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64 Dossier | Ishikawa Energy Research ARE series hybrid throttle bodies – we were familiar with them from when we worked with Honda, so we had good knowledge of their quality and consistency, as well as good relations going in,” Ishikawa notes. Ishikawa has personally engaged in considerable design and testing of different valve systems for ICEs over the years, and he oversaw similar r&d for the ARE, eventually selecting poppet valves. As he recounts, “We produced versions with ball valves and rotary valves, but they all failed to meet our requirements. For example, we tried constant-speed rotary valves, but we had to supply them with inordinate amounts of oil to prevent them from seizing. “Not only did that put too much demand on the lubrication system, it caused excess oil to leak into the combustion chamber, causing smoking problems that you’d usually only get with a two-stroke. We worked on circumventing that issue, and gradually saw an improvement, but we never got those issues when using poppet valves.” Various types of ball valves were also tested, with positive results for their clearances and improvements to volumetric efficiency, but they could get stuck in their seats when exposed to combustion pressure and heat. Poppet valves on the other hand would merely close from that pressure. “We also tested some ceramic ball valves to resolve the heat problem, but they were problematic in terms of how they could be connected to the metals around them. Studying how best to integrate them would have taken far too long, and by then the poppet valves already seemed to be the best overall.” Forced-air cooling The original patent for IER’s engine included plans for a water-cooling system, but the ARE in its current form has an active air-cooling system instead – a switch made with the aim of saving weight. “We haven’t realised as much weight reduction in our prototypes as we had projected in our simulations. The passive element of our air-cooling relies on adding a lot of fins all over the engine block to act as heat sinks, so our fans can then cool them,” Ishikawa notes. “We may iterate versions with water cooling, as it can be more thorough, more reliable and more suitable if the hybrid multi-rotor is to be used in climates exceeding 40 C. It would also not add very much weight.” As the ARE is designed to be used on multi-rotor UAVs rather than fixed- wing craft or helicopters, IER needed to account for the absence of ram-air, propeller downwash, or forward motion- induced airflow available for cooling the engine as the multi-copter hovers or banks at relatively slow speeds. This shortfall is therefore accounted for by installing two cooling fans, one on each generator’s rotor, so that they turn as the ARE generates electricity. As the fans spin, each one draws air into a plastic housing shroud, which forces the air over the generator windings – a critical point of thermal losses if left unmanaged. The fan shrouds also act as compressors to direct and push the airflow onto the centre of the top of the engine block, near TDC. “The basic design of the forced-air cooling system was developed through CFD simulation and analysis. CFD isn’t perfect though, so we did a lot of experimenting to study the distribution of heat and airflow, as well as the peak temperatures at critical points of thermal concern, such as the spark plug seat,” Ishikawa notes. “At Honda, we produced a lot of air- cooled engines, and we had a set of development criteria for temperature limits around the spark plug seat, so that’s something we carried over to our practices at IER.” Through testing and analysis, the team gradually redesigned and optimised the number and dimensions of the cooling fins on the IER into their current form. Lubrication A wet-sump oil pan constructed from two sections sits under the block, with the oil pump installed behind the right camshaft’s gear. From the pump, oil is fed directly to the two crankshafts, as keeping the plain bearings lubricated is the first priority. A split-off from each oil line to the crankcase also leads to the camshafts. As the four shafts rotate, centrifugal force spreads the oil to the con rods, pistons, piston rings and gears. An oil filter is mounted next to February/March 2020 | Unmanned Systems Technology As multi-copters are often stable or slow-moving, each generator has a forced-air fan mounted on its top for cooling