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The priciple of the pulsejet is very simple. A long exhaust pipe is needed so that at the end of the combustion the moving mass of air sucks in fresh mixture.
What if you replace the long pipe with an axial turbine?
After combustion in the combustion chamber the hot gases exit trough a short pipe
where they spin a small turbine.
At the end of combustion normally the fast moving gas in the pipe creates an underpressure wich sucks in fresh mixture, but now instead the fast spinning turbine starts to act like a compressor and sucks fresh mixture in to the combustion chamber where it gets ignited again.
Pulse frequency would be much lower i guess, and i'm not sure if the flame wild hold long enough for igniting the next cycle. (maybe spark plugs needed?)
It could make a small lightweight emergency generator with only two moving parts (mainshaft and inlet reed-valves)
I've had the idea to try and build one for a long time, and till now i couldn't find anything about it on the net.
Because this idea is very simple, and there's nothing written about it, must i conclude it's a stupid idea?...
In that case please stop me before i hurt myself.
(What i'm most curious about is the sound. This must be the engine from hell)
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A gasturbine is much more complex and has to be build much stronger because it has to deliver most of it's power to it's own compressor. And it has to maintain a minimum speed and pressure. |
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My simple design is from left to right:
One way inlet valve, combustion chamber, turbine. |
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The turbine doesn't have to be as complex and efficient as in a gasturbine. it doesn't have to power itself, it only has to keep spinning to act like a ventilator to suck in fresh mixture. |
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I don't see how "the fast spinning turbine starts to act like a compressor". In normal engines, the turbine lowers the pressure in the combustion chamber. |
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Windmill's reaction to the above:
Resonant tuning would be absolutely spoiled if you put the turbine at the end of the pipe. |
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But the turbine is directly connected to the combustion chamber, and im convinced of two things:
a>Combution will spin the turbine. (why should it not?)
b>The spinning turbine will create a vacuum that sucks in mixture through a reed valve. (why should it not?) |
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There's no tuned resonance needed. The turbine acts as a flywheel that keeps the cycle going. |
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You'd need someone who knew what he was doing, but I think it could be done. As mentioned, it would not be terribly effecient, but it would be very simple and durable. Basically the idea is that instead of using the resonance to pull in incoming air, you use the turbine (still spinning from the last pulse, moving air out of the combustion chamber) to create a vacuum in the combustion chamber that will then pull new air in from the non-turbine side (which will then combust, but because the intake has a one-way valve, the force will only exit through the turbine, which spins the turbine... etc). |
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You'd have to get the turbine spinning first, but most pulsejets require some sort of external starter to get them going. |
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My only question is how much energy you could extract from the turbine and still have it be spinning fast enough to draw in fresh air. The concept seems to work in principle though. [+] |
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Pulsejets are typically re-ignited by re-sucking-in hot exhaust gasses. A turbine MIGHT interfere with this. I don't know, but it's something to consider. |
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The efficiency would be absolutely horrendous. //turbine starts to act like a compressor// Um, no. The blades of a turbine and compressor are extremely different. A well designed turbine stage will perform extremely poorly as a compressor, and vice-versa. |
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If you build a valveless pulsejet which exhausts onto a turbine, you might be able to get this to work, but it certainly won't be efficient. |
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Fantastic idea. Only problem is I didn't though it myself. It will work OK. To start it, an electric engine will turn the turbine, which will suck air allright. We are not using the turbine as a compressor, which is not adecuately designed for, but as a vacuum pump, sucking air through the inlet valves, and that´s different. You can make a test model with a car's turbo, taking off the compressor and replacing it with
your standard pulse-jet tube. Only delicate part are the intake valves. Perhaps you could use existing two stroke engine intake lames.
What you can do is contact my friend "turboman Jerry". See his page "turbojer". He's a very hands-on man, makes turbojets out of chickenwire and tins. You should see his turbojet powered lawnmover. At his side we are very sane. |
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Spot on!, I was actually playing with the same idea, I am currenly building a pulse jet and my idea was that instead of letting the open end "return" the presure wave I would cut half a wavelength of the exhaust pipe and let the turbine wheel act as a sort of back wall to return the presure wave to the combustion. |
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Windmill, just been baked. See ( www.angelfire.com/mi3/gmpr/ptbc7.htm ) . Uses a pulse combustion chamber with your standard four stroke valves and injects some water to increase volume and lower temperature. As I said, the idea is
very good.
Wertyfrog, no resonance isnecessary since the turbine will provide the inertia reserve required, as windmill said.
Other turbine using less than the usual parts was featured long ago in "Popular Mechanics" and it used the exaust gases from a liquid rocket ( Turbonique engine ).
Power and fuel consumption were impressive. |
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An early type of piston engine worked by having the first part of the power stroke induct the fuel and air mixture which was then ignited. The resulting explosion then drove the piston the remainder of the stroke to produce power. It was simple but very inefficient because the expansion ratio was so low. I think the pulse jet is inefficient for the same basic reason. |
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