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

58 Dossier | Liquid-Piston X-Mini efficiency, we converted an X-Mini to CI and ran it at 7000 rpm, which is very high for a diesel. We used a modified diesel truck injector with just one hole in the nozzle. “Investigation with a high-speed camera shows that most fuel does hit the wall – normally a real no-no – yet even then it mixes and combusts well. We measured 90% combustion efficiency at full load; 99% at part-load, comparable to a diesel piston engine [3]. “We modelled the 70 cc engine and a 750 cc version using GT Power software. That correctly predicted 18% brake thermal efficiency on the 70 cc X-Mini and predicted 45% when scaled up to a 750 cc CI engine. That result gave DARPA the confidence to pursue the 750 cc X-4, which will fit into a 10 in box and weigh 40 lb. “The X-Mini is designed for 35 bar of cylinder pressure; using CI the X-4 is designed for 120 bar. So far, we have built a prototype without cooling, and successfully operated it at 150 bar [exceeding the objective] at a 26:1 compression ratio while naturally aspirated. “Currently, in other projects we are working with the energy group at General Atomics to hybridise the X-Mini, as well as with HDT to package the engine into a portable generator. “While we have had a lot of interest in the X-Mini from potential buyers, including those in the lawns and gardens industry, many ask questions such as, how do you start it; how responsive is it to throttle control? We have therefore packaged it into a go-kart demonstrator. People can come to our facility in Bloomfield, Connecticut, and drive it for themselves! “We plan to do something similar on a UAV platform. That will be a means of proving the life of the engine and its performance at altitude – both major concerns in the UAV market.” Shkolnik adds that a multi-rotor version of the X engine “is quite feasible”, although scaling up a single-rotor design is likely to prove more beneficial at sub- 1.5 litre displacements. Supercharging is another possibility. The X engine could develop in many ways. X-Mini gas exchange The Liquid-Piston rotor is geared 3:2 with the eccentric shaft, consequently there are 1.5 combustion events per revolution of the shaft. The shaft is supported by a bearing in each of the housing’s side plates while there are a pair of bearings between the shaft and the rotor. For the spark ignited X-Mini a throttle body holds a servo-operated butterfly throttle and a single injector. The current prototype fuel injection system is based on the Honda PGM-F1 developed for 50 cc four-stroke scooters, and uses a compact high-pressure electric pump mounted in the tank and a single solenoid-type injector controlled by the ECU. The throttle body is located ahead of a hollow section of the eccentric shaft feeding the charge into the engine. The shaft has radial drillings that communicate with a port in the base of the rotor (located between the two bearings on which the rotor runs). From there the charge is channelled through the body of the rotor to the port in its radial (working) surface and thus into the respective working chamber. At the time of writing, Liquid-Piston was investigating different ignition systems. “The use of jet fuel or JP8 calls for a CDi ignition system,” notes Shkolnik. “There is no camshaft, and without a half-speed sensor – only a full-speed sensor on the eccentric shaft – the ECU does August/September 2018 | Unmanned Systems Technology The X-Mini as applied to a UAV