Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

67 battery can provide enough current, and an onboard starter controller has been properly wired in. With compression ignition engines, this torque requirement can be sizeable. Alternatively, an external combination of battery and starter controller can be used to save weight. Ignition systems Both spark-ignited piston engines and gas turbines need some means of providing an electric current of sufficiently high voltage to generate a spark at the plug in the combustion chamber to ignite the fuel-air mixture. Timing the spark is more critical in piston engines, which carry out the induction, compression, power and exhaust strokes in sequence in the same place, while turbine engines carry them all out at the same time in different places. Controlling spark timing is critical to the performance of the engine, as the plug must fire several degrees of crankshaft rotation before the piston reaches top dead centre. That can be a fixed value in very basic engines, but modern computerised ignition systems take many variables into account, including engine rpm, throttle position, air temperature, coolant temperature (in the case of liquid-cooled engines) and air pressure. The major choice to be made these days is between standalone capacitor discharge ignition (CDI) systems and ECUs that can either control CDIs or trigger the ignition coils directly using integral ignition drivers, which are solid- state power electronics devices. CDI systems are fairly simple in that they use a charging circuit to charge a capacitor. Stopping the charging process causes the capacitor to discharge rapidly into the primary windings of an ignition coil to trigger the spark. One supplier offers a range of CDI systems for singles, twins and multi- cylinder UAV engines. The version for a single-cylinder engine consists of a single box containing the capacitor circuitry and the coil, the high-voltage lead and the spark plug cap plus the low-voltage wiring and block connector. Operating at between 6 and 8.4 V, it draws 300-400 mA at an engine speed of 10,000 rpm and can operate at an rpm range from 400-25,000. Static ignition timing is set at 28-30° before top dead centre, although the automatic advance feature can adjust the timing to prevent kickback on start-up as well as matching the advance setting to engine speed and load. For starting, it provides 30-35 kV at 400-800 rpm, dropping to 12-15 kV by 10,000 rpm. The box measures 52.5 mm long by 40 mm wide by 26 mm high, the high- voltage lead is 310 mm long and the power and sensor wiring is 240 mm long. The whole system weighs 85 g. Exhausts How exhaust gases leave an engine, and the work to which they are put, can have a profound effect on how the engine performs. The arrival in motorcycle racing in the 1960s of two-stroke engines with expansion chamber exhausts, and the impact of turbocharging on top-level rallying from the 1980s, were the most remarkable examples of harnessing exhaust energy, and led to huge increases in power output. The first used changes in exhaust pipe cross-section to reflect sound waves to improve the two-stroke engine’s ability to scavenge spent gases from the cylinder while minimising the loss of fresh charge through the exhaust port, while the second used energy in the exhaust gas to drive a compressor to supercharge the engine. Developments in UAV exhaust systems have not been quite so dramatic, but they are increasingly seen as integral to the power plant rather than as afterthoughts, whether for performance tuning or silencing. At least one engine manufacturer develops systems for its own engines in close cooperation with UAV manufacturers over the installation. Muffler designers have always sought to minimise noise without compromising engine power. One manufacturer, for example, ensures that its muffler internal volume is ten to 15 times larger than that of the engine’s cylinder. A smaller ratio can stifle the engine’s performance, a conclusion arrived at through calculation and experiment. This volume ratio makes the exhaust a major contributor to the power plant’s overall size, weight and proportions, however, so a lot of customisation tends to be carried out. It also mandates making them from lightweight aluminium alloy. Instead of packing the muffler with sound-absorbing material such as glass fibre, some feature components described as ‘turbine wheels’ inside them. As they do not rotate, they UAV engine ancillary systems | Focus Unmanned Systems Technology | June/July 2017 Exhaust systems are increasingly viewed as integral to the power plant, with the ratio between engine displacement and muffler volume being a critical parameter for reducing noise output without sacrificing power (Courtesy of 3W International)