Fuel systems | Focus that injectors made for small, scootertype vehicles often approximate closely to the ideal flow rates and efficiencies for UAS powertrains. Sizing injectors accurately for gasoline two-strokes is especially critical owing to the very slim injection windows available at 8000 rpm. It follows that a very high flow rate can be extremely helpful in this regard. Also helpful are some key innovations in EFI control systems produced for the UAV world. For instance, some more advanced EFI software modules now include programming for greater control over injection timings. In addition to assisting with finer gasoline injection times, this provides valuable opportunities for safety and efficiency in heavy-fuel operations, as is popular among defence and particularly maritime users. During transients, for instance, injectors can be commanded to ‘skip’ injection rounds if heavy fuels are pooling too much inside cylinders owing to the vaporisation problems inherent in heavy fuels, to help regulate the richness of the fuel during the next combustion cycle. Sensing such pools in real time is challenging, so accurate maps of pooling ‘probabilities’, as indicated by exhaust and performance readings, are very valuable for heavy-fuel engines. This technique can also be modified to compensate for projected fuel deficits, to build up a pool for when more fuel will be needed than can be typically injected in one go. In general, air-assisted direct injection is still considered the best approach to heavy-fuel efficiency, by virtue of not short-circuiting fuel to the exhaust and hence substantially improving the specific fuel consumption (SFC). However, they remain heavy and expensive. By reducing SFCs by around 20-25%, they can add 50% to powertrain mass as well as quite a few additional moving parts. The alternative approach of having a fine and flexible EFI control paired with indirect injection therefore stands to make a great difference for the value operators derive from their UAVs when preferring heavy fuel. Although automotive injectors can work well for UAV applications, automotive EFI controllers can suffer issues such as mistaking altituderelated air pressures for errors or failures, unnecessarily triggering backup measures, and can contain some unsuitable or proprietary modules that make them difficult to work with. By contrast, ECUs designed from (or mostly from) scratch for UAV applications can be tailored for the engines and aircraft unique to the industry. Future of fuel Although IC engine powertrains retain an advantage of around 10x over batteries in terms of specific energy, it is generally agreed that batteries will eventually overtake fuel systems in that respect. However, advances in some key IC technologies will delay that. One of those is the use of alternative fuels to traditional gasoline, diesel and heavy fuels that can provide higher efficiencies and lower emissions. One obvious route is themixing of biofuels into conventional fossil fuels, as some airlines already do, as well as using 100% biofuel (with Demcon already planning to do so with its upcoming 7mUSV – see page 62). Synthetic fuels produced through captured carbon are also likely to become viable, with recent EU automotive legislations having enshrined the use of these ‘e-fuels’ in car engines beyond the 2030s. Also, more engines are being designed to burn hydrogen. While not as efficient as using it to produce electricity via a PEM fuel cell, it still guarantees true zero carbon emissions shaft power. Whether hydrogen or biofuel is better environmentally depends on the source – ‘grey’ hydrogen for instance can be derived from processing fossil fuels, while ‘green’ hydrogen comes from electrolysers powered by solar and wind. Similarly, biofuels can be carbonnegative when considering the CO 2 capture and photosynthesis of the biomass fromwhich they are produced, although any advantage there is cancelled out if rainforests for example have to be cleared for farming the biofuels. Determining the best fuel is therefore a matter of case-by-case calculation. 99 Uncrewed Systems Technology | June/July 2023 Developing UAV EFI controllers from a blank sheet can be vital for avoiding errors that can stem from using EFI parts of automotive origin (Courtesy of Power4Flight)
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