The automotive industry is in a state of transition with new emissions reducing policies and regulations having been brought into force.
However, one of the ongoing disagreements across many platforms it what constitutes a ‘zero-emission’ vehicle?
Fundamentally there is no such thing as an entirely ‘zero-emission’ vehicle. However, until now any electric vehicle has been deemed zero-emissions as it does not produce any tailpipe emissions. The arguments in and around the automotive sector have been based on the simple fact that although electric vehicles have no tailpipe emissions, these vehicles still produce emissions from their brakes, tyres and from their manufacture and their life cycle.
Life Cycle Analysis (LCA) is now a common phrase in and around the automotive manufacturing sector.
However, it is still currently not being used as a unit of measure to assess the environmental impact of so called ‘zero-emissions’ vehicles. No matter which way you look at this it means that the information about ‘zero-emissions’ is flawed and not an accurate representation of a ‘zero-emissions’ vehicles real impact on the environment.
One area of research which is now being focussed on for the measurement of vehicular emissions is brake and tyre wear.
Emissions Analytics focussed on this subject and their 2022 webinar ‘Tire wear, composition and toxicity’. Throughout the webinar they assessed the environmental impact of tyre wear and how tyre composition and toxicity could impact directly on the environment.
According to Emissions Analytics research (2022), some 300,000 tonnes of tyre rubber are released into the atmosphere in the US/Europe annually. Despite tailpipe emissions of new ICE’s falling and the uptake of EV’s reducing this further according to the Emissions Analytics research tyre emissions are increasing.
According to the EA research the increase in tyre emissions can be due to the increased weight of EV’s due to heavy battery technologies.
Passenger car EV’s are Road Tax exempt in the UK due to being zero tailpipe emissions. However, this does not take into account the emissions caused by the EV’s excessive tyre and road wear due to their increased weight distribution.
The Emissions Analytics research (2022) tested a series of 100 tyres from differing manufacturers. Their findings discovered that the compositions of all the tyres varied widely, with no seeming commonality in the compounds. Ingredients such as fragrances having been added which were found to be an eye irritant and high order carbon compounds were also found to be prevalent.
One tyre in the study was found to have a compound known as phenal 2 present, which is a known respiratory irritant, and another contained 6PPD quinone, which has been linked to the death of coho salmon in California, as it interferes with their reproductive cycles.
The Emissions Analytics data was based on using 14 different brands of tyre and 100 tyres in the entire study. With tests being formed over 1000+ miles, the research discovered that each tyre lost a total mass of 37mg per km. In on road ‘normal dynamics’ the Emissions Analytics research showed that 11% of the fine particle mass emitted by the tyres were less than 2.5 microns in diameter. The material would therefore not only be hanging in the air as an air pollutant but would undoubtedly then find its way into soil and water as the particles fell.
Taking the EA research into account, a new Department for Transport (DfT) report has dug deeper into the non-exhaust emissions topic and according to the report collaboratively collated by Ricardo Plc and the Arup AECOM consortium, non-exhaust emissions (NEE) are now the primary source of PM10 and PM2.5 from road transport in the UK. (1)
With a global focus having been primarily on how tailpipe emissions can affect emissions and human health, the research into how brake and tyre compound emissions has until now been somewhat lacking. The new Ricardo and Arup AECOM consortium report is one of the most comprehensive insights into this topic to date.
NEE of particles from road vehicles primarily arise from a combination of brake wear, tyre wear, road surface wear, and the resuspension of dust particles. Emission data from the UK National Atmospheric Emissions. (1)
Inventory (NAEI) estimate that NEE are now the dominant source of coarse and fine particulate matter (PM10 and PM2.5) from road transport in the UK. Therefore, it is important to improve knowledge on NEE and address the gaps so it can be used to inform policy and legislation aimed at reducing tyre and brake wear particulate emissions, particularly as the percentage of electric and hybrid vehicles on the roads is increasing. (1)
Although there are still gaps in understanding the extensive sources of particulates from non-exhaust emissions, the DfT are seeking ways to close the gaps on non-exhaust emissions and in doing so inform policy and legislation aiming at reducing tyre and brake wear emissions.
The Ricardo Plc and Arup AECOM consortium data was based on a prototype sampling and measurement system-based approach which consisted of:
- A sampling duct for tyre wear, fixed to the wheel hub carrier (to allow movement with steering) to draw samples from behind the tyre-road contact patch into the measurement system.
- Brake enclosures for sampling emissions from the brake pads and discs – three designs were tested with differing interfaces of static and rotating components.
- Sampling probes for background ambient particulate sampling.
- Sampling tunnel, from which the analysers sub-sampled from.
- A pair of Dekati Electrical Low Pressure Impactor (ELPI+) analysers for real-time measurement of particle number and size distribution. One ELPI (cold) was run at ambient temperature to measure total particles, the second ELPI (hot) was heated to 180°C and sampled via a heated line to provide a measurement with volatile particles removed.
- A Dekati eFilter was used to measure particle mass in real time, along with cumulative gravimetric mass via a glass-fibre filter. During chassis dynamometer testing an external Horiba MEXA-2100SPCS Solid Particle Counting System (SPCS) was also used to measure entirely non-volatile particles. (1)
The prototype system was installed to a small light duty van (front wheel) and testing was undertaken in a chassis dynamometer facility; on a nearby test-track; and on-road in an urban environment.
A brake enclosure was subsequently installed on a passenger car (rear wheel) to assess the transferability of the enclosure and to measure emissions from a different set of pads and discs. (1)
The report in its entirety is an extensive insight into the data capture from the commissioned DfT report. However, here are a few of the highlights of the report and how the data collated has been interpreted.
- Emissions increased with extreme braking events. The extreme braking events also resulted in considerably higher pad/disc temperatures (up to ~300°C for the pad and ~400°C for the disc) and indicated that at these temperatures the continuous release of particle emissions can be observed.
- Reducing the tyre pressures (from 2.9 to 2.0 bar) resulted in an 8-fold increase in particle number sampled by the cold ELPI. The eFilter observed an increase in mass (by ~2 times). The decreased tyre pressure also resulted in an increase in particles in the nucleation mode (< 20nm), suggesting an increase in volatile / semi-volatile materials under these conditions.
- Particle mass emissions from the tyre during the moderate braking events were ~3.2 mg/km/tyre. During urban driving the levels were lower at ~2.5 mg/km/tyre.
- Particle number emissions from the sampled tyre, measured in the chassis dynamometer, were ~3.3×109 #/km/tyre for the cold ELPI and ~0.6×109 #/km/tyre for the hot ELPI illustrating a strong emission of volatile particles.
- Measured brake particle mass emissions were lower when sampling the rear wheel on the
passenger car compared to the front wheel of the van. In contrast, non-volatile particle number emissions were higher from the rear wheel. The differences may be related to variations in composition of the brake pads and discs.
Note: These are only a few highlights from the extensive report.
Non-exhaust emissions continue to be highlighted across the automotive and manufacturing sectors. With increasing pressure on legislators and policy makers to recognise and include LCA (including brake and tyre emission analysis) in their emissions directives; there is now considered shift towards a need for more transparency in the measure of the automotive sectors emission standards.
Written by Katy Mason for and on behalf of Dolphin N2.