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36 Focus | Engine ancillary systems achieving this without affecting the probe’s dielectric stability takes significant r&d as well as manufacturing precision. CAN bus is also making its mark in the field of level sensors. While the sensors would typically have produced a 0-5 V analogue signal, a CAN-enabled digital signal enables the sensors to output additional key information for diagnostics and analysis. For example, the temperature of the sensor’s flange head or its base can indicate its health or help assess the condition of the liquid. Other information such as runtime, service life or minimum and maximum temperatures can also be communicated and combined with these data through CAN bus. Compared with older level sensor designs, the CAN bus also reduces wiring requirements. That helps cut weight and points of mechanical failure in engines. More intelligent comms standards are likely to be a precursor to further analytics and autonomy in fuel sensing. For example, at least one UAV has integrated an automatic subroutine to automatically switch to an auxiliary fuel tank once its fuel level sensor detects a certain level of depletion. Fuel systems Secure fuel delivery remains a critical priority for UAV engineers, who are continuing to opt for fuel injection instead of carburettors. The key problem with carburettors is that they can cause fuel starvation if there is air in the fuel line. As UAVs are subject to intermittent changes in acceleration, including catapult launches at 10-20 g , fuel starvation presents an unacceptable hazard for most users. UAV fuel injection requires a high- pressure fuel rail that weighs as little as possible while also consuming as little power as possible. Most automotive systems use a gerotor pump, which (as with carburettors) cannot tolerate air; they will rapidly lose their suction capability if they ingest air, and will take a while to re-prime. Cars can solve this by putting the gerotor pump at the bottom of the fuel tank (keeping it submerged in fuel), but UAVs cannot rely on this approach as their fuel is constantly splashing around because of accelerations and other aircraft movements. UAVs can therefore benefit from switching to positive-displacement piston pumps, which can pump air without disrupting their operation or losing fuel pressure. This enables the pump to be located anywhere around the tank (as needed for the UAV’s centre of gravity or lack of internal space), simplifying a key design concern. Piston pumps will process any sucked- in air at rail pressure, and dissolve it into the fuel; the air will then exit through the injector at considerable speed. As a result, any lull in fuel delivery is minimal in quantity and short in duration, allowing the engine to operate without interruption. A fuel pressure accumulator is often installed in the fuel line directly after this type of fuel pump. This is required mainly to mitigate the significant pressure spike that occurs each time the pump cycles, so that there is an accumulating volume of fuel output from the pump, pooling at a high but steady pressure level in the accumulator. However, including an accumulator can also mean that if the pump should stop working for any reason, the rail pressure will be sustained so long as the accumulator still has some fuel in it. This keeps the engine running and potentially buys time for an operator (or the autopilot) to steer the UAV safely towards the ground – assuming that the fuel pump isn’t simply facing a temporary issue, after which it might resume normal operations. Also critical for smooth and efficient fuel delivery is the use of electronic fuel injection (EFI) control modules that can account for the differing requirements between engines. Two engines of the same series might be required to run on different fuels and perform in different ways, while using differently calibrated sensors. To that end, the use of EFI systems with adaptive control algorithms can be vital for testing and mapping a new engine’s fuel injection strategy before operating it in flight. Generators Growing calls for hybridisation are moving the standard starter/generator from the 12 V, 120-250 W system to a 48 V, 500-2000 W configuration – or in some cases, a 270 V, 6-20 kW generator for powering heavier and more sophisticated payloads. Recent advances in related April/May 2020 | Unmanned Systems Technology Advanced EFI systems are becoming more popular than carburettors, as they prevent the risk of fuel starvation and take advantage of the diagnostics possible with CAN bus (Courtesy of Moscat Ingenieria)