74 Dossier | INNengine Rex-B director at INNengine, recounts, “Even when we were 9 or 10 years old, we were already taking engines apart and playing around with them, and by the age of 12 Ruben had built his own kart but was more interested in making a faster and more efficient engine.” After a series of iterations and arriving at the architecture described above, Juan continued optimising the E-Rex design for another 8 years or so, until he and Manuel Luna Manu (mechanical and electronics engineer at INNengine) felt the time was right to spin-off a version of the engine for RC aircraft. That soon turned into a version called the Rex-B, a 125 cc two-stroke axial engine measuring 21 cm long, 15 cm in height and diameter, and 4.6 kg in weight, producing 21.5 bhp (16 kW) at 6000 rpm. As the design is scalable, however, INNengine has also briefly shown a 288 cc version 27 cm long, 19 cm tall and wide, and weighing 9 kg. That version develops 55 bhp at 6000 rpm, and the company plans to integrate a 20 kW motor/generator into it. Testing and performance “The Rex-B was originally meant just for RC aircraft, but we soon discovered that the engine needed a large displacement to be worth it,” Ruben explains. “Also, the axial shape of the engine means a linear factor increase in its total size and weight gives a quadratic order increase in displacement and hence power output. “So making 20-40 cc engines for RC made no sense. In fact, 125 cc is the smallest we feel the Rex-B should be for it to still be a significant value proposition for small aviation craft, but we realise that’s a bit too big for RC.” However, customers from the UAV manufacturing and integration sector heard about the engine and began approaching INNengine, to the point that the company paused its plans to promote it for general RC and is now focused on optimising it for professional UAVs. The first designs of the Rex-B were simulated and validated in CAD, using dynamometer and other test bench data on the E-Rex as a foundation. “Then we built a prototype and a test rig for it, and cycled it against simultaneous tests of a DLE Engines model with identical propellers on both to compare and map power and torque over speed,” Manu adds. “And although we’re now ready for manufacturing with the second Rex-B prototype we also want to find a professional dyno small enough to run further tests on it. “We’ve also validated both the 125 cc and 288 cc versions to the point that we’re satisfied that larger versions will scale well. Given that more than doubling the smaller Rex-B’s displacement meant just a 2 cm increase in radius, quite large UAVs could run on it. It has a mechanical simplicity thanks to its two-stroke cylinder configurations that we feel the UAV industry will want more than the E-Rex’s complexity [being an opposedpiston, uniflow-scavenged and far heavier engine].” Tests have confirmed that the engine can continue running if power from up to two cylinders is cut, with just one opposed pair of cylinders or just one from either pair still working. “We’re still mapping the fuel consumption, but we are finding better SFCs at lower rpm than 6000 rpm; we’re having to explain that to many prospective customers,” Ruben muses. “All of them want the maximum performance with the maximum efficiency, but like a roadcar December/January 2024 | Uncrewed Systems Technology As each piston strokes four times per shaft revolution, one piston ring is enough for sufficient oiling and compression of the cylinders The Rex-B’s rods run in a guide plate that also has apertures for transferring charge, and points for mounting carbon reed valves
RkJQdWJsaXNoZXIy MjI2Mzk4