Uncrewed Systems Technology 048 | Kodiak Driver | 5G focus | Tiburon USV | Skypersonic Skycopter and Skyrover | CES 2023 | Limbach L 2400 DX and L 550 EFG | NXInnovation NX 100 Enviro | Solar power focus | Protegimus Protection

83 The ‘G’ in EFG stands for gearbox, one of which has been installed on the L 550 EF to reduce its speed and increase its torque at the output shaft more reliably than a belt or chain. The engine has also been fitted with a new starter/generator from ePropelled, to better satisfy the growing demand for high electrical power in UAVs for payloads and back-up electrical propulsion systems for greater redundancy in flight. The L 550 EFG displaces 548 cc, produces 50 bhp at a crank speed of 7500 rpm, and weighs 15 kg without its gearbox, or 28 kg with it. It is predominantly aluminium, with a die- cast crankcase and cast aluminium alloy pistons and cylinders (as well as a nickel silicon carbide coating in the latter’s barrels). Although further rig testing is needed to fully validate numbers, present calculations indicate a peak torque of 96 Nm at the top crank speed. The other engine is the L 2400 DX, and it is Limbach’s most powerful unit. It has a dry weight of 86 kg, and at its peak power of 118 kW and 3000 rpm crank speed it produces 375.63 Nm of torque. It also comes with options such as a dry sump. The bulk of Limbach’s r&d since our previous investigation has gone into four- strokes, hence our principal focus on the L2400 DXhere, although we will also delve into the new features in the L 550 EFG. L 2400 DX operation The L 2400 DX runs using electronic fuel injection and engine management, as is commonly requested by the UAV industry. It also comes with a ‘two-tier’ cooling system, which combines air and liquid cooling in different methods. Air distribution for combustion occurs via an airbox on top of the engine, with an intake collector manifold sitting near the upper rear. The manifold houses the throttle, fromwhich the airbox’s four distribution pipes run across the engine block and into the cylinders. The manifold is cut from aluminium to save weight while maximising strength and thermal stability, although a fresh set of flow simulations is planned to optimise its design and construction, and to see if newer materials or technology could be exploited to improve the flow. Intake boost comes from an exhaust gas turbocharger at the rear of the engine block, installed principally for altitude compensation as more and more UAVs conduct survey and mapping missions at increasingly high altitudes. “We’ve therefore selected amodel that can generate good boost pressure even at high altitudes and relatively low engine speeds of 3000 rpm,” says Sven Simmerkuss, development and test manager at Limbach. The turbocharger is a standard, single- scroll design, with each cylinder featuring an exhaust pipe running backwards and feeding into a single manifold collector that connects with the turbine inlet to drive the compressor shaft, and an EGT sensor installed in the collector. Simmerkuss notes here that a twin- scroll turbo had also been tested with this engine when it was first designed, but it brought no advantages. The present design itself is a proprietary Limbach Limbach L 2400 DX and L 550 EFG | Dossier The turbocharger model we’ve selected can generate good boost pressure even at high altitudes and relatively low engine speeds Uncrewed Systems Technology | February/March 2023 Intake boost for the L 2400 DX comes from a single-scroll turbocharger at the engine’s rear, fed by a single manifold that collects the exhaust from all four cylinders