Algorithms for Recuperation and Traction in Electric Vehicles

Abstract

Depending on the design of the braking system foundation, a hydraulic state-of-the-art braking system in conventionally driven passenger cars distributes braking forces between front and rear axle at a ratio of about 70:30 (installed brake force distribution) [2], ❶. In hybrid and electric vehicles the brake energy can be partially recuperated. The distribution of axle specific recuperated brake energy depends on the drivetrain topology. In case of electric braking on the front axle only, a strong understeering of the vehicle would occur. When braking electrically on the rear axle only, an oversteering or even instable brake force distribution is likely. In general, this can lead to more frequent interventions of ABS/ESP. ❶ The installed brake force distribution is defined by the design of the hydraulic brake and by the wheel radius, the ideal brake force distribution results from the pitching of the vehicle [1]; the ratio of the brake force on the rear axle to the vehicle weight Fb,RA/G over the ratio of the brake force on the front axle to the vehicle weight Fb,FA/G [2] is depicted For electric vehicles with one electric motor per axle, every possible brake force distribution can be established as long as the maximum recuperation potential is not exceeded. Additional consideration of the hydraulic braking system can reduce the frequency of ABS/ESP interventions thus achieving a better balanced performance near the installed brake force distribution. Based on the brake pedal application a vehicle related target brake force Fb,Veh can be derived. For the generation of brake force four actors are available: one generator and the hydraulic brake per axle. The target brake force needs to be distributed to the four actors meeting suitable and prioritised rules: : vehicle stability : realisation of target brake force : maximum recuperation efficiency : realisation of demanded brake force distribution. The vehicle stability criterion results from axle individual acting of the generator brakes and their responsiveness, in contrast to the wheel individual ABS control of a hydraulic brake. In the following, a situation dependent reduction of the recuperation potential on one or both axles is assumed. This can be realised by a suitable pre-processing algorithm. The criterion of building up a target brake force is given by Eq. 1: The maximum recuperation efficiency demands for a preferably electric braking. Next to the stability restrictions here the degrees of freedom of the hydraulic brake are to be considered. Under consideration of the remaining recuperation potential of both electric motors and of the restrictions coming from the hydraulic braking system, the installed brake force distribution given by Eq. 2 and Eq. 3 can be established: Cp,FA, Cp,RA: brake force coeffi cient on front (FA) and rear axle (RA) τrwhl,FA, τrwhl,RA: wheel radius on front (FA) and rear axle (RA) The hydraulic brake force is separated in an unavoidable part Fb,HydBase and a freely adjustable part Fb,HydAdd. Hereby, the brake force of each axle is given by Eq. 4 and Eq. 5.