Thermal Management Lies at the Heart of EV Innovation

Cooling strategies are evolving to meet the needs of tomorrow’s improved EVs.

April 1, 2025

At the heart of innovations that have powered improvements in EV range and performance is the pursuit of efficiency. Today’s electric drivetrains convert over 85 percent of a battery’s electrical energy into mechanical energy. By comparison, IC engines convert less than 40 percent of their fuel’s chemical energy into mechanical energy.

As a result of this greater efficiency, we have seen major gains in electric motor performance, with today’s motors operating at up to 25,000 rpm compared to the 15,000 rpm common 10 years ago. So, what’s driving this increase in efficiency? As with any complex system, there are several variables at play: optimized winding configurations, improvements in magnet layout and materials, and better integration between components. However, one of the major heroes of EV efficiency improvements has been thermal management technologies.

Why Thermal Management Matters

Simply put, temperature represents the average kinetic energy of particles in a substance — that is, how fast particles move around. For electrical components like wires, this means that their constituent ions vibrate more as the temperature rises. This, in turn, means that these ions are more likely to collide with one of the free electrons that make up an electric current. As a result, electrical resistance rises with temperature, reducing an electric motor’s efficiency.

Despite having fewer moving parts than traditional ICE architectures, EV motors still produce a significant amount of heat in their operation. This is due to intrinsic electrical resistance, eddy currents, and other factors like mechanical heat. Combined with the fact that electrical resistance increases heat output, this means that electric motors risk being trapped in a feedback loop of declining efficiency.

At its most basic level, energy lost through heat dissipation in a closed-loop system is wasted energy that could be useful elsewhere, for example, by finding marginal gains in extending a vehicle’s range. Finding ways to improve thermal management is thus a major driver of efficiency gains in electric motor design.

Generally, to maintain efficiency and maximize an electric motor’s longevity, engineers target maintaining a temperature of less than 365 °F (180 °C). In practice, this is typically done via indirect or direct cooling.

Indirect Cooling

Indirect cooling methods are techniques that don’t allow a coolant to directly touch the motor or heat sources, instead using a heat exchanger to run coolant in a separate closed loop. This is analogous to the traditional cooling setup that we see in ICE vehicles, which circulates coolant from a jacket around cylinder blocks and heads to a radiator that dissipates the heat into the surrounding air.

For EVs, indirect cooling methods were historically the preeminent technology until recently, building off the decades of expertise automotive engineers have with comparable ICE setups. A typical indirect cooling setup for EVs uses a water jacket, embedded in the stator of an electric motor. This water jacket, usually filled with a water-glycol coolant, helps provide constant passive cooling to the copper windings in the stator to prevent them from overheating.

The pursuit of efficiency is a major driver of innovations in eDrive technologies to improve EV range and performance. (Image: GKN)

A water jacket is a simple, reliable, and cost-effective way to cool an electric motor. However, its inherently passive nature means that it cannot ramp up activity in the face of temperature spikes. More problematically, water jackets are also less effective in helping to cool the rotor and active parts of an electric motor, since they can only surround the stator.

As a result, we’ve seen a far greater focus on the development of new direct cooling methods for EVs, particularly for high-performance drivetrains.

Direct Cooling

In contrast to indirect cooling methods, direct cooling methods directly expose a liquid to a motor and respective heat sources. Because water is a conductor, this means putting an oil-based coolant in direct contact with a motor’s windings, stator and rotor.

The major benefit of direct cooling is that it helps deliver exactly what indirect cooling via water jackets cannot. Direct coolant can be sprayed inside an electric motor, providing cooling to a rotor and the interior surface of a stator. It can also be throttled up or down, helping stabilize a motor’s temperature in the face of spikes, such as if a motor is having to do extra work to haul an EV up a steep hill.

There are a variety of direct cooling methods in development and undergoing deployment. One is manifold drip cooling, which precisely and uniformly sprays coolant onto the electric motor’s winding heads. Rather than bathing the rotor in coolant, manifold dripping requires minimal coolant and reduces drag on the rotor. The downside is that this method requires a higher-pressure oil pump to properly apply the coolant to the winding heads.

