Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

60 Dossier | Insitu N20 UAV 50 cc two-stroke single existing engine used in the ScanEagle, Insitu wanted a larger displacement for greater take-off power, and for the same cruise power level to lower the engine’s operating speed. That in theory would enhance reliability and fuel efficiency, and reduce vibration and (propeller and engine) noise levels. Running slower also allows use of a larger diameter propeller, which is more efficient as well as quieter. Consequently the move was made from a 28 cc to a 50 cc displacement. Beloy says, “The weight penalty associated with that increase is marginal considering the improved specific fuel consumption provided by Orbital’s direct injection system. Moreover, the larger displacement of the N20 can provide additional climb power for heavier payloads, for more extreme applications as well as significantly more electrical power for more demanding payloads.” FlexDI Orbital calls its unique compressed-air assisted DI, which is used in conjunction with its own spark ignition and engine management systems, FlexDI. The engine management system embraces its own ECU, sensors and attendant actuators. It has had a number of different applications, starting with a two- stroke Mercury marine engine in 1996 and including high-volume applications such as two-stroke scooters made by Aprilia and Piaggio. Its reliability and effectiveness are thus well proven. There have been two- and four-stroke applications of FlexDI. Orbital’s chief technology officer Geoff Cathcart admits that over the years the advantage of FlexDI has proved greater in two-stroke than in four-stroke applications. Typically, he says, a two-stroke switching from port injection can obtain a 50% reduction in fuel consumption and a 90% reduction in emissions. He explains that in the case of a port injected two-stroke, a significant proportion of the fuel-oil mixture in the incoming charge is lost from the cylinder through the exhaust port during the scavenging process. Using FlexDI instead there is no fuel in the charge during the scavenging phase, and excess air can be used to help extract spent gas. The basic approach to injection is that of stratified charge combustion, with the incoming fuel being spray-guided. The ingress of fuel is assisted by a dedicated compressed air supply running through the delivery nozzle. The pressure of the compressed air transports the fuel into the combustion chamber, in the process creating very small droplets. Cathcart says, “Think of an aerosol spraying a very well atomised mist. The air accelerates as it expands, and that acceleration shears the fuel droplets, creating a very finely atomised mist. Compared with the 120 microns representative of port injection, our average fuel droplet size is 8 microns, which is the industry benchmark.” Cathcart notes that in the case of conventional high-pressure direct injection, the fuel is broken up by fine holes in the delivery nozzle, which doesn’t inherently create as good atomisation as FlexDI. “Rather than having an inward-opening nozzle with many small holes, our outward-opening pintle acts like a poppet valve, forming an annulus through which the fuel is sheared by the action of the compressed air. Our fine atomisation results in better vaporisation through increased surface area of the fuel,” he says. “Heavy fuel is less volatile than gasoline, so is much harder to spark- ignite. Given our droplet size, spark rather than compression ignition works fine. Owing to the atomisation we obtain, and the fact that we keep the fuel off the cold bore wall, heavy fuel can be spark ignited down to -30 C in 5 s or less without the need for a glow plug or similar. Our stratified combustion provides a wide June/July 2016 | Unmanned Systems Technology Schematic of Orbital’s FlexDI compressed- air assisted direct injection