Pulse Jet

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The pulsejet is a powerful air breathing jet engine that is composed of a simple reed valve and a properly shaped pipe.  The pulsejet functions by combusting an air/fuel mix at intervals that are regulated by the opening and closing of the reed valves.  The pulsejet has an operating range from 0 to approximately 400 miles/hour.  At this high speed the air pressure keeps the valve permanently open and the pulsejet can no longer operate.

Here is a sound of our pulse jet in operation

Here are some good tips from people on pulsejet operation:

"If you cut the tailpipe off at an angle, it will reduce the acoustic
energy delivered to the ambience (lot less noisy:-) ) Apparently converts
the spherical wave to a distorted shape with less distributed energy"

-Bill Colburn of the SORAC project

"The way you have the engine set up with the valves closed is correct.
They should not be open at all. You might be able to get them to
resonate by blowing air in the front to measure the frequency. You need
the frequency of the valve, retainer and valve plate all together which
is tricky to get.
    When the engine is running, the through flow of air used by the engine
itself atomizes the fuel. The fuel needs to be atomized, but the air
through put on a running engine does this, especially if the fuel is
supplied ahead of the valve in a diffuser or venturi.
    As to the flying, pulsejets need good fuel systems, but are reliable
with the right fuel system. I've had one flame out in the last three
years. As to the valves, they seem to need replacing about ever 10
flights, that's about forty minutes of run time. When I leave it longer
the valves can be damaged. Valve damage can lead to valve head damage
if the head is made of aluminum so I err on the cautious side and
change them every 10 flights. I electro etch valves which is a tricky
procedure to get good results, but when you sort out how to do it
produces excellent results. I also block ram air into the engine which
gives much more consistent engine runs as the mixture does not change
with model speed. I recommend this at least on the first RC model.

    A tuned pulse jet can run without having air forced into it.  To tune the pulse jet the resonant frequencies of the reed valve and the pulsejet tube must be matched as close as possible.

To do this the pulse jet is ran with compressed air and propane and the sound is recorded.  This sound is later analyzed on a PC with spectrum analyzer software.  The reed valve resonant frequency is found by blowing compressed air through the front of the pulse jet and

Recording the reed valve frequency of bouncing with a sound recorder.  This is analyzed with the PC's spectrum analyzer as well.  To raise the tube frequency, a slice is taken off the end of the pipe.  To raise the reed valve frequency, the reed valve holder tension can be increased or the size of the reed valve holder can be increased as well.

    Big pulse jets sometimes inject the fuel right into the combustion chamber but smaller pulsejets will inject it in front of the reed valve."

-Richard Caine