70 Dossier | UAVHE RW1 rotary cylinders they’re placed inside, potentially thousands of hours. They even make for very good lubrication, as the molecular structure of cast iron gives better surface tension for oil than nickel silicon carbide does, as well as more desirable friction and heat distribution properties,” he adds. To UAVHE’s knowledge, no-one had previously attempted a cast iron sleeve in a rotary engine, but the company viewed the Wankel’s epitrochoid chamber as analogous to a reciprocating engine’s cylinder in certain respects; a typical Wankel uses an aluminium epitrochoid housing with a coating of nickel silicon carbide, or some other ceramic, similar to the use of aluminium and nickel silicon carbide in a piston engine cylinder. However, while cylinders may be cooler at their intake ports and hotter at their exhaust ports, Bogdanov says: “Wankels have a much bigger thermal differential, as the combustion and exhaust sections of the housing get really hot while the intake and compression regions stay relatively much, much colder. And the border between the hot and cold sections poses an incredibly heavily loaded and difficult to manage part of the engine, maybe the most stressed part overall, so it’s a classic point of failure for nickel silicon carbide-coated rotaries.” Sealing UAVHE has forgone the typical, 400- to 800-micron nickel silicon carbide coating for a cast iron sleeve with a thickness of a few millimetres. This is heavier than nickel silicon carbide, and more costly, as discussed, but as per the sleeves in UAVHE’s piston engines, they are now one of the longest-lasting components in RW1 engines. “One of the key choices in typical Wankel development is whether to make your apex seals harder than your housing liner, or vice versa,” Bogdanov says. “Most companies choose soft seals, because sooner or later they will make contact with the coating, almost definitely at that point between the hot and cold sections, so they’re choosing for the seals to wear first and to just replace them when that happens. “But our liner is a monolithic iron piece – we don’t get that expansioncontraction differential, so we don’t have to choose a soft seal. In fact, we get to choose the hardest, longest-lasting apex seal we can to enhance the engine’s time between overhauls (TBO) and the time between maintenance intervals.” UAVHE’s choice has been a tungsten carbide apex seal, which is CNC-milled in-house from commercially available end mill bits, as these feature a circular and professionally polished outer surface. This gives a curved shape to UAVHE’s seal tips, unlike conventional apex seals, which often feature a square cross-section at their tips. As they take varying angles relative to the housing wall, the rounded outer shape minimises scraping at the point of contact, and hence friction and wear between the tip and the liner, helping to extend the lifespan of both components. To date, the RW1 engines have been cycled to just over 690 hours before overhauls, but the company is confident in its projections (based on repeated disassemblies every 20-50 hours to study component degradation) that the two engines’ core components can be optimised to go for 1000 hours. “We’ve taken several units of our rotary engines apart after 600 hours and inspected them under microscopes April/May 2024 | Uncrewed Systems Technology To cool the rotor and its bearing, supercharged air (with fuel and oil) passes at high speed through the ‘lemon’ and into the rotor’s apexes, where it lubricates and cools its surroundings
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