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

38 Dossier | Hirth S1218 UAV two-stroke twin terms, if revs are higher then the tube will be shorter. The flow has to be reflected at the appropriate time. A fuel injector is located near the top of the tube and is angled, and its operation is timed such that it sends a slug of fuel into the flow of escaping burnt charge. Kehe says, “You spray the fuel into the tube as the window opens, and the fuel has most of the length down and all of the length of the tube back to become vaporised. Because of the very hot exhaust gas it vaporises very easily. You shoot back vaporised fuel into the cylinder, which is ready to ignite [mixing with the fresh charge air that has come into the cylinder through the transfer ports].” It follows that only air is pumped through the crankcase and the transfer ports, while the opening and closing of the window to the resonance tube – which introduces the fuel to the combustion chamber – is controlled solely by the piston. The timing of the injection into the tube is controlled by the engine management system (EMS) operating a regular Bosch solenoid-type injector with a fuel pressure of only 3.0 bar. “Normal direct-injected gasoline engines use high fuel pressure, which means heavy components and the complication of a high-pressure injection pump,” Kehe says. “With this system we don’t need that. Plus, with additional preparation time – our ‘pre-vapouring’ of the fuel – the droplets are smaller and the charge will burn faster. We only need the injector to meter the fuel at the right time and in the right quantity.” The fuel is timed to arrive in the combustion chamber after the exhaust port has closed, avoiding loss of fuel through scavenging. On the other hand, an apparent drawback with mixing the fuel with exhaust gas is that the resultant charge will contain a proportion of exhaust gas. Kehe however says that in fact this is an advantage, since it has the effect of reducing knock sensitivity. “Kerosene-based fuel is extremely sensitive to knock, so its effective octane rating is very low – in fact it is not rated at all,” he explains. “With our use of exhaust gas to vaporise the fuel you get a damping effect. Even if we have [uncontrolled] ignition in one area, it cannot spread so fast that you have a knock issue; it is damped by the presence of the exhaust gas.” There is still the challenge of cold starting though, which is notoriously November 2014 | Unmanned Systems Technology If we have [uncontrolled] ignition in one area, it cannot spread so fast that you have a knock issue Underside view of the Hirth S1218