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Frontiers in Mechanical Engineering ·

Volume 12 - 2026 | https://doi.org/10.3389/fmech.2026.1951365

A review of structural design for direct oil cooling and integrated thermal management in electric motors for new energy vehicles

  • Xueming Gao1
  • Jingsong Tan1
  • Hua Li1*corresponding author
  1. Shandong Huayu University of Technology, Dezhou, China
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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.

Keywords

1 Thermal management challenges and the evolution from water jacket to direct oil cooling

The thermal management of electric traction motors has become a critical bottleneck in the pursuit of higher power densities for new energy vehicle (NEV) applications. As motor power densities (kW/L) continue to increase to match the performance benchmarks of internal combustion engines (ICEs), the heat generation per unit volume has escalated dramatically, with winding temperatures reaching critical limits that threaten insulation integrity, magnet stability, and operational efficiency (Sarlioglu et al., 2017). Conventional water jacket cooling, which circulates water-glycol mixture through channels machined into the motor housing, has served as the industry standard for over a decade. However, this indirect cooling approach suffers from fundamental thermal resistance limitations because the heat must conduct through the stator core laminations, insulation layers, and slot liners before reaching the coolant, creating temperature gradients that leave the winding copper—the primary heat source—at significantly elevated temperatures (Davin et al., 2015; Lim and Kim, 2014). Experimental comparisons have demonstrated that water jacket cooling fails to achieve the desired efficiency for high energy density motors, with the highest temperatures appearing in the winding copper core shrouded by insulation layers that lead to poor heat dissipation (Zajac et al., 2024).

Direct oil cooling represents a paradigm shift by bringing the dielectric coolant into direct contact with the heat sources, eliminating the intermediate thermal resistances and enabling heat transfer coefficients (HTCs) exceeding 10,000 W/(m2·K) compared to approximately 500 W/(m2·K) for simple water jacket channels (Davin et al., 2015; Park and Kim, 2019). The transition from water jacket to direct oil cooling is driven by three primary structural advantages: first, oil’s high electrical resistivity allows safe direct contact with live components without insulation barriers (Lindh et al., 2016); second, oil’s minimal impact on magnetic fields preserves electromagnetic performance (Reinap et al., 2019); and third, the ability to integrate oil cooling with the transmission system reduces overall vehicle complexity and cost (Lundmark et al., 2021). The structural evolution of motor cooling has progressed from air cooling, through water jacket cooling, to the current generation of direct oil cooling systems that use spray nozzles, hollow rotor shafts, and oil groove immersion to achieve unprecedented heat removal rates. The continuously increasing energy demand has forced excessive utilization of fossil fuels, causing depletion of energy sources and environmental issues of CO2 emissions and global warming, which has intensified the need for effective thermal management in electric vehicles to improve motor efficiency and extend operational lifetime (Garud and Lee, 2025). Cooling requirements are fundamentally governed by power density (kW/L or kW/kg) rather than absolute motor power (kW). Increasing power density—driven by the need to reduce motor mass and volume in space-constrained EV platforms—directly escalates heat generation per unit volume, necessitating advanced direct oil cooling technologies that can achieve heat transfer coefficients exceeding 10,000 W/(m2·K).

Figure 1 distills the structural evolution of EV motor thermal management from air cooling through water jacket to direct oil cooling, revealing an order-of-magnitude improvement in power density (≥10 kW/L versus ≤3.5 kW/L) enabled by eliminating intermediate thermal resistances. The comparative analysis of spray configurations demonstrates that manifold integrated spray achieves ultra-high performance through uniform core cooling, while drip cooling offers a simpler design at moderate performance levels. Hollow-shaft centrifugal cooling extends direct oil contact to rotating components, and synthetic oil selection balances thermal conductivity, dielectric strength, and viscosity for EV-optimized applications. The following section examines spray cooling configurations and nozzle optimization in greater technical depth, with particular attention to multi-nozzle placement, injection angle, and flow rate optimization that maximize heat transfer while minimizing parasitic losses.

