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Simulation Leads to Better Cooling of Electronic Components in Toyota Hybrid Vehicles

One glance under the hood of a modern automobile is all it takes to realize that free space in the engine compartment is a thing of the past.

If carmakers could reduce the number, size, and weight of the components in there, better fuel economy would result. A case in point is the design and development of optimized cooling structures, or advanced heat sinks, for thermally regulating the growing number of power electronics components used in the electrical system of Toyota hybrid vehicles.

altTo save the time and expense associated with analytical design methods and trial-and-error physical prototyping, researchers at the Toyota Research Institute of North America (TRI-NA) in Ann Arbor, MI, instead used numerical simulation and multiphysics topology optimization techniques to design, fabricate, and test possible prototypes of a novel heat sink for future hybrid vehicle generations.

One example prototype combines single- phase jet impingement cooling in the plate’s center region with integral hierarchical branching cooling channels to cool the periphery. The channels radiate from the device’s center where a single jet impinges, and carry liquid coolant across the plate to dissipate heat evenly throughout and with minimal pressure loss.

Numerical simulations enabled Dr. Ercan (Eric) Dede, Principal Scientist in TRI-NA’s Electronics Research Department, and colleagues to produce the optimized branching cooling channel patterns in an automated fashion using advanced simulation tools as opposed to a traditional trial-and-error design approach. He carried out this work as part of TRI-NA’s mission to conduct accelerated advanced research in the areas of energy and environment, safety, and mobility infrastructure. TRI-NA is a division of the Toyota Technical Center, which in turn is part of Toyota Motor Engineering & Manufacturing North America, overseeing R&D, engineering design and development, and manufacturing activities for Toyota’s North American plants.

TRI-NA’s Electronics Research Department focuses on two main areas: sensors and actuators, and power electronics. Among its resources are powerful modeling and simulation capabilities and prototype design tools, which enable its staff to develop effective solutions in the compressed timeframes demanded by the highly competitive automotive markets.

Hot Under the Hood

altToyota hybrid vehicles have sophisticated electrical systems in which many power diodes and power semiconductors such as insulated gate bipolar transistors (IGBTs) are used for power conversion and other applications. These components are standard planar silicon devices measuring a few centimeters per side, with high power dissipation.

In these hybrid vehicles, they are mounted on aluminum heat sinks, or cold plates, through which a water/glycol coolant mixture is pumped. In earlier model years, the cold plate design featured a fluid inlet on one side of the plate, an outlet on the other side, and in between were arrangements of mostly straight cooling channels through which the coolant flowed. The long channels provided adequate heat transfer but it came at the cost of a significant pressure drop across the plate.

However, the technology roadmap for these power components calls for them to shrink to about half their current size while dissipating the same amount of power, meaning that heat fluxes will have to increase. In addition, although they have a 150 °C maximum operating temperature, typical silicon devices are kept at lower temperatures for greater component reliability. Moreover, the role of such devices is becoming more important as the electrification of vehicle systems increases.

All of these factors mean that thermal management of these devices will become more difficult than it has been to date. It might seem reasonable to simply redesign the cold plates so that more coolant can be pumped through them. But that would require more pumping power, and with space already at a premium in the engine compartment where the pump is located, moving to a larger, more powerful pump or adding an additional pump is unacceptable.

Instead, Toyota decided to look at reengineering the cold plate with an eye toward achieving optimum heat transfer and negligible additional pressure drop simultaneously. If both could be achieved, thermal objectives could be met at no significant increase in system pumping capacity.