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60 Dossier | Ishikawa Energy Research ARE series hybrid “One of the first changes we made therefore was to raise the engine speed, from about 1800 rpm at peak power to 6800 rpm. Also, to reduce the weight we incorporated a lot of lightweight materials and alloys, with magnesium covers, and aluminium cylinders with Nikasil liners, new lightweight pistons, and we redesigned the crankshafts to be lighter,” Ishikawa notes. “In addition, we changed the valve layout. For the co-generator we had one intake valve and one for exhaust, but for the ARE we needed to increase the volumetric efficiency to draw in more air and generate more power. So the UAV engine has double the valve count.” To further improve the strength-to- weight ratio, the pistons and engine block were changed to aluminium, with the latter being manufactured in four parts by sand-moulded casting. The pistons have been manufactured by pore-free (PF) die casting, a technique prized by Ishikawa and IER. “Normally when casting, you just inject aluminium into your mould, and the liquid metal is exposed to the air,” Ishikawa explains. “Oxygen in the air reacts with the aluminium, which gives it its protective layer of aluminium oxide, but the other 79% of air is nitrogen, which doesn’t react with the metal, it just forms bubbles or pores in the component, lowering its integrity.” The team considered low-pressure casting, which involves a kind of vacuum- forming of the cast, but Ishikawa notes that this would still have left some nitrogen bubbles in the pistons. On the other hand, PF casting involves injecting 100% pure oxygen into the casting die, leaving no nitrogen to contaminate the aluminium. The result is a physically stronger part. Compression analysis A key challenge with any opposed- piston engine is achieving the desired compression ratio, given the dynamics of two pistons thrusting away from each other compared with one piston thrusting away from a stationary surface. The ARE has a 10:1 compression ratio, and considerable development time was dedicated to studying how best to control the volume and flow of oxygen to achieve that within the relatively small form factor of this engine. Ishikawa says, “We conducted a lengthy study to explore the different manufacturing approaches and tolerances we could incorporate to get the compression right. For example, we could have completely CNC-machined the combustion chamber, which would have been the best way to consistently ensure that 10:1 ratio, but it would also have been prohibitively expensive. “So we opted for a high-quality cast for cost efficiency, studying a range of casting methods to choose the right one. And we built up a huge body of geometric reference points for the casting die, with by far the highest density of points in the combustion chamber, as the dimensional, mechanical and thermal tolerances are tightest there. “Also, the compression ratio depends not just on the volume and geometry of the combustion chamber but also February/March 2020 | Unmanned Systems Technology The pistons have been manufactured using pore-free casting to prevent formation of air bubbles in the aluminium An assembly of timing gears and a chain on the back of the engine keeps the left and right cranks and camshafts in time with each other