Structural overview of direct oil cooling and integrated thermal management for electric vehicle traction motors. **(a)** Evolution of thermal management strategies from air cooling through water jacket to direct oil cooling, illustrating the 3× improvement in volumetric power density. **(b)** Four principal oil spray cooling configurations for stator end windings: (i) outer spray with nozzles on housing exterior directing oil inward; (ii) front-rear combined axial spray targeting both end windings; (iii) annular manifold with 12 nozzles providing uniform radial distribution; (iv) gravity-assisted drip cooling with collection tray. **(c)** Hollow-shaft centrifugal rotor cooling system where oil enters through the shaft center and is driven by centrifugal force through radial channels to the rotor inner surface, with drainage at the periphery. **(d)** Integrated vehicle thermal management architecture showing the shared oil circuit (e-motor and gearbox), water-glycol circuit (inverter and battery pack), and waste heat recovery pathways to cabin HVAC.
Figure 1. Structural overview of direct oil cooling and integrated thermal management for electric vehicle traction motors. (a) Evolution of thermal management strategies from air cooling through water jacket to direct oil cooling, illustrating the 3× improvement in volumetric power density. (b) Four principal oil spray cooling configurations for stator end windings: (i) outer spray with nozzles on housing exterior directing oil inward; (ii) front-rear combined axial spray targeting both end windings; (iii) annular manifold with 12 nozzles providing uniform radial distribution; (iv) gravity-assisted drip cooling with collection tray. (c) Hollow-shaft centrifugal rotor cooling system where oil enters through the shaft center and is driven by centrifugal force through radial channels to the rotor inner surface, with drainage at the periphery. (d) Integrated vehicle thermal management architecture showing the shared oil circuit (e-motor and gearbox), water-glycol circuit (inverter and battery pack), and waste heat recovery pathways to cabin HVAC.

2 Spray cooling configurations and nozzle optimization

Spray cooling has emerged as the most effective direct oil cooling strategy for high-power-density electric motors, utilizing pressurized nozzles to impinge oil droplets directly onto end windings and stator surfaces where heat generation is most concentrated. The structural design of spray cooling systems involves careful optimization of nozzle placement, spray pattern, oil flow rate, and injection angle to maximize heat transfer while minimizing parasitic losses and oil degradation. Multi-nozzle-based oil spray cooling configurations have been systematically investigated, with comparative studies evaluating outer spray cooling, front-rear combined spray cooling, and manifold integrated spray cooling arrangements (Lim and Kim, 2014; Davin et al., 2015; Park and Kim, 2019). The combined spray cooling configuration, which employs nozzles at multiple locations simultaneously, has demonstrated superior thermal management characteristics, achieving winding temperatures of 128.9 °C with torque output of 255.59 Nm when using Novec dielectric oil, representing a 20.3% improvement in cooling performance compared to water jacket cooling (Park and Kim, 2019). The heat transfer coefficient of oil spray cooling has been measured at approximately 10,800 W/(m2·K), which is substantially higher than 479 W/(m2·K) for simple cooling channels, highlighting the dramatic improvement in thermal performance (Park and Kim, 2019; Davin et al., 2015).

