Non-exhaust emissions from brakes and tyres...an EV killer?

Reducing air pollution is one of the most important reasons to transition to electric vehicles

Transport emissions are directly linked to the early deaths of 1715 Australians each year. Incredibly, this number is higher than the annual road accident toll, but without the highly-visible safety campaigns.

Despite the significant effects on human health, there is very little awareness around the dangers of the gasses and particles produced from our cars each time we drive or idle our engines.

EVs have no exhaust emissions, giving them a clear advantage in terms of reducing air pollution over even the cleanest combustion engine, which will still releases greenhouse gasses into the atmosphere any time the engine is running.

EVs do produce slightly more emissions during manufacture than internal combustion engine (ICE) vehicles, however, this is offset over the lifespan of the vehicle, meaning that lack of exhaust emissions outweighs the initial greenhouse gasses created in manufacturing. This is highlighted in the graph below from CarbonBrief.org which outlines the carbon cost of the average European car versus a Toyota Prius and Nissan Leaf.  

Part of the reason EVs produce more emissions in the manufacturing process is because they are yet to see widespread adoption, so automotive manufacturers are unable to take advantage of the same economies of scale as with ICE manufacturing. 

However, as manufacturing techniques continue to evolve and manufacturers make an increasing amount of EVs each year, this gap is expected to close rapidly, making EVs a much greener alternative to ICE cars. 

The world is acting on tailpipe emissions

Public awareness and desire to act on climate change and pollution has become increasingly strong in the last few years, to the point where ICE manufacturers have been forced to take drastic steps to lower their exhaust emissions. 

Public concern around air pollutants, such as diesel particulate, has driven policymakers to adopt new policies to limit the effect of vehicle emissions on public health. This has seen automakers scramble to develop radical improvements in particulate filtering, which has led to significant reductions in the harmful effects of vehicle emissions. 

While ICE vehicles will never reach zero exhaust emissions like that of an EV, they are making significant progress in reducing the health impact of their pollutants.

As research into exhaust emissions has matured, researchers have begun to focus on other harmful emissions and pollution caused by vehicles; known as non-exhaust emissions.

What are non-exhaust emissions?

Firstly, it's important to identify the distinction between gaseous and non-exhaust emissions.

Gaseous emissions, such as carbon dioxide and nitrogen, are caused by the chemical processes inside a combustion engine. As fuel ignites in a vehicle’s cylinders, the carbon in the fuel combines with oxygen from the air to produce C0­₂ molecules. These gaseous emissions are why ICE vehicles are such a major contributor to greenhouse gas emissions.

Non-exhaust emissions, on the other hand, consist of microscopic particles dispersed into the air. They are produced by mechanical wear and tear – as various parts of the engine and car produce friction between the road and each other, they wear down. This abrasion happens all the time and, like exhaust emissions, are largely invisible to the naked eye.

Non-exhaust emissions can be created by combustion engines, due to the huge number of moving parts and chemical reactions. However, the friction-dependent nature of braking and tyres is of particular concern. 

Tyres:

A study performed by Emissions Analytics attempted to measure the mass lost by tyres of a VW Golf. The researchers were initially unsure if there would be a measurable effect, so the test was designed to intentionally simulate a worst case scenario using a heavily loaded car with poor quality tyres, high speeds and a lot of cornering over a 320 km test drive. In this worst case test, they discovered that 5.8g of tyre mass was lost per kilometer – a figure 1000 times higher than regulated exhaust emission limits.

Even though this test was conducted in an intentionally poor environment, in the real world tyres are often underinflated and drivers routinely break speed limits and take corners too quickly. Cheap, low quality tyres are also commonly seen in the real world, meaning that the results from this test may not be as unrepresentative of real world conditions as initially assumed by the researchers .

Brakes:

Like tyres, brakes undergo wear and tear as part of their normal use. Brakes work by causing friction between the brake rotor and brake pads, which then causes the vehicle to slow down. As this occurs, the brake pads wear down, releasing particles into the atmosphere.

