EV Braking System Explained
We all know that the braking system in a car is super important! That has never changed however, in the ever-evolving landscape of automotive technology, the advent of electrified vehicles has ushered in a new era of sustainability and efficiency.
Every electric and plug-in hybrid vehicle available in the U.S. today features a regenerative braking system. Some conventional hybrid vehicles also incorporate this technology. In this article, we will unravel the mysteries behind the energy management of electrified vehicles. Focusing on the fascinating world of vehicle braking systems and energy recuperation.
What Is an EV Braking System?
An EV braking system slows the car using two methods working together: regenerative braking, which uses the electric motor to slow the vehicle and recapture energy back into the battery, and traditional friction braking, which uses brake pads and rotors like a gas-powered car. Regenerative braking handles most everyday slowdowns, while the friction brakes step in for hard stops and emergencies. The rest of this article covers how each part works, how the two blend together, and how an EV braking system compares to the traditional braking system in a gas-powered car.
History of the Automotive Braking System
In 1899, Gottlieb Daimler, an engineer, conceived a novel concept for brakes. He proposed that a drum wrapped in cables and anchored to the car’s chassis could effectively halt a moving vehicle. Louis Renault later implemented this concept and constructed the first mechanical drum brake in 1902. This invention laid the foundation for contemporary automotive brakes.
The Braking System in a Car
To comprehend the dynamics of energy management in EVS, it’s essential to grasp the fundamentals of a car’s braking system. The goal of the braking system is to dissipate the vehicle’s energy. It does this by using friction between brake pads and rotors to slow down and eventually stop the vehicle.
Currently, most vehicles use a traditional hydraulic system. However it is becoming more and more common for automakers to incorporate newer higher tech brake-by-wire systems into their vehicles. With the rise of electrified vehicles, an even more sophisticated approach is taken to maximize efficiency and minimize energy wastage.
Vehicle Braking System Physics: A Balancing Act
When a driver applies the brakes in a conventional car, the vehicle’s kinetic energy is converted into heat energy. This is done through the friction between the braking components. In contrast, electrified vehicles provide an opportunity to harness this energy. It does this by using a combination of regenerative and friction braking. This allows EVs to achieve a delicate balance between slowing down the vehicle and recouping energy.
Vehicle Braking System Forces
To achieve this, engineers carefully calibrate the braking forces, considering factors such as vehicle mass, speed, and the desired rate of deceleration. Understanding braking forces is essential for designing effective braking systems that can safely stop a vehicle in various conditions. The braking forces can be divided into two groups: longitudinal braking and vertical braking force.
• Longitudinal forces
𝐹𝑥,𝑓 = 𝑀𝑎𝑥𝛾, 𝐹𝑥,𝑟 = 𝑀𝑎𝑥 (1 − 𝛾)
• Vertical forces
𝐹𝑧,𝑓 = 𝑏𝑟/𝐿 (𝑀𝑔) − ℎ/𝐿 𝑀𝑎𝑥, 𝐹𝑧,𝑟 = 𝑏𝑓/𝐿 (𝑀𝑔) + ℎ/𝐿 (𝑀𝑎𝑥 )
Despite the advancements in braking technology, there are inherent challenges in managing the forces involved. Braking constraints are crucial in maintaining stability and control during braking. If the rear wheels lock up while braking, the vehicle can become unstable, leading to oversteer. To prevent this, Regulation 13 of the United Nations Economic Commission for Europe (UNECE) outlines conditions to avoid such situations. One important constraint specified by this regulation is the ratio of the front axle braking force to the total braking force, represented by the symbol 𝛾. This constraint helps ensure that braking forces are distributed properly between the front and rear axles. Thus optimizing stability and preventing instability during braking. Where 𝐹𝑥,𝑡𝑜𝑡 is the total braking force demand.
𝛾 = 𝐹𝑥,𝑓 / 𝐹𝑥,𝑡𝑜𝑡
What is Braking intensity
Braking intensity (Z), which is the ratio of the total braking force demand to gravitational force acting on the vehicle (Mass *gravity). see the equation below:
Adhesion utilization
Adhesion utilization, denoted as 𝑘, is another key factor, representing the ratio of the braking force to the maximum braking force at a given axle. According to Regulation 13 of the United Nations Economic Commission for Europe (UNECE), requirements 3.1.1 and 3.1.2 stipulate specific constraints to be met. These include the condition 𝑘𝑓 ≥ 𝑘𝑟 (A), ensuring that the front axle’s braking intensity is greater than or equal to the rear axle’s braking intensity. Additionally, it condition (B) specifies that the ratio 𝑘 must be within the range of 0.85k < 𝑧 + 0.07 for 𝑧 values ranging from 0.1 to 0.61, further optimizing braking efficiency and safety.
