Study of an optimization criterion of mounting blocks for drivability evaluation of an electric vehicle

In order to lead properly an electric vehicle project, it is very important to assess and optimize the driver perception during manoeuvres such as tip-in and tip-out. This aspect of vehicle customer perception is called “drivability”. Precedents papers have shown that the motor block (i.e. electric motor and speed reducer) rolling motion onto its mounting blocks has an important effect on the drivability rating. Moreover, this drivability aspect is crucial for an electric vehicle (because of the high torque gradient at low speeds or between regenerative and motor modes). A usual criterion used to study the impact of the motor block rolling on the drivabilty rating is the energy decoupling between roll and surge modes. This criterion, called “EcTx criterion” warrants to limit the oscillations of the longitudinal force transmitted from the motor block to the car body. As this criterion is independent of the torque command, it is used for any manoeuvre. Its application to a Key-On/Key-Off manoeuvre on conventional vehicle is well-known and enables to predict the rating of this manoeuvre. Nevertheless, questions are remaining about the appliance of this criterion to a tip-in manoeuvre. Simulations have shown that the EcTx criterion can also be used for a tip-in manoeuvre and gives a good prediction of the oscillations amplitude for the longitudinal force transmitted from the motor block to the car body. However, the driveline mode is also involved in the vehicle dynamics during a tip-in manoeuvre. Moreover, this paper shows that the oscillations of the motor block onto its mounting blocks can also damp the driveline mode oscillations thanks to a specific phase shift between the two modes. Therefore, EcTx criterion can be improved by taking into account the driveline equivalent stiffness and inertia into its calculation.