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Improving energy-efficiency of electric vehicles by over-actuation

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Time: Fri 2020-05-08 09.00

Location: Live-streaming:, Stockholm (English)

Subject area: Vehicle and Maritime Engineering

Doctoral student: Peikun Sun , Fordonsdynamik

Opponent: Professor Gordon Timothy, University of Lincoln

Supervisor: Annika Stensson Trigell, VinnExcellence Center for ECO2 Vehicle design; Lars Drugge, VinnExcellence Center for ECO2 Vehicle design; Jenny Jerrelind, Farkostteknik, Farkost- och flygteknik, VinnExcellence Center for ECO2 Vehicle design

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As the concerns regarding environmental pollution, climate change and the fossilfuel crisis have grown, electric vehicles (EVs) have attracted a great deal of attention. However, for EVs to be more competitive compared with internal combustion engine vehicles (ICEVs), their driving range needs to be increased. Therefore,energy-efficient control of EVs is considered to be a very important research field.

Electrification of actuators for EVs makes decentralized driving systems possible,such as systems providing individual wheel torque, individual wheel steering andindividual wheel camber control. Electrified vehicle actuators can provide morecontrol possibilities than the normal control of motion in the longitudinal and thelateral direction. EVs equipped with these kinds of actuators are thereby overactuated and more advanced motion controls focusing on energy-efficiency andvehicle directional stability can be realised.

The research objective of the work presented in this doctoral thesis was the exploration of energy-efficient control methods for over-actuated EVs. In order toevaluate the contribution of different control strategies to energy saving, modelsfor energy loss during driving, especially during cornering, were developed. Both alinear tyre model with a camber effect and a non-linear tyre model with a cambereffect were studied. The powertrain efficiency based on a motor efficiency map ofin-wheel motors, was considered.

On the basis of vehicle dynamics with a linear tyre model, camber’s contributionto tyre slip loss reduction during cornering and the corresponding contributionof direct yaw moment control (DYC) when neglecting the powertrain losses werestudied and compared. The results show that camber control can reduce the tyreslip loss significantly and DYC in this case has a small contribution to tyre slip lossreduction. Using a non-linear tyre model, a camber controller based on the lateralacceleration was developed, the effectiveness of this controller was evaluated andthe results show a promising energy saving.

In addition, to investigate the influence of the powertrain losses, a DYC for energyefficiency considering a motor efficiency map was proposed. While satisfying thesame cornering demand, by actively distributing the wheel torques using DYC forenergy-efficiency, the overall energy-efficiency during non-safety-critical manoeuvres could be improved.

Since there might be a higher risk of accidents when applying DYC for energyefficiency in safety-critical manoeuvres, a DYC for safety was developed based ona stability judgement considering the yaw rate and slip angle. To handle this compromise, a switching principle for alternating between DYC for energy-efficiencyand DYC for stability was then proposed. Furthermore, a method for designing arule-based DYC for energy-efficiency was developed to reduce the calculation task and thereby enable real-time implementation.

This research has resulted in an increased understanding of how to improve energyefficiency using different actuators in an over-actuated EV. Different ways in whichover-actuation of EVs can improve the energy-efficiency are presented and analysed in this thesis. Altogether, potential energy loss reductions (up to 22% usingcamber control) for the studied vehicles and manoeuvres have been found usingthe developed methods. Even though the actual improvement will depend on theconsidered vehicle, actuators and driving conditions, the methodology presented inthis thesis can be applied to other vehicles and driving situations.