Issue 54 Uncrewed Sytems Technology Feb/Mar 2024 uWare uOne UUV l Radio and telemetry l Rheinmetall Canada medevacs l UUVs insight DelltaHawk engine l IMU focus l Skygauge in operation l CES 2024 report l Blueflite l Hypersonic flight

74 “We also use four oil squirters, one per cylinder, to jet a good amount of oil at the bottoms of the pistons to cool them. They are basically the kind you’ll find in any high-performance engine, and in most of our competitors in the four-stroke diesel world,” Webb adds. “The second section of our oil pump functions as a very high-power scavenge pump: it sucks the oil out via the bedplate, whether that’s at the top because the end-user chose the inverted-vee or at the bottom of a regular vee integration. The inlet to the scavenge pump has a second filter – a wire-screen device to remove any particles from inside the engine before the oil can return to the tank.” Both the oil tank and pump were designed in-house. The pump is a standard gear-drive device, running from a gear that sits on the engine’s accessory cover, and it is driven by the crank’s output shaft. The pump also features an integral spring-and-piston component for pressure regulation, so the piston moves in response to excess oil pressure, allowing some of the oil to bypass entry into the engine, relieving pressure that could otherwise overwhelm the seals. Inside the crankcase A total of eight plain journal bearings require lubrication in the DHK180. Needle and ball bearings can have trouble lasting in compression-ignition engines, whereas plain bearings (assuming optimal design of components and sufficient oil viscosity) run with a hydrodynamic film separating crankshaft from bearing and preventing metal-to-metal contact. The rear-most main bearing sits at the back of the engine near the highpressure fuel pump, forward of which are two more bearings sitting in the third and fourth con-rod big ends. Ahead of those is a central main journal bearing and bulkhead, separating the rearward third and fourth con-rod crankcase half from the first and second con-rod crankcase half, followed by two more big-end bearings in each of those con rods. Two more combined main and thrust bearings run in the front of the crankcase (which collectively handle gyroscopic and thrust loads from the propeller, be it a pusher or tractor prop), making for four main bearings and about 8 in of bearing length in total across the crankshaft. The crankshaft is designed with two crankpin journals, each holding two con-rod big ends, with the surfaces polished smooth to prevent wear. Webb notes that having more journals would have added more weight, and thanks to the 90° vee two-stroke configuration, a smooth and manageable firing order can be achieved with two con rods and hence cylinders per journal. Moving up the con rods, one does not find the usual free-floating gudgeon pins (held by three bearing surfaces across the con-rod small end and the two piston bosses) that are typical in four-strokes. The two-stroke and highly loaded nature of the DHK180 gudgeon pin has motivated DeltaHawk to instead opt for a trunnion-style gudgeon pin. “That means our con-rod small end is U-shaped instead of circular, and it bolts to the wrist pin [US English for gudgeon pin],” Webb explains. “There’s a number of reasons why we do that. Number one, it forces articulation of the wrist pin; it’s rolling back and forth with the motion of the con rod. Second, we don’t have an exhaust stroke; all strokes into the cylinder are compression strokes, and that means we can have a full-length wrist pin-to-piston bearing surface. “A highly loaded two-stroke will impart a lot of force on the wrist pin, and that needs to distribute over the surfaces it is in contact with. The traditional pistonto-pin interface concentrates loading in very tight sections of the piston body and circlips, whereas the metal of the bottom of our piston goes across the entire wrist pin. To ensure enough oil for all these components, we cut some very precise grooves into the piston-bearing surface, such that squirted oil runs full-length across the wrist pin as it articulates.” This approach was first proposed for the DHK180 by Doug Doers, with subsequent engineers and iterations optimising the bearing surface and adding the piston’s lubrication grooves. The company also cites the Detroit Diesel Series 71 and Electro-Motive Diesel 645 engines as precedents for this type of gudgeon pin. The pistons additionally feature two compression rings (an upper ring for February/March 2024 | Uncrewed Systems Technology The plenum is integral to the cast aluminium engine block, as are the cylinders