I think I need help. A lot. Really.

[Ape]

Hi.
I am an apprentice precision mechanic, who is lucky enough to have a foreman so generous as to give me a day a week to make whatever I fancy, as incentive to further develop my skills on mill and lathe.
I have been fascinated by the concept of pulsejets since I was about 8, when I saw a V1 on display in a museum, and thought it could be supercool to make a small stand with all the nessecary components to start and run a miniature pulsejet.
I figured, being me, that since the concept of the engine itself is rather simple, it should be relatively easy to make one and get it to run.
However, since I started looking into it, I have been having serious doubts as to the feasability of my project.
Actually, I have reached a point where I don't even know what to ask for, in terms of advice, and so I guess I'm down to: "HELP!!"
I have been thinking it would have to be small. really small. about 20 by 160 mm., valved, liquid fueled, glow plug, using the airflow in the intake to vaporize the fuel.
But then I came here. And read the topics in the forum. I still want that jet, really. But. . can I even get a pulsejet that small to run? Will I need a leafblower to start it? Will it blow up in my face? Are there "better" alternatives to liquid fuel? If I look at the thread with all the physics mumbo-jumbo again, will my head cave in? Would I need to sell my soul to whatever resides in the twisting nether to ever see. . no, hear it run? I haven't even really started yet, and I'm already intimidated beyond what I could possibly imagine, and fresh out of steam.
Well. I'm gonna try, for sure, but any and all help will be appreciated. If nothing else, I can add this to the list of failed projects.
Im gonna turn the entire body in stainless, and I think I might need to know if the thickness of the material is significant relative to the pulse, if there are certain relations between combustion chamber measurements and exhaust that won't work, or that'll work better, and all the other stuff that I don't even know I should be asking.
pretty please with sugar on top, help this poor lad!

Ape

[larry cottrill]

Ape -

Valved pulsejets HAVE been made really small. The only part of your idea that might be a problem is trying to make the engine very short. Short lengths make it hard to develop good pressure swings in the engine, basically because the deflagration speed (the speed at which the flame front advances through the air) of most fuels is actually not very fast. HOWEVER, if you're skilled in handling higher energy fuels such as acetylene or hydrogen, this problem is taken care of (VERY small valveless pulsejets have been recently built using these kinds of fuel). If you need to stick with "safer" fuels like propane or gasoline (petrol), though, I greatly doubt that you'll even come close to getting such a short length to run. Probably (I am just guessing), nothing shorter than about 400 mm could be expected to work using "ordinary" fuels.

Here's a weird attempt of mine at building a little valved engine, but I never did finish it:
http://www.cottrillcyclodyne.com/SFOA1.html

L Cottrill

[Ape]

Ah.
Bummer.
I have been reading a bit more on the subject, and even though I don't really get physics, it seems to me that a pulsejet doesn't sound right because it runs proper, it runs proper because it sounds right (oversimplified, and only part of the truth, of course, but still.). Is that completely wrong?
I have access to acetylene, and could propably use that, but with a proper container, it just won't fit on any shelf in my home. Besides, that would sorta ruin the charm of the thing, to me, anyways. But 400mm would be. . well. . complicating matters a bit. It's gonna be a %"#"¤%& to turn, especially the inside. On top of that, a 400mm engine would have to have a different diameter as well, now wouldn't it? I have been looking at the plans available on this site, and it seems to me the proportions are pretty much identical. Of course I just thought that it was a tried and tested layout, and thus "easy". .. Hmmm. And now you're telling me it's not that it's "easy", it's that it's possible. Well. Thank you for bearing with me, I know how testing it can be to deal with newbies. But; if I go and try anyway, do you know if there's some set range of frequencies I should be aiming for?
Thanks again.

Ape

[larry cottrill]

Yeah, I wouldn't want to do anything where I had to send a boring bar WAY in. That's pretty difficult, especially where the ID is small. That's mainly why I went with standard pipe - something I could chuck at one end, tailstock the other end, and just carve away the OD over most of the length and then just chuck the piece to finish the ends. Crude but effective, as I like to say. But then, you have to weld a chamber on, which isn't what you want to do.

I would guess that if you did one at a 400mm length with reasonable proportions that would actually run, you'd be at 300 hz, and maybe much higher! Contrary to popular belief, frequency is not just a matter of length, so it's really hard to estimate something without actual analysis, and for that you need a full set of pipe internal dimensions.

