Trouble with new pulsejet design

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HPSCL
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Trouble with new pulsejet design

Post by HPSCL » Tue Mar 31, 2009 2:35 pm

I've built several pulsejets, by simply upscaling Eric Beck's plans for the 3lb. Thermojet. I've never had any real problem that couldn't easily be figured out, except with this latest design;

Image Image... (Sorry, for the lack of an actual picture. - Still need to load the software for my camera into this computer).

The combustion chamber was fabricated, using a discarded 30 lb. R134a bottle. The bottle holds 14100cc of water, so it's volume is estimated at 860.43 cu. in. The tailpipe is an 18" length of 6" diameter mild steel exhaust tubing. It's volume is estimated at 456 cubic inches. The twin intake tubes are 6" (each) of 2" mild steel exhaust tubing. (.063" wall thickness).

The fuel nozzles are fabricated out of 1/4" steel brake line. I originally tried to taper them down to a 1/8" diameter, but the fuel flow wasn't fast enough. Most of the flame simply belched back out of the front of the intake tube. By pinching them almost closed, the "Rosscojetter" design worked out the best. The fuel nozzle openings are now 1/2" wide by .025".

No matter what I've tried, the engine will not sustain a high idle, nor will it transition into a high cycle. The only thing I can think of, is that the tailpipe is simply way too short (compared to its diameter).

Does anyone agree that adding more length to the tailpipe, will help. As it is right now, most of the flame just blows out of the end of the tailpipe (about 2 feet) when I use a leaf blower blowing into one of the intake tubes. Any comments would be greatly appreciated.

Mike Everman
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Re: Trouble with new pulsejet design

Post by Mike Everman » Tue Mar 31, 2009 5:04 pm

Well, the confinement seems ok, but it sure seems fat for its length. Are the length dims scaled as the diameters were? Have you tried plugging an intake for starting? No flares on the intakes? This beast will need A LOT of propane, like two tanks and don't expect the pressure to stay up long, so you may need to switch to vaporizer coil and liquid propane feed.
Mike
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Graham C. Williams
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Re: Trouble with new pulsejet design

Post by Graham C. Williams » Tue Mar 31, 2009 11:30 pm

Hi.
I have to go along with Mike.
Without testing the model I'd say 'The Exhaust looks very short'. Try making it about 2 to 3 times as long as you have it now.
and
The induction pipes look a little too small, I mean the induction dia. This is an odd one because you can get motors running with very small induction pipes - not to best performance but they will run none the less. Those induction pipes should also have Bell Mouths (flares); it's a boundary layer and gas-flow thing, it'll help a lot. Yes, you can run these motors without them but it makes the whole system a lot harder to get correct.
You may have to change the induction pipe length a little too.

How are you fuelling this motor? The question is not trivial.

To sum-up.
Try a longer exhaust pipe
Add Bell Mouths to the induction pipes
Make the Induction Pipe length Adjustable.
Check your fuel system - you're going to need a lot of it.

Graham.
Dark days nurture new
light. Productions begin.
Now open your eyes.

HPSCL
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Re: Trouble with new pulsejet design

Post by HPSCL » Wed Apr 01, 2009 5:16 am

Gentlemen, I truly appreciate the quick replies. As I've stated in previous posts, I'm new to the design and fabrication of pulsejets.

Mr. Everman - to answer your questions first; The LENGTH dimensions are not scaled - I'm scaling these according to a chamber-to-barrel ratio (C:B ratio, in the "spudgun" world).
The 3lb. Thermojet plans have an aproximate 1.9:1 "C:B ratio" and this has worked out great, on all of my other pulsejet designs.

I have not tried plugging an intake for starting. I use a hand-held propane torch to start, at one of the intake tubes, and it immediately lights both sides. As the propane line pressure is increased, both are creating a nice, even blue flame.

To begin to answer Graham now and still answer another of Mr. Everman's questions; Yes, the intake tubes are flared, as well as the tailpipe.

I finally got around to getting some pictures taken, as seen below:

Image Image Image

To answer yet another question; I am fueling this engine with a 17lb. (7.71kg) "BBQ" propane bottle. It was approx. 60 degrees F. outside the other day, so after many failed attempts, I installed an inline pressure gauge... I was only getting 20psig. flowing out of the bottle. (The low pressure confirmed my guess that the combined area of the openings of the twin fuel nozzles were a little too large for this size bottle).

Placing the bottle in a 22 gallon plastic tote, I added warm water and tested the temperature at a respectable 80 degrees F. The line pressure (wide open) was now 40psig. -Still low, but should be enough to sustain a high-cycle of combustion.

You guys absolutely rock! My wife listens to me babble all the time about my pulsejets and even she implied that the tailpipe length look "awkward" compared to the diameter of the pipe.

I had already figured that the volume of the fuel reservoir seemed to small, but that a full bottle would (at the very least) allow me to see if the design would work at all. I didn't want to buy a larger bottle if it didn't work, as this size is great for all the other smaller engines. I'll definitely add more length to the tailpipe, as the pipe is pretty inexpensive, at only $16 foot.

As an aside:
A respected member of these forums, recently sent me a PM, which explained that:
"...the scaling of Eric Becks engines is a bit of a taboo subject , he was kind enough to post the 3 lb engine way back , and gets a bit touchy when people scale it up and then publicly complain that it wont run..." --- "Build a "known to run engine to some plans" and there will likely be much more help , the truth is that it takes a lot of work to calculate all the variables for an oddball engine, and frequently the person asking the questions never returns to the forum..."
If somehow I may have implied (in any way) to be complaining about Beck's design, then let me apologize. Beck is, for lack of a better explanation, my inspiration for building these engines. His site (The first that I ran across, after researching and watching many a "youtube" video) looked really interesting and his products looked like works of art. I've known (from the beginning) that the ONLY problem with upscaling Eric's design, were MY miscalculations and general lack of knowledge. I've based all of my "upscaled designs" off of his plans and they all work awesome now.

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Re: Trouble with new pulsejet design

Post by Mike Everman » Wed Apr 01, 2009 6:04 am

Scaling is interesting business. Generally it goes like this: a good little motor will scale up nicely, all dims just factored. As you go up, though, you can accomplish more open confinements (larger intake and exhaust areas compared to CC area). A good large motor will have trouble scaling down, unless it's non-optimal already and the scaling process brings it in.

If you scale your "barrel" to 4x the length of your chamber, then you can start playing with shortening it. I don't have much of a recommendation for the intakes; they look OK for a start.

The Thermojet is widely considered a very "forgiving" motor. I believe this is because the volume of the intakes are such a small percentage of the combustor volume, that their local acoustic behavior is less critical, that is, dominated by the rest of the duct. When the intake volume gets up to 1/4 of the combustor, things are more sensitive to intake length, and the intakes drive the bus.

As to fueling, you can use a BBQ tank per intake to do your gas tests. The inevitable imbalance won't be critical and getting LOTS of gas in there is.
Mike
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Jutte
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Re: Trouble with new pulsejet design

Post by Jutte » Thu Apr 02, 2009 6:04 pm

Hi there,
Noticed your pulse jet was made off an old propane bottle.
I made one similiar to Erics one (picture)- as you can see the tail pipe
is longer and narrower.Hope the picture helps to get your one going.
Attachments
propjet1_173.jpeg
Erics creation

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