74 To further optimise gas flow, the inlet and exhaust components are largely bespoke designs. The former is optimised to enable rapid, direct delivery of atomised fuel, from a single port injector running via the throttle body, onto the back of the inlet valve bell shape. The exhaust manifold integrates an oxygen sensor for measuring O2 in burnt gases to determine whether the fuel mixture must be made leaner or richer (by the ECU shrinking or extending the injector’s pulse duration), depending on altitude. The engine is typically started using a handheld starter in front of the propeller, and a toothed crown on the crankshaft’s flywheel is used by the ECU for ignition timing. “Most engines use a Hall sensor combined with a bit of magnetic material in their crankshafts to trigger the ECU for ignition timing, but you may remember that one of our primary applications was mineral detection. Well, carrying a magnetometer for mineral hunting means you can’t have any ferromagnetic materials anywhere near that payload, so we couldn’t use a Hall sensor,” Slater says. “Instead, we do tooth counting. There’s 24 teeth on the crown, equally spaced, with one extra one inserted between, and an optical sensor counts the teeth and bases the spark timing on when it sees that extra one. “The optical sensor works more than quickly enough to advance or slow the ignition timing however we’d like. At lower rpm, we might have it firing 6° or 7° before TDC, but by the time we reach 4000 rpm, we might be at 27° pre-TDC.” Also connected to the ECU is a manifold absolute pressure (MAP) sensor to track engine load and inform injection cycle lengths. An air tube connecting the inlet manifold to the ECU enables the necessary measurement of the difference between ambient atmospheric pressure and how much air the engine throttle is demanding. The throttle itself is an adapted COTS unit, positioned under the engine by the firewall, to match the high-prop pusher positioning of this engine at the top rear of the Prion Mk.3’s fuselage, and an injector housing installed on the side of the engine feeds into that. Crankshaft balancing While the crankcase is conventionally designed and milled from a single piece of 6061 aluminium alloy, the crankshaft is unconventional in a few ways, not least for being supported at only one point. “Most crankshafts have a bearing or two at each end to support and hold them in place, but we have three bearings at the middle and those act as a slight pivot point – something like the stationary mid-section of a seesaw – while the output shaft is free-floating,” Slater explains. “On one end you have the reciprocating mass of the piston, con rod and so forth, along with the flywheel and the drive gear for the valve train; on the other you have the propeller. The result is that the stressed components of the powertrain perfectly balance each other out, and those three bearings sitting almost equidistant between them effectively manage the loads and stresses transmitted from them.” Two of the bearings sit as a pair near the crank nose, while the other, larger bearing sits on the other side of the counterweight from the big-end bearing. The smaller pair of bearings and the larger one also sit equidistant from a gear, machined at roughly the middle of the crankshaft’s length, for driving the camshaft. A critical part of optimising the DS120’s performance (culminating in 2021) was balancing the engine, particularly balancing the crankshaft, without having access to significant in-house crankshaftbalancing equipment. To do that, UAVE partnered AMB Engineering, a Sheffieldbased specialist in crankshaft balancing; the process involving significant remote collaboration due to COVID restrictions at the time. In-house, Slater and his team stripped down the engine, measuring and weighing each part of the reciprocating assembly, down to its circlips and piston ring. “We then designed and built a jig to keep the crankshaft balanced, and learn what weight distribution was necessary December/January 2025 | Uncrewed Systems Technology The crankshaft features three bearings, which handle the balance of loads from the propeller and the reciprocating piston mass
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