Towards a wood fueled pulsejet

[vturbine]

Yeah, but if it starts running, that would mean the injector was outside the CC, wouldn't it? The burps were with the injector outside but very near the hole.

I'll try to arrange more pipe on top of the tailpipe, but the flare there is making it hard to use a hose clamp, etc. Maybe I'll cut off the flare and weld on an outer sleeve. Then I can just slip another length on flush with the bottom pipe.

Pouring outside now.


EDIT:

Do you think it's the premixing with air that gets it burping? Or the fact that I get a little further away from the inlet by going outside the hole?

I'm kinda wondering what if I weld say a 1" piece of 3/8" pipe to the outside of the injector hole. Then slide the injector down that looking for a tuned spot. A close snorkel for the injector.

See, that's why I think it's maybe blowing out on the burp. Sort of jam-jarruing out the injector hole, with the injector acting as a lip feed. Probably a snorkel would do the same thing. But there is some interesting stuff here.

[GRIM]

Hey , yes the burp, I had forgotten all about that ,
when you have the injector close to the hole it does do that, a kind of rattle, fairly high frequency , I think its a bit of a jam jar thing , where it is pulling the flame in and out of the hole ,

What I found worked best was to light the injector , with maybe a 2" high flame and slide this quickly into the hole about 1/2" , the smoke from the flame should now be coming out of the inlet stack , If the flame goes out , try again with a little bigger flame ,

Then turn the gas up , how quickly or slowly you open the gas valve is something you have to "feel" but it is obvious when it is right ,

The angle of the injector wrt horizontal and the depth into the cc is something you will have to experiment with , and does make a big difference ,

Another way I have started mine is to open the gas valve a bit with the injector in the hole and light the top of the exhaust , you should get a fairly large yellow flame , then virtually close the gas valve , the flame will die down , and go blue and pointy , and then "organ pipe" , down the exhaust , this is an unmistakable , but impossible to describe sound, get it right and open the gas valve just at the right time and you will get her resonating , this is tricky to do though, the timing is critical ,

[vturbine]

Rain let up for a half hour so I tried again. Tried the top of the pipe lit trick -- no problem with the flame dropping but went all the way to the injector.

Tried inserting 1/2" w/ 2" flame, does pretty much the same as always, acts mostly like a yellow flame heater, not oscillator until the gas is turned up far enough, then goes blue momentarily and goes out.

However I think I got pretty close this way:

Lit the injector adjust for 2" flame and insert 1/8" into hole. Gradually throttle up. VERY gradually. I start hearing pulsations, and they gradually get louder and speed up as it warms and I throttle higher. Sounds like a motorboat, not a pulsejet, so I assume it is jam jarring mode. I can keep it pretty stable if I don't touch the throttle. But overall it's pretty weak. Definitely don't need the ear protection. No visible flames out of the tubes -- at least in daylight. The CC is certainly not glowing. It's hot but nowhere near as hot as a pulsejet.

It seems to work best if I angle the injector so it points as far as possible toward the bottom of the CC. This is a little over 45 deg because it is limited by the 1/4" hole from swinging farther.

If I try to add more throttle the pulsing speeds up but weakens until it blows out. Tried this many times.

I think the injector is too close to the inlet, and the gas jets are exceeding flame speed at higher throttle levels and crossing the inlet. The downward flowing air is chopping them off. That's my guess.

Also, when making the injector, the pins were pretty much parallel with the tube. I see in the drawing of the injector that the pins should have been angled out some.

[GRIM]

I think the injector is too close to the inlet,

Yes you could be right about that ,being as the cc is smaller and its not centred , I remember that the injector position was quite touchy for starting , once running it was much more forgiving ,

It seems to work best if I angle the injector so it points as far as possible toward the bottom of the CC. This is a little over 45 deg because it is limited by the 1/4" hole from swinging farther.

Enlarge the hole slightly with a round file , kind of oval , any leakage there wont affect the running to any extent

Also, when making the injector, the pins were pretty much parallel with the tube. I see in the drawing of the injector that the pins should have been angled out some.

