64 Pulsejet engines are, mechanically speaking, among the simplest combustion engines in existence. They typically consist of a short, stubby intake pipe, a longer exhaust pipe, a combustion chamber, an ECU, and some fuel valving and injection parts possibly present in modern versions. At a high level, such engines do not function dissimilarly to Braytoncycle models such as the gas turbines previously covered in this publication: air is drawn into the combustion chamber, where it is compressed and mixed with fuel, and the resulting fuel/air mixture is ignited, triggering a combustion event that generates pressure and temperature, which is used to create thrust and exhaust. The main difference is that the process in a pulsejet is unsteady and time-variant, and the compression for the fuel/air charge is provided by the acoustic waves from the previous combustion event, rather than by spinning turbomachinery, as in a gas turbine engine. Additionally, the hot thrust/exhaust gas generated from a pulsejet’s combustion will shoot out of both the intake and exhaust tubes. Hence, referring to them as such may be considered a misnomer, except for the practical aspect of easy referencing. However, rather than the air pressure in the combustion chamber simply dropping back to atmospheric level after combustion, the inertia of the exhausting gases causes them to overshoot, creating a vacuum effect. This means that immediately after the exhaust phase, fresh air is sucked back into the chamber through both pulsejet tubes, enabling the combustion cycle to repeat (although most of the air subsequently burned will have come via the intake tube, given the shorter distance to travel than from the exhaust tube). All of this largely occurs as a consequence of air being a compressible medium with a spring-like nature – that quality also being the key to how humans vocalise. “That combustion cycle can repeat indefinitely, its frequency depending on the size of the pulsejet engine; the bigger the engine, the lower the frequency, due to acoustic principles. Blow a large horn, you get a low-frequency blast of music; blow a small horn, you get something more high-pitched,” says Daanish Maqbool, CEO of Wave Engine Corporation. “Most practical pulsejet engines operate around 50-250 Hz, so our J-1 engine product operates at around 100 Hz, regardless of where it is in its power curve, as a pulsejet’s combustion or operating frequency is dependent upon its geometry, not on how or when we throttle up or down.” One company is designing pulsejet engines with UAVs in mind, as Rory Jackson reports On the pulse October/November 2024 | Uncrewed Systems Technology Wave Engine has tested its J-1 pulsejet via a simple external integration on a test aircraft, although internal integrations are possible (Images courtesy of Wave Engine)
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