98 circulation, and overall the systemwould be constantly pulling excessive electric power, making it rather energy-inefficient. In UAVs though, constant fuel circulation is not only prohibitively inefficient but unsafe, as outgassing could occur owing to the smaller tank sizes (compared with those in cars) causing bubbling of their fuel. Also, mechanical pumps such as gerotor or gear systems are difficult to scale down to sizes appropriate for UAVs (ideally less than 100mm in length, width and height), largely because of the tight clearances needed. Even when well-designed, they can draw 10-20Wper pump, and UAVs in the late 1990s to early 2000s would rarely have hadmore than 60-90Wof electric power available, making such pumps a considerable burden. UAVs therefore need dead-headed fuel pumps, driven by highly efficient electric motors regulated to control pressure, rather than by mechanical systems, and that in turn means having to use fuel pressure sensors. This efficiency helps to lower power consumption – a must-have for flight endurance as well as keeping running mission costs reasonable. Positive-displacement pumps are among the most efficient designs. They have minimal leakage and strong self-priming. They are also often designed to run dry, as air can pass into the fuel line during moments of high or negativeg-forces. Also, end-users will occasionally not refuel their UAVs completely before running their engines, which can break the pumps if dry running causes a failure mode. This pump type can require a damper because of their pulsing manner of operation, similar to those sometimes used for the pulses caused by the pressure fluctuations from the injectors opening and closing. A highly efficient UAV fuel pump will transfer around 45% of the electrical power it consumes into mechanical work, and it must tolerate the severe temperatures that can build up around UAV engines, as well as moments of low fuel pressures, down to the vapour pressure level, beyond which fuel can no longer be pumped. Resilience to sand ingress is also vital; should a filter fail to block that, integrating meshes internally can be key here. With such robustness, a good fuel pump will last for 700-1500 hours in a UAV, rivalling the lifespan of a high-end UAV engine. Some fuel pumps have been designed to be integrated somewhere below the fuel tank, with gravity providing assistive pressure to the fluid flow. This is of course unsuitable for UAV-bound pumps though, as not only will extended pitching and rolling interfere with the direction of gravity, but the fuel tank or bladder is often mounted at one of the lowermost sections of the airframe, making it impossible to mount the pump anywhere below the tank. Higher-end UAV pumps are therefore engineered to function no matter where they are integrated relative to their tanks. As heavy fuel becomes used to power more and more UAVs, demand for more rugged pumps designed to operate at up to 10 bar (with 8 bar being ideal for atomising such fuels) will grow. Positivedisplacement pumps with a higher cylinder count and a stronger crankshaft than their gasoline-handling counterparts have a track record of meeting this need. Fuel injection and EFI control Some UAS suppliers opt for automotive fuel injectors, although it is recommended that they be modified for ensuring the appropriate flow rates for the 25-50 cc engines that are common among UAVs – changes would include replacing the director plates and drilling fresh sub-mm holes. Also, some note June/July 2023 | Uncrewed Systems Technology More robust and advanced EFI control systems are critical for long-endurance autonomous vehicle operations (Courtesy of Moscat Ingenieria) Sizing injectors for gasoline twostrokes is critical owing to the slim injector windows at 8000 rpm, so a high flow rate can be helpful in this regard
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