Unmanned Systems Technology 025 | iXblue DriX I Maintenance I UGVs I IDEX 2019 I Planck Aero Shearwater I Sky Power hybrid system I Delph Dynamics RH4 I GCSs I StreetDrone Twizy I Oceanology Americas 2019

62 Dossier | Sky Power hybrid system won’t have quite as much power density as with the Wankel hybrid, because the range extender system has different cooling requirements and temperature limitations to the two-stroke – but the platform concept and architecture remains the same,” adds Seidl. In the case of the SP-180 SRE, the permanent magnet MGU is mounted directly on one end of its extended rotor shaft without the need for a belt drive or other transmission mechanism. “That’s why we call it a complete, functional hybrid solution – because of this direct drive system,” Schudt points out. Hybridisation remains optional, however. Customers can choose to have just a starter-generator or opt for full hybrid functionality – there are no major differences in powertrain architecture, as the company integrates an MGU and an alternative starter- generator in the same manner. Currently, three of Sky Power’s two- stroke engines have been upgraded, tested and made available with hybrid functionality, and the rest are on course to receive the same upgrade. Differences between the power management boards will occur depending on the power of the engine, on the model of MGU (or starter-generator) integrated, and other components of the powertrain. “We can combine different engines with different types of motor- generators,” Seidl notes. “Sometimes there will be more of a focus on electricity generation. “Say a UAV developer needs 2-5 kW of electricity; in that case we’ll put one of our bigger generators on the engine shaft, or if a smaller UAV is constrained for weight or space, we can select and integrate one of our smaller ones. The smallest one we have is used as a starter for our two-stroke engines.” The SP-180 SRE The 20 kW SP-180 SRE is a Wankel- type single-rotor unit with a four-stroke equivalent displacement of 180 cc. It weighs 6.8 kg, and when integrated with its generator, the overall hybrid system weighs 9.5 kg and is 222 mm long. “We’ve compared this Wankel design to four-cylinder UAV engine designs we’ve worked on in the past,” notes Schudt. “The Wankel is a more expensive engine, but it gives performance levels – and in particular a power-to-weight ratio – that the four-cylinder two-stroke cannot match, which is obviously vital for UAVs. “With a Wankel, you have the power density of a two-stroke engine but the gas exchange and emissions of a four- stroke,” notes Seidl. He adds that by going above the power specifications of a starter-generator, from 500 W to between 10 and 20 kW, the company could develop a well-integrated hybrid solution. The rotary offers inherent advantages regarding generator integration, such as straightforward packaging and low vibration compared with reciprocating engines. “Between 10 and 20 kW, you can easily generate more than enough power for a wide array of electronic systems, while always reserving at least 50% of the engine’s power output to thrust,” Seidl notes. “Wankel engine designers in the past innovated in perhaps one or two specific ways on designs established in the 1970s and 1980s, but none of them have taken a comprehensive approach that addresses all the mechanical, chemical, vibration, sensory and other specifics of the engine.” By contrast, the engineering team at Sky Power was determined to develop a rotary engine from a blank sheet, rather than use an existing design, to try to address the various and growing needs they saw in the UAV market. “We looked at the advantages and disadvantages of the engine types we considered before going into this,” says Seidl. “For instance, in a piston engine, the Otto cycle phases all occur in April/May 2019 | Unmanned Systems Technology The SP-180 SRE comes with a closed- loop water cooling system to help thermal management of the trochoid housing, which can suffer mechanical stresses owing to zones of different temperatures around the cylindrical structure