Tyres prove to be a major source of emissions

14 November 2019 - 11:57 By Michael Taylor
Electric vehicles, like the battery-powered London taxi, generate high emissions from tyre and brake wear. Picture: REUTERS.
Electric vehicles, like the battery-powered London taxi, generate high emissions from tyre and brake wear. Picture: REUTERS.

A study in England has found that exhaust fumes account for just a third of all emissions from vehicles, with the rest from tyre, road and brake wear.

With the balance of opinion swinging against combustion fuels in cars, environmental groups are taking the next leap and bundling the $240bn (R3.57-trillion) a year tyre industry into their emissions battles.

The British government’s Air Quality Expert Group (AQEG) emissions research organisation has recommended that tyre wear be prioritised as a “source of ambient concentrations of airborne particulate matter, even for vehicles with zero exhaust emissions of particles.”  

Research from Europe’s Emissions Analytics indicates that emissions from normal tyre wear will be the next big environmental mountain to climb, regardless of whether cars were electric or combustion powered.

In fact, electric cars could make the phenomenon worse, with most EVs weighing considerably more than similarly sized combustion vehicles, plus having far more initial torque from a standing start.

The AQEG found that more than a quarter of all microplastics in the oceans came from normal tyre wear and Emissions Analytics insists it could be an even bigger environmental problem than disposing of old tyres.

“Tyres also sit uniquely at the intersection of air quality and microplastics,” the Emissions Analytics study document claimed, insisting it could also spark a new public health scare.

“You cannot see tyres wearing and therefore ‘tyre wear’ in this sense remains imperceptible except in deliberately extreme use such as branches of motorsport such as drag racing and drifting.

“Regarding public health, there is a tentative emerging consensus among epidemiologists and other medical researchers that ultrafine particles are potentially more injurious to human health than coarse particles, owing to their ability to translocate to the bloodstream through the lungs.  

“A [normal] tyre abrades owing to the friction between its contact patch and the road surface. It ‘emits’ particles across a broad size spectrum, from coarse to fine to ultrafine to nanoscale.”

Yet, the study found that the biggest issues with tyre wear came with both coarse particles falling quickly to the ground and fine particles remaining airborne, blown away from the road, before settling on the ground.

“Particle dispersion and deposition eventually occurs, but that is not the end of the story,” Emissions Analytics said.

“The particles typically pass into the watershed through street drainage and are estimated to be a primary source of as much as 28% of microplastics found in the marine environment.”

SOME TYRES CONTAIN NATURAL RUBBER, BUT TO ALL INTENTS AND PURPOSES WE LIVE IN THE AGE OF THE PLASTIC TYRE

The report relied heavily on new real-world microplastics testing equipment and techniques, which the tyre industry itself insists it never had access to.

New equipment has helped Emissions Analytics to measure 14 microparticle sizes from six nanometres to 10 micrometres (1,000 of which make up a millimetre) for both mass and number.

Instead, just like the car industry up until last October’s Worldwide HarmoniSed Light Vehicle Test Protocol (WLTP) regulations, the tyre industry relied on laboratory tests to measure its emissions.

“We support the recommendation of further inquiry into the feasibility of a standardised test method for measuring tyre abrasion rate, which is currently being looked into by the European Commission with support of Industry,” the European Tyre and Rubber Manufacturer’s Association wrote in a statement.

“However, as mentioned by the report, the tyre industry’s efforts alone will not be enough to address the tyre and road wear particles (TRWP) challenge. The ETRMA fully agrees that the debate considers both tyre design and the diverse external factors at play when finding solutions for reducing TRWP generation, such as driving behaviour, vehicle and road characteristics and traffic conditions.”

However, it strongly disagreed with the AQEG’s test methodology, insisting it relied on 1990s tests that were “no longer scientifically robust, which highlights the need for additional research.”

Emissions Analytics claimed that over a typical 20,000km-50,000km lifespan, a road tyre would lose up to 30% of its rubber, or up to 2kg of weight.

The organisation insisted the emissions free pass given to tyres over the last century came about because of the perception that they were made from a natural product, rather than crude oil.

“The recent re-characterisation of tyre wear emissions as ‘microplastic pollution’ corrects the broadly misleading public idea that tyres are composed principally of natural rubber. Instead, tyres are a close derivative of crude oil and their wholesale pricing typically tracks it.  

“A typical car tyre comprises 45% oil-derived synthetic rubber (polymer), 40% oil-derived carbon-black (filler), and 15% various additives to aid production processes, some of which typically contain heavy metals and some of which are also oil-derived. Some tyres contain natural rubber, but to all intents and purposes we live in the age of the plastic tyre.”

THE TYRE INDUSTRY’S EFFORTS ALONE WILL NOT BE ENOUGH TO ADDRESS THE TYRE AND ROAD WEAR CHALLENGE

The problem is expected to become worse with more electric cars on the road, except where (in rare cases like BMW’s i3) the cars are made with lightweight material to offset the battery mass.

The AQEG report suggested that nanoparticulate emissions from tyres rise up to 1.8% for every 10kg increase in a vehicle’s mass. That’s bad news in the era of SUVs and crossovers, but far worse for the looming era of electric cars, which are estimated to be around 29% heavier than similarly sized combustion vehicles.

“A whole generation of new EVs is hitting the roads with considerably larger and heavier battery packs than in the past,” Emissions Analytics reported.

 “Electric vehicles offer instant torque and higher kerb weights, implying higher tyre wear rates, even while regenerative braking is expected to reduce brake wear emissions. But a Tesla Model S or Model X, Mercedes EQC, Audi e-tron or Jaguar i-Pace, EVs with larger ranges and battery packs in the range of 60kWh-100kWh, weigh 2.3 tons to 2.6 tons. 

“The 600kg battery pack in the Mercedes EQC would potentially increase nanoparticle emissions from tyres by 48%-108%, compared to a conventional vehicle weighing 600kg less. “The same argument can be extended to internal combustion engine vehicles. A heavier vehicle increases tyre wear, whereas lightweighting mitigates it. This has implications for the broader market trend towards SUVs, where often particularly large rim tyre sizes are adopted.”

X