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

54 Dossier | Liquid-Piston X-Mini way that the complete four-stroke cycle is executed with two shaft revolutions for each of the three chambers. It follows that the X engine completes 1.5 combustion events per revolution of the shaft, much like a three-cylinder four-stroke engine, as compared to one combustion event per shaft revolution in the case of the Wankel. As with the Wankel engine, the three power strokes for each revolution of the rotor offer high power density, while the lack of reciprocating motion of piston assembly and valves reduces the inherent vibration and the lack of complexity assists durability. In the case of a single-rotor version, the only moving elements (aside from ancillaries) are the rotor and the eccentric shaft (which carries counterbalance weights). The HEHC Liquid-Piston was founded by a father and son team after they developed a more efficient than conventional thermodynamic cycle, which they dubbed the High Efficiency Hybrid Cycle (HEHC). Founder and CEO Alexander Shkolnik reports, “My father [Nikolay] was trained in mechanical engineering in Ukraine, where he was raised, then emigrated to the USA, where he did a PhD in Physics at the University of Connecticut. “He then became a consultant in a field called TRIZ, which is a Russian acronym and in English means the theory of inventive problem-solving. In other words, it is a systematic methodology to solving problems. “My father assisted Fortune 500 companies in which engineers had become stuck on a particular problem. Without being an expert in the particular field he would be able to help the company become unstuck, giving its engineers new ideas and concepts. “TRIZ really comes down to reformulating the problem – once you really understand the core problem you are trying to address, a new answer can pop out. My father even has a patent in bread-making – one of the oldest technologies in the world!” Shkolnik, who himself has a degree in computer science and a PhD in Robotics from MIT, explains that, fascinated by thermodynamics, he and his father invented and patented the HEHC. This combines high compression ratio (CR), constant-volume (isochoric) combustion and over-expansion. A high CR is characteristic of compression ignition operation, while over-expansion is a feature of the Miller and Atkinson cycles. Constant volume combustion is not so straightforward to achieve. “With a reciprocating engine, if you could magically stop your piston at TDC and, having brought it to a halt, combust all the charge at that constant volume, you would – all other factors equal – get 50% more energy conversion compared even to a diesel cycle,” explains Shkolnik. “That is why Professor Gordon Blair, in his book, Design and Simulation of Four- Stroke Engines [1], described it as the holy grail of engine research. “In the real world, a gasoline engine has a faster combustion process than a diesel engine, so it is closer to constant volume, but the diesel exploits a higher compression ratio, which on balance makes it more efficient. The HEHC aims to combine the concept of constant-volume combustion with a high compression ratio. “Then there is a lot of pressure energy left over in the exhaust. So the HEHC continues expansion until there is no energy left – as per the Atkinson and Miller cycles. If you combine high- compression, constant-volume August/September 2018 | Unmanned Systems Technology Key elements of the X-Mini