89 weight can mean a lower SFC for their required power output, hence less onboard fuel and a lower total wet weight than if they’d gone for an engine with a lower dry weight. Multiple engines can be optimal for a UAV in different ways.” All four engines are spark-ignited two-strokes built as twin-cylinder boxers, air-cooled, lubricated by premixing, and running on gasoline, with the designation for each coming from its total displacement in cubic centimetres. Ancillary engine systems All four engines are managed via an ECU based on a 32-bit microprocessor with 512 kbytes of flash memory and 96 kbytes of RAM. Its IP50 housing measures 115.2 x 82.5 x 31.6 mm, which enables it to operate in temperatures from -45 C to +80 oC. Its CAN bus has a baud rate of 500 kbit/s and enables persistent diagnostic feedback and monitoring of critical health-related parameters. These include crankshaft rpm, measured by a Hall or inductive sensor, throttle valve width as measured by a potentiometer, exhaust gas temperature, intake manifold air temperature, manifold absolute pressure, cylinder head temperature and generator temperature. The ECU is also programmed for tracking parameters such as coolant water temperature or camshaft speed in Sky Power’s other engines. “Having one ECU for all our engines means anyone with a fleet of different UAV models needs to program only the CAN comms between the autopilot and the ECU once. From then on they can swap the engines in an almost plug-andplay manner,” Schudt says. The same models of throttle servos, fuel pumps and ignition plugs are used in all the engines, simplifying inventories and integration work compared with using different fuel and air components for each engine. “We’ve also run tests with fuels of varying condition, to check for any mismatches between gasoline type or quality and the ECU mapping in terms of what power output you should get for the fuel you’re injecting – 95 octane won’t always be available where our users are operating,” Schudt says. For fuel-oil mixing, Sky Power uses products from Aspen, particularly the pre-mixed Aspen 2 fuel solution at a 50:1 ratio. The Swedish company’s products are synthetic, alkylate fuel-oil mixtures, which it says are formulated in a proprietary manner to eliminate harmful components found in other fueloil mixtures to achieve a high purity and octane rating, as well a particularly clean burn, all useful qualities for small engines with a high power bandwidth. Schudt adds that the engines exhibit the least carbon deposits inside the cylinder, on the piston and in the exhaust when running on Aspen’s fueloil mixtures, compared with any others the company has tried. That therefore contributes to all four engines having similar lifespan benefits from using the mixture; a 700-hour TBO is achieved when testing them with it. Components and materials Going deeper into the engines, differences between the engine parts begin to emerge, with crankshafts, cylinders and pistons being sized as appropriate for the various displacements and crankcase lengths of the four engines. Looking closer, however, common metals and treatments can be found among these core parts of the engines. For instance, Sky Power points out that all the cylinders are coated internally with nickel silicon carbide, and the cooling fins surrounding them are designed to be as thin as possible – 1.6 mm thickness at absolute maximum – for optimal heat absorption and dissipation. Each cylinder is cast from aluminium as a single piece with the head, with up to four transfer ports depending on factors such as fuel types, altitude and cost. Schudt notes that casting the complete cylinder as a single piece is a more complicated process than casting SkyPower’s SP engines | Insight The same ECU is used across all of Sky Power’s engines, to enable quick reconfigurations between unit installations Uncrewed Systems Technology | October/November 2023
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