76 Engine dossier | UAVE 120 cc four-stroke The engine ran 10 C cooler to the pair’s delight, and so they gradually worked through Honda’s contacts until its kartracing component company identified the part numbers for the piston, piston rings, gudgeon pin and circlips, sending samples to Slater to confirm. “The great thing about using a Honda Racing-cast piston is that it’s already dimensioned to thermally expand and contract consistently within a perfectly cylindrical bore, so that it doesn’t seize or cause wear effects as a less sophisticated piston might when overexpanding on the exhaust side, compared with the cooler inlet side,” Slater says. “We do mill out surplus material from the piston skirt to reduce weight, as Dave taught us, because we’d already done our weight-balancing calculations for the tungsten alloy inserts that go into the crankshaft, so we needed a like-for-like piston weight, including bearings, rings, gudgeon pin and so on.” The piston features two rings – both for compression with no oil scraper, as UAVE wants oil to run past the piston, rather than keeping it in the combustion chamber (to date, no issues relating to blowby have had a measurable impact on any part of the engine). The gudgeon pin is held in place by two bronze ‘top hat’ collars, as well as the bronze bushing, drilled oil path in the small end lubricates those components. Cylinder in cylinder The cylinder is composed of a steel liner, inserted into a shrunk-fit cooling jacket cut from aluminium (including cooling fins). The main body of the cylinder head is cut from aluminium and finned externally for cooling. Once cut, a smaller, cast-iron cylinder is inserted into the head and pinned in place. This small cylinder serves multiple purposes. For one, it functions as a valve seat or floor for the valve springs to sit and compress against. It additionally serves as part of the rocker box housing, and an aperture is cut into part of it, which functions as the exhaust outlet. “We are also experimenting with a bronze insert for that cast-iron component, which will form a valve guide, as added insurance against valve stems sticking. The bronze is a great material for mild steel to pass through. The valves themselves are excellent, and I’ll explain why later, but the bronze valve guide could prove really helpful for getting that final 1% of reliability into this engine,” Slater says. The head clamps atop the aluminium cylinder cooling jacket via a squish ring made of steel, dimensioned and tuned to optimise the compression ratio for the DS120’s 3600 rpm cruising speed. One-lobed camshaft The valvetrain has been kept as mechanically simple as possible, while incorporating a number of high-end components, both of these qualities contributing to the engine’s lengthy TBO. A single camshaft actuates two side-by-side, upright valves – one for the intake, one for the exhaust – via two conventional sequences of pushrods and rockers, as mentioned. As previously indicated, the camshaft is driven by a crank gear, with each full rotation of the crankshaft rotating the camshaft by 180° per the Otto four-stroke cycle. The cam shaft and gear are made from ground, flat, stock steel (also known as ‘gauge’ or ‘tool’ steel), heat-hardened and precision-fitted together. Unusually, the camshaft has only one lobe, rather than two. One cam follower rests on the lobe’s left side and another on its right, with their differing shapes and dimensions determining the sweep of the engine (the timing by which the inlet valve opens and closes, the piston compresses, and the exhaust valve opens and closes). “Because of how we’ve differently shaped the inlet valve’s cam follower and that of the exhaust valve, as well as the corresponding shape of the cam December/January 2025 | Uncrewed Systems Technology The connecting rod is milled steel, while the piston is cast aluminium from Honda Racing
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