I need some help

[rockooby]

I made 35% (all dimensions times .35) version of Bruce Simpson Lockwood PJ design. I cannot seem to get it to pulse properly. I made an account on the pulse jet forum but I don’t know if the forum is unused or not, because I have not had my account approved by a moderator yet. Back to the PJ, I can only seem to get a really low frequency pulse sound when I have the leaf blower at full chat. I added 10 inches more to the exhaust pipe and still got the low frequency pulse noise, and I even got a split second or two of proper pulsing with the leaf blower but it immediately died. In addition I added more holes to my fuel rail now going from only one 4mm hole to five 4mm holes including the original. That helped with me not accidentally blowing out the pj when I was trying to get it started. I’m still stuck with the low pitch pulsing noise I had a few more split seconds of proper pulsing but still have not been able to get it to idle. I have videos and photos of it before I made the previously mentioned modifications completely sure how/if I can share them on this forum. Any help to get it to idle properly and get rid of this low pitch pulse for the proper idle. The propane system is direct to the pj with no regulator or valves.

[racket]

Hi

Video please, so we can see whats happening .

35% ..........is that for length and diameters , but what about cross sectional areas ??

Cheers
John

[rockooby]

Hi racket got the video and some pictures

I put them in the original post

and only 35% of length and radius, not cross sectional. The cross sectional would be about 12% of the original cross section.

Here is also what I went off originally, I did not have any design for the fuel rail if you were wondering. Just this photo I went off of. 

[racket]

Hi

Thanks for the video , yep a bit frustrating , I think you needless start air , just enough to get a positive movement through the engine so that the propane can burn in the combustion chamber , it should soon colour up and start to glow , then maybe add more air , PJs are frustrating to start until a system is worked out then they'll start easy :-)

By "scaling" you've change the acoustics :-(

You could try pre heating the combustion chamber , get it glowing red .

Also your propane supply , is there a supply safety incorporated in the cylinder valving

[rockooby]

If I wanted to scale and keep the same acoustic properties how would one go about that? Is it even a viable option to scale down most larger scale designs? Do you have any good sources where I could learn more about PJ acoustics? Maybe to make my PJ idle and maybe have a shred of ease of uses. Also by safety do you mean a excess pressure release system, or an automatic kill switch or valve of some sort? Because honestly I do not know what you mean. In this video I had the propane going straight to the tube of which I showed the end of. I did have a 100psi propane regulator on in previous tests but I was testing it without the regulator after reading a forum posting about the regulator possibly killing the PJ after or while it was trying to start. In earlier tests I was using 1lb propane bottle like for a camping fire but was unsure of available pressure after they reached 0 degrees Celsius and water vapor started to freeze on the sides of the tank. I borrowed a much larger bottle out of the shed filled with flammable things and kept testing. Do you have a good safety system maybe I could copy? What does it include? All fittings are 1/4 in pipe threads except for the transition between flexible tube and hard tube which is a 1/4inch 45 degree SAE fitting with a double flare. All fittings seal good.

Thanks

[racket]

Hi

In some countries theres an inbuilt safety device in the cylinder valve to prevent excessive flow in case of an accident .

Do you have a drawing of your 35% engine with all the dimensions ??

I think your fuel injection holes might be a tad large , I think several 1.5 mm holes would produce better distribution across the entire combustion zone.

Cheers
John

[racket]

I've had a bit more of a look around the Net for injector ideas and I'm pretty certain you've got such large holes that the propane isn't "squirting" far enough , similar problems occur in our turbine engines where the propane injector must squirt across the entire flametube , large holes produce a core of propane with minimal mixing with the air, generally resulting in a flame going down the flametube and through the turbine wheel.

As your engine is only ~1/8th the power of the one you scaled off , the injector flow areas will need to be similarly scaled , one example had several 3mm holes , say a total of 50 sq mms , 50/8 = 6.25 , if you use 1.5mm dia holes of 1.76 sq mms each , that would only equate to 3.5 holes , maybe go to 1mm holes and use 8 holes arranged so they squirt across the entire combustion area ..

Cheers
John

[rockooby]

Hello John

In my testing today I changed the nozzle to your recommended eight ~1mm (their actually imperial drill bits size 60 or 1.02mm) holes. I do not believe my propane bottle has a safety excess flow mechanism only a excess cylinder pressure release. I've still attached photos of the propane bottle valve if I did miss something about that aspect. When testing I was only able to get a broadly pulse jet noise when I lit the intake and the exhaust of the pulse jet then started blowing. When I began to blow, a massive fireball that would quickly appear then disappear came out,  leaving a small blue flame protruding about 2 or 3 cm from the exhaust outlet. If I blowed harder, the blue flame would go away and the pulse noise would begin. I will have attached a video of it after I got it going and to make the noise. Though it did make the proper pulse jet sound the volume of sound fluctuated frequently often becoming much louder for a few second only to become much quieter again. In another test I did not record, I used a tuning app on my phone and found that the PJ pulse sound was roughly in the range of 160-164 HZ (don't know if this helps anyone thought It might.) I do have a screen recording of the tuning app, though annoyingly no sound. The length of my PJ is 47.3 inches with a 15 inch long intake, combustion chamber, and initial straight pipe to the bend. The bend length I estimated to be about 7.8 inches with the diameter of the tube being 1inch and outer diameter of the bend about 5.5 inches and inner diameter of the bend about 4.5inches so I just found half the circumference at 5 inches diameter. The exhaust cone with extension tube is 24.5 inches long. Lastly I noticed after doing the frequency test that only the second half of my PJ's exhaust was hot. No other preceding parts were exceptionally hot and could be easily handled. In a photo I put my finger at the point where if I continued to run my finger down the exhaust I run the risk of burning myself. I know the PJ should be using about all of the exhaust length for combustion any recommendations on what to do?