Yes , I found that it worked best with the pins angled a little outwards, try crimping one hole shut with pliers , and just run on two ,

Sounds like you are really close , you might want to try a sliding sleeve in the inlet, and yes as you said earlier you could cut the flare off of the exhaust and make a coupler from sheet steel , and just add length that way ,

Have you tried some compressed air ? light the tail and shoot a little air in the inlet , (expect some bangs,) and the wire wool trick is well worth a shot ,

It is frustrating I know , but we have all been through this ,

[vturbine]

Thanks again, Grim.

Tomorrow promises to be a fine day. I'll either get it started with opened out injector hole for more down angle, and extended exhaust pipe or do major surgery and move that inlet about 3/8" towards the draft pipe. Will try better injector, too.

I also had a thought tonight that because the engine was sitting on rain wet sand instead of on legs, that maybe the bottom of the CC never warmed up to what it normally does on a free standing version. Maybe hot metal at the bottom of the CC is a big part of clean ignition and transitioning to pulse jet mode.

So tomorrow , too, maybe add the legs. Of course if it does start up with multiple changes, I won't know which was the culprit. But I'll be glad to move on past the starting and running on propane stage to more interesting stuff. However with an engine finicky on the injector, it doesn't seem likely it will take well to the kind of massive plumbing a woodgas feed would seem to require. But you never know until you try things.

Sorry to cut up and patch my nice first gas welded anything, but if she don't work, out comes the torch.

[GRIM]

I also had a thought tonight that because the engine was sitting on rain wet sand instead of on legs, that maybe the bottom of the CC never warmed up to what it normally does on a free standing version. Maybe hot metal at the bottom of the CC is a big part of clean ignition and transitioning to pulse jet mode.

Might be , I never had mine flat against the ground , but I think really humid conditions do make things a little harder , similar to jam jars but not as severe , pulsejets will start and run in torrential rain,

I think moving the inlet is not a bad move , but will require a fair bit of work , might be better to cut the inlet and exhaust off flush and make another combustor close to the drawing , put yours to one side , and go back to it later maybe ,

HOWEVER Your description of the engine "almost" running in a previous post suggests to me that it should be possible to get your current combustor running without modifications , although it will likely need some compressed air to push it over that dead spot ,

I still have my engine , it is very rusty but should still run , will have to weld a piece of exhaust on , If I get time tomorrow I will have a go at firing it up , the memory is fragile , maybe I am forgeting some detail ,

Injectors are strange animals , I must have made dozens of them , some look the part , but dont work , others that are all beaten up work great , the only advice I can give re the 3 point is make sure the end is crimped up good and tight you only want the gas to come out of the 3 holes ,

If you can , make a small video of it nearly running , a 10 second video is worth a 100 posts ,

Good luck for tomorrow , hope the weather is kind ,

[PyroJoe]

May be useful to make three injectors, each one crushed a little more than the last. A rich, medium and lean. Try to have a crushed injector end length of 3/8", at least it works best here. Is the needle valve still in place? Mine runs fine on the ground. Does sound like your very close. Gaining some good experience along the way. Imagine that thub thub thub in a 10" CC draft engine, like a big snorkler that just won't quit, rain or shine.
Joe

Picked up some stainless steel to start on a gasifier tube. Rained yesterday, bringing the measure up to 0.30" for this year. Bring the rain Ace.

[vturbine]

SUCCESS!!!!

Yahooooooooooo!


Uh,
Didn't take too much changing after all.

It needed an exhaust extension and both inserts so the pipe was 1-1/2" OD stuff.

I tried a lot of combinations before I got that all together.

Makes sense now, hindsight... I have a smaller cc, therefore I need to restrict pipes more -- otherwise I just have a free-breathing burner. If however you restrict pipes, you reduce draft. So extend the draft pipe to compensate, increasing vacuum at inlet. Then it can climb out of jam jar mode with enough air to continue, and enough exhaust restriction to guarantee oscillation, rather than dropping into burner mode.

