Efficiency at the Limit: How Highly Integrated Designs and High-Speed Applications Are Redefining Thermal Management in Electric Drives

Mr. Schwarz, a modern electric transmission seems almost alarmingly simple compared to a conventional 8-speed automatic. But why is designing it for mass production so challenging?

Bjarne Schwarz: At first glance, this visual impression may seem accurate, but it is deceptive. At the component level, the observation holds true: fewer parts, a clearer structure. But the complexity hasn’t disappeared; it has merely shifted. A conventional transmission operates within a well-researched operating range. Rotational speeds, lubricant properties, power loss behavior – all of this has been experimentally validated over decades and incorporated into established equations and calculation methods. With high-speed electric drives, however, we face entirely new requirements. The operating points shift, and the lubricant becomes an even more critical variable. Additionally, the electric motor is usually mounted directly with the transmission in a shared housing and can thus alter the thermal behavior of the entire system. What looks simpler on paper has become considerably more complex in calculations. Today, electric transmissions operate in speed ranges where, in some cases, entirely different types of power losses dominate, and additional thermal management requirements apply. As a result, components such as the electric motor, the transmission, and often the power electronics must now be considered holistically, as they can influence each other thermally depending on their design.
 

A current trend in electric motor development seems to be moving toward ever-higher speeds. In your opinion, will the “sound barrier” of 20,000 revolutions per minute soon be a thing of the past?

Bjarne Schwarz: Ultimately, this depends on the design approach and strategic positioning of the respective OEM. The trend toward high speeds is attractive because a higher speed requires less torque for the same power output. Since torque scales directly with the diameter and weight of the electric motor, this allows for a massive increase in power density. However, at significantly higher peripheral speeds, power loss behavior can differ significantly from the operating conditions considered so far. Not every manufacturer is following this path; some instead rely on heavier motors with higher torque or use entirely different concepts, which creates a highly dynamic market. We are researching the high-speed range so intensively because this is where existing technological standards are reaching their limits.
 

Are established simulation and calculation methods, which have been the standard for decades, still suitable for future transmission designs?

Bjarne Schwarz: Today, there are established calculation methods that have been extensively validated for conventional parameters and application areas. However, when applied to electric drive units, we often operate outside the validated application range, which frequently requires extrapolation and for which sufficient validation tests and experimental findings do not yet exist. Consequently, the calculation methods established to date may not be readily applicable, which can lead to inaccuracies when using simulation tools.
 

You already mentioned thermal management as a critical factor. When power dissipation increases, cooling becomes the bottleneck. How can this problem be solved through design?

Bjarne Schwarz: In the past, conventional gearboxes often relied on splash lubrication due to its low complexity and cost, as well as its generally adequate cooling and lubrication capabilities. In electric drive units, injection lubrication is increasingly being used, or a combination of both lubrication methods. In addition, the lubricant itself is becoming a focus of research. In our studies, for example, we are comparing a synthetic polyalphaolefin with an extremely low operating viscosity of just 9 mm²/s at 40 °C. This is significantly lower than what was previously used, for example, in torque converter automatic transmissions.
 

You are also experimenting with water-based lubricants. Doesn’t that sound almost like sabotage to a traditional transmission engineer?

Bjarne Schwarz: It is indeed a research approach that may seem extreme. However, a water-based polyalkylene glycol offers enormous advantages for the design. Calorimetrically, water cools significantly better than oil. Even more exciting, however, is what is known as “superlubricity.” With this, we achieve friction coefficients of less than 0.01 in tooth contact at certain operating points. This can significantly reduce power loss for each operating condition. Of course, this is currently still a research topic for potential future applications. Yet this approach demonstrates how broadly we need to think in order to meet the requirements of future drives.
 

Modeling such highly integrated systems is very computationally intensive. How do you model the thermal interactions between the engine and the transmission without having to wait weeks for simulation results?

Bjarne Schwarz: We use thermal network models for this purpose. You can think of it as an electrical circuit diagram in which the various components are divided into isothermal nodes. Between these nodes, we calculate thermal resistances or conductance values. This 1D modeling is very computationally efficient and significantly faster than a full-scale CFD simulation. We also take external factors into account, such as heat conduction across shaft ends, room size, or ambient air. In our model, we consider an electric drive unit consisting of a transmission and an electric motor.
 

As you mentioned, your study doesn't just look at the transmission, but at the entire electric drive unit. What changes when the electric motor is factored in?

Bjarne Schwarz: This is a crucial point. In addition to the transmission, the electric motor must be taken into account during the design process in the future, so that the overall efficiency of the electric drive unit is considered – and not just the transmission on its own. Integrating the electric motor creates additional cooling potential, which must also be factored into the design. At operating points where the transmission reaches its thermal limit due to its limited oil volume, the electric motor can help dissipate heat more reliably. Increasing system efficiency is the design goal that must be pursued.
 

How do the various electric motor designs differ – whether they are separately excited, permanently excited, or asynchronous?

Bjarne Schwarz: The design influences the materials, power loss distribution, and thus the thermal modeling. In my publication, I deliberately chose not to include these differences as variables in the analysis, but instead focused on the influence of the lubricant and the type of lubrication. However, we designed the modeling structure so that it can, in principle, be adapted to various electric motor concepts. This is relevant for future studies. For example, the electric motor has separate nodes for the rotor, stator, windings, and winding heads, for each of which the power loss can be specified locally.
 

You mention power losses, thermal management, and lubricant optimization. How does NVH fit into this picture – that is, the acoustic and dynamic behavior of the powertrain?

Bjarne Schwarz: This is an important question because it illustrates how each subdiscipline is embedded within a larger whole. Here at FZG, we work with what I like to call the “design triangle”: three key research areas – efficiency, NVH, and load-bearing capacity or service life – with cost, of course, at the center, so that we can also take industry needs into account. Every powertrain is a compromise between these three poles, and depending on how you shift the design priorities, you gain on one side and tend to lose on another. My research clearly focuses on efficiency. A colleague from the NVH field might design the same drive differently, with different priorities, and he would be just as right. The key is to understand and take these interactions into account. For a complete production design, you need all three perspectives and, in the end, the compromise for the desired drive concept. The value of basic research lies precisely in this: we identify the limitations of existing methods, determine the relevant influencing factors in novel operating ranges, and pinpoint the parameters that must be taken into account if one truly wishes to understand the entire system – not just the transmission, not just the electric motor, but both working together. This is the foundation upon which the next generation of calculation standards will be developed, as we are investigating, for example, in the course of the Opt4E research project.
 

You mention experimental validations. What is the current status of the comparison between simulation and measurement?

Bjarne Schwarz: This is the next logical step. We already have the Opt4E test bench, where the simulation results can be experimentally verified in the future. The calculation results from my publication will thus be experimentally validated. This is crucial, because even the most carefully constructed model contains simplifications of reality and assumptions. For example, heat transfer coefficients, oil distribution, and flow behavior within the housing can only be described approximately through simulation. Only by comparing the results with measured data can we determine where the model is reliable and where it may need to be adjusted. This process is not yet complete, but the results I will present in Baden-Baden form the computational basis upon which the experimental validation is built.