Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

56 A single injector projects into one side of the throttle body, positioned to discharge just under a servo-operated butterfly, into the reed opening in the top or bottom of the crankcase (according to how the engine is positioned). From the crankcase the charge travels through two main transfer posts, one each side and with a central divider to support the piston ring, and two rear ports. Schnuerle-type loop scavenging is used. Of the twin rear transfers, Johnson notes, “One of our smaller engines has just one rear transfer but we like the twin arrangement. There is a little offset in terms of how they are shooting across the top of the piston.” The cylinders are cast in aluminium alloy 356, and combine the barrel and the head. Johnson says, “We use investment casting to obtain the transfer ports we require – die casting limits you. Although more expensive than die casting, investment casting in wax allows us to get the 3D forms we need. “The rear boost ports are ‘finger ports’, meaning they go up the side of the cylinder and can be machined after casting, whereas the main transfers are true dedicated tunnels exploiting investment casting.” The combustion chamber in the head positions the spark plug centrally and upright. It has a central dome surrounded by a squish band that has a slight taper. The piston crown has a shallow dome. Its outer area is not directly parallel to the squish band formed by the head, although it is tapered, forming a wedge- shaped squish area. The single exhaust port feeds directly into an exhaust system that is application-dependent. “We have various configurations that can be used, including long tuned pipes and short canisters,” explains Johnson. We asked, if a customer wants a really quiet system, what do you recommend? “We use what we call a header and canister system, whereby we have a fairly long canister fed by a header pipe,” Johnson says. “Once you get into longer systems you have a tuning effect on the engine, and you need a specific length of canister to maintain optimum engine performance. It isn’t like a full tuned pipe on a road racer but it does have a significant effect on the operation of the engine.” Crankshaft and piston assembly The steel crankshaft is forged and is machined in-house before going out for heat treatment. “It has a proprietary heat treatment process,” reports Johnson. “Basically case hardening. Then the journals are ground in-house.” The crankshaft is made in five parts (including the two pins) that are pressed together. It runs in three ball bearings. The crankcase is two-piece, split transversally, and the bearings are a press fit, using heat expansion on assembly. “It is an art, getting the bearings not too tight but not too loose,” remarks Johnson. “The bearing seats are wide lands, and the bearings are each captured by a lip formed by the crankcase that stops any fore/aft thrust movement.” Rather than having an open bearing lubricated by the pre-mix, all three are sealed greased ball bearings. Johnson notes, “That is a lighter option but you are heavily dependent on the quality of the seals. However, we don’t get many bearing issues. If a bearing gets hot and abused we might lose a seal and have to replace it, but overall the success ratio is excellent and the simplicity is an advantage.” Two of the bearings sandwich the web- crankpin assembly while the third is August/September 2017 | Unmanned Systems Technology Dossier | Desert Aircraft DA150 EFI A DA150 cast aluminium alloy piston