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

60 not know where the engine is within the cycle, so to overcome that we use the wasted spark principle when using an off-the-shelf ECU.” The ECU controlling the injection and ignition is a Microsquirt. After the expansion stroke, exhaust products are routed radially through an exhaust port within the rotor and then axially through a side cover to the exhaust collector. “Direct injection is far more feasible for our engine than for the Wankel, with its moving combustion chamber,” Shkolnik says. “Combustion tends to grow like a ball, and when it hits the wall it extinguishes – you can’t make a success of it in the flattened-banana- shaped combustion chamber that the Wankel offers. “I have seen Wankel engines using eight spark plugs in an effort to obtain decent combustion. There are some Wankels where you see flames shooting out of the exhaust because they haven’t completed combustion on time. “Our combustion geometry can really be anything you want, without the constraints of a piston engine chamber that has to accommodate valves. Also, most piston engines generate swirl and tumble motion during the intake stroke, which persists until ignition as a result of flow momentum. “The kinetic energy of turbulence associated with the flow is at its peak during the intake stroke, and tends to decrease by the time ignition takes place. Our architecture fully displaces the air and fuel mixture into a chamber of any size or geometry, favouring squish as a more effective way to generate swirl and tumble just before ignition. “We have tried a symmetrical chamber, in which the air comes in from both sides and the air motion cancels itself out, and then with little motion in the chamber combustion is relatively slow. We have instead shaped the chamber like a teardrop, so that the air is now coming in at different angles. It builds on the motion, and you end up with four times the kinetic energy experienced by a typical piston engine. You get extremely rapid combustion [4]. “The optimum solution might be in the middle of those two options. We are still investigating.” Compared to a CI X engine, the SI X-Mini has a lower compression ratio – its current compression ratio of 9.5:1 reflects the use of SI and it can run on petrol as low in octane as 87 (RON+MON/2), Shkolnik says. “We have yet to experience knock on gasoline, even running 87 octane pump fuel – the lowest we can get hold of,” he adds. “We have run the X-Mini on kerosene, which is even lower in octane, and it doesn’t knock. “We have also run it on jet fuel, and it will run on JP8 without any change to the 9.5:1 compression ratio. Having said that, JP8 can be very inconsistent – we ran two batches without knock, then a third batch caused it. For that reason we may lower the CR for JP8 moving forward.” Operating with SI, the reduction in compression ratio causes a reduction in efficiency compared to CI. Nevertheless, Shkolnik observes, “The dwell in August/September 2018 | Unmanned Systems Technology The X-Mini’s eccentric shaft Liquid-Piston inventor Nikolay Shkolnik (left) and his son Alexander, who is the CEO of the company