Regenerative braking it’s affecting your brake pads more than you think
Regenerative braking it’s affecting your brake pads more than you think
From an engineering point of view, aBrakerefers to a device that primarily performs the function of braking or stopping a moving object, such as a vehicle. Going back to physics, a brake refers to converting kinetic energy into other types of energy. And this energy conversion mechanism should be fast, available and reliable. Braking is one of the most important components of security of any vehicle. Braking events are not always predictable and any braking system must guarantee the stopping of a vehicle within the limits of current regulations.
Friction braking:
Road mobility has been based mainly on the friction braking since its conception. A friction brake transforms kinetic energy mainly into friction and thermal energy, by pressing a friction material against a moving part, generating an opposing friction force andheat. Commercial friction materials are complex compounds and can contain more than20 different ingredients and a phenolic resin as a binder. General friction characteristics are defined by the synergy of brake pad component properties and the development of new friction materials always requires a primarily empirical, and is mainly based on previous experiences, because the behavior of the friction material, under different operating conditions, is difficult to predict due to its stochastic nature. It is necessary to have the following characteristics in a simultaneous manner (i) short bedding period, (ii) cold coefficient of friction stability, (iii) friction coefficient stability during speed changes, (iv) friction coefficient stability with increasing temperature, (v) stability of the coefficient of friction after heating, (vi) stability of the coefficient of friction under pressure changes, (vii) relationship between the static and dynamic coefficient of friction, (viii) coefficient of friction in wet conditions, etc. Engineering is an important part of brake pad design, but tribology and tribochemistry are critical to achieving your goals. And tribology and tribochemistry depend on the working conditions and the composition and properties of the friction pairs (friction material/counterpart).
Oversimplifying, Europe and America have followed two different paths in automotive friction materials since asbestos was banned in the last century. In Europe, friction materials often contain steel (known as Low Steel or LS) to achieve greater resistance to high temperatures, which makes them easily suitable to pass the AMS standard test used by the OEM industry to validate the safety of their products. brake pads in extreme conditions in their vehicles for the European market. In North America, friction materials do not use steel (being known as non-asbestos-organic or NAO), oriented in a greater comfort (which again is a huge simplification).
Regenerative braking:
With the introduction of electricity as part of a vehicle’s main propulsion system, other types of kinetic energy transformation are possible. Electromagnetic regenerative braking is one of the main features of an electric vehicle. The vehicle’s driveshaft is connected to an electrical generator, which uses magnetic fields to restrict the rotation of the driveshaft, slowing the vehicle and generating electricity. During regenerative braking, the car’s electric motor becomes a generator, converting the car’s kinetic energy into electrical energy that can be stored in the battery and charged.
Regenerative braking is not yet a replacement for friction brakes. It may never do. They are two complementary technologies with a common characteristic (reducing the speed of the vehicle), but with different purposes (increasing the autonomy of the vehicle and guaranteeing safe braking in all types of braking). Each technology has its lights and shadows.
Regenerative braking can only be installed on the drive wheels, as a powertrain is required for energy recovery. The waste heat generated is not significantly reduced unless the vehicle is an all-wheel drive model. The braking force of an electromagnetic regenerative braking system is often insufficient to cover any type of stop. The maximum recharging speed of the circuit and the battery capacity will be the main determinants of the braking capacity of said system. The evolution of materials and engineering will reduce this drawback of regenerative braking. As an example, the Gen3 Formula E car from the 2023 season has a second engine on the front axle with more focus on braking than acceleration. Under hard braking, the regenerative braking will be so powerful that these cars won’t even have friction brakes on the rear wheels, and the FIA projects that a staggering 40 percent of the total electrical power used in racing will come from the regenerative system. . Of course, the time you spend braking a single-seater on most race tracks is a different scenario than most electric mobility users face.
The design of a regenerative braking system includes a variety of sensors and logic control units to adjust the operation. Also to consider the reliability of these electronic components and their integration with mandatory security systems such as the ESC. Therefore, a traditional friction brake system is required to convert excess energy from the vehicle. The friction brake can also prevent the loss of braking capacity in the event of a failure of the regenerative braking system.
Evolution:
For decades, from the beginning of the 20th century, friction materials in brake systems have been designed as the main sacrificial part of the brake system. Working conditions on PHEVs and especially EVs are very different from conventional ICE vehicles in terms of temperature, achieving comparatively lower operating temperatures for most of the time. In practice, and in normal driving, regenerative braking should be the preferred way to decelerate a hybrid or plug-in car, capable of stopping the vehicle without the participation of the mechanical brake in 90% of the braking. This could mean that the car’s traditional friction brakes are not being used as much as they should be. Under different working conditions and due to the stochastic nature of the friction braking process (which depends on the actual contact area, the existence of a transfer layer between the friction pairs, changes in pressure, temperature, speed, deformation and wear), the behavior of friction materials in a PHEV or EV can be affected.
