Cool Collaborations

MATERIAL SCIENCE

Cool Collaborations Adam Banks

Direct oil cooling is helping eMobility to push new boundaries and not just via the eMotor. Batteries, power electronics and charging systems can also benefit from this more effective cooling method. With the right coolant capable of delivering efficiency as well as reliability, future design is best optimised when developing hardware and fluids side by side. eMobility is, quite literally, a hot topic. Electrified vehicles (EVs) generate considerable heat that, if allowed to build up, can impact sensitive systems.

Air-cooling is the most basic, low-cost way to keep temperatures down. However air is less effective at conducting heat away from vital parts than fluids such as glycol, which have excellent thermal conductivity. Glycol’s superior cooling action is diminished by distance, however, as it must be contained within a water-glycol jacket that keeps the coolant a step removed from the heat source.

Although water-glycol jackets are commonplace in earlier EVs, being able to cool a component directly is more effective and enables lower peak temperatures, reduced temperature gradients and greater responsiveness.

Cooling eMotors Direct cooling is increasingly common in eMotors. The heat generated in motor windings must be conducted away quickly; immersing it directly in coolant offers the best solution. The coolant must be dielectric (non-electrically conducting) so water-glycol is out of the question.

Hydrocarbon-based oils possess the most appropriate characteristics.

The advantages of direct oil-cooling eMotors are not limited to protecting components from heat. Modelling by Afton Chemical and Ricardo predicted that more effective cooling could increase eMotor peak efficiency. This was further demonstrated through rig testing with a pre-

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prototype eMotor, in work carried out by Afton together with a major OEM. Efficiency benefits may be substantial and would be welcomed by OEMs for enhancing a vehicle’s driving range or enabling battery cost savings.

Of course in the case of eAxles, particularly multi-speed eAxles, direct cooling the eMotor from a common sump means the coolant must be compatible with an even broader range of materials, while also protecting gears and bearings – and delivering appropriate friction characteristics for any shift devices. Fluids also need excellent thermal and oxidative stability to maintain essential properties throughout their lifespan.

Afton has already carried out extensive in-house development and testing to ensure the new generation of eLubricants demonstrates the right balance of performance, for even the most demanding multi-speed eAxle applications.

Battery performance Direct cooling benefits are being explored in EV applications where even greater value can be realised, such as the batteries themselves.

Batteries produce large amounts of heat while discharging and charging. BEVs typically already use water-glycol coolant channels running through and around the battery pack to extract heat via tabs or a bus bar. However, immersing battery cells directly in coolant enables power to be released or replenished even faster without build-up of damagingly high temperatures.

Battery immersion with custom coolant has already been successfully employed to enhance power output in racing and high performance vehicles, such as the McLaren Speedtail1 and the Mercedes-AMG E-PERFORMANCE2 hybrids and battery electric vehicles (BEVs). AMG states that the 14 litres of dielectric fluid used to cool individual

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cells enables a constant 45oC temperature, no matter how often the battery is charged or discharged. It credits direct cooling with doubling its battery power density to 1hp/lb.

The resulting lighter, more compact battery system weighs just 196 lbs and would not be possible with conventional cooling.

It is likely that immersion technology will ultimately be adopted by mainstream EV manufacturers too; although potentially to optimise efficiency and charge time rather than boost driving performance.

Improving battery power transfer speeds through cooling can also make charging faster. Time spent at recharging stations could be minimised by employing effective cooling inside fast chargers and even in the charging cables.

Power electronics Power electronics are key components in any hybrid or battery electric powertrain, and play a key role in increasing efficiency. Traditionally these components have also been cooled using water-glycol jackets. But as the target is to improve performance by converting higher levels of electrical energy, heat input increases and the increased heat transfer area offered by direct cooling becomes more attractive.

Afton Chemical has worked in close collaboration with a major OEM to assess the impact of direct cooling power electronics, using transmission fluids with strong heat transfer properties. This joint research indicates that direct cooling with appropriately specified transmission oils could increase the heat transfer coefficient by a factor of three to eight, compared to indirect cooling with water-glycol.

Fuel cell cooling Hydrogen fuel cells are designed to work optimally in a narrow temperature range, somewhere between 60oC and 80-90oC. As the exothermic reactions within a fuel cell generate only water and waste heat, removal of excess heat is paramount for efficient function. Cells work better at higher temperatures, but only until excessive heat causes the cell membrane to dry out; after this, efficiency drops dramatically.

