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59 Origins of the project IER was founded in May 2010 by CEO Mitsuru Ishikawa, who along with his head of design Taro Fukuda and most of his engineers had previously worked in Honda’s r&d centres across racing, general automotive and other applications. “I specialised in engine development as well as thermodynamics engineering for HVAC systems, and Mr Fukuda was a design engineer specialising more in general-purpose machinery, including co-generation engine systems for combined heat and power [CHP],” explains Ishikawa. “As a result of that, one of the first major projects we developed together was a CHP engine, which output roughly 1.2 kW of electricity and 2.7 kW of heat, for powering and heating households and other buildings. That unit was effectively the progenitor of the ARE engine,” he says That engine, the 135 cc GE1B LOT, had to be designed in order to recover all the waste heat – and indeed, it had an energy efficiency of 93% – and it was small so that it would fit indoors. Furthermore, it needed to produce an absolute minimum of vibration, so as not to cause discomfort or long-term damage to people’s homes. “While there’s a lot of buzz over fuel cells for CHP, they are expensive, they need unfeasibly pure hydrogen gas to work well, and they aren’t necessarily reliable in cold weather. ICEs are reliable, proven methods for CHP – we just needed to produce little to no vibration so that they could be used in people’s homes,” Ishikawa notes, hinting at the parallels with UAV propulsion. Opposed-piston benefits As development of that system was ongoing, Ishikawa and Fukuda sought other applications for the engine they had designed. During their search, they soon noticed widespread demand from multi- copter UAV users for far longer flight times than the 10-15 minutes that battery- electric craft could manage. “So we proposed our engine for UAVs as an alternative, using gasoline to generate far more electricity than a battery pack of equal size. That project was then picked up and sponsored by the Japanese Ministry of Economics for two-and-a-half years,” Ishikawa recalls. In general, the concept of an opposed-piston four-stroke has a few key advantages over more widely adopted and conventional alternative configurations, which has contributed to a renewed interest in the design approach. For example, the absence of cylinder heads eliminates the weight and manufacturing cost associated with it. This, combined with the resulting reduced surface-to-volume ratio of the two combined cylinder areas (compared with two separate cylinders), theoretically means higher thermal efficiency. Also, the counterbalancing of the two counter-rotating crankshafts against each other have been found through testing to help greatly in minimising vibration in all three axes (at 7000 rpm the ARE produces below 2 g of vibration). This is a critical benefit for flight stability, particularly for UAVs gathering high- resolution video or delivering potentially fragile cargo such as electronics or medical supplies. From co-generator to UAV engine The IER team needed to perform numerous alterations to re-engineer their system from a co-generation unit into a UAV engine. “For the GE1B LOT, we were very focused on thermal efficiency, so we used pistons made from stainless steel 303 for their low thermal conductivity [about one-seventh of aluminium’s], with semi-adiabatic combustion in mind,” Ishikawa explains. “We also used a cast iron engine block, for greater thermal efficiency compared with aluminium and other materials.” However, when the UAV engine project began, Ishikawa and Fukuda agreed that their targets needed to change: the engine now needed lower weight and greater power density – higher priorities for UASs than thermal efficiency. Ishikawa Energy Research ARE series hybrid | Dossier Unmanned Systems Technology | February/March 2020 Each of the engine’s twin crankshafts directly drives a camshaft by way of a gear on its back