72 also the roller bearing near the bottom of the cylinder – reduces the impact on it of very high thrust loads from combustion,” Dardalis says. “And keeping the head and cylinder integral, and rotating together, means we avoid the issue of sealing and oiling between them – we bond the two parts together with a conventional static seal between them. “But, importantly, there’s nothing special about the crankshaft-gear system, other than that there’s two counter-rotating cranks. A 2:1 gear reduction inbuilt to the design enables direct drive of propellers, as the 3500-4000 rpm of the engine drops to 1750-2000 rpm for a prop.” In addition to Dardalis’ extensive use of CFD via Converge and Matlab to optimise the XMD-250 design, Avadi has invested heavily in its in-house CNC machining capabilities for rapid prototyping and control over adjustments where necessary. “New valve systems tend to be difficult to get right in new engines, so if we succeed with the valve design, we anticipate the remaining optimisations shouldn’t be overly difficult or complex,” Wilkinson says. Rotary ups and downs General advantages of well-optimised rotary valve designs over poppet valves are well-established in literature, and Dardalis points to several examples. For one, the rate at which they open and close can be higher than that achievable with poppets, improving volumetric efficiencies at high rpms while retaining good torque at lower speeds. Fuel and air get more direct intake flow paths without needing to travel around the intake valve seat, increasing flow coefficients and volumetric efficiencies at high speed. The absence of a hot exhaust valve above the piston increases the engine’s tolerance of possible detonations, and allows for tighter compression and a higher compression ratio. “The XMD-250 has similar operating principles to RCV Engines’ original designs. They also once used a rotating cylinder, with ports in the cylinder side walls, and all the advantages that we’ve discussed from that approach above. Only relatively recently did they move the valving to the cylinder heads,” Dardalis notes. “Regarding TBOs, the cylinder rotation should minimise ring and bore wear, and once the rotary valve is refined it too may have very low wear. Furthermore, if the intake air is kept clean or well filtered, and the oil frequently changed, we may have a TBO multiple times the standard in UAV engines.” The disadvantages of rotary valves relate largely to sealing and oil-control challenges. A closer look at the XMD-250 reveals how Avadi resolves such issues. Sealing The transfer port inside the cylinder head runs upwards from approximately TDC, offset by roughly 45° relative to the angle of the stroke, to form a side August/September 2024 | Uncrewed Systems Technology The cylinder head (left) rotates in a stator (right) to open and close the intake and exhaust ports to the combustion chamber It’s an idea called a sleeve seal, which didn’t exist in the World War II heyday of sleeve valve engines, nor did their designs need one
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