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

86 A waterless coolant from Evans is pumped through liquid jackets inside the cylinders. The team refers to these as its “precision-cooling jackets”, notably with separation of coolant flow between the head and walls. “Liquid enters from below the exhaust port, and then there are three different passageways,” explains Hilbert. “The middle passageway directs coolant to the cylinder walls, and the two outer passageways lead up to the cylinder head.” This approach achieves two goals. First, to maximise cooling efficiency for heavy-fuel handling, it can be useful to run a cylinder’s head hotter than its walls, as excess heat at the walls can degrade and even cause breaches of their oil film. Second, more heat at the head helps to vaporise the heavy fuel before the spark event. More generally, the internal layer of liquid helps to muffle some of the combustion noise from the cylinders, and the improved heat dissipation from liquid cooling gives the engine overall higher durability and lifetime thanks to reduced thermal expansion and contraction, particularly at its seals. “We’ve done FAR33 endurance testing, and found a direct causal relationship between how hot you run an engine and how long it lasts,” Vaglienti says. “Temperature affects carbon deposits, how well metal surfaces tolerate wear, and other things, and you see an exponential rise in the severity of these issues as you go up the operating temperature curve. “Air-cooled engines will run for hours at 220 C, but not for 500 hours, and they can maintain 160 C for a few hundred hours but won’t give a 1000 hour TBO,” he adds. “We haven’t determined for certain what the maximum TBO of the A99H is, but it should go a long way – of the order of thousands of hours.” While the A33 had an air plate that allowed cooling air through the cowling, the A99H has a servo-controlled mixer valve that can divert outgoing heated liquid to either the radiator to cool off, or back to the inlet area near the exhaust if a higher coolant throughput is more important than cold coolant – indeed, heavy fuel requires 40-60 C higher minimum coolant temperatures than gasoline engines. A custom thermostat is installed to monitor what action the mixer valve should take. Exhaust manifold While the A33 featured an active, variable bypass venting system in its exhaust, this has been temporarily shelved (pending ongoing r&d to optimise it) in favour of a simpler and less costly fixed exhaust for Cobra’s production engines. As indicated, the A99 instead uses an exhaust assembly that has a channel running from each cylinder linking into a common manifold, with cross-stuffing meaning that one cylinder’s exhaust gas wave helps the containment and compression of the next cylinder’s charge, akin to an exhaust valve. “Also, the three blow-down processes aren’t fighting each other, they are cleanly separated so a thin, lightweight packaging is sufficient,” Hilbert says. Vaglienti adds, “We didn’t invent cross-stuffing, it’s from older racing days, but we knew from early results we could achieve it at low rpm. A lot of the iteration on the exhaust manifold has focused on maintaining the cross- stuffing at higher rpm.” The manifold is produced using metal AM, enabling strikingly curved, complex, aerodynamic geometries to minimise drag and hence noise from the exhaust gases. The companies note that advances in metal AM are such that designing new iterations takes longer than printing them. “Right now, they’re aluminium, but we may go to titanium in the future, which might be better for thermal stability,” Vaglienti continues. “We’ve left ourselves a lot of options in terms of materials and geometric tweaks, but we’re happy with our design methodology, and what we can achieve aerodynamically and acoustically versus traditional milling approaches.” Engine and generator management Engine management is achieved using Power4Flight’s IntelliJect ECU system, with a minor resistor redesign to increase the number of controllable injectors from June/July 2022 | Uncrewed Systems Technology The generator can cold-start the engine or maintain a given crank speed in the event of a fault or failure