Environmental and economic considerations primarily drive the transition towards electric vehicles (EVs). Given the current limitations in EV and fuel-cell vehicle (FCV) technologies, hybrid electric vehicles (HEVs), which are approximately 25% more efficient than internal combustion engine vehicles (ICEVs), can contribute to advancing the broader EV industry. Today’s EVs and HEVs use regenerative braking systems (RBS) to recapture some of the energy spent during driving. With the advancement in connectivity and autonomous driving technologies, there is a significant opportunity to harness these features further to boost efficiency. 

A paper presented at SoutheastCon 2024 introduces a novel approach to maximize the recaptured energy from the regenerative braking system. The method involves determining the optimal braking torque by pre-processing vehicle dynamics and electric powertrain data, aiming to find the most effective braking torque relative to the vehicle's speed and the required distance for a complete stop. Researchers revealed a substantial increase in the energy stored in the battery in a comparative analysis between the energy recaptured in standard braking procedures and the proposed braking method.

Current Research

Despite the considerable attention and research dedicated to EVs, they have yet to entirely replace fossil fuel-based vehicles, mainly due to ongoing challenges in battery technology.

The trade-off between extending battery size for increased range and the associated environmental impacts and carbon footprint is a significant concern, as is the increased electricity demand. Addressing these challenges is vital, and enhancing the energy efficiency of EVs is seen as a key factor in improving both their market penetration and operational range. This can be achieved through advancements in battery efficiency, power electronics, and vehicle control systems.

Approaches to enhance the efficiency of EVs include the improvement of electric motor efficiency, advanced battery thermal management, optimized charging strategies, and vehicle routing efficiency. Another method involves optimizing energy consumption through tailored energy management strategies for different driving conditions. However, none of these methods are optimized for maximum energy regeneration through an efficient regenerative braking system (RBS).

The RBS, known for enhancing vehicle range and reducing energy dissipation as heat, addresses a critical issue in automotive design. Traditionally, brake systems are identified as a significant source of energy loss in vehicles, contributing to as much as 50% of the total power losses in the traction system. Recent advancements have led to the development of more efficient RBS. Whether controlled by a driver or an automated system, the vehicle must be informed about upcoming driving conditions or events. Research has shown that an energy-optimal deceleration system can significantly enhance the effectiveness of regenerative brakes, with a reported 33% improvement over human drivers. 

Innovation in Braking

The researchers examine the energy principles, regenerative braking system model, and energy collection in braking before presenting an efficiency map for RBS tailored to the vehicle's specific characteristics and electric powertrain components.

Fitted map on output data of RBS system capability of recovering SOC based on braking torque and initial velocity.

 

This efficiency map, developed through a comprehensive understanding of the vehicle's specific characteristics and electric powertrain components, serves as a guide for braking commands. It is a graphical representation of the vehicle's energy recovery efficiency under different braking conditions. It considers the vehicle's speed and the proximity to its final stopping point. Location-sharing technologies or the vehicle's detection mechanisms, such as visual detection systems or radar, can determine this stopping point. This practical application of the efficiency map in guiding braking commands demonstrates the real-world relevance of the research. 

The study conducts simulations at two distinct levels to provide a comprehensive understanding of applying a combined model of a vehicle and its electric powertrain in determining the optimal braking torque. The initial level of simulation focuses on developing an efficiency map for the vehicle. Subsequently, in the second level, this established map is utilized to execute optimal braking along a predetermined path to compare the energy recaptured during braking. This detailed and thorough approach allows for a comprehensive analysis of the effectiveness of the proposed model in enhancing energy recuperation through optimized braking strategies.

Comparative graph showing recaptured SOC in vehicles at 40 m/s initial speed: with vs. without optimized braking controllers across different stopping distances

 

Results and Discussion

The study presented a three-dimensional efficiency map for energy recovery in EV batteries, developed through vehicle dynamics simulation in conjunction with the electrical power transmission system. The map then acted as a benchmark for determining the optimal braking torque, given a specific initial vehicle speed and distance to the stop location. 

Various scenarios were analyzed using this method, and the comparisons showed a notable increase in the amount of energy recaptured by the vehicle. This significant boost in energy recapture directly impacts the overall range of the simulated vehicle, demonstrating the potential of this innovative automated braking system to extend the operational range of EVs. What's more, the practicality of this system is underscored by its minimal hardware requirements, needing only a device to determine the final stop location. This simplicity in implementation not only makes the system more accessible but also positions it as an effective tool for boosting the efficiency of EVs.

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