Electric Flytrain EFT-Hybrid-1x | Dossier much power from the engine and cause it to stall. Otherwise, the demand on the system could actually run much higher than the hybrid powertrain can supply.” For thermal management, the PMU has been designed with a thermally efficient selection, layout and packaging of board-mount components, as dictated by Acutronic’s discussions with EFT, with heat dissipation typically also achieved by mounting it within the flow of the forcedair cooling system. “Speaking of those board-mount parts, at the time this system was being designed, there wasn’t a perfect MOSFET that could perform optimally in both a starting functionality and a rectification functionality,” Peryea recounts. “So we have one bank of MOSFETs that drives the starting mode – they are very robust in order to handle the spikes of hundreds of amps when cranking the engine – and then we have a second bank of MOSFETs that run more efficiently when running at lower current levels. “The latter are ideal for rectification, because they’re better tuned for getting every last bit of current out of the alternator and into the battery with minimal losses, which is what you want most in a hybrid arrangement. Our component selections, design layout, timing and switching strategies are all oriented around optimising efficiency.” Battery and BMS The battery is a 14S1P pack, with 800 mAh of capacity, and consists of cells tuned for high discharge so that the pack can deliver high power during emergency descents lasting up to 2 minutes or possibly longer. In addition to storing electrical energy and delivering it to the UAV, the battery also powers the alternator to drive the crankshaft during start-up. It also balances the peaks and troughs of power received from the PMU, which can be particularly critical for instance during strong gusts, when a multi-copter’s rotors must suddenly run at very high power to try to maintain its position. While the engine’s crank speed and electric output will accelerate slowly relative to the rotors’ speeds, the battery’s energy will last until the alternator output catches up, and continue absorbing the surplus power after a gust has passed. “The battery itself comes from Maxpower, while we’ve designed the BMS along with the full junction box that comes with it,” Kahnert says. “That contains a relay for switching the battery in and out of the powertrain, like an automotive HV interlock switch, and the HCU controls that relay. The junction box also allows us to distribute power to different downstream subsystems across the UAV by way of different voltage dividers.” While UAS batteries are smaller and therefore safer in many regards than those Kahnert and his colleagues encountered in the EV industry, temperature management is still critical for preventing issues such as overheating during an emergency descent, or under-temperatures when the IC engine is not running and suddenly needs to be started. In addition to mounting the pack away from any potential sources of heat (or puncturing), the BMS therefore had to be developed with greater safety and thermal stability than those in all-electric UAVs. “Designing the BMS was made easier though by the fact that many typical BMS functions were already handled elsewhere in the powertrain,” Kahnert says. “For instance, we were already tracking the incoming current, outgoing current and the voltages through the PMU. “Similarly, the HCU already ran battery temperature sensors. So tasks left to the BMS included individual cell voltage measurements and balancing.” Future plans Development of the EFT-Hybrid-1x is ongoing, and as mentioned the company continues to seek and accumulate partners for performing the first rounds of flight tests, after which it will look to bring the powertrain into productionisation. “We don’t just want to fully productionise it ourselves and then show it to themarket; we want to define the production line in cooperation with pilot customers, because everything about UAV powertrains down to themost minute details is so heavily defined by niche end-user requirements,” Kahnert notes. As well as manufacturing and supplying its UAV powertrains in the long term, EFT also plans to provide full maintenance services for its customers and maintain component stocks in-house for the fleet managers who buy from them. 91 Uncrewed Systems Technology | June/July 2023 EFT-Hybrid-1x Series hybrid Four-stroke, two cylinder boxer Spark-ignited Gasoline Naturally aspirated Shaft-mounted, PMAC alternator 14S1P battery pack Lithium-ion cells Weight: 19.5 kg Maximum continuous electrical power: 11.7 kW Specific fuel consumption: 290 g/kWh Maximum fuel consumption: 3.8 kg/h Maximumbattery capacity: 800 mAh Some key suppliers IC engine: HC Concepts Alternators: Acutronic Inverters: Acutronic Batteries: MaxPower CAD: Dassault CAD: Ansys Specifications
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