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78 Dossier | Flygas Engineering GAS418S constructed from fibreglass composite and aluminium for optimal strength and rigidity, and has been carefully designed in CFD software to divide the airflow equally between the four cylinders. The camshaft sits in a dedicated chamber next to and below the crankshaft, with a gear on the latter driving one on the former in a standard 1:2 rotation ratio (22 teeth on the crank gear, 44 teeth on the cam gear). Two pairs of cam lobes are machined on the camshaft, and each cam alternates between lifting a tappet on its left and another on its right. Each tappet moves a pushrod and hence a rocker arm to lower a poppet valve; each cylinder accordingly has one intake valve and one exhaust valve. The valvetrain is quite standard and simple from a mechanical standpoint, in terms of the types of components used as well as the sequences in which they have been arranged. “However, each part of the valvetrain has been made using a careful and proprietary selection of materials and thermal treatments,” Gamberini says. “It took about 10 years of r&d to devise our valvetrain, to make sure we had something that would meet our requirements on rigidity and lifespan.” He adds that each intake valve head is 36 mm wide, and the exhaust valves are 42 mm. The exhaust manifold sits at the bottom if the engine is mounted horizontally (or at the back if in a vertical installation). In both cases, the manifold connects and mounts to the cylinder heads, and exhaust pipes are customised to meet the end-user’s requirements – standard issue includes double pipes with exhaust gas temperature and lambda sensors (depending on the preferred ECU arrangement). “We’ll give suggestions for exhaust pipe diameter, length, geometry and the muffler depending on the use-case; depending on the expected altitude and back-pressure, noise might not be a problem,” Gamberini says. Zucchini adds, “Back-pressure should not exceed 0.5 bar or else performance will suffer, so in addition to the fairly standard Y-shape exhaust manifolds we supply, we’ll make recommendations for exhaust pipes and mufflers, and we can supply them if requested, but we don’t manufacture them.” Fuel injection versus carburettor The standard-issue GAS418S is port injected at a typical pressure of 3 bar, although Gamberini notes that Flygas does have a direct injection version of the engine in development, which currently exists as a first prototype undergoing tests. “And although the injector sits in the port, essentially we spray directly into the cylinder, at an angle of about 45 º ,” he says. “If you don’t set that kind of spray angle and trajectory at the factory you February/March 2022 | Unmanned Systems Technology The plenum is constructed from fibreglass and aluminium for optimal strength and rigidity The gear-driven mechanical fuel pump is also supplied as a standalone product for use on other engines