Thermal Management Lies at the Heart of EV Innovation

October 1, 2024

Andreas Mair

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% of a battery’s electrical energy into mechanical energy. By comparison, IC engines convert less than 40% 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 ten 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 degrees 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.

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 towards best-fit cooling methods that reflect the unique needs and architectures of EVs.

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 (FEM) tools. With computing resources becoming cheaper and more efficient amid the AI revolution, 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.

Andreas Mair is the director of mechanical engineering at GKN Automotive.

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