The structural design of spray cooling systems must balance cooling effectiveness against manufacturing complexity and cost. Oil pipe spraying designs use separate oil pipes to spray directly onto winding ends, with oil rings primarily used to adjust the spray angle; neither requires machining oil grooves in the stator core, making them simpler and less costly to implement (Reinap et al., 2019). Alternatively, machining oil grooves into the stator core allows oil to flow through the core itself, enabling simultaneous cooling of both winding ends and the interior, though this requires designing an oil circuit within the stator and machining grooves into the original stator core. Different manufacturers have adopted varying designs for oil flow paths and spray angles, with some combining both approaches—using stator core oil grooves together with oil pipes for comprehensive cooling coverage (Acquaviva et al., 2020). The nozzle geometry significantly impacts heat transfer performance, with flat jet injectors showing considerable temperature reductions of approximately 50 °C on end windings when coupled with standard water jacket cooling, and the coil end-winding temperature decreasing from 113 °C to 62 °C at an oil flow rate of 360 L/h and temperature of 40 °C (Davin et al., 2015). The optimization of hollow shaft structures for oil injection has been investigated using Taguchi analysis, with optimized structures showing reductions of 15.1% and 10.3% in average and maximum winding temperatures, respectively, along with a 77.3% reduction in injection flow rate difference compared to conventional cooling structures (Liu et al., 2020). The manifold drip cooling method precisely and uniformly directs cooling fluid onto the motor winding heads, requiring less cooling fluid than rotor bathing approaches and therefore removing parasitic losses due to drag, though it does require a higher-pressure oil pump (Garud et al., 2022). The annular ring with holes design achieves maximum volumetric torque density, volumetric power density, specific torque density, and specific power density of 17.22 Nm/L, 7.19 kW/L, 4.69 Nm/kg, and 1.95 kW/kg, respectively, demonstrating the performance potential of optimized spray cooling architectures (Liu et al., 2020).

Oil spray cooling performance depends on droplet atomization (SMD 50–500 μm) and surface impingement governed by Weber number (Chauhan and Singh, 2019a; Chauhan and Singh, 2019b). Unlike water, dielectric oils remain in single-phase convection as their boiling point (>200 °C) and Leidenfrost temperature (>300 °C) exceed motor operating limits, with cooling enhancement from droplet surface area and thin-film flow.

Table 1 consolidates the comparative landscape of EV motor thermal management architectures based on published experimental and numerical studies, revealing a clear progression from indirect to direct cooling strategies that trade manufacturing simplicity for thermal performance. Water jacket cooling remains the baseline with mature supply chains but is fundamentally constrained by thermal resistance barriers that limit power density to ≤6 kW/L (Davin et al., 2015; Lindh et al., 2016). Among direct oil approaches, manifold integrated spray achieves the highest heat transfer coefficient (∼10,800 W/(m2·K)) and lowest winding temperatures, though at elevated manufacturing complexity that currently restricts it to motorsport applications (Park and Kim, 2019). The hollow-shaft centrifugal architecture addresses the critical rotor thermal challenge that spray configurations largely neglect (Lundmark et al., 2021), while the closed-circuit stator-rotor integration represents the most comprehensive—yet most complex—thermal envelope solution (Liu et al., 2020). Notably, drip cooling emerges as a pragmatic near-term candidate for cost-sensitive platforms where the performance premium of manifold spray remains economically prohibitive (Garud et al., 2022). The selection of cooling architecture must therefore balance not only thermal performance targets but also manufacturing feasibility, parasitic loss tolerance, and system integration requirements across the diverse NEV platform landscape.

TABLE 1. Comparative overview of direct oil cooling architectures for electric vehicle traction motors across cooling mechanism, thermal performance, and application suitability.

Cooling architectureMechanismHeat transfer coefficient (W/m2K)Winding temp. (°C)Power densityManufacturing complexityKey advantageKey limitationBest application
Air coolingNatural/forced convection50–150180≤3.5 kW/LVery lowSimpleLow costPoor thermal performance
Water jacket coolingIndirect: Housing channels479150–180≤6 kW/LModerateMature technologyEstablished supply chainHigh thermal resistance through laminations/insulation
Outer spray coolingNozzles at stator outer housing5000–8000140–1608–10 kW/LModerateDirect end winding contact
Front-rear combined sprayDual-sided nozzles targeting winding ends8000–10000128–14510–12 kW/LModerate–HighBalanced thermal distribution
Manifold integrated sprayMulti-nozzle manifold with uniform core distribution10800113–12812–15 kW/LHighUltra-high HTCUniform cooling
Drip coolingGravity-assisted oil drip onto windings3000–5000130–1507–9 kW/LLowSimple design
Hollow-shaft centrifugalRotor-driven oil through shaft channels6000–9000120–140 (rotor)10–13 kW/LHighRotor cooling
Semi-flooded statorPartial oil filling of stator cavity4000–6000125–1458–11 kW/LModerateLarge heat removal
Closed-circuit stator-rotorIntegrated loop: stator slots + hollow shaft8000–12000<12015–20 kW/LVery highComplete thermal envelope coverage

