69 liner technologies of its CTO, Dr. Dimitrios Dardalis, to overcome some key limitations of the sleeve valve and achieve its collective vision of an ideal engine for modern UAVs, with improvements in fuel economy and maintenance. That vision is presented in the XMD250 four-stroke, spark-ignited, naturally aspirated engine, which is anticipated to weigh 25 lb or less (14.1 kg), and produce 13 kW of indicated power and 31 Nm of torque at an operating speed of 4000 rpm. Wilkinson says power may increase by 15% or more after the intake and exhaust ports have been optimised via CFD. As standard, the system has a horizontally disposed single cylinder and piston at the back, with the piston thrusting forwards into a crankcase, which features an oil pan beneath and an output shaft for directly driving a propeller in front. From MA-250 to XMD-250 Avadi’s prototypes have consistently combined rotary valving systems with single-cylinder approaches in which a piston thrusts along the same forward, longitudinal axis that the engine’s output shaft and directly driven propeller rotate in. Hence, a slightly different mechanical approach (compared with that in conventional reciprocating engines) is needed for turning combustion power into shaft horsepower to account for the unconventional angle (or lack thereof) between piston stroke and shaft rotation. The MA-250 design is operated using a self-centring, rotating piston, linked to a pair of counter-rotating, selfbalancing connecting rods. Each one moves with a scissor-like motion and drives a respective shaft, with each shaft mounting a pinion gear. Both gears would run in a circular track fixed to the housing, hence driving the piston and con rods to rotate with the vertical stroke. Mounted between the con rods on a pair of bearings (and hence directly driven by their rotational motion) was a ‘half-shaft’, serving as an output shaft for propellers, alternators or both. Additionally, a housing connected to the half-shaft and running upwards (or backwards) about the cylinder provided the valving for the system, opening and closing the cylinder’s intake and exhaust ports as it rotated. This approach was claimed to minimise side pressure on the cylinder wall from the piston, and thus reduce friction, heat and wear compared with standard arrangements of the piston, con rod and crankshaft. This is in addition to the traditional advantages of rotating valve systems, such as part minimisation and hence reduced points of failure or maintenance, compared with camshafts and their comparatively complex valve trains (consisting of lifters, pushrods, rockers, tappets, springs, valves and the like). After several years of private development in Canada, the design was brought into Avadi Engines in Washington, where iterations on several aspects (primarily on the rotating valve configuration) resulted in the MA-250 prototype. Five were built and put through 30 hours of bench testing, including third-party trials, but ultimately, Avadi determined that issues of heat and a low flow coefficient merited a new approach. Following Dardalis joining as CTO, by January 2023, the MA-250 was redesigned with his rotating cylinder Avadi Engines | Engine dossier The MA-250 design is operated using a self-centring, rotating piston, linked to a pair of counter-rotating, self-balancing connecting rods Uncrewed Systems Technology | August/September 2024 The XMD-250 is an evolution of Avadi’s previous engine design, the MA-250, in which the piston, cylinder and output shaft all rotated in unison
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