74 Engine dossier | Avadi Engines “In our approach, we’ve built a super-simple, positive displacement pump, which delivers a very small and fixed amount of oil per cylinder rotation very consistently. It sits in the stator and contacts the sleeve seal via a small plunger. An eccentric lobe on the sleeve drives that plunger in and out by around 0.25 mm on average, with one reciprocation per valve rotation [corresponding to two crank rotations].” Behind the plunger are channels with two check valves: one connected to the oil pump that allows oil to come in, and one that lets the oil come out. The former is regulated such that the amount of oil that comes in is physically triggered by the backwards motion of the plunger, which mechanically opens its check valve, so there is a fixed limit on how much oil can enter. “The design of this valve system is intended to help us achieve a longer TBO than most small engines, because in theory it completely prevents metal-onmetal contact between the sleeve and stator wall,” Wilkinson notes. Dardalis continues: “It is like the injection you’d see in an old diesel, which spits a small amount of diesel for each rotation. It’s just been reconfigured to spit incoming oil out of the system whenever oil pressure inside is high enough to overcome the extractor check valve. “We don’t need the super-high fuel atomisation of the old diesel-injection pumps, just oil flow in very small amounts, and then an insert in the sleeve seal along with the oiler holes diverts the oil coming in past the plunger to specifically run in the circumferences above and below the transfer port opening, instead of just coating and overrunning everything indiscriminately.” As a final measure against excessive oil around the sleeve seal and head, an oil-recovery channel in the stator lets oil escape to the pan for recirculation. The channel is thin due to the oil film being micron-thick (so total oil is minimal). While some sleeve valve engines used oil for cooling as well as sealing and lubrication, thermal loads on the XMD250’s sleeve seal are minor. The cylinder head takes most of the combustion heat, and an air-cooling approach (explained below) dissipates that. “Going forwards, either the sleeve or stator will have a hard chrome treatment,” Wilkinson notes. “And while the stator is currently steel, there’s no reason we couldn’t make it from aluminium to save around 2 lb [900 g] of weight, with nickel silicon carbide on the inner wall for hardness. We might then go with another steel for the sleeve seal, or even remove it if the nickel silicon carbide’s hydrodynamics provide for our oil control and sealing needs.” Starting up The rest of the engine’s operations and ancillaries are largely conventional. Depending on the application, the engine can start via “a typical motorcycle sprag clutch or flywheel with an electric motor [or] alternatively we have a relationship with ePropelled, who can supply a hybrid starter-generator and electric-motor configuration, which starts the engine with the motor and then converts to a generator once the engine is running”, Wilkinson says. The hybrid starter-generator of choice in the latter is ePropelled’s SG12000, a 4.3 kg system delivering up to 150 A or 14 kW, although further integrations of starter generators in series are being designed, which would achieve a combined power output exceeding 16 kW. August/September 2024 | Uncrewed Systems Technology The main engine housing is presently CNC-milled from aluminium 6160 in three parts: two main crankcase halves split lengthways (parallel with the piston stroke and output shaft), and a ‘jug’ that mounts and supports the stator. Once the engine is productionised, Avadi will probably cast the housing parts for more cost-effective, durable manufacturing. The rotating cylinder is CNC lathe-cut from steel and heat-treated, with a hard chrome plating inside (hardened steel has been used in past iterations and, as discussed, nickel silicon carbide is being studied as a future solution). The sleeve seal is cut from polished 1045 steel, while the cylinder head is CNC-milled from 7075 aluminium, with a precision finishing procedure on the outer diameter’s surface to ensure a tight fit and good hydrodynamics with the sleeve. Due to the cooling fins’ thinness, the head may be additively printed in future, but as their shape is not complex, the company will first study casting (with finishing processes afterwards) for feasibility, possibly using aluminium 7071, before going to additive printers. The two-piece stator meanwhile is cut from 4140 steel. The piston is CNC-machined aluminium for weight, while the connecting rods are machined as single pieces from hardened steel. The three rings about the piston are described as very standard components (both chromoly steel and stainless steel rings have been used), but Avadi plans on adding pins to fix them in place (as sometimes seen in two-strokes) to limit their floating and interference with oil hydrodynamics. The oil-control rings around the top and bottom of the cylinder head are iron. Down in the crankcase, the pinion gears, ring gears (at the top of the output shaft and the underside of the rotating cylinder), and shafts are all cut from heat-treated and hardened steel. Both pinion-geared shafts and the output shaft are cut as single pieces. The oil pan is currently cut from aluminium in two parts (split lengthways as the crankcase is), although it may be cast or printed from a high-temperature polymer in later versions. Anatomy
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