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

57 Liquid-Piston X-Mini | Dossier combustion and over-expansion, then in theory, under air-standard assumptions and using CI at a CR of 18:1, you can obtain an ideal thermodynamic cycle efficiency of 74%, which is about 30% higher than comparable Otto or Diesel cycles. “Of course, you can’t achieve constant- volume combustion with a conventional piston engine; you can’t just stop the piston. Our first idea of how to obtain it was to have gas acting on a body of liquid, which is why the company is called Liquid-Piston. “That idea was inspired by the Humphrey pump, but didn’t go beyond the conceptual stage, and in 2004 we moved on to a split-cycle rotary engine. That was using a small rotor for compression, a constant volume combustor and a large volume expander. “It wasn’t until 2007 that we got some funding, from a US Army SBIR contract, and that allowed us to build the compressor. My father and a technician built that, and it worked very well. “However, the problem with a split- cycle engine is that when you transfer charge between the different operating zones you lose a lot thermodynamically. We tried various concepts and then, in 2013, we had the idea of the X engine. We built a 1.6 litre version, and from the first motoring of it we knew we had a winner. We have focused on the X engine concept ever since.” The X-Mini Shkolnik says the original 1.6 litre X was a compression ignition (CI) engine. “Certain companies in the lawns and gardens sector expressed an interest in a small spark ignited [SI] derivative for chainsaws and leaf blowers, thus the X-Mini was born in 2015. With that we went down from 1600 to 70 cc, and so sealing and heat transfer became far greater issues,” he says. The X engine also attracted the attention of the US Defense Advanced Research Projects Agency (DARPA), which was interested in a 750 cc CI version, to be known as the X-4. First though, it wanted to evaluate key technical challenges of the rotary, including the sealing and heat transfer of the X-Mv3 (Mini version 3) as well as its potential combustion efficiency as a CI engine. Given funding from DARPA, Liquid- Piston was able to address these areas in 2016. “A rotary has a lot of surface area so potentially a lot of heat transfer,” notes Shkolnik. “Will that rob the engine of the efficiency you are trying to get from HEHC given the high pressure and temperature associated with the cycle? “On a small piston engine, you would expect 30-35% heat transfer into the piston and surrounding structure; we were hoping for less than 30%. We produced an SAE paper in 2016 based on our modelling and experimental investigation [2]. “It shows that only 18% of our fuel energy goes as heat into the rotor and the walls of the working chamber. So heat transfer was under control, even without resorting to thermal barrier coatings. “Sealing was a big focus of the first DARPA programme, and we managed to reduce blow-by to 20% of its original value. To study CI combustion Unmanned Systems Technology | August/September 2018 Comparison of the spark ignited X-Mini (left) and the compression ignition X