3 Hollow-shaft centrifugal cooling and rotor thermal management

While spray cooling primarily addresses stator end-winding temperatures, the rotor represents an equally critical thermal challenge that requires specialized structural solutions. Hollow-shaft centrifugal cooling leverages the rotational motion of the rotor to distribute oil through internal channels, using centrifugal force to spray oil onto the inner end-winding surfaces and rotor components. This approach offers the same cooling capacity as external spray systems but with significantly reduced oil flow rate requirements, as the rotor shaft itself becomes the distribution mechanism (Garud et al., 2022). The structural design of hollow-shaft cooling systems involves machining axial channels through the rotor shaft, with radial exit holes positioned to direct oil toward critical heat sources. Computational fluid dynamics (CFD) analysis has been employed to predict the convective heat transfer coefficient of oil cooling through hollow shafts, with Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (RANS) software used to model the complex fluid dynamics under rotational conditions (Lundmark et al., 2021). The rotational effects of high-speed motor shafts on heat transmission from shaft to coolant have been investigated, revealing that the coolant oil passing through the rotor shaft hole experiences enhanced heat transfer due to the centrifugal acceleration and turbulent mixing induced by rotation (Lundmark et al., 2021). The churning effect of oil splashing from cooling pipes onto motor components has been analyzed under various conditions of flow rate, temperature, and rotational speed, with mean temperature decreases of 48.9 °C observed as oil temperature reduces from 100 °C to 50 °C, and decreases of 8.7 °C with flow rate increases from 5 L/min to 10 L/min (Han et al., 2023).

The integration of hollow-shaft cooling with spray cooling creates a comprehensive thermal management system that addresses both stator and rotor heat sources. A closed-circuit design flowing through the stator (including slot and end windings) and the rotor hollow shaft has been developed for Formula 1 applications, with a thin seal sleeve added to separate the rotor from the stator and prevent oil spilling in the air gap (Ye et al., 2016). This integrated approach demonstrates that direct oil cooling can maintain motor temperatures below reference values even without active water jacket cooling, though the temperature gradient in the stator increases as the hot spots shift toward the stator center when oil is sprayed on both end windings (Davin et al., 2015). The structural challenge of rotor oil cooling lies in maintaining dynamic balance while incorporating internal oil channels, and in preventing oil accumulation in the air gap that could cause viscous friction losses and electromagnetic performance degradation. The semi-flooded stator approach numerically analyzes the cooling action of oil partially filling the stator cavity, with prototype testing showing larger heat removal and more uniform temperature distribution when adding spray cooling to the semi-flooded configuration (Ye et al., 2016). The oil spray cooling concept with nozzle on the hollow shaft of an induction machine has allowed a doubling of the continuous power of the motor, demonstrating the significant performance gains achievable through integrated rotor-stator oil cooling (Reinap et al., 2019).

