Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

7 How difficult is fuel flow measurement? The answer clearly depends on the required accuracy. In internal combustion engine test cells, fuel flow is traditionally measured with an extremely high degree of accuracy using a Coriolis-type meter. In the context of a UAV though, that is impractical, not only because of the weight of such a meter but its size – which is as large as a bathroom cabinet. The challenge then is to develop an alternative means of measurement that is feasible within the confines of a small UAV, without significantly affecting the craft’s weight. After all, the whole point of precise fuel measurement in this context is to allow the craft to exploit the maximum flying time it can obtain from its fuel load. Clearly, that task is complex and challenging. UAV internal combustion engines are typically small-displacement two- strokes, sometimes using a carburettor but more often nowadays equipped with electronically controlled fuel injection, under the command of an ECU. It is quite normal for that ECU to incorporate data logging and to send that data to the operator; from the logging of injector opening times, most systems will calculate fuel consumption. That’s all well and good but how accurate is such measurement? The answer depends on the approach to it. Using sophisticated techniques, ECU supplier Performance Electronics has developed what it says is accurate fuel flow measurement as a modestly priced adjunct to its control systems. Those systems are used in automotive applications as well as internal combustion-engined UAVs. Performance Electronics does a lot of work for OEMs and others, supplying engine control components with branding by the customer; however, it also offers its own brand of ECU, a multi- purpose unit that incorporates ignition coil and injector drivers, offers other outputs according to the specific version, accepts readings from all necessary engine sensors and has a diagnostics capability via CAN together with data acquisition. It can be mapped using a PC in the normal manner. UAV versions of this control unit – the PE4 – use similar hardware and firmware modules as automotive versions, and similar tuning software. The PE4 is fully configurable and adaptable to specific applications. Performance Electronics engineer Brian Lewis noted that, for example, some UAV applications require more thermocouples than others. “We have a daughterboard that can accommodate that and lots of other custom requirements. We also offer options such as Mil-spec connectors.” It is the UAV market that has driven Performance Electronics’ development of integral fuel flow measurement. “We found that our UAV customers wanted more accurate fuel consumption calculation,” explained Lewis. “They wanted to know how much fuel they are using, so they can work out how long they can continue to fly.” Lewis also said that in its own right fuel injector opening time as set by the injector drivers within the ECU is insufficient data from which to accurately calculate consumption, even when referenced to fuel pressure, engine speed and the prevailing mapping. The injector dynamics have to be understood, starting with the calibration of each injector with reference even to battery condition. He noted that it often isn’t appreciated that battery voltage compensation is a vital aspect of injector driving. “The ECU can only alter mass flow indirectly via the injector open time, and if battery voltage isn’t what it thinks it is then a required, 10% change in mass flow for example might in reality be a 15% change.” Thus it is that an appreciation of actual injector dynamics is central to Performance Electronics’ fuel flow measurement, which can be incorporated into closed-loop control of an engine run by one of its units. Platform one This new NW-44 multi-fuel UAV engine produced by Northwest UAV is using Performance Electronics’ ECU with fuel measurement technology Going with the flow Fuel management Unmanned Systems Technology | November 2014