6 Mission-critical info for uncrewed systems professionals Platform one Suter Industries has developed a new engine for UAVs, which is its largest yet, and has been optimised for running on heavy fuels (writes Rory Jackson). The company decided to convert its TOA-288, a spark-ignited, two-cylinder, two-stroke boxer – unpacked in our 32nd issue (June/July 2020) – into a heavy fuel version. And, with customers calling for more engine power, Suter increased the cylinder displacement while keeping the crankcase and much of the rest of the architecture the same. “It’s now a 330 cc engine; we’ve called it the TOA-330. Where the first engine ran on 95 octane Avgas and certain synthetic fuels that customers felt were right for their needs, we’ve now been able to run on kerosenes, mainly through switching away from injecting fuel into the crankcase from the intake manifold to a semi-direct injection solution, by which we spray into the fifth transfer port,” says Dietrich Kehe, CEO of Compass Aircraft Europe (CAE), Suter Industries’ partner. The arrangement of five transfer ports in the engine is such that the fifth sits in a central position opposite the exhaust port, giving it the longest distance from the exhaust aperture of any port, with the other four transfer ports disposed elsewhere in the sides of the cylinder, functioning as ‘support ports’. “That’s a common approach in two strokes for closely controlling the fuel intake for quantity and timing – being able to put the exact minimum amount of fuel necessary into the combustion chamber precisely when you need it helps minimise fuel waste,” Kehe says. “Kerosene doesn’t vapour well, so if you try to pass it through the crankcase, you’ll have uncontrolled vapouring, and a poorly controlled fuel delivery and combustion quality.” Suter reports that tests show no significant carbon deposits indicative of incompatibilities between the heavy fuel and the engine’s oil. “We had to reduce the compression ratio and take a slight trade-off in performance to guarantee safe operation on heavy fuel. As kerosenes have no octane or cetane reading, their performance in spark-ignited engines can vary, so you have to assume the worst-case scenario and tune your engine’s compression to not cause knocking in the kerosene. For added safety, we’ll likely integrate a knock sensor,” Kehe notes. Design and testing of the larger cylinders was first performed via gasoline operations before swapping over to kerosene. While dynamometer tests are ongoing, initial trial units of the TOA-330 have been delivered to customers. Suter’s flight tests of the engine are slated to begin by the summer. ENGINES Two-stroke converted to run on kerosene April/May 2024 | Uncrewed Systems Technology Suter’s TOA-330 has been engineered for heavy fuel and the largest displacement of any of its engines yet (Image courtesy of Suter Industries)
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