4 Dielectric coolant selection and integrated system architecture

The selection of appropriate dielectric coolant represents a critical structural design decision that directly impacts thermal performance, electrical safety, and system longevity. Three primary oil types have been investigated for direct motor cooling: automatic transmission fluid (ATF), specialized thermal oils, and engineered dielectric fluids such as Novec. Comparative thermal management studies have evaluated these coolants under identical operating conditions, with combined spray cooling using Novec oil demonstrating optimum efficiency of 96.84% and achieving the lowest winding temperature of 113.7 °C at higher oil flow rates and lower inlet temperatures (Park and Kim, 2019). The direct cooling with Novec dielectric oil has shown improved heat transfer and economic characteristics compared to transmission oil and shell thermal oil, restricting maximum winding temperatures within 80.21 °C, 67.65 °C, and 116.82 °C under Urban Dynamometer Driving Schedule (UDDS), New European Driving Cycle (NEDC), and US06 driving cycles, respectively (Garud and Lee, 2025). The enhanced insulation lifetime, heat transfer coefficient, levelized cost of energy (LCOE), and net cumulative savings of 7.91 × 105 h, 23,036.73 W/(m2·K), 0.170 USD/kWh, and 47,151.13 USD, respectively, have been demonstrated for direct cooling with Novec dielectric oil (Garud and Lee, 2025).

The physico-chemical properties of oils significantly impact cooling performance, with viscosity affecting flow distribution and pressure drop, and electrical resistivity determining safety margins for direct contact with energized components (Davin et al., 2015). Key properties include dielectric strength, thermal conductivity, viscosity, density, specific heat capacity, flash point, oxidation stability, compatibility with insulation materials, and long-term degradation characteristics (Davin et al., 2015; Lindh et al., 2019). Experimental characterization of oil aging under direct motor cooling conditions has revealed that electrical resistivity and viscosity degrade with extended use, with one test fluid showing a loss of approximately 34 MΩ m in resistivity at 90 °C between new and aged states, though the aged fluid remained suitable for continued motor operation (Davin et al., 2015). The integration of direct oil cooling with the broader vehicle thermal management system offers significant structural advantages. In eAxle systems, oil from the gearbox can be shared with the motor cooling circuit, eliminating the need for separate cooling loops and reducing system weight, cost, and complexity (Lundmark et al., 2021). The electrically driven oil pump allows precise control of cooling capacity for maximum efficiency, with the pump operating as a stand-alone unit or shared with the vehicle’s transmission depending on application requirements (Garud et al., 2022). However, oil cooling introduces challenges including higher viscosity (20× water) requiring larger pumps and more pumping power, oil hygroscopicity leading to potential corrosion failures, and parasitic windage losses when oil enters the rotor-stator air gap (Garud et al., 2022; Han et al., 2023). The comparison of thermal power absorbed by water and oil at identical operating points reveals that oil begins to absorb more heat at a flow rate of 240 L/h, which is one-third of the water flow rate of 720 L/h, and this trend increases with oil flow rate, demonstrating the superior heat extraction capability of direct oil contact (Davin et al., 2015). The stator temperature comparison clearly shows that oil reduces significantly the coil end-winding temperature by direct contact, while the stator center temperature increases slightly as it is more distant from the oil impact zone (Davin et al., 2015). Alternative water-based direct cooling systems have been proposed that achieve direct contact with heat sources without the disadvantages of oil, though these require sophisticated sealing and insulation strategies to prevent electrical short circuits (Chen et al., 2020).

Recent advances in integrated vehicle thermal management have extended beyond single-component optimization to system-level architectures that couple multiple heat sources and sinks. Integrated motor-inverter cooling leverages the shared dielectric or water-glycol circuits to balance thermal loads between power electronics and traction motors, reducing the number of heat exchangers and pumps (Kim, 2013). Transmission oil cooling can be integrated with motor cooling through shared oil circuits, enabling heat recovery from the gearbox to precondition motor oil during cold starts (Lundmark et al., 2021). Battery-motor thermal integration utilizes waste heat from the motor and inverter to maintain battery pack temperature within the optimal 15 °C–25 °C range, improving both charging efficiency and cycle life. Multi-physics optimization frameworks that simultaneously evaluate electromagnetic, thermal, and fluid dynamic performance across the complete eAxle system have emerged as essential design tools, enabling trade-off analysis between cooling performance, efficiency, and packaging constraints (Chauhan and Koorata, 2024).

