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

41 Hirth S1218 UAV two-stroke twin | Dossier based fuel is used. That makes it a genuine multi-fuel engine, and the next development will be to use a vibration- based knock sensor, so that it isn’t even necessary to change the mapping. The one drawback with this multi- fuel approach however is that the compression ratio has to be a compromise when running on gasoline. Running on gasoline, ideally it is approaching 10:1. “For a two-stroke it makes no sense to go higher than 10.5:1,” remarks Kehe. “There is no gain in going higher; you only stress the engine more. Around 9.5:1 is about the limit for an air-cooled two-stroke; only with water cooling do we go to 10.5:1.” On the other hand, says Kehe, even at 9.5:1, “on kerosene-based fuel there would be so much knocking it wouldn’t even run”. All he will say about the compression ratio for running on kerosene-based fuel is that it is well under 9.5:1 – “somewhere between 5:1 and 9.5:1”. The engine will run satisfactorily on gasoline at this lower compression ratio but, he says, “If you optimise the compression ratio for gasoline you will gain performance, including enhanced fuel efficiency.” If an engine is to be run solely on gasoline then the base of each cylinder is machined as a means of adjusting compression ratio. Also, there are different chamber forms in the head, providing different squish areas as appropriate for the low- and high- compression versions of the engine. S1218 base engine The S1218 is a flat-twin operating as a boxer, which means its con rods are on individual crankpins phased at 180 º so that they ‘box’ together and apart again during each 360 º revolution. Since the two cylinders flank the crankcase, air is fed into the centre of it, so it follows that it has to go through the transfer ports on both sides at the same time, and that both cylinders have to fire at the same time. In theory, this configuration eliminates unbalanced forces in the reciprocating parts of the drivetrain, but having the crankpins offset from each other causes an unbalanced moment on the crankshaft. No vibration dampening is used to offset that; Kehe notes that this would not represent a useful addition of weight. A small weight penalty is introduced by the fact that each cylinder has twin plugs that fire simultaneously, simply as a flight safety measure. However, weight is saved by using a combined cylinder and head, which also overcomes the problem of head sealing. It allows each head/ cylinder to be sealed to the crankcase by means of a simple paper gasket. The S1218 has an all-aluminium structure with linerless cylinders. This is a lighter option than using an iron or steel liner and, given the use of low-expansion 4032 alloy aluminium pistons, it avoids the need to warm the engine up before launching. In some military applications, to minimise the possibility of the operator being detected, as soon as the engine starts running the UAV needs to be sent on its way. Kehe notes that two-strokes are very sensitive to back-pressure, and that exhaust design is consequently a compromise between noise suppression and back-pressure. “If the customer wants an extremely quiet engine then we can also use a separate silencer cartridge, Unmanned Systems Technology | November 2014 Weight is saved by using a combined cylinder and head, which also overcomes the problem of head sealing Engine assembly at Hirth