Kinetic Energy Recovery System

1. What is a Kinetic Energy Recovery System?

A Kinetic Energy Recovery System (KERS), or regenerative braking system, is designed to capture the kinetic energy generated during vehicle braking, convert it into electrical or mechanical energy, and store it for subsequent use. Originally developed for Formula 1 racing, the technology was first introduced in the 2009 season before making a full-fledged return to the grid in 2011. Today, regenerative braking has become one of the cornerstone technologies in new energy vehicles (NEVs).

In conventional internal combustion engine (ICE) vehicles, kinetic energy is dissipated into the atmosphere as wasted heat through brake friction whenever the vehicle slows down or brakes. In battery electric vehicles (BEVs) and hybrid vehicles, engineers have developed systems that harvest and store this kinetic energy, repurposing it as propulsion power whenever required. This not only recovers otherwise wasted energy effectively, but also enhances acceleration response from a standstill.

2. How It Works

The core operating principle of kinetic energy recovery relies on the reversibility of electric motors. An electric vehicle's drive motor is inherently reversible—under normal driving conditions, it acts as an electric motor, drawing energy from the traction battery to deliver mechanical drive to the wheels; during deceleration or braking, the wheels drive the motor's rotor in reverse, turning the motor into a generator that produces alternating current (AC) via electromagnetic induction. The inverter then converts the AC output into direct current (DC), which is regulated and fed back into the high-voltage traction battery.

Simultaneously, the induced counter-current charges the battery, while the counter-electromotive force applies reverse torque across the motor and driveshafts, generating electromagnetic braking force to retard the vehicle. This regenerative braking action not only recovers energy, but also significantly offloads work from the conventional friction brakes.

3. Main System Types

Kinetic energy recovery systems generally adopt several technical architectures:

1. Motor-Battery Regenerative Braking System

This is the most widespread configuration found in mass-market BEVs and hybrids today. Equipped with an electric motor and an inverter, the vehicle applies negative torque during braking to switch the motor into generator mode, converting kinetic energy into electrical energy stored within the battery pack. This stored energy can subsequently power initial take-off, top up the auxiliary battery, or run onboard electrical ancillaries.

2. Mechanical Flywheel Energy Recovery System

Extensively deployed in F1 motorsport, this setup operates on a principle akin to a friction toy car—reverse wheel rotation winds up an internal mechanism to store mechanical potential energy, which is rapidly unleashed as kinetic energy upon release. Because it avoids repeated conversions between mechanical and electrical energy, thermal losses are minimal, yielding an energy conversion efficiency of 65% to 70%.

3. Electro-Mechanical Flywheel Recovery System

Pioneered by Williams, this hybrid design bridges both mechanical and electrical concepts. By substituting conventional chemical batteries with a high-speed composite flywheel rotor, it offers high energy density, compact packaging, low weight, ultra-fast charge rates, extended service life, and zero chemical footprint. Cutting-edge electro-mechanical flywheel setups boast net operating efficiencies exceeding 95%.

4. Two Operating Trigger Modes

In real-world driving conditions, kinetic energy recovery is engaged via two primary modes:

Coasting Regeneration: Automatically engaged the moment the driver lifts off the throttle pedal; the electric motor introduces electromagnetic drag to slow the vehicle down while generating charge. Tesla's "One-Pedal Drive" is a prime example of high-level regen—yielding a distinct deceleration feel upon throttle lift-off that can replace up to 70% of routine light braking.

Braking Regeneration: Lightly depressing the brake pedal prompts the system to prioritise motor regenerative braking, with the hydraulic friction brakes only blending in when deceleration demand exceeds the motor's regen capacity. This "regen-first" calibration typically captures around 70% of available braking energy.

Under the GB 21670-2025 standard ("Technical Requirements and Test Methods for Passenger Car Braking Systems") taking effect in 2026, electric regenerative braking is officially classified into Type A and Type B. Type A is not considered part of the primary service braking system and is activated solely by lifting off the accelerator pedal, whereas Type B forms an integral part of the service braking system and is engaged via the brake pedal.

5. Advantages and Drawbacks

The advantages are substantial: lower electrical consumption and improved fuel economy; reduced wear and longer service life for brake components; sharper overall braking response; judicious use of high regen in urban stop-and-go traffic can boost real-world driving range by 10% to 30% ; on extended downhill mountain passes, it can even deliver sustained battery recharging; brake pads can comfortably last 80,000 to 100,000 km before requiring replacement—effectively doubling the service intervals seen on conventional ICE cars.

Drawbacks, however, must also be considered: deceleration feels significantly sharper than in traditional petrol cars, yet on certain models the brake lamps do not illuminate during lift-off regen, raising rear-end collision risks; aggressive regen profiles can cause sudden pitch and deceleration, inducing motion sickness among passengers; over-reliance on lift-off regen can dull driver reflexes during emergency panic-stop situations; when the battery state of charge (SoC) exceeds 90% , the management system will curtail or disable regenerative braking entirely.

6. Efficiency and Operational Constraints

Overall regen efficiency is governed by multiple variables. Since motor/generator conversions and battery charging cycles never achieve 100% efficiency, coupled with thermal losses in complex power electronics, sustaining a net system efficiency of over 50% is commendable. Most modern EV energy recovery systems operate at an average efficiency between 60% and 80% .

Permanent magnet synchronous motors (PMSM) generally exhibit higher regen efficiency than asynchronous induction motors. Furthermore, the bulk of recoverable braking energy occurs within the 10 to 50 km/h speed window. In sub-zero conditions below 0°C, suppressed chemical activity inside the battery pack can degrade recovery efficiency by roughly 30% .

7. Regulatory and Safety Updates

The updated automotive braking standard GB 21670-2025, taking effect on 1 July 2026, introduces stringent mandates for regenerative systems: whenever regenerative deceleration exceeds 1.3 m/s², the vehicle must automatically illuminate the rear brake lamps; this illumination signal must remain active for as long as deceleration continues. This regulatory change directly addresses safety concerns surrounding unlit brake lamps during strong single-pedal regen on older EV models.

Summary

The Kinetic Energy Recovery System remains an indispensable technology for maximising energy efficiency in modern electrified vehicles. Leveraging the reversible characteristics of electric drive motors, it repurposes discarded braking energy into usable charge, extending driving range while significantly reducing mechanical brake wear. As powertrain hardware and international safety standards continue to evolve, regenerative braking systems are becoming increasingly refined, intuitive, and efficient.