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

54 integration. Twin spark plugs have also been introduced, although a single plug remains an option. The A33’s original displacement was 38 cc. “We downsized it because by using active exhaust technology we found we could obtain sufficient power for potential applications from 33 cc, with greater efficiency given the 12% smaller displacement,” Hilbert says. “On the intake side we went from a conventional reed valve with a single petal to a multi-petal design, taking advantage of our axial inlet [relative to the crankshaft axis]. We could use a rotary valve but we wouldn’t be interested in one with fixed timing. However, there are interesting things you can do in the rotary valve ‘windowing’ system to vary the timing, and that is something we will investigate in the future, with Currawong Engineering being the lead on those mechanisms.” The porting went from five transfer ports to three. Cobra motorcycle engines typically have two transfers each side plus one booster at the back, but that initial approach was changed to just one each side. “This engine isn’t as highly strung as our race engines, and doesn’t have the requirement for as much transfer port area,” Hilbert explains. “With a primary and a secondary transfer on each side you can aim them differently to manipulate scavenging, but we didn’t find the need for that with this engine. We could therefore simplify the porting.” The A33’s 250 W continuous (500 W intermittent duty) generator is a tubular unit located parallel to the crankshaft axis and outrigged from the adjacent crankcase wall. It is driven off the crankshaft’s nose pulley by a toothed composite belt. Thought was given to incorporating a starter/generator into the front of the crankshaft. In the end the ‘modular’ approach won out on the basis that it is easier to accommodate differing customer requirements and keep up with progress in the technology of this key item. It also eases maintenance. While using a single plug is a simpler, slightly lighter and more affordable option than twin plugs, the latter option provides more than redundancy. The use of a second spark plug decreases the burn duration, and in turn that provides an efficiency gain of about 3-5%. “We can advance ignition timing by as much as 10 º with the dual plug, but the spiky loads on the cranktrain go up dramatically,” remarks Hilbert. “So we get more efficiency and more power but we’ve had to develop the cranktrain around that increase in peak loading.” Cranktrain The crankshaft is high-tension steel, machined from a forging with the journal surfaces superfinished; it is not nitrided. There are a number of reasons why the crankpin is cantilevered. As well as providing more flexibility over intake architecture, it allows the crankcase to be made in one piece, aside from a rear plate, to structural advantage. It also allows the crankshaft to have as integral the throw and the pin, to its structural advantage. Another advantage is the scope for pre-stressing the crankshaft. Between the two main bearings is a spacer, upon which tension is drawn when the relatively large nut at the front is tightened. Hilbert explains, “What that does, with prop loading and with engine loading, is it pre-stresses the output spar to the point where it won’t ever experience a load February/March 2017 | Unmanned Systems Technology Cobra A33i Cobra UAV A33i Spark-ignited two-stroke Single cylinder 39.0 x 28.0 mm = 33.45 cc Naturally aspirated Pump gasoline Aluminium structure Linerless, nickel silicon carbide coated bore Two main bearings, undisclosed type Steel crankshaft, one pin Steel con rod Reed valve Light alloy piston; one ring Twin plug electronic ignition Electronic port injection Engine management system 8.5:1 compression ratio Maximum rpm, 9000 Data sheet Head, barrel and crankcase: in-house Bore coating: Millennium Technologies Crankshaft: undisclosed Piston: Vertex Ring: Vertex Piston pin: Vertex Circlips: Vertex Con rod: undisclosed Crankshaft bearings: undisclosed Crankshaft seals: undisclosed Gaskets: Cometic Ignition system: Currawong Engineering Spark plug: NGK Fuel injector: Bosch Fuel injector metering plate: Currawong Engineering Engine management system: Currawong Engineering Sensors: Currawong Engineering CAN bus: Currawong Engineering Fuel pump: Currawong Engineering Exhaust: AXS Exhaust: in-house Air filter: in-house Generator: Maxon Motor Dynos: Tygerdyne Some key suppliers to this engine