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

62 Dossier | Liquid-Piston X-Mini to the sealing surfaces. “You are not burning much oil, simply maintaining an oil film,” Shkolnik notes. “Our goal is for oil consumption to be on par with a four- stroke piston engine.” However, he explains that owing to its smaller size the X-Mini is currently charge- lubricated using 50:1 pre-mix. Oil goes to the apex seals, the bearings and the gear. “That works reasonably well,” he adds. “We are investigating moving to a direct lubrication of the apex seals, the bearings and the gear using a small electric pump. The X-4 will have plain bearings, with oil coming in through the shaft to the bearings and the gear, and also cooling the rotor, and then we will have oil metered in tiny amounts to the seals.” Cooling On the one hand, over-expansion reduces cylinder operating and exhaust gas temperatures; on the other, constant- volume combustion increases combustion temperatures. However, unlike the Wankel combustion zone, each of the three in the Liquid-Piston engine are subject to the cooling effect of an intake stroke on each revolution of the rotor. The X-Mini is satisfactorily cooled by air blown axially through the rotor, across numerous ribs. A fan is driven by the eccentric shaft – it is directly mounted on the shaft, and blows from six outlets. Three go into the centre of the rotor, where there are three windows. The other three go to the outside of the housing, where fins increase the surface area. Performance The X-Mini engine core weighs 1.7 kg and currently produces 2 kW (2.7 bhp) at 9000 rpm, the maximum operating speed at present. “We have so far only run the X-Mini to 9000 rpm,” Shkolnik confirms. “It is rated for 14,000 rpm, at which we hope to obtain around 5 bhp [3.73 kW]. “The next version of the engine currently in prototype manufacture will have a displacement of 79 cc total (about 65 cc trapped), and is expected to increase power to 3.5 bhp at 9000 rpm.” At 9000 rpm the current indicated mean effective pressure and friction mean effective pressure are about 5.0 and 1.0 bar respectively. The piston in a four-stroke reciprocating engine momentarily comes to rest four times per cycle as its direction of motion changes. In contrast, the moving parts in a rotary engine remain in continuous unidirectional rotational motion, offering low vibration. As the X-Mini does not have poppet valves, and the gas is fully expanded before the exhaust stroke starts, it has the potential to be quiet in operation. An apparent disadvantage of the Liquid-Piston approach to the rotary engine is that it requires three spark plugs, increasing the complexity of the ignition system, although Wankel engines often use a multi-plug approach as well. On the other hand, there is redundancy in that failure of one (or even two) spark plugs should allow a UAV to get back home rather than fall out of the sky. “With a Wankel you have a lot of blow- by from the seals and poor lubrication of them, which leads to poor durability and high emissions,” Shkolnik says. “The blow-by also leads to poor efficiency. You have very poor combustion geometry, which also leads to high emissions and again poor efficiency. Plus the Wankel uses the conventional Otto cycle. “We have a fundamentally more efficient cycle. Compared to a Wankel we also have better lubrication, lower emissions and inherently superior durability. We have solved the problems of the rotary while keeping its key advantages of high power density, simplicity, compact size, low weight and smooth running.” References 1. “Design and Simulation of Four- Stroke Engines”, Blair, Gordon P., Society of Automotive Engineers, Warrendale, PA, 1999 2. “Measurement and Prediction of Heat Transfer Losses on the XMv3 Rotary Engine”, Tiago J. Costa, Universidade do Minho, Mark Nickerson and Daniele Littera, Liquid-Piston, Jorge Martins, Universidade do Minho, Alexander Shkolnik and Nikolay Shkolnik, Liquid- Piston, Francisco Brito, Universidade do Minho. SAE International 2016 DOI 10.4271/2016-32-0033 3. https://vimeo.com/172430180 4. http://liquidpiston.com/wp-content/ uploads/2017/09/Measurement-and- Prediciton-of-Heat-Transfer-Losses-on- the-XMv3-Rotary-Engine.pdf 5. https://www.sae.org/publications/ technical-papers/content/ 2018-01-0372/ August/September 2018 | Unmanned Systems Technology Overhead view of the X-Mini