Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

58 Breathing, burning and control Situated within the rear cover, since the intake port is coaxial with the crankshaft, it reduces flow losses as well as providing reed and rotary valve options. The throttle body attaches to the crankcase back plate, which extends rearwards to enclose the reed housing. A flange at the back of the reed body attaches it to its housing, the assembly sandwiched between the crankcase and the throttle body. The throttle is of the butterfly type with the plate laser-welded to its shaft. The throttle body incorporates mounts for the servo operating the throttle and, in the case of the A33i, that operating the active exhaust. It also carries various sensors. Upstream of the throttle body is a velocity stack with an air filter beyond that forms the entrance for the charge air. How the air reaches the filter is airframe-specific. The transfer porting geometry is such that scavenging is of the nowadays conventional Schnuerle loop type. The piston crown has a large-radius low dome. The combustion chamber in the head has a conventional squish band parallel to the flat outer area of the crown. The single plug is on the central axis, whereas the twin plugs straddle that within a recess which is oval and which encroaches into the squish band. 10 mm NGK CM6 plugs are used – either one or two (in other words the same plug spec is used in the twin as in the single option). The ignition is an inductive system, fully controllable. There is a coil for each plug, located off the engine but close to it: the HT leads need to be as short as possible from the standpoint of avoiding EMI. A single solenoid-type injector feeds at a forward angle into the throttle body between butterfly and reed valve. Currawong adapts an off-the-shelf injector, having the ability to make and weld in place its own metering plate, which fits beneath the nozzle to vaporise the fuel. “That is critical to making an injector suitable for an engine this small,” remarks Hilbert. The fuel pump is a Currawong self- priming electrically driven piston pump. That creates up to 3.0 bar fuel pressure, and the system is return-less. The fuel passes through the ECU housing, where there is a fuel pressure sensor that allows the unit to feed back to the pump to control supply pressure. The system incorporates a bellows-type accumulator. The injector driver is integrated into the ECU, which is Currawong’s own hardware. The ECU also controls the ignition coils and the throttle and active exhaust servos. It can optionally govern a servo that operates a flap to adjust airflow into the engine cooling shroud according to real-time requirement. February/March 2017 | Unmanned Systems Technology The exhaust port valve and exhaust flap valve are linked by a control rod, which in turn is operated by a rod linked to the servo below