78 lobe, each valve opens in a different way to the other,” Slater says. Within each cam follower is a pushrod, running up to the cylinder head and interacting at their top ends with conventional tappets (integrated with rocker arms of EN32 case hardened, heat-treated steel), with a valveclearance adjustment screw embedded in one end. A single spindle in the rocker box allows the valve tappet to rest in a simple collet arrangement atop its valve. “Most valve collets nowadays involve a two-piece collet, which clamps around recesses in the end of the valve stem, and the valve spring typically holds that recess in an increasingly tight clamp. We have a simpler system than that,” Slater explains. “We have a single disk, which presents over the valve, and the valve moves towards the centre of that disk, after which the valve-spring pressure keeps that located correctly. It’s a very simple approach, not overly sophisticated – put that in a Formula One engine, and you’d have problems with dislocation and valve bounce – but as we never go above 5200 rpm, it works great for us.” Valve to survive The valves are supplied by G&S Valves, which also makes valves for Formula One engines. “At first, as with many components in the engine, Dave made many of the components in-house, but as I worked on enhanced fuel mappings and approaches to getting oil from the cylinder into the crankcase, down to the valvetrain and into the rocker box, I started to get better performance out of the engine until a couple of valve stems snapped and fell into the cylinder during a test run,” Slater says. “Fortuitously, I was soon introduced to G&S Valves, based in Guildford, which is a hotbed for Formula One technical expertise, and after hearing about my problem, they manufactured – at a reasonable cost – incredibly strong, customised valves, based on our specifications, so they’re a near-direct fit to our valve seats.” The bodies of the valves are of 1.4882 steel with a pulsed plasma nitriding treatment. Chemically referred to as X50CrMnNiNbN21-9, this is an austenitic steel, specified for engine valves, containing chrome, manganese, nickel, niobium, nitrogen, phosphorus, silicon, sulphur and tungsten. The valves are held closed using standard COTS valve springs, although UAVE experimented with highcompression springs before passing on them due to issues with pushrod bowing. The cylindrical component functioning as a valve seat is cast iron, and it is precision-machined and refaced with a diamond-cutting machine to fit closer to the dimensions of the G&S valve. “A local technician has the diamondcutting machine, as well as a scanning tool for measuring the geometry of the valve guide and to gauge how much excess movement or stiction might occur during thermal expansion. We’ve got aluminium, cast iron and steels working closely together at TDC, so it has paid to stay mindful of the differing coefficients of thermal expansion those all have,” Slater adds. Electric power While the engine is typically manually started, without a starter-alternator, an inline alternator is integrated and runs on the crankshaft. The alternator is a three-phase, 500 W system, supplied by Sullivan UV of Acutronic Power Systems, and the AC power it generates runs into a Sullivan power regulator. The resulting DC is regulated and distributed by the device December/January 2025 | Uncrewed Systems Technology A pair of overhead poppet valves, made of steel with a pulsed plasma nitriding treatment, govern intake and exhaust We’ve got aluminium, cast iron and steels… it has paid to stay mindful of the differing coefficients of thermal expansion those all have
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