Please put your "I am new and need help" question here.

[PyroJoe]

no worries

[Rich Birch]

What guage mild steel would you recommend , in general, for a valveless pulse jet ?
Are there any plans for a self starting thermojet with 10-12 lb. thrust?

[Tonskulus]

I have seen lockwood designs of two types, one having larger diameter intake and some others has smaller diameter compared to the first section of exhaust.
My lockwood has intake diameter of 3" and exhaust is 2.5" in the beginning and enlarged to 3" rest of it. I will make it conical anyway sooner or later..
So is there any reason using smaller or larger diameter intake than exhaust?

[ganuganu]

I have seen lockwood designs of two types, one having larger diameter intake and some others has smaller diameter compared to the first section of exhaust.
My lockwood has intake diameter of 3" and exhaust is 2.5" in the beginning and enlarged to 3" rest of it. I will make it conical anyway sooner or later..
So is there any reason using smaller or larger diameter intake than exhaust?

remember this in pulsejets the intake diameter should be always smaller than the exhaust if not the starting of the engine WILL be more difficult. And if we use smaller dia of intake than exhaust most of the combusted gases WILL be concentrated towards the exhaust so that engine comes to life soon.

[Tonskulus]

I have seen lockwood designs of two types, one having larger diameter intake and some others has smaller diameter compared to the first section of exhaust.
My lockwood has intake diameter of 3" and exhaust is 2.5" in the beginning and enlarged to 3" rest of it. I will make it conical anyway sooner or later..
So is there any reason using smaller or larger diameter intake than exhaust?

remember this in pulsejets the intake diameter should be always smaller than the exhaust if not the starting of the engine WILL be more difficult. And if we use smaller dia of intake than exhaust most of the combusted gases WILL be concentrated towards the exhaust so that engine comes to life soon.

Ok thank you. So that was the reason why my engine is not easy to start. But how about maximum thrust?

Anyway, i see many lockwoods having much larger intake:
http://www.cnccookbook.com/img/OthersPr ... twins2.jpg

http://www.home.no/andreas.sunnhordvik/ ... rawing.jpg

Just wondering if there is some advantages making intake larger diameter than exhaust..?

[kevin lee]

Hmmm interesting, all the designs that i have looked at have had smaller intake inner diameter than the exhaust. However i havent looked at any lockwood plans at all yet!!


You should get a feel for the dimensions using pyrojoes caliber method, it may help some of the more knowlegable guys make some suggestions also.

I have these following details from the pyrojoe recipe imprinted on my brain haha

Exhaust id = 1 caliber
Exhaust length = 20 caliber
Inlet id = .77-.88 caliber
intake length - 2.67 caliber
cc id= 2 caliber
cc length = 4 caliber

anyways i just saw the video of it running from your other thread and it doesnt sound too bad, lockwoods are well known for being a biatch

[ganuganu]

I have seen lockwood designs of two types, one having larger diameter intake and some others has smaller diameter compared to the first section of exhaust.
My lockwood has intake diameter of 3" and exhaust is 2.5" in the beginning and enlarged to 3" rest of it. I will make it conical anyway sooner or later..
So is there any reason using smaller or larger diameter intake than exhaust?

remember this in pulsejets the intake diameter should be always smaller than the exhaust if not the starting of the engine WILL be more difficult. And if we use smaller dia of intake than exhaust most of the combusted gases WILL be concentrated towards the exhaust so that engine comes to life soon.

Ok thank you. So that was the reason why my engine is not easy to start. But how about maximum thrust?

Anyway, i see many lockwoods having much larger intake:
http://www.cnccookbook.com/img/OthersPr ... twins2.jpg

http://www.home.no/andreas.sunnhordvik/ ... rawing.jpg

Just wondering if there is some advantages making intake larger diameter than exhaust..?

In my opinion there are not much advantages, try to use intake diameter 80% of the exhaust.

[Storm]

Hi I am new here,

Thank you to the many informative members.

I have Some questions:

Is there any recipe for starting build from a known combustor volume, regardless of shape?

Thrust to Size: Are larger engines exponentially more powerfull than smaller engines?

Blended bodies ie... multiple intakes and/or exhausts: Reading of Mikes tripple pocket jet with blended cc and other similar multiport engines not producing the equivalent multiple of thrust can I assume single port engines of the same dimentions will be more powerfull, what am i missing? are there other benefits to multiporting?

[Storm]

Hello??

[metiz]

You mentioned not only "recipe" but also "blended" in your post. look at the top of the valveless pj forum to find the answer to all of your questions: "Pyrojoe's recipe for beginners" read on about half a page and you will find info on blended bodies. That's probably why no-one replied.

[PyroJoe]

Well the "regardless of shape" can have many problems, to many to really go into.

Thrust to size has problems also, in that if your fuel source is limited, then the size will have a limited thrust, so you could actually see a smaller engine over thrust a large engine that lacks of fuel.

Also in larger engines, it is difficult to have fuel mix well with the intake air to achieve a large even chamber fill with properly mixed fuel/air charge.

