70 Engine dossier | Wave J-1 “Our engine runs at 100 Hz – that’s the equivalent of a four-stroke engine running at 12,000 rpm in combustion speed terms. It brings some challenges in the physics of it all, and in how tightly one has to electronically control the injection timing and pressure, but the highly turbulent environment in the combustion chamber helps us pull off a quick and thorough mixing.” Further gains will be achieved through CFD optimisation of the combustion chamber in future versions of the engine. Productionising the J-1 has been prioritised over ‘completing’ CFD simulations for now, as performing CFD work on pulsejets is inherently complicated due to the challenges of modelling the properties of the airflow inside the engine. “The airflow in pulsejets’ innards is unsteady, compressible, multiphase and reactive – that’s every type of quality that can make CFD complicated. There is literally no way left to make our CFD more difficult,” Maqbool says. “That said, it makes absolute sense to open up CFD investigations more and more, and we do want to engage heavily in that in the future. We just haven’t prioritised it, so as to avoid splitting focus away from core functions, reliability and controllability.” Squish and burn The pressure waves of air coming from either pipe contributes some compression to the fuel/air mixture in the combustion chamber. If one placed a pressure sensor inside, one would see a spike in pressure during and immediately after the combustion event, and before intake one would read negative pressure. “Some have assumed a Lenoir cycle for pulsejets in which there is no compression, just a constant volume combustion event where pressure spikes and then gradually comes down, and that’s it, never any negative pressure in the chamber,” Maqbool says. “Frankly, I don’t agree with that: there is exhaustive evidence indicating that a compression wave comes back during intake. It’s not huge, and the two columns of air ramming into each other aren’t quite a full-on mechanical piston and cylinder head, but it does provide some squish, which assists with the combustion.” A modified COTS capacitive discharge ignition system is used to spark the fuel/ air mixture, although a customised and proprietary system is being developed for embodiments where heavy fuel is preferred. With the air inlets being open and valveless, the engine is throttled up and down by regulating the fuel flow up and down via the EFI. Maqbool reports that unlike most gas turbines, which can take 5-15 seconds to go from idle to maximum rpm due to the mass and inertia of the rotating machinery, the J-1’s throttle response is extremely rapid. October/November 2024 | Uncrewed Systems Technology Air intake happens via both pipe apertures, meaning two sources of pressure waves meet in the combustion chamber and contribute compression to the fuel/air mix
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