Unmanned Systems Technology 028 | ecoSUB Robotics AUVs I ECUs focus I Space vehicles insight I AMZ Driverless gotthard I InterDrone 2019 report I ATI WAM 167-BB I Video systems focus I Aerdron HL4 Herculift

71 compression and valve-less ports for exhaust and air intake, the WAM-167BB has exhaust valves to give an end-to-end or uniflow scavenge arrangement. That leads to better scavenging and hence less contamination of fresh charge air by exhaust products, which is vital to enable high power to be developed from a relatively small and light engine. Each exhaust valve is 35 mm in diameter, and sits atop a 7 mm stem. The port throat is about 31 mm across, and the camshaft lifts each valve by about 11 mm. “Each piston obviously has to come up to ‘open’ the air ports before coming back down for compression and combustion. After combustion, the cam lobes open the valves for the exhaust gas to come out,” Newton says. A system of cam-follower ‘buckets’ and tappet shims sits between the cam lobes and the valve stems. The exhaust camshaft runs in the cylinder heads, in plain bearings, and is machined from EN40B nitriding steel, hollowed to save weight. “The Gen 1 engines used Nimonic- headed exhaust valves, but the Gen 2 engine doesn’t need the Nimonic, just standard-grade exhaust valve steel, which we found perfectly adequate,” says Franklin. “Given the bigger bore in the Gen 2 engine, we took the opportunity to separate the valves a bit more. By increasing valve separation, we get less heat concentration between the valves and thus better cooling. That was actually what largely eliminated the need for the Nimonic.” “Since the bore was widened by about 7 mm we could have spread the valves 7 mm further apart, but we found 3.5 mm to be good enough for the broader cooling passage we wanted.” The typical operating oil pressure of the engine is 3 bar, provided by a multi-lobe gerotor-type pump installed at the end of the camshaft, and driven by the large cam gear. As lubricant comes out of the oil pump head, it is sent to an external oil cooler on the UAV airframe, then routed back through an oil filter on the forward lower-left of the engine before going into the oil galleries. “The oil galleries carry oil up to the crankcase, which ferries it to the crankshaft and its bearings, to the nose bearing and the prop governor,” Franklin explains. “And there is an oil feed to all the pistons – oil cooling jets squirt oil at the aluminium alloy pistons. Because the pistons are upside down, the oil gets thrown back out and, like the rest of the oil, finds its way by gravity down through drains to the sump.” In technical terms, the engine has integrated a lossless oil system, as no oil needs to be mixed in with the fuel and burned, unlike two-stroke engines with crankcase compression. As Newton and Franklin note, however, as in a four-stroke engine a minute quantity of oil does get past the piston rings and is burned. Some of the oil lubricates the three large timing gears at the back on its way down before arriving in the sump. Some of that is captured in a small vee- shaped ‘catcher’ though, to direct it back towards the fuel pump and through the roller bearing carrying the fuel pump shaft, before it continues by gravity down into the sump. Most of the oil, however, runs through a series of ‘drain points’ directly or semi-directly to the sump. Because an open wet sump sits just below the Unmanned Systems Technology | October/November 2019 Apple Tree Innovation WAM-167BB | Dossier Three timing gears sit at the back of the engine. The top sets the timing for the crankshaft, the middle for the fuel pump (which also drives the supercharger) and the bottom holds the exhaust camshaft (and also drives the oil and coolant pumps)