A typical brake system found in a contemporary passenger car might weigh up to 80kg. Of that figure, the foundation brake dominates, at over 80% of the weight. A typical “brake down” is shown in Table 1.
|
Actuation |
6% |
|
Modulation |
11% |
|
Foundation |
83% |
|
(of which) |
|
|
Discs |
51% |
|
Callipers |
31% |
|
Pads |
7% |
Table 1: Brake system weight ratio
When we think about what a circular approach for braking systems means, it is useful to know what we are dealing with in terms of final system weight. In this post, we will consider what we can do today to make our brakes as environmentally efficient as possible. We’ll look at the latest thinking and best practice in key areas, and consider also the current shortcomings.
Reduce, reuse, recycle.
Before we dive into the detail, lets’ zoom out a bit to consider the wider green horizon. To do that, we must first orientate ourselves in the fundamentals of sustainable products, namely the Reduce, Reuse, Recycle concept. This concept is a fundamental principle of sustainability aimed at minimizing waste and conserving resources, and should inform our thinking on any considerations of greener brakes.
Reduce: This involves lowering the amount of waste produced by consuming fewer resources. In consumer products, reducing can mean choosing products with minimal packaging or material usage, or opting for digital versions of items instead of physical ones. In industrial settings, it could involve streamlining production processes to use less raw material or energy, such as using energy-efficient machinery or reducing water consumption in manufacturing. It is important to state that reducing our use of a product or resource is the most fundamental improvement we can make to a system. The cheapest component in your system is the one you can design out. The most powerful energy efficiency is the energy you don’t need in the first place. If we are to make a greener brake system, reduce must be front and centre of our ambitions.
Reuse: Reusing involves finding new ways to use items that might otherwise be discarded. For consumers, this could mean repurposing glass jars for storage, donating old clothes, or using reusable shopping bags. In industrial contexts, reusing might involve refurbishing machinery, repurposing by-products from one process as inputs for another, or using pallets or stillages multiple times in logistics. If we can find a second life for a component or resource, this makes significant savings on having to gather and process raw materials, and significantly reduces the waste associated with our original product or component. Where we can’t easily reduce the (need for a) component, then our next best recourse is to consider how the component can achieve a second or subsequent reuse.
Recycle: Recycling involves processing used materials into new products. For consumers, recycling includes sorting paper, plastics, and metals to be processed into new materials. In industrial settings, recycling could involve melting down scrap metal to create new components or using recycled plastic in product packaging. In the automotive context, material recycling is the de facto option for components at the end of their life, and stringent regulations in this area have greatly improved both the recycling rates and efficiency here. In the EU, over 85% of automotive waste must be recycled at the end of life, and further improvements are envisaged, including minimum use of recycled materials in new vehicles and circular design approaches. While material recycling is important in reducing waste and recovering (raw) materials, it comes at significant processing cost (and energy input), so should be seen as the lowest priority for green system design.
Reducing our brakes
Now that we have set the scene, let’s dive deeper into each of the three pillars. For the first pillar, here we need to consider what can be done to reduce the brakes altogether. And the good news is there is potential here, and meaningful improvements we can make.
Friction brakes, at a fundamental level, are required to provide a highly reliable method to decelerate and stop a vehicle. The friction braking process works by converting kinetic energy into heat, with some by-products of wear particles, noise and possible vibration. Today’s braking systems use pneumatics or hydraulics to transmit force, and have either air or electrical actuation and modulation systems.
We have covered brake light weighting in previous posts, so we won’t revisit the topic in great detail here. But we can consider ways to share the braking load across other systems. Regenerative braking can be utilised even in deep slip conditions, allowing for reductions in single stop and fade temperatures. Regenerative braking can also play a role in reducing disc and pad wear, so less margin must be included. Power management of acceleration can be a useful way to limit peak fade temperatures. Cooling on demand is becoming increasingly important for balancing vehicle efficiency with lighter foundation brakes. All these measures together allow for reduced foundation systems, with associated weight and raw material savings.
Figure 1: Average annual precipitation
Another high potential area is the hydraulic fluid used in most road vehicles. Today, fluid service life varies greatly across the world, even for the same fluid type used in the same vehicle model. Brake fluid absorbs water, which affects two key technical aspects – the wet boiling point and the viscosity. Water typically enters the fluid through the brake hoses and the reservoir cap breather. Factors such as annual precipitation levels, therefore, could play a significant role in predicting fluid service life. But in many cases, this is not the case.
Fig.2: Hyundai Ioniq5
Taking the example of an Ioniq5 from Hyundai Motor Company, the same vehicle requests fluid replacement every 2 years in Europe but every 4 years in the USA (with a fluid condition check every year). Similarly, the Cadillac Lyriq from General Motors is sold in both the EU and USA. Again, the Lyriq service schedule for the EU requests fluid replacement every 2 years, but the equivalent USA schedule is 5 years (again with a fluid condition check). Both these vehicles specify DOT 4 fluid type. Therefore, reducing the fluid swaps can easily reduce the overall resource consumption of the brakes during the life of the vehicle.
Reusing our brakes
Next let’s consider the Reuse pillar. Here we are concerned with how to find a second life for components, materials or by-products in the brake system.
A leading example of this is the work of Green Friction. Here, scrap or used brake pads are collected from service centres and centrally processed. Any remaining pad material is separated from the pad back plate, and processed to recover homogeneous raw materials. These materials can then be used to generate another brake pad, with equivalent technical performance. This intervention means the end of life treatment (typically incineration for friction material) is avoided, and of course the raw materials required for the new pad are significantly reduced. Studies suggest that between 80% and 90% of brake pad emissions are Scope 3, so even a small amount of material reuse makes a significant impact. This sits well with Green Friction’s own findings that over 90% of CO2 equivalent can be reduced through reuse of this friction material.
Reuse of components in braking is a much harder nut to crack. As we saw earlier, the foundation components make up 4/5ths of the system mass, and here reuse opportunities are limited. The nature of the braking process means the components nearest to the friction pair can see considerable heat, which is a significant factor in determining their remaining endurance. Without meaningful ways to quantify this factor, component reuse in this area is difficult to achieve with today’s approaches. Unlike in railway wheels, for road vehicles the friction braking is achieved with dedicated elements, without scope for refurbishment or reconditioning. (Brake discs or drums can be skimmed, usually to deal with excessive thickness variation or corrosion build up, by removing disc mass and thermal capacity) While a brake pad and disc can be replaced once they reach a wear limit, it is not possible to refurbish these elements to their original condition.
When considering our actuation and modulation components, the reuse topic seems even more inaccessible. In the case of the major components (booster, modulation unit), continuous innovation in these areas largely preclude reuse. For those components with more stable functional outlooks (pedals, pipes and hoses), package integration presents significant barriers.
Fig. 3 – Second life use of brake discs
Tip of a clag iron iceberg
If we circle back to our brake system weight roll-up, we can see that many of our major components don’t easily allow for material recovery or component reuse. Design for efficiency has always been important, and optimisation of braking system sizing will continue to play a major role in the development stage.
But designing a system that has material or component reuse at its core is still some way off. We have discussed some innovative approaches, and the meaningful benefits they bring. We have also discussed some low hanging fruit to reduce our braking system and its maintenance needs. Together, these ideas represent a small percentage (or tip of an iceberg) of our system mass, but they show that some innovative approaches are both possible and worthwhile in this area.