Evaluation of Torque-Dense Electric Machine Technology 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
$\text{kNm/m}^{3}$
) of conventional electric machines compared to mobile hydraulic machines (up to approximately 1000
$\text{kNm/m}^{3}$
). This article reviews emerging variants of electric machines that offer a step improvement in torque density and potential pathway to enable electrified off-highway vehicles. First, sizing approaches for electric and hydraulic machines are developed, and torque-dense electric machines reviewed in literature are compared to commercial hydraulic machines to identify design trends in terms of speed, torque density, and power density. Next, key metrics are identified for the electric machine, based on which the following four emerging electric machine variants that promise to improve torque density are reviewed: i) multi-harmonic machines injection, ii) combined radial-axial flux machines, iii) magnetic gears, and iv) magnetically-geared machines. The findings from this review show that these new electric machines can achieve upwards of 300% improvement in electric machine torque density, with several designs exceeding 100
$\text{kNm/m}^{3}$
, making them worthy candidates for further research to bridge the torque density gap with hydraulic machines.