INNengine Rex-B | Dossier all four cylinder heads point forward, which (ideally) positions their cooling fins in the direction of the airflow coming from the propellers and the forward motion of the aircraft. “That of course also means the hottest parts of the engine – the cylinder heads – are completely exposed to the airflow,” Juan says. “When you cycle a multi-cylinder inline, vee or boxer engine to failure, almost always the cylinder that fails is the one at the back, because any air reaching it has already been heated by the forward cylinders. “We don’t get that issue, nor even any differential in coolant air temperatures, because all four cylinders are equally exposed to the air and hence equally cooled.” Also, while some two-stroke UAV engines have their cylinders and heads manufactured together as a single part, INNengine avoids that for thermal management reasons. By keeping the head separate from the rest of the cylinder, with a standard sheet gasket between the two, thermal conductivity from the head down into the engine is minimised, confining most heat to the cylinder head where it can be dissipated by propeller downwash. While that air flows around the engine, and hence tends to cool the outer portions of the cylinder heads and cylinders more than their inner sections and walls, the transfer ports are shaped such that the charge enters the combustion chamber from the inner walls of the cylinders – that is, from the sides facing the shaft. The inner parts of the cylinders and heads are therefore cooled by the charge to offset the thermal differential between the outer and inner wall areas. Also, the portion of each transfer port where the reed valve sits is deliberately positioned close to the exhaust gas outlet to cool the surrounding metals. Conclusions Readers familiar with normal crankcase compression, crankshaft-operated engines but unfamiliar with axial engines will no doubt have questions over some aspects of the Rex-B’s performance. For one, some may feel that using a sinusoidal cam and rollers to convert linear piston strokes into shaft rotations could make Rex-B seem less inherently reliable than normal UAV engines using con rods and a crankshaft (while some might argue the opposite). In any case, INNengine claims having tested the Rex-B to a 500 hour TBO threshold, and anticipates being able to prove it out to 1500 hours between overhauls (and plans to certify the engine going forwards). This lifespan can be attributed to its mechanical simplicity and low number of parts. Granted, two-strokes are inherently simple; however, in the Rex-B for instance the cam track, output shaft and air-fuel intake chamber are all one part, rather than having a separate crankcase and crankshaft, each typically being made of multiple parts, as is typical in UAV engines. Interactions between rollers and a cam track might however present a source of wear in cam engines, particularly if loft and bounce of the rollers are not controlled for. Instead, the Rex-B’s rollers run in constant contact with the cam track – no lifting off from the crests or bouncing occurs as combustions triggered by the ECU-controlled ignition timing cause the piston’s head and body to thrust downwards as appropriate. In addition to this constant contact, the metals in both the roller and cam track are treated, lubricated and thermally managed with the aim of minimising wear, and a secondary cam and a guide plate sit above the rollers to physically block excess upward movement, such that INNengine uses the analogy of balls in a bearing to describe the relationship between the Rex-B’s rollers and its cam track. A question remains over the efficiency of the engine, particularly regarding how efficiently the cam and roller system 79 Uncrewed Systems Technology | December/January 2024 The cylinders are CNC-milled from T6 aluminium separately from the heads for thermal efficiency
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