Abstract
Direct oil cooling has emerged as a transformative thermal management strategy for new energy vehicle (NEV) traction motors, addressing the fundamental limitations of conventional water jacket cooling in achieving the high power densities required for next-generation electric propulsion. This review systematically examines the structural design aspects of direct oil cooling systems, encompassing spray cooling configurations, hollow-shaft centrifugal cooling, stator oil groove immersion, and integrated multi-physics thermal management architectures. Particular emphasis is placed on the heat transfer characteristics, electromagnetic-thermal coupling, and system-level integration of oil cooling with power electronics and transmission systems. The paper synthesizes recent advances in multi-nozzle spray optimization, dielectric coolant selection, coupled CFD-thermal finite element analysis, and economic evaluation of cooling strategies under various driving cycles. By analyzing comparative performance metrics between water jacket and direct oil cooling, and identifying persistent technical barriers including parasitic windage losses, oil degradation, and sealing reliability, this review aims to provide a comprehensive reference for researchers and engineers engaged in high-performance electric drive thermal system development.