Rotary valve pulsejet?

[Johansson]

Anyone who has thought about this? Instead of a reed valve you use a ported disc that is rotated by a stepper motor so the ports open and close at the right frequency.

It should have a very long lifespan compared to the peckly reed flaps and huge sized engines could easily be built. Plus you can do the final tuning simply by adjusting the valve frequency instead of cutting or lengthening the tailpipe.

[racket]

Hi Anders

Theres some interesting comments about these over on the Pulse Jet Site www.pulse-jets.com/ in the Valved Threads

Sounds like its not such a simple thing, the "frequency" can vary a lot and often , probably best tried on a fairly large thrust unit( 20 kg plus ) where the frequency of pulses is lower .

Cheers
John

[Johansson]

I was thinking 200+kg engines, so fairly low Hz.

[enginewhisperer]

I think it might be tricky to make it work, since a pulse jet relies on resonance of the whole system - but in theory if you can get the valve timing right it should run.

It will change with power, temperature, weather, forward speed, altitude, etc, but maybe there's a wide enough operating window to make it happen.

There should be definite advantages in valve opening time due to the lack of inertia compared to a reed valve - but the closing time could be an issue (losing combustion pressure out the front while the valve is still closing)

With a low frequency engine it should be possible to measure the combustion pressure pulses with a pressure sensor (or piezo transducer) to assist with the valve timing, and maybe it could even be a closed loop system.

[Johansson]

But couldn´t the variations in frequency be caused by the nature of the self aspirating reed valve system?

If a mechanical intake valve would set the pace to exactly 60Hz and you had a tailpipe built to run at that frequency I cannot see why the engine wouldn´t run at least as well as with reeds, since a pulsejet is tolerant to slight frequency changes it should also be tolerant to a fixed Hz. At least in my narrow mind.

[enginewhisperer]

yeah it's quite possible


I think starting it might be tricky if you have the valve up to speed, because the air flow characteristics will be very different when the engine isn't resonating.

The resonant frequency will also change as it warms up, but I don't know how much.

[Johansson]

That is probably true, only one way to find out I guess. :-)

A large stepper motor that drives the rotary disc would make the air port frequency very stable, and you could inject the fuel through a hollow drive shaft and use a single spray nozzle for fuel feed.

Hmmmmmm........

[enginewhisperer]

I am already on the lookout for large stainless tube

[jeffreyguy]

You could probably set up some sort of electronic pulse timer with an automatic ramp up setting, it starts spinning at a very low frequency, then once combustion is detected it ramps the frequency up to 60hz then it could be fine adjusted with a potentiometer

[dominik]

Dear Johannsen,
have you advanced in this project? I planned something similar but not like the described self sustaining pulse jet.
However the parts are packed in a box for later usage because first I have to get my turbine running, I just found
this place of rich information a few days ago, seems that my try and error phase could have been much shorter if I
had found it earlier, anyway I´m confident to get it running until the end of this year, if not I will do some reseach
here and will of course have some questions...

Back to rotary pulse jet:

My plan was to charge the jet with an EDF and do the ignitions with a spark plug timed by a small µC.
Rather than having it run on resonance. (Kind of a "weak" PDE).

During my theoretical planning I had the following doubts:
- 60 hz means 3600 rpm with a rotaray valve with just one window, that´s very high for a normal stepper
- one window without special design will result in an unbalance
- more windows will significiantly reduce rpm but will be more difficult to manufacture (to get a reasonable area), for me this means external work (laser, watercut, etc.)
- In my design the rotary disk is on the combustion side, so the combustion chamber must be dismountable
- with a "constant" rotational speed, will the valve block/stop during the combustion phase and the applied pressure?
- if so, how to mount a sensor to get the actual valve position for correct timings?

Perhaps something of that is useful for you.
If you already have a working version of your idea, it would be very interesting for me to see.

Regards

[Johansson]

Hi!

I have projects up to my neck already so this was only a wild idea I wanted to get out for discussion, perhaps in the next life I will have time to actually build it.

You have some interesting ideas, I would absolutely love to see a project thread for a rotary valve engine!

[dominik]

Dear,

yes I know what you mean, I had that Kamps book lying around for several years and 3 month´s ago I said now or never (this life),
even with projects open for two lifes (or more).
Planned 4 weeks for it, after two weeks of nightly work I had a 10 second run on my first turbine, that was the point of no return
and the infection had been taken place. Total rework of first Turbine because turbine wheel suffered damage and eroded and I just hat
a bigger one on hand, that eroded 5 weeks later after 30 seconds. Now it´s been 12 weeks on the turbine project and the second turbine is a total different approach but I need to have a selfmade turbine running ;-) Not to mention what this means to the other projects...

Perhaps I will find the time during the christmas holidays to extract some pictures from my CAD planning to show some more details on the EDF charged "PDE" with Rotary valve.

Nice Christmas and a good start into the next year to everyone.

[monty]

I don't see any reason why this idea wouldn't work. Using a microprocessor for controlling IC engines should do the trick. You could use automotive stuff. A PWM motor on the valve with a timing wheel and regular fuel injectors should be a trivial thing to make work. Ramp up/down and timing should all be easily controlled.

Monty

[Johansson]

What if the rotary valve was shaped as an axial fan that pulls air into the engine? Compression would be very low but compared to a normal suction valve with pressure drop losses it could mean a significant power increase.

[enginewhisperer]

There could be an axial fan, then the rotary valve on the same shaft (each end of a motor?) with a bit of plenum volume in between to smooth out the pulses.

It'd certainly help with starting if not actual running!