High temperature friction stability and wear resistance have always been a key feature of friction materials, and in the future perhaps some of the ingredients used in their formulations for this specific operating range (such as high temperature lubricants). temperatures) decrease in use or be substituted by other ingredients to focus on functionality at lower temperatures. The consumption of brake pads is very low in electric vehicles. The brake pads therefore tend to be thinner and thinner..
Another very noticeable effect is the appearance of excessive corrosion of the brake discs, which generates noise and safety problems.Brake discs are basically made of gray cast iron, due to its good heat resistance and ability to damp vibrations, forming an oxide transfer film on its surface and on the surface of the brake pad. Strategies are foreseen to mitigate these problems from the friction materials industry, such as the introduction of corrosion inhibitor additives. But another strategy is also being considered, such as the extensive use of corrosion and wear resistant coatings on brake discs.
An external factor is also contributing to this last strategy. As part of the EU Commitment initiative to accelerate the transition towards smart and sustainable mobility, Euro7 will implement stricter emission standards for all cars, vans, trucks and buses (regardless of powertrain), including for the first time the brake emissions. The Adoption Roadmap has recently reached a major milestone in Commission Adoption, setting the brake emissions limit at 0.7 mg/km PM10 by 2025. Rotor wear is responsible for up to 80% of particles emitted by a mechanical brake system, and LS-type friction materials cause a higher number of particle emissions than NAO-type ones.
In the US, an industrially well-established rotor coating process, the so-called FNC (Iron Nitride-Carbide), seems to be the winner to be standardized for all electric vehicles and PHEVs. Since its introduction in brake rotors in 2008, FNC has helped reduce brake warranty claims by 70 percent at General Motors. In Europe, with LS friction materials, and especially in vehicles with intensive use of mechanical brakes (such as ICE), the very small thickness of 25 micrometers can easily be eliminated, producing unwanted noise and requiring disc replacement. It is the Laser Cladding technology that seems to be best positioned for its industrial implementation in Europe for the hard coating of rotors. Thicker, harder and wear resistant coatings, based on Metal Matrix Composites with Carbides or precipitation hardening alloys. Able to last working with an LS friction material. Rotor coating development process.has been generally led by the OEM companies themselves with leaders in coating technology companies, but not necessarily including the friction industry in the process from its inception.
And remember what I said earlier about friction material design?“…working conditions and composition and properties of the friction pairs (friction material/counterpart)….” “…Brake discs are basically made of gray cast iron…forming an oxide transfer film on their surface and on the surface of the brake pad…”.And yes, you’re right…
Emerging Technologies:
The industry is now trying to catch up with the need for simultaneously (i) short bedding period, (ii) cold coefficient of friction stability, (iii) friction coefficient stability during speed changes, (iv) friction coefficient stability under temperature loading, (v) stability of the coefficient of friction after temperature loading, (vi) stability of the coefficient of friction under activating pressure changes, (vii) relationship between the static and dynamic coefficient of friction, (viii) coefficient of friction under wet conditions, etc. . It seems much simpler in the case of a combination of NAO and FNC (already used since 2008), but it seems more complicated in the case of LS with Laser Cladding, at least until a cladding is fixed. Is it a new fork in the development of brakes between Europe and America?
What seems clear is that regenerative braking and friction braking are going to be complementary systems for a while, but with changes in the friction pairs and in the composition and thickness/geometry of the brake pads. The introduction of regenerative braking can provide a safety advantage in time and distance kinematic deceleration over traditional service braking. Results from initial studies showed that regardless of which level of regenerative braking is used, it gives drivers a braking advantage and also showed that driver foot behavior differed under the high regenerative braking condition. Can this change generate an expectation of a different driving experience on the part of users? Is it possible that the AMS test is no longer as important as it is today? Could NAO formulas that simplify the reduction of brake emissions one day become popular in Europe? Can this lead to a simplification in the formulation of friction materials and at the same time a redesign from scratch for new friction pairs? Could this be the starting point for the commercialization of mechanical braking systems? Well, as Socrates said: “The secret of change is to focus all your energy, not on fighting the old, but on building the new.”
By: Carlos Lorenzana Agudo, Chief Innovation Officer.
