 2020 ITN proposal. Leeds is a major partner in this project which is planned for submission in January 2020.
Figure 2 – Brake head assembly
Brake dust capture and analysis system – General dynamometer description at Leeds:
The EPSRC funded brake dynamometer and ducting at Leeds is shown in Figure 1 with the brake head assembly shown in Figure 2. A brief description of the airflow system is as follows. A fan on the top of the building takes the air from the laboratory and passes it through a HEPA filter into the enclosed brake test chamber. The speed of the fan can be adjusted to create an average air speed in the range of 6 to 15 m/s at a velocity test point
Within the brake test chamber, the generated brake wear particles (debris) are emitted from the brake friction interfaces and may follow three different tracks. Some of the heavier particles fall down to the floor of the chamber whereas some of lighter particles remain attached to the disc surface to re-enter the friction interface – increasing abrasion and wear. The remainder of the finer particles become airborne and mix with the induced airflow through the chamber and directed to an outlet. The outlet air is then ducted to a sampling tunnel where a sampling probe is used to extract representative samples from a range of tapping points allowing immediate (real time) analysis (Figure 3). The probe is designed to make use of the isokinetic sampling concept to avoid biased results and errors. As such, the probe geometry and air sampling volume flow rate are tailored to comply with this concept. Along with a state-of-the-art particle capture and measuring system (Dekati ELPI®+ Electrical Low-Pressure Impactor) this arrangement allows measurement of airborne particle size distribution and concentration in real time with a sampling rate of 1 Hz or 10 Hz.
The instrument measures particles in 14 size stages from 6 nm to 10 µm. The 14 impactor stages (including a back-up filter stage) make it possible to collect airborne particles on filters for later gravimetric and chemical analyses. A sample flow rate of 10 litres/minute is needed to maintain correct operation of the unit.
Particles of size less than 6 nm emitted from the vacuum pump are discharged back into the duct and then into the external atmosphere through a final HEPA and carbon activated filter.
CFD Analysis:
In conjunction with the mechanical design of the test rig, it was appreciated by the research team at Leeds that a firm understanding of airflow within the entire system was necessary if a thorough understanding of potential dust circulation (and capture) was to be understood and indeed predicted.
A standard ANSYS Fluent software package was used to predict such information with the general circulation as shown in Figure 4. In addition, the velocity maps were predicted across the ducting at the 4 tapping points and the velocity test point. These velocity maps are shown in Figure 5 with the changing velocity profiles along the tunnel shown in Figure 6.
It is clear that a more uniform velocity distribution becomes established the further away it is from the ducting bend. It is worth noting that the measured air velocity at the middle of the duct is within 8% of the air inlet velocity. A CFD analysis was also carried out on the probe which indicated a small but insignificant flow distortion around the inlet of the probe.
Overview:
Initial trials using the system have proved more than encouraging. As expected, the probes detect a significant increase in captured wear debris when the brake experiences arduous tasks. Analysis of the dust has yet to be undertaken and will only be attempted when a consistent test procedure has been formulated.
The complete CFD analysis indicates near uniform airflow at the measuring points and significant, desired, turbulence within the brake head test chamber. As a result of this turbulence it is expected that the dust measurements will show a wide range of captured particulates for all braking events.
Planned research:
A research programme has been established that will include additional CFD analysis and dynamometer validation. Brake dust emissions will focus on monitoring emissions with a brake operating over a range of braking events – light drag braking to arduous mountain descents. Dust analysis will extend beyond particulate size and number count to investigate constituent variance following each diverse testing regime. A particular ongoing investigation will compare the particle emissions from a conventional cast iron rotor brake with those emitted from an alumina-coated aluminium alloy rotor. It is anticipated that the lighter, aluminium, rotor will release far less wear debris than the cast iron equivalent.
The CFD analysis will concentrate on the air flow around the brake head when open running and when shrouded by a wheel centre. This will include rotational speed and vehicle linear velocity variations. There will be parallel CFD modelling monitoring air flow through a rotating vented disc and radially across the friction surface. The subsequent models will be validated and optimised on a purpose designed “spin rig”. Thermal considerations will follow.
Opportunities:
The University are keen to work with interested parties with the primary aims of extending knowledge on brake emissions, develop risk and full cycle cost models in the use of differing materials and working towards the realisation of practical solutions to mitigating dust emissions either at the design stage or on-vehicle collection.
Contacts:
Dr Carl Gilkeson (Thermofluid Dynamics) – [email protected]
Carl is a lecturer in aerospace engineering with an interest in braking technology. He has an extensive research background in applied CFD with specialist knowledge in airborne pathogen transport and infection risk in hospital wards.
Dr Peter Brooks (Braking Dynamics) – [email protected]
Peter is an Associate Professor in Mechanical Engineering. He has an established record of research in automotive brake systems engineering that is underpinned by a blend of industrial experience drawn from across the road and rail vehicle sectors.
Professor David Barton (Solid Mechanics and Materials) – [email protected]
David is Professor of Solid Mechanics. His research is centred around the experimental derivation and numerical implementation of complex modelling of materials and their integrity, including the understanding of tribological interactions at friction pair interfaces within braking systems. His current interest is directed towards lightweight braking systems and mitigating brake dust emissions at the design stage
Shahriar Kosarieh (Surface Engineering) – [email protected]
Shahriar is a Lecturer in Mechanical Engineering where he is highly active in the areas of tribology and surface engineering. His primary focus is currently on friction braking systems and the tribological interactions at the friction interfaces in the creation of brake dust and particulates.
Bibliography:
“Design and Assessment of a Test Rig for Airborne Brake Wear Debris Measurements”
AB Sanuddin, S Kosarieh, CA Gilkson, PC Brooks, DC Barton
Paper No. EB2019-MDS-005, EuroBrake 2019, Dresden, May 2019.
“Measurement of brake wear particle emissions with a novel test rig”
AB Sanuddin, S Kosarieh, CA Gilkson, PC Brooks, DC Barton
5th Annual Conference on Recent Advances in Automotive Engineering, Malaysia, August 2019.
“Towards Environmentally Friendly Brakes” DC Barton. University of Leeds UK. Keynote Address, 5th Annual Conference on Recent Advances in Automotive Engineering, Malaysia, August 2019.