L Cottrill

[Ape]

hehehe, I thought popular belief was that frequency was a matter of diameter first, length second

Anyhoo, I could make a conical fit that would propably hold the pieces well enough together, if I first applied some serious pressure to it. And as long as I don't weld or solder or whatnot, but stick to threading and fitting parts, standard tubing would most likely be allowed. From what I have gathered on this forum, the smaller the engine, the proportionally larger the combustionchamber has to be? Should I up the diameter of the combustion chamber to 30mm or even 40mm, and stretch the overall length to about 250mm, as a sort of first try, and then see if I can learn anything from that? .. . I think I will.
Your help is greatly appreciated, and when there is a filmable result, it will be posted right away. Though some time will pass, considering the one day a week I have for the project ;D

Cheers
Ape

[Tim36]

Hey, I saw pulse jets and thought it would be a piece of cake to make one... Silly Tim. I used a program called Pulsejet Engine Calculator 1.4 to work out the dimensions for my jet. I made the mistake of thinking that smaller would be easier, so I made mine about 650mm in over all length. My major trouble was fuel delivery, as I tried to use a carbie style arrangement. Long story short, i'm sure it could be done, but the whole thing needed to be pretty fine, and I didn't have the right bits to machine it properly. A lot of people seem to use hydrogen or accetylene, but I've been thinking about trying using a fuel injector from a small car, and using petrol or ethanol. I'll see if I can put up some pics of mine. It's pretty rough around the edges, and was really just a practice run that I wasn't expecting to be too exciting. I don't feel so bad now knowing I'm not the only newbie...

[Tim36]

This is my first attempt. I think I will start over and go a bit bigger, to try out the injection thing. My understanding is that larger jets have a lower resonance frequency... correct?

[Mike Everman]

Hey, Tim, that doesn't look too rough at all. You could use a flare on the tail, which can be essential for starting. That wood is going to be a fire when you get it running, so be warned, though I like the wire suspension idea.
You may be closer than you think. What's the valve thickness? did you lap in the valve plate for a good seal? You can most likely run this on propane with a simple brake line injector, and save the liquid fueling for the next thing.

[Ape]

Seeing as how you already build one, Tim, I still consider myself more of a newbie than you!

Well. . I'm not going for small because I think it's easier, but because the project calls for a size that'll fit on the shelf in my formans office , that already holds the projects of former apprentices. (He refers to that shelf as "the Impressionator".) There has been a sort of tradition that all apprentices should make at least one project, not just to hone skills, but to show off as well. So I don't wanna let the man down, especially since he's giving me a day a week to work on the project (with pay and all, mind you.).

I was thinking I would use whatever liquid fuel I could get my hands on. . nitromethanol, diesel, gasoline, kerosene, ethanol, whatever. And maybe have the fuel line run around the exhaust a few times to vaporize the fuel before letting it into the engine, to make fuel delivery a little easier. The only issue I foresee in doing so is preheating the engine to actually vaporize the fuel before I can even start it proper. . or well. . TRY to start it proper. Heh. But then again, I AM a newbie.

Best of luck to you, I think messing with pulsejets means you'll need it

Ape

[Tim36]

yeah, I know the wooden stand is pretty bad... Don't worry, I'll be building a proper stand before I try anything more I did lap the valves on a granite plate. I used 6'' valves. I'll try the flare on the tail, and propane. I'll let you know how it goes. cheers guys.

[Tim36]

My bad, I meant to say 0.006'' valves. I originaly used 0.005'', but they blew up, before the engine even looked like running. probably had something to do with not having enough overlap of the valves. how much overlap should the valves have, and do you coat the plate with a thin layer of high temp silicone or anything after you've lapped it? the plans I was half following said to.

[Ape]

I had two hours to spare today, and decided to spend them making a mockup of the intake, just to get an idea of what I wanted, shapewise.
Please bear in mind that it IS just that, it's not exactly right yet. But overall, I think it's gonna be something pretty much like it, just smaller, ofc.
Any feedback will be appreciated, and most likely needed.

[larry cottrill]

Very distinctive style -- I like it very much.

I wouldn't want to be in the way once it starts moving.

L Cottrill

[Ape]

hehehe, the tip, yeah. Well, I was planning to shorten it somewhat, flatten it, and mount the venturi there, but thats not gonna happen, have decided to have the venturi comming in from the side. And then I thought, you know, what the heck, why not just leave it like that? I'm not really old enough to have Flash Gordon childhood memories, but still

Cheers

[Ape]

haha. last post shows clearly how much of newbie I am ;D