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

42 Dossier | Hirth S1218 UAV two-stroke twin but packaging is always a problem.” A catalytic converter doesn’t come into the equation. Given the associated temperature level, there would be too great a risk of fire. A single central reed valve-controlled inlet feeds the charge to both cylinders via the crankcase. Each cylinder is then fed via four transfer ports. “With our 6500 rpm maximum operating speed there is no need to go for more than four transfer ports,” explains Kehe. Since the crankshaft carries roller (main) and needle (big-end) bearings there is no need for a pressurised lubrication system. Oil is metered into the charge air in the intake manifold and travels with the flow to lubricate both the bottom and the top end. An electric solenoid-type oil pump controlled by the EMS meters fuel into the incoming charge air one small shot at a time. The pump is pulsed at a frequency that corresponds with the engine speed and load. Each piston has a crown that is only very slightly domed, while the squish area formed by the chamber in the head is considered crucial in terms of avoiding knock. Since there are no valves in a two- stroke cylinder head a large squish area can be provided. “You have to be careful – if you overdo it, you get knocking. If you make it too ‘soft’ then you don’t get any benefit from it,” Kehe says. The light-alloy piston carries two identical steel compression rings which have the plain working surface “recommended by Mahle for use with its Nikasil-type bore coating,” according to Kehe. They have a 1.2 mm tall working face and taper back within a groove that has a matching taper. Essentially the floor is flat while the roof tapers down towards the inside. This influences the ‘angle of attack’ of the working face in the dynamic situation. Doubling the number of rings clearly increases ring-generated friction but it helps with wear. It is the rings rather than the bore where most wear occurs, so the twin-ring solution tends to extend maintenance intervals. Engine operation Hirth’s own EMS runs the injection and ignition, and also controls the fly-by-wire throttle (a butterfly, the opening of which is servo-operated). Engine control is closed loop, the EMS reacting to mission parameter requests from the autopilot and to various engine sensors to obtain the requested rpm (propeller speed) at any given time while keeping the engine within safe operating conditions. There is no lambda sensing but the temperature of the exhaust exiting each cylinder is accurately measured using a thermal probe. “That is a key engine sensor on a two-stroke,” remarks Kehe. “We also read crankshaft position, throttle position, intake air temperature and pressure above the throttle, cylinder head temperature and other [undisclosed] temperature readings.” If necessary the engine can run in ‘limp home’ mode without any input to the EMS aside from crank position. The normal operating engine speed range is 3500-6500 rpm, and the engine is designed to run no faster than that because of the implicit increase in wear and noise. That 6500 rpm limit also assists the challenge of running on kerosene-based fuel, and allows many customers to mount a propeller directly on the nose of the crankshaft. Some UAVs using this engine have the propeller driven via reduction gearing instead, in order to spin a bigger propeller November 2014 | Unmanned Systems Technology Hirth makes a range of two- stroke UAV engines, some of which are water-cooled, such as this inline twin, the S1214 There is no lambda sensor but the exhaust-exit temperature is measured using a thermal probe