Energy-efficient torque distribution for axle-individually propelled electric vehicles

Abstract

We propose a novel operation strategy for electric vehicles with axle-individual electric machines to improve their energy efficiency in typical driving situations. The developed algorithm is allocating a total torque requested by a velocity controlling system or the driver to the electric machines such that the energy loss is reduced compared to an equal distribution. By taking near-future forecasts into account, the predictive nature of the algorithm leads to a minimized number of clutching processes compared to previous work and thereby contributes to increased comfort and minimized component wear. Overall, an average reduction of up to 25 % in the electric machine losses can be achieved for the ARTEMIS driving cycles. At the same time, a reduction of the clutching operations by 70 % is possible due to the forecast, compared to algorithms only considering the momentary state. I. INTRODUCTIONAn increased market penetration of electric vehicles (EV) can offer a major contribution to CO 2 reduction, provided the energy used to charge these vehicles is generated from regenerative sources. However, a primary obstacle for current EVs is the low energy density of the storage media. The dominating application of batteries as energy storage devices leads to a strongly limited range of these battery electric vehicles (BEV). Current middle-class BEVs offer a range of around 150 km, which is significantly less than that of conventional vehicles propelled by an internal combustion engine (ICE). Besides improving the battery technology itself, research is focusing on both operation and driving strategies to advance energy efficiency of BEVs and hence increase their cruising range.The presented work introduces an operation strategy to optimize the efficiency of BEVs propelled axle-individually by two electric machines (EM). The drive elements are used both for propulsion and for regenerative braking. If a longitudinal acceleration is requested, the availability of two drive elements for providing a combined torque leads to an over-actuated system. A distribution factor is introduced, deciding how much torque each machine is generating to satisfy the request. Using this degree of freedom in an adaptive way -based on the powertrain characteristicsleads to an efficiency gain of the powertrain. It is shown in the course of the paper that the energy losses due to machine inefficiency can be reduced in average by up to 25 % by utilizing an optimized torque distribution. As the bulk of the energy benefit is gained by switching between the use of one or two EMs, a naive approach can lead to frequent clutching

Torque Distribution Strategies for Energy-Efficient Electric Vehicles With Multiple Drivetrains

“…‚ ,`/a , Vƒ /" ,`/a > 0. These hypotheses were experimentally verified for the vehicle demonstrator of Figure 2 (see Section 2) and are confirmed by other studies [27].…”

“…Nevertheless, in practice the same paper recommends to use the drivetrain/s of a single axle at low torque demands, and a 50:50 front-to-rear torque distribution at medium-high torques, without specifying the threshold for implementing the transition between the two modes. This control policy is indirectly confirmed by the experimental tests in [26] and [27], but these studies do not critically analyze the resulting optimal torque distribution. [28] presents a simple control allocation algorithm for electric vehicles with identical power loss characteristics on the front and rear axles.…”

“…Eq. (27) implies that the optimal solution R• T * depends on the longitudinal and lateral vehicle accelerations, whilst R•T * does not. Figure 9(c).…”

On the Investigation of Energy Efficient Torque Distribution Strategies through a Comprehensive Powertrain Model

“…where η inv is the efficiency of the inverter, η em is the efficiency of the electric motor, η t is the efficiency of the transmission. Power losses associated with the drivetrain represent the major contribution to vehicle power losses [6,8,13].…”

“…Such studies differ in many aspects-the considered sources of power losses, the drivetrain layout with in-wheel [2][3][4] or on-board motors [5][6][7], the proposed optimal torque distribution configurations. For instance, [4,5] focus on the motor power losses, while [2,8] also consider losses in the drive. References [3,7,9,10] look at the whole powertrain, including tyre power losses.…”