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

40 difficult with kerosene-based fuel. “The military likes to operate at temperatures as low as -30 C so you do need a cold- start strategy,” remarks Kehe. “For the first few cycles you have to prepare the fuel in a different way; after that you have the hot exhaust gas, and the injection performs normally. “So we have a special cold-start mechanism – that is our ‘trick’, which we haven’t patented. It is a strategy to vaporise the fuel before we put it into the engine. I wouldn’t like to reveal more than that!” In addition to fuel preparation, the duration of the spark is an important aspect of i-power engine control. The S1218 uses CD ignition, and the EMS provides control not only over the timing of the spark but also its duration, according to the burn rate of the fuel. The spark duration can be as long as 1 s if required. Using i-power, an engine can run on any fuel, with a change of mapping in the case that gasoline rather than kerosene- November 2014 | Unmanned Systems Technology Anatomy: Hirth S1218 UAV engine • Flat twin • 54 x 40 mm = 183.2 cc • Naturally aspirated two-stroke • Multi-fuel, including kerosene-based • Aluminium structure • Linerless with nickel silicon carbide coating • Two main bearings, roller • Steel crankshaft, two pins • Steel con rods • Light alloy pistons; two rings • Four transfer ports per cylinder • Two plugs per cylinder • CDI ignition • Reed valve • Compression ratio undisclosed • Maximum rpm, 6500 The S1218 is a boxer twin that has an all-aluminium construction with linerless cylinders that have a nickel silicon carbide bore coating. The heads are integral, and each cylinder is attached to the crankcase by four bolts with a paper gasket at the crankcase interface. The crankcase is split laterally (not, as might be expected, longitudinally). Beyond the front and rear sections is a nose section that extends out to the propeller. Thus does the three-section crankcase support the crankshaft and the propeller, which is mounted to the very nose of the crankshaft. Liquid sealant is used between the crankcase sections. Each side carries a roller main bearing in its wall. The crankshaft runs in just those two bearings, although it carries two pins, phased at 180 º . Each con rod, which carries needle roller bearings top and bottom, is H-section. A steel piston pin, optionally DLC coated, runs directly in a light-alloy piston, which has a full skirt to control the various ports. The exhaust is stainless steel. The upper centre section of the crankcase incorporates the common intake port, within which is located the reed valve. The EMS-controlled butterfly is located above the reed valve, while an air filter is used upstream. The EMS also controls twin-plug CD ignition and Hirth’s i-power direct injection system. Ready to run, the S1218 weighs 7.6 kg (including just 67 g for the EMS), excluding propeller assembly and an e-machine on the rear of the crankshaft. The e-machine is either a permanent magnet starter/generator or simply a generator, according to customer choice. It supplies all the electrical requirements of the craft, including the engine ignition and EMS. The wiring is according to customer requirement. The S1218 is designed to be mounted semi-stressed in the airframe, using four bolts only. These are positioned at the rear, so the engine and propeller assembly is cantilevered, and the thrust comes through the engine structure to the airframe To strip and rebuild a S1218 takes about six man-hours. Some key suppliers to this engine Head/cylinder castings: Gilardoni Crankcase casting: MH Gusstech Crankshaft: in-house Reed valve: Moto Tassinari Pistons: Mahle Rings: Mahle Piston pins: Mahle Circlips: Mahle Con rods: in-house Big-end bearings: FAG Big-end bearings: Koyo Main bearings: FAG Main bearings: Koyo Gaskets: Pilz Ignition system: undisclosed Spark plugs: NGK Fuel injectors: Bosch Engine management system: undisclosed Servo actuators: Volz Servos Throttle: in-house Oil pump: Dellorto Oil filter: Mikuni Alternator: customer-specific Exhaust: undisclosed Air filter: K&N Air filter: Pall Fuel pump: undisclosed Dynos: Schenk Dynos: Magtrol QC equipment: Mitutoyo