Another method, for instance, is shaft centrifugal cooling. This method builds the coolant sprays into the rotor shaft, spraying oil onto the winding heads as the rotor turns and works. Once a rotor is turning at full normal speed, this uniformly covers the winding heads and provides excellent cooling. However, shaft centrifugal cooling does produce some drag in the motor and does not provide even cooling coverage when the motor is only turning slowly.

Given that they can cool the interior of a motor in a way that indirect cooling methods currently struggle with, direct cooling methods are seeing a major surge in development. In a way, this transition reflects the overall maturity of the EV category as a whole: moving from a reliance on ICE-derived indirect cooling methods toward best-fit cooling methods that reflect the unique needs and architectures of EVs.

GKN’s Net-Zero Roadmap

GKN Automotive builds more than just EV driveline components. It also makes all-wheel drive systems and plug-in hybrid (PHEV) systems. GKN’s work on EVs is just part of the company’s environmental efforts. One of GKN’s biggest announcements in 2024 was the signing of a 10-year virtual power purchase agreement with Recurrent Energy that will cover 65 percent of GKN’s European electricity load. The deal will add approximately 2 million megawatt-hours (MWh) of renewable electricity to the European grid over the next decade (200,000 MWh a year). The actual project GKN’s deal will fund is called Rey I, a large, new solar farm in Spain.

The recurrent announcement, along with the announcement of a SBTi-approved net-zero roadmap and stronger sustainable procurement practices and reporting processes in its annual sustainability report were all reasons that independent sustainability assessor, EcoVadis awarded GKN a Gold rating in 2024, up from a Silver rating in 2023 and a Bronze in 2022. GKN has announced plans to reduce its direct greenhouse gas (GHG) emissions by 45 percent by 2030 and achieve net zero across its entire value chain by 2045. GKN’s 2024 sustainability report, detailing its efforts in 2023, found that it reduced CO₂ emissions 10.5 percent compared to 2022 and that already “87 percent of R&D expenditure was on products that contribute to the decarbonization of the industry, exceeding 50 percent target.”

GKN has also targeted that 50 percent of electricity it consumes will be certified renewable by 2025 and 100 percent of it waste will be diverted from landfills by 2030. When it comes to sourcing, in 2023, GKN asked the top 80 percent of its strategic suppliers for their own sustainability roadmaps and targets.

Expect to see more advances like the thermal management technologies mentioned here, as GKN has said it will commit 90 percent of its research and development budgets to sustainable innovation. The supplier is also “actively pursuing” other VPPA deals in Europe and South America.

“By decarbonizing our products through innovation and resource efficiency, we can help our customers reduce their own emissions and enhance the long-term sustainability of our people, business, and industry” the company said in a statement.

-Sebastian Blanco, Editor-in-Chief, Automotive Engineering magazine

GKN Automotive’s eMotor Hybrid Cooling Prototype delivers efficiency gains by providing effective direct cooling via three different configurations: dripping through manifolds, shaft splashing, and shaft centrifugal cooling. (Image: GKN)

There likely is a future for indirect cooling in EVs. However, the rise of direct cooling and the continued miniaturization and integration of components like oil pumps is making this technique an ever-more appealing option. As a result, direct cooling is enabling many manufacturers to jettison the need for a water jacket altogether, to deliver lighter, higher-performance, and more affordable powertrains than ever before.

Modeling and Digitization

Along with rapid innovation in cooling methods, a major trend in EV thermal management is the use of technology to better design more thermally efficient vehicle architectures. AI and digital technologies have been major boons to the engineering toolkit for better design, helping to radically speed up system testing and development.

One of the best ways to computationally model structural, thermal, and fluid performance is through Finite Element Method tools. With computing resources becoming cheaper and more efficient, engineers are finding it easier than ever to optimize the placement and configuration of EV components to provide the best thermal performance.

AI offers the chance to further democratize and speed up the development and testing process. Rather than having to manually analyze options in response to customer requests, AI can reliably help select the most efficient electric motor and drive system for a given customer’s needs in minutes or seconds. AI can also help designers, engineers, and customers model and understand what cooling method may be best for their needs in rapid time, helping them navigate the trade-offs that may exist between various direct and indirect cooling techniques.

This article was written by Andreas Mair, Director of Mechanical Engineering, GKN Automotive (London, United Kingdom). For more information, visit here .