A Review of Torque-Dense Electric Drive Systems with Potential to Bridge the Gap with Hydraulics for Off-Highway Vehicle Electrification

Electrification of off-highway vehicles offers the benefits of improved energy efficiency, enhanced control, and reduction in greenhouse gas emissions. However, progress towards electrification has been limited by the low torque-density (30 kNm/m
³
) of conventional electric machines compared to mobile hydraulic machines (340 - 600 kNm/m
³
). This paper reviews emerging torque-dense electric machine variants and techniques to improve the torque-density of electric machines for enabling electrified off-highway vehicles. Key metrics are identified for the electric machine, based on which the following four technologies that promise to improve torque-density are reviewed: i) multi-harmonic current injection, ii) combined radial-axial flux machines, iii) magnetic gears, and iv) magnetically-geared machines. The findings from this review show that these technologies can achieve upwards of 300% improvement in electric machine torque-density, with several designs exceeding 100 kNm/m
³
, making them worthy candidates for further research to bridge the torque-density gap with hydraulic machines.