Avadi Engines | Engine dossier opening that passes over the stator’s air inlet and exhaust outlet as the head rotates. “The transfer port’s opening is circular, and the inlet and outlet have convex edges matching the opening’s curvature, enabling rapid opening and closing of the flow area for maximum volumetric efficiency,” Dardalis explains. “However, the key question is around how we seal that tiny, thin gap between the head and the stator, preventing blow-by into the rest of the engine, because in both intake and exhaust, high-pressure gas passes through that port, and it’s going to slip into every crack and gap it can find.” One aspect of the current method is a ring seal around the opening of the transfer port, which is pushed by spring force to plug the interface between port and inlet/outlet, but Avadi is adopting another system now as the primary seal (the ring seal being a backup measure), which may be harder to machine and tune, but brings better oil control. “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. We first saw something similar in a motorcycle engine from England in the late 80s,” Dardalis explains. “It’s a spring-loaded sleeve, encircling the cylinder head, with a machined opening matching and fitting over the transfer port. Like a piston ring, it wants to open outwards, and that expansive tendency enables it to plug the gap between cylinder head and stator.” Above and below the sleeve seal’s inlet/outlet port are two oiler holes (four in total), which pass over oil-injector chambers elsewhere in the stator, with their positioning ensuring a preferential buildup of oil film specifically above and below the port. The positions of the oiler holes prevent oil from entering the transfer port, and air from passing upwards or downwards into the rest of the engine. “If a minutely thin channel or bubble of fuel air escapes into the narrow band between the upper and lower oil films, that’s fine – it doesn’t interfere with lubrication or lead to blow-by,” Dardalis affirms. “We’re also researching spiral grooves or other surface features for improving the oil control about the sleeve seal, and helping the oil to handle loads and prevent metal-on-metal contact.” As a final measure, two oil-control piston rings – one atop the outside of the cylinder head and one at the bottom – block axial blow-by flow at the top and bottom of the valve. Oiling Two main oil injectors atop the midsection directly lubricate the pinion gears and shafts, with splash lubrication covering the five SKF roller element bearings (plus the two needle bearings in the con-rod big ends). The output shaft’s two bearings are less exposed to the splashing; to compensate, a small, additional oil flow is delivered via an injector near the propeller governor. An oil pan connects to the bottom of the crankcase via a small, central aperture, where oil can enter via gravity. The pan has a slightly conical shape to encourage recovered oil to pass into a lower outlet for recirculation. Cooling fins are cut into the pan’s underside to help remove heat. The oil pump is a mechanical system standard in small engines, chain-driven off the output shaft, as Dardalis favours this simplicity and reliability over the vulnerability of an electric pump. He says: “Like in other small aircraft engines today, all our bearings are either deep-groove or angular-contact roller element devices, which lets us use a relatively low-capacity oil pump to keep them lubricated and cooled.” The rotating cylinder head has critical lubrication requirements of its own, with strict oil-regulation limits that past sleeve valve engines could not overcome. “Meeting those requirements comes down to controlling oil pressure, which is very difficult. Oil pressure changes depending on engine rpm, oil temperature, the state of wear in the engine’s bearings, the grade of oil you’re using, and other factors,” Dardalis says. 73 Uncrewed Systems Technology | August/September 2024 Gears and shafts in the engine are directly lubricated via two main oil injectors atop the mid-section
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