EV Issues With Traditional Braking System
During normal braking in any vehicle, moisture naturally accumulates on brake pads. In internal combustion engine (ICE) vehicles, the braking system is the primary means of slowing down, leading to frequent use of the brakes. This usage generates heat from the friction between the brake pad and the brake disc or drum, causing any accumulated moisture to evaporate and prevent corrosion of the pads.
However, electric vehicles (EVs) use the brakes much less frequently. As a result this means that any moisture buildup may not evaporate as quickly. Without corrosion-resistant brake pads, rust can accumulate, leading to deterioration of the pad backing and breakdown of the adhesive used to attach the backing to the friction material. Ironically, this can result in traditional brake pads wearing out faster on EVs than on ICE vehicles.
The Engineering Trade-Off: Rotor Corrosion vs. FNC Treatment
Reduced brake usage can also affect the rotors themselves. Because EVs rely on their friction brakes so infrequently, a thin layer of surface rust can build up on the rotors — sometimes called “lot rot” — which can lead to noise, vibration, and harshness (NVH) concerns when the brakes are eventually used. Engineers can design around this with ferritic nitrocarburizing (FNC), a specialized thermochemical surface treatment applied primarily to iron brake rotors that significantly hardens the rotor surface, prevents rust, and extends the rotor’s lifespan — but the fix isn’t free, and deciding whether to specify it is a classic automotive engineering trade-off.
On one side of the trade-off, FNC rotors solve the corrosion and NVH problem and last longer in service. On the other, the treatment process adds cost, and unlike a standard rotor, an FNC rotor can’t be turned (resurfaced) during routine service, which hurts serviceability. FNC rotors also have a lower coefficient of friction against the brake pads, which lowers the torque output of the braking system — often forcing engineers to increase the caliper or rotor diameter to make up the difference, adding size, weight, and cost elsewhere in the brake package. Automakers have to weigh the corrosion resistance and refinement benefits against these cost, serviceability, and packaging penalties when deciding whether FNC rotors are worth specifying on a given vehicle program.
Regenerative Braking: Harnessing the Power of Deceleration
At the heart of efficient energy management in electrified vehicles lies the concept of regenerative braking. Regenerative braking employs electric motors instead of traditional friction braking systems to decelerate and stop a vehicle. In a conventional hydraulic braking system, typically utilizing disc brakes or drum brakes, braking results in energy wastage. A traditional braking system converts the kinetic energy propelling the car forward into heat. While effective at slowing down the vehicle, the heat generated from braking friction is no longer able to be harnessed or utilized.
Regenerative brakes capture the kinetic energy during braking and transfer it to the car’s batteries, minimizing energy wastage compared to friction braking. It’s estimated that in most hybrids, up to 90 percent of the energy normally lost as heat during braking is captured and used to recharge the battery through electrical motor resistance, thereby assisting in preserving and replenishing the range of an electric or plug-in hybrid vehicle.
In a conventional hybrid, the energy recovered from regenerative braking helps power various auxiliary functions in the car, such as the audio and climate control systems. This reduces the load on the engine and electrical system, enhancing overall efficiency.
How Does a Regenerative Braking System Work
Regenerative brakes work by reversing the electric motors that propel a vehicle, functioning as a generator. This process feeds energy back into the hybrid or electric system, assisting in replenishing a portion of the vehicle’s range. These incremental boosts in battery range can accumulate and enhance efficiency over time with regular use.
Regenerative braking systems intelligently switch between regenerative and friction braking based on driving conditions. During light braking or coasting, the vehicle primarily relies on regenerative braking to maximize energy recovery. However, in more aggressive braking scenarios, such as sudden stops or emergencies, friction brakes are engaged to provide the necessary deceleration.
While regenerative brakes operate differently from friction brakes, they serve the same purpose of slowing down and halting a moving vehicle. Because regenerative brakes produce a similar effect to conventional brakes, the brake lights are still activated when using regenerative braking as a safety measure. This means that when you release the accelerator, the regenerative brakes engage automatically, causing the brake lights at the rear of the car to illuminate, just as they would if you were to press the brake pedal.
One-Pedal Driving and Blended Braking
Many EVs let the driver control how strongly regenerative braking slows the car down when the accelerator is released. At the strongest setting, the car decelerates hard enough that the driver can bring it to a full stop using only the accelerator pedal, rarely touching the brake pedal at all — a driving style known as one-pedal driving. Tesla popularized this as its “regen braking” mode, Nissan calls its version e-Pedal, and Kia and Hyundai let drivers adjust the regen strength in steps using steering wheel paddles. In vehicles without one-pedal driving, the car instead uses blended braking, automatically combining regenerative and friction braking behind the scenes whenever the driver presses the brake pedal, so the driver feels one smooth stop rather than two separate systems taking turns.
