Development and control of electrified axle (eAxle) powertrains for all-terrain heavy-duty vehicles
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Date
2025
Authors
Healy, Conor
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Publisher
University College Cork
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Abstract
This thesis summarizes the design, control and validation of a modular electrified axle (eAxle) for all-terrain heavy-duty vehicle applications. The eAxle is designed for 4x4, 6x6 and 8x8 vehicle configurations but can also be used in drivelines where only some axles are powered, such as 6x4 and 8x6 vehicles. The eAxle maintains compatibility with an existing independent suspension system and is suitable for use in multiple on-highway and off-highway sectors, including agriculture, construction vehicles, forest machinery, fire trucks and mining and powered trailers. The eAxle can be used in series-hybrid, battery-electric and hydrogen fuel cell powertrains.
In Chapter 2, a novel method, based on axle loading and weight transfer for powertrain sizing, is proposed and validated through dynamometer testing. This is demonstrated by focusing on a 27-tonne 6x6 vehicle as a case study to determine the eAxle’s tractive effort requirements for one of the possible vehicle applications. A novel weight transfer-based approach is used for determining the worst-case (most heavily loaded) axle operating conditions which in turn are used for sizing the powertrain. A three-speed transmission is required to meet the vehicle performance requirements. The performance of the hybrid 6x6 vehicle is evaluated at steady-state operating points and during transient acceleration events. The powertrain sizing method is validated through a comprehensive dynamometer testing program.
In Chapter 3, a novel eAxle packaging concept is developed. The eAxle uses an over-the-shoulder configuration where the pinion-gear, used to drive the differential, is located on the opposite side to the motor and transmission. This enables the eAxle to be packaged more symmetrically on each side of its driveshafts, a key enabler for meeting tight vehicle wheelbase requirements. The over-the-shoulder concept provides access to the pinion-gear’s shaft, facilitating the installation of auxiliary drives on the pinion-gear side of the differential. The auxiliary drives include inboard parking brakes and ground-driven steering pumps, both of which are required to meet vehicle safety and redundancy requirements. The eAxle can be mounted with the motor facing the front or rear of the vehicle. This allows for more driveline packaging concepts to be considered and increases the vehicle’s maximum approach angle.
In Chapter 4, an efficiency-optimized gear selection map is developed for the eAxle. Raw power loss data for the eAxle’s inverter and motor system is analysed and processed allowing power loss and efficiency maps for the inverter and motor system to be developed. The resulting efficiency maps for each of the eAxle’s gears are calculated and mapped to a common set of lookup vectors before the most efficient gear for each operating point is selected. While this gear selection map is optimal from an efficiency perspective, practical considerations prevent it from being directly implemented on vehicles. These issues are resolved by developing efficiency-derived upshift and downshift curves for each gear transition. The performance of these gear shift curves is compared with the efficiency-optimized map in simulation. Dynamometer testing is used to validate the gear selection strategies.
In Chapter 5, control algorithms are developed for vehicles with multiple modular eAxles. The algorithms are implemented on a Driveline Control Unit (DCU). The DCU acts as an interface between a vehicle integrator’s Vehicle Control Unit (VCU) and the eAxle Control Units (EACUs). Some key DCU features include speed measurement, wheel-slip calculation, available torque calculation, torque vectoring, wheel-slip limitation, driveline efficiency optimization, gear selection and staggered gear-shifting. The DCU also provides additional benefits to vehicle integrators, such as reducing vehicle Controller Area Network (CAN) bus requirements and allowing the VCU to maintain a conventional torque control algorithm focused on vehicle-level torques. The latter is of particular importance for removing the barriers into the electrification space posed by distributed propulsion, a term used to describe a powertrain with multiple independent torque sources.
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Keywords
eAxle
Citation
Healyer, C. 2025. Development and control of electrified axle (eAxle) powertrains for all-terrain heavy-duty vehicles. PhD Thesis, University College Cork.