The promise of electric vehicles has long been associated with emission free driving. And while it is true that the lack of a tailpipe dramatically improves air quality, it is not true to say that electric propulsion is free of emissions. Brakes and tyres play a significant role in local emissions from electric vehicles, and the creation, fuelling and servicing must be considered when looking at the balance of carbon and other emissions for electric vehicles. The purpose of this article is to present the situation today, and point to where improvements could be made in the coming years.
Long tailpipes
Perhaps the most obvious and oft debated area to consider is where the fuel for electric vehicles comes from. Indeed, this element is often cited by those who claim fossil fuel vehicles can still play a role in low carbon transport. The central element here is that if your electricity production is carbon intensive, it’s difficult to see how electric vehicles are less polluting than internal combustion. Often cited is Germany’s fuel mix, where a heavy reliance on coal and lignite means EVs powered from the national mix were once creating as much CO2 as an efficient diesel.
However, this analysis falls short on a number of counts. Firstly, it assumes that electricity production in the future will be no cleaner than electricity of today. As can be seen from the graph above, it is likely that power generated over the life of a vehicle built in 2019 will be significantly cleaner by the end of the vehicle’s life than it is today. Therefore, the fuel to run an electric vehicle will become greener as the vehicle is used. Indeed, it is possible for consumers to choose exclusively renewable electricity for their EVs, and it is likely that EVs with Vehicle2Grid (V2G) support will enable an even greener energy mix in years to come.
A second area of discussion for the Long Tailpipe is the CO2 balance of vehicle production. Here, the EV comes off at a distinct disadvantage with respect to an internal combustion vehicle. The short version is that battery packs are more difficult and complex to produce than existing internal combustion systems, so there is more energy expended in producing them. This then leads to an EV having a much higher CO2 footprint when it reaches the end of the production line, and will take a long time to get past this initial handicap. But this picture isn’t yet complete. Relative to internal combustion, battery production is still scaling up, and so it is expected become more efficient. Also, the second-life usage of a battery pack (for example in a grid-buffer role) could significantly change this overall life-cycle calculation.
Clean Air and local emissions
If we’ve covered how the car is produced, and where the fuel comes from, can an EV claim to have zero emissions when it is being driven – so called zero local emissions? Well, not really, but the difference to an ICE vehicle is significant. Where emissions will come from is those parts of the vehicle that are designed to wear in use – tyres, brakes, bearings and bushes – and those sacrificial elements which are designed to keep the vehicle working – lubricants, filters, fluids, soft protective covers.
Of these elements, tyres and brakes dominate the emissions – tyre wear is intrinsic to providing grip, and so is a constant source of emissions. And while tyres role across roads, the road surface also wears – creating a compound and complex problem to analyse. Brake dust is created when brakes are applied, and while this is not as frequent as tyres, most particles created are airborne, therefore of greater immediate concern. The vast majority of tyre particles are large enough to fall to the roadside, and so have no immediate impact on air quality.
When considering clean air, a few more points need to be considered – particle size, weight and shape, and of course what a particle is made of. Brake emissions tend to dominate the make up of particulate matter in urban areas (where traffic must brake more regularly) – being responsible for over 55% of non-exhaust related emissions. This is due to brake particles being smaller in size (less than 0.1mm) and light enough to be caught in air turbulence, and can easily enter human airways. While these particles are difficult to visualise, if you have spent time in an underground train station, you probably have an appreciation of how these particles can affect the environment.
In a previous article, I looked at how brake linings were developed, from simple leather straps and wooden blocks for low power braking, to complex chemical compounds for higher power applications. Over the years friction material formulations have contained many harmful substances (including harmful levels of asbestos, mercury, lead, cadmium, chromium and copper). While modern trends have seen limits or bans on some of these substances, the complex chemistry involved in friction braking is not something that can be considered fundamentally healthy.
Brake dust already forms a significant portion of vehicle emissions, and the decades long trends in controlling tailpipe emissions has meant that braking has played an increasingly significant role in the overall emissions mix. And while legislators have a good grip on tailpipe emissions, they are now turning their attention firmly towards brake emissions. Recent work from the California Air Resources Board, and the EU Joint Research Committee look set to quickly move in the direction of new legislation and testing regimes imposed on vehicles.