Pros and Cons of Regenerative Braking
Regenerative braking brings real advantages over relying on friction brakes alone, but it comes with a few tradeoffs drivers should know about:
- Extended brake pad life — since the electric motor handles most everyday slowing, the friction brake pads and rotors wear out much more slowly.
- Improved efficiency and range — energy that would otherwise be lost as heat is instead recaptured and sent back to the battery.
- Less brake dust — lighter, less frequent use of the friction brakes means less dust generated from pad wear.
- A learning curve — one-pedal driving feels different from a traditional car and takes some drivers time to get used to.
- Reduced feel in hard stops — regenerative braking alone isn’t strong enough for emergency stops, which is why every EV still relies on friction brakes for hard braking.
- Rotor rust (“lot rot”) — an engineering trade-off — infrequent friction brake use can let a thin layer of surface rust build up on the rotors, causing NVH (noise, vibration, harshness) concerns. Engineers can specify FNC-treated rotors to solve this, but that trades away cost, serviceability (FNC rotors can’t be resurfaced like a standard rotor), and some braking torque (requiring larger brakes to compensate for their lower coefficient of friction).
Regenerative Braking vs. Traditional Friction Braking
| Regenerative Braking | Traditional Friction Braking | |
|---|---|---|
| How it slows the car | Reverses the electric motor to act as a generator | Friction between brake pads and rotors or drums |
| Energy | Captures kinetic energy and sends it back to the battery | Converts kinetic energy into heat, which is wasted |
| Component wear | Very low — no pad-to-rotor contact | Wears pads and rotors over time |
| Best suited for | Everyday, gradual slowing and stopping | Hard stops and emergency braking |
Energy Demand – Recuperation: A Symbiotic Relationship
Remembering the vehicle dynamics equations, we know that the aerodynamic and the rolling-resistance forces are always dissipative, i.e., 𝐹𝑎,𝑖 + 𝐹𝑟,𝑖 ≥ 0. If a vehicle is equipped with an energy recuperation device, some of the vehicle’s kinetic energy can be recovered during driving.
The success of energy management in electrified vehicles hinges on the harmonious interplay between energy demand and recuperation. A well-designed system anticipates the driver’s energy requirements and adjusts the braking strategy accordingly. Advanced algorithms analyze various parameters that can impact driving cycles, including driving habits, terrain, and traffic conditions, to optimize the balance between energy consumption and recuperation.
Even in the case with no additional recuperation device, the vehicle utilizes some of its kinetic energy to drive parts of the cycle. 𝐹𝑚,𝑏 is later dissipated as heat by the brakes.
Conclusion
In conclusion, the energy management of electrified vehicles, especially braking systems, represents a captivating synergy of engineering precision and environmental consciousness. As we witness the continued evolution of electric mobility, understanding these intricate systems becomes not just a technical curiosity but a key to unlocking the full potential of sustainable transportation. The roads ahead are paved with innovations, and the efficient management of braking energy is steering us toward a future where every deceleration is an opportunity for progress.
Frequently Asked Questions
Does regenerative braking wear out brakes faster or slower?
Regenerative braking makes friction brake pads and rotors last longer, not faster. Because the electric motor handles most everyday slowing, the friction brakes are used far less often, so pads and rotors wear down much more slowly than on a gas-powered car.
Can you turn off regenerative braking?
In most EVs, regenerative braking can be reduced or set to a lighter setting, but it generally cannot be fully disabled, since it’s part of how the car manages energy. Some vehicles, like the Kia EV6 or Hyundai Ioniq 5, let drivers adjust the regen strength in steps using paddles behind the steering wheel.
Why do EVs still need regular friction brakes?
Regenerative braking alone isn’t strong enough to bring a car to a hard, fast stop. Friction brakes are still needed for emergency braking, low-speed stops, and situations where the battery is already full and can’t accept more recaptured energy.
Is regenerative braking as good as normal braking?
For everyday slowing and stopping, regenerative braking works very well and adds the benefit of recapturing energy. For hard or emergency stops, EVs blend in the friction brakes automatically, so overall stopping power is comparable to a traditional car.
What is one-pedal driving?
One-pedal driving is a mode, popularized by Tesla, that uses strong regenerative braking to slow and often fully stop the car when the driver lifts off the accelerator, with little or no need to press the brake pedal. Nissan’s version is called e-Pedal.
What is “lot rot” on EV brake rotors?
“Lot rot” is a thin layer of surface rust that builds up on brake rotors because EVs use their friction brakes so infrequently, which can cause noise, vibration, and harshness (NVH) when the brakes are eventually applied. Fixing it is a classic engineering trade-off: automakers can specify ferritic nitrocarburizing (FNC), a surface treatment that hardens the rotor and resists rust, but FNC rotors cost more, can’t be resurfaced like a standard rotor, and have a lower coefficient of friction that can require larger brakes to make up the lost torque.
