Uncrewed Systems Technology 044 l Xer Technolgies X12 and X8 l Lidar sensors l Stan UGV l USVs insight l AUVSI Xponential 2022 l Cobra Aero A99H l Accession Class USV l Connectors I Oceanology International 2022

83 Cobra Aero A99H | Dossier the pistons’ downward pressure forcing charge through the transfer ports, and the crankcase is separated into three chambers by the two main bearings and seals (otherwise, the I3’s crankcase pressure would always be the same). As with the exhaust phenomenon alluded to above, the cross-stuffing effect ensures that each cylinder breathes independently. As one cylinder draws in air, the others are either compressing or shut entirely, ensuring dense air delivery where appropriate. Regarding fuel, most boxer twins have a single injector that feeds into the crankcase, with the induction charge being blown into both cylinders simultaneously. Vaglienti notes, however, “The quality of charge transfer is a function of how each cylinder blows down on its previous cycle. So you can get an imbalance where one cylinder gets more charge than the other. “In a gasoline engine, maybe that’s not so much of an issue: gasoline burns pretty well, even if your mixture isn’t perfectly on target for your maps. But a spark-ignited heavy fuel engine is much less tolerant of mixture error. Luckily, with an inline engine you’re obliged to fuel each cylinder independently, with an air seal between each cylinder.” As such, the A99H has three injectors rather than throttle body injectors, which spray at 3 bar into the crankcase. Each cylinder scavenges and pumps on its own, as in a single-cylinder, giving the precise control over the charge mixture for all cylinders that’s needed for heavy- fuel combustion. This approach also helps performance in high transients. Heavy fuels are challenging to vaporise, which is why diesel engines often have extremely high pressure injectors to atomise droplets that are small enough to burn effectively. The conventional tactic of spraying fuel onto the reed valve would therefore not achieve sufficiently precise charge mixing for any significant changes in speed. “One answer is to just cut back the performance of the UAV, but a better one is to move the injectors past the reed and downstream, directly into the crankcase,” Vaglienti says. “Then we worked hard on our IntelliJect software over several months to model how heavy fuel could be puddling on the crankcase surfaces, until basically we’d mapped what the sizes of puddles would be based on the operating conditions and how they’d change over transients. “So now we can transition almost as well as a gasoline engine. At worst, moving the throttle from idle to 100% might take half a second, compared with twice as fast for a gasoline engine. Our software also allows us to do other clever things with the injection, such as deficit-track and skip-fire to account for puddling.” Aiding this further is a glow plug mounted at the upper-rear of the A99H that serves as a pre-heater for the engine metal. As a high-current device, it can heat the heavy fuel rapidly, and the heated fuel sitting in the line above the cylinders can also warm the latter to help with a cold start. Heavy-fuel handling Hilbert notes that companies such as Orbital (see UST 11, December/January 2017) and Hirth ( UST 34, October/ November 2020) have switched to using air-assisted direct injection rails to handle their heavy fuel pre-ignition needs. “I worked closely with Orbital’s system back in my Ford days, and it confers fantastic control advantages by moving the injector right to the head, as well as really fine atomisation; it’s a great solution,” he says. “That said, such systems can be energy-intensive, problematic in packaging terms, and they’re also a little heavy and complex. “And when you inject directly, you reduce the time in which the fuel can vaporise, so the actual quantity of useful air in the cylinder drops, unless you upsize the engine to compensate for the reduced power at the top end. Again, that means packaging and weight issues, albeit with the benefit of very efficient atomisation and timing control; the trade- off we’ve gone with is targeting lighter, simpler and more aerodynamic.” Naturally, when dealing with heavy fuels, an even bigger issue leaps to the forefront: knocking. As the probability of knocking increases with time and temperature, there are three typical approaches to preventing such pre- ignition (or outright explosion) events. The first is to run at low loads, which many heavy-fuel engine designers achieve by expanding their displacements by 30-40% so that the load is lower relative to the engine and airframe volume. The second is to Uncrewed Systems Technology | June/July 2022 Designing the split needle bearings in-house was a critical step toward reducing the crankshaft part count