USE Network launch I UAV Works VALAQ l Cable harnesses l USVs insight l Xponential 2020 update l MARIN AUV l Suter Industries TOA 288 l Vitirover l AI systems l Vtrus ABI

74 directly onto the bottom end of the piston to cool it down, but in a two-stroke you don’t have that,” Kehe says. He also points to a growing trend among operators to run UAV engines too lean, which produces excessive heat, often damaging the pistons. The team therefore wanted piston materials and components that would withstand very high heat levels over their lifetime. “We came up with the project targets for the piston components internally, and these included some incredibly important requirements,” Kehe recalls. “For instance, we wanted half-keystone piston rings, not rectangular rings. The latter are widely supplied and easy enough to obtain, but the former are much harder to find and difficult to get in the sizes we were looking for. “It sounds like a small detail, but it’s so important. When you’re running an engine near the top of its power output band for a long time, the two-stroke oil can cause the rings to start locking up and become stuck in the cylinder – you’ll quickly ruin the engine that way. That’s why we chose to use two half-keystone sealing rings per piston, to keep the piston and cylinder clean over the long term.” The piston itself is hypereutectic, made from a high-temperature alloy of silicon and aluminium, containing what Kehe describes as a high percentage of silicon. It’s a rare material which the team found challenging to source, but it is key to ensuring the piston’s longevity versus repeated thermal expansion and contraction. “It isn’t a standard piston; it too June/July 2020 | Unmanned Systems Technology Dossier | Suter Industries TOA 288 The engine block is CNC-machined from hardened and tempered aluminium in two parts with a vertical split down the middle, with a silicon- based liquid gasket and six fasteners bonding the two halves together. The crankshaft is CNC-machined as three parts from hardened and tempered high-strength steel. The forward section includes a power take-off of 25 mm diameter with a 1:10 taper for mounting propellers (and additional generators), and joins to the crankcase via a ball roller bearing. The back section is held in the crankcase by a needle bearing. The starter/generator is mounted on the rearmost part and housed in a separate chamber from the crankcase, while a circular crank web holds the front and back shaft sections together by socketing their crank pins. The con rods are made from the same material as the crankshaft, and are coated in copper. They attach to the crank pins and piston pins using needle bearings in their big and small ends, with a tighter grip at the big end between the crank journals to enable a greater range of axial motion at the piston pin (and thus less lateral friction between the pistons and cylinder liners). Each of the two pistons is made from an undisclosed silicon-aluminium alloy for high thermal resistance, given a coating of an undisclosed nature and fitted with two half-keystone compression rings. The cylinders are cast from hardened and tempered aluminium, then water-cut to exact measurements and given a Nikasil coating. Four corner bolts fasten each cylinder to its respective side of the engine block. Each of the two cylinder heads is bolted on by four fasteners and CNC-manufactured from hardened, tempered aluminium, with fins for cooling and inserts for the spark plug and cylinder head temperature sensor. All fasteners used in the engine are titanium, a selection made for the material’s light weight and corrosion resistance. Anatomy Exploded view of the TOA 288’s crankcase