78 Engine management Having a combustion event occur in one pair of cylinders with every 90o rotation of the shaft makes for a highly reactive engine, although INNengine has some tricks to monitor and manage the engine via the ECU. “We’ve used phonic wheels in the past, but we’re now using a system of two magnets, spaced 180° apart about the shaft,” Ruben explains. “A sensor linked to the ECU detects those magnets’ passes, and that makes the ECU think the engine is running twice as fast as it actually is. “That allows us to run a very normal twostroke management system in the ECU, despite the extra number of combustion events per shaft rotation that we have. It also minimises the amount of new programming we have to do, and prevents end-users from having to familiarise themselves with software they’ve had no experience with to work on our engine.” The ECU hardware is designed and assembled in-house by Manu (with a Bosch Motronic unit used for the timing and power of the CDI ignition system), embedded with an open source speed density algorithm. CM-6 spark plugs are installed, one in the centre of each cylinder head, although INNengine is designing an update for dual spark plug integration, as standard in UAV engines. Cam power The cam followers roll against the cam track for a very low friction, low vibration and low-temperature transfer of the pistons’ linear force into the rotation of the cam track and shaft. “Just as a crankshaft has kinematics, we have our own, dictated by the curvature of the cam lobes,” Juan explains. “The geometry of the cam track is calculated as a derivative function of factors like the piston stroke length and the engine speed,” he says. “That means we always end up using simple harmonic [sinusoidal] curves. When the roller reaches the top of a lobe curve, its over-acceleration reaches zero, creating the perfect, low-vibration condition for the next combustion event to push the roller down the downward slant past the apex of the curve. “Both the rollers and cam tracks are made from hardened and treated steel, and very smooth and lubricated enough to enable the rollers to run for hundreds of hours in what is essentially a curving raceway, as in a bearing. We just have some proprietary parameters in cam geometry and roller size we need to adhere to for efficient power output and engine life.” As mentioned, a second cam track is designed onto the shaft, a few centimetres forward from the main cam track. The second track is theoretically never used during normal operations, but if engine power should cut off suddenly while running at very high rpm, to the point that the inertial forces exceed the inner pressure, the second track acts as a ‘brake’, catching the cam followers to prevent the pistons crashing upwards. “It’s a very particular and unexpected situation, but we know that users are going to put engines through conditions we would never think of trying during inhouse dyno testing, so it’s good to have that to prevent damage in that extreme circumstance,” Ruben comments. A range of exhaust and muffler attachments can be used to suit different airframes and integrations. At the time of writing, INNengine was testing a couple of custom-designed systems in which each cylinder has its own exhaust pipe and manifold to prevent interference between each cylinder as it scavenges. “But while it’s common for each cylinder in a two-stroke to have its own exhaust, there aren’t many two-strokes out there in which two cylinders fire at once,” Ruben says. “So we’re working on designs with two exhaust manifolds instead of four, to see if we can reduce part count and weight while keeping the same power output.” Air cooling Although the engine is cooled passively by airflow, major design choices have been made to minimise the creation and propagation of hotspots. As mentioned, December/January 2024 | Uncrewed Systems Technology The cam track and shaft are manufactured as a single part, with a second ‘brake’ cam track above the main power cam for safety The rods’ BDCs integrate cam followers that run in and actuate the cam track
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