What makes a good coolant?

All these electrical systems within EVs operate at different temperatures and employ different materials in their design. The varied performance and compatibility needs of each application must be taken into account when formulating the coolant, especially when considering a common sump.

Hydrocarbon-based dielectric fluids possess the right electrical characteristics for direct cooling of electrified components but, just as every application differs, not all fluids are equal in terms of cooling capacity.

Simulations confirm that the four ideal fluid properties are higher density, heat capacity and thermal conductivity, plus lower viscosity. These are inherent in the base oil, which is why synthetic fluids such as polyalphaolefins (PAO), polyalkylene glycol (PAG) and esters make the most efficient coolants.

Experimentation supports these findings, with polyalphaolefins (PAO) showing superior cooling performance to Group III base oils, keeping the temperature in a heated block (used to simulate the motor) around 8% lower.

Figure 1. The above shows the setup for the measuring heat transfer coefficient and pressure drop (A) and enlarged view of the power electronics with chiller (B).

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Viscosity

Fluid solutions

Low viscosity oils make better coolants because cooling efficiency increases by 4% with every two centistoke drop in viscosity. Lower viscosity fluids have a higher Reynolds number: this indicates that the fluid is more likely to experience turbulent flow through the cooling system, making for much more efficient heat transfer.

Using commercial eCoolant development expertise, Afton has developed a set of example fluids with performance data that help to illustrate potential trade-offs and the

Turbulent flow can also be encouraged (or discouraged) by the characteristics of the cooling system pipework.

Maximising cooling performance therefore requires an understanding of hardware design and fluid dynamics as well as chemistry, so benefits from close collaboration between OEMs and lubricant developers.

Trade-offs and testing While lower viscosity oils may have better cooling capability, this can present challenges for other oil functions. Where sumps are shared, such as in an eAxle, coolant may also need to provide effective gear and bearing protection, which is easier to achieve with thicker oil.

The volatility of thinner synthetic fluids can also lead to issues with loss of volume and importantly fire safety. Low viscosity fluids are also at greater risk of leakage around seals and of greater aeration while in use.

Any coolant must also be compatible with all materials used in the system, including copper and elastomers.

Certain types of synthetic fluid can be particularly challenging for elastomer materials, causing seals to swell and/or lose their elastic properties – ultimately causing coolant loss.

While the inherent properties of a base oil will largely determine its suitability for electrical systems and its capacity for cooling, being able to balance this with antiwear performance, corrosion protection, thermal stability and antioxidancy requires additives.

expected performance of different coolants.

This growing library highlights the impact of different base oils, viscosities and additive systems to demonstrate what is possible, based on system needs such as high or low temperature, mechanical protection and cooling. It also helps to quantify efficiency benefits vs aspects such as fire safety and can be used to help calculate system cost, not just coolant cost.

A cool payoff Ultimately, taking advantage of direct oil cooling to optimise EV performance can be viewed as a simple costbenefit exercise.

Let’s say a next-generation oil appropriate for direct eAxle cooling gives a 1% efficiency gain. (For context, when Afton tested this oil versus a standard manual transmission fluid, the actual increase in efficiency was 3%).

For a 55kWh car with a 2023 estimated battery price of $100 per kWh, the total battery cost would be $5,500. Assuming the efficiency increase is used to reduce battery size by 1%, this translates into a substantial saving of $55 per car – per 1% efficiency gain.

Getting the coolant right can offer improved reliability, responsiveness and peak efficiency throughout an electrified powertrain. Early collaboration between OEMs and fluid formulators, informed by insights from in-house testing, should therefore be not the exception but the rule.

Adam Banks, eMobility Marketing Manager, Afton Chemical

Trading-off the differing needs of systems cooled by the same fluid is essential for getting the final balance right, and is best done through collaboration between hardware designer and chemist at an early stage. Expertise testing the interaction of fluid and prototype hardware is valuable in generating meaningful insights. Collaborative testing with experts can find solutions to specific issues and evaluate risks, but can also for optimise potential efficiency gains.

Refrences 1 www.mclaren.com/applied/news/secretsspeedtail/ 2 www.greencarcongress. com/2021/03/20210331-amg.html

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