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58 A s hybridisation stands to improve the reliability and lower the operating costs of fuel engines, demand for hybrid powertrains is spurring numerous engineering approaches to lightweight, energy-dense airborne electricity generation. In line with this trend, Ishikawa Energy Research (IER), based near Tokyo, has developed a single-cylinder opposed-piston engine, which will be used initially in a heavy-lift hexacopter UAV being made by Aichi Steel Corporation. The four-stroke ARE (AiR Engine) weighs 12 kg, measures 548 x 352 x 328 mm, displaces 350 cc and produces a maximum continuous output of 11 kW. It will directly power six heavy-lift electric motors for an hour, via a module of supercapacitors that regulate the load from the onboard systems. While traditional reciprocating engines incorporate a single piston per cylinder, opposed-piston engines have two per cylinder – one at each end – that thrust outwards from the cylinder, each into a separate crankcase. Opposed-piston engines also typically have two crankcases and two crankshafts, with the number of cylinders and the combustion cycle varying between designs. In the ARE, each twin crankshaft drives a camshaft, each of which has two cam lobes; the left camshaft drives two inlet poppet valves, while the right drives two exhaust poppet valves. These sit above the top dead centre (TDC) of the pistons, in a joint space (or ‘sub-combustion’ chamber) where the spark plug also sits. Rory Jackson reports on the development of this opposed- piston engine designed to boost flight times for multi-copters Powerful opposition February/March 2020 | Unmanned Systems Technology The AiR Engine four-stroke hybrid has generators mounted on its twin crankshafts (Images courtesy of IER)