What is it about?

Almost every jet engine flying today works the same way: a compressor and a turbine spin at enormous speed, air is squeezed, fuel is burned at constant pressure, and the hot gas pushes the aircraft forward. That rotating machinery is heavy, expensive, and made of exotic materials. Pulsejets — the engines that powered the German V-1 in the 1940s — take the opposite approach. They have essentially no moving parts. Fuel and air enter a tube, ignite from the hot gas left over from the previous burst, and the pressure rise slams the intake shut and blows exhaust out the back. The cycle repeats on its own, hundreds of times a second, driven by acoustics rather than by machinery. The result is a loud but astonishingly simple engine that costs very little to build. In this paper we used computational fluid dynamics to ask whether that simplicity can be kept while fixing the pulsejet's main weakness, which is poor efficiency. Starting from a hobby-scale pulsejet, we replaced the long acoustic tailpipe with a converging-diverging nozzle and added two simple rotating valves so the burning happens in a sealed chamber at constant volume rather than at constant pressure. We simulated the combustion, the fluid flow, the valve timing, different chamber and nozzle shapes, several operating frequencies, several ways of injecting propane fuel, and the behavior of the engine sitting still and flying at speeds up to Mach 0.8.

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Why is it important?

Small, fast unmanned aircraft are currently stuck between two bad options: propellers, which are cheap but slow, and miniature turbojets, which are fast but costly, fragile, and hard to shrink. An intermittent-combustion engine sidesteps that trade. It is a tube, a chamber, a nozzle, and two valves spun by a small electric motor — no turbine discs, no high-temperature blades, no tight tolerances. It tolerates damage, it is cheap enough to be expendable, and because its performance depends on the shape rather than on blade physics, it can be scaled up or down without losing efficiency. That last property matters enormously for miniaturization. The numbers from the simulations support the idea. The engine ran at 194 Hz with a thermodynamic cycle efficiency averaging 49 percent, which is competitive with far more complicated engines, and reached specific impulse values as high as about 4,750 seconds in a 275 m/s airstream — putting it in the company of efficient turbojets and pulse detonation engines. Just as useful operationally, it showed roughly a 4-to-1 thrust turndown, so the same engine can loiter economically for a long time and then accelerate hard when needed.

Perspectives

What I enjoyed most about this work is that it revisits an old, somewhat disreputable idea with modern tools. The pulsejet was dismissed for decades as a crude noisemaker, and in its original form that judgment was fair. But the underlying principle — let the combustion itself do the pumping, in bursts, instead of paying the weight and cost penalty of continuous rotating machinery — never stopped being attractive. Having CFD available means we can now ask precisely where the losses were and whether careful valve timing and nozzle design can recover them, which the original designers had no way to do. I was also struck by how much of the performance came down to seemingly small details. How far the chamber expands before the intake opens, whether the two valves close together or in sequence, where the fuel is injected and with what velocity — these shifted thrust and fuel consumption far more than I expected, and the single most valuable change was simply moving the air inlet onto the engine centerline, which created a vortex that mixed fuel and air and stopped fuel from escaping out the nozzle. My hope is that someone builds this and finds out where the simulations are wrong, because that is the next necessary step.

Andrey V Kuznetsov
North Carolina State University

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This page is a summary of: Computational Fluid Dynamics Simulations and Validation of a Novel Constant Volume Combustion Jet Engine, July 2016, American Institute of Aeronautics and Astronautics (AIAA),
DOI: 10.2514/6.2016-4811.
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