5 Multi-physics optimization, economic evaluation, and future perspectives

The design of direct oil cooling systems requires sophisticated multi-physics analysis that couples electromagnetic, thermal, and fluid dynamic simulations to optimize performance across the complete operating envelope. Coupled numerical analysis frameworks have been developed to simulate electromagnetic losses, temperature distributions, torque output, and efficiency simultaneously, with experimental validation showing maximum errors within 5.62% (Liu et al., 2020). The integrated electromagnetic-thermal numerical model using commercial software such as ANSYS Motor-CAD enables simulation of heat transfer and economic characteristics under various driving duty cycles, with reliability assured by validating simulated results against experimental data within error ranges of ±5% (Garud and Lee, 2025). The comparison of direct oil cooling versus water jacket cooling has demonstrated significant efficiency advantages: the electrical machine efficiency during motor operation is approximately 1% higher for urban and rural drive cycles and up to almost 3% higher for highway driving, with total system energy efficiency up to 3.4 percentage points higher for highway driving (Lindh et al., 2019). These efficiency gains translate directly to extended vehicle range and reduced battery capacity requirements.

The economic evaluation of cooling strategies must consider not only the initial hardware cost but also the long-term operational savings from improved efficiency and extended component lifetime. The levelized cost of energy (LCOE) analysis for direct oil cooling with Novec dielectric oil has demonstrated favorable economics compared to conventional water jacket cooling, with net cumulative savings of 47,151.13 USD over the system lifetime (Garud and Lee, 2025). The direct oil-cooled design has been shown to enable doubled current density (23 A/mm2) and doubled power density compared to water jacket cooling for hairpin winding machines, with the continuous power over a wider speed range significantly exceeding that of water-jacket-cooled motors (Reinap et al., 2019). The in-slot direct cooling between coils in double-layer concentrated non-overlapping winding machines has achieved current densities of 20 A/mm2 with flat wires, while direct water-cooled designs using steel pipes carrying coolant have reported maximum current densities of 14 A/mm2 at 2 L/min coolant flow rate for 205 kW bus applications (Chen et al., 2020). Future directions in direct oil cooling research include the development of advanced nozzle geometries for more uniform oil distribution, the integration of phase-change materials for transient thermal absorption, and the application of machine learning algorithms for predictive thermal management. The convergence of direct oil cooling with wide-bandgap power electronics, ultra-high-speed motors, and integrated eAxle architectures positions this technology as a foundational enabler for the next-generation of high-performance electric vehicles. Standardized test protocols and design methodologies will be essential for accelerating the commercial adoption of direct oil cooling across the diverse landscape of NEV platforms. The continuous power capability enabled by effective direct cooling allows motor designers to reduce oversizing and rare-earth permanent magnet (PM) content, contributing to both cost reduction and supply chain sustainability in the electrified transportation ecosystem.

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Footnotes

Edited by: Yatao Ren, Harbin Institute of Technology, China

Reviewed by: Vikash Kumar Singh Chauhan, National Institute of Technology Karnataka, India

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. We gratefully acknowledge financial support from the New Energy Vehicle Intelligent Network Technology Shandong Province Higher Education Institutions Future Industry Engineering Research Centre and Shandong Province Higher Education Institutions Marine Vessel Special Motor Key Technology Development and Dezhou Key Laboratory of Industrial Robot Control.

Author contributions

XG: Conceptualization, Data curation, Writing – original draft. JT: Investigation, Methodology, Writing – original draft. HL: Formal Analysis, Funding acquisition, Writing – review and editing.

References

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Citation

Gao, X., et al. (2026). A review of structural design for direct oil cooling and integrated thermal management in electric motors for new energy vehicles. Frontiers in Mechanical Engineering, Volume 12. https://doi.org/
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12 - 2026

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Copyright © 2026 Gao, Tan and Li.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Hua Li

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