Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

50 Dossier | Northwest UAV NW-44 made in one piece rather than built up, with the main bearings sliding onto one end and the big-end bearing sliding onto the other, so no con rod cap split is needed. The main bearings are ball bearings, “sized to make the engine robust”, notes Harris. Left open at the sides, they are lubricated by oil carried in the charge rather than by grease, avoiding the friction associated with sealing for the latter. The needle roller big end is similarly lubricated by oil mixed into the fuel, at a ratio of 1:50. This avoids the complication of any form of lubrication system. Harris says, “Our trade study revealed that the additional complexity, weight and failure modes associated with an independent lubrication system didn’t provide any benefit over trained technicians mixing the fuel.” Likewise, the additional weight, package size and complexity represented by a water-cooling system sees the NW-44 air cooled. The use of heavy fuel leads to lower cylinder temperatures than running on gasoline; nevertheless, Harris admits that the NW-44 has more than enough cylinder head finning for the use of all fuels, to provide a safety margin. “Heat dissipation is one of the biggest challenges of this type of engine,” he remarks. Opting to make this 44.2 cc displacement engine with its upright cylinder over-square helped keep it squat; low in frontal area. It is only slightly over- square since ultimately the cylinder bore size was limited by the need to avoid the danger of detonation. The piston crown has a slight dome, and at TDC the outer portion of it comes tight to the squish area surrounding the hemispherical chamber formed in the head. The NW-44 was designed from the outset for multi-fuel operation, and Harris remarks that running on the likes of Jet A1, JP5 or JP8 doesn’t imply any compromise in performance compared with gasoline. Mechanically the engine remains unchanged; it is simply remapped for the fuel to be used. “This is down to our experience and the understanding we have gained of the physics of the combustion process,” Harris says. “The physics is something that is not well known. We have spent five years researching it, working with a university at PhD level.” Harris notes that the impressive performance obtained on heavy fuel is a function of many factors, including the shape of the combustion chamber in the head, the shape of the piston crown, the squish band, the fuel droplet size and so on. “There are many, many details that you need to get right. In some instances we actually get better performance on heavy fuel than on gasoline,” he says. The NW-44 was also designed from the outset to use twin plugs for redundancy, anticipating future FAA UAV certification requirements. Both plugs enter the hemispherical chamber at an angle, one each side. While this twin-plug solution adds weight and complexity, it has been found to enhance fuel efficiency to the extent that it is of net benefit, as the twin ignition points promote more complete combustion. When running any two-stroke on heavy fuel it is normal to preheat the charge to assist starting from cold. The approach taken by NWUAV is to fit a small (diesel engine-type) glow plug into one side of the NW-44 cylinder head casting (it does not reach into the combustion chamber). This warms the top end sufficiently for cold starts on heavy fuel, even in sub- zero temperatures. Harris notes that the NW-44’s porting has been developed with fuel economy as the primary concern. There is no standard number of transfer ports for a small-displacement two-stroke, and NWUAV keeps the NW-44 February/March 2016 | Unmanned Systems Technology The NW-44 powers Brican Flight Systems’ TD-100LE UAV platform Working on an NW-44 at NWUAV’s facility near Portland, Oregon