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 short bursts, and pressure waves bouncing inside the engine pull in fresh air and push hot gas out of the tailpipe, over and over again. The valveless pulsejet has no moving parts at all. This makes it a very attractive candidate for powering small, fast drones, where conventional engines become inefficient, expensive and prone to wear as they are shrunk down. In this paper we built and tested a valveless pulsejet only about 8 cm long, which to our knowledge is the smallest working pulsejet ever reported. Running on hydrogen, it fires roughly a thousand times per second with a high-pitched hum. We compared two designs: a single inlet facing forward and a pair of inlets facing backward. We measured pressure, temperature, thrust and operating frequency, and we built detailed computer simulations that closely matched the experiments. The simulations let us follow the full cycle step by step, from ignition to the intake of fresh air, inside an engine far too small to probe easily with instruments.
Featured Image
Photo by Hermeus on Unsplash
Why is it important?
Shrinking an engine is hard. At this scale, the fuel and air spend very little time in the combustion chamber, and heat escapes quickly through the walls. Both effects can stop the engine from running at all. We showed that a pulsejet can still sustain itself at this size, and we identified what makes it work. Turning the inlets to face backward was critical. With a forward-facing inlet, hot gas escapes out of the front and cancels most of the thrust. With rearward-facing inlets, all the exhaust pushes in the same direction, and the engine produced about 1 N of net thrust. The efficiency is still low, but this is a meaningful result for an engine weighing so little and containing no moving parts. We also found a new design rule. In a traditional valved pulsejet, the running frequency depends only on the total engine length, much like an organ pipe. In the valveless engine, the inlet length also matters: the frequency scales with the inlet length raised to the power of minus 0.22. Our simulations also showed that nearly all the oxygen for combustion enters through the inlet, so inlet design largely controls how well these engines perform. These findings give engineers practical guidance for designing miniature, low-cost propulsion for small high-speed drones.
Perspectives
It was exciting to see an engine small enough to hold between two fingers start up and run on its own. Many people doubted that a pulsejet could work at this scale, because the chemistry and heat losses seemed to work against it. The close agreement between our simulations and experiments gave us confidence that we really understand what goes on inside such a tiny engine. I am grateful to my co-authors Tao Geng, Fei Zheng, Andrew Kiker and Will Roberts for their skill and persistence. I also thank DARPA for supporting this work. Small drones are becoming increasingly important, and the search for simple, light and inexpensive ways to propel them is far from over. Turbines and piston engines lose efficiency and reliability as they shrink, while a valveless pulsejet stays remarkably simple at any size. I hope this paper encourages others to take miniature pulsejets seriously. With better fuel mixing and inlet design, I believe their efficiency can be improved considerably.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: Experimental and numerical investigation of an 8-cm valveless pulsejet, Experimental Thermal and Fluid Science, July 2007, Elsevier,
DOI: 10.1016/j.expthermflusci.2006.06.005.
You can read the full text:
Contributors
The following have contributed to this page
