73 each crest spaced evenly around the circular track at 180o (and the same for the troughs). This means that, across the four cylinders (numbering them from 1 to 4 clockwise for reference purposes), cylinder 1 and 3’s pistons will be at TDC while 2 and 4’s will be at BDC, giving it a firing order that INNengine styles as (1+3) and (2+4). It also means this sequence occurs twice per revolution of the output shaft. In other words, each piston strokes four times per shaft revolution, giving this two-stroke four a total of eight combustion events per revolution. Air first enters the engine through an inlet at the back, passing initially into a chamber machined into the back of the 3D cam track, inside which the fuel injectors also sit, making this chamber analogous to a typical two-stroke crankcase, in that fuel and air meet here before being drawn into the cylinders. Transfer ports run from this intake chamber to each cylinder, allowing the top end of each (and by extension, the engine’s scavenging and exhaust) to operate as per a normal, crankcase compression, twostroke cylinder. Replacing the crankshaft of a four-cylinder two-stroke by a cam track and attendant axial configuration in this manner makes for a very compact and relatively light engine with consequently high power density. Indeed, INNengine claims to have produced the most gravimetrically power-dense UAV engine we have ever seen. At the time of writing, its fuel efficiency and durability had yet to be confirmed, although the inherent lack of piston side forces promises to lower friction and wear, as does the elimination of a conventional fourcylinder engine crankshaft. Background Arguably the scarcity of axial cam engines has been due to either insufficient technological advances in design tools and CNC machining to precisely and consistently manufacture power cams capable of smooth, low-vibration power output, or difficulty in producing them in quantities large enough for commercial or military aircraft operators. INNengine understands these problems, and has been tackling them over the past several years from its base in Spain. In addition to optimising the designs of its cam engines, the company has been manufacturing its components to meet the orders and maintenance requirements from professional vehicle manufacturers. At the time of writing, two engines were being offered by the company, each with a different design but centred on cam power. The first concepts originated from research about 8 years ago, the first result being an engine now designated the E-Rex and to be supplied to the automotive and advanced air mobility industries. Here we will focus on the spinoff UAV-oriented E-Rex. The E-Rex was invented by Juan Garrido Requena, co-founder and project manager at INNengine, who developed and prototyped the concept through the final thesis project of his bachelor’s degree in Mechanical Engineering, but he had considered engine architectures since his childhood, out of a passion for karting and later interests in two-stroke motorcycles and RC aircraft. Ruben Garrido Requena, business INNengine Rex-B | Dossier Uncrewed Systems Technology | December/January 2024 The Rex-B is a cam-driven axial engine, its cylinders parallel to and concentric around the shaft, with its rods acting on the cam via rollers (or cam followers)
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