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60 April/May 2018 | Unmanned Systems Technology UAV Factory UAV28-EFI UAV two-stroke Single cylinder 36 x 28 mm = 28.5 cc Naturally aspirated Gasoline fuel All-aluminium structure Linerless Three main bearings, ball Steel crankshaft, single pin Steel con rod Light alloy piston, one ring One spark plug Reed valve Electronic ignition Port electronic fuel injection Engine management system 10:1 compression ratio Maximum rpm, 8500 The UAV28-EFI is a single-cylinder, air-cooled two-stroke that directly drives a pusher propeller at its rear. The crankcase is formed in two portions with a split line that is perpendicular to the crankshaft axis; thus it has front and rear halves, which are joined by four bolts. The cylinder is mounted on top while to one side is the mounting for the generator, which is belt-driven by a pulley on the rear of the crankshaft, where it exits the crankcase. The crankshaft runs in three sealed all-metal main bearings, one at the front and two at the rear, to provide additional support for the propeller drive. The generator pulley hub connects the propeller drive to the rear of the crankshaft. Running at the speed of the crankshaft, the propeller shaft has a conical seating arrangement to ensure concentricity. The APC-supplied two- blade propeller is sandwiched between a back plate and the front attachment nut/serrated washer that drives it. The back plate and a spinner cone attached to it rotate with the propeller. The cylinder is attached to the crankcase via four bolts, with a conventional paper gasket between the cylinder and the crankcase. The three transfer ports are as per the supplied core engine with some subtle working by UAV Factory to maximise the quality of the passages. The single transfer on each side, plus the rear one, are angled to create a Schnuerle loop-type flow of the incoming charge. That means the gas flow is a circular loop, flowing in and across the piston crown from the transfer ports, up and around the combustion chamber and then out through the exhaust port. That avoids the need for a deflector on the piston to discourage the flow from passing directly through the exhaust, which would waste charge. Such a deflector spoils the effectiveness of the combustion chamber. In the manner of most contemporary two-strokes, the UAV28-EFI has a flat piston crown with the chamber formed as a bowl in the head surrounded by a slightly angled squish area, and with the spark plug centrally mounted. The steel crankshaft has 10 mm main bearing journals and 12 mm big- end journals. It is built up with the big- end journal being a press fit into the webs supporting it. That enables the use of a one-piece steel con rod. The I-section rod has needle roller bearings at both its big and small ends. The small end accepts an uncoated, through-drilled steel piston pin, which runs directly in its piston bores, retained by conventional round wire circlips. The pin is 10 mm in diameter and 27 mm long. The TiN-coated cast aluminium alloy piston runs in a chrome-plated cylinder bore and has just a single steel ring, which is uncoated and is of square section. The charge is admitted through a reed valve at the front of the crankcase. A throttle body housing a butterfly throttle, and below that the fuel injector, is attached to the reed housing. In turn, a filter unit attached to the throttle body admits air from the engine bay. Anatomy The UAV28-EFI prepares to power a Penguin C from launch