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72 Dossier | MVVS 116 NP LC INJ using the piston’s downstroke, with a single exhaust port for evacuating gases as fresh charge rushes in. Dimensions and heights of the ports are confidential. The crankcase, it is worth noting, splits vertically – that is, with the two halves coming apart in the longitudinal axis – which can be useful during servicing for helicopter UAV users given the engine’s vertical orientation in such applications. “The crankcase is CNC-machined from duralumin, [EN AW 7075],” Dundalek says. “It’s an aluminium alloy with small percentages of copper, manganese and magnesium, which gives it better characteristics in loading, strength and machinability. “We went with machine-cutting duralumin billet rather than casting because of the cost. The number of engines we produce across the UAV market is too high for mass-producing such a small engine from a casting, but through CNC cutting we can flexibly customise the crankcase in minor ways from unit to unit, to ensure a tight footprint for integration into different UAVs. Trying to do that with casting mould tooling would be very expensive and challenging.” The bore and stroke of the 116’s cylinders are both 42 mm; they feature no cooling fins, being an unnecessary burden in terms of material weight and cost when liquid cooling is used. Instead, open channels run inside the circumference of each cylinder, around both the transfer ports and exhaust port as well as the combustion chamber and spark plug. A single liquid inlet port sits on one side of each cylinder, with an outlet on the opposite side, and each protrudes a couple of centimetres outwards. “We use two pumps for the liquid cooling,” Dundalek says. “One is a gear pump, which is mechanically driven off a gear on the front of the crankshaft, and we tend to use that more often for the single-cylinder 58 cc engines. In the two- cylinder engines we more often use April/May 2022 | Unmanned Systems Technology The 116 NP LC is a boxer twin two-stroke engine made predominantly from aluminium, featuring two linerless 58 cc cylinders, each of which has a nickel silicon carbide bore coating. Its crankcase is made from two halves, machine cut from an aluminium alloy containing copper, magnesium and manganese. The halves split perpendicularly to the engine’s main axis, and are joined by four long steel bolts – two from the front and two from the back. The heads are integral to the cylinders; each cylinder and head is cast simultaneously as a single part from aluminium alloy. Each cylinder is linerless, and mounts to the crankcase by way of four small tailor-made screws and nuts, as well as a gasket for sealing, with a nickel silicon carbide coating galvanically applied inside the bore. Also, under each cylinder a flange gasket is secured in place by two nuts and screws for mounting exhaust pipes. The pistons are cast from an aluminium alloy with a high percentage of silicon, and coated in molybdenum sulphite. A single compression ring from hardened cast iron is installed on each piston. Like the crankshaft, the piston pins are also cut from steel, and heat treated (given their proximity to the combustion chamber), then pressed into needle bearings to bond them in the con rod small ends. The pins float in the pistons, with a retaining clip on each side of each pin to lock it in place and keep it from the cylinder walls. The con rods themselves are cut from Certal aluminium alloy, with needle bearings also installed in their big ends. The crankshaft is machine-cut from steel alloy before being machine-ground, heat-treated and then pressed together (with the con rods already mounted, as they are single-piece rods machined from AlZn 5 Mg 3 Cu). The shaft consists of five key parts – two crank pins, a central crank web, a forward web that features the main bearing journal and the PTO, and a rear web which has the rear bearing journal (and can also feature a rear PTO). Three steel bearings – two ball roller bearings on the front journal, and a needle roller bearing on the rear journal – are used to securely mount the crankshaft in the crankcase. In front of the forward bearing, a large packing ring is installed, with a drive washer enclosing the front end of the crankcase and locking into the front end of the crankshaft by way of a standard semicircular washer key piece. A propeller screw inserts into the front of the drive washer, and a propeller washer slides onto the length of the prop screw before six large steel screws are used to fasten the propeller and drive washers in place. The intake assembly bolts onto the crankcase using two long steel fasteners, through either a throttle and carburettor assembly or a throttle and manifold with a single fuel injector to the side. In both cases, a flange and flange gasket connect with an upper reed valve gasket to seal this section to the reed valve case. The reed valves themselves mount to the case via two small screws and a strap, and a large flange sandwiched by an additional pair of reed valve gaskets is installed beneath the case for mounting with the crankcase. Anatomy