UST030

62 February/March 2020 | Unmanned Systems Technology IER series hybrid Opposed-piston four-stroke Gasoline and propane-fuelled Dual crankshaft, dual-piston, single-cylinder 62 x 58 mm bore and stroke 350 cc 12 kg Aluminium-Nikasil liners Naturally aspirated Air cooling Spark ignition 11 kW at 6800 rpm Compression ratio, 10:1 Maximum rpm, 7000 The IER ARE hybrid is built around a cast aluminium engine block that consists broadly of four sections, with each of the two inner sections forming the inner half of the crankcase and one half of the cylinder. The two inner parts are joined by a metal gasket part, with six 8 x 32 mm titanium stud bolts and two 8 x 14 mm dowel pins holding them together. The two outer parts of the engine block serve as the outer halves of either crankcase, and each is fastened onto its inner half by eight flange bolts: four 8 x 35 mm bolts, and four 6 x 20 mm bolts. One of each flange bolt is also used to install a mounting bracket on the outside of each crankcase for installing the system on a UAV airframe. Each cylinder half has a 62 mm bore and two 58 mm stroke, for a total displacement of 350 cc. The cylinders have Nikasil liners, and the pistons have been cast from aluminium using pore-free die casting. Each piston has three rings, with two compression rings on top and an oiling ring underneath. Each also has a piston pin, which is installed into a slot beneath the piston crown. The pins (and the monolithic con rods) are then held in place by piston clips on either side. Each of the ARE’s twin crankshafts is assembled from three main parts – a front shaft section, a crank pin and a back shaft section. Each of the four ‘shaft sections’ throughout the engine also features a counterweight to help reduce vibration. Both crankshaft front ends have a Woodruff key and an oil seal, with a crank gear fitted on the back ends to drive the camshafts. Also on the back end of the left crank is (going outwards) an additional oil seal, a timing gear (for the timing belt running from the left crank to the right cam) with a flange on either side, and a magneto rotor for the ARE’s ignition. All of these are held in place by two 5 x 25 mm socket bolts, a collar and a 4 x 8 mm dowel pin. A stator coil installed under the magneto connects to the 8 mm spark plug via a wire to the front exterior of the cylinder. The timing belt runs from the left crank’s pulley to a larger timing sprocket, which is fixed behind the right cam gear using a washer and a 12 mm flange nut. The cam gears are fitted onto the backs of the camshafts, with the former being driven by the crank gears to turn the latter at half-rpm speed. Each camshaft is machined with two lobes for actuating both of its valves simultaneously. Each valve assembly consists of (from base to valve seat) a tappet lifter, a shim, a cotter, a spring retainer, the spring itself, a stem seal, a spring seat and then the valve itself. The two generators are mounted and driven directly via the crankshafts. The forced-air fan shrouds sit on the front of the crankcases, each shroud atop four 6 mm flange collars, with a combination of 6 x 10 mm titanium pan screws and 6 x 20 titanium flange bolts fitting into the collars. Each stator is installed into its fan shroud using four 5 x 40 mm titanium socket bolts, with the outer rotors mounted atop (and around) their stators. Eight 6 x 25 mm flange bolts affix the cooling fans to the rotors, and over the top of these, six push-pull rivets fasten the fan covers to the fan shrouds. Anatomy The engine block is cast from aluminium in four parts, with the cylinder formed at the centre via a gasket, two dowel pins and six titanium stud bolts