Unmanned Systems Technology 021 | Robot Aviation FX450 l Imaging Sensors focus l UAVs Insight l Liquid-Piston X-Mini l Riptide l Eurosatory 2018 show report l Zipline l Electric Motors focus l ASTS show report

53 to the rotor is transferred to an eccentric shaft that provides the power take-off from the unit. To understand the basic operation of the Liquid-Piston rotary, however, anyone familiar with the Wankel type needs to be clear on certain fundamental differences. The Wankel rotor forms three rotating working chambers within its housing, each of which is fired once for each revolution it makes. The rotor carries transverse ‘apex’ seals spaced at 120 º that divide the three chambers, each of which completes the four-stroke cycle in one rotor revolution (or three shaft revolutions). Thus each chamber’s charge passes the spark plug(s) once per revolution of the rotor, and there is one shared intake port and one shared exhaust within the housing. The geometry of Liquid-Piston’s rotor and housing are fundamentally different. The Wankel has an epitrochoid-shaped housing surrounding an (approximately) triangular rotor. Liquid-Piston reverses that by having an epitrochoid-shaped rotor moving within a three-lobed (approximately) triangular housing. There are no apex seals on the rotor, but the housing does have three transverse seals spaced at 120 º attached to it, against which the rotor runs. Those stationary seals (hereafter referred to as its apex seals) define three working chambers, and it follows that the rotor housing is characterised by three ignition points spaced at 120 º intervals. Moreover, within each working chamber the housing has a recess that forms a stationary combustion chamber in the manner of that of a conventional four- stroke. The intake and exhaust is routed through the rotor, and ports are formed by windows in the rotor’s radial working surface, rather than in the housing. The Liquid-Piston rotary can therefore be likened to an ‘inside-out’ Wankel. In Liquid-Piston’s rotary engine the charge travels through the rotor to enter the given working chamber. As the rotor moves, its geometry is such that the volume of the working chamber increases, drawing in charge while the intake port is in operation. The port subsequently moves past the next apex seal to serve the next working chamber. Once the intake stroke of the working chamber is complete, the charge is compressed by another section of the rotor. Approaching TDC, the arc of the rotor matches that of the housing recess forming the combustion chamber. In fact, the volume of the combustion chamber remains approximately constant over about 20 º of rotor rotation. The rotor is therefore turning while most of the compressed charge is in the stationary combustion chamber, and it is ignited at this time. Having been expanded to power the rotor, the spent gas passes through another port in the top surface of the rotor, this one opening into an exhaust escape route through the rotor. Meanwhile, the rotor’s intake port will have charged the next working chamber and the one after that. As with the Wankel, there are therefore three ignition events per rotation of the rotor, and there is a constant flow through the shared intake and exhaust ports. The X engine is geared in such a Liquid-Piston X-Mini | Dossier Unmanned Systems Technology | August/September 2018 Liquid-Piston’s 70 cc four- stroke rotary is air-cooled Comparison of Wankel (left) and Liquid-Piston rotary operation