I'll try to get video later today, I'd like to know more about the "boat leaving dock jam-jarring mode" at start, as well as the frequency of the pulsejet once it's going.

I'm wondering now how to go about reducing flow-through and heat/fuel consumption (mass flow, I guess) in favor of pulsing/ waves amplitude.

[vturbine]

I'm guessing that the jam jar/reynst will always be better at pulsation than a two pipe pulsejet. By its very nature of trading exhaust with intake it is a true oscillator. The pipe pulsejet uses oscillation to create flow through. So it will tend towards a ramjet. In some ways it is the intermediary between pot and ram.

[PyroJoe]

Wooo hooo, full gusto, ha. Nothing compares to that first one firing up.
The nature of kinetics next? No moss grows on your heels.

[GRIM]

CONGRATULATIONS ,

Makes sense now, hindsight... I have a smaller cc, therefore I need to restrict pipes more -- otherwise I just have a free-breathing burner. If however you restrict pipes, you reduce draft. So extend the draft pipe to compensate, increasing vacuum at inlet. Then it can climb out of jam jar mode with enough air to continue, and enough exhaust restriction to guarantee oscillation, rather than dropping into burner mode.

Yes I reckon thats about it ,

I'm wondering now how to go about reducing flow-through and heat/fuel consumption (mass flow, I guess) in favor of pulsing/ waves amplitude.

Thats going to be a little more tricky , , most of whats been done here and elsewhere has been aimed at increasing mass flow,

Again well done and please do post a small vid if you can , theres been very little noise and flames round here just lately,

[ace_fedde]

vturbine,
Congratulations! Just hope that the dance of joy that you probably made will not be interpretated as a rain dance
I know that adrenaline rush creates strange dances!
Fedde

[Viv]

I'm guessing that the jam jar/reynst will always be better at pulsation than a two pipe pulsejet. By its very nature of trading exhaust with intake it is a true oscillator. The pipe pulsejet uses oscillation to create flow through. So it will tend towards a ramjet. In some ways it is the intermediary between pot and ram.

Hi V

Insightful summation but not sure I agree about the ram jet bit, it depends on what you mean.

Congrats on your first run

Viv

[PyroJoe]

I''ll take a stab at explaining the kinetics,
My approach is to look at the CC as a Helmholtz chamber supported by combustion, both the intake and exhaust pipes are basically nozzles, the tail pipe is a combination of inertial pump and nozzle. Nozzle design in PJ are different than rockets due to the pressures involved. PJs produce a few psi swings for brief moments, rockets produce continuous pressure of 100s of p.s.i. for its duration, so nozzle design is reasonably different.

Putting the tail pipe upwards in draft mode, aids in the pumping effect, needing less combustion energy to drive the pump, with a good capacity to clear exhaust. The taller the draft pipe, the less combustion energy needed to sustain pulsed operation.

With less desire for mass flow and more desire for Helmholtz action, what would the layout be? Maybe a stupidly big CC with a smallish tailpipe to clear exhaust?

Also contemplate how much fuel a locked in engine is consuming, compared to the slower thub thub thub operation. Don't take my word. Build, test and consider.

Joe

[vturbine]

Thanks grim, pyro ace viv! That's quite a thrill!

Funny I ran up the hill to the house afterwards and asked my wife and daughter if they heard it. Duh. They did.

I just meant a ram jet where air is flowing through without pulsation, and requiring some outside force to move air through. Pulsejets using restriction and resonance to act as the compressor or fan pushing air through, even if in a lating oscillating net gain kind of way. And jam jars trading contents through one hole, exhaust, intake, in, out. A pulsejet folded in on itself radially.

Now as I see it there are three modes of oscillation described so far:

jam jar
pulse jet

and one other.

It's buried in the odd metal jar thread. I think it has potential here, not just a novelty. Something possibly important. PJ probvably knows right off the bat, and Mark, too. Involves hollow all-thread.