What is it about?
Pulsejets are among the simplest jet engines ever built. Instead of the heavy compressors and turbines of a conventional jet, they rely on intermittent combustion driven by sound waves. Fuel burns in a short burst, pressure waves race back and forth along the tube, and these waves draw in fresh air and push hot gas out of the tailpipe, hundreds of times per second. The valveless version has no moving parts at all, not even the flapping reed valves that wear out in ordinary pulsejets. This makes it a promising, low-cost and rugged way to power small, high-speed drones, where conventional engines become expensive and inefficient as they shrink. In this paper we built and studied a valveless pulsejet only about 15 cm long, a scaled-down version of a popular 50 cm hobby engine. At this size the engine had to run on hydrogen, because ordinary fuels such as propane burn too slowly to keep up with the rapid pressure cycle. We systematically changed the length and width of the air inlet and the length of the exhaust pipe to find which combinations would run on their own. We measured pressure, temperature, thrust and operating frequency, and we built a three-dimensional computer model that reproduced the measured frequency to within 3 to 7%.
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Why is it important?
When we started this work, little was known about how valveless pulsejets behave at such small sizes. We found that the inlet and exhaust lengths are tightly linked. The shorter the inlet, the shorter the whole engine can be while still running, and our smallest working engines were under 16 cm long. Inlet length mattered far more than inlet width, although wider inlets raised the operating frequency, which reached over 1,000 cycles per second, and allowed a wider throttle range. Adding a flared exhaust nozzle let us shorten the engine by about 16% more. We also showed that the inlet can face sideways or even backwards, which helps keep the hot gas from escaping out of the front and wasting thrust. The simulations revealed what happens inside the engine during each cycle. All the oxygen for combustion enters through the inlet, while the exhaust pipe holds hot gas from several previous cycles. The incoming cold air forms a strong swirling vortex that speeds up the mixing of fuel and air. We concluded that the key limit on how small a pulsejet can be is a race between the speed of the chemical reaction and the speed of the pressure waves. These results provide practical design rules for shrinking pulsejets and served as a stepping stone toward even smaller engines for miniature drones.
Perspectives
This paper was an important bridge in our research program, connecting the familiar hobby-scale pulsejets to the truly miniature engines we built later. I remember how much trial and error went into finding geometries that would run on their own. Each working combination felt like a small victory, and the careful table of what worked and what did not became one of the most useful outcomes of this study. I am grateful to my co-authors Tao Geng, M. A. Schoen and Will Roberts for their dedication, and to DARPA for supporting this work. What fascinates me most is that the success of such a simple device depends on a delicate balance between chemistry, fluid flow and acoustics. Get the timing right, and a plain metal tube with no moving parts produces thrust; get it wrong, and the engine falls silent. I hope this paper helps engineers see valveless pulsejets as a realistic option for low-cost propulsion of small, fast drones, and encourages further work on making these engines more efficient.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: Combined Numerical and Experimental Investigation of a 15-cm Valveless Pulsejet, Flow Turbulence and Combustion, August 2006, Springer Science + Business Media,
DOI: 10.1007/s10494-006-9032-8.
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