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67 into what others had already done; what worked and what didn’t. On this basis a concept was made to suit the kart application, and parts were designed accordingly. The first dyno runs of the 294 cc kart rotary were carried out in the winter of 2000. After three prototypes the design went through a review loop and then an initial batch of 20 kart engines was built and sold.” Woelfle then teamed up with Aixro, in Aachen, Germany, to start engine production. Distributed by Woelfle Engineering, of Krefeld, Germany, and called the Aixro XR50, this engine develops 44 bhp (33 kW) at 8750 rpm yet weighs only 17 kg. With a maximum eccentric shaft speed of 10,500 rpm and power band width and response well- suited to karting, it proved popular at home and abroad. Having thus proven their Wankel, in 2006 Woelfle Engineering and Aixro turned their attention to the emerging UAV market. The arrangement with Aixro for manufacture continues to this day, and technical developments are still cooperatively handled, Woelfle Jnr says. The UAV version Compared to an equivalent gasoline- fuelled naturally aspirated reciprocating engine, a Wankel offers certain inherent advantages. The speed of operation of a Wankel’s rotor relative to that of its output shaft is such that the main moving parts move more slowly than their counterparts in a reciprocating engine. Mechanical stress is lower, rotating parts are in dynamic balance and the Wankel has also the advantages that power is not wasted operating a valvetrain or to crankcase windage. The way the charge is whipped around by the rotor and the fact that it is always moving arguably aids combustion. Moreover, an appropriate shape of combustion chamber and the turbulence induced by the rotor combine to prevent the formation of localised hotspots, keeping the engine free from the dangers of detonation and pre-ignition. Fuel quality is therefore less of an issue than for an equivalent reciprocating engine. On the other hand though, the long chamber is not in the best interest of flame propagation, and combustion can be incomplete, releasing unburned hydrocarbons into the exhaust. Woelfle Engineering/Aixro XF40, XH40 and XP40 | Dossier The key was lots of research into what others had already done. On this basis a concept was made to suit the kart application Unmanned Systems Technology | October/November 2020 The Aixro kart and subsequent UAV engines are fundamentally the same, save for different charge- cooling bypass controls and power outputs The first XH40 was developed and flown in 2008 for an agricultural helicopter UAV in South Korea (Courtesy of SungWoo Engineering)