76 Dossier | INNengine Rex-B capable of 200 bhp, it only uses that maximum power when it’s going flat out or uphill with a full trunk, which you don’t do for 8 hours straight. If you are coupling the engine with a constantrpm generator you should choose a lower power demand. The UAV’s full power is for take-off at maximum payload capacity. Cruising efficiency will be somewhere below that, and if users want higher efficiency with higher power, they can have that: they just need a bigger Rex-B, with cylinders that can breathe better.” Intake Manu explains that the Rex-B uses port injection with commonplace saturated injectors chosen for high impedance, which typically spray at about 3.5 bar. Juan adds, “We’re using standard 50:1 two-stroke pre-mix gasoline at the moment, but we’re planning an update to the Rex-B that will feature a dedicated oiling system, lossless and contained in the upper cam track where the piston’s rollers [called cam followers by INNengine] make contact with the cam. “At that point, the ratio of fuel to oil will greatly increase as only a tiny amount of oil will be needed in the fuel, just for lubricating the liner via the piston ring. “And because each piston strokes four times per shaft revolution, compression and expansion happen in half the time of usual gasoline two-strokes, meaning less time to lose compression in the cylinder, so we’ve found one piston ring is sufficient per cylinder.” Two holes are drilled into opposing sides of the cam track, so as it rotates the air passes through the holes into channels running in the outer sides of the cylinders – effectively serving as transfer ports, with three per cylinder – to each pair of cylinders. The mixing chamber is opened to each pair of cylinders’ inlet ports during their pistons’ downstrokes for effective scavenging. The guide plate for keeping the rollers aligned with the track is fixed between the cylinders and upper engine block (see Anatomy) to hold the rods in place, and has apertures for transferring the air-fuel mixture to the cylinder inlets; carbon reed valves are fixed to these apertures. The cam followers are designed with hollows that also form part of the transfer port; this contact with the air-fuel mixture means they are directly lubricated and cooled, and deposit oil against the cam tracks as they run in them. “We’ve also designed the transfer ports and sized the cylinders in such a way that there’s no dead space in the engine,” Manu explains. “Some crankcases use attachments inside to reduce dead space for faster response. Many of the Rex-B’s advantages come not from the two combustions per rev, but reducing dead space for air travel, which increases our maximum rpm, reduces vibrations, and eases cooling.” December/January 2024 | Uncrewed Systems Technology The Rex-B’s engine block is cut in two parts – a lower section called the ‘sump’ that serves as a structural base and air manifold, and an upper section analogous to a crankcase (in that it surrounds the shaft) on top of which sit the cylinders. The sump and upper section are made from 7075-T6 aluminium billet and hard anodised, then held together with the cylinders and cylinder heads by 16 small rods (four per cylinder) that are made from hardened, rectified and superfinished steel. The four cylinder heads are precision CNC-machined from aerospace-grade 6082 T6 aluminium, INNengine forgoing casting to avoid any porosities from cast parts and ensure internal control over production. The cylinders are similarly CNC-machined from T6 aluminium, before being coated internally with 70-80 µm of nickel silicon carbide that is then burnished. In future, however, the cylinders and heads will be cast by a third party for production at scale, potentially using another aluminium to maintain similar thermal conductivity in the finished products post-casting. The pistons are precision machine-cut from 7075 aluminium, and are untreated as they are not load-bearing and only experience friction at the piston ring. The pistons are cut with a truncated conical shape, and with a slightly ovular geometry, as INNengine has found that internal temperature differentials cause the pistons to expand slightly more at the crown than at the skirt. Machining them in this way therefore prevents deformation and friction at the crown over time. The crown is otherwise symmetrical, as the absence of a crankshaft means minimal second-order (side to side) movements of the pistons occur. The rods are cut from the same material, untreated and fastened via a single piston pin and circlip. The piston body is given an undisclosed treatment to endure friction from the guide plate and from the roller and cam track. While much of the particulars about the cam track and shaft component are proprietary information, INNengine does say they are cut from a hardened and springtempered steel. A nine-axis CNC machine is used ,along with selected suppression of acceleration profiles over standard CNC programs to achieve the exact curvature of the cam track. After cutting the part, it is low-temperature nitrided and superfinished to improve the hardness, corrosion resistance and smoothness of its surface. Although the cylinders and heads might be cast in the future, INNengine anticipates keeping cam track and shaft production in-house to protect its IP. Anatomy
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