Unmanned Systems Technology 018 | CES show report | ASV Global C-Cat 3 USV | Test centres | UUVs insight | Limbach L 275 EF | Lidar systems | Heliceo DroneBox | Composites

65 Limbach L 275 EF | Dossier bearing is actually a pair of bearings, each two-thirds the width of the rear bearing. Limbach explains of the twin front bearing arrangement, “We wanted to take some of the bending load that might be introduced by the propeller out of the crankshaft.” The crankshaft is forged steel. Originally it had induction-hardened journals, but better results were obtained by moving to case hardening. Sandwiching its two pins, the crankshaft has a full-circle centre web and ‘pork chop’ style outer webs. The outer webs each provide a balance factor of the order of 30%. The central full-circle web does not fully seal the crankcase into front and rear sections and (despite the 22 mm bank offset) each cylinder’s intake port width is such that the incoming charge accesses both sections. Limbach says, “We have a common crankcase for both cylinders – you can consider it as operating like a single- cylinder two-stroke [both cylinders firing at the same time] – so you don’t need to seal the crankcase into separate sections. “But we did see on the carburettor engine that if one carburettor fails then that really only affects the cylinder on which the carburettor is mounted. In practice, there is little if any mingling of the individual cylinder fuel-air mixtures.” Even though the con rod has a split big end, it runs on steel needle roller bearings, using split cages. The con rod cap is fracture-split. “We introduced that long before fracture-split rods were first used for production car engines,” Limbach notes. The small end has a needle roller bearing between the con rod and the piston pin, even though the steel pin is DLC coated. The pin runs directly in the aluminium alloy piston, and after a couple of decades of running uncoated a problem of accelerated pin wear occurred “out of the blue”, Limbach explains. The pin was case hardened steel yet was found to be wearing faster than its piston bores. This followed a change of process by the pin manufacturer. “There had been a problem with the hardening oven, and the amount of carbon in the pin had been far too high, causing other customers to complain and forcing the manufacturer to repair the oven,” Limbach says. “Nevertheless, for us, from then on the pins didn’t work anymore. We considered increasing the amount of carbon but the repaired oven wouldn’t allow us to do that. The problem was overcome by introducing a DLC coating.” The cylinder bores have always been nickel silicon carbide coated, while the piston and its two cast iron rings are uncoated aside from a phosphate coating for the rings that wears off as they run in. The piston crown is flat while the chamber in the head has a tri-oval rather than circular plan form surrounded by a slight taper for the squish band. Both the chamber and the spark plug are slightly offset from the cylinder axis, the plug being located towards the base of the chamber’s tri-oval form. The L 275 EF is run by a full engine management system, and ignition timing is mapped according to load demand and crankshaft speed: it is within 37 º BTDC (high rpm) and 25 º BTDC (low rpm). Limbach says detonation is not an issue with this engine. The fact that it is a two- stroke, the way it is operated and its use at altitude mean that the effective compression ratio is less than 8:1, he observes, and running on 92 RON (or better) gasoline keeps the threat of detonation at bay. He says the introduction of port fuel injection for the engine was straightforward, given that his company had already developed the technology for its four-stroke aero engines. “We knew from that experience what was required. The main challenges we had were the packaging of the system and the manufacturing of in-house components. We make our own throttle bodies, for example.” Between the circular exit of the throttle body and the elliptical intake port entrance below it is a short manifold made of a thermoplastic material. That provides a thermal barrier between the cylinder and the throttle body, where the fuel is injected into the incoming air. Limbach notes that any heat reaching the throttle body will adversely affect cylinder filling. The ignition system uses a pair of induction coils, mounted off the respective plug (on the airframe) and with the driver integrated into the engine control unit Unmanned Systems Technology | February/March 2018 The L 275 EF throttle body houses a butterfly throttle and a fuel injector