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68 undisclosed. Also undisclosed are the surface treatments applied to the piston skirt, crown and ring groove. All Cathcart will say is that “the piston design minimises thermal expansion, and the cylinder design [with integral head] ensures that the bore stays closer to round when hot. That means we can run a tight clearance between the skirt and cylinder, maximising heat transfer, resulting in a cooler-running piston and enhancing durability.” The single ring is chrome-plated steel with a symmetrical barrel profile. Its other design features are undisclosed. Cathcart does report though that the manufacture of the ring, the piston and the cylinder are such that ring end-gap adjustment is not required during assembly. Lubrication, cooling and exhaust Engine lubrication is total loss, with an electric oil metering pump controlled by the ECU such that its supply varies in accordance with the requirements of speed and load. The supply line from an external oil tank housing the pump serves various galleries in the crankcase, with Cathcart unwilling to disclose the oiling arrangement in detail. “There is no spray jet involved, but the key is in where the galleries release the oil as the crankcase pressure pulses above and below atmospheric,” he says. “All the lubricant ends up as an oil mist within the crankcase and, having made its way to the combustion chamber, is either burned or simply expelled as part of the scavenging process.” In the interest of noise attenuation the muffler is of a ‘multi-pass’ type, which means that the exhaust gas has a much longer exit path than appears to be the case when viewing the engine. Cathcart explains that it consists of inner and outer shells forming acoustic chambers and is “without any service items inside – there are no absorption items to get blocked”. The clever compact muffler bolts directly to the barrel, with no other support, hence no danger of thermal expansion causing cracking. “This approach is simple and robust,” Cathcart observes. All cooling is obtained from air flowing over the engine. In a pusher configuration the cooling duct intake faces the oncoming airflow with its exit just ahead of the propeller. That means the propeller pulls air through the duct, ensuring a supply even when the craft is stationary before take-off with the engine running. The duct directs air over the combined head and barrel, and its effective size is not adjusted in flight, when it benefits from the ram-air effect of the craft’s forward motion; there is an additional ram-air cooling flow to the power supply unit. The charge air filter does not have a forced supply, it breathes from ambient air within the engine compartment. Electronics The generator creates a three-phase alternating current that is rectified to dc by the power supply unit (PSU), which provides a 14 V output to the engine bus and a 28 V output to the bus serving the rest of the craft. Those outputs are separately controlled by the engine’s management system, while the PSU has the ability to operate on two-phase input should one of the generator’s phases fail. There is no battery in the engine’s power supply system; the PSU takes power from the generator as required. The mechanical load thus imposed on the engine is proportional to the electrical load, where torque is inversely proportional to engine speed. The generator is sized for the maximum requirement of the entire craft. The ECU reacts to the instantaneous demand for current from the PSU and adjusts engine power accordingly, via the fuelling or the throttle position. Unlike the throttle, the FlexDI fuelling provides scope for an instantaneous reaction to any change in loading from the generator. Developed specifically for UAV applications, the Orbital ECU operates the coil(s), the two injector solenoids, the throttle servo, the fuel pump and June/July 2016 | Unmanned Systems Technology N20 engine assembly at Orbital in Australia The compact Orbital FlexDI engine control unit