74 Dossier | UAVHE RW1 rotary leaks and hence oil consumption, even when they are designed with their rotor bearings sitting in an oil bath, supposedly sealed from fuel and air, UAVHE has opted for the two-stroke approach of mixing oil into the fuel, such that the oil can contribute to cooling and then burn up, instead of contaminating the rest of the engine via blow-by or other leakage. The cooling of the outer housing is far more straightforward in both engines. The RW1-79 is cooled by air flow, with the supercharger pulling its intake air via the electric motor, and the latter’s rotor dispersing part of that air into the channels and ribs about the engine housing. Those heat-sink structures are geometrically optimised via CFD and thermal FEA to compensate for the non-uniform heat distributions of Wankel engines. The RW1-300, being a larger and higher-power engine, is liquid-cooled using a standard, water-glycol jacket running internal to the housing midplate. It typically integrates with dualredundant cooling loops, radiators and pumps, the lattermost devices having variable capacities to ensure temperature optimisation during flight and a controlled rate of cooling after landing. Fuel delivery In addition to the first stage of fuel injection shortly after air enters the engine, a second stage is sprayed directly into the combustion chamber, precisely calculated for speed and position about the twin spark plugs. The RW1s (as with all UAVHE engines) integrate a proprietary fuel-injection system. “It is similar to how inkjet printers distribute ink,” says Bogdanov. “We’ve built the system module around a matrix injector that can mechanically alter the focal length and width of the spray nozzle, and it’s around 100 times faster than conventional injectors in its ability to control outputs of droplets. “That allows us to closely control the size of each droplet, and hence control fuel pressure, which hugely helps us achieve the ideal fuel/air mixture and high-quality combustion conditions. The ability to maintain such conditions, including the ideal MAP via our supercharger, was something we wanted to be sure we could achieve before we started cutting metal for rotary engines.” Each module features nine injectors (in a 3x3 matrix) and functions, such that each column and row can be rotationally actuated to control the spray angle. This also helps control droplet size, as thanks to UAVHE’s CFD modelling of airflow inside the chamber, the ECU can aim a given injector to spray into the airflow to ensure a particular coagulation or dispersion of fuel henceforth. “To improve fuel burn, the second fuel stage doesn’t involve just one fuel spray per rotor compression,” Bogdanov says. “The internal volume of the Wankel combustion chamber changes at each moment of the crank rotation, and not in the relatively simple way that reciprocating engines’ spaces between piston and cylinder head change, so the curve of the air/fuel mixture over time resembles a letter ‘m’, to keep the air/fuel ratio efficient and stable. “In essence, the changing shape of the Wankel combustion chamber is such that when the first stage of April/May 2024 | Uncrewed Systems Technology UAVHE’s proprietary fuel system is described as similar to inkjet printers, and enabling close control of droplet sizes, fuel pressure and hence combustion conditions A matrix injector that can mechanically alter the focal length and width of the spray nozzle, around 100 times faster than conventional injectors
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