Regenerative braking to the rescue
A common idea for electric vehicles and brake emissions is that regenerative braking will remove the topic of brake dust, or indeed, remove the need for brakes altogether. Unfortunately this is not the case – regenerative braking will never be able to bring a vehicle to a stop on it’s own, the friction brakes are always in play. For a more detailed look at what happens during regenerative braking, previous blog entries on the subject can be found here and here.
However, it is fair to say that regenerative braking can greatly reduce the need to use friction brakes, particularly in urban areas (where speeds are lower). Which brake system a vehicle uses is decided by software – and from the context of brake emissions, this can be considered an emissions control device. And as we know from tailpipe emissions, such software must rightly be subject to a strict and intelligent oversight regime – so as to ensure that brake emissions controls are applied consistently.
Other ways to clean up brakes
Apart from the promise of electrification, there are a host of other ways that cleaner brakes can be created – autonomous driving, material, tribology and dust filters. These innovations are all gathering momentum, and while some come with extra investment or unit costs, others promise better performance without significant changes in price, and in some cases, savings over the lifetime of the vehicle.
In terms of automated vehicles, the effect on emissions is down to driving style and traffic flow recognition. Autonomous vehicles don’t tend to miss braking points for pre-planned turns or speed limit changes. They are also (mostly) better at paying attention to traffic flow around them – so see the need to brake sooner. Their driving “style” is conservative, patient and passive. All this means that brake events are less intense, and can more readily rely on factors like engine braking, regen or rolling resistance to decelerate the vehicle in a timely manner. In short, machines brake better.
But when the brakes need to be applied (and they will eventually always need to be applied) – how the brake materials interact can greatly change the amount of emissions created. Harder brake disc surfaces typically mean less disc wear, and a suitable hard-wearing pad will also reduce this. Recent innovations in this area include adding hardened surface layers to traditional iron discs, or creating similar effects using different material mixes (for example CSiC discs).
Alternatively, using an aluminium alloy to create a hard wearing and light weight brake disc can have significant benefits. Here, the material properties of the friction pair act in such a way that there is almost no wear of the brake disc, once the pad has deposited a working layer on the disc. This is an example of how tribology (the study of friction) can play an important role in brake emissions.
While brake parts wear, it is possible to create systems to capture this wear on the vehicle – using air filters. These filters are designed to capture the majority of brake particles, and hold them in the vehicle until such time as the vehicle is service – and the material can (presumably) be safely disposed of. There are a number of nascent solutions in development, and tests on development vehicles are on-going.
A final consideration is the service interventions for brakes. This includes the need to replace components as wear limits are reached, but the most obvious area of concern is brake fluid. Current best practice asks for fluid to be replaced biannually, regardless of use. The reason behind this is that brake fluid absorbs moisture from the atmosphere (hygroscopy), which over time can lower the boiling point of the fluid. This requirement stands out on the service schedule of an EV like a sore thumb – like a hangover from the ICE age. Alternative approaches do exist here, with air brakes being common on heavy goods vehicles, and by-wire braking systems starting to be used in vehicles (albeit backed up by hydraulics).
What’s next for brakes?
If the trends towards cleaner transport are set to continue, it seems clear that brake emissions will be firmly in the cross-hairs of public debate. Vehicle manufacturers will need to show they are serious about all forms of emissions, and particularly in areas where there are known solutions. As discussed above, if the production and fuelling of electric vehicles can lead to cleaner acceleration, it seems incumbent on the brakes to play a role in cleaning up the deceleration element.
The good news is that the industry is striving towards this goal, across multiple fronts simultaneously. The adoption of regenerative braking will see a dramatic shift in the levels of brake emissions, and the adoption of electric powertrains will see similar changes to the expectations on building, using and re-using vehicles. The addition of focused legislation will help refine this approach, giving all interested parties a clear imperative and direction for their research efforts.