(below are some questions)
I found this air column resonance calculator on another forum and when I input my PJ's length of 1.201 meters and the temp today of 28 degrees Celsius it says my f1 would be at 145.12 HZ and my f2 at 290.24 Hz. I don't know if I'm going about this wrong or not but I'll include a link to the calculator for reference. (Also I was looking at the open cylinder[now that I think about It I don't know if the specific forum I got this from was referring to valved PJs and does this still apply to my PJ as I have a massive bend in it unlike most valved PJs?] ) On a similar note I saw ideas about making a trombone like pulsejet exhaust slide, and that gave me the thought that spurred this frequency data collection. Can I tune my PJ like a musical instrument, but instead of slides I would add a piece of tubing or take away a piece. Maybe even copying the trombone slide idea for the exhaust. If I can tune my PJ, what do I tune it to? Am I trying to get my pulse jet to pulse at 145 Hz like the calculator says should be my resonance? Can I tune the PJ by changing the cross-sectional areas of some parts as changing the length would seem to change the frequency I'm aiming for. If so how can I tune my PJ? Also is there any good source recommendations for learning more about pulsejets and how resonance affects their operation. Also how did you estimate that my PJ was 1/8 power?

Thanks

Calculator









This is the scaled down drawings and list of piece cutout dimensions made for cutting out the metal I made (bit of a Picasso)






This is the video of the the PJ kind of running with the leaf blower sustaining its pulse ( sorry it is in the short format, made the mistake of recording vertical and now I can't change it.)

[racket]

You might need to remove the extension piece on the exhaust , its changing the ratio of inlet to outlet volumes and the acoustics that are required for the pulsing to work correctly .

Have you has a good read of Bruces stuff , he's been around for decades , he's not a great fan of making small PJs as they get "complicated" .

Valved engines can be played around with but valveless ones need to be "as designed".

Just been looking at the PJ Site pulse-jets.com/ and my old 2005 turboprop PJ Thread is still there www.pulse-jets.com/phpbb3/viewtopic.php?f=2&t=2349

Maybe you need to read through the valveless threads , maybe this one www.pulse-jets.com/phpbb3/viewtopic.php?f=3&t=6758

[rockooby]

Hello 

So the testing today yielded no new results, the only change I made was to revert to the original pulse jet exhaust length. I was able to get the whooshing sound and greyish smoke and some pops and bangs if I was lucky. When I added more air or fuel respectively the flame would die out and I would have to relight it. Also no pulsing noise this time as with previous tests with the exhaust extension. I was reading through Bruce Simpson's website I saw a page about him turning one of his valved PJs into a Valvless PJ just for the sake of making a valveless PJ. I included the website link and probably a screenshot of the specific excerpt I was looking at. I know he already had a working valved PJ as a base. Though initially as a valveless PJ it didn't work, just like mine. I was thinking of copying the process maybe extending my exhaust pipe again as it was making a proper pulse noise (though intermittently) before I removed it for todays test. Then tuning my inlet to best fit the exhaust pipe. I know I only have one inlet unlike his four inlets I still think the general idea would apply to my PJ it would just be a bit harder to get the right length of inlet. I think maybe getting it to work with a longer exhaust and then tuning the inlet to fit it might be a valuable option to get my PJ to at least run. Is there is a better way I have not considered or some obvious short coming of this method? Bruce also talks about tuning the frequency of the inlet to the exhaust section.

    I did not realize that the frequency calculator I mentioned last post is used excluding the length of the inlet and just the exhaust and combustion chamber lengths combined. With a combined exhaust and combustion chamber length of 41.9 inches (this is including the 10 inch exhaust extension present in yesterdays test.) In addition to setting the temp variable on the calculator to 28 degrees Centigrade as it was the temp yesterday when I ran my test and recorded my frequency test. The calculated F1 is 163 HZ which is right in line with what my phone tuner was reading yesterday while testing the pulse jet pulse. Is this just a coincidence or can I actually use this information to maybe match the frequency of my inlet and exhaust and hopefully make the PJ work.  Lastly, I included another photo of my PJ today. Taken after I finished testing, for a reference.











link to Bruce Simpsons Page I provided the excerpt from



Thanks

[racket]

Hi

It might be helpful if you fitted a spark plug so that theres more opportunities to have ignition whilst you are adjusting fuel and blower.

Tuning your engine will be just trial and error as you've constructed it to a "design" that does work.

Did you try my recommendation of heating the combustor section ??

Cheers
John

[rockooby]

Hey John

I can fit a spark plug easily. I already have a plug and a few nuts with the same thread too to weld on. I also did see your recommendation on heating the chamber, but yesterday I had no real way of heating the chamber efficiently as I didn't want to drain the MAP blaster I was using to light PJ to heat it up. I'll try to think of a better way to heat up my combo chamber with what we have. I'll try to heat up the combo chamber and add a spark plug for better fuel mixing opportunity, though after that I might need to change the "design" to get it to work. Probably doing it trial and error style unless I come across a better method.

Thanks 

[racket]

Hi

The Pulse Jet Site is the best place to find info on what you can do as well as what you can't do :-)

The fact that you've had some moments where the engine seems to want to work is encouraging, once its hot and having a continuous spark hopefully it'll do what you want it to do, make lotsa noise .

Cheers
John