Analysis of Safety Relevant Wheel Individual Brake Torque Requirements for City EVs

Abstract

Current electric vehicles (EVs) already perform most braking maneuvers by recuperation using the electric powertrain. In order to generate additional benefits regarding cost, weight, brake dust emission and design freedom, there might be the option to omit the brake system and solely brake by recuperation. The potential elimination or downsizing of the friction brakes results in multiple questions concerning deceleration capabilities, availability of brake torques as well as driving dynamics. Especially for EVs with the electric motor located centrally at the axle, wheel individual braking interventions may not be possible without additional measures. This study investigates the brake torque requirements for the rear axle of an electrically driven urban vehicle with rear axle drivetrain. The focus of the analysis is targeted on wheel individual brake torque generation as such differential brake torques may be relevant for state of the art (SoA) driving safety and electronic stability control (ESC) interventions. In order to examine the wheel individual brake torque requirements a Simulink based software in the loop (SiL) simulation environment for vehicle dynamics is utilized. It simulates the dynamic behavior of vehicles with focus on the brake system. The main feature is the integration of software in the loop control algorithms of an ESC system with powertrain, vehicle behavior and electronics also being included. To maintain expert knowledge and application effort, a simulation model and ESC software of a SoA series production urban EV is used. This model is applied to a vehicle test catalogue for ESC software release covering maneuvers that allow testing of different driving stability functions. Based on the simulation results and supported by real world measurement data, the most critical driving maneuvers concerning the amount of differential brake torque, its direction and dynamics are identified. The test catalogue includes driving scenarios such as acceleration on inhomogeneous surfaces. In case of a vehicle equipped with a conventional open differential the maximum drive torque of the entire axle is limited by the lower friction wheel. Wheel individual brake applications can increase this drive torque. Such intervention may not be possible in vehicles without a conventional brake system topology. As a result, acceleration on such surface is restricted. Further maneuvers examined are dynamic cornering situations that may require wheel individual brake torque to ensure driving stability and safety. An in-depth analysis of the SoA vehicle behavior and its control strategy is necessary to understand potentials and limitations of EVs with non-conventional brake topologies.