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

88 Dossier | Limbach L 2400 DX and L 550 EFG Tiers of cooling The oil cooling is key to removing the heat generated by friction throughout the crankcase, cylinders, generator and turbo, which Limbach estimates accounts for roughly 5% of the engine’s heat. Around 70% is dissipated at the cylinder heads through a liquid-cooling system, with the rest being dissipated through two direct-air cooling systems: one for the cylinders, the other for the generator. A mixture of water and ethylene glycol (50% glycol, for frost and corrosion protection) is pumped through 25 mm pipes into the cylinder heads at a rate of about 0.1 m 3 /minute when the engine is running at 3000 rpm. The pump is belt- driven by the crankshaft, and a tensioning adjustment arm and jockey wheel sustain the tension on the drive belt. After encircling the combustion chambers, including the spark plugs and exhaust valves, the water exits the heads through tubes opposite the inlet pipes and on to a collector, a thermostat and then a radiator before returning to the pump. The standard-issue radiator is sized for an anticipated airspeed of 40 m/s (144 kph) during climb and a temperature differential of 60 oC between the liquid coolant entering the radiator and the ambient air. A water-glycol temperature of 110 oC is the maximum tolerable. However, the radiator can also be resized to suit each end-user’s climb airspeed, their air inlet and outlet designs, and what positions are available for mounting it. For instance, a smaller radiator can be used if the UAV flies much faster than 144 kph during ascent. “The liquid cooling as well as the cylinder heads are optimised for the pressure of the water-glycol mix, and are unique to our 2400 series engines,” Simmerkuss says. “They were developed specifically to enable the higher power output of this engine, and to prevent the kind of wear that cylinders and cylinder heads can suffer without a rigorous design of the cooling system. Most of our other engines use only air cooling.” Ram-air cooling for the cylinders requires only 25% of the airflow necessary for cooling a conventional air-cooled engine, although some additional pressure to around 1.2 bar is ideal. That is a guide value: the important end result is that air flows over the cylinders at roughly 0.2m 3 /s, or 0.17 kg/s. To enable that, a pair of inlets made fromfibreglass-reinforced plastic composite sit on top of the engine and connect to ducts that direct the airflow to the cylinders’ surfaces. “We could have designed the cylinders such that the water flowed from the heads down into the cylinder walls, but this would have meant excessive cooling relative to the design and manufacturing changes needed – air is sufficient for the cylinders,” Simmerkuss adds. “Ideally, the engine should be used with a tractor propeller, as airflow coming from the prop will directly enter the two inlets and go towards the cylinder fins, and that tends to provide sufficient air pressure for cooling the cylinders. But a pusher configuration is also no problem, so long as sufficient cooling air can be directed to the air inlets.” In addition to the two ducts that send air coolant to the four cylinders, a third duct directs air to the generator. Once the high- pressure air has flowed through the stator and the cylinder fins, it returns to a pressure closer to ambient levels and exits through an outlet in the engine cowling. February/March 2023 | Uncrewed Systems Technology While much of the L 2400 DX is cooled by the oil and by water-glycol, the cylinders use ram-air, ideally generated by the propellers pushing air through dedicated inlets on the engine’s front The engine should be used with a tractor propeller, as the airflow from the prop provides sufficient cooling, but a pusher set-up is also no problem