Exhaust particulates now pale in comparison to non-exhaust emissions, said Duncan Kay of the UK Department for Transport in his EuroBrake 2021 keynote
The automotive industry has worked hard – and successfully – to cut exhaust emissions. Now that the level of exhaust emissions in particulate matter have been reduced, regulators are turning their focus to so-called non-exhaust emissions (NEE) – that is, other emissions caused by driving, such as particulates from brakes, tyres, and road surfaces.
Together, brake wear, tyre wear, road surface wear, and resuspended road dust currently comprises about 10% of UK primary particulate emissions, said Duncan Kay, UK DfT in a keynote presentation at EuroBrake 2021. At current rates, he added, the proportion of non-exhaust emissions is expected rise.
Exhaust particulates pale in comparison to non-exhaust emissions
According to the United Nations Economic Commission for Europe (UNECE), vehicle exhaust emissions have been the subject of "increasingly stringent" particulate (PM 2.5 and 10) emissions regulations over the last two decades. The result: a significant reduction in volumes of exhaust emissions of particulate matter in a number of countries, according to the UNECE citing data submitted to its Convention on Long-range Transboundary Air Pollution (CLRTAP).
Emissions regulators are now turning to non-exhaust sources from road traffic, including harmful debris and particulate matter from road, tyre, and brake wear. UNECE notes a significant rise in the contribution of these sources to total particulate matter (PM) emissions from road transport, and expects the rise in electric and hybrid vehicles to increase this trend.
The UNECE’s Working Party on Pollution and Energy (GRPE) has, through its World Forum for Harmonization of Vehicle Regulations, begun developing "a rigorous test procedure to measure brake particle emissions under standardized conditions."
Thanks to stringent emissions regulation, and the fitment of diesel particulate filters, exhaust particulates have declined in recent years, to the extent that exhaust particulates in total particulate matter (PM) emissions from road transport now pale in comparison to NEEs.
NEE reduction: the UK as a case study
The UK is making a major push on shifting road transport away from gasoline and diesel, noted Kay. A new Environment Bill is currently under scrutiny in the UK Parliament; the UK government has passed law committing to achieving net zero greenhouse gas emissions by 2050; the Prime Minister’s November 2020 ten-point plan for a green industrial revolution included commitments to accelerate the transition to electric vehicles and invest in zero emissions public transport; and the UK government has brought forward by 15 years to 2035 a target for a 78% cut in carbon emissions, and banned the sale of gasoline and diesel cars and vans by 2030.
On the subject of NEEs, said Kay, a UK Department for Environment (DEFRA) call for evidence in 2018 highlighted a lack of quantitative data on the effectiveness of different approaches, and an accompanying degree of uncertainty surrounding this field.
Emissions from braking systems will be under ever greater scrutiny as tail-pipe emissions continue to decline, said Kay, adding that "any new regulation in this area designed to address non-exhaust emissions must be designed in a way that addresses the future new vehicle market which will be dominated by electric vehicles."
The United Nations Particle Measurement Programme – in which the UK is an active member – is developing a proposed laboratory test procedure to measure brake particulate emissions, based on a similar driving test cycle to the real-world driving database used to develop the WLTP (World harmonized Light-duty vehicles Test Procedure).
And the UK government has recently commissioned a 2.5-year, £1.2m (US$1.7m) research project led by Ricardo, which focuses on real world measurements of brake and tyre particulate emissions.
Considerations when introducing NEE regulation
Kay noted specific considerations that need to be taken into account when introducing new regulation on NEEs. These include:
Vehicle level approval: Assessing at a vehicle level is more likely to comprehensively assess vehicle emissions, he said, and can take into account technologies installed to reduce brake emissions, such as regenerative braking.
Aftermarket type approval: Type approval of aftermarket brake products at a component level may be required to maintain low emissions over the vehicle lifetime.
Different limits for PM and PN? NEE results in a wide range of particle sizes, raising the idea of separate limits for particulate matter (PM) and particulate number (PN).
Future-proof the regulation: Real world measurements could potentially provide a uniform approach across propulsion types and even support the assessment of newer technologies. Kay noted that advanced driver assistance systems (ADAS), vehicle-to-vehicle (V2V) communication, and even self-driving technologies could change the way brakes are used, and thus the emission of NEEs from the braking system.
Safety first! The primary role of braking systems is safety, so regulation cannot compromise brake performance.
Questions for the auto industry
Kay, of course, spoke about the need to regulate NEE from a regulatory perspective. His keynote speech raised some interesting questions for automakers and suppliers.
What are automakers and suppliers doing to get ahead of regulation and reduce NEE? Developing solutions before regulations are imposed is of course more efficient than reacting to regulation, but until the details of proposed regulation are known, automakers and suppliers will be left aiming at a potentially moving target.
Second, how fundamental will the changes need to be in tyre and brake design and technology to reduce vehicle-based NEE? Stakeholders will be concerned that any changes may have notable cost implications; consumers will of course be hoping that changes are broadly affordable within existing budget parameters.
And third, is there anything that automakers and suppliers can do to mitigate road surface NEE? Kay confirmed at EuroBrake 2021 that road wear particle emissions are estimated to be roughly equal to tyre wear emissions; he also noted that evidence shows considerable variances in road wear emissions in different countries.
Collaboration required
This is a challenging and complex area, said Kay. "It's important we find a way to address and reduce brake emissions," he said, "and I believe that if regulators and industry work closely together, we can find the most appropriate way of ensuring that any new regulation still allows for innovative solutions to be brought forward."
