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71 Woelfle Engineering/Aixro XF40, XH40 and XP40 | Dossier cold conditions. If it’s too narrow then of course you risk blow-by.” The design of the two-piece apex seal, which consists of a long ceramic segment with a much shorter iron segment at the other end, enables those segments to ‘float’ to an extent, to compensate for thermal or other deviations, Woelfle Jnr says. “We use ceramic for the longer piece and iron for the shorter piece, because what you use for the shorter piece doesn’t make much difference in terms of friction or shape stability. The iron segment acts as a guide piece that can run in and adjust a little if required. “Mazda used ceramic apex seals on its Le Mans-winning engine, and they were of huge benefit there. Nevertheless, these days you don’t see ceramic seals as often as you might expect, given their strength and low friction, and the benefits they confer for long-term engine health. There is a significant cost factor to consider, which can make it tough to integrate ceramic seals on a production engine. “The other thing is that the material really has to be matched to the design, or vice versa. For example, the Mazda RX-8 engine’s apex seals are only 2 mm thick, so if you got a bending force on that material it could crack. Ours are 3 mm thick, and that extra millimetre helps a lot in terms of stability.” He notes that some Wankel developers have tried incorporating ceramic apex seals, only to find a very marginal gain in performance (if any), leading them to take a different approach to sealing. Indeed, he attests, the real power benefits of ceramic seals in a new engine are only apparent when running upwards of 8000 rpm, which is of no benefit to the Aixro UAV engine. “However, that is only true when the engine is new,” he explains. “Over hundreds of hours of the apex seals being exposed to the exhaust flame, the tip of the iron [or other non-ceramic] seals will often deform thermally and become convex, such that the middle of the tip sticks out and will wear away. “Since the middle of the tip gets worn away, as the seal cools and the deformation is reversed, the tip shape will go concave. That creates a sort of running ‘pocket’ along the seal tip, which allows leakage of gases across the middle, except when the engine is running hot and the seal tip re-expands outwards. “Some would say that is a minor problem – and we do say we try to go for the simpler solution wherever possible – but for us, proper sealing is really important, especially since our engines are going to be flying for hundreds of hours, potentially over inhabited areas. Our ceramic seals will never suffer that kind of wear or deformation; they are robust enough for safe aviation power output in pretty much any operating environment.” Aside from the use of ceramic apex seals, the Aixro UAV’s package of apex and side seals is conventional, and Woelfle Jnr remarks, “Some competitors with similar engines buy seals from us.” Combustion The Aixro combustion chamber is fed a gasoline-air mixture by a combination of side port and peripheral port. As we will investigate in the cooling section, the latter operates only at over 70% throttle and receives denser air than the former due to the engine’s rotor cooling system. Each combustion pocket in the rotor takes the form of a tub on the leading side, plus a narrower, shallower section through to the trailing end. “This shape is useful because you get very high air velocities around TDC; beyond TDC you have the narrowing of the chamber,” Woelfle Jnr says, explaining that the air from the trailing side will receive a little less flame propagation and that a better concentration of the fuel-air mixture and a more complete burn is accomplished with this asymmetric shape, which offers a compression ratio of just under 10:1. In keeping with a pragmatic design approach, as standard the Aixro UAV has a single Dell’Orto carburettor and a single spark plug. Woelfle Jnr says, “In most load conditions the engine achieves a specific fuel consumption [SFC] of about 350-370 g/kWh. We have run electronic fuel injection [EFI] on prototypes and it improves the SFC by a small amount, but operationally we’ve seen no major advantage. “EFI means adding 20-30 wires and several extra sensors – all potential points of failure. So, maybe you add two of everything for redundancy, then how much extra weight have you added, how much extra space have you used up, all so you can carry a bit less fuel? “Carburettors aren’t perceived to be as modern as EFI but they’re generally quite error-proof and don’t need redundancy. Wankels are inherently insensitive to fuel quality – as long as they don’t run too lean, which raises internal temperatures – so what few errors there might have been have never posed an issue.” The ignition system is digital, a PVL electronic module operating the single coil and spark plug – “typically a Denso U22ETR”, says Woelfle Jnr. This Unmanned Systems Technology | October/November 2020 Specifications Aixro UAV Wankel Single rotor 294 cc Naturally aspirated Liquid- and charge-cooled Gasoline premix Aluminium structure Nickel silicon carbide coated working surface Needle rotor bearing Steel eccentric shaft Iron rotor Single carburettor Single spark plug Digital ignition Compression ratio, about 10:1 Maximum rpm, 7000 Datasheet