Unmanned Systems Technology 018 | CES show report | ASV Global C-Cat 3 USV | Test centres | UUVs insight | Limbach L 275 EF | Lidar systems | Heliceo DroneBox | Composites

60 February/March 2018 | Unmanned Systems Technology Limbach L 275 EF Flat twin 66.0 x 40.0 mm = 273.7 cc Naturally aspirated two-stroke Gasoline, RON 92+ Aluminium structure Linerless with nickel silicon carbide bore coating Two main bearings, ball Steel crankshaft, two pins Steel con rods Light alloy pistons; two rings Two transfer ports per cylinder One plug per cylinder Electronic ignition Port fuel injection Engine management system 8.0:1 compression ratio Maximum rpm, 7500 The L 275 EF is a boxer twin that has an all-aluminium construction with linerless cylinders. The heads are integral, and each cylinder is attached to the crankcase by four M6 bolts while eight M6 bolts tie the two halves of the crankcase together. Each cylinder is topped by a finned heat sink attached by a pair of M5 bolts. Cast in AlSi7Mg T6, the crankcase supports a single main bearing at the rear and twin main bearings at the front. These ball bearings are 20 mm in diameter. The pair of front main bearings are 8 mm wide each while the rear bearing is 12 mm wide. An oil seal is fitted between bearing and respective crankcase bulkhead. Ahead of the crankcase, on the forged 16MnCr5 steel crankshaft is a threaded spacer ring, and then ahead of that is a flange for the timing wheel, and on a common spool with that flange, the propeller hub. Retained by a bolt at the nose of the crankshaft, that brass spool assembly is a press fit with a keyway as well. Normally, a generator is mounted in line with the crankshaft axis and to the rear of the crankcase, with the drive going through a flexible coupling attached to the back of the crankshaft. The inner section of the generator drive coupling is attached to the rear of the crankshaft via a single screw with a press fit and also a keyway. The con rod, which runs on big- and little-end needle roller bearings, is I-section steel, made from the same alloy as the crankshaft and likewise case hardened. A pair of M5 bolts attaches the fracture-split cap. The DLC-coated steel piston pin is 15 mm in diameter and 46 mm long, and is through-drilled for lightness. The circlips retaining it are round wire spring steel with ears (in conventional fashion) and are free to rotate within a semicircular groove. The piston has a full depth but not a full round skirt (the skirt is recessed where not required to operate the ports). The top ring is half-keystone while the lower ring is square section. The standard 200 mm exhaust bolts directly to the respective cylinder; it is an individual pipe open to the atmosphere. Along its route there is an axial canister containing packing material interacting with the main pipe running through it via numerous holes. This unit includes provision for an exhaust temperature sensor that sends its readings to the flight control unit. In addition to the exhaust port and a pair of transfer ports, each cylinder forms its own intake port, which sends Anatomy