Unmanned Systems Technology 036

75 Hirth 3507-01M | Dossier updated for the changes to the exhaust and transfer ports unique to the 3507. The open deck extends from the base of the cylinders, approximately where they meet the crankcase, allowing the water-glycol to flow up through the walls and into the cylinder heads. Naturally the key areas of thermal concern – and thus the densest concentrations of the water-glycol flow – are around the cylinder heads and the piston’s outlet side. It follows that the first point of entry for the water-glycol is directly beneath the exhaust ports, thus cooling the hotter side of the cylinder first to prevent the aluminium suffering heat stress. The heated coolant from the engine exits via ports on the cylinder heads, running to a multi-input/output filler pipe. This distributes the coolant to and from a reservoir, and to a second water pump for delivering liquid to the radiator for cooling. Also, the coolant flowing to the compressor enters just above its piston’s TDC, flowing up through its cylinder and exiting through a pair of output hoses at its top. Both of those then flow to the filler pipe for redistribution. Control The ECU controls the ignition and injection functions using an Alpha-N strategy, as with most of Hirth’s engines (including the 4201 and 4202). Readings for this strategy are delivered from the TPS and a crankshaft position sensor installed by the engine’s flywheel. Other key functions mapped and controlled via the ECU include altitude compensation, using an air temperature sensor and air pressure sensor to detect drops in either of them, as well as coolant temperature compensation, Unmanned Systems Technology | February/March 2021 Five ball bearings support the length of the crankshaft – two on the frontal PTO section, two at the central journal, and one on the rear accessory section. This latter section is also where the starter/generator and compressor drive pulley are mounted. The con rods are non-split, single- piece parts (as is common among UAV engines, and as typically follows from having a multi-piece crankshaft). They are CNC-machined from 16MnCr5 case-hardened steel, and are coated in copper to prevent the formation of deposits from fuel or oil. Held within both the big end and small end of each con rod is a needle bearing, for mounting onto the crank pin and piston pin. Each piston is forged from a standard aluminium silicon alloy by Vertex. It is the same technique, material and supplier used for the pistons in Hirth’s 35 Series, and is designed and produced with a dome-shaped crown. Each piston has two half-keystone compression rings, with this cross- section ensuring they do not become blocked or stuck by carbon deposits. The piston pins are made from case- hardened steel; they measure 52 mm long and 18 mm in diameter, and are held in place with round wire circlips. Specifications 3507-01M Two-stroke inline two-cylinder Naturally aspirated Direct injection Liquid-cooled Fuel: Jet A-1, JP-4, JP-5, JP-8 Weight: 38 kg Dimensions: 543 x 442 x 380 mm Total displacement: 625 cc Maximum power: 41.5 kW Maximum speed: 6600 rpm Compression ratio: 9.5:1 (static), 6.9:1 (dynamic) TBO: 250 hours Man-hours to strip and rebuild: 36-40 (estimated) Some key suppliers Heads: Aluminiumgiesserei Hanke Block/crankcase: Grabert Aluformguss Liners/bore coating: Gilardoni Auxiliary drives: Acutronic (formerly Sullivan UV) Pistons: Vertex Rings: Vertex  Piston pins: Vertex  Circlips: Vertex  Con rods: Meidlinger Main bearings: SLF Fraureuth Seals: Schwarz Fasteners: DIN-standard parts Gaskets: Athena Ignition system: EVS Engine management system: Microtec Sensors: TMG Throttle servo: Volz  Water pumps: Sandtler Oil pumps: Dellorto Oil filter: SF Motorsport Technik  Alternator: Acutronic (formerly Sullivan UV) Exhaust: Tusche Maschinenbau Air filter: A&B Filters Fluid lines: ProLiquid Wiring loom: Looms Fuel pump: HP-Tech Surface treatments: Osswald Aluminiumverdedelun Dynos: Schenk