Unmanned Systems Technology 042 | Mayflower Autonomous Ship | Embedded Computing | ElevonX Sierra VTOL | UUVs insight | Flygas Engineering GAS418S | Ocean Business 2021 report | Electric motors | Priva Kompano

77 Flygas Engineering GAS418S | Dossier “Volumetric superchargers are generally great for maximum torque at 1000-2000 rpm, but upon reaching cruise, you’d heat up the air and lose so much power, and you’d still be carrying that very heavy blower. A centrifugal supercharger by contrast can be precisely mapped through the design of the internal transmission parts for very high engine volumetric efficiency at every crank rpm, without any thermally induced power losses.” Another critical consideration lies in how different forced induction systems perform at higher altitudes or anywhere with low ambient air pressure. The turbine in a turbocharger can be forced into overspeed in order to deliver the required air to the engine, which leads to expansion and contraction of the wheel, causing fatigue and eventually failure. The only feasible countermeasure is an electronic control system for shutting off turbo power, similar to the Rotax system described above, which adds weight and another point of failure prone to EMI. “A gear-driven centrifugal supercharger, by contrast, is by its nature machined, ground and assembled to tolerate extremely high speeds, so there’s no overspeeding to worry about. The impeller’s maximum safe speed is 92,000 rpm, and we never need to get anywhere close to that,” Gamberini says. “It will achieve the necessary boost speeds for the ECU mapping’s air intake densities without inducing fatigue on the impeller, and we’ll continue discharging exhaust gases without any build-up of back-pressure, as we’ve tested using sensors at our exhaust valves. Thus high power is sustained efficiently at altitude.” As a final point, Gamberini notes the importance of a large flywheel to four- stroke engines. The stability gained via the flywheel’s centrifugal mass does much for the lifespan of effectively every component inside and around the engine, but he points out that for aerospace engines, flywheel mass must be reduced somewhat to improve the weight efficiency of the aircraft. “You have to keep the flywheel to no more than 1 kg – you can’t have a UAV running on less than 200 bhp with a 3, 4 or 10 kg flywheel, let alone the 25 kg flywheels in car diesel engines,” he says. “So if we want to use a small flywheel but maintain a smooth, stable torque output, we first have to increase the cylinder count – having four cylinders helps in this regard – but also the inertia of our supercharger’s impeller has a stabilising effect on the running of the engine, much like a flywheel. “It only weighs 180 g, but it spins at 52,000 rpm at peak power, complementing our small flywheel and keeping the engine as reliable as if we had a 3-4 kg flywheel.” Intake systems The engine uses a 14 V, 32 A electric starter motor that works via a sprag clutch (a one-way freewheeling clutch) and absorbs only 1.2 bhp during operation, enabling a very small and lightweight battery to be used for start-up. Air intake is managed by a 70 mm- diameter throttle, which sits between the supercharger and the plenum atop the block. Flygas’ plenum is Unmanned Systems Technology | February/March 2022 The GAS418S is built with a closed-deck engine block that has been die-cast from aluminium alloy as two parts, with the split running lengthwise, parallel with the crankshaft and camshaft. A total of six M10 studs and six M8 bolts fastens the two halves together. The alloy itself contains about 5-10% silicon. It is not added specifically for mechanical or performance advantages but for improving the casting characteristics, as adding it reduces the molten aluminium’s viscosity without significantly affecting overall cost. Three main bearings hold the crankshaft in place, one at either end and one in the middle of the crankcase. The crankshaft itself is manufactured as a single piece, with the four con rods split at their big ends, each big end being held together by two studs. The con rods are cast from a commercial grade of steel, with sleeve bushing-type plain bearings installed in the big and small ends. Each con rod actuates its piston by way of a chrome-plated steel piston pin, to ensure smooth motion between the two components; a circlip holds each pin in place. The pistons themselves are forged from aluminium to maximise the effective material strength, by aligning the crystalline structure to handle the loads from combustion. The cylinders feature iron liners that are cast directly into the engine block, making each liner integral to its cylinder. The cylinder heads are die-cast from an undisclosed aluminium alloy that has been specially selected to prevent air voids forming in the metal during the casting process. They are then CNC-machined to precise tolerances for their valve guides, mounting points and other orifices. A gasket sits between the cylinders and their heads; it consists of two stainless steel nets, one atop the other, bonded within a rubber casing. Each cylinder head is then tightened in place using nine 12 mm studs of the same make as those used in the con rod big ends. All the studs and bolts used to fasten together the block halves and cylinder heads are made from an undisclosed non-aluminium metal. The high number and quality of the studs were chosen to prevent issues arising from the cylinder head sealing, such as leaking or burning, and such problems have yet to occur in the engine. Anatomy