Typically what we see in thrust is near 1 pound for each square inch of combustion chamber cross sectional area. A very few are much higher, I think back in the forum somewhere there were reports of 2 pounds per square inch for some engines. And that is where I think the future of PJ will remain, in high peak CC pressures, at 40 psi or above.

There are some conflicting ideas on multiple intakes versus single intakes. One problem with multiples is increased boundry layer, and intake flow resistance. One advantage that I have noticed is increased ability to mix the air fuel. I wouldn't recommend blended bodies or multi-intakes until some skill has been achieved with the single intakes. Single intakes are the simpler design.

It is important to define your design goals and start working towards that end. If your goal is fuel efficiency then certain engines require less fuel than others. If it is heat generation, then other designs work best. You need to know what you want.

These aren't very good answers to your questions, I guess thats why I didn't answer previously.

[Storm]

Thankyou for replying, your answers are good and appreciated, the questions can only be answerd through trial and error which I have none (except JJs) so I humbly ask those who have come before me.

Metiz I have read all pages of PyroJoes recipe, Mikes Blended bodies and various notes on Kentfield and have used the search function extensivly, my research has left me with these questions and that is why I ask.

There seem to be many varieties of combustion chambers that work from a cylinder with flat plates on the ends merging into ones with a cone on one or both ends, to full cones and spheres and even boxes(electra). My question related to someone having a volume how would they determine an exhaust diameter to suit? PyroJoes recipe may be followed on from there.

My interest is lightweight, compact, thrust.

PyroJoe you say thrust to size is not exponential. 1-2lbs/inch^2 CCarea, looking up Argus V1 660lbs thrust 33inch diameter (pie r ^2 =855inch^2) had a thrust to area ratio of less than 1lb/inch^2. Thankyou for that.

How does length of CC affect thrust, Shorter fatter Vs longer skinnier?

Regarding multiple intakes, A/F mixing is said to be better but does that actually increase the TSFC? I guess if it did you would still need a larger, heavier, louder engine to produce the equivalent thrust of the single intake counterpart, for the efficiency benefits to outweigh the other negatives it would depends on the application for sure.

Do single intakes provide for the greatest thrust in the smallest lightest package or is it still undecided?

[PyroJoe]

Lightweight, compact and thrust lend the design to multi intake blended bodies.

Shorter fatter, or longer skinnier? In blended bodies or single bodies CCs, I typically see better performance when they start resembling Helmholtz resonators. Very nearly approximating the Dyna Jet CC in shape per volume. I guess that would be logical, as that combustor has had many decades of development.

TSFC has alot hinged on peak temperature and pressure generation. Which ever cycle you choose, typically the lowest and highest temperatures/pressures define many of the parameters of efficiency. If your mix is good and you have a good CC fill ratio, the core gas will expand and splash against the tailpipe, and then typically when near peak throttle, the core gas will spill onto the walls of the CC.

I used to be happy to run an engine up to red throughout the most of the duct, until I was able to bring a few up to yellow. One of my blended bodies is a complete freak engine and runs very near white. I dare not run it more than a few seconds as metal errosion sets in quick. I can only imagine the "delta T" as the temperature jumps from ambient to near white hot!

Of course as the engine size increases, the pressures will tend to come into play, typically thinner sheet metal engines will
have a tail collapse as the internal temperatures increase, and the external pressure will eventually crush the tailpipe. Also as the temperature and pressure increases in the CC, any seams that have 90 degree angles will start to load, and have a possibility of failure.

If you have good welds, its actually a badge of honor to blow out a CC plate! Just think about it, you blew apart a piece of steel with air and fuel.

Ok so you can go with a heavier sheet metal tail, and a little thicker CC reinforced with some external gusset plates, which add more weight, but increase the ability of the engine to produce more thrust more efficiently. So you end up trying to balance how hot, how reinforced and how lightweight you want it.

[Storm]

Thankyou PyroJoe,

A quick google search shows the basic Helmholtz resonators to be spherical in shape.


Helmholtz Resonators.

Where these 2 peice sliding cylindrical resonators are used to aid tuning to find a frequency


Tunable Helmholtz Resonators

My thoughts are the resonator frequency is very important (volume) and the intake an exhaust length and diameter should match the acoulstic properties precisely.

[Storm]

Can you please help me understand the relationships between various parts some more?

Researching your recipe again PyroJoe and not knocking it just trying to understand the basics here, you say in your opening post there are variations used to achieve different engine objectives (like differing boats etc) so maybe this is one of them. Ok Larry Cottrill points out on this forum that intake length works well at L3 (Hinote criteria) but doing the sums of your recipe (E=1.5"ID) x 20 = 30" + (CC 1.5" x 4 =6"ID), Total length=36" Therfore L3 =12" where you set intake length (E 1.5" x 2.67) = 4" ie: 1/3 of L3. Can you explain how your engine works with such short intake, is it a wave distance or something like that?

How do you measure the wave distance anyway, is it volume of combustion chamber x burn rate of fuel or length of exhaust?

I have a spherical CC of 5 liters, any help to size an exhaust diameter to suit it?