88 Dossier | Electric Flytrain EFT-Hybrid-1x Forced-air cooling As heat rises in electric powertrains, efficiency rapidly drops, and as multicopter UAVs are generally either slow-moving or stationary, they rarely experience or produce sufficient airflow to cool their power systems using anything other than the downwash directly below their propellers, so the EFT-Hybrid-1x uses a forced-air thermal management approach. “At the heart of it is a cooling fan design, which consists of fins mounted straight onto the outrunner rotor with a housing around both,” Kahnert explains. “That allows us to use the rotational speed of the crankshaft and rotor to drive the fan as an air impeller. It pulls air into the housing and through different spiralled channels, so some air goes to the alternator and some to the cylinder heads.” This means that in principle, the cooling system is similar in concept to those installed on ULPower’s UL350iHPS (issue 45, August/September 2022), and by Ishikawa Energy Research in its ARE opposed-piston series hybrid (issue 30, February/March 2020). Both used cooling fans driven directly by their crankshafts, with minor differences – the former was not a hybrid, and the latter had two crankshafts and hence two fans. “By having a very efficient and targeted set of pathways for the air, we make sure we only cool the systems that we need to cool, and dissipate their heat in as concentrated a way as possible,” Kahnert says. “The housing around the fan and alternator forms a critical part of the cooling, as its geometry and outlets dictate the flow rate and mass of air going to the alternator and heads. Crucially, its frontal aperture integrates a filter that prevents particles and moisture from reaching the alternator.” Dassault’s CATIA was initially used to design the housing and the fan, and simulate how air could be pulled, collected and redirected to the hottest areas of the powertrain. It also helped to determine how this could be achieved without undue leaks of air elsewhere across the system, as that could induce unwanted vacuum or pressure effects across the engine or UAV. As CATIA’s simulations fleshed out the design concept, the results were increasingly exported to and run in Ansys for more detailed analyses of how targets such as more energyefficient air suction could be managed. This second set of simulations was indispensable for optimising the thermal management, especially in retuning the fins (or blades) of the fan and the air distribution channels for efficient highvolume intake of air and cooling of the cylinder heads. “That’s not necessarily just because the pull of air was sometimes inefficient,” Kahnert adds. “Without exhaustive simulation, it would have been really easy to make the fins too big and induce a load of parasitic losses on the crankshaft, and hence waste energy at the shaft level even if we were saving some at the thermal level.” SLS (selective laser sintering) June/July 2023 | Uncrewed Systems Technology The use of additive manufacturing has been key to the design of the cooling housing, with future use of AM potentially enabling metal or composite housings The PMU integrates one bank of MOSFETs to drive the starting mode, and another for rectification that runs more efficiently at lower current levels (Courtesy of Acutronic)
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