DeltaHawk DHK180 | Dossier 71 scavenged and piston ported from the plenum (forgoing traditional crankcase compression). Copious amounts of CFD simulation and testing went into optimising the combustion chamber’s internal geometry to ensure sufficient swirl and tumble of intake air coming from each cylinder’s six intake ports, such that any unwanted losses via the three open exhaust ports are prevented (although some ‘losses’ of forced air helps provide cooling, and fuel injection is mechanically timed so some slight air loss does not constitute a parasitic loss in the engine’s power mapping). “The CFD simulations were vital in optimising port height and placement, and the design of the steering ports. Two of the intake ports fire air in directions that ensure a good, vertical loop as part of the airflow. Without those, all the air would just shoot out of the exhaust,” Webb says. “Uniflow-scavenged two-strokes can ensure 100% scavenging efficiency, but we still get 97% without needing poppet valves, tappets, rockers, pushrods and so on. It took a lot of computer modelling, cylinder iteration and bench testing of slightly different engines, but it means we’ve maximised the amount of burnt gas evacuated and fresh charge introduced before the pistons close over the exhaust ports. “And that 3% ‘leftover’ exhaust in the cylinder is useful to an extent, because the radicalised particles become free nuclei in the cylinder for enabling faster light-off and combustion.” While many details inside the combustion chamber are proprietary, DeltaHawk discloses that it uses a recumbent bowl design, typical of diesel combustion pockets, with a flat rim around it. As the piston completes its compression, the outermost air gets squeezed into the coalescing squish of fuel and air, giving a final rush of turbulence that optimises the mixture and helps it ignite. “But, of course, diesel is controlled cetane. Jet fuel isn’t. It can be all over the map, so we did a lot of optimising on top of basic recumbent bowl technology, so our internal geometry and steel choices would work for mixing and igniting fuels of varying cetane levels, including very low cetane fuels,” Webb adds. Liquid cooling The DHK180 operates at roughly 200 F (93.33 C). To keep to that, a cooling pump on the front of the engine pumps water-glycol in two lines – one goes to each of the V4’s two cylinder banks – where it enters jackets running around the cylinders, and up through the cylinder heads to cool the areas heated most by combustion and exhaust. The coolant exits via the back of either bank into two pipes that combine into one for running to the integrator’s radiator. “In addition to the cooling from the forced induction, and some oil cooling that we’ll get into, but we’ve generally tried to keep the cooling as simple as possible, albeit not as simple as all-air cooling, which wouldn’t have worked for several reasons,” Webb explains. “Air-cooled engines require really extensive metallurgy and excessive clearances between the piston and rings and cylinders to work well, and it has to work at negative and hot Uncrewed Systems Technology | February/March 2024 The oil pump integrates a high-power scavenge pump to recover oil via the bedplate, whether the engine is installed as an inverted-vee or an upright-vee In addition to the liquid cooling concentrated about the cylinders and heads, squirters in the crankcase cool the undersides of the pistons by shooting oil at them
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