Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

72 Dossier | Strange Development REVolution company wanted to be able to use as little oil as possible, and eliminate even the marginal oil consumption levels that four-strokes are prone to. As discussed, the REV forms one key part of the cylinder airflow management. The piston forms the other, and has thus received almost as much attention. The crown is coated using a ceramic, to protect its exhaust side from being damaged by the high-velocity, superheated gases that rush out of the exhaust port as it opens. Krzeminski notes such damage is a perennial issue in the two-strokes he has observed over the years, and that Strange Development is also considering coating the valve in a ceramic to provide similar protection. The piston skirts are coated in molybdenum disulphide (or ‘moly’) for long-term durability and low friction during their movements against the cylinder walls, as well as being able to survive the extreme heat of the cylinder. Each piston pin has three trapezoidal piston rings, a shape that allows enough room for the rings to float stably in their grooves. The bottom ring sits at the end of the moly coating, preventing intake and exhaust gases from entering the crankcase, and keeping the crankcase’s oil out of the cylinder’s airstream. “We want to stop excess oil flow from touching the moly, because that would contribute to emissions, smoke output and other problems,” Krzeminski explains. Two compression rings sit near the piston crown, with a holding pin in each one to prevent them from getting caught on either port as they float. Each pin rides on a needle bearing in its single- piece, non-split con rod’s small end, with another needle bearing in the big end around the crank pin. The crankshaft separates into three sections at the two crank pins. In addition to press-fitting the shaft pieces together, customised holding pins are routed through the crank throws and pins to stop rotational differentials occurring between the sections, otherwise they might start to spin at different speeds near maximum power output. Each con rod sits between two counterweights, and in the centre of the crank is a gear for driving the water pump, which sits at the bottom of the crankcase, perpendicular to the crankshaft. That propels water up through the cylinder jackets, above and below the rotary valve seats, and up to the cylinder heads. On either side of the water pump gear is a seal for separating the undersides August/September 2020 | Unmanned Systems Technology A system of reed valves is installed under the engine – not for intake but to push excess air and oil out from the crankcase, to recover the oil and prevent drag-induced losses The aluminium cylinder head is designed as two plates that form an internal cooling jacket between them