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

60 Dossier | Cobra Aero A33i to maintain that engine/propeller speed, provides enhanced fuel efficiency and, using the silencer, much-reduced noise. Thus open-open is used to increase power for take-off, climbing and sprinting from one target to the next and sometimes to react to unexpected transient conditions, such as gusts of wind. Those are scenarios in which noise is normally less of an issue, and since they typically represent a small percentage of a typical mission, fuel efficiency is also less crucial. The majority of a typical mission is spent cruising and loitering, when by contrast the UAV needs maximum fuel efficiency and minimum noise. The closed-closed option logically requires the throttle to be open slightly more to obtain a sustained crankshaft/propeller cruising speed of 5500 rpm, yet at that speed it provides a 3 dB reduction in noise and a 10% gain in brake-specific fuel consumption (BSFC) . The operation of the exhaust port valve is purely mechanical. In effect it is a rectangular-section rod that moves up and down a channel that has a carefully contoured nose that in the down (‘closed’) position projects into the port. The orientation of the rod’s channel and the shape of its nose are such that in the closed position it masks the top section of the port close to the cylinder wall. Thereby is the area of the port exit reduced with a reduction in its height. The rod is moved up and down by a gear working on a rack that it incorporates; in turn the gear is rotated by the servo- driven linkage. The servo, designed and supplied by Currawong is controlled via CAN (Controller Area Network), and it has an accelerometer in it to identify if it has been put under too much load. The exhaust flap valve is connected via the same mechanical control linkage such that when the port valve is open, so is the bypass. The fact that a rotary servo is used means that interim positions for the valve and flap could be explored, but that hasn’t been done yet. For engine starting, the exhaust port valve can be indexed back to uncover a decompression passage. That allows the generator to act as a starter motor with minimum draw on the battery and even in flight. Performance The A33 runs on pump gas, ideally with a RON/MON of around 96 with lubricant pre-mix at a ratio of 50:1. “We have been working with Sunoco on developing a high-octane pump gas that has certain features appropriate to this type of engine, including detergent packages that reduce or even eliminate carbon deposits,” notes Hilbert. “That is important to maintain time between overhauls when using the exhaust port valve.” The compression ratio is 8.5:1. Hilbert says, “We have developed the engine to avoid detonation, which would potentially have been an issue, especially under hot conditions. If we do sense an increase in temperature, which would be an indication of detonation, we can dynamically alter the effective compression ratio by opening the exhaust port valve, so in that respect it is also a detonation control valve.” The A33i typically runs from 3000 rpm to a redline of 9000. “The limitation at the top is more propeller-based than a cranktrain or durability consideration,” remarks Hilbert. “Without the propeller load it could certainly go well above 9000 rpm. Our motocross engines run to 14,000-plus rpm.” The A33i weighs only 1.45 kg as a full system including the fuel pump, the engine control electronics and the starter/ generator. Its maximum output is 2.4 kW at 8500 rpm (running with both exhaust valves in the open position), while the BSFC at cruising speed, with exhaust valves closed, is 450-500 g/kW/h. Hilbert says, “You can see from our fuel map [above] the depth of data we are able to obtain from our fully instrumented motoring dyno, which provides highly accurate fuel flow readings. “From the consumer’s perspective, they look at information on power and BSFC, and a vendor will normally offer readings that are the best points for each on the engine map. However, they are not related to each other: what you need is a plot like ours that covers all the operational conditions. Other engine manufacturers don’t provide that information because they don’t have the same measurement capability.” That and innovations such as the CNC transfer ports and active exhaust are why the A33i should have a bright future. February/March 2017 | Unmanned Systems Technology The A33i’s brake-specific fuel consumption