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74 conventional electrically driven pumps as standard, which can be installed anywhere convenient, inlet and outlet hoses permitting. “There is no liner in our cylinder, because liners increase the weight. It makes far more sense to coat the working surface of the cylinder in nickel silicon carbide, which is a common, well- understood approach to protecting the aluminium cylinder walls. We also opt for high-quality, more expensive coating suppliers because that pays dividends in engine lifespan,” Dundalek explains. “There’s only one potential problem with that: our cylinders aren’t open. As with many two-stroke petrol engines, our cylinder heads form part of the cylinders, and most nickel plating suppliers are used to doing their coating with cylinders that are open from top to bottom. Only a few do ‘blind hole’ nickel coating; it’s challenging and we can’t do it in-house. “But we don’t want to switch to a design that uses separate cylinders and heads, even if that would mean more security regarding our coating suppliers, because then we’d need additional screws to bolt the heads on. Those are often very long screws, made from steel in order to reliably tighten the heads and the cylinders right down against the crankcase with no gas leaks, and that would greatly increase the weight, as well as the part count.” Piston and crank Most of MVVS’ piston manufacturing is carried out by fellow Czech company Almet, although some Italian companies such as Meteor Pistons and Vertex also serve as suppliers to its other engine models. “The piston is made from aluminium alloy, and we use one ring per piston,” Dundalek says. “What is perhaps most interesting is the molybdenum sulphite coating we use on the outer surface of the piston. It enables better running-in of the pistons in the cylinders, with reduced friction and wear from contact with the nickel silicon carbide, and hence a longer lifetime for these parts of the engine.” The piston faces are slightly convex, with the indentation of the combustion chamber designed into the cylinder head. That helps keep flame front distribution even across the piston crown, and maintains a middle ground in terms of compression between a dish-top piston and one with a more strongly pronounced dome (the latter of which would increase compression but also weight). Also, following their manufacture, the quality of the pistons is validated using testing tools such as ultrasonic devices, spectrometers and automated measurement lines, to ensure that physical, geometric and void tolerances for each unit are being met. The pistons connect to the con rods via piston pins that float in the former but are fixed into the latter via needle bearings. Each con rod is produced as a single piece from Certal, an aluminium alloy (AlZn 5 Mg 3 Cu, sometimes known as grade 7022) which has high tensile strength and shape stability, and good machinability. It is well-suited to heat treatments and welding, although no heat treatment is considered necessary for the 116’s rods. The crankshaft is produced largely in- house as multiple pieces before being pressed together, largely to enable the use of single-piece con rods as well as easier installation of both the rods and bearings. Dundalek comments that the shaft is CNC- machined from steel alloy before being ground to exact tolerances, and then heat- April/May 2022 | Unmanned Systems Technology The cylinder was originally designed for MVVS’ single- cylinder 58 cc engines, and is used in the boxer twin 116s for customer familiarity and simplicity The pistons are coated in molybdenum sulphite to reduce friction and wear against the nickel silicon carbide that coats the inside of the cylinders