Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

37 of the cylinder and then the transfer port windows on the other side. The ports remain open while the piston reverses direction at bottom dead centre, until its return on the compression stroke brings its skirt back up to close them again. The transfer ports allow in a fresh charge that has been pre-compressed in the crankcase, and unavoidably there is a lot of overlap with the action of the exhaust. The challenge of simultaneous charging and scavenging is aided by tuning the unsteady gas dynamics via exhaust geometry, which in effect pushes any fresh charge trying to escape back into the cylinder. It is also aided by the design of the transfer ports, which turbulates and directs the incoming charge, again to minimise overlap loss. In the case of a Hirth i-power engine, there is an extra window in the cylinder, above the height of the exhaust and transfer ports. That window opens into an aluminium resonance tube. As the piston’s descent opens it, hot and high- pressure exhaust gas rushes into it. The gas runs down the tube to the end, where it is reflected back to the cylinder. The length of the tube is a function of the rpm operating range – in simple Hirth S1218 UAV two-stroke twin | Dossier kerosene once it had warmed up. Its hot exhaust was routed around the intake to heat the kerosene to help vaporise it, so that it could be spark-ignited. Modern kerosene-based fuels on which a UAV might be asked to fly include the civilian Jet A and Jet A1 and the military JP-5 and JP-8 blends. JP-8 is similar to A1 and is the regular NATO airforce fuel, whereas JP-5 is a fuel developed in 1952 specifically for use in aircraft stationed on carriers, where the risk from fire is particularly high. JP-5 isn’t as flammable as JP-8 and A/A1; as Kehe notes, “It is almost impossible to burn JP-5 outside of an engine!” i-power A two-stroke engine has exhaust and (incoming charge) transfer ports accessed via windows in the bore wall, the opening of which are controlled by the action of the piston skirt. As the piston descends on the power stroke, it opens the exhaust port windows on one side Unmanned Systems Technology | November 2014 The key aspect of the i-power system is that it allows the engine to ignite all forms of kerosene- based fuel The S1218 powers various UAVs, including the BAT seen here taking off