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

62 operating range and is very tolerant of changing conditions.” Cathcart reports that the robustness of FlexDI is such that engine calibration doesn’t have to be individually tailored to each example of a production engine batch. That means there is no requirement for any end-of-line tuning. Kerosene-based heavy fuel is low in octane rating, which means it has a propensity to knock; using FlexDI, the fuelling can be tailored to avoid knocking. In effect, the amount of time the fuel spends in hot chamber conditions can be reduced by delaying the injection event. This ability to avoid knock enables the use of a compression ratio in excess of 10:1, which in turn provides improved fuel efficiency. Beloy notes that the fuel efficiency advantage of the N20 is seen most prominently when the craft is in cruise and loitering modes. He adds, “A further advantage of FlexDI is that since the engine can maintain robust, reliable combustion across a relatively wide range of cylinder temperatures, an active cooling system is not required and wider temperature limits are established, affording users the ability to operate the rest of the system without the need to constantly monitor engine temperature.” This, Beloy notes, is particularly significant in the case of a UAV, which typically experiences a wide temperature variation between ground level and flight altitude. N20 FlexDI fuelling system The N20 draws air into its crankcase through an air filter, a throttle body with a butterfly throttle and a reed valve (in that order). Cathcart notes that intake tuning revolves around the length of the throttle body and its interaction with the air filter. However, he adds that “most tuning is on the exhaust side”, as with any two-stroke that exploits pressure waves to enhance cylinder scavenging and thus cylinder filling. The relative simplicity of a reed valve compared with a rotary is seen as advantageous in terms of saving weight. At the same time, its inherently superior response at lower engine speeds is seen as the basis of a flatter torque curve, which is considered more important than any peak power gain that could be made by using a rotary valve. The exhaust port is to the right-hand side of the cylinder, while the transfer ports are adjacent, four in total – two main outer ports sandwiching a pair of secondary ones. The transfer ports are arranged to create loop scavenging. Avoiding the need for a piston crown deflector, loop scavenging directs the incoming charge air towards the combustion chamber such that it follows its curvature and is deflected downwards again. Thus the air makes a loop that discourages it from escaping through the exhaust port but helps push out the exhaust gas and at the same time creates a swirling turbulence that enhances combustion. The combustion chamber in the N20 head is a recess with a tapered squish band as its extremity. The form of the piston crown is undisclosed. The fuel injector is vertical and on the central axis of the cylinder, while the plug is angled at 45 º and is located above the exhaust port. Both the injector and the plug enter the chamber in the recess, which can accept either one or two plugs. The FlexDI fuel injector has twin, independently controlled solenoids. An upper solenoid meters the fuel supply into the chamber where the compressed air is admitted; a lower solenoid controls the outward-opening pintle that admits fuel to the combustion chamber. The air can only flow when the pintle opens and, as noted, it carries the fuel into the chamber. Beloy remarks that a lot of development has gone into fuel delivery, in ensuring that pick-up in the fuel tank works down to a very low level despite the g-forces to which a lightweight UAV is subject in flight. This has meant paying close attention to the control of slosh in the tank and to the pick-up position. It has also led to the use of a header tank inside the main tank. Fuel is supplied by an electric pump submerged in the header tank to a pressure regulator in the fuel rail assembly atop the N20. The pressure regulator has a spring-loaded diaphragm that lifts to return excess fuel to June/July 2016 | Unmanned Systems Technology Dossier | Insitu N20 UAV 50 cc two-stroke single The N20 two-stroke has air cooling of its single cylinder