This has become a key concern of vehicle emission regulators as, according to research by the University of Cambridge, brake pads are typically made from a variety of metals including iron, copper, titanium and magnesium. These particles can cause stress to human cells, and could be major contributors to chest infections and lung problems. The research estimated brake dust is estimated to make up about 20% of all traffic-related particle pollution.

Why are non-exhaust emissions a problem?

In their 2016 study, Timmers and Achten concluded that EV non-exhaust emissions are comparable to ICE vehicles.  What's clear is a large proportion of roadside traffic pollution is actually from non-exhaust emissions, most of which comes from the disintegration of brake pads and tyres.

As well as the impacts of non-exhaust emissions on human health - an impact that may be as bad as petrol or diesel particulate emissions - there is a waste issue.

Tyres, in particular, break down into microplastics which enter watercourses, compromising our already stressed oceans. Ultrafine brake dust particles can also significantly worsen local air quality.

Do EVs produce more non-exhaust emissions than ICE vehicles?

We have to accept that all vehicles are going to cause non-exhaust emissions simply due to the nature of mechanical parts.

However, when comparing electric vehicles to internal combustion engine vehicles, it's not a straightforward yes or no answer to the question of non-exhaust emissions.

EVs tend to weigh approximately 10% more than an equivalent ICE vehicle due to the heavy battery packs needed to store electricity. This suggests that they do tend to cause more tyre degradation than a comparable ICE vehicle. 

However, the increase in particle emissions is offset in comparison to the additional gaseous tailpipe emissions produced by ICE vehicles. For example, as highlighted in the above graph, the average European car produces more than double the total emissions of a 2019 Nissan Leaf.

In addition, EVs use regenerative braking technology that doesn’t exist in ICE cars. In your current petrol or diesel car, the brakes work by applying friction to slow the vehicle; that kinetic energy is converted into heat and is lost to the surrounding air. 

In an electric vehicle, regenerative braking converts some of this lost energy and stores it within the vehicle’s battery, reducing wear and tear on your brakes.

What can we do to reduce both exhaust and non-exhaust emissions?

This is an area with a lot of room for growth. While there's been a concentrated focus on exhaust emissions and the decarbonisation of transport, brake and tyre emissions have largely gone unnoticed.

However, the nature of technological progress means that as more time and effort is put into reducing exhaust emissions, the law of diminishing returns will encourage automotive manufacturers to look elsewhere to improve efficiency. And, now that brake and tyre particulates are being put under the microscope, and as awareness of particle pollution grows, it's hoped that government regulations will shift to take emissions source into account, forcing tyre and brake manufacturers to adjust their practices.

We will likely also see improvements to brake and tyre wear as the overall design of electric transport shifts. EVs are still an emerging technology, meaning there is still a lot of designing and engineering needed to fully adapt to the different drive-trains and challenges associated with having to accommodate heavy battery packs.

Most companies have focused on producing high-performance luxury EVs in order to offset production costs associated with the small volumes they expect to sell. The Tesla Model S, Audi E-Tron and Jaguar i-Pace are a few examples of EV manufacturers targeting the luxury SUV market. High performance vehicles require wide, low profile tyres that have more contact with the road and consequently generate more tyre particles.

Battery size limitations are also a major factor in EV design, and the chunkier profile of an SUV gives manufacturers a lot more room to manoeuvre when installing battery packs. Energy storage is one of the most pressing areas of research in the electric car sector today, and any decrease in battery size and mass will have a positive effect on non-exhaust emissions. Similarly, as battery costs drop and more EV manufacturers shift to producing accessible hatchbacks and sedans such as the Nissan Leaf or Hyundai Ioniq or Kona, their tyre profiles will change accordingly.

Overall, the solution to the problem is really pretty simple – don’t drive as much. This goes for every vehicle on the road; any car, truck or bus is going to produce a certain amount of non-exhaust emissions simply because it’s a physical system. 

We believe that electric vehicles will lead the decarbonisation of our commercial and personal transport, but the uptake of public transport, cycling and walking are still the most efficient ways to reduce the impact of both exhaust and non-exhaust transport emissions. 

With thanks to Mov3ment